Volume 30, Issue 3, May–June 2021

CONTENTS-Volume 30, Issue 3, May–June 2021

Springer Link: https://link.springer.com/journal/11630/volumes-and-issues/30-3

1. A Review on Modeling of Bionic Flow Control Methods for Large-Scale Wind Turbine Blades

ZHANG Mingming, ZHANG Yi’nan, CAI Chang, CAO Huijing, ZHANG Ziliang

Corresponding author: ZHANG Mingming

E-mail: mmzhang@iet.cn

Journal of Thermal Science, 2021, 30(3): 743-757.

https://doi.org/10.1007/s11630-021-1444-1

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1444-1

Keywords: riblets, leading-edge protuberances, trailing-edge serrations, modeling method

Abstract: Due to complicated working conditions, the normal operating large-scale wind turbine blades are often suffering from some inevitable problems, i.e., friction adhesion, flow separation and acoustic noise, which may significantly affect the aerodynamic performance of the blades and thus the wind turbine system. Therefore, effective measurements must be taken to solve these issues. Correspondingly, several novel bionic flow control methods by mimicking shark skin, whale fin and owl wing, i.e., riblet, leading-edge protuberance and trailing-edge serration, have been recently studied, and good progresses have been made in terms of effectiveness, analysis and mechanism. However, these potential techniques are unable to be widely applied within wind energy community due to the lack of reasonable modeling methods, clearly reflecting the effect of bionic structures on the flow field around, which results in incapability to carry out further optimal design of bionic blade. To this end, this review paper first concentrated on a summary of the control mechanisms of three bionic techniques. Based on this, some feasible ideas of model buildup were proposed. Finally, the flow analyses around the typical blade airfoils were chosen as case studies to verify the feasibility and accuracy of these simulation methods.

2. Wind Tunnel Experiments and Numerical Study on Performance Characteristics of an H-type Vertical Axis Wind Turbine in the Spanwise Direction

ZHANG Yanfeng, LI Qing’an, ZHU Xinyu, SONG Xiaowen, CAI Chang, GUO Zhiping

Corresponding author: LI Qing’an; E-mail: liqingan@iet.cn

GUO Zhiping; guozhp@imut.edu.cn

Journal of Thermal Science, 2021, 30(3): 758-771.

https://doi.org/10.1007/s11630-021-1435-2

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1435-2

Keywords: VAWT, wind tunnel experiments, CFD, spanwise direction, pressure distribution, power coefficient

Abstract: The aerodynamic forces and vortex characteristics of an H-type Vertical Axis Wind Turbine (VAWT) become complicated because of dynamic stall, particularly in the three-dimensional impact on the blade spanwise direction. This study focused on the evaluation of the aerodynamic performance and vortex characteristics of an H-type VAWT in the spanwise direction by numerical simulations and wind tunnel experiments. Pressure acting on the blade surface was obtained from multiport pressure measurement devices by wind tunnel. Meanwhile, the vortex field around different blade sections was investigated through numerical simulations. The stall behavior was analysed by comparing the results of numerical simulations and experiments. As a result, the tangential force of single blade was mainly contributed at the chordwise position of x/c≤0.4c and the power of single blade was mainly contributed at the azimuthal angle range of 60°≤θ≤150° in the blade section position region of 0≤z/(H/2)≤0.7. At the section position of z/(H/2)=0.5, the initial flow separation was found at the suction side and progressed forward to the leading edge. With the increase of the tip speed ratios, the decreasing position of the averaged local power coefficient of each section was closer to the middle section of z/(H/2)=0, and the attenuation speed became faster. The power coefficient reductions at the blade section position of z/(H/2)=0.9 were 38.29%, 46.78% and 56.42% when the tip speed ratios were 1.38, 2.19 and 2.58, respectively. The results of this study provided a better understanding of the development of the performance characteristics and vortex characteristics of H-type VAWT.

3. Experimental Study on the Heating Effect of a Wind-Energy Stirring Heater

LIU Xingran, SUN Xianpeng, LI Jianming, QIU Xinyang, YANG Kang, CAO Yanfei

Corresponding author: SUN Xianpeng

E-mail: sunxianpeng@nwsuaf.edu.cn

Journal of Thermal Science, 2021, 30(3): 772-781.

https://doi.org/10.1007/s11630-021-1344-4

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1344-4

Keywords: wind-heating, heating power, heating effect, working fluids, structure design

Abstract: In this paper, an experiment system of wind-energy stirring heater has been designed and built. Its heating performance under the rotation speed of 300 r/min has been studied through changing stirring rotor’s layer number or using different working fluids. With the help of CFD numerical simulation method, we studied the influence factors including temperature rise, total heat, heating power of each experimental group, and analyzed why these factors have such an effect. The results show: increasing the layer number of the stirring rotors can increase the motion intensity of working fluid and improve the heating effect; the quantity of effective working fluid in the rotor area can be increased significantly with the increasing of liquid level; the working liquid with high density, low viscosity, low specific heat capacity will be the ideal one.

4. A Critical Review of Real Gas Effects on the Regenerative Refrigerators

CAO Qiang, LUAN Mingkai, LI Peng, WEI Li, WU Yan

Corresponding author: WU Yan

E-mail: yan.wu@tongji.edu.cn

Journal of Thermal Science, 2021, 30(3): 782-806.

https://doi.org/10.1007/s11630-020-1381-4

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1381-4

Keywords: real gas effects, regenerative refrigerators, loss mechanism, reduction methods

Abstract: The regenerative refrigeration is an important reverse work-heat conversion cycle with a theoretical coefficient of performance (COP) identical to the Carnot efficiency. Practical regenerative refrigerators are capable of working down to 4 K and largely fulfill the refrigeration requirement of modern technologies in many fields, especially for space applications. However, the enthalpy flow associated with the pressure dependence, abbreviated as pressure-induced enthalpy flow, brought about by real gas effects degrades the theoretical COP of the refrigerator to below about 30% of the Carnot efficiency at the temperatures of below the critical point. This paper reviews the long history of exploring the real gas effects which dates back to the 1970s and continues to now. Important explorations of uncovering the loss mechanism and reducing such losses are summarized. The theories that are in accordance with experimental results and simulation results are expounded. We further carry out analyses on the expansion components, including the pulse tube and the clearance gap. Several inferences are made in order to explore the long-lasting puzzles about real gas effects. It is emphasized that the underlying cause of the loss in the regenerator is an indirect effect of the real gas properties. Further study about carrying out a direct verification of the theory is proposed.

