Heat Transfer Performance and Structural Optimization of a Novel Micro-channel Heat Sink

With the advent of the 5G era,the design of electronic equipment is developing towards thinness,intelligence and multi-function,which requires higher cooling performance of the equipment.Micro-channel heat sink is promising for the heat dissipation of super-thin electronic equipment.In this study,thermal resistance theoretical model of the micro-channel heat sink was first established.Then,fabrication process of the micro-channel heat sink was introduced.Subsequently,heat transfer performance of the fabricated micro-channel heat sink was tested through the developed testing platform.Results show that the developed micro-channel heat sink has more superior heat dissipation performance over conventional metal solid heat sink and it is well suited for high power LEDs application.Moreover,the micro-channel structures in the heat sink were optimized by orthogonal test.Based on the orthogonal optimization,heat dissipation performance of the micro-channel radiator was further improved.

Heat Dissipation Performance of Metal Core Printed Circuit Board with Micro Heat Exchanger

A Metal Core Printed Circuit Board with Micro Heat Exchanger(MHE MCPCB)was introduced for thermal management of high power LED.A comparative study was performed between 4 W/(m·K)regular MCPCB and this novel MCPCB to investigate the heat dissipation performance of this novel MCPCB.It was found that MHE MCPCB can obviously enhance the comprehensive optical properties of LED in comparison with 4 W/(m·K)regular MCPCB.Additionally,thermal contact resistance confining a dominant part of heat within the micro heat exchanger to achieve high efficient heat dissipation was proved.

Conceptual design and heat transfer performance of a flat-tile water-cooled divertor target

The divertor target components for the Chinese fusion engineering test reactor(CFETR)and the future experimental advanced superconducting tokamak(EAST)need to remove a heat flux of up to20 MW m-2.In view of such a high heat flux removal requirement,this study proposes a conceptual design for a flat-tile divertor target based on explosive welding and brazing technology.Rectangular water-cooled channels with a special thermal transfer structure(TTS)are designed in the heat sink to improve the flat-tile divertor target’s heat transfer performance(HTP).The parametric design and optimization methods are applied to study the influence of the TTS variation parameters,including height(H),width(W*),thickness(T),and spacing(L),on the HTP.The research results show that the flat-tile divertor target’s HTP is sensitive to the TTS parameter changes,and the sensitivity is T>L>W*>H.The HTP first increases and then decreases with the increase of T,L,and W*and gradually increases with the increase of H.The optimal design parameters are as follows:H=5.5 mm,W*=25.8 mm,T=2.2 mm,and L=9.7 mm.The HTP of the optimized flat-tile divertor target at different flow speeds and tungsten tile thicknesses is studied using the numerical simulation method.A flat-tile divertor mock-up is developed according to the optimized parameters.In addition,high heat flux(HHF)tests are performed on an electron beam facility to further investigate the mock-up HTP.The numerical simulation calculation results show that the optimized flat-tile divertor target has great potential for handling the steady-state heat load of 20 MW m-2under the tungsten tile thickness<5 mm and the flow speed7 m s^(-1).The heat transfer efficiency of the flat-tile divertor target with rectangular cooling channels improves by13%and30%compared to that of the flat-tile divertor target with circular cooling channels and the ITER-like monoblock,respectively.The HHF tests indicate that the flat-tile divertor mock-up can successfully withstand 1000 cycles of20 MW m-2of heat load without visible deformation,damage,and HTP degradation.The surface temperature of the flat-tile divertor mock-up at the 1000th cycle is only930℃.The flat-tile divertor target’s HTP is greatly improved by the parametric design and optimization method,and is better than the ITER-like monoblock and the flat-tile mock-up for the WEST divertor.This conceptual design is currently being applied to the engineering design of the CFETR and EAST flat-tile divertors.

Effects of Heat Exchange Tube Structural Parameters on Performance of Vehicle Radiator

In order to improve the performance of vehicle radiators, a two-dimensional heat transfer steady-state model of the radiator was set up. The influence of the structural parameters (axial ratio) of the heat exchange tube on the windward side on the heat transfer performance of the radiator was studied. With the increase of the axial ratio of the heat exchange tube on the windward side, the heat exchange capacity of the heat exchange tube surface slightly decreases. The heat exchange area increases significantly, which increases the total heat exchange of the radiator and improves the heat transfer performance of the radiator. When the axial ratio increases from 1.0 to 2.0, the average surface heat transfer capacity decreases from 5664.16 W/m2 to 5623.57 W/m2.

