Performance Evaluation of Small-channel Pulsating Heat Pipe Based on Dimensional Analysis and ANN Model

The pulsating heat pipe is a very promising heat dissipation device to address the challenge of higher heat-flux electronic chips,as it is characterised by excellent heat transfer ability and flexibility for miniaturisation.To boost the application of PHP,reliable heat transfer performance evaluationmodels are especially important.In this paper,a heat transfer correlation was firstly proposed for closed PHP with various working fluids(water,ethanol,methanol,R123,acetone)based on collected experimental data.Dimensional analysis was used to group the parameters.It was shown that the average absolute deviation(AAD)and correlation coefficient(r)of the correlation were 40.67%and 0.7556,respectively.For 95%of the data,the prediction of thermal resistance and the temperature difference between evaporation and condensation section fell within 1.13K/Wand 40.76K,respectively.Meanwhile,an artificial neural networkmodelwas also proposed.The ANN model showed a better prediction accuracy with a mean square error(MSE)and correlation coefficient(r)of 7.88e-7 and 0.9821,respectively.

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

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℃ to 90℃),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.

Magnetic Field-induced Enhancement of Phase Change Heat Transfer via Biomimetic Porous Structure for Solar-thermal Energy Storage

Multifunctional phase change composites are in great demand for all kinds of industrial technologies and applications,which have both superior latent heat capacity and excellent solar-thermal conversion capability.In this research,biomimetic phase change composites are made by inspired by natural systems,successfully getting the high thermal conductivity of carbon foam and magnetism of composites together,to establish a novel solar-thermal energy storage method.The morphology and the thermal characteristics of biomimetic phase change composites have been characterized.The results showed that the maximum storage efficiency of the biomimetic phase change materials increased by 56.3%compared to that of the based materials,and it can further be improved by the application of magnetic field.Meanwhile the heat transfer process of solarthermal conversion and energy storage in biomimetic porous structure under the external physical fields has been explained by simulation.Thus,the magnetic field-induced method applied in this research has better solar-thermal energy storage characteristics within a porous structure by dynamically controlling the magnetism,which has potential uses for various sustainable applications,including waste-heat recovery,energy conservation in building,and solar-thermal energy storage.

Prospect Evaluation of Low-GWP Refrigerants R1233zd(E)and R1336mzz(Z)Used in Solar-Driven Ejector-Vapor Compression Hybrid Refrigeration System

In this paper,the entrainment ratio,pump work,heat loads of heat exchangers and COPthermal were theoretically evaluated for a solar-driven ejector-vapor compression hybrid refrigeration system with R1233zd(E)and R1336mzz(Z)as the working fluids.The evaluation of the utilization potentials of R1233zd(E)and R1336mzz(Z)was presented by comparing the system performance with that of R245fa,a commonly used refrigerant in the ejector system.The results indicated that the systems with R1233zd(E)and R1336mzz(Z)had a higher entrainment ratio and lower pump work.The pump works when using R1233zd(E)and R1336mzz(Z)can be up to 14.59%and 38.05%lower than those of R245fa,respectively.Meanwhile,the system showed the highest COPthermal utilizing R1233zd(E)followed by that of R245fa,with the R1336mzz(Z)system having the lowest value.The differences between R1233zd(E)and R1336mzz(Z)systems,R1233zd(E)and R245fa systems were 4.33%and 2.0%,respectively.This paper was expected to provide a good reference for the utilizing prospect of R1233zd(E)and R1336mzz(Z)in ejector refrigeration systems.

Thermal Performance of a Micro Heat Pipe Array for Battery Thermal Management Under Special Vehicle-Operating Conditions

The thermal management of battery systems is critical for maintaining the energy storage capacity,life span,and thermal safety of batteries used in electric vehicles,because the operating temperature is a key factor affecting battery performance.Excessive temperature rises and large temperature differences accelerate the degradation rate of such batteries.Currently,the increasing demand for fast charging and special on-vehicle scenarios has increased the heat dissipation requirements of battery thermal management systems.To address this demand,this work proposes a novel micro heat pipe array(MHPA)for thermal management under a broadened research scope,including high heat generation rates,large tilt angles,mild vibration,and distributed heat generation conditions.The experimental results indicate that the temperature difference is maintained 3.44°C at a large heat generation of 50 W for a limited range of tilt angles.Furthermore,a mild vehicle vibra-tion condition was found to improve temperature uniformity by 3.3°C at a heat generation of 10 W.However,the use of distributed heat sources results in a temperature variation of 3.88°C,suggesting that the heat generation distribution needs to be considered in thermal analyses.Understanding the effects of these special battery-operating conditions on the MHPA could significantly contribute to the enhancement of heat transfer capability and temperature uniformity improvement of battery thermal management systems based on heat pipe technologies.This would facilitate the realization of meeting the higher requirements of future battery systems.

