基于最小熵增法编程确定非共沸混合工质组分比

以非共沸混合工质在蒸发器中沿程温度分布变化所导致传热不可逆熵增为目标函数,建立混合工质与冷媒水在蒸发器中的稳态换热模型;以换热温差最小值为基准,编程分析计算,得出二元混合工质R290/R600在不同组分比下的相对熵增,选取其中最小值对应组分比为最佳组分比.     

空调工况下非共沸制冷剂在蒸发器内的传热窄点研究

为了揭示15种常用非共沸制冷剂在空调工况下蒸发器中的传热窄点特征,探明它们和换热流体间温差的沿程变化规律,以及如何防止传热窄点的产生进行了理论分析,建立了制冷剂的蒸发换热模型,明确了蒸发过程中传热窄点产生的条件,并根据标准的空调工况确定了15种制冷剂的蒸发物性参数;然后,利用制冷剂状态方程得到蒸发过程中温度与焓值的对应关系,并分段计算了焓随温度的变化率进而得到15种制冷剂蒸发过程中发生传热窄点现象的机率;以R409A和R407D为例,计算了制冷剂和换热流体在蒸发过程中的温度分布;最后针对2种可能发生传热窄点现象的制冷剂给出了换热流体温差控制范围.     

非共沸工质非完全冷凝实验现象及判据的验证

针对非共沸工质可能出现的非完全冷凝现象，首先针对两种非共沸工质进行了实验研究，得到制热量和COP随冷凝器水流量逐渐增加的变化规律，证明非完全冷凝现象存在的可能性和普遍性；然后根据已经提出的此现象是否发生的理论判据（极限流量判别式）进行计算，并用实验测得极限流量对其进行验证；最后，分析了实验与理论计算的差异和原因，并提出完善非完全冷凝现象理论判据的改进方向，即考虑工质在冷凝器中的压降、工质和换热流体间的实际温差和实际换热过程中的各种外力。     

采用非共沸混合工质的原奶速冷式系统热力学仿真与性能分析

通过利用Helmholtz自由能状态方程,模拟计算6种非共沸混合工质的原奶速冷式系统制冷性能系数(COP),并与纯工质作比较,分析非共沸混合工质的优缺点。结果表明,非共沸混合工质的原奶速冷式系统COP普遍大于采用纯工质的原奶速冷式系统COP,R436A是6种混合工质中综合性能最好的。尽管采用非共沸混合工质可以有效地改善原奶速冷式系统传热温差过大的问题,但原奶速冷式系统的传热温差仍旧偏大。指出在将来的研究中若采用组合式原奶速冷式系统,可在某种程度克服这些问题,进而改善原奶速冷式系统的能源效率。     

过冷度对地热发电非共沸工质有机朗肯循环热力性能的影响

有机朗肯循环（organic Rankine cycle,ORC）是利用中低温地热能（< 150℃）发电的主要途径,在实际运行中,非共沸工质往往会冷凝至过冷状态。分析了冷凝过冷度对非共沸工质ORC热力性能的影响,建立了ORC、内回热（internal heat exchanger,IHE）ORC的热力学模型,以净输出功最大为目标函数优化了工质的蒸发压力,并开展了系统的㶲分析。结果表明：过冷度影响了工质与冷源换热流体间的温度匹配特性,受夹点温差的限制,随着过冷度的增加,工质的冷凝压力上升;过冷度亦改变了预热器和蒸发器的热量分摊,随着过冷度的增加,最佳蒸发压力亦上升。混合工质异丁烷/异戊烷的质量配比为0.4：0.6时,净输出功受过冷度的影响最大,当过冷度为2℃时,净输出功下降了4.36%。IHE回收膨胀机排汽的余热,提高了预热器入口温度,可提高过冷ORC系统净输出功0.55%。过冷度增大了冷凝器的㶲损失;采用内回热冷凝器的㶲损失降低了24.7%。     

蒸发器中非共沸混合工质的换热特性

为了阐明非共沸混合工质在制冷、空调系统蒸发器中的换热过程,以及混合工质蒸发时的温度滑移现象为工程实际带来的某些特殊性,运用传热及热力学原理进行了相应的理论分析,发现非共沸混合工质的蒸发过程中蒸发介质存在极限流量的现象,并得到此类工质在可用能角度相比纯工质具有节能效果的结论(一般情况下,相对可用能损失减少40%～55%),最后将理论分析结论应用于几种常用的混合工质上,如R407c、R405a和R414b,并预测了这些工质在实际使用中的极限流量和可用能损失情况.     