5. Thermal-Hydraulic Calculation and Analysis on Evaporator System of a 1000 MW Ultra-Supercritical Pulverized Combustion Boiler with Double Reheat

WAN Li, YANG Dong, ZHOU Xihong, DONG Le, LI Juan

Corresponding author: YANG Dong

E-mail: dyang@mail.xjtu.edu.cn

Journal of Thermal Science, 2021, 30(3): 807-816.

https://doi.org/10.1007/s11630-020-1322-2

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1322-2

Keywords: double reheat, ultra-supercritical boiler, water wall, flow network method, thermal-hydraulic characteristic

Abstract: A double reheat ultra-supercritical boiler is an important development direction for high-parameter and large-capacity coal-fired power plants due to its high thermal efficiency and environmental value. China has developed a 1000 MW double reheat ultra-supercritical boiler with steam parameters of 35 MPa at 605°C/613°C/613°C. Reasonable water wall design is one of the keys to safe and reliable operation of the boiler. In order to examine the thermal-hydraulic characteristics of the double reheat ultra-supercritical boiler, the water wall system was equivalent to a flow network comprising series-parallel circuits, linking circuits and pressure nodes, and a calculation model was built on account of the conservation equations of energy, momentum and mass. Through the iterative solving of nonlinear equations, the prediction parameters of the water wall at boiler maximum continue rate (BMCR), 75% turbine heat-acceptance rate (THA) and 30% THA loads, including total pressure drops, flow distribution, outlet steam temperatures, fluid and metal temperatures were gotten. The results of calculation exhibit excellent thermal-hydraulic characteristics and substantiate the feasibility of the water wall design of the double reheat ultra-supercritical boiler.

6. A Feasible and Novel Multi-Generation System Taking Advantage of Two-Stage Compressor for Luxury Hotels

LI Hongqiang, ZHOU Saiqun, HE Changjie, DENG Lu, LI Lu, LI Shuisheng, ZHANG Guoqiang

Corresponding author:

LI Hongqiang; lhq@hnu.edu.cn

DENG Lu; denglu@hnu.edu.cn

https://doi.org/10.1007/s11630-020-1328-9

Journal of Thermal Science, 2021, 30(3): 817-827.

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1328-9

Keywords: two-stage compression, building energy conversion, heat recovery, district energy systems, energy saving, multi-energy output

Abstract: In order to satisfy heating, cooling and domestic hot water demand and energy conversion in luxury hotels, a feasible and novel multi-generation system taking advantage of the two-stage compressor was proposed and analyzed. The feature of this novel system is that the compressing power can be reduced and domestic water can be generated at the same time by adopting two-stage compressors instead of conventional one-stage compressor with a high compressing ratio. The energy-saving mechanism and two operating modes were introduced in detail. In a further way, the thermodynamic performance was calculated by Aspen Plus so as to make a better understanding of this novel system. And the calculation results showed that, in summer and transition seasons operating mode, the system can obtain COP (Coefficient of Performance) 7.3, and 60°C domestic hot water, when evaporating temperature is 4°C; while, in winter season operating mode, the system can obtain COP 3.1, and 58°C domestic hot water, when evaporating temperature is ?24°C. Compared with the conventional independent systems, this system can achieve higher energy-efficient, higher waste heat recovery, reduce energy consumption, and meet multiple energy demands simultaneously.

7. Experiment Investigation and Control Strategies on Two-Phase Refrigerant Injection Heat Pump System for Electric Vehicle in Start-up Stage

QIN Fei, LIU Hanzhou, ZOU Huiming, CHEN Deqi, TIAN Changqing

Corresponding author:

ZOU Huiming; zouhuiming@mail.ipc.ac.cn

LIU Hanzhou; hzliu@cqu.edc.cn

https://doi.org/10.1007/s11630-021-1458-8

Journal of Thermal Science, 2021, 30(3): 828-839.

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1458-8

Keywords: refrigerant injection heat pump system, heating performance, electric vehicle, start-up stage, control optimization

Abstract: Due to the poor heating performance and operating safety in low ambient temperature, traditional Air Source Heat Pump (ASHP) for Electric Vehicles (EVs) has many limits in cold region, which can be solved by the ASHP with refrigerant injection. During the start-up stage of EV in winter, the inlet air temperature of the in-car condenser is the same as the ambient temperature. At this situation, the performance and control strategy of the heat pump require special attention. In the present study, a series of experiments were carried out on the heating performance of the Refrigerant Injection Heat Pump (RIHP) system in start-up stage of EV, at the ambient temperature from –20°C to –5°C. The effects of compressor speed and injected refrigerant state on the heating performance of the system were discussed in depth. According to the results, the control strategies during start-up stage have been discussed in the end of the article. The study provides a practical control strategy for the RIHP system during the start-up stage of electric vehicles, helping to efficiently operate electric vehicles in cold regions.

8. Knowledge Mining of Low Specific Speed Centrifugal Pump Impeller Based on Proper Orthogonal Decomposition Method

ZHANG Renhui, CHEN Xuebing, LUO Jiaqi

Corresponding author: ZHANG Renhui

E-mail: zhangrhlut@163.com

https://doi.org/10.1007/s11630-020-1356-5

Journal of Thermal Science, 2021, 30(3): 840-848.

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1356-5

Keywords: centrifugal pump, multi-objective optimization, proper orthogonal decomposition, knowledge mining

Abstract: To clarify the complex relation between the pump blade shape and its corresponding hydraulic performance, the knowledge mining method of centrifugal pump impeller based on proper orthogonal decomposition (POD) was proposed. The pump blade shape was parameterized by cubic Bezier curve. The Latin hypercube design method was employed to supply the necessary samples for producing the perturbations of blade wrap angle, and blade angle at inlet and outlet. The hydraulic efficiency and head were optimized by NSGA-II and RBF hybrid algorithm. The Pareto-optimal solutions were obtained. In order to further illustrate the relationship between the centrifugal pump blade shape and its hydraulic performance, the POD method was used to discover the effects of optimized blade shape to the flow solutions. For the optimization of centrifugal pump MH48-12.5, blade shape and relative velocity field in impeller from Pareto-optimal solutions were analyzed. The results demonstrate that larger blade angle and smaller wrap angle increase the average kinetic energy in impeller, resulting in higher pump head design. Smaller blade angle and larger wrap angle decrease the velocity gradient from the pressure side to suction side, resulting in smaller hydraulic loss and higher efficiency design.

