Experimental and Simulation Studies on Cold Welding Sealing Process of Heat Pipes

Sealing quality strongly affects heat pipe performance, but few studies focus on the process of heat pipe sealing. Cold welding sealing technology based on a stamping process is applied for heat pipe sealing. The bonding mechanism of the cold welding sealing process （CWSP） is investigated and compared with the experimental results obtained from the bonding interface analysis. An orthogonal experiment is conducted to observe the effects of various parameters, including the sealing gap, sealing length, sealing diameter, and sealing velocity on bonding strength. A method with the utilization of saturated vapor pressure inside a copper tube is proposed to evaluate bonding strength. A corresponding finite element model is developed to investigate the effects of sealing gap and sealing velocity on plastic deformation during the cold welding process. Effects of various parameters on the bonding strength are determined and it is found that the sealing gap is the most critical factor and that the sealing velocity contributes the least effect. The best parameter combination （AIB3CID3, with a 0.5 mm sealing gap, 6 mm sealing length, 3.8 mm sealing diameter, and 50 mm/s sealing velocity） is derived within the experimental parameters. Plastic deformation results derived from the finite element model are consistent with those from the experiment. The instruction for the CWSP of heat pipes and the design of sealing dies of heat pipes are provided.

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.

An Energy Analysis on Gasification of Sewage Sludge by a Direct Injection in Supercritical Water

Supercritical Water Gasification is an efficient technology in converting wet biomass into H2 and CH4 in comparison to other conventional thermochemical processes. Coke deposition, however, remains as a major challenge in this technology. Coke formation is the result of polymerization reactions that take place at sub-critical conditions. Directly injecting the relatively unheated wet biomass feed into supercritical water increases the heating rate and reduces the residence time of the feed in the sub-critical condition. This leads to a minimized coke formation in the process. However, a non-isothermal mixing takes place during this direct injection that is less energy-efficient. In addition, the biomass feedstream experiences less pre-heating that means less heat recovery from the product gas. These two aspects might reduce the overall process performance. Parametric studies of key operating parameters, such as operating temperature, dry matter content, bypass water ratio and heat exchanger effectiveness, are carried out to investigate the influence of direct injection to the thermal efficiency of the system. Subsequently, optimization using pinch analysis is conducted to the system with direct injection. Finally, an operating window for optimum performance of the optimized direct injection gasification system is proposed.

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.

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.

Euler-Euler CFD modeling of fluidized bed:Influence of specularity coefficient on hydrodynamic behavior

Euler-Euler two-fluid model is used to simulate the hydrodynamics of gas-solid flow in a bubbling flu- idized bed with Geldert B particles where the solid property is calculated by applying the kinetic theory of granular flow （KTGF）. Johnson and Jackson wall boundary condition is used for the particle phase, and different amount of slip between particle and wall is given by varying the specularity coefficient （φ） from 0 to 1. The simulated particle velocity, granular temperature and particle volume fraction are compared to investigate the effect of different wall boundary conditions on the hydrodynamic behavior, Some of the results are also compared with the available experimental data from the literature. It was found that the model predictions are sensitive to the specularity coefficient. The hydrodynamic behavior deviated sig- nificantly for φ = 0 and φ = 0.01 with maximum deviation found at φ = 0 i.e. free-slip condition. However, the overall bed height predicted by all the conditions is similar.

Wetting transition energy curves for a droplet on a square-post patterned surface

Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical, experimental and numerical studies on wetting transitions have been carried out by researchers, but the mechanism of wetting transitions between Cassie-Baxter state and Wenzel state, which is crucial to develop a stable superhydrophobic surface, is still not fully understood. In this paper, the flee energy curves based on the transition processes are presented and discussed in detail. The exis- tence of energy barriers with or without consideration of the gravity effect, and the irreversibility of wet- ting transition are discussed based on the presented energy curves. The energy curves show that different routes of the Cassie-to-Wenzel transition and the reverse transition are the main reason for the irre- versibility. Numerical simulations are implemented via a phase field lattice Boltzmann method of large density ratio, and the simulation results show good consistency with the theoretical analysis.

Numerical Study of Wetting Transitions on Biomimetic Surfaces Using a Lattice Boltzmann Approach with Large Density Ratio

The hydrophobicity of natural surfaces has drawn much attention of scientific communities in recent years. By mimicking natural surfaces, the manufactured biomimetic hydrophobic surfaces have been widely applied to green technologies such as self-cleaning surfaces. Although the theories for wetting and hydrophobicity have been developed, the mechanism of wetting transitions between heterogeneous wetting state and homogeneous wetting state is still not fully clarified. As understanding of wetting transitions is crucial for manufacturing a biomimetic superhydrophobic surface, more fundamental discussions in this area should be carried out. In the present work, the wetting transitions are numerically studied using a phase field lattice Boltzmann approach with large density ratio, which should be helpful in understanding the mechanism of wetting transitions. The dynamic wetting transition processes between Cassie-Baxter state and Wenzel state are presented, and the energy barrier and the gravity effect on transition are discussed. It is found that the two wetting transition processes are irreversible for specific inherent contact angles and have different transition routes, the energy barrier exists on an ideally patterned surface and the gravity can be crucial to overcome the energy barrier and trigger the transition.

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.

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 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.

Scale-resolving simulations and investigations of the flow in a hydraulic retarder considering cavitation

Cavitation has a significant influence on the accurate control of the liquid filling rate and braking performance of a hydraulic retarder;however,previous studies of the flow field in hydraulic retarders have provided insufficient information in terms of considering cavitation.Here,the volume of fluid(VOF)method and a scale-resolving simulation(SRS)were employed to numerically and more comprehensively calculate and analyze the flow field in a retarder considering the cavitation phenomenon.The numerical models included the improved delayed detached eddy simulation(IDDES)model,stress-blended eddy simulation(SBES)model,dynamic large eddy simulation(DLES)model,and shear stress transport(SST)model in the Reynolds-averaged Navier-Stokes(RANS)model.All the calculations were typically validated by the brake torque in the impeller rather than the internal flow.The unsteady flow field indicated that the SBES and DLES models could better capture unsteady flow phenomena,such as the chord vortex.The SBES and DLES models could also better capture bubbles than the SST and IDDES models.Since the braking torque error of the SBES model was the smallest,the transient variation of the bubble volume fraction over time on a typical flow surface was analyzed in detail with the SBES model.It was found that bubbles mainly appeared in the center area of the blade suction surface,which coincided with the experiments.The accumulation of bubbles resulted in a larger bubble volume fraction in the center of the blade over time.In addition,the temperature variations of the pressure blade caused by heat transfer were further analyzed.More bubbles precipitated in the center of the blade,leading to a lower temperature in this area.

Mathematical and Experimental Investigation of Water Migration in Plant Xylem

Plant can take water from soil up to several metres high. However, the mechanism of how water rises against gravity is still controversially discussed despite a few mechanisms have been proposed. Also, there still lacks of a critical transportation model because of the diversity and complex xylem structure of plants. This paper mainly focuses on the water transport process within xylem and a mathematical model is presented. With a simplified micro channel from xylem structure and the calculation using the model of water migration in xylem, this paper identified the relationship between various forces and water migration velocity. The velocity of water migration within the plant stem is considered as detail as possible using all major forces involved, and a full mathmetical model is proposed to calculate and predict the velocity of water migration in plants. Using details of a specific plant, the velocity of water migration in the plant can be calculated, and then compared to the experimental result from Magnetic Resonance Imaging （MRI）. The two results match perfectly to each other, indicating the accuracy of the mathematical model, thus the mathematical model should have brighter furore in further applications.

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.