Heat Transfer Characteristics and Pressure Drop in a Horizontal Circulating Fluidized Bed Evaporator

A vapor-liquid-solid horizontal circulating fluidized bed evaporation setup was constructed to study the thermal-exchange properties and pressure change.The influences of the operating variables,including the amount of added particles,heat flux,and circulating flow velocity,were systematically inspected using resistance temperature detectors and pressure sensors.The results showed that the heat transfer eff ect was improved with the increase in the amount of added particles,circulating flow velocity,and particle diameter,but decreased with increasing heat flux.The pressure drop fluctuated with the increase in operating parameters,except circulating flow velocity.The enhancing factor reached up to 71.5%.The enhancing fac-tor initially increased and then decreased with the increase in the amount of added particles and circulating flow velocity,fluctuated with increasing particle diameter,and decreased with increasing heat flux.Phase diagrams showing the variation ranges of the operation variables for the enhancing factor were constructed.

Research on the Flow and Heat Transfer Characteristics of a Water-Cooling Plate in a Heat Dissipation System

The water-cooling heat dissipation technology can solve the heat dissipation and noise problems of the calculation plate.Therefore,the structural design of the water-cooling plate directly affects its flow and heat transfer characteristics,which restricts the promotion and application of the technology.To this end,the water-cooling plate of a heat dissipation system was taken as the research object,and its flow and heat transfer characteristics were numerical simulated and experimental studied.Through comparative analysis,the rationality of the numerical simulation method was verified.Based on this,three improved schemes of water-cooling plate structure were proposed and numerical simulation was carried out,and the optimal model was verified by experiments.The results of the study show that compared with the original water-cooling plate,the optimized water-cooling plate has increased internal flow velocity and distributes uniformly,increased heat transfer amount by 4.2%,and the average temperature of the calculation plate decreased by 5.3%.

Effect of Heat Treatment on the Corrosion Characteristics of Al-Li Alloys

Experiments were conducted to study the general room temperature corrosion characteristics of heat-treated and non-heat-treated Al-Li alloys with different Li compositions. Corrosion rate was measured using both the polarisation method and the weight-loss method. It was observed that the samples with higher Li content had lower corrosion resistance than those with lower Li content. Moreover, for all the specimens tested, it was fOund that heat treatment at 180℃ for 2 h (for the purpose of precipitation hardening) severely reduced the corrosion resistance,whereas heat treatment at 180℃ for 6 h significantly increased the corrosion resistance.

High-temperature characteristics of SiC module and 100 kW SiC AC–DC converter at a junction temperature of 180 ℃

High-temperature,high-power converters have gained importance in industrial applications given their ability to operate in adverse environments,such as in petroleum exploration,multi-electric aircrafts,and electric vehicles.SiC metaloxide-semiconductor field-effect transistor(MOSFET),a new,wide bandgap,high-temperature device,is the key component of these converters.In this study,the static and dynamic characteristics of the SiC MOSFET,half-bridge module,are investigated at the junction temperature of 180℃.A simplified experimental method is then proposed pertaining to the power operation of the SiC module at 180℃.This method is based on the use of a thermal resistance test platform and is proven convenient for the study of heat dissipation characteristics.The high-temperature characteristics of the module are verified based on the conducted experiments.Accordingly,a 100 kW high-temperature converter is built,and the test results show that the SiC converter can operate at a junction temperature of 180℃in a stable manner in compliance with the requirements of high-temperature,high-power applications.

