Numerical simulation of heat transfer of latent functionally thermal fluid in tubes with coaxially inserted cylindrical bars in laminar

The heat transfer of latent functionally thermal fluid in three kinds of tubes with coaxially inserted cylindrical bars is numerically researched using equivalent specific heat model, and the flow fields are analyzed with field synergy field. It is found that in the tubes with coaxially inserted cylindrical bars, the heat transfer effects of functionally thermal fluid become more and more pronounced with the Ste decreasing. This is similar to be case of functionally thermal fluid flowing in smooth straight tubes. Compared with the results receiving from smooth straight tubes, the heat transfer of functional thermal fluid in tubes with coaxially inserted cylindrical bars has been significantly enhanced. And this effect becomes more apparent as the diameter of coaxially inserted cylindrical bars increases meanwhile, however, energy consuming of the tubes shows the same trend. Supported by the Key Project of National Natural Science Foundation of China (Grant No. 50436020)     

Convective heat transfer enhancement of laminar flow of latent functionally thermal fluid in a circular tube with constant heat flux: internal heat source model and its application

Latent functionally thermal fluid is a kind of new two-phase fluid, in which phase change material is microencapsulated and suspended in a conventional single-phase heat transfer fluid (fig. 1). Such a slurry is of much larger apparent specific heat than conventional single-phase fluids during the phase change period, which enhances the heat transfer rate between the fluid and the tube wall. The flow rate and pump power required can be reduced evidently by using the slurries …     

铜-水基纳米流体环路热管太阳能热水系统的实验研究

采用"二步法"制备了铜-水基纳米流体,并对纳米流体进行了透射电镜分析。对不同质量浓度下(0.1%、0.15%、0.2%)铜-水基纳米流体的环路热管太阳能热水系统进行了实验研究,分析了环路热管太阳能热水系统中的水箱水温变化、瞬时光热效率,并与去离子水系统进行了对比分析。实验结果表明,纳米铜颗粒的加入增加了液体的导热系数,铜-水基纳米流体更适合作为太阳能重力环路热管热水系统的相变传热工质,且存在一个最佳的纳米流体工质质量浓度(最佳质量浓度为0.15%),可使得环路热管热水系统的传热性能最佳。     

复合相变调湿材料的制备与热湿性能

制备一种兼具调热调湿功能的新型复合相变调湿材料（CMPCM）,该材料由相变微胶囊（MPCM）与多孔调湿材料合成。作为被动式节能材料,能够有效平抑室内温湿度波动和降低建筑能耗。其中,相变微胶囊由癸酸和十八烷酸的混合物为芯材、二氧化硅为壁材通过溶胶凝胶法合成,多孔调湿材料为硅藻土。通过扫描电子显微镜（SEM）、差示扫描量热法（DSC）、热重分析法（TGA）,正杯蒸发法和等温吸放湿法分析表征了复合相变调湿材料的组成结构、热性能、热稳定性以及传湿系数和湿缓冲值。DSC和TGA结果显示,复合相变调湿材料比普通相变材料有更小的过冷度和更好的热稳定性。传湿特性实验显示,该新材料比单纯的多孔调湿材料有更大的传湿系数和湿缓冲值。     

工质对180～230 K低温环路热管工作特性的影响

摘 要：低温环路热管（Loop heat pipe, LHP）是一种高效的两相传热装置,常被用在航天热控系统中。为探究用于空间探测项目中180～230 K环路热管的合适工质,采用乙烯、乙烷和丙烯为工质对LHP在不同热沉温度下的启动特性、工作温度对热负荷增大的响应变化及稳态传热热阻进行了实验研究。实验和分析结果表明启动前蒸发器和补偿器温度低于工质临界温度至少10 K时,LHP均能以5 W的热负荷成功启动。启动前蒸发器温度相近时,乙烷LHP和丙烯LHP的启动温升均在2 K以内,乙烷LHP启动时间短于丙烯LHP,工质汽化潜热和液体粘度差异是造成LHP启动时间差异的重要因素。工质对LHP工作温度随热负荷增大的响应影响不明显：蒸发器温度均随热负荷的增加而先减小后增大,最低工作温度出现于20～30 W,LHP达到稳定状态的时间随热负荷增大而缩短。工质能明显的影响LHP的稳态传热热阻,工质在外环路的压降是其影响LHP稳态传热热阻的重要因素,而工质的气相压降占外环路总压降的绝大部分：小的气相压降对应小的稳态传热热阻,同热负荷下的乙烯LHP气相压降小于乙烷LHP和丙烯LHP,且气相压降均随温度升高而减小,对应的乙烯LHP稳态传热热阻小于乙烷LHP和丙烯LHP,且传热热阻都随温度升高而减小,实验得到的LHP最小稳态传热热阻达到0.21 K / W。     

