Volume 29, Issue 6, November-December 2020

CONTENTS-Volume 29, Issue 6, November-December 2020

Springer Link: https://link.springer.com/journal/11630/volumes-and-issues/29-6

1. Analysis of Hydrocarbon Mixture Performance of a Dual-Channel Swirl Engine

YUAN Wenhua, HUANG Qilin, FU Jun, LIAO Jingjing, LI Yu, HE Yong, ZHANG Zengfeng, MA Yi

Corresponding author: YUAN Wenhua; FU Jun

E-mail: ywh6308@163.com; 4160@hnsyu.edu.cn

Journal of Thermal Science, 2020, 29(6): 1391-1397.

https://doi.org/10.1007/s11630-020-1372-5

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1372-5

Keywords: gas mixing performance, dual-channel, vortex chamber, combustion system

Abstract: In order to understand and improve the oil and gas mixing performance of a dual-channel vortex chamber diesel engine, the BH175F dual-channel vortex chamber combustion system was used as the research foundation, and the oil-gas mixture process of the combustion system was numerically analyzed. By analyzing the cylinder temperature, cylinder pressure, mixing process and combustion process of the combustion system, the mixture performance of the combustion system was studied. Results indicated that: The mixture of the compression Top Dead Center (TDC) started to enter the main combustion chamber through the start-up hole; when the piston reached 4° After Top Dead Center (ATDC), the mixture started to enter the main combustion chamber through the connecting channels A and B, and the high-concentration mixture entered the main combustion from the start-up hole; when the piston continued running down to 20° and 25° ATDC, it could be seen that the main combustion chamber mixture was already relatively uniform; besides, when the equivalence ratio was between 0.8 and 1, the air-fuel mixture was unevenly distributed in the main combustion chamber. It provides guidance for further improvement of the combustion system.

2. Numerical Study on Effects of Key Factors on Performance of CeO2-based Catalyzed Diesel Particulate Filter

WU Gang, LI Zonglin, ABUBAKAR Shitu, LI Yuelin, LI Yuqiang

Corresponding author: LI Yuqiang

E-mail: csulyq@csu.edu.cn

Journal of Thermal Science, 2020, 29(6): 1398-1409.

https://doi.org/10.1007/s11630-020-1338-7

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1338-7

Keywords: concentrating solar thermal (CST), concentrating solar power (CSP), line-focus, parabolic trough collect (PTC), linear Fresnel reflector (LFR)

Abstract: In the present review, parabolic trough collector (PTC) and linear Fresnel reflector (LFR) are comprehensively and comparatively reviewed in terms of historical background, technological features, recent advancement, economic analysis and application areas. It is found that although PTC and LFR are both classified as mainstream line-focus concentrating solar thermal (CST) technologies, they are now standing at different stages of development and facing their individual opportunities and challenges. For PTC, the development is commercially mature with steady and reliable performance; therefore, extension of application is the main future demand. For LFR, the development is still in rapid progress to commercial maturity, yet indicating very promising potentials with high flexibility in novel designs and remarkable reduction in capital and operational costs. The question, which has to be answered in order to estimate the future perspectives of these two line-focus CST technologies, becomes which of these characteristics carries more weight or how to reach an optimal trade-off between them.

3. A Review on Recent Development of Cooling Technologies for Photovoltaic Modules

ZHANG Chunxiao, SHEN Chao, WEI Shen, WANG Yuan, LV Guoquan1, SUN Cheng

Corresponding author: SHEN Chao; SUN Cheng

E-mail: chaoshen@hit.edu.cn; suncheng@hit.edu.cn

Journal of Thermal Science, 2020, 29(6): 1410-1430.

https://doi.org/10.1007/s11630-020-1350-y

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1350-y

Keywords: solar energy, PV modules, cooling technologies, nanoparticles, phase change materials