Research on Flow and Heat Transfer Characteristics Inside Spiral Pile-honeycomb Structures with Different Shapes

Spiral plate heat exchanger is a traditional compact heat exchanger,and widely used in the occasion of the high heat transfer capacity requirements,such as waste heat recovery. In the situation of energy shortage and rapid development of heat transfer technology^(1-3),cylindrical,elliptical and rhombic spiral pile-honeycomb heat transfer models are established^(4-5). The internal flow and heat transfer are simulated by using CFD software FLUENT15. 0 and RNG k-ε turbulent model,and then the three models are analyzed. The results show that the secondary flow and vortex are induced again in the secondary honeycomb,which further increases the turbulence intensity of the fluid. The thickness of the boundary layer is reduced twice,and the heat transfer effect is better than that of the honeycomb spiral structure. The spiral pile-honeycomb model for the rhombus is better than the models of cylinder and ellipse in heat transfer performance.

Experimental Investigation of the Operating Characteristics of a Pulsating Heat Pipe with Ultra-Pure Water and Micro Encapsulated Phase Change Material Suspension

The specific heat capacity of working fluid is an important influence factor on heat transfer characteristic of the pulsating heat pipe(PHP).Due to the relatively large specific heat capacity of micro encapsulated phase change material(MEPCM) suspension,a heat transfer performance experimental facility of the PHP was established.The heat transfer characteristic with MEPCM suspension of different mass concentrations(0.5% and 1.0%) and ultra-pure water were compared experimentally.It was found that when the PHP uses MEPCM suspension as its working fluid,operating stability is impoverished under lower heating power and the operating stability is better under higher heating power.At the inclination angle of 90°,the temperature at heating side decreases compared to ultra-pure water and the temperature at heating side decreases with the raising of MEPCM suspension mass concentration.The heat transfer characteristic of the PHP is positively correlated with the inclination angle and the 90° is optimum.The unfavorable effect of the inclination angle decreases with heating power increasing.When the inclination angle is 90°,the PHP with MEPCM suspension at 1.0% of mass concentration has the lowest thermal transfer resistance and followed by ultra-pure water and MEPCM suspension at 0.5% of mass concentration has the highest thermal transfer resistance.When the inclination angles are 60° and30°,the effect of gravity on the flow direction is reduced to 86.6% and 50% of that on the inclination angle of 90°,respectively,and the promoting effect of gravity on the working fluid is further weakened as the inclination angle further decreases.Due to the high viscosity of MEPCM suspension,the PHP with ultra-pure water has the lowest heat transfer resistance.When the inclination angles is 60°,the thermal resistance with MEPCM suspension at0.5% of the mass concentration is lower than that at 1.0% at the heating power below 230 W.The thermal resistance of MEPCM suspension tends to be similar for heating power of 230-250 W.At the heating power above 270 W,the thermal resistance with MEPCM suspension at 1.0% of the mass concentration is lower than that at 0.5%.

Study on Electromagnetic-Fluid-Temperature Multiphysics Field Coupling Model for Drum of Mine Cable Winding Truck

Aiming at solving problems of low efficiency,low cable capacity in current 300m open-pit mine cable winding truck,a 900 m cable winding plan was proposed.In this paper,the mechanism of the thermal effect of the cable was described,and a two-dimensional axisymmetric electromagnetic-fluid-temperature multiphysics coupling model of the cable reel was established regarding the 900m cable reel as independent system.Considering the structure of the drum,the number of cable winding layers,the factors of heat conduction,heat radiation and convective heat transfer in the actual working process,the steady state analysis of the multi-physical field coupling was carried out.The sum of the losses of each part of the cable was obtained through the calculation of electromagnetic field,which was used as a heat source to calculate and analyze the temperature distribution of different layers of cable winding,as well as the temperature distribution and heat dissipation characteristics of different structures of the drum.The results show that three layers of cable winding is the best design.The lowest temperature of closed cylindrical drum is 70℃after heat dissipation,which has obvious advantages compared with the lowest temperature of 85℃after heat dissipation of squirrel-cage cylindrical drum.The results provide a reliable theoretical basis for the research and development of a new type of mine cable winding truck with 900 m cable capacity.

