Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

机柜服务器热管背板换热性能的试验研究

通过测试热管背板系统在不同进风温度、制冷剂冷凝单元（RCU）冷冻水进水温度及水流量工况下的换热能力,分析环境温度升高、系统供水不足等对高密度机柜服务器安全运行的影响,采用模拟负载机柜测试机房机柜安装热管背板后不同位置的换热能力。结果表明,热管背板系统对进风温度、冷冻水温度及流量具备良好的换热适应性,发热量大的服务器优先安装在机柜中下部,发热量小的服务器安装在机柜上部,可以最大限度发挥热管背板的换热能力。     

Effects of intermediate pressure on the heating performance of a heat pump system using R410A vapor-injection technique

Recent heat pumps are actively equipped with a refrigerant vapor-injection technique to acquire better performance in severe operating conditions. In this study, the target system which has an additional expansion valve at the outlet of condenser was presented. The concept of intermediate pressure was generated in the target system. Effects of the intermediate pressure on the heating performance with various injection ratios were measured and analyzed according to the compressor frequencies ranging from 60 to 100 Hz. Unlike conventional vapor-injection cycle, the maximum injection ratio was highly affected by the intermediate pressure. The high intermediate pressure provided high initial heating capacity and COP; but it restricted the available range of vapor-injection within narrow limits. The result indicates that a proper operating strategy is needed for the vapor-injection cycle.     

Enhancement of thermal performance in a sintered miniature heat pipe

Abstract

The performance of a sintered miniature heat pipe is enhanced. With the capillary limitation, porosity takes priority over the wick structure parameters that would affect the heat transfer capacity. Since sintered dendritic copper powder has higher porosity, it is used to mix with pore former (Na₂CO₃) in experiments for increasing porosity, and hence enhancing the thermal performance. The results show that, for a heat pipe with a 3mm outer diameter and 200 mm effective length, the heat transfer rate is up to 16.5W and the thermal resistance is 0.9°C/W. In comparison with the unmixed case, the performance increases about 40%.     

Operational performance of a cryogenic loop heat pipe with insufficient working fluid inventory

A cryogenic loop heat pipe (CLHP) has been developed for future aerospace applications at the Technical Institute of Physics and Chemistry (TIPC). It has been demonstrated that this CLHP, when placed horizontally, can operate in liquid-nitrogen temperature range and have a heat transfer capability of up to 12 W with proper working fluid inventory. This paper presents some particular characteristics of the CLHP when the compensation chamber is half-filled with liquid-phase working fluid before startup. The device has been tested at different orientations using nitrogen as the working fluid in order to compare its thermal behavior, specially related to the heat transfer capability, the operation temperature and the thermal resistance, as well as to investigate its operational characteristics under power level as low as 1 W. Tests were performed for the CLHP at horizontal position and with the liquid line 3.4 and 6.4 cm below the vapor line, respectively. The experimental results show the operationability of the CLHP tested at three orientations and tests with the liquid line 6.4 cm below the vapor line show lower operation temperatures and higher heat transfer capability.     

The heat transfer performance of horizontal tube bundles in large falling film evaporators

The performance of the horizontal heat transfer tube bundles in falling film evaporators in large compression refrigeration systems was investigated with numerical simulation in this paper. Four types of tubes, including plain tubes, and enhanced surface tubes of Turbo-B, Turbo-BII and Turbo-EHP, were employed in the simulation. Some factors, such as tube kind, tube pass arrangement, dry patch area on tube surface, liquid refrigerant flow rate, and number of flooded tubes, were analyzed based on simulated results. In the study, the maldistribution of liquid refrigerant flow caused by the distributor apparatus was discussed, which severely affects the performance of falling film evaporators according to the simulation. Some calculated results were verified by the experiment. These discussions and results can be used to guide the design of falling film evaporators under realistic flow conditions.     

槽道热管在加快低温回路热管主蒸发器降温过程中作用的实验研究

介绍液氮温区低温回路热管结构设计的1种方案.首次采用槽道热管作为低温回路热管的次蒸发器,并对其在加快主蒸发器降温过程中的作用进行了详尽的实验研究.这种次蒸发器的结构简单,并使低温回路热管的整体设计变得简易.实验表明,经过合理的阻力设计,槽道热管形式的次蒸发器能起到加快主蒸发器降温的作用,为研究低温回路热管的传热特性提供了前提条件.     

Comparison of the heating performance of air-source heat pumps using various types of refrigerant injection

The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.     