使用非共沸二元混合制冷剂的压缩式热泵

近年来,使用非共沸混合制冷剂以提高压缩式热泵COP值的可能性成为人们的议题。迄今为止的计算和实验表明,纯采取此项措施很有可能使多种热泵减少能量的损失。以非共沸混合制冷剂代替纯制冷剂对热泵热力性能的影响在图1所示,图中绘出了在冷凝器、蒸发器中的制冷剂以及与热交换有关的加热源、受热源的温度变化情况。因为在冷凝器、蒸发器内的压力降被忽略时纯制冷剂的温度值是常数,所以在图中以水平线表示。另一方面,由两种制冷剂混合而成的非共沸制冷剂的温度在冷凝器中沿程下降;而在蒸发     

ODP为零的热泵混合工质性能匹配及非共沸特性研究

针对一种臭氧层破坏潜能(ODP)为零的非共沸中高温热泵混合工质HTR00,研究了其实际应用中的三个关键问题:R134a压缩机直接充灌HTR04的匹配性问题;HTR04的组分迁移问题;温度滑移特性利用问题.结果表明:对于一个配备R134a压缩机的HTR04热泵机组,能够稳定且高效地提供80℃的热水;HTR04组分迁移导致的系统性能变化在工程允许范围之内;通过HTR04滑移温度与冷凝器水侧温差的最优匹配,可提高热泵系统性能.研究可为绿色环保中高温热泵工质HTR00的实际应用提供理论基础和技术支持.     

自动复叠制冷系统非共沸混合工质组分变化特性

非共沸混合工质冷凝是温度不断降低的等压冷凝过程,气相和液相组分不断变化。当冷凝器出口温度为300 K时,液相混合工质R600a/R23/R14的质量分数为78.04/12.62/9.34,冷凝液大部分为R600a,但含有相当数量的中低沸点工质。冷凝温度降低至280 K时,R600a在气相中的比例为9.8%,即使冷凝温度降低到质量分数35/35/30的泡点温度249.49 K,R600a在气相的质量分数仍然占2.67%。相分离器I能够分离78.04%的高沸点工质R600a,但低沸点工质R14在液相中占9.34%。相分离器II只能分离30.27%的R23,12.62%依靠相分离器I分离,其余的R23都被带入到蒸发器。R600a在蒸发器内仍然有6.31%的含量,低沸点工质R14在蒸发器内占45.64%。     

制冷空调系统应用混合工质的性能和泄漏研究

尽管非共沸混合制冷剂的优点很多,但其工业应用至今仍是很有限的。主要原因是制冷装置内混合工质的泄漏以及由此引起的浓度变化。另一个缺点是,尚缺乏一种有效简便的方法以确定循环系统内工质的浓度。本文介绍了作者的研究结果。研究的非共沸混合物具有大的沸点差。其目的是评估上述问题的严重性。研究内容为: 1.进行泄漏试验和计算机摸拟,以确定泄漏引起的浓度变化; 2.简单地确定循环系统内混合物的浓度。为了与沸点差极大的最恶劣情况比较,作者对沸点差较小的实际情况进行了研究。在此实际情况中,用一种新开发的三元混合物代替超级市场的制冷系统中使用的R502。     

Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump systemPerformance et caractéristiques de transfert de chaleur des frigorigènes hydrocarbures utilisés dans un système de pompe à chaleur

Performance of a heat pump system using hydrocarbon refrigerants has been investigated experimentally. Single component hydrocarbon refrigerants (propane, isobutane, butane and propylene) and binary mixtures of propane/isobutane and propane/butane are considered as working fluids in a heat pump system. The heat pump system consists of compressor, condenser, evaporator, and expansion device with auxiliary facilities such as evacuating and charging unit, the secondary heat transfer fluid circulation unit, and several measurement units. Performance of each refrigerant is compared at several compressor speeds and temperature levels of the secondary heat transfer fluid. Coefficient of performance (COP) and cooling/heating capacity of hydrocarbon refrigerants are presented. Experimental results show that some hydrocarbon refrigerants are comparable to R22. Condensation and evaporation heat transfer coefficients of selected refrigerants are obtained from overall conductance measurements for subsections of heat exchangers, and compared with those of R22. It is found that heat transfer is degraded for hydrocarbon refrigerant mixtures due to composition variation with phase change. Empirical correlations to estimate heat transfer coefficients for pure and mixed hydrocarbons are developed, and they show good agreement with experimental data. Some hydrocarbon refrigerants have better performance characteristics than R22.     