CFD Analysis of Fluid-Dynamic and Heat Transfer Effects Generated by a Fixed Electricity Transmission Line Interacting with an External Wind

The flow past a fixed single transmission conductor and the related heat transfer characteristics are investigated using computational fluid dynamics and a relevant turbulence model.After validating the method through comparison with relevant results in the literature,this thermofluid-dynamic problem is addressed considering different working conditions.It is shown that the resistance coefficient depends on the Reynolds number.As expected,the Nusselt number is also affected by Reynolds number.In particular,the Nusselt number under constant heat flux is always greater than that under a constant wall temperature.

Experimental Investigation on Pouch Lithium-ion Battery Thermal Management with Mini-Channels Cooling Plate Based on Heat Generation Characteristic

An electrochemical thermal coupling model of lithium battery was established to study the heat generation characteristic in this study.The simulation results showed that the heat generation density of the battery increased with the discharge rate.With the discharge process,the heat generation density of the battery increased continuously.With 2.5C discharge rate,the heat generation density at the end of discharge was 1.82 times of that at the beginning of discharge.The heat generation density at different areas of the battery was not uniform and 46%of the total ohmic heat was generated near the electrode tabs.A cooling plate with variable mini-channels was designed to improve the temperature non-uniformity caused by the heat generation characteristic.A cooling plate with uniform mini-channels was designed for compared experiment.The experiments were conducted with deionized water and refrigerant R141b and carried out with 1.5C,2C and 2.5C discharge rates.Experimental results showed that the cooling plate with variable mini-channels had a better cooling performance in both single-phase and two-phase cooling conditions.

Thermal characteristics and thermal stress analysis of solar thermochemical reactor under high-flux concentrated solar irradiation

Nowadays, using a solar-driven thermochemical reaction system to convert greenhouse gases into high-quality liquid fuels has been proven to be an effective way to address the growing depletion of traditional fossil fuels. However, the utilization of highlyconcentrated solar irradiation runs the high risk of reactor damage issues resulting from thermal stress concentration, which seriously threatens the security and reliability of the total reactor system. In this study, the thermal radiation distribution and thermo-mechanical process in a volumetric reactor were numerically investigated by combining Monte Carlo ray-tracing method with computational fluid dynamics method. Based on the experimental results and thermal characteristic analysis, the formation mechanism of thermal stress concentration and the strategies of improving thermal stress distribution were discussed in detail.The simulation results indicate a great possibility of reactor damage at about 1000℃ operating temperature and 9.0 k W lamp power, which is well-matched with related experimental results. The ceramic damage typically occurs at the inner edges of the through-holes, including the aperture, the gas inlet, and the thermocouple hole, then extends along the lines connecting these holes and finally causes brittle fracture. By reasonable control of the opening direction and the distance between the throughholes, the maximum compressive stress can be reduced by 21.78%.

Heat transfer characteristics in a narrow confined channel with discrete impingement cooling

Heat transfer characteristics in a narrow confined channel with discrete impingement cooling were investigated using thermal infrared camera. Detailed heat transfer distributions and comparisons on three surfaces with three impact diameters were experimentally studied in the range of Reynolds number of 3000 to 30000. The experimental results indicated that the strong impingement jet leaded to a high strength heat transfer zone in the ΔX=±2.5D;range of the impact center,which was 1.3–2.5 times of the average heat transfer value of the impingement wall. With the same coolant mass flow rate, small diameter case had lower heat transfer coefficient on both inner wall and outside wall, while the impingement wall was insensitive to the impact diameter. The surface averaged Nusselt number of inner wall was only 43%–57% of impingement wall, while the outside wall can reach up to 80%–90%. The larger the diameter, the higher heat transfer enhancement and the smaller the channel flow resistance was observed in term of Reynolds number. The surface averaged Nusselt numbers were developed as the function of Reynolds number and the impingement height-to-diameter for further engineering applications.