表面活性剂对脉动热管传热特性的影响

本文试验研究了表面活性剂脉动热管的运行性能。采用浓度为0.125%的十六烷基三甲基溴化铵(CTAB)水溶液作为工质。试验结果表明,表面活性剂脉动热管的启动时间和启动温度随输入功率的增加而降低。脉动热管的传热特性与输入功率有关,且随着输入功率的增加而增强。与纯水脉动热管相比,充液率为50%的CTAB脉动热管在加热功率100 W时热阻降低52%。     

液氮温区重力辅助深冷回路热管的实验研究

给出了液氮温区重力辅助深冷回路热管结构设计方案,建立了实验系统,对其启动特性和工作性能进行了实验研究.深冷回路热管以高纯氮作为工作液体,工作温区为90 K～126 K.实验结果表明,深冷回路热管能在重力作用下快速启动,在气体管线高于液体管线20 mm的情况下,最大可传送的功率为11W.     

Entropy generation on MHD flow and convective heat transfer in a porous medium of exponentially stretching surface saturated by nanofluids

This paper examines the unsteady boundary layer magnetohydrodynamic flow and convective heat transfer of an exponentially stretching surface saturated by nanofluids in the presence of thermal radiation. The combined effect of stratifications (thermal and concentration) in the unsteady boundary layer flow past over a stretching surface embedded in a porous medium is analyzed. The system of coupled nonlinear differential equations are solved numerically by developing finite difference scheme together with the Newton’s linearization technique, which allows us to control nonlinear terms smoothly. The study shows that the thermal boundary layer thickness significantly increases with the increase of Brownian motion, thermophoresis number and magnetic field strength. The unsteadiness behavior of the flow of nanofluid has reducing effect on both momentum and thermal boundary layer thickness. The Brownian motion has controlling effect on nanoparticle migration. The entropy generation by means of Bejan number has strong impact on the applied magnetic field, dissipation of energy, thermal radiation and Biot number.     

微通道冷凝器低温回路热管的实验研究

设计了一套以乙烷作为工质,使用微通道冷凝器的低温回路热管原理样机,并对样机的降温过程、传热性能以及再启动特性进行了实验研究。结果表明:在5 W的驱动功率下该低温回路热管可实现快速降温,在降温过程中通过增大蒸发器的加热功率可以加速回路热管的降温;该样机可以在200 K时能够稳定传输50 W的热量,并且随着加热功率的增大,回路热管的热阻不断减小;在回路热管停止工作后,重新施加热量,该样机仍能够正常启动;该样机在本实验中的最大传热功率为54W,此时的热阻为0.46 K/W。     

Experimental thermal performance study of an inclined heat pipe heat exchanger operating in high humid tropical HVAC systems

In an earlier paper [Y.H. Yau, Application of a heat pipe heat exchanger to dehumidification enhancement in tropical HVAC systems – a baseline performance characteristics study, International Journal of Thermal Sciences 46 (2) (2007) 164–171], the author had established the baseline performance characteristics of the eight-row wickless heat pipe heat exchanger (HPHX) for a vertical configuration under a range of conditions appropriate for a tropical climate. Now, the same basic experimental set-up was to be used in the present research with the HPHX tilted 30°. In this configuration, the gravitational force would be expected to enhance drainage of any condensation forming on the extended fin surfaces of the HPHX evaporator section, and therefore, the effectiveness of the HPHX could be anticipated to be better than the vertical configuration, particularly when processing inlet air with high RH. The investigation has been carried out for 32 experiments with typically high RH and the results are presented in this paper. The results suggested that the possibly adverse influence of condensate forming on the fins of the HPHX was negligible, and therefore the HPHX in a typically-used vertical configuration could perform equally as well as it would if the HPHX was installed in an inclined position.     

U形热管换热器对空调箱机组除湿性能影响的试验研究

为分析U形热管换热器的工作性能,通过改变迎面风速和回风温度,测出U形热管换热器前后的参数,计算换热温差和换热效率,然后测试相同条件下无U形热管换热器的状况,分析得出增加热管换热器后空调箱机组除湿量可提高15%左右。     

The new thermo-ecological performance optimization of an irreversible three-heat-source absorption heat pump

The new thermo-ecological performance optimization of an absorption heat pump operating between three temperature levels with the losses of heat resistance, internal irreversibility and leakage is analyzed by taking the ecological coefficient of performance (ECOP) as an objective function. The new thermo-ecological criterion takes into account the first and second law of thermodynamics and is defined as the heating rate per unit loss rate of availability. The ecological coefficient of performance has been expressed and maximized in terms of the temperatures of the working fluid in the main components of the system. The corresponding optimal temperatures and other optimum performance parameters have been derived analytically, and the effects of the internal irreversibility, the heat leakage coefficient and the source temperature ratio on the global and optimal performances are discussed. The obtained results may provide a general theoretical tool for the ecological design of absorption heat pumps.     