9. Investigation of Self-Priming Process of a Centrifugal Pump with Double Blades

QIAN Heng, MOU Jiegang, REN Yun, ZHU Zhibing, LIU Nuojia, ZHENG Shuihua, WU Denghao

Corresponding author: WU Denghao

E-mail: wudenghao@aliyun.com

Journal of Thermal Science, 2021, 30(3): 849-858.

https://doi.org/10.1007/s11630-020-1370-7

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1370-7

Keywords: self-priming process, gas-liquid two-phase flow, centrifugal pump, gas-liquid mixing and separation, transient numerical simulation

Abstract: The gas-liquid two-phase flow patterns of a centrifugal pump during the self-priming process were investigated numerically and experimentally. The Euler-Euler multiphase model and SST k-ω turbulence model were applied for simulating the self-priming process. Meanwhile, the changes of motor speed and self-priming height were considered in the simulation. The overall transient two-phase flow features and water level distributions were mapped. Results showed that the self-priming process was divided into three stages. The liquid level in inlet-pipe rose in oscillation during self-priming process. The variations of water level during self-priming process of numerical simulation and test result agreed well. The inlet-pipe (Ver) was filled at 22 s and 24 s respectively numerically and experimentally. The bubble cloud circulated in the volute during middle stage of self-priming process, and breakup into smaller bubbles by shear force and tongue, and then discharged into chamber. The bubbles in the outlet-pipe mainly included bubbly flow and slug flow at the last stage of self-priming process, which is morphologically consistent with the test results. Also, during the self-priming process, the reflux liquid was pressed by blades and fully mixed with gas; that is the way to realizing the function of self-priming.

10. A Novel Approach for Energy Efficiency Prediction of Various Natural Draft Wet Cooling Towers Using ANN

SONG Jialiang, CHEN Yongdong, WU Xiaohong, RUAN Shengqi, ZHANG Zhongqing

Corresponding author: CHEN Yongdong

E-mail: chenyongdong@hgmri.com

Journal of Thermal Science, 2021, 30(3): 859-868.

https://doi.org/10.1007/s11630-020-1296-0

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1296-0

Keywords: natural draft wet cooling tower, artificial neural network (ANN), energy efficiency, evaporation loss of water, power plant

Abstract: Cooling tower is crucial equipment in the cool-end system of power plant and the natural draft counter-flow wet cooling tower (NDWCT) gets wide application. The artificial neural network (ANN) technique is becoming an effective method for the thermal performance investigation of cooling towers. However, the neural network research on the energy efficiency performance of NDWCTs is not sufficient. In this paper, a novel approach was proposed to predict energy efficiency of various NDWCTs by using Back Propagation (BP) neural network: Firstly, based on 638 sets of field test data within 36 diverse NDWCTs in power plant, a three-layer BP neural network model with structure of 8-14-2 was developed. Then the cooling number and evaporation loss of water of different NDWCTs were predicted adopting the BP model. The results show that the established BP neural network has preferable prediction accuracy for the heat and mass transfer performance of NDWCT with various scales. The predicted cooling number and evaporative loss proportion of the testing cooling towers are in good agreement with experimental values with the mean relative error in the range of 2.11%–4.45% and 1.04%–4.52%, respectively. Furthermore, the energy efficiency of different NDWCTs can also be predicted by the proposed BP model with consideration of evaporation loss of water in cooling tower. At last, a novel method for energy efficiency prediction of various NDWCTs using the developed ANN model was proposed. The energy efficiency index (EEI) of different NDWCTs can be achieved readily without measuring the temperature as well as velocity of the outlet air.

11. Thermodynamic Evaluation of Transcritical CO2 Heat Pump Considering Temperature Matching under the Constraint of Heat Transfer Pinch Point

LIU Ruijian, GAO Fengling, LIANG Kunfeng, WANG Lin, WANG Moran, MI Guoqiang, LI Yachao

Corresponding author: LIANG Kunfeng

E-mail: lkf@haust.edu.cn

https://doi.org/10.1007/s11630-021-1373-z

Journal of Thermal Science, 2021, 30(3): 869-879.

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1373-z

Keywords: heat pump, CO2 transcritical cycle, temperature matching degree, exergy, COP, pinch point

Abstract: The non-linear temperature glide in the supercritical CO2 cooling process makes the heat transfer pinch point of heat exchanger show multiplicity, like size, location distribution and quantity, which makes the thermodynamic performance of the CO2 transcritical cycle more complex and eventually affects the evaluation of the optimal operating state of the system. Based on the second law of thermodynamics and the constraints of heat transfer pinch, a thermodynamic evaluation method of CO2 transcritical heat pump system was proposed according to the degree of temperature matching. The influence mechanism of multi-characteristic change of heat transfer pinch point on temperature matching degree and the effect of temperature matching degree on thermodynamic performance of CO2 transcritical heat pump system were discussed. The relationship between temperature matching degree, COP and exergy efficiency of the system was analyzed. It is considered that the change of temperature matching index value can clearly characterize the change trends of COP and exergy efficiency. That is, the smaller the temperature matching degree is, the closer the temperature distribution of heat transfer fluids on both sides of the heat exchanger is to Lorenz cycle, and the greater the COP and exergy efficiency are. Furthermore, by monitoring the outlet temperature of the CO2 cooler, which has an essential relationship with the temperature matching degree during the heat exchange process, the deviation between actual and optimal working condition can be judged online, which is beneficial to real-time evaluation of the working state of the system.