Characteristic studies of heat sources and performance analysis of free-cooling assisted air conditioning and ventilation systems for working faces of mineral mines

High temperature heat hazard at mineral mine becomes more and more serious as the increase of mining depth.Heat sources at working faces of mineral mines are complex and are of different characteristics,presenting new challenges for air conditioning systems.In this paper,heat sources at four types of working faces are summarized and their characteristics are investigated.Based on this,simplified equations,which are linear with length of working faces,are proposed to calculate heat dissipation rates.So that the main heat sources of different working faces can be found,and cooling load of air conditioning systems can be calculated.Then,considering main heat sources of coal mines,a typical working face is designed to investigate performances of different ventilation systems and air conditioning systems.Simulation results show that segmented ventilation systems(SC)and heat shield assisted centralized ventilation systems(CCHS)can realize much better temperature distributions at working faces.However,cooling load can be greatly reduced for CCHS,when untreated air is supplied to the coal seam side.Based on this,free cooling assisted air conditioning systems are designed,and annual average energy efficiency ratio(EERann)of the systems are investigated and compared between direct evaporate cooling and indirect evaporate cooling(IEC).For SC,as compared with scenarios without free-cooling,IEC can increase EERann by 15%-23%and 22%-32%under Benxi and Datong ambient conditions,respectively.Besides,to ensure high EERann,CCHS is preferred and it is essential to increase thermal insulation of air ducts.

A comprehensive study on the heat transfer characteristics of windward bend lattice frame structure

The windward bend lattice frame structure(WB structure)is characterized by a high heat transfer coefficient and low friction factor.The WB structure can be applied for ther-mal protection system,protecting outer walls of afterburner and nozzles from being damaged by the heating load of hot gas,for air cooling system of the power battery module,dissipating the heat generated during its charging and discharging.In this paper,the heat transfer charac-teristics of the windward bend lattice frame structure have been comprehensively studied.A systematic 3D numerical simulation has been conducted to investigate the effects of the struc-tural parameters of the WB structure,including the pitches in both flow direction and transverse direction,the diameter and the inclination angle of windward bend ligament,on its flow resis-tance and heat transfer enhancement,which has been evaluated by comparing its Nusselt num-ber under an equal pumping power.Furthermore,the contribution of an important parameter,i.e.,the ratio of the interstitial heat transfer rate to the end-wall heat transfer rate(RQ),to the overall heat transfer rate has been fully discussed.As a result,the case of 6 units in the longi-tudinal direction and 2.5 units in the transverse direction,i.e.(nx Z 6,nz Z 2.5)exhibits the best performance in the light of the value of the Nusselt number. Moreover, the structure with aratio of RQ ranges in 4.5e5.0 achieves a better heat transfer performance. Finally, two colorcontour graphs showing an optimal range of Nusselt number coordinated by unit numbers(nx, nz) for pumping powers of 2500 and 3000 have been presented. The graphs correctly reflectthe variation of Nusselt numbers of structures with different nx and nz, and the conclusionsremain consistent with the discussion in sections 4.2 and 4.3, instructing the reasonable selec-tion of structural parameters of a thermal protection system embedded with WB structure.

Experimental Study on a Multi-Evaporator Loop Heat Pipe with a Dual-Layer Structure Condenser

A loop heat pipe(LHP)is a kind of passive heat transfer device that uses the latent heat of the working fluid and the capillary forces of the capillary wicks.It demonstrates high heat transfer efficiency,long-distance heat transfer,and high pipeline flexibility.The multi-evaporator loop heat pipe(MeLHP)is a special loop heat pipe with multiple evaporators so that heat collection and emission from multiple heat sources can be achieved.In this paper,a new type of the multi-evaporator loop heat pipe prototype with a dual-layer condenser was designed,which can ensure the uniform and symmetrical layout of pipelines.The working temperature was 20℃,and propylene was used as the working fluid.The performance of the same evaporator in a single-loop LHP was considered as a reference.The experiment was conducted under two heating modes,i.e.single-evaporator heating and multi-evaporator heating,and the working stability of the prototype was verified by applying periodic heating power change and adverse elevation condition.It was observed that the prototype can be successfully started in different heating modes with a heat transfer limit of 230 W.In the test,the four loops were different in heat transfer limit due to the differences of flow resistance,and less power distribution to the loop with lowest heat transfer limit was considered to be beneficial to the prototype’s performance.Meanwhile,the prototype showed good heat sharing characteristic as the maximum temperature difference is low(smaller than 2 K in single-evaporator heating mode and 0.5 K in multi-evaporator heating mode).The prototype was of good operational reliability and found to be adaptable to the adverse elevation and cyclic variation of the heating power to a certain extent.