Abstract: When converting solar energy to electricity, a big proportion of energy is not converted for electricity but for heating PV cells, resulting in increased cell temperature and reduced electrical efficiency. Many cooling technologies have been developed and used for PV modules to lower cell temperature and boost electric energy yield. However, little crucial review work was proposed to comment cooling technologies for PV modules. Therefore, this paper has provided a thorough review of the up-to-date development of existing cooling technologies for PV modules, and given appropriate comments, comparisons and discussions. According to the ways or principles of cooling, existing cooling technologies have been classified as fluid medium cooling (air cooling, water cooling and nanofluids cooling), optimizing structural configuration cooling and phase change materials cooling. Potential influential factors and sub-methods were collected from the review work, and their contributions and impact have been discussed to guide future studies. Although most cooling technologies reviewed in this paper are matured, there are still problems need to be solved, such as the choice of cooling fluid and its usability for specific regions, the fouling accumulation and cleaning of enhanced heat exchangers with complex structures, the balance between cooling cost and net efficiency of PV modules, the cooling of circulating water in tropical areas and the freezing of circulating water in cold areas. To be advocated, due to efficient heat transfer and spectral filter characters, nanofluids can promote the effective matching of solar energy at both spectral and spatial scales to achieve orderly energy utilization.

4. Micro-Channel Heat Sink: A Review

ZHOU Jinzhi, CAO Xiaoling, ZHANG Nan, YUAN Yanping, ZHAO Xudong, HARDY David

Corresponding author: YUAN Yanping

E-mail: ypyuan@home.swjtu.edu.cn

Journal of Thermal Science, 2020, 29(6): 1431-1462.

https://doi.org/10.1007/s11630-020-1334-y

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1334-y

Keywords: micro-channel, heat transfer, pressure drop, effect factor, application

Abstract: The method to cool a high heat flux device is an important research direction for the heat exchanger design. Micro-channels are an effective heat exchange structure both for single-phase and two-phase flow. In this paper, the heat transfer correlations of single-phase, two-phase and nanofluid in a micro-channel are discussed and analyzed. The correlations of pressure drop for single-phase and two-phase fluids are also presented. Excluding the different working fluids used in the micro-channel, the diameter and aspect ratio, shape and structure, surface roughness, internal and external factor and layout of micro-channel pipe are considered to analyze their influence on the heat transfer performance and pressure drop. Micro-channel technology applications include industry, air-conditioning, solar energy systems, heat pipe technology and computer data center cooling. Compared to the conventional heat exchangers used in these fields, a micro-channel heat sink showed a much better heat transfer coefficient and low volume, indicating that it is a good choice and has huge potential for cooling application. Finally, existing problems and future scopes are described, and drawing up design standard, experimental and simulated methods for evaluating its performance are the urgent actions which need to be carried out. This review paper serves as guidance for researchers to design and predict the performance of micro-channel heat sinks.

5. Experimental Investigation of the Secondary Flow in a Rotating Smooth Channel Subjected to Thermal Boundary Conditions

LI Haiwang, YOU Haoliang, YOU Ruquan, TAO Zhi

Corresponding author: YOU Ruquan

E-mail: youruquan10353@buaa.edu.cn

Journal of Thermal Science, 2020, 29(6): 1463-1474.

https://doi.org/10.1007/s11630-020-1251-0

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1251-0

Keywords: flow dynamic, rotating smooth channel, secondary flow, buoyancy force

Abstract: In the current study, thermal boundary conditions are considered in a rotating smooth channel with a square cross-section to investigate the secondary flow and compare it to that of the same channel without heating. The measurement is conducted at three streamwise planes (X=445 mm, 525 mm, 605 mm). The flow parameters are the Reynolds number (Re=4750, which was based on the average longitudinal or primary velocity U and the hydraulic diameter D of the channel cross-section), the rotation number (Ro=ΩD/U, where Ω is the rotational velocity, ranging from 0 to 0.26), and the aspect ratio of the channel cross-section (AR=1, which is calculated by dividing the channel height by the channel width). The leading and trailing walls are heated under a constant heat flux qw=380 W/m2, and the top and bottom walls are isothermal at room temperature. This work is in a series with our previous work without thermal boundary conditions. Based on the experimental data, we obtained a four-vortex regime. There is a counter-rotating vortex pair near the leading side and the trailing side. Because the leading and trailing walls are heated, the buoyancy force increases the relative vertical position of the vortex pair near the trailing side from 5% to 12.5% of the hydraulic diameter. When moving upstream along the streamwise direction, the upper vortex near the trailing wall becomes weaker, whereas the lower vortex becomes stronger. As the rotational speed increases, the vortex pair near the trailing side is inhibited by the Coriolis force. Under heated thermal boundary conditions, the vortex pair near the trailing side reappears due to the effect of buoyancy force. These results indicate that the buoyancy force has a substantial effect on the secondary flow regime under thermal boundary conditions.