12. Heat Transfer Characteristics of Printed Circuit Heat Exchanger with Supercritical Carbon Dioxide and Molten Salt

LAO Jiewei, FU Qianmei, WANG Weilong, DING Jing, LU Jianfeng

Corresponding author: LU Jianfeng; E-mail: lujfeng@mail.sysu.edu.cn

DING Jing; dingjing@mail.sysu.edu.cn

Journal of Thermal Science, 2021, 30(3): 880-891.

https://doi.org/10.1007/s11630-020-1374-3

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1374-3

Keywords: heat exchanger, molten salt, supercritical carbon dioxide, numerical simulation

Abstract: Molten salt and supercritical carbon dioxide (S-CO2) are important high temperature heat transfer media, but molten salt/S-CO2 heat exchanger has been seldom reported. In present paper, heat transfer in printed circuit heat exchanger (PCHE) with molten salt and S-CO2 is simulated and analyzed. Since S-CO2 can be drove along passage wall by strong buoyancy force with large density difference, its heat transfer is enhanced by natural convection. In inlet region, natural convection weakens along flow direction with decreasing Richardson number, and the thermal boundary layer becomes thicker, so local heat transfer coefficient of S-CO2 significantly decreases. In outlet region, turbulent kinetic energy gradually increases, and then heat transfer coefficient increases for turbulent heat transfer enhancement. Compared with transcritical CO2 with lower inlet temperature, local heat transfer coefficient of S-CO2 near inlet is lower for smaller Richardson number, while it will be higher for larger turbulent kinetic energy near outlet. Performance of PCHE is mainly determined by the pressure drop in molten salt passage and the heat transfer resistance in S-CO2 passage. When molten salt passage width increases, molten salt pressure drop significantly decreases, and overall heat transfer coefficient slightly changes, so the comprehensive performance of PCHE is improved. As a result, PCHE unit with three semicircular passages and one semi-elliptic passage has better performance.

13. A Correction Factor-Based General Thermal Resistance Formula for Heat Exchanger Design and Performance Analysis

HAO Junhong, CHEN Qun, LI Xia, ZHAO Tian

Corresponding author: CHEN Qun

E-mail: chenqun@tsinghua.edu.cn

Journal of Thermal Science, 2021, 30(3): 892-901.

https://doi.org/10.1007/s11630-021-1369-8

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1369-8

Keywords: general thermal resistance formula, correction factor, heat current model, linear-transfer law, heat exchanger

Abstract: Methods for the analysis of heat exchangers with various flow arrangements modeling, design, and performance are essential for heat transfer system modeling and its integration with other energy system models. This paper proposes the use of the linear-transfer law for the heat exchanger design and performance analysis as a function of the thermal resistance related to the ratio of a linear temperature difference to the total heat transfer rate. Additionally, we derived a correction factor that represents the influence of the flow arrangement on the heat transfer performance by the effective thermal conductance, as a function of correction factor, heat transfer coefficient, and surface area. Based on the effective thermal conductance, we propose the hot-end NTU and cold-end NTU for deriving a standardized and general thermal resistance formula for different types of heat exchangers by the combination of the correction factor with linear-transfer law. Moreover, for parallel-flow, cross-flow, and 1-2 Tubular Exchanger Manufacturers Association (TEMA) E shell-and-tube heat exchangers, we derived and obtained alternative correction factor expressions without introducing any temperatures. Two cases about heat exchanger design and performance analysis show that the calculation processes using the correction factor-based general thermal resistance are straightforward without any iteration and the calculation results are accurate. Finally, the experimental validation shows that the general thermal resistance formula is appropriate for analyzing the heat transfer performance. That is, the correction factor-based general thermal resistance formula provides a standardized model for heat exchanger analysis and heat transfer/integrated energy system modeling using the heat current method.

14. Effect of Blockage Ratio on Heat Transfer and Pressure Drop in Rotating Ribbed Channels at High Rotation Numbers

DENG Hongwu, LI Hua, TAO Zhi, QIU Lu, ZHU Jianqin

Corresponding author: ZHU Jianqin

E-mail: zhujianqin@buaa.edu.cn

Journal of Thermal Science, 2021, 30(3): 902-913.

https://doi.org/10.1007/s11630-020-1275-5

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1275-5

Keywords: thermal performance, rib blockage ratio, high rotation number

Abstract: At high rotation numbers, the rotational effects on heat transfer and flow could be diverse among the channels with different blockage ratios. However, most studies are conducted under low rotation number (less than 0.25) and selected blockage ratio. This paper experimentally investigates the effect of rib blockage ratio (ranges from 0 to 0.3) on pressure loss and heat transfer in a rotating square channel under high rotation number (up to 0.81). The ribs staggered on leading and trailing walls were oriented 90° to the mainstream flow. The Reynolds number and the wall-to-fluid temperature ratio varied from 20 000 to 40 000 and 0.08 to 0.2, respectively. The results showed that a larger blockage ratio resulted in a better heat transfer but a higher pressure drop. The optimum blockage ratio was 0.1 for the best thermal performance. The rotational effects were weakened in the passage with a higher blockage ratio, where the critical rotation number could not be observed. Moreover, the heat transfer enhancement induced by rotation was more significant when the temperature ratio increased. Finally, the correlations were developed for the pressure drop and the convective heat transfer on the leading and trailing edges.

15. Investigation on the Transient Thermal Performance of a Mini-Channel Cold Plate for Battery Thermal Management

FANG Yidong, SHEN Jiali, ZHU Yue, YE Fei, LI Kang, SU Lin

Corresponding author: LI Kang; E-mail: lklk789@usst.edu.cn

SU Lin; linsu@usst.edu.cn

Journal of Thermal Science, 2021, 30(3): 914-925.

https://doi.org/10.1007/s11630-020-1280-8

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1280-8

Keywords: battery thermal management, cold plate, transient analysis, temperature distribution, mini-channel

Abstract: Cold plate is an important component for a liquid battery thermal management system. In order to study the transient thermal performance of the cold plate under conditions with sharply increasing heat loads, the numerical model of a battery cold plate is established. The validation experiment shows that the error between the simulation and experiment is around 2.5% to 5%. Effects of the coolant flow rate, the increase in heat flux, and the channel number are analyzed to study the transient thermal performance of the cold plate. Results show that the average temperature of the cold plate at 540 s is lowered from 28.3°C to 26.9°C when the coolant flow rate is raised from 0.065 kg/s to 0.165 kg/s. The temperature deviation is decreased when the coolant flow rate is increased from 0.065 kg/s to 0.115 kg/s; however, it is slightly increased if the coolant flow rate is further increased. Both average temperature and temperature deviation are raised if the final heat flux is increased from 11 000 W/m2 to 16 500 W/m2, which are 2 and 3 times of the initial, respectively. In addition, increasing the channel number has slightly positive effect on the average temperature of the cold plate, while the temperature deviation is increased when the channel number is increased from 3 to 11 due to the non-uniform velocity distribution between each channel. The results of this study will be helpful during the design of cold plate for battery thermal management, especially for transient conditions with sudden rising heat loads like thermal runaway.