Numerical investigation of heat transfer performance and bubble behavior of nucleate pool boiling in a confined space under rolling conditions

Nucleate pool boiling process is widely used in heat exchangers because of its excellent heat transfer performance.With the gradual increase of applications,more and more equipments work in a non-static state,but there is little research under rolling conditions.Therefore,it is necessary to investigate the influence of rolling motion on the nucleate pool boiling process.In this study,a numerical investigation of the nucleate pool boiling process under static and rolling conditions is performed based on the volume-of-fluid(VOF)method.Physical fields and phase distribution under static state and rolling motion are compared to investigate the effect of rolling motion on the nucleate pool boiling process.The results show that rolling motion greatly influences the bubble behavior and void fraction owing to the differences between flow fields.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°,and by 11.84%,22.27%,and 21.81%as the rolling period increased from 1 s to 3 s.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°.The heat transfer coefficients of different cases are compared,and it is found that the effects of rolling motion on heat transfer coefficients can be ignored.

Experimental investigation of a latent heat thermal energy storage unit encapsulated with molten salt/metal foam composite seeded with nanoparticles

Molten salt has been widely used in latent heat thermal energy storage(LHTES)system,which can be incorporated into hybrid photovoltaic/thermal solar system to accommodate the built environment.Solar salt(60 wt.%NaNO 3 and 40 wt.%KNO 3)was employed as the phase change materials(PCMs)in this study,and both aluminum oxide(Al_(2)O_(3))nanopowder and metal foam were used to improve the properties of pure solar salt.The synthesis of the salt/metal foam composites seeded with Al_(2)O_(3)nanopowder were performed with the two-step and impregnation methods,and the composite PCMs were characterized morphologically and thermally.Then pure solar salt,the salt/2 wt.%Al_(2)O_(3)nanopowder and salt/copper foam composite seeded with 2 wt.%Al_(2)O_(3)nanopowder were encapsulated in a pilot test rig,respectively,where a heater of 380.0 W was located in the center of the LHTES unit.The charging and discharging processes of the LHTES unit were conducted extensively,whereas the heating temperatures were controlled at 240℃,260℃and 280℃respectively.Temperature evolutions at radial,angular and axial positions were recorded,and the time-durations and volumetric mean powers during the charging and discharging processes were obtained and calculated subsequently.The results show that physical bonding between Al_(2)O_(3)nanopowder and nitrate molecule has been formed from the morphological pictures together with XRD and FTIR curves.Slight changes are found between the melting/freezing phase change temperatures of the salt/metal foam composites seeded with Al_(2)O_(3)nanopowder and those of pure solar salt,and the specific heats of the salt/Al_(2)O_(3)nanopowder composite slightly increase with the addition of Al_(2)O_(3)nanopowder.The time-duration of the charging process for the salt/copper foam composite seeded with Al_(2)O_(3)nanopowder at the heating temperature of 240℃can be reduced by about 74.0%,compared to that of pure solar salt,indicating that the heat transfer characteristics of the LHTES unit encapsulated with the salt/copper foam composite seeded with Al_(2)O_(3)nanopowder can be enhanced significantly.Consequently,the mean volumetric powers of the charging process were distinctly enhanced,e.g.,the volumetric mean power of heat storage can reach 110.76 kW/m 3,compared to 31.94 kW/m 3 of pure solar salt.However,the additive has little effect on the volumetric mean power of heat retrieval because of the domination of natural air cooling.