6. Enhanced Heat Transfer of Carbon Nanotube Nanofluid Microchannels Applied on Cooling Gallium Arsenide Cell

ZHANG Huiying, YAN Suying, WANG Tao, WU Yuting, ZHAO Xiaoyan, ZHAO Ning

Corresponding author: YAN Suying

E-mail: yansy@imut.edu.cn

Journal of Thermal Science, 2020, 29(6): 1475-1486.

https://doi.org/10.1007/s11630-020-1303-5

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1303-5

Keywords: concentrated photovoltaic solar cell, thermal conductivity, carbon nanotube nanofluids, field synergy principle, heat transfer property

Abstract: Carbon nanotube nanofluids have wide application prospects due to their unique structure and excellent properties. In this study, the thermal conductivity properties of carbon nanotube nanofluids and SiO2/water nanofluids were compared and analyzed experimentally using different preparation methods. The physical properties of nanofluids were tested using a Malvern Zetasizer Nano Instrument and a Hot Disk Thermal Constant Analyzer. Combined with field synergy theory analysis of the heat transfer performance of nanofluids, results show that the thermal conductivity of carbon nanotube nanofluids is higher than that of SiO2/water nanofluids, and the thermal conductivity of nanofluid rises with the increase of mass fraction and temperature. Moreover, the synergistic performance of carbon nanotube nanofluids is also superior to that of SiO2/water nanofluids. When the mass fraction of the carbon nanotube nanofluids is 10% and the SiO2/water nanofluids is 8%, their field synergy numbers and heat transfer enhancement factors both reach maximum. From the perspective of the preparation method, the thermal conductivity of nanofluids dispersed by high shear microfluidizer is higher than that by ultrasonic dispersion. This result provides some reference for the selection and use of working substance in a microchannel cooling concentrated photovoltaic and thermal (CPV/T) system.

7. Heat Transfer Enhancement of Supercritical Nitrogen Flowing Downward in a Small Vertical Tube: Evaluation of System Parameter Effects

ZHU Xiaojing, LYU Zhihao, YU Xiao, LI Qiang, CAO Maoguo, REN Yongxiang

Corresponding author: ZHU Xiaojing

E-mail: zhuxiaojing@dlut.edu.cn

Journal of Thermal Science, 2020, 29(6): 1487-1503.

https://doi.org/10.1007/s11630-020-1377-0

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1377-0

Keywords: supercritical nitrogen, heat transfer enhancement (THE), numerical simulation, vertically downward flow, system parameter evaluation

Abstract: In this paper, the heat transfer enhancement (HTE) of supercritical nitrogen flowing downward in a vertical small tube (diameter 2 mm) is studied using the commercial software CFX of Ansys16.1, to provide theoretical guidance on the design of high-performance heat transfer systems. An effective numerical simulation method, which employs the SSG Reynolds stress model with enhanced wall treatment, is applied to study the heat transfer of supercritical nitrogen under typical working conditions. The objective is to evaluate the effect of the main parameters taking into account the buoyancy and flow acceleration effects. Simulation results are compared with results calculated from three well-known empirical correlations and the applicability of empirical correlation is discussed in detail. It is discovered that the Watts and Chou correlation accurately fits the simulation results of supercritical nitrogen and the Dittus-Boelter and Jackson correlations can only be used for high-pressure conditions. The HTE of supercritical nitrogen is closely related to the laminar sub-layer and buffer layer of a boundary layer. The buoyancy effect on the HTE should be considered at low mass flux conditions, and thermal acceleration can be completely ignored for the cases studied. The special HTE featured by the increment in heat transfer coefficient with increasing heat flux is discovered at low pressure, and simulation results proved that this HTE is caused by the combined actions of buoyancy as well as significant variations in specific heat and viscosity.