Numerical simulation of flow and heat transfer of n-decane in sub-millimeter spiral tube at supercritical pressure

The flow and heat transfer characteristics of n-decane in the sub-millimeter spiral tube(SMST) at supercritical pressure(p = 3 MPa) are studied by the RNG k-ε numerical model in this paper. The effects of various Reynolds numbers(Re) and structural parameters pitch(s) and spiral diameter(D) are analyzed.Results indicate that the average Nusselt numberNu and friction factorNu increase with an increase in Re, and decrease with an increase in D/d(tube diameter). In terms of the structural parameter s/d, it is found that as s/d increases, the Nu first increase, and then decrease. and the critical structural parameter is s/d = 4. Compared with the straight tube, the SMST can improve Nu by 34.8% at best, while it can improve Nu by 102.1% at most. In addition, a comprehensive heat transfer coefficient is applied to analyze the thermodynamic properties of SMST. With the optimal structural parameters of D/d = 6 and s/d = 4, the comprehensive heat transfer factor of supercritical pressure hydrocarbon fuel in the SMST can reach 1.074. At last, correlations of the average Nusselt number and friction factor are developed to predict the flow and heat transfer of n-decane at supercritical pressure.

Tailoring of a robust asymmetric aramid nanofibers/MXene aerogel film for enhanced infrared thermal camouflage and Joule heating performances

The development of infrared(IR)surveillance technology has led to a growing interest in thermal camouflage.However,the trade-off relationship between low IR-emissivity and thermal insulation hinders the advance of thermal camouflage materials.Herein,guided by multi-physics simulation,we show a design of asymmetric aramid nanofibers/MXene(ANF/MXene)aerogel film that realizes high-efficient thermal camouflage applications.The rationale is that the asymmetric structure contains a thermal-insulation three-dimensional(3D)network part to prevent effective heat transfer and a low IR-emissivity(~0.3)dense surface layer to suppress radiative heat emission.It is remarkable that the synergy mechanism in the topology structure contributes to over 40%reduction of target radiation temperature.Impressively,the tailored asymmetric ANF/MXene aerogel film also enables sound mechanical properties such as a Young’s modulus of 44.4 MPa and a tensile strength of 1.3 MPa,superior to most aerogel materials.It also exhibits great Joule heating performances including low driving voltage(4 V),fast thermal response(<10 s),and long-term stability,further enabling its versatile thermal camouflage applications.This work offers an innovative design concept to configure multifunctional structures for next-generation thermal management applications.

Heat Storage Performance of PCM in a Novel Vertical Pointer-Shaped Finned Latent Heat Tank

The heat storage performance of latent heat storage systems is not good due to the poor thermal conductivity of phase change materials.In this paper,a new type of pointer-shaped fins combining rectangular and triangular fins has been employed to numerically simulate the melting process in the heat storage tank,and the fin geometry parameter effects on heat storage performance have been studied.The results indicate that compared with the bare tube and the rectangular finned tank,the melting time of the phase change material in the pointer-shaped finned tank is reduced by 64.2%and 15.1%,respectively.The closer the tip of the triangular fin is to the hot wall,the better the heat transfer efficiency.The optimal height of the triangular fin is about 8 mm.Increasing the number of fins from 4 to 6 and from 6 to 8 reduces the melting time by 16.0%and 16.7%respectively.However,increasing the number of fins from 8 to 10 only reduces the melting time by 8.4%.When the fin dimensionless length is increased from 0.3 to 0.5 and from 0.5 to 0.7,the melting time is shortened by 17.5%and 13.0%.But the melting time is only reduced by 2.9%when the dimensionless fin length is increased from 0.7 to 0.9.For optimising the design of the thermal storage system,the results can provide a reference value.

Numerical Simulation and Optimization of a Mid-Temperature Heat Pipe Exchanger

In this paper,we take the mid-temperature gravity heat pipe exchanger as the research object,simulate the fluid flow field,temperature field and the working state of heat pipe in the heat exchanger by Fluent software.The effects of different operating parameters and fin parameters on the heat transfer performance of heat exchangers are studied.The results show that the heat transfer performance of the mid-temperature gravity heat pipe exchanger is the best when the fin spacing is between 5 mm and 6 mm,the height of the heat pipe is between 12 mm and 13 mm,and the inlet velocity of the fluid is between 2.5 m/s to 3 m/s.

Optimization of Heat-Sink Cooling Structure in EAST with Hydraulic Expansion Technique

Considering utilization of the original chromium-bronze material, two processing techniques including hydraulic expansion and high temperature vacuum welding were proposed for the optimization of heat-sink structure in EAST. The heat transfer performance of heat-sink with or without cooling tube was calculated and different types of connection between tube and heat-sink were compared by conducting a special test. It is shown from numerical analysis that the diameter of heat-sink channel can be reduced from 12 mm to 10 mm. Compared with the original sample, the thermal contact resistance between tube and heat-sink for welding sample can reduce the heat transfer performance by 10%, while by 20% for the hydraulic expansion sample. However, the welding technique is more complicated and expensive than hydraulic expansion technique. Both the processing technique and the heat transfer performance of heat-sink prototype should be further considered for the optimization of heat-sink structure in EAST.

