临界热流密度流体模化方法与分析

为比较模化工质和实际工质的参数,计算R12、R22、R123、R134a、水在液气密度比172～4条件下的压力变化范围,简单介绍流体模化理论基础并评述已有典型临界热流密度流体模化模型的特点。分析认为氟利昂类物质汽化潜热低,温度压力参数比高温高压水蒸气低一个数量级,作为模化工质可降低实验困难70%～80%,节能80%左右,考虑到环保性能,替代制冷剂R134a等可作为理想的模化工质。最后讨论了CHF流体模化方法理论和实验方面的几个问题及其改进方向。     

一种新的保护同温层安全的工作介质：134A制冷剂的热力学性质

在过去的十年中人们十分关注替代氟氯烷(R12)制冷剂的研究,理论上已经揭示了可以为保护大气层上部使臭氧层不致严重消耗作出贡献的可能性。这就是“可保护同温层安全”新的制冷剂——四氟乙烷(R·134a),它的热力学性质与R—12相似,并且它被认为是R—12在空调和冰箱应用中具有潜力的直接的替代品。 本文提出了R—134a的p—V—T,饱和蒸汽压力、液体密度,理想气体热容量以及临界参数等热力学数据。另外,将热力学数据关联成状态方程,通过状态方程再计算出熵与焓。采用合适的计算方法,开发了一个Fortran计算机硬件包,据此建立了一个大的温度、压力范围内的热力性质图。     

混合工质R134a/R23替代R22的可行性研究

叙述了基于新型环保型混合制冷剂R134a/R23替代制冷剂R22的问题,以及通过REFPROP7.5,对混合工质R134a/R23从物性和热力学特性进行的理论计算分析,指出,由质量分数为70%的R134a和质量分数为30%的R23组成的混合制冷剂与R22性能最为接近,在变工况运行条件下,其COP值比R22高8%左右,其冷凝压力比同条件下用R22作为循环工质低21%～36%,理论上完全具有替代R22的可能性。     

超临界R134a在ORC系统中的换热特性分析

采用有限元分析软件FLUENT对超临界有机朗肯循环系统中的换热器进行数值分析,进而对其进行设计和优化。超临界R134a在长为2 100 mm,直径为4 mm的光管换热器中分别进行向上流动和向下流动。换热边界条件是定热流密度,流体的进口压力P为4.5 MPa,进口温度Tin为349 K。分别探究热流密度q、质量流量G、热流质量比q/G和浮升力对传热特性的影响。采用无量纲数Bo来预判浮升力对传热的影响。虽然Morky的工作流体不是超临界R134a,但他们的经验公式也能用来更好地预测超临界R134a在光管中向上流动的传热特性。     

新型制冷剂R1234yf/R134a可燃性实验研究

根据美国材料试验学会标准研制可燃性实验装置,对实验装置进行性能和误差分析,并进行校验。R1234yf环境性能好,臭氧消耗潜能值为零,其三元混合物R1234yf/DME/R134a可作为一种新型空调制冷剂。测试不同体积比时二元混合工质R1234yf/R134a的可燃性数据,得到其爆炸极限和临界抑爆比,并给出三元混合物R1234yf/DME/R134a的临界爆炸极限曲线图。结果表明:当R134a和R1234yf的体积比约为2时,二元混合物R1234yf/R134a达到临界抑爆点。     

制冷工质在低温发电中的热力学分析

基于常规能源日益稀少,环境污染日益严重,主要研究开发利用太阳能来进行低温发电.采用了CO2、R123、R134a、R290、R600、R236fa、R245fa、NH3等8种常用的制冷剂,选用等工况(same working condition, SWC)和最佳蒸发压力(optimal evaporating pressure,OP0)两种研究方法,分析各个工质的循环特性.并以CO2的跨临界循环和R134a的亚临界循环为例,进行分析,对比了不同太阳辐射量和工质质量流量下效率、温度及膨胀比的变化.结果表明,CO2的膨胀比最小,膨胀性能优于其他7种工质,最差的为R245fa.OP0法的计算方法只适用于亚临界循环,不适用于跨临界循环.在进行非常规循环(跨临界循环)与常规循环(亚临界循环)热源条件变化的对比中,SWC法更有说服力.     