高压绕管式换热器中换热管漏气后的返修

从某套空分设备上卸下的DR87XX型高压绕管式换热器的换热管胀接部分焊缝出现漏气现象,原因是胀接压力过大。制定了详尽的返修方案,具体实施后高压绕管式换热器恢复正常运行。文章简介了DR87XX型高压绕管式换热器中管板与换热管的"胀接+强度焊"模式,详细叙述了返修前的胀接试验、检验和返修工艺过程,最后总结了返修中的注意事项。     

液氢温区脉动热管试验台设计

总结了前人对低温脉动热管的研究成果,同时提出了液氢温区低温脉动热管试验台的设计方法,主要包括热负荷计算和力学校核.该试验台可以研究多种参数(如弯折数、充液率、绝热段长度、加热量等)对传热性能的影响.     

MHD convective heat transfer of nanofluids through a flexible tube with buoyancy: A study of nano-particle shape effects

This paper presents an analytical study of magnetohydrodynamics and convective heat transfer of nanofluids synthesized by three different shaped (brick, platelet and cylinder) silver (Ag) nanoparticles in water. A two-phase nanoscale formulation is adopted which is more appropriate for biophysical systems. The flow is induced by metachronal beating of cilia and the flow geometry is considered as a cylindrical tube. The analysis is carried out under the low Reynolds number and long wavelength approximations and the fluid and cilia dynamics is of the creeping type. A Lorentzian magnetic body force model is employed and magnetic induction effects are neglected. Solutions to the transformed boundary value problem are obtained via numerical integration. The influence of cilia length parameter, Hartmann (magnetic) number, heat absorption parameter, Grashof number (free convection), solid nanoparticle volume fraction, and cilia eccentricity parameter on the flow and heat transfer characteristics (including effective thermal conductivity of the nanofluid) are examined in detail. Furthermore a comparative study for different nanoparticle geometries (i.e. bricks, platelets and cylinders) is conducted. The computations show that pressure increases with enhancing the heat absorption, buoyancy force (i.e. Grashof number) and nanoparticle fraction however it reduces with increasing the magnetic field. The computations also reveal that pressure enhancement is a maximum for the platelet nano-particle case compared with the brick and cylinder nanoparticle cases. Furthermore the quantity of trapped streamlines for cylinder type nanoparticles exceeds substantially that computed for brick and platelet nanoparticles, whereas the bolus magnitude (trapped zone) for brick nanoparticles is demonstrably greater than that obtained for cylinder and platelet nanoparticles. The present model is applicable in biological and biomimetic transport phenomena exploiting magnetic nanofluids and ciliated inner tube surfaces.     

Numerical study on the steady state and transient performance of a multi-type heat pump system

Numerical methods are provided to analyze dynamic characteristics as well as steady-state performance of a multi-type heat pump system. Lumped parameter method was applied to simulate the compressor and expansion device. Additionally, fully distributed method was used for analyzing the condenser and evaporators. The transient terms in the governing equations for heat exchangers were solved adopting the wave equation solutions. The simulation results predict the steady-state performance of a heat pump system with the deviation of ±10%. The transient simulation showed satisfactory responses of temperatures and pressures for various compressor speeds or expansion valve openings. From the results, it was shown that the operating conditions of secondary fluid of one indoor unit had minor influence on the performance of the other unit, while opening of expansion valve affected the transient responses of both evaporators significantly.     

A minichannel aluminium tube heat exchanger – Part III: Condenser performance with propane

This paper reports heat transfer results obtained during condensation of refrigerant propane inside a minichannel aluminium heat exchanger vertically mounted in an experimental setup simulating a water-to-water heat pump. The condenser was constructed of multiport minichannel aluminium tubes assembled as a shell-and-tube heat exchanger. Propane vapour entered the condenser tubes via the top end and exited sub-cooled from the bottom. Coolant water flowed upward on the shell-side. The heat transfer areas of the tube-side and the shell-side of the condenser were 0.941 m² and 0.985 m², respectively. The heat transfer rate between the two fluids was controlled by varying the evaporation temperature while the condensation temperature was fixed. The applied heat transfer rate was within 3900–9500 W for all tests. Experiments were performed at constant condensing temperatures of 30 °C, 40 °C and 50 °C, respectively. The cooling water flow rate was maintained at 11.90 l min⁻¹ for all tests. De-superheating length, two-phase length, sub-cooling length, local heat transfer coefficients and average heat transfer coefficients of the condenser were calculated. The experimental heat transfer coefficients were compared with predictions from correlations found in the literature. The experimental heat transfer coefficients in the different regions were higher than those predicted by the available correlations.