16. Visualization of Thermo-Hydrodynamic Behavior in Flat-Plate Pulsating Heat Pipe with HFE-347

YAN Lipei, ZHANG Ping, XU Hui, MA Wei, SHENG Jiang

Corresponding author: ZHANG Ping

E-mail: pingzhang@guet.edu.cn

Journal of Thermal Science, 2021, 30(3): 926-938.

https://doi.org/10.1007/s11630-021-1409-4

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1409-4

Keywords: flat-plate pulsating heat pipe, HFE-347, visualization, thermal performance, infrared thermal imaging

Abstract: In this paper, the visualization of the thermo-hydrodynamic behavior in flat-plate pulsating heat pipe (FP-PHP) with HFE-347 is experimentally investigated. The FP-PHP is vertically placed with filling rate of 20% to 70% and heating power of 20 W to 140 W. A high-speed camera is used to record the two-phase flow in the FP-PHP. Four flow pattern types and four flow directions are observed. The flow directions of the two-phase flow inside the FP-PHP with medium filling rate (40%–60%) are the most complex, and the FP-PHP with high filling rate (70%) is most likely to form a directional circulating flow. At high heating power (100 W to 140 W), the flow patterns in FP-PHP with medium (40%–60%) and high filling rate (70%) are dominated by mixed flow. The wall temperature fluctuates greatly at moderate heating power (60 W to 80 W) owing to the uncertainty of the flow direction. The temperature distribution of the FP-PHP is highly affected by the heat transfer intensity of the working fluid under different flow states, so that the state of fluid flow and the thermal performance of FP-PHP can be evaluated through the infrared thermal image of the FP-PHP.

17. Thermo-Mechanical Simulation of Self-Heating of a High-Power Diode Made of Ti3SiC2 (MAX) Phase-on-4H-SiC Substrate

ABOU HAMAD Valdemar, SOUEIDAN Maher, HAMAD Hassan, GREMILLARD Laurent, FABREGUE Damien, ZGHEIB Charbel, ZAATAR Youssef

Corresponding author: SOUEIDAN Maher

E-mail: msoueidan@cnrs.edu.lb

Journal of Thermal Science, 2021, 30(3): 939-949.

https://doi.org/10.1007/s11630-020-1254-x

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1254-x

Keywords: thermo-mechanical simulation, self-heating, 4H-SiC, MAX Phase, electrical contact

Abstract: The fact that traditional semiconductors have almost reached their performance limits in high power applications, is leading to failure in high power devices. This failure results from self-heating effects, leading to higher temperature and a breakdown of the electrical contact. The good thermal and mechanical properties of 4H-SiC and Ti3SiC2 and their good performance at high temperatures make them good candidates for high power applications. In order to improve the performance of electrical contacts, a thermo-mechanical simulation was carried out using the finite element method to study the self-heating effects in a high power PN diode made of a 4H-SiC substrate with a Ti3SiC2 electrical contact and Al3Ti metallization. The three-dimensional model took into account the temperature dependency of several thermal and mechanical properties of the different materials to improve calculation accuracy. To simulate the self-heating, the power loss in the diode was calculated from the corresponding direct I-V characteristic. In addition, the interfacial thermal resistances (ITR) between the different layers were varied and studied in the thermo-mechanical investigation, in sequence to determine their effects on the heat dissipation and the resulting stresses in the model. The results show that for realistic ITR values, the ITR barely affects heat diffusion mechanical stresses of the model. Whereas, ITR may cause serious problem to the functionality and the efficiency of some electronic components. On the other hand, extremely large ITR leads to a decrease in the thermal stress in the diode. Good control on the ITR may help to improve the performance of high-power devices in the future, in addition to providing more efficient electrical contacts.

18. Mixed Convection in a Ventilated Enclosure by Considering Both Geometrical Parameters and Thermo-Physical Properties of Water/Cu Nanofluid

MEHDI Miansari, AFSHIN Halimi, MORTEZA Miansari, DAVOOD Toghraie

Corresponding author: MEHDI Miansari; m.miansari@qaemiau.ac.ir

DAVOOD Toghraie; Toghraee@iaukhsh.ac.ir

Journal of Thermal Science, 2021, 30(3): 950-961.

https://doi.org/10.1007/s11630-020-1267-5

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1267-5

Keywords: heat transfer, mixed convection, open enclosure, rectangular obstacle, nanofluid

Abstract: In this study, mixed convection heat transfer and fluid flow is numerically investigated in an open system enclosure containing a heated obstacle. Effects of the positions of both obstacles and fluid flow outlet on the flow field and heat transfer within the enclosure were studied. Richardson number ranging from 0.1 to 10 was examined for a constant Grashof number at different Reynolds numbers in both base fluid and fluid mixed with Cu nanoparticles (0 to 4% of volume fractions). It is found that the obstacle and outlet position as well as the Richardson number greatly affect the flow field and heat transfer rate. The maximum heat transfer rate occurs for Richardson number 0.1, when the outlet and obstacle position are located at the bottom of the right side of the enclosure (P3) and near right side of the enclosure (8H), respectively. Also, the heat transfer rate is higher when the obstacle is placed close to the inlet (2H) and outlet (8H) compared to the center of the enclosure (5H). It is shown that although the change of volume fraction of nanofluid does not have significant effect on the flow pattern, however, it enhances the heat transfer rate through improving the effective thermo-physical properties of the base fluid.