8. Prediction of Thermal Conductivity of Various Nanofluids with Ethylene Glycol using Artificial Neural Network

WANG Xuehui, YAN Xiaona, GAO Neng, CHEN Guangming

Corresponding author: YAN Xiaona

E-mail: yxn0828@163.com

Journal of Thermal Science, 2020, 29(6): 1504-1512.

https://doi.org/10.1007/s11630-019-1158-9

Springer Link: https://link.springer.com/article/10.1007/s11630-019-1158-9

Keywords: thermal conductivity, nanofluids, ANN model, heat transfer

Abstract: The nanofluid has been widely used in many heat transfer areas due to its significant enhancement effect on the thermal conductivity. Therefore, the methods that can accurately predict their thermal conductivities are very important to evaluate and analyze the heat transfer process. In this paper, a novel artificial neural network (ANN) model was proposed to predict the thermal conductivity of nanofluids with ethylene glycol and could be used in a wide range with excellent accuracy. A total of 391 experimental data with a wide range of temperatures (4°C to 90°C), nanoparticles (metal, metal oxide, etc.), volume concentrations (0.05% to 10%), and particle sizes (2 nm to 282 nm) were collected. To build the ANN model, the temperature, thermal conductivities of the base fluid and nanoparticles, the size and volume concentration of the nanoparticles were selected and used as the input parameters. There were 5 nodes, 10 nodes and 1 node in input layer, hidden layer and output layer, respectively. The predicted results of the ANN model coincided with the experimental data very well with the correlation coefficient and mean square error (MSE) were 0.9863 and 3.01×10–5, respectively. The relative deviations of 99.74% data were within ±5%. The model was expected to be a good practical method to predict the thermal conductivity of nanofluids with ethylene glycol.

9. Numerical Analysis of Unsteady Natural Convection Flow and Heat Transfer in the Existence of Lorentz Force in Suddenly Expanded Cavity Using OpenFOAM

SINGH Ranjit J., GOHIL Trushar B.

Corresponding author: GOHIL Trushar B.

E-mail: trushar.gohil@gmail.com

Journal of Thermal Science, 2020, 29(6): 1513-1530.

https://doi.org/10.1007/s11630-020-1190-9

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1190-9

Keywords: OpenFOAM, Lorentz force, Boussinesq approximation, natural convection

Abstract: The present study reveals the significance of the magnetic field or Lorentz force on the unsteady natural convection flow and heat transfer in the suddenly expanded cavity. The Lorentz force based magnetohydrodynamics (MHD) solver using electric potential formulation coupled with the energy equation by the means of Boussinesq approximation is developed in the open-source CFD tool OpenFOAM. The unsteady flow is generated by the buoyancy force keeping the Rayleigh number (Ra) at 109, at the fixed Prandtl number (Pr) of 0.71. The effects of the magnetic field on the flow and heat transfer are explained for various orientations of magnetic field (Bx, B45, and By) in terms of Hartmann number (Ha=0, 50, 100, 300 and 500). The increase in the magnetic field increases the strength of the Lorentz force, which regulates the flow pattern and suppresses down the unsteady nature of flow and heat transfer into the steady-state. It is perceived that the average Nusselt number decreases as the intensity of Bx and B45 magnetic field increases. However, for By magnetic field the average Nusselt number increases up to Ha of 100 as compared to the non-MHD case (Ha=0). The distribution of Lorentz force in the domain plays a significant role in the governing of the fluid flow and heat transfer.

10. Natural Convection in a Horizontal Cylinder with Partial Heating: Energy Efficiency Analysis

MAZGAR Akram, JARRAY Khouloud, HAJJI Fadhila, BEN NEJMA Fay?al

Corresponding author: MAZGAR Akram

E-mail: mazgarakram@yahoo.fr

Journal of Thermal Science, 2020, 29(6): 1531-1550.

https://doi.org/10.1007/s11630-020-1238-x

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1238-x

Keywords: natural convection, cylinder, partial heating, energy efficiency, Rayleigh number

Abstract: The current study reports a numerical analysis of free convection of air in an isothermal horizontal cylinder, cooled and heated at different wall locations. Three heater sizes are discussed in this study. The first heated zone is spread across one-quarter of the sidewall; the second is uniformly distributed over the half of the wall and the third active wall covers three-quarters of the cylinder. Five various locations are considered and examined for each active zone of the sidewall. The computation is carried out for Rayleigh number ranging from 102 to 106. Numerical results characterizing heat transfer and flow features are achieved using an iterative model developed in COMSOL Multiphysics. The effect of Rayleigh number on heat transfer and fluid flow characteristics within the cavity are investigated. Particular attention is paid to the influence of heater location and heater size on energy efficiency. It is found that the mean Nusselt number and dimensionless velocity increase when increasing the Rayleigh number. Moreover, the optimal level of energy efficiency is achieved if the heating zone is centered at the upper part of the cylinder, regardless of the heater size. It is also shown that the optimal configuration providing higher energy efficiency is obtained when three-quarters of the sidewall are locally heated, and more precisely, if the active zone is centered at the top of the cylinder.