基于模糊层次分析法的制冷剂选择

制冷剂选择是复叠式热泵研究的一个重要方向。在分析模糊、灰色关联在制冷空调领域应用及复叠式热泵制冷剂选择研究现状基础上,采用模糊层次分析法构建了包括安全性、热力性能、环保性能、成本和热物性5个方面共27个指标组成的制冷剂评价指标体系,建立制冷剂性能综合评价模型,利用该模型在47种纯工质范围内为复叠式热泵选择合适的工质。结果表明,综合性能较好的制冷剂组合为R134a/R601a、R134a/R601、R1234yf/R236ea等。     

二甲醚制冷性能分析及其lgP-h图

文章将二甲醚作为制冷剂与R22、R134a、R600a在热力性质、安全性、对环境的影响和制冷循环性能等方面进行了对比分析，发现虽然DME可燃，但只要安全措施得当并减小泄漏的可能性，相对R600a更为安全，因此DME作为制冷剂具有环保、高效、无毒和与材料兼容的特点。文章还根据DME的物性方程及状态方程进行了热力学分析，绘制出DME的lgP-h图。     

不同制冷剂对喷射制冷系统性能的影响

选择R717、R290、R600a、R134a和R1234yf五种沸点相近的制冷工质,基于MATLAB软件,引入制冷剂物性参数的Cleland拟合模型,编写了喷射制冷系统的仿真模型,比较了五种制冷剂在不同冷冻水进口温度及不同冷却水进口温度下,喷射系数u和COP值的变化趋势。结果显示:冷冻水进口温度的改变,使制冷剂的制冷性能随之升高而增大;且制冷剂制冷性能随冷却水进口温度的升高而减小。R717在五种制冷剂中,喷射器喷射系数和系统性能COP都是最大的,R1234yf和R134a则具有相近的喷射系数。     

混合制冷剂在空气源热泵热水器系统中替代R22的可行性研究

空气源热泵热水器系统所用制冷剂主要包括R410A,R407C,R134a,它们的GWP值较高,会对大气环境造成不利影响.将R1234ze/R152a,R290/R1234ze,R152a/R134a分别按一定比例组成混合制冷剂,通过理论分析混合制冷剂的环境影响指数、热力学性能、安全性以及经济性能等因素,并与R410A,...     

Flow boiling and condensation of tetrary refrigerant mixtures inside water/refrigerant enhanced surface tubing

This work presents the results of an experimental study concerning the heat transfer characteristics of two-phase flow condensation and boiling of tetrary (R-32/R-125/R143a/R134a) refrigerant mixtures inside water/refrigerant horizontal enhanced surface tubing. Heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of the tetrary refrigerant mixtures, have been predicted and compared with other mixtures during flow condensation and boiling inside enhanced surface tubing. It was found that the tetrary refrigerant blend has higher transfer coefficients than R-502, and the lowest pressure drop among the refrigerants studied. © 1997 by John Wiley & Sons, Ltd.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Boiling on a Tube Bundle: Part II—Heat Transfer and Pressure Drop

An extensive experimental study was undertaken to measure nucleate pool boiling heat transfer coefficients, local bundle boiling heat transfer coefficients, and two-phase bundle pressure drops for R134a and R236fa on one plain tube bundle configuration. The experimental database allowed the refinement of frictional pressure drop models previously developed at the Laboratory of Heat and Mass Transfer. Together with the new onset of dryout prediction method presented in part I (preceding article in this issue), this constitutes a significant improvement in such prediction methods. The local bundle boiling heat transfer data highlighted the dependency of the heat transfer coefficient on the heat flux as expected for the present conditions. The new method was proposed and worked well versus the present database and was also validated against additional refrigerants from independent studies. It was proven to also work reasonably well for falling film evaporation data, proving the new prediction method is applicable for a wide range of operating conditions.     