19. Heat Transfer Analysis for Dropwise-Filmwise Hybrid Surface of Steam on Vertical Plate

ALHASHEM Abdulwahab, KHAN Jamil

Corresponding author: ALHASHEM Abdulwahab

E-mail: ai.alhashem@paaet.edu.kw

Journal of Thermal Science, 2021, 30(3): 962-972.

https://doi.org/10.1007/s11630-021-1146-z

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1146-z

Keywords: dropwise-filmwise hybrid surface, dropwise condensation region width, filmwise condensation region width, drop-size distribution, maximum radius to region width ratio

Abstract: A model is proposed to predict and evaluate the heat transfer characteristics of dropwise-filmwise hybrid surface. This is approached by modifying the original drop-size distribution, which is defined for fully dropwise condensation (DWC) and making it applicable for dropwise-filmwise condensation (DFC). The modification is achieved by simulation work to determine two parameters: the area fraction occupied by large drops f and the ratio of the maximum radius of newly formed drops to that of larger drops ?. Simulation results show a good agreement with the literature for fully DWC and provide a correlation for each parameter with respect to DWC region width in the DFC hybrid surface. The present model evaluates the heat transfer performance of DFC by utilizing these correlations. A comparison is made between the proposed model with experimental work from the literature and results show a good agreement. While changing filmwise condensation (FWC) region width significantly affects the overall heat transfer performance, utilizing smaller width to that of DWC region has the advantage over fully DWC. Furthermore, surface renewal within the hydrophobic region is controlled by adjusting the DWC region width. When the ratio of drop maximum diameter to DWC region width is unity, surface renewal is achieved by drops merging to adjacent FWC regions only. When the ratio is less than unity, surface renewal is achieved by sweeping of departing drops within the hydrophobic region and by merging. For each case, an optimum DWC region width which corresponds to the maximum DFC heat flux is defined.

20. Effect of Rotor-stator Axial Gap on the Tip Seal Leakage Flow of a Steam Turbine

LI Pan, CAO Lihua

Corresponding author: LI Pan

E-mail: 283395346@qq.com

Journal of Thermal Science, 2021, 30(3): 973-982.

https://doi.org/10.1007/s11630-021-1469-5

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1469-5

Keywords: steam turbine, axial gap, tip leakage, labyrinth seal, prediction formulation

Abstract: The tip leakage flow in different axial gaps is numerically investigated based on the Reynolds-Averaged Navier-Stokes equations and k-? SST turbulent model using commercial CFD software ANSYS Fluent. The characteristics of vortexes in the flow field of tip seal and the distribution law of rotor tip leakage loss are analyzed under the change of axial gap. The results show that the flow field in the blade tip seal undergoes complex changes with the increase of axial gap. There are two opposite vortices in the seal cavity with small axial gap, namely, the large vortex in the cavity and the small vortex near the wall of the shroud. The small vortex gradually disappears with the increase of the axial gap. When the axial gap is increased to a certain value, the reverse large vortex is formed in the cavity behind the low teeth of seal, and the leakage rate increases significantly; the steam flow angle and the mixing loss also show different changes. Based on the traditional calculation formula of tip leakage rate, a more accurate calculation formula of tip leakage rate is constructed by introducing equivalent labyrinth seal teeth to consider the variation of axial gap.

21. Effect of Unsteady Secondary Vortices on Performance of a 1.5-Stage High- Pressure Turbine

WANG Qingsong, MA Can, SU Xinrong, YUAN Xin

Corresponding author: SU Xinrong

E-mail: suxr@mail.tsinghua.edu.cn

Journal of Thermal Science, 2021, 30(3): 983-998.

https://doi.org/10.1007/s11630-020-1371-6

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1371-6

Keywords: clocking effect, secondary vortices, entropy generation, unsteady flow

Abstract: In modern high-load high-pressure turbine, the secondary flow in the blade channel is very strong and occupies a large spanwise region. Although high-quality experimental data at the stage interfaces have been obtained in previous research, the influence of the clocking position on the secondary flow patterns is not fully understood. This paper investigates the clocking effect in a 1.5-stage high-pressure turbine and focuses on the variations of secondary flow patterns and their effect on the turbine performance. The detailed flow fields of various clocking positions were obtained by carrying out unsteady flow simulations using an in-house code. Among the four clocking positions in this work, the highest entropy generation was observed when the wakes from stator 1 hit the leading edges of stator 2, which is opposite to the well-known conclusion for the turbine with high-aspect-ratio blades. Detailed flow analysis showed that the wakes and the near tip secondary vortices from stator 1 showed different traces when entering the stator 2 channel and the secondary vortices clearly have a more important influence in determining the performance. The different behaviors of the secondary vortices explained the performance variations due to the clocking effect.

22. Using Tip Gaps on Tandem Diffuser to Broaden the Operation Range of a Centrifugal Compressor

FANG Junyi, JI Chunjun, LI Chunyang, SUN Qi

Corresponding author: FANG Junyi

E-mail: 719183952@qq.com

Journal of Thermal Science, 2021, 30(3): 999-1009.

https://doi.org/10.1007/s11630-021-1436-1

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1436-1

Keywords: centrifugal compressor, tandem diffuser, tip gap flow, operation range, surge, choke

Abstract: Gaps in compressors are generally unfavorable because they lead to energy loss. However, gaps on diffuser blades can broaden the operation range of a compressor. To broaden the operation range of a centrifugal compressor with a tandem diffuser, gaps with different sections and widths on the diffuser blades were examined to determine their influence on the compressor performance. The results show that the compressor’s surge margin increased from 33.28% to 49.2% for a 2.5% tip gap ratio, while its stability margin increased from 44% to 60.5%. However, pressure ratio and efficiency losses of 1.16% and 1.14%, respectively, were incurred. Gap widths less than 5% of the height of the diffuser blade are adequate for broadening the operation range, whereas gap ratios greater than 5% cause more energy loss without broadening the operation range further. Gaps can also reduce the Mach number in the diffuser blade channels and can be influenced by the impeller-tip gap. Moreover, diffuser gaps on the shroud side yield better performance than those on the hub side. An experiment was conducted to investigate the reliability of the calculation and conclusions in this study, and the results proved that the conclusions are valid for different types of centrifugal compressors.