11. Influence of Air Humidity on Transonic Flows with Weak Shock Waves

DYKAS S?awomir, MAJKUT Miros?aw, SMO?KA Krystian

Corresponding author: DYKAS S?awomir

E-mail: slawomir.dykas@polsl.pl

Journal of Thermal Science, 2020, 29(6): 1551-1557.

https://doi.org/10.1007/s11630-019-1182-9

Springer Link: https://link.springer.com/article/10.1007/s11630-019-1182-9

Keywords: dry air, moist air, transonic flow, weak shock wave, steam condensation

Abstract: The paper presents CFD results for the transonic flow of dry and moist air through a diffuser and a compressor rotor. In both test geometries, i.e. the Sajben transonic diffuser and the NASA Rotor 37, the air humidity impact on the structure of flows with weak shock waves was examined. The CFD simulations were performed by means of an in-house CFD code, which was the RANS-based modelling approach to compressible flow solutions. It is shown that at high values of relative humidity, above 70%, the modelling of the transonic flow field with weak shock waves by means of the dry air model may produce wrong results.

12. Performance Analysis of Inter-Stage Leakage Flows at Rotating Conditions in an Axial Compressor

KONG Xiaozhi, LIU Yuxin, LU Huawei, TIAN Zhitao, XIN Jianchi

Corresponding author: LIU Yuxin; KONG Xiaozhi

E-mail: liuyuxin_lz@163.com; kongxiaozhi_lx@163.com

Journal of Thermal Science, 2020, 29(6): 1558-1568.

https://doi.org/10.1007/s11630-020-1378-z

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1378-z

Keywords: shrouded stator blade, inter-stage seal, rotor-stator cavity, leakage flow, windage heating, swirl ratio

Abstract: For the compressor with shrouded stator blades, the stator well is a rotor-stator space between the rotating drum and the stationary shroud. Due to the pressure difference, a reverse leakage flow would travel through the stator well and inject into the main flow path. Although, the labyrinth seal is commonly placed under the shroud, the rotation effect and seal clearance variation in actual operation process have great impact on the characteristics of this inter-stage leakage, as well as the compressor performance.

In this paper, experiments were conducted at a compressor inter-stage seal test rig. The leakage flow rates, total temperatures and swirl ratios were obtained at different speeds and working clearances. The proportions of rotation effect and the clearance reduction effect were analyzed by data processing. Comparisons indicate that the working clearance and leakage flow reduce about 43% and 50% respectively, when the rotational speed ω=8100 r/min. The proportion of reduction caused by the rotation effect is around 15%, while the influence of working clearance variation is much greater, accounting for about 35%. The windage heating coefficient and swirl ratio in the outlet cavity are almost in exponential relationship with the rotor speed. The increases in total temperature and swirl ratio generated by the rotation effect are found to be about 80%. In addition, the swirl and radial velocity profiles in the cavities were discussed by validated numerical simulations to reveal the typical flow characteristics. The data presented can provide guidance for better leakage conditions prediction as well as the inter-stage seal design enhancement.

13. Accuracy and Efficiency Assessment of Harmonic Balance Method for Unsteady Flow in Multi-Stage Turbomachinery

ZHANG Zhen, MA Can, SU Xinrong, YUAN Xin

Corresponding author: SU Xinrong

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

Journal of Thermal Science, 2020, 29(6): 1569-1580.

https://doi.org/10.1007/s11630-020-1201-x

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1201-x

Keywords: multi-stage turbomachinery, unsteady flow, harmonic balance, accuracy, efficiency