Evaluation on Excess Entropy Scaling Method Predicting Thermal Transport Properties of Liquid HFC/HFO Refrigerants

The application of the excess entropy scaling(EES)method to predict the viscosity,thermal conductivity and thermal diffusivity of HFC/HFO refrigerants is evaluated in this paper.The universal coefficients of the EES model were firstly obtained from the properties of HFC refrigerants,and the accuracy of the model was further investigated with HFO properties.It was suggested that the EES model correlated the viscosity very well with the average absolute deviations(AADs)of most HFC refrigerants lower than 6.55%except R32.The correlations also provided very good prediction on the viscosity for R1234yf and R1234ze(E),but not for R1336mzz(Z).The prediction of thermal conductivity for both HFC and HFO refrigerants was generally well with the maximum AAD of 11.44%.However,the paper also indicated that there were no universal thermal diffusivity coefficients for even HFC refrigerants,and the linear function could not fit the thermal diffusivity curve very well.Therefore,the exclusively two-order polynomial correlations based on the EES model were presented for each HFC/HFO refrigerant.

A Novel Design of Thermoelectric Generator for Automotive Waste Heat Recovery

With progressively stringent fuel consumption regulations,many researchers and engineers are focusing on the employment of waste heat recovery technologies for automotive applications.Regarded as a promising method of waste heat recovery,the thermoelectric generator(TEG)has been given increasing attention over the whole automotive industry for the last decade.In this study,we first give a brief review of improvements in thermoelectric materials and heat exchangers for TEG systems.We then present a novel design for a concentric cylindrical TEG system that addresses the existing weaknesses of the heat exchanger.In place of the typical square-shaped thermoelectric module,our proposed concentric cylindrical TEG system uses an annular thermoelectric module and employs the advantages of the heat pipe to enhance the heat transfer in the radial direction.The simulations we carried out to verify the performance of the proposed system showed better power output among the existing TEG system,and a comparison of water-inside and gas-inside arrangements showed that the water-inside concentric cylindrical TEG system produced a higher power output.

考虑气蚀的液力缓速器湍流流动尺度解析模拟

目的:液力缓速器高速运行时会产生气蚀侵蚀现象,进而对缓速器的缓速制动及平稳运行产生不利影响。本文旨在对液力缓速器的气蚀湍流场进行分析,探究气蚀侵蚀发生的原因,为进一步探究减轻气蚀侵蚀的措施提供理论基础。创新点:1.引入目前先进的尺度解析模拟方法来模拟湍流场,使湍流场数值计算结果更加真实;2.采用气液两相流模型和气蚀模型相结合的方法模拟气蚀现象,并通过流场中的气泡体积来衡量气蚀侵蚀程度。方法:1.采用不同的湍流模型解析液力缓速器四种转速下的湍流场,并通过比较流场结果得出不同湍流模型模拟流场的区别(图3~5和7);2.采用应力混合涡(SBES)模型模拟高转速下的气蚀流场,并提取流场处理结果来分析缓速器内部气泡体积的瞬态演变规律(图8和9);3.提取不同时刻的叶片温度来分析气蚀引起的能量变化(图12和13)。结论:1.在四大湍流模型中,SBES模型模拟湍流场涡旋的能力最强且提取出的制动转矩结果与实验值最接近;2.高转速下的气蚀侵蚀情况严重,流场中出现的气泡体积较大,并且,随着时间推移气泡体积累积对缓速器运行将产生不利影响。3.气蚀流场中出现的气泡会影响缓速器湍流场中的涡旋,并且影响缓速器的叶片温度变化。

Experimental Investigation of the Ejector Refrigeration Cycle for Cascade System Application

Ejector refrigeration cycle(ERC)with advantages of simple structure and low cost holds great application potential in cascade/hybrid cycles to improve the overall system performance by removing or recovering the heat from the main cycle.In this paper,a theoretical and experimental investigation of the ERC as a part of a cascade system was carried out.The operating parameters were optimized.The experimental ERC test rig was designed,developed and investigated at high evaporating temperatures and wide ranges of operating conditions.The influence of operating conditions on the efficiency of the ejector and ERC was analyzed.Experimental results and analysis in this study can be helpful for the application and operating condition optimization of ERC in cascade/hybrid refrigeration systems.

A Study of the Truncated Square Pyramid Geometry for Enhancement of Super-hydrophobicity

Super-hydrophobic surfaces are quite common in nature,inspiring people to continually explore its water-repellence property and applications to our lives.It has been generally agreed that the property of super-hydrophobicity is mainly contributed by the microscale or nanoscale(or even smaller)architecture on the surface.Besides,there is an energy barrier between the Cassie-Baxter wetting state and the Wenzel wetting state.An optimized square post micro structure with truncated square pyramid geometry is introduced in this work to increase the energy barrier,enhancing the robustness of super-hydrophobicity.Theoretical analysis is conducted based on the wetting transition energy curves.Numerical simulation based on a phase-field lattice Boltzmann method is carried out to verify the theoretical analysis.The numerical simulation agrees well with the theoretical analysis,showing the positive significance of the proposed micro structure.Furthermore,another novel micro structure of rough surface is presented,which combines the advantages of truncated pyramid geometry and noncommunicating roughness elements.Theoretical analysis shows that the novel micro structure of rough surface can effectively hinder the Cassie-Baxter state to Wenzel state transition,furthefly enhancing the robustness of the surface hydrophobicity.