Numerical study on convective heat transfer of supercritical CO2 in vertically upward and downward tubes

The experimental measurement of supercritical pressure carbon dioxide(sCO_2) heat transfer in vertical downward flow was performed in a circular tube with inner diameter of 10 mm. Then, a three-dimensional numerical investigation of sCO_2 heat transfer in upward and downward flows was performed in a vertical heated circular tube. The influence of heat flux, mass flux,and operating pressure on heat transfer under different flow directions were discussed. According to the "pseudo-phase transition" viewpoint to supercritical fluids, the analogy to the subcritical inverted-annular film boiling model, the physical model to sCO_2 heat transfer was established, where fluid region at the cross-section of circular tube was divided into gas-like region covering heated wall and core liquid-like phase region. Then, the thermal resistance mechanism which comprehensively reflected the effect of multiple factors including the thickness of the gas-like film or liquid-like region, fluid properties and turbulence on heat diffusion was proposed. Surprisingly, thermal resistance variation in upward flow is well identical with that of wall temperature and heat transfer deterioration is predicted successfully. In addition, compared with thermal resistance in the core liquid-like region, gas-like film formation is determined to be the primary factor affecting heat transfer behavior. Results also show that total thermal resistance in upward flow is always larger than that in downward flow. The investigation can provide valuable guide to design and optimize sCO_2 heaters.     

中温相变蓄热装置的蓄放热性能研究

以相变温度为80℃的相变蓄热装置为研究对象,根据建立的相变传热分析模型,应用数值模拟的方法,研究了蓄热体——相变材料的热物性参数、几何尺寸、填充率,以及热媒体——热水的流速等因素对蓄热装置蓄、放热规律及其特性的影响．研究结果表明,当板状定形相变材料的导热系数在0．4W/(m·K)左右、板厚为8 mm、相变材料的填充率在65%～75%、热媒体流速为5～8mm/s时,蓄热装置可获得较高的蓄、放热效率．     

Experimental study on the new type of electrical storage heater based on flat micro-heat pipe arrays

A new type of electrical storage heater that utilizes latent heat storage and flat micro-heat pipe arrays(FMHPAs) was developed.The thermal characteristics of the heater were tested through experimentation. The structure and operating principle of the storage heater were expounded. Three rows of FMHPAs were applied(three rows with five assemblies each) with a mass of 28 kg of phase change material(PCM) in the heat storage tank. Electric power was supplied to the PCM in the range of 0.2-2.04 kW, and air was used as heat transfer fluid, with the volume flow rate ranging from 40-120 m~3/h. The inlet temperature was in the range of15-24℃. The effects of heating power, air volume flow rate, and inlet temperature were investigated. The electrical storage heater exhibited efficiencies of 97% and 87% with 1.98 and 1.30 kW of power during charging and discharging, respectively. Application of the proposed storage heater can transfer electricity from peak periods to off-peak periods, and the excess energy generated by wind farms can be stored as heat and released when needed. Good economic and environmental benefits can be obtained.     

Lee相变传质方程中传质系数取值的分析

针对Lee相变传质方程中传质系数主要依靠经验取值的问题,本文尝试提出一种确定传质系数的方法,给出了评价传质系数合理性的2个指标———潜热份额（潜热换热量占总换热量的份额）和饱和温差（流体温度与饱和温度的差值）,建立了稳态分析模型,通过理论推导,得出了潜热份额和饱和温差的理论表达式,据此分析了传质系数对模拟结果的影响,已有特定条件下的理论解验证了本文分析结果的正确性。分析结果表明传质系数越大,计算精度越高。基于分析结果解释了不同文献对该系数取值差异很大的原因,并给出了传质系数的通用取值方法,最后推荐了常见工况模拟中传质系数的合理取值范围。     