Abstract: With the relative movement of neighboring blade rows, flows in multi-stage turbomachinery are unsteady and periodic in time at the design condition. As an alternative to the widely used time domain time marching method, the harmonic balance (HB) method has been successfully applied to simulate the essentially unsteady flow of multi-stage turbomachinery. By modelling various number of harmonics, the accuracy of this method could be adjusted at different level of computational cost. In this article, accuracy of the harmonic balance method is not only validated against the time domain time marching method, as in most previous works on this topic, but also against the data from an experiment campaign of a two-stage high-pressure turbine where strong tip leakage flow exists. Efficiency of this method is also assessed in detail by adjusting the number of harmonics and comparing with time domain time marching solution results. Results show that the harmonic balance method is a flexible tool with adjustable accuracy for fast-turnaround unsteady flow simulation of multi-stage turbomachinery. Results from this work can provide a guidance in applying the harmonic balance method with balance between accuracy and computational cost.

14. Direct Numerical Simulation of the Pulsed Arc Discharge in Supersonic Compression Ramp Flow

SONG Guoxing, LI Jun, TANG Mengxiao

Corresponding author: LI Jun

E-mail: apsl87324@163.com

Journal of Thermal Science, 2020, 29(6): 1581-1593.

https://doi.org/10.1007/s11630-020-1380-5

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1380-5

Keywords: direct numerical simulation, pulsed arc discharge, shock waves, turbulent boundary layer, flow separation

Abstract: Direct numerical simulation (DNS) of shock wave/turbulent boundary layer interaction (SWTBLI) with pulsed arc discharge is carried out in this paper. The subject in the study is a Ma=2.9 compression flow over a 24-degree ramp. The numerical approaches were validated by the experimental results in the same flow conditions. The heat source model was added to the Navier-Stokes equation to serve as the energy deposition of the pulsed arc discharge. Four streamwise locations are selected to apply energy deposition. The effect of the pulsed arc discharge on the ramp-induced flow separation has been studied in depth. The DNS results demonstrate the incentive locations play a dominant role in suppressing the separated flow. Results show that pulsed heating is characterized by a thermal blockage, which leads to streamwise deflection. The incentive locations upstream the interaction zone of the base flow have a better control effect. The separation bubble shape shows as “spikes”, and the downstream flow of the heated region is accelerated due to the momentum exchange between the upper boundary layer and the bottom boundary layer. The high-speed upper fluid is transferred to the bottom, and thus enhances its ability to resist the flow separation. More stripe vortex structures are also generated at the edge of the flat-plate. Furthermore, the turbulent kinetic disturbance energy is increased in the flow filed. The disturbances that originate from the pulsed heating are capable of increasing the turbulent intensity and then diminishing the trend of flow separation.

15. A Hydraulic Performance Comparison of Centrifugal Pump Operating in Pump and Turbine Modes

DAI Cui, DONG Liang, LIN Haibo, ZHAO Fei

Corresponding author: DONG Liang

E-mail: dongliang@ujs.edu.cn

Journal of Thermal Science, 2020, 29(6): 1594-1605.

https://doi.org/10.1007/s11630-020-1236-z

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1236-z

Keywords: Computational Fluid Dynamics, centrifugal pump, impeller modification, pump mode, turbine mode

Abstract: The modifications of impeller may show diverse impact on centrifugal pump operating in pump and turbine modes. To clarify this problem, the hydraulic performance of a low specific speed centrifugal pump operating in both modes was firstly obtained by CFD method and verified by experiment. Then, based on the single-factor design method, a series of calculations have been conducted to identify the effects of impeller geometry parameters on the hydraulic performance in different modes. The variations of head, shaft power and hydraulic efficiency curves with different impeller parameters were explored. It is found that compared with turbine, the pump shows a more obvious variation of head. The outlet angle has positive impact both on the head consumed by pump or generated by turbine. The change of turbine shaft power is apparently smaller than that of pump for different impeller geometry parameters. Only the outlet width somewhat changes the turbine shaft power. The hydraulic efficiency in both modes shows different variation under different impeller geometric parameters, while the hydraulic efficiency of both modes is reduced with the outlet angle increasing. Meanwhile, the response amount of hydraulic efficiency caused by certain change of impeller parameters was estimated by sensitivity analysis method. It is found that only the appropriate blade number and outlet width can improve the hydraulic performance both in pump and turbine modes. Eventually, the hydraulic loss, skin friction loss and theoretical analysis were performed to explore the reason of hydraulic performance variation due to different impeller parameters. The change of slip factor, impeller inlet area, impeller outlet area or hydraulic loss results in the change of hydraulic performance in both modes. The results can be useful for hydraulic performance improvement for both pump and turbine modes through impeller geometry modification.