Application of Technology of Additive Manufacturing in Radiators and Heat Exchangers

Radiators and heat exchangers play a key role in the long-term and stable operation of the equipment. The emergence of additive manufacturing technology has released the freedom of design, enabling many innovative structures of radiators and heat exchangers to be manufactured. The paper reviews the application of additive manufacturing in new radiators and heat exchangers. The technology of additive manufacturing boosts the development of new radiators and heat exchangers, which improves heat dissipation performance and heat exchange efficiency. This paper will provide a new idea and method for the development of radiators and heat exchangers via the application of additive manufacturing.

Temperature distribution of fluids in a two-section two-phase closed thermosyphon wellbore

Compared with a conventional single section two-phase closed thermosyphon （TPCT） wellbore, a two-section TPCT wellbore has better heat transfer performance, which may improve the temperature distribution of fluid in wellbores, increase the temperature of fluid in wellheads and even more effectively reduce the failure rate of conventional TPCT wellbores. Heat transfer performance of two-section TPCT wellbores is affected by working medium, combination mode and oil flow rate. Different working media are introduced into the upper and lower TPCTs, which may achieve a better match between the working medium and the temperature field in the wellbores. Interdependence exists between the combination mode and the flow rate of the oil, which affects the heat transfer performance of a two-section TPCT wellbore. The experimental results show that a two-section TPCT wellbore, with equal upper and lower TPCTs respectively filled with Freon and methanol, has the best heat transfer performance when the oil flow rate is 200 L/h.

Numerical study on the heat transfer performance of coaxial shallow borehole heat exchanger

In the present work,a numerical investigation on the coaxial shallow borehole heat exchanger based on Com-putational Fluid Dynamics(CFD)technique in Hefei city of China has been performed.The effects of design parameters,including inlet flow rate,inlet fluid temperature,inner pipe material and outer pipe diameter,on the heat transfer performance were systematically studied.Besides,the thermal behavior along the pipe has been carefully examined with focus on the thermal short-circuiting phenomena.When the fluid inlet velocity is less than the critical value,the turbulence intensity increases and the Nusselt number increases with the inlet flow rate increasing.However,there is sufficient time for heat transfer between the fluid in inner pipe and outer pipe because of low flow rates,leading to large heat loss,i.e.,thermal short-circuiting phenomenon.It is found that with the inlet flow rate increasing,the heat transfer increases first and then decreases,and the rate of reduction slows down gradually.When the inlet flow rate increases,the pumping power undergoes exponential growth.As the inlet temperature increasing,the heat transfer decreases almost linearly.Moreover,when the soil temperature at the top of the casing is lower than that of the fluid in the casing,heat is transferred from the fluid in the casing to the soil,and the heat loss increases with the increase of the inlet fluid temperature.The material of inner pipe with high heat conductivity would result in large heat loss under the influence of thermal short-circuiting.The heat load increases while the pumping power required decreases with the increasing of outer pipe diameter.This study is very beneficial for the coaxial shallow borehole exchanger designs and energy conservation of buildings.

Chinese Economists’ Confidence Survey Economic Performance is on the Heated Side

As shown in the results of the Survey of the Confidence of 100 Chinese Economists,conducted by China Economic Monitoring and Analysis Center,National Bureau of Statistics,the confidence index of the economists in the fourth quarter of 2007 was 5.52,an average index since 2004.

Experimental evaluation of the effect of an internal heat exchanger on a transcritical CO_2 ejector system

This study presents experimental results focused on a performance comparison of a transcritical CO2 ejector system without an internal heat exchanger(IHX) (EJE-S) to a transcritical CO2 ejector system with an IHX(EJE-IHX-S) . The comparison includes the effects of changes in operating conditions such as cooling water flow rate and inlet temperature. Experiments are conducted to assess the influence of the IHX on the heating coefficient of performance(COPr) ,heating capacity,entrainment ratio,pressure lift,and other parameters. The primary flow rate of the EJE-IHX-S is higher than that of the EJE-S. The pressure lift and actual ejector work recovery are reduced when the IHX is added to the transcritical CO2 ejector system. Using a more practical performance calculation,the compression ratio in the EJE-S is reduced by 10.0%-12.1%,while that of EJE-IHX-S is reduced only by 5.6%-6.7% compared to that of a conventional transcritical CO2 system. Experimental results are used to validate the findings that the IHX weakens the contribution of the ejector to the system performance.