Comparative study on evaporator heat transfer characteristics of revolving heat pipes filled with R134a, R22 and R410A

Heat pipes are used extensively in various applications including the heating, ventilating and air conditioning (HVAC) systems. The high thermal conductivity of the device, attributed from the two-phase heat transfer processes within the heat pipe, made them superior heat exchanger devices. Heat pipes had been widely used in HVAC applications in energy conservation, dehumidification enhancement, heat dissipation, etc. A number of researches have been conducted to expand the applicability of heat pipes in HVAC in Malaysia, especially in air-to-air heat recovery using stationary heat pipes. However, the potential usage of rotating heat pipe in heat recovery in tropical countries like Malaysia was yet to be explored. Hence, the potential of rotating heat pipe in the HVAC systems used in tropics was explored through a parametric study that incorporates rotational speeds, off-axis displacements and varied refrigerants. The rotating heat pipes charged with R134a, R22 and R410A were tested with varied radial displacement from the rotational axis. The straight and leveled heat pipe with the furthest radial displacement yields the most significant heat transfer, which was attributed to the magnitude of the generated centrifugal force, and effective distribution of liquid in the evaporator.     

Prediction of two-phase condensation in horizontal tubes using probabilistic flow regime maps

A flow regime based condensation model is developed for refrigerants in single, smooth, horizontal tubes utilizing a generalized probabilistic two-phase flow map. Flow map time fraction information is used to provide a physically based weighting of heat transfer models developed for different flow regimes. The developed model is compared with other models in the literature, with experimentally obtained condensation data of R134a in 8.92 mm diameter tubes, and with data found in the literature for 3.14 mm, 7.04 mm, and 9.58 mm tubes with R11, R12, R134a, R22, R410A, and R32/R125 (60/40% by weight) refrigerants and a wide range of mass fluxes and qualities.     

Development of a diabatic two-phase flow pattern map for horizontal flow boiling

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. Based on new flow pattern data for three different refrigerants covering a wide range of mass velocities, vapor qualities and heat fluxes. The new flow pattern map includes the prediction of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters and is an extension to the flow pattern map model of Kattan et al. [J. Heat Transfer 120 (1998) 140]. The proposed modifications allow an accurate prediction of the flow pattern for very different fluids which are the substitute refrigerants (HFC-134a and HFC-407C) and the natural refrigerant R-717 (ammonia).     

Condensation Heat Transfer and Pressure Gradient Inside Multiport Minichannels

Abstract

In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A.

Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.     

Dimensional analysis on the evaporation and condensation of refrigerant R-134a in minichannel plate heat exchangers

Two-phase flow analysis for the evaporation and condensation of refrigerants within the minichannel plate heat exchangers is an area of ongoing research, as reported in the literatures reviewed in this article. The previous studies mostly correlated the two-phase heat transfer and pressure drop in these minichannel heat exchangers using theories and empirical correlations that had previously been established for two-phase flows in conventional macrochannels. However, the two-phase flow characteristics within micro/minichannels may be more sophisticated than conventional macrochannels, and the empirical correlations for one scale may not work for the other one. The objective of this study is to investigate the parameters that affect the two-phase heat transfer within the minichannel plate heat exchangers, and to utilize the dimensional analysis technique to develop appropriate correlations. For this purpose, thermo-hydrodynamic performance of three minichannel brazed-type plate heat exchangers was analyzed experimentally in this study. These heat exchangers were used as the evaporator and condenser of an automotive refrigeration system where the refrigerant R-134a flowed on one side and a 50% glycol–water mixture on the other side in a counter-flow configuration. The heat transfer coefficient for the single-phase flow of the glycol–water mixture was first obtained using a modified Wilson plot technique. The results from the single-phase flow analysis were then used in the two-phase flow analysis, and correlations for the refrigerant evaporation and condensation heat transfer were developed. Correlations for the single-phase and two-phase Fanning friction factors were also obtained based on a homogenous model. The results of this study showed that the two-phase theories and correlations that were established for conventional macrochannel heat exchangers may not hold for the minichannel heat exchangers used in this study.     

R1234ze(E)与R134a在水平双侧强化管外的凝结换热对比实验

搭建了水平单管降膜蒸发试验台,以第四代制冷剂R1234ze(E)和第三代制冷剂R134a作为工质,在新型水平双侧强化管管外分别进行了改变管内水速、热流密度和冷凝温度条件的凝结换热实验。使用Wilson-Gnielinski图解法计算得到管内表面传热系数h_i,进一步采用热阻分离法分离出两种制冷剂的管外表面传热系数,并分析了管内冷却水水速、冷凝温度和壁面过冷度的变化对其换热性能的影响。实验结果表明:同根实验管下不同制冷剂凝结换热性能的差异与制冷剂物性与强化管结构之间的匹配特性有关,实验管型下,R1234ze(E)的管外凝结换热性能高于R134a。     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.