16. Analysis and Optimization of Unsteady Flow in a Double-Suction Centrifugal Pump for a Cooling-Water Supply System in a Nuclear Reactor

YAN Hao, SU Xiaozhen, SHI Haixia, CHENG Maosheng, LI Yunqing

Corresponding author: SU Xiaozhen

E-mail: suxiaozhen870727@126.com

Journal of Thermal Science, 2020, 29(6): 1606-1616.

https://doi.org/10.1007/s11630-020-1252-z

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1252-z

Keywords: unsteady flow, reactor coolant pump (RCP), radial force, pressure pulsation

Abstract: The management of a cooling-water supply system in a nuclear reactor is performed by valve and reactor coolant pump (RCP) control, which regulates both the pressure and the discharge between certain limits. However, the RCP has a significant unsteady flow when operating at different conditions. The unsteady pressure pulsation and radial force vector are difficult to calculate because these are affected by the transient properties of the unsteady flow. This study explores the use of a commercial Computational Fluid Dynamics (CFD) code to comprehensively estimate the unsteady flow of the RCP. The full 3D-URANS equations were solved for different flow rates, and some optimised cases for the unsteady flow were proposed. The results showed that the numerical predictions were validated with the experimental data of a model pump. The code was used to estimate the velocity streamlines, pressure pulsation and radial force vector in the steady and transient conditions. The flow rates were not equal for the inner and outer passage in the double volute casing. Additionally, the pulsation of the pressure and radial force was effectively reduced by optimising the staggered angle α. An optimal case was observed when α =30°.

17. Optimization Design of the Grate Cooler Based on the Power Flow Method and Genetic Algorithms

MA Xiaoteng, CAO Qun, CUI Zheng

Corresponding author: CUI Zheng

E-mail: zhengc@sdu.edu.cn

Journal of Thermal Science, 2020, 29(6): 1617-1626.

https://doi.org/10.1007/s11630-019-1188-3

Springer Link: https://link.springer.com/article/10.1007/s11630-019-1188-3

Keywords: power flow method, genetic algorithm, grate cooler, entropy generation minimization, multi-objective optimization

Abstract: As an important process during the cement production, grate cooler plays significance roles on clinker cooling and waste heat recovery. In this paper, we measured experimentally the heat balance of the grate cooler, which provided initial operating parameters for optimization. Then, the grate cooler was simplified into a series-connected heat exchanger network by power flow method. Constructing the equivalent thermal resistance network provided the global constraints by Kirchhoff’s law. On this basis, with the objectives of the minimum entropy generation numbers caused by heat transfer and viscous dissipation, solving a multi-objective optimization model achieved the Pareto Front by genetic algorithm. Then selecting the scheme of the lowest fan power consumption obtained the optimal operating parameters of the grate cooler. The results showed that the total mass flow of the optimized scheme did not change significantly compared with the original scheme, but the fan power consumption was 25.44% lower, and the heat recovery efficiency was 88.43%, which was improved by 11.35%. Furthermore, the analysis showed that the optimal operating parameters were affected by the local heat load. After optimizing the diameter of clinker particles within the allowable industrial range, the clinker with particle diameter of 0.02 m had the optimal performance.

18. Synergistic Effect of Alkali Metals in Coal and Introduced CaO during Steam Gasification

LIU Yang, YANG Xinfang, LEI Fulin, XIAO Yunhan

Corresponding author: LEI Fulin

E-mail: leifulin@iet.cn

Journal of Thermal Science, 2020, 29(6): 1627-1637.

https://doi.org/10.1007/s11630-020-1250-1

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1250-1

Keywords: Zhundong coal, alkali metals, CaO, synergistic effect, steam gasification