Comparison of thermal and light performance in two typical Chinese solar greenhouses in Beijing

Solar greenhouses have been used for producing vegetables in northern China during early spring,late autumn or over-winter.To improve the thermal performance of solar greenhouses,a traditional type and a retrofitted design were comparatively evaluated.In the retrofitted design,three adjustments were incorporated:the material and structure of the walls,south-facing roof angle,and structure of the north-facing back-roof.The results indicated that the thermal and light performance of the retrofitted greenhouse was much better than that of the traditional greenhouse.Specifically,the daily mean temperature,minimum air temperature,and soil temperature inside the greenhouses after retrofit ting were increased by 1.3,2.4,and 1.9℃,respectively,meanwhile,the daily total solar radiation and PAR were increased by 28.2%and 9.2%,respectively.The wall temperature and its daily variation range were reduced with increasing depth and height.The characteristic analysis of heat storage and release indicated that higher locations have longer heat storage,and shorter heat release time in vertical direction,as well as a lower ratio of heat release to storage.In horizontal direction,the western wall has the shortest heat storage time but the highest heat release flux density.Altogether,the heat storage time of the wall is 1.5 h less than that of the soil.The heat storage flux density of the wall is 1.5 times of that of the soil,but the heat release flux is only 61%of the soil’s value.The total wall heat storage is half of that of the soil in the greenhouse;the total wall heat release amount is only a quarter of that of the soil.Therefore,the thermal environment of solar greenhouses can be further improved by improving the thermal insulation properties of the wall.

Enhancement of Thermo-Flow Performances by Windbreakers for Two-Tower Indirect Dry Cooling System

The winds will greatly weaken the cooling performance of indirect dry cooling system with twin towers.The exterior windbreakers are recommended to restrain the wind adverse effects in this paper.The macro heat exchanger model was adopted to simulate the heat exchange between circulating water and ambient air.The performances of natural draft dry cooling system(NDDCS)with and without exterior windbreakers were numerically studied.The pressure,velocity and temperature distribution of cooling air in wind angles of 0°,45°and 90°was obtained and presented.The results show that in all wind directions,the performances for lateral sector of towers with windbreakers are significantly improved,but the low-pressure zone appears unexpectedly for the rear sectors,which reduces the air flow rate.The cooling performances of the twin towers with or without windbreakers decrease at first but then recover with the wind velocity increasing.Besides,the optimal flow and heat transfer performances appear in the wind angle of 0°.The cooling performances can be significantly improved in all three wind directions due to windbreaker configuration.

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

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℃ to–5℃.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.

Experimental study on effect of inclination angles to ammonia pulsating heat pipe

In this paper, a novel study on performance of closed loop pulsating heat pipe（CLPHP）using ammonia as working fluid is experimented. The tested CLPHP, consisting of six turns, is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter. The filling ratio is50%. The visualization investigation is conducted to observe the oscillation and circulation flow in the CLPHP. In order to investigate the effects of inclination angles to thermal performance in the ammonia CLPHP, four case tests are studied. The trends of temperature fluctuation and thermal resistance as the input power increases at different inclination angles are highlighted. The results show that it is very easy to start up and circulate for the ammonia CLPHP at an inclining angle.The thermal resistance is low to 0.02 K/W, presenting that heat fluxes can be transferred from heating section to cooling section very quickly. It is found that the thermal resistance decreases as the inclination angle increases. At the horizontal operation, the ammonia CLPHP can be easy to start up at low input power, but hard to circulate. In this case, once the input power is high,the capillary tube in heating section will be burnt out, leading to worse thermal performance with high thermal resistance.

Simulation of performance of intermediate fluid vaporizer under wide operation conditions

The intermediate fluid vaporizer(IFV)is a typical vaporizer of liquefied natural gas(LNG),which in general consists of three shell-and-tube heat exchangers(an evaporator,a condenser,and a thermolator).LNG is heated by seawater and the intermediate fluid in these heat exchangers.A one-dimensional heat transfer model for IFV is established in this paper in order to investigate the influences of structure and operation parameters on the heat transfer performance.In the rated condition,it is suggested to reduce tube diameters appropriately to get a large total heat transfer coefficient and increase the tube number to ensure the sufficient heat transfer area.According to simulation results,although the IFV capacity is much larger than the simplified-IFV(SIFV)capacity,the mode of SIFV could be recommended in some low-load cases as well.In some cases at high loads exceeding the capacity of a single IFV,it is better to add an AAV or an SCV operating to the IFV than just to increase the mass flow rate of seawater in the IFV in LNG receiving terminals.