23. Numerical Investigations of Film Cooling Characteristics of Interrupted Slot and Trench Holes on a Vane Endwall

XIAO Xiangtao WANG Pei, DU Qiang, XU Qingzong, LIU Jun, ZHU Junqiang

Corresponding author:

WANG Pei; wangpei@iet.cn

DU Qiang; duqiang@iet.cn

Journal of Thermal Science, 2021, 30(3): 1010-1024.

https://doi.org/10.1007/s11630-021-1468-6

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1468-6

Keywords: film cooling, trench holes, blowing ratio, lateral pressure gradient, step vortex, axial position

Abstract: This paper describes film cooling characteristics of the novel combined configuration employing interrupted slot and trench holes on a vane endwall. Interrupted slot, formed by uneven thermal expansion between combustor and high-pressure turbine vane, can improve adiabatic film cooling effectiveness of the leading edge and pressure side-endwall junction by inhibiting the development of horseshoes vortex. Holes embedded into a straight trench were introduced to improve film efficiency comparing to cylindrical holes on vane passage endwall. The influences of lateral pressure gradient, slot coolant and step on crossflow of the hole coolant in trench were mainly discussed. Three dimensional Reynolds-averaged Navier-Stokes equations with shear stress turbulence model (SST k-ω) were used to obtain the flowfields and adiabatic film cooling effectiveness of a cascade model. Four hole blowing ratios M=0.5, 1.0, 1.5, 2.0 and two axial positions X/Cax= –0.05, 0 were considered. The coolant crossflow in trench on flat endwall of cascade passage trends to flow towards suction side due to the lateral pressure gradient rather than both sides on flat plate. For combined configuration of interrupted slot and trench holes, the step vortex rolls up hole coolant upstream of trench which changes η distributions comparing to that on flat passage endwall. Comparing to the cylindrical holes, better film cooling performance can still be obtained when arranging trench holes under high blowing ratios. The influence of hole’s axial positions was also discussed.

24. Influence of Tip Clearance on Centrifugal Compressor IGV Regulation and Improvement

SUN Qi, WANG Huishe, ZHU Junqiang, JI Chunjun, LI Chunyang, YAO Ka

Corresponding author: SUN Qi

E-mail: sunqi525@126.com

Journal of Thermal Science, 2021, 30(3): 1025-1033.

https://doi.org/10.1007/s11630-020-1343-x

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1343-x

Keywords: centrifugal compressor, inlet guide vane, tip clearance

Abstract: In order to avoid collision with the end wall of the casing when the guide vane rotates around its own rotating axis in a centrifugal compressor, a circular gap will be left at the top of guide vane. Part of the gas flow through the clearance cannot be effectively regulated, and the tip clearance leakage vortex is formed, which has an impact on the mainstream and makes the compressor performance decline. In order to solve this problem, in this paper, three improved schemes of guide vane channel structure, including rectangular channel, cambered channel, and globular channel, were put forward, which can effectively improve the regulation ability of guide vane. Comparing these three improved channels with the original one, the stable operating range of the compressor was enlarged; the power consumption of the compressor was reduced and the efficiency of the compressor was improved. By comprehensive comparison, the cambered channel has better performance and is easier to process, which has potential to be widely used in practice.

25. Application of a Lumped Multi-Section Model for Analyzing the Thermal Performance of a Small-Scale Biomass Boiler

POLESEK-KARCZEWSKA Sylwia, WARDACH-?WI?CICKA Izabela, KARDA? Dariusz, TURZY?SKI Tomasz

Corresponding author: POLESEK-KARCZEWSKA Sylwia

E-mail: sylwia.polesek-karczewska@imp.gda.pl

Journal of Thermal Science, 2021, 30(3): 1034-1045.

https://doi.org/10.1007/s11630-021-1385-8

Open Access -

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1385-8

Keywords: lumped cell-model, biomass boiler, helically coiled tube heat exchanger

Abstract: The stationary lumped-cell model was developed and used to simulate the thermal characteristics of domestic biomass boiler with helically coiled tube heat exchanger (HCHE). The device serves as the heat source for ORC (Organic Rankine Cycle) unit and utilizes the thermal oil as the medium transferring the heat to the unit. Most of studies available in the literature refer to the CFD simulations for water flow in tube coils or in one- or two-turn coil elements. These investigations are basically focused on the determination of Nusselt number. The proposed herein model aims at determining the thermal performance of flue gas-oil HCHE while providing low CPU time. To demonstrate the model possibilities, it was used to predict the flue gas temperatures at the inlet and outlet from the heat exchange zone, based on measurement data regarding the outlet temperature of thermal oil. Six test series were considered. The computation results appeared to be in satisfactory agreement with experimental results (the discrepancies do not exceed 12%). The investigations showed that the used approach may be recommended as an alternative method that allows for fast prediction of thermal parameters for units of complex geometries, in particular the multi-coil heat exchangers.

26. Effects of Flame Temperature and Absorption Coefficient on Premixed Flame Interactions with Radiation

KANG Sang Hun

Corresponding author: KANG Sang Hun

E-mail: aeroksh@sejong.ac.kr

Journal of Thermal Science, 2021, 30(3): 1046-1056.

https://doi.org/10.1007/s11630-021-1393-8

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1393-8

Keywords: radiation, high-fidelity numerical simulation, discrete ordinates method, premixed flame

Abstract: The effects of the flame temperature and the absorption coefficient on interactions between radiative heat transfer and the flame behavior were numerically investigated using high-fidelity numerical code with the discrete ordinates method. To study the effects of the flame temperature and the absorption coefficient, three different flame temperatures and four different absorption coefficient conditions were selected, so twelve test cases were studied in total. In the numerical test results, radiation effects resulted in preheating of the reactant gases and heat loss from the product gases. A higher flame temperature resulted in stronger preheating effects in reactants near the flame. Due to the preheating effects, with the appropriate absorption coefficient, the peak temperature appeared at the flame front. Lower flame temperatures resulted in larger reabsorption effects in the product zone. The peak temperature at the flame front and the flame speed were influenced by the combined effects of preheating and radiative heat loss at the flame front. Depending on the conditions, due to those effects, the peak temperature and the flame speed could increase or decrease. When the absorption coefficient was sufficiently large, the temperature decrease was reduced in the product zone.