碳纳米管纳米流体对液冷式CPU换热性能的改善

针对液冷式CPU（central?processing?unit）散热器散热效果差的问题,设计了液冷式CPU散热器的换热性能实验系统. 该实验系统使用基液丙二醇-水,Al2O3纳米流体和多壁碳纳米管（MWCNTs）纳米流体进行换热实验,采用单变量法对实验条件进行控制. 当加热功率为18.26 W时,基液丙二醇-水的热阻值为0.859 ℃/W,质量分数0.135%的Al2O3纳米流体的热阻值为 0.751 ℃/W,质量分数0.135%的多壁碳纳米管纳米流体的热阻值为0.739 ℃/W,质量分数0.32%的MWCNTs纳米流体的热阻值为0.457 ℃/W. 结果表明:在基液中添加纳米粒子能提高基液的换热能力,MWCNTs纳米流体的换热效果随着质量分数的增加而增强.     

Peristaltic flow with convective mass condition and thermal radiation

The primary objective of present investigation is to introduce the novel aspects of convective mass condition and thermal radiation in the peristaltic transport of fluid. Magnetohydrodynamic(MHD) fluid was considered in a symmetric channel. Heat and mass transfer characteristics were analyzed in the presence of Soret and Dufour effects, and the results were presented via two forms of thermal radiation. The temperature, concentration and pressure rise per wavelength were examined. It is observed that the velocity slip and magnetic field parameters have opposite effects on the pressure rise per wavelength. Temperature of fluid is a decreasing function of the radiation parameter. Further, the temperature of fluid decreases by increasing the heat transfer Biot number. It is notified that the heat transfer rate at the wall is a decreasing function of radiation parameter.     

Radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina RPC structures as integrated solar thermochemical reactor

This paper investigated radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina reticulated porous ceramic structures as integrated solar thermochemical reactor. High-flux solar radiation absorption and axial temperature distribution in the ceramic foams reactor were analyzed by adopting surface-to-surface radiation model coupled to the P1 approximation for radiation heat transfer. The radiative heat transfer and thermal characteristics of different foam-type RPC structures, including SiC, CeO₂, FeAl₂O₄, NiFeAlO₄, Fe₃O₄/SiC, and NiFe₂O₄/SiC were evaluated. The mass flow rate and foam structural parameters, including the permeability, pore mean cell size, and extinction coefficients have significantly affected the axial temperature distribution, pressure drop, heat transfer, and fluid flow. Integrated porous structure to the solar receiver could maximize the incorporation of redox powder in the reacting medium, lower the pressure drop, and enhance the thermal performance of the thermochemical reacting system. SiC structure was the candidate materials in the case where more heat flux and high axial temperature distribution is needed. However, Fe-based oxide coated Al₂O₃ structure could be considered regarding the heat transfer enhancement along with the catalyst activity of oxygen carriers for solar thermochemical reacting system performance.     

Theoretical investigations on heat transfer to H_2O/CO_2 mixtures in supercritical region

The supercritical H_2O/CO_2 mixture is the working fluid to drive a turbine in a novel power generation system with coal gasified in supercritical water. This system is promising because of zero pollution emission in contrast to the conventional coal-fired power plant. Heat transfer coefficients of the supercritical H_2O/CO_2 mixtures are important to design heat transfer devices in this system, which is similar to the role of heat transfer to supercritical water in conventional systems. However, heat transfer to supercritical mixtures has received less attention. Here, we show that the supercritical mixtures with H_2O being one of the components, have similar convection heat transfer behavior to supercritical pure fluids for temperatures and pressures above the critical point of H_2O. This phenomenon was demonstrated from two aspects. Firstly, the forced convection heat transfer coefficients of supercritical mixtures were numerically calculated using the simulation model for supercritical pure fluids and using the thermophysical properties(density, heat capacity, thermal conductivity and viscosity) of supercritical mixtures as input.The calculated results agree well with experimental data in the supercritical region. Secondly, the calculated results also agree well with the correlations for supercritical pure fluids. The mechanisms were investigated by molecular dynamics simulations on the diffusion of supercritical mixtures. These results lay the foundation for predicting convection heat transfer coefficients of supercritical mixtures and for designing heat transfer devices with supercritical mixtures as heat transfer fluids.     