Flow Boiling Heat Transfer Characteristics and Delayed Dry-Out Ability of Non-Azeotropic Mixtures R245fa/R134a in Microchannels

Microchannel flow boiling heat transfer has the advantages of strong heat dissipation capacity,good temperature uniformity,and compact structure.It is an excellent way to thermally manage electronic devices,but when the heat flux exceeds CHF(Critical Heat Flux),the heat transfer performance deteriorates as the working fluid dries out.Non-azeotropic mixtures have the potential to effectively delay or avoid dry-out during the boiling process due to their temperature slide characteristics which causes the mass transfer resistance.To understand the influence of non-azeotropic mixtures on microchannel flow boiling,using the phase-change microchannel heat sink as the research object,the experiments on the flow boiling heat transfer performance of R245fa/R134a mixtures under different working conditions were carried out,and the characteristics of flow boiling heat transfer were obtained under the different working conditions,and comparison was developed with those of pure substance R245fa.The results demonstrated that a small amount of low-boiling-point components in the high-boiling-point working fluid inhibited boiling heat transfer to some extent,and lowered the average heat transfer coefficient under the non-dryout condition slightly lower than that of the pure substance;however,it also effectively delayed the onset of local dry-out and prevented significant deterioration in thermal transfer performance under the lower mass flow rate and higher heat flux,which could enhance the heat sink's stability.

Comparative Experimental Study on Heat Transfer Characteristics of Building Exterior Surface at High and Low Altitudes

The external surface heat transfer coefficient of building envelope is one of the important parameters necessary for building energy saving design,but the basic data in high-altitude area are scarce.Therefore,the authors propose a modified measurement method based on the heat balance of a model building,and use the same model building to measure its external surface heat transfer coefficient under outdoor conditions in Chengdu city,China at an altitude of 520 m and Daocheng city at an altitude of 3750 m respectively.The results show that the total heat transfer coefficient(h_(t))of building surface in high-altitude area is reduced by 34.48%.The influence of outdoor wind speed on the convective heat transfer coefficient(h_(c))in high-altitude area is not as significant as that in low-altitude area.The fitting relation between convection heat transfer coefficient and outdoor wind speed is also obtained.Under the same heating power,the average temperature rise of indoor and outdoor air at highaltitude is 41.9%higher than that at low altitude,and the average temperature rise of inner wall is 25.8%higher than that at low altitude.It shows that high-altitude area can create a more comfortable indoor thermal environment than low-altitude area under the same energy consumption condition.It is not appropriate to use the heat transfer characteristics of the exterior surface of buildings in low-altitude area for building energy saving design and related heating equipment selection and system terminal matching design in high-altitude area.

A review on heat transfer in nanoporous silica aerogel insulation materials and its modeling

Silica aerogels are widely used in many fields for thermal insulation,such as building insulation,electric power energy,energy storage systems,and high-temperature thermal protection due to their excellent insulation per-formance.Therefore,the heat transfer in silica aerogels and its modeling in recent years have attracted much attention and many valuable achievements have been acquired.The heat transfer in nano-porous silica aerogels and its modeling methods are reviewed in this work.This review starts with a brief introduction of heat transfer characteristics in silica aerogels,including the multi-component and multi-mode coupling effect,size effect,and multiscale effect.Then the heat transfer mechanism of each mode,including heat transfer via gas phase,solid phase,and thermal radiation,is reviewed,and the models for predicting the gaseous thermal conductivity in nanoscale pores,gas-contributed thermal conductivity,the apparent thermal conductivity of solid skeleton,and finally the effective thermal conductivity are collected and discussed in details.Besides,modeling of transient heat transfer in silica aerogels is also briefly introduced.Finally,the conclusions and some problems which need to be further investigated in the future are provided.