27. Effect of Operating Parameters on Carbon Dioxide Depressurized Regeneration in Circulating Fluidized Bed Downer using Computational Fluid Dynamics

SAKAUNNAPAPORN Chattan, CHAIWANG Pilaiwan, PIUMSOMBOON Pornpote, CHALERMSINSUWAN Benjapon

Corresponding author:

CHALERMSINSUWAN Benjapon

E-mail: benjapon.c@chula.ac.th

Journal of Thermal Science, 2021, 30(3): 1057-1067.

https://doi.org/10.1007/s11630-021-1437-0

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1437-0

Keywords: carbon dioxide, circulating fluidized bed downer, computational fluid dynamics, depressurized regeneration, statistical experimental design

Abstract: Typically, heating or high-temperature treatment has been used to regenerate solid sorbent. In this study, the depressurized regeneration using a circulating fluidized bed downer was proposed and the significance of its operating parameters was identified. Two-dimensional computational fluid dynamics were employed to systematically investigate the effects of operating parameters on carbon dioxide depressurized regeneration with potassium carbonate solid sorbent particles. The simulated model was based on a laboratory scale circulating fluidized bed downer. The chemical equilibrium model for predicting the highest outlet carbon dioxide mass fraction was then used. A central composite design was employed to identify the main, quadratic, and interaction effects of operating parameters to the regeneration process. The operating parameters consisted of the outlet system pressure, inlet gas velocity, and inlet solid circulation rate, while the response variable was the released outlet carbon dioxide mass fraction. Among the multiple operating parameters, there were two main operating parameters and their combinations, namely the inlet gas velocity, outlet system pressure, square of inlet gas velocity, and interaction between inlet gas velocity and outlet system pressure, which had great impacts on the regeneration. All the main, quadratic, and interaction effects were explained. Then, the optimal operating conditions were obtained through the response surface method.

28. Miniaturization of Bubbles by Shock Waves in Gas-Liquid Two-phase Flow in the Venturi Tube

JIANG Xin, YASUNOBU Tsuyoshi, NAKAMURA Arisa, SHIMAZU Masaki

Corresponding author: JIANG Xin; E-mail: jiangxin@kct.ac.jp

YASUNOBU Tsuyoshi; yasunobu@kct.ac.jp

Journal of Thermal Science, 2021, 30(3): 1068-1076.

https://doi.org/10.1007/s11630-021-1432-5

Springer Link: https://link.springer.com/article/10.1007/s11630-021-1432-5

Keywords: fine bubble, shock wave, gas-liquid two-phase flow, venturi tube

Abstract: Fine bubbles have been widely applied in many fields such as industry, medical engineering and agricultures. Therefore, many attentions have been paid to the study of fine bubble generations in order to increase the yield while decrease the cost. However, the generation process of fine bubbles is a quite complicated process in which multiple hydrodynamic forces are interacted in the gas-liquid two-phase flow. Many studies focus on the techniques of the converging-diverging nozzle (venturi tube) generator, which is famous for its simple and cheap features, and generates fine bubbles by using the miniaturization phenomenon of bubbles occurring in the venturi tube. However, the impact conditions on the amount and size of bubbles such as nozzle geometry and bleed air haven’t been investigated clearly. In this work, we implement many experiments on the venturi tube fine bubble generators with different geometries and generating conditions, and evaluate different factors impacting the production components such as the volume and the bubble size. The experimental results show that the supersonic flow filed in the venturi tube promotes the miniaturization of the bubbles, and the convergent angle of the nozzle and air bleed have a great impact on the size and volume of bubbles.

29. Polygeneration Study of Low-to-Medium Enthalpy Geothermal Reservoirs in Mexico

CHáVEZ Oscar, GODíNEZ Francisco

Corresponding author: GODíNEZ Francisco

E-mail: fgodinezr@gmail.com

Journal of Thermal Science, 2021, 30(3): 1077-1087.

https://doi.org/10.1007/s11630-020-1271-9

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1271-9

Keywords: polygeneration system, geothermal reservoir, local thermal equilibrium, low-to-medium enthalpy

Abstract: This study combines the thermodynamic analysis of a polygeneration system along with the numerical modelling of the thermal behavior of geothermal reservoirs in Mexico to exploit their energy. Each reservoir was modeled as a porous medium assuming a five-spot well configuration and local thermal equilibrium. The heat conduction-convection along with the Laplace equations were solved to compute temperature distributions, the useful life and the optimum distance between injection-extraction wells. The predicted temperature and pressure of the geothermal fluid at the outlet of the reservoir were exploited in the polygeneration system consisting of: (1) a Rankine cycle, (2) an absorption refrigeration cycle, and (3) a heat exchanger. The developed approach allows calculating both the optimal distance between injection-extraction wells and the global (utilization) efficiency of six arrangements (each composed by a reservoir connected to a polygeneration system) by assuming that reservoirs have a lifespan of 30 years. Results also show that: (a) due to the low efficiency of the Rankine cycle, very little thermal energy is converted into electrical one; (b) not only the temperature and the size are important when evaluating the power production performance of reservoirs, but also the permeability plays a fundamental role; (c) the first law efficiency of the polygeneration system ranges from 41.9% to 43.7%; (d) the utilization efficiency of the six arrangements lies in the range between 25.8% and 31%.

30. Analysis of Influencing Factors on NOx emission in Gas-Fired Heating and Hot Water Combi-Boilers Based on Orthogonal Method

ZHOU Weiye, LIU Wenbo, WANG Yan, LYU Xinyu, YANG Lin

Corresponding author: ZHOU Weiye

E-mail: zhouweiye@tju.edu.cn

Journal of Thermal Science, 2021, 30(3): 1088-1095.

https://doi.org/10.1007/s11630-020-1329-8

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1329-8

Keywords: gas-fired heating and hot water combi-boilers, nitrogen oxides, orthogonal method, flue restrictor diameter, fan power, nozzle ejection distance, nozzle aperture, air relative humidity

Abstract: In order to determine the sensitivity of influencing factors on NOx emission in gas-fired heating and hot water combi-boilers, the orthogonal method was adopted in this paper. Five predominant factors affecting the formation of NOx and four levels were selected to be analyzed, including the diameter of flue restrictor, power of fan, ejection distance of nozzle, aperture of nozzle and relative humidity of air. The test plan was designed by employing L16(45) orthogonal array, and 16 groups of experiments were conducted. The test results were analyzed with range analysis and variance analysis. The results indicated that the power of fan has the greatest influence on the formation of NOx of the gas-fired combi-boilers, followed by the diameter of flue restrictor and the relative humidity of air, while the ejection distance of nozzle and aperture of nozzle have little effect on the formation of NOx. In addition, with the selected optimal combination of five factors, a minimum NOx volume fraction is obtained, which confirms the superiority of the orthogonal test. The research finding has a certain guiding significance for the reduction of NOx formation of gas-fired heating and hot water combi-boiler.