Analysis of the effects of the heat of sorption on the process of heat transfer in moisture exchange across a membrane

A theoretical study has been conducted to investigate the effects of mass transfer on heat transfer in moisture exchange across a membrane and a mathematical model describing the heat transfer process with consideration of the heat of sorption was established. A dimensionless variable, Ψ =JLλ /δ (T ₁₀−T ₂₀), which controls the effect of the heat of sorption on the heat transfer in membrane process, was obtained through theoretical analysis, and the effects of Ψ on the heat transfer process were analyzed. Results showed that in the case that the temperature gradient and mass transfer are in the same direction, the effective heat flux changes the direction at Ψ=1. For Ψ<1, the heat transfers from high to low temperature sides through the membrane, and the total thermal resistance increases with increasing the moisture flux across membrane or reducing the temperature difference between the bulk flows on the two sides of membrane. For Ψ>1, the overall effect of the heat and mass transfer is that the effective heat flux points from low to high temperature sides and the total thermal resistance decreases with increasing the mass flux or reducing the temperature difference. In the case that the temperature gradient and mass transfer are in the opposite directions, the existence of the heat of sorption acts to enhance the heat transfer from high to low temperature sides, causing a reduced total thermal resistance, and the greater the mass flux or the smaller the temperature difference, the smaller the total thermal resistance. Supported by the National Natural Science Foundation of China (Grant No. 50576040)     

Shape-stablized EP/paraffin composite for latent heat storage

62# and 56# paraffin mixtures were impregnated into expanded perlite (EP) by vacuuming method. Effects of impregnation with/without vacuum, vacuuming time, and thermal cycles were discussed. The appropriate mass fraction of paraffin mixture in the composite and vacuuming time were found respectively to be 80% and 20 min. Fourier transform infrared spectrometer (FT-IR) analysis shows that it has a good compatibility between paraffin mixture and EP. From differential scanning calorimetry (DSC) analysis, the latent heat of EP/Paraffin mixture composite is almost linearly related with the mass fraction of paraffin mixture in the composite. After 100 thermal cycles, the deviation of phase change temperatures is acceptable, but more deviation of latent heat appears. The calcium stearate dispersing granule coated by epoxy resin can effectively lower leakage during thermal cycles.     

R245fa传热特性的实验研究

对R245fa水平光滑管管内流动沸腾换热特性进行了实验研究,主要包括加热水质量流速、工质质量流速、蒸发温度以及干度对局部换热系数的影响,结果表明在相同蒸发温度及加热水质量流速下,随着工质质量流速增大,管内流动沸腾换热系数迅速升高;在相同工质质量流速及蒸发温度下,随着加热水质量流速的增大,管内流动沸腾换热系数升高;在相同工质质量流速及加热水质量流速下,随着蒸发温度的升高,管内流动沸腾换热系数降低。分别采用Chen公式、Liu-Winterton公式、Shah公式计算了与本实验相同工况下R245fa的管内流动沸腾换热系数,其结果表明:3个预测公式的计算结果与实验值之间的平均误差分别为31.6%、6.3%、37.4%。采用Liu-Winterton公式计算R245fa的流动沸腾换热系数满足工程实际的要求。     

Experiment investigation on visualization and operating characteristics of closed loop plate oscillating heat pipe with parallel channels

Using ethanol or acetone as the working fluid, visualization of oscillations in steady state was observed visually by high-speed cameras, and temperature oscillating and heat transfer characteristics of closed-loop plate oscillating heat pipe with parallel channels(POHP-PC) were experimentally investigated by varying liquid filled ratios(50%, 70%, 85%), section scales(1 mm×1 mm and 1 mm×1.5 mm), inclination angles, working fluids and heating inputs. It was found that during operating there was mixed flow consisting of plug flow and annular flow in channels of oscillating heat pipe at steady-state. There was an equilibrium position for working fluid of condenser during oscillating, and periodic oscillations occurred up and down in the vicinity of equilibrium position. With heat input increasing, equilibrium position rose slowly as a result of vapor pressure of evaporation.Evaporation temperature oscillating amplitude possessed a trend of small-large-small and frequency trend was of small-large during steady-state. It may be generally concluded that temperature, whether evaporator or condenser, fluctuated sharply or rose continuously when oscillating heat pipe coming to dry burning state. Simultaneously, it was found that temperature difference of cooling water possibly dropped with heat input rising during dry burning state. Thermal resistance of No. 2 with acetone was lower than that of No. 1 during experiments, but No. 2 achieving heat transfer limit was earlier than No. 1. However, with ethanol, thermal resistance of No. 1 and No. 2 were similar with the heating input less than 110-120 W and filling ratios of 50% and 70%. And with filling ratio of 85%, heating transfer performance of No. 2 was better compared to No. 1 during all the experiments.     