Performance evaluation of a heat pump cycle using NARMs by a simulation with equations of heat transfer and pressure drop

Simulation analyses for a vapour compression heat pump cycle using nonazeotropic refrigerant mixtures (NARMs) of R22 and R114 are conducted under the condition that the heat pump thermal output and the flow rate and inlet temperatures of the heat sink and source water are given. The heat transfer coefficients of the condensation and evaporation are calculated with empirical correlations proposed by the authors. The validity of the evaluation method and the correlations is demonstrated by comparison with experimental data. The relations between the coefficient of performance (COP) and composition are shown under two conditions: (1) the constant heat transfer length of the condenser and evaporator; and (2) the constant temperature of refrigerant at the heat exchanger inlet. The COP of the NARMs is higher than that of pure refrigerant when the heat transfer lengths of the condenser and evaporator are sufficiently long.     

不同结构布置空调用翅片管冷凝器性能的仿真研究

在空调用翅片管冷凝器的几何结构尺寸相同,空气的进口状态和流量相同的条件下,采用计算机仿真技术,研究了支路数、管排数对翅片管冷凝器的传热与流动特性的影响,结果表明:随着支路数的增大,压降随之减小,最大值为2个支路时的33.8kPa,最小值为6个支路时,仅为0.9kPa;空气与制冷剂间传热温差增大,总传热系数减小,冷凝器的换热量递减,最大值比最小值大32.1%。随着排数的增多,压降增大,4排管的压降是1排管的4.3倍;空气侧换热系数与制冷剂侧换热系数的变化呈相反趋势,但传热温差增大,换热量也增大,4排管的换热量是1排管的2.45倍。     

R404A和R410A应用于平行流冷凝器的模拟分析比较

采用分布参数法对平行流冷凝器建立数学模型，对目前广泛使用的制冷剂R134a和低温制冷剂R404A和R410A在平行流冷凝器中的换热和流动性能进行模拟计算和分析比较。分别在相同和不同工况下。比较3种制冷剂的换热系数及压降等换热和流动性能参数。结果表明，在采用平行流冷凝器的汽车空调工况范围内，R410AR404A的流动和传热性能均优于R134a，更适宜用于汽车空调用平行流冷凝器。     

Cooling performance of several CO2/propane mixtures and glide matching with secondary heat transfer fluid

This paper presents the cooling performance of several CO₂/propane mixtures measured in air-conditioning test rig at several conditions. The discharge pressure of CO₂/propane mixtures is reduced with increasing mole fraction of propane and their reduced values coincide approximately with the circulation concentrations of propane. Since propane is the refrigerant having a higher refrigerating effect and a much lower vapor density than CO₂, adding propane to CO₂ improves the system efficiency and reduces the cooling capacity. The temperature glide effect of CO₂/propane mixtures on the cooling performance was analyzed based on the experimental data. To utilize the temperature glide effect successfully, a sufficient heat exchange area is required, and the temperature gradient of refrigerant must be similar to that of secondary heat transfer fluid. It is better the temperature change of refrigerant can prevent pinching with that of the secondary heat transfer fluid.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures

A study on the prediction of heat transfer coefficient (HTC) and pressure drop of refrigerant mixtures is reported. HTCs and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis. Results indicate that nucleate boiling is suppressed at qualities greater than 20.0% for all mixtures and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8.0–10.0% increase in HTCs. Some mixtures with large volatility difference exhibit as much as 55.0% reduction compared with R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32.0%) and reduced mass flow rates (23.0%). Other mixtures with moderate volatility difference exhibit 20.0–30.0% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall HTC of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10.0% in transport properties causes a change of less than 6% in heat transfer prediction.     

A numerical investigation of flow boiling of non-azeotropic and near-azeotropic binary mixtures

The flow and heat transfer characteristics of binary refrigerant mixtures in a heated horizontal tube were investigated numerically. The pressure drop, temperature profile, and heat transfer coefficient for non-azeotropic and near-azeotropic mixtures of different bulk compositions were obtained. It is found that the non-linear physical properties of the mixtures strongly affect the pressure drop characteristics. Both the fluid properties and mass transfer resistance are responsible for the heat transfer characteristics. The mass transfer resistance has a more significant influence on the nucleate boiling than the convective evaporation for non-azeotropic mixtures, while the resistance can be neglected for near-azeotropic mixtures.     

载冷剂低温热物性测试方法综述

航天器内的温度通过热控制系统中载冷剂的强制对流换热来调节。在研制和选择载冷剂时,根据载冷剂的性能要求,需要对流体工质在低温下的热物性参数进行测量。对现有的载冷剂物性测试方法进行了综述,总结了测量载冷剂密度、凝固点、沸点、黏度等参数的方法,并通过比较得出绝热量热法是低温下测量液体比热容的最佳方法,径向热流法适合于低温下测量液体工质的导热系数。