Abstract: The steam gasification kinetics of Zhundong raw coal and the washed coal by deionized water or hydrochloric acid with/ without addition of CaO were tested by thermogravimetric analyzer (TGA) at medium temperatures (650°C to 800°C). The cation contents of potassium and sodium in samples were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The Brunauer-Emmett-Teller (BET) surface area of the samples was tested by N2 adsorption, and the morphologies of the samples were characterized by scanning electron microscopy (SEM). Experimental results showed that the organic sodium was the main catalyst for the gasification of the pyrolysis char, and the gasification rate of the char could be enhanced further with the introduction of CaO. The inherent alkali metals in coal and the introduced CaO showed a synergistic effect that occurred obviously above 735°C. The char conversion curves with or without CaO were fitted by the modified volumetric model (MVM). The calculated results indicated that the addition of CaO increased the pre-exponential factor obviously, but made little changes on the activation energy. It was proposed that the synergistic effect was resulted from the co-melting of the sodium and CaO, which facilitated the migration of the catalyst ions and the generation of C(O) intermediates for the gasification.

19. Mechanism of Methane Addition Affects the Ignition Process of n-heptane under Dual Fuel Engine-Like Conditions

LIU Zongkuan, ZHOU Lei, ZHAO Wanhui, QI Jiayue, WEI Haiqiao

Corresponding author: ZHOU Lei; WEI Haiqiao

E-mail: lei.zhou@tju.edu.cn; whq@tju.edu.cn

Journal of Thermal Science, 2020, 29(6): 1638-1654.

https://doi.org/10.1007/s11630-020-1260-z

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1260-z

Keywords: ignition, methane addition, n-heptane, low-temperature reaction pathways

Abstract: For saving energy and protecting the environment, natural gas has been widely used in internal combustion engines, which makes the study on the ignition characteristics of natural gas/diesel mixtures important. In this work, the effects of trace methane addition on the ignition delay of n-heptane/air mixtures are numerically studied using a detailed n-heptane mechanism under marine engine-like conditions. The simulations are carried out based on the software CHEMKIN-PRO 18.0 with a closed homogeneous reactor. Results show that the prolonged ignition delay times (IDs) of n-heptane/air mixtures are observed over the whole initial temperature range after methane is added, and the increment of IDs in the negative temperature coefficient (NTC) region is significantly higher than that in high temperature region. The sensitivity analysis indicates that both inhibition and promotion effects of important elementary reactions on n-heptane oxidation are weakened because of methane addition. However, the weakening influence on the promoting effect is more prominent. In addition, the inhibition effect of some elementary reactions that are related to the methane oxidation is enhanced. Thus, the IDs of n-heptane/air mixture are prolonged. The analyses of reaction rate of production (ROP) show that the both the production and consumption rates of key radicals decrease significantly in NTC region after methane is added, but it is negligible in the high temperature region. The study can extend the theoretical basis of ignition characteristics of methane/n-heptane blends under elevated temperatures and pressures.

20. Experimental Investigation for Co-Combustion Characteristics of Semi-Coke and Bituminous Coal in a 3 MWth Tangential Combustion Facility

GUAN Jingyu, YU Qiang, SUN Rui, SHEN Tao, WANG Minghao, YAN Yanfei, SONG Xin

Corresponding author: SUN Rui; GUAN Jingyu

E-mail: sunsr@hit.edu.cn; guanjy@hbc.com.cn

Journal of Thermal Science, 2020, 29(6): 1655-1662.

https://doi.org/10.1007/s11630-020-1187-4

Springer Link: https://link.springer.com/article/10.1007/s11630-020-1187-4

Keywords: tangential combustion, semi-coke, blending ratio, combustion characteristics, experimental investigation

Abstract: An experimental investigation was conducted in a 3 MW pilot-scale tangential combustion facility to explore the co-combustion characteristics of bituminous coal mixed with semi-coke. The thermal gravimetric analyzer (TGA) was used to obtained fuel thermal analysis. The results presented effects of semi-coke blending ratio (BR) on average furnace temperature, ignition temperature, NO emission and combustion efficiency. The excess air coefficient in main combustion sections and outlet were fixed at 0.85 and 1.2 while BR increased from 0% to 50 wt.%. The temperature profiles of combustion decreases along the height of furnace while average furnace temperature fluctuates slightly with an increasing BR. The concentration of NO has an increasing tendency with the increasing of BR. The ignition temperature obtained from TGA measurement agreed well with experiment result. In addition, combustion efficiency was not sensitive to BR and decreased slightly with the increasing BR.