Thermal performances of non-equidistant helical-coil phase change accumulator for latent heat storage

Helical-coil is a common structure of heat exchanger unit in phase change heat accumulator and usually has the equal coil pitch between adjacent coils.Its thermal performances Thus,could be improved by improving the uniformity was of unit the phase change material(PCM)temperature distribution.flow a novel non-equidistant helical-coil made structure heat proposed area in this study.Its coil pitch match decreased along the direction of heat transfer fluid,which the exchange in that volume increase to the decreasing temperature difference between was the heat transfer and fluid and PCM.The structure was optimized using numerical simulation.helical-coil temperature An heat experimental accumulator system was developed the experiment results indicated latent the proposed non-equidistant uniformity more effective than equidistant helical-coil for heat storage.The of the distribution was also confirmed by simulation results.     

Characterization and thermal conductivity of modified graphite/fatty acid eutectic/PMMA form-stable phase change material

We prepared and characterized a form-stable composite phase change material (PCM) with higher thermal conductivity. Capric acid(CA)-myristic acid(MA) eutectic as core, poly-methyl methacrylate (PMMA) as supportive matrix and modified graphite (MG) powders serving as the thermal conductance improver were blended by bulk-polymerization method. The composite PCMs with different MG mass fraction (2%, 5%, 7%, 10% and 15%) were characterized by FT-IR, SEM, DSC technique and mechanical tests. Thermal conductivities of the composites were measured by transient hot-wire method. The results indicate that MG powders have been successfully inserted into the CA-MA/PMMA matrix without any chemical reaction with each other. The MG/CA-MA/PMMA composites maintain good thermal storage performance while the thermal conductivity has been enhanced significantly. The composite PCM added with 15 wt% MG powders increases approximately as 195.9% in thermal conductivity. Moreover, the thermal conductivity improvement of the composite PCMs is also verified by the melting-freezing experiment, which is profitable for the heat transfer efficiency in latent heat thermal energy storage system.     

Dynamic analysis of heat extraction rate by supercritical carbon dioxide in fractured rock mass based on a thermal-hydraulic-mechanics coupled model

Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation. Compared to the thermal-physical characteristics of water, supercritical CO₂ (scCO₂) has a higher heat storage capacity over a wide temperature-pressure range and may be favored as a heat transfer fluid. Singularly characteristic of scCO₂-based heat extraction is that the hydraulic-thermal properties of the scCO₂ vary dramatically and dynamically with the spatial pressure gradient during unsteady-state flow along fracture. This highly nonlinear behavior presents a challenge in the accurate estimation of heat extraction efficiency in scCO₂-based EGS. In this paper, a thermal–hydraulic-mechanical (THM) coupled model is developed by considering deformation of the fractured reservoir, non-Darcy flow and the varying thermal-physical properties of scCO₂. The proposed model is validated by matching the modeling temperature distribution with published data. The results show that during continuous injection of scCO₂, the fracture first widens and then narrows, ultimately reopening over the long term. The sequential fracture deformation behaviors are in response to the combined impacts of mechanical compression and thermally-induced deformation. By controlling the injection parameters of the scCO₂, it is found that the heat extraction rate is positively correlated to its pore pressure or mass flow rate. The heat extraction rate can be significantly enhanced, when the inlet temperature of scCO₂ is below its critical temperature. As a result, the heat increment recovered per unit mass of scCO₂ decreases as the hot rock is gradually cooled. Meanwhile, the heat increment recovered per unit mass of scCO₂ decreases by increasing the inlet temperature of scCO₂ or its mass flow rate, but increases as the outlet pressure rises. Furthermore, multi-linear regression indicates that controlling the inlet temperature of the scCO₂ can significantly improve the thermodynamic efficiency of heat extraction.