New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator

This paper presents a proposal for a venting reverse vapor in flash gas removal A/C system in order to improve refrigerant distribution and reduce pressure drop in microchannel evaporator and thus increase system efficiency. Introduction to the reverse vapor flow observed in parallel flow microchannel evaporator was presented in earlier IJR paper by the authors. An experimental comparison of the A/C system with new approach to an FGB system revealed that vapor venting provided a 5% increase of cooling capacity and 3% of COP when operated at identical test conditions, while the maximum COP improvement was approximately 10%–12% when capacity is matched by reduction of compressor speed. The improvement compared to direct expansion system was significantly higher.     

Air-side heat transfer and pressure drop characteristics of accelerated flow evaporators

We investigate an alternative tube-fin evaporator design (accelerated flow evaporator, AFE) in which the air-side local heat transfer coefficient is increased due to a progressive reduction of the free-flow area. For a given heat transfer rate, this enhancement is expected to partially compensate the decrease in surface area, which reduces the evaporator material cost. The air-side thermal-hydraulic behavior of nine AFEs was determined experimentally, and a calculation method that relies on j and f correlations for plain tube-fin heat exchangers and on mass and momentum balances has been proposed. The method enables the calculation of the by-pass fraction associated with the air stream through the clearance between the tube bank and the outer edge of the fins. The agreement between the model, which introduces no additional fitting parameters, and the experiments is within ±10% for all heat transfer data and ±15% for the majority of the pressure drop data.     

Investigating performance of microchannel evaporators with different manifold structures

In this paper, the performances of microchannel evaporators with different manifold structures are experimentally investigated. Eight evaporator samples with 7 different designs of the I/O manifold and 5 different designs of the return manifold are made for this study. The performances of the evaporator samples are tested on a psychometric calorimeter test bench with the refrigerant 134A at a real automotive AC condition. The results on the variations of the cooling capacity and air temperature distribution of the evaporator due to the deflector designs in the I/O manifold and flow hole arrangements in the return manifold are presented and analyzed. By studying the KPI’s for the performance of an evaporator, the design trade-off for an evaporator designer is summarized and discussed.     

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.     

Effect of twisted tape insert on heat transfer and pressure drop in horizontal evaporators for the flow of R-134a

An experimental study has been carried out on the heat transfer enhancement and pressure drop characteristics in presence of twisted tape inserts, during flow boiling of R-134a, inside a horizontal evaporator. The test-evaporator was an electrically heated 1260 mm long copper tube with 7.5 mm inside diameter. The experiments were performed for plain flow and four tubes with twisted tapes of 6, 9, 12 and 15 twist ratios and four refrigerant mass velocities of 54, 85, 114 and 136 kg/s m² for each tape. It has been found that the twisted tape inserts enhance the heat transfer coefficient on relatively higher pressure drop penalty, in comparison to that for the plain flow.     

叶脉型微通道热沉设计及散热特性分析

为改善相控阵天线多热源阵面温度的均匀性,基于植物叶脉优良的传质特性,提出一种用于相控阵多热源阵面的叶脉型微通道热沉。首先,对叶脉型微通道热沉的流动特性和散热特性进行仿真分析,得到热沉的热源温度分布。然后,以热源温度标准差最小化为目标,进一步优化叶脉型微通道结构,得到了非对称叶脉型微通道拓扑结构。最后,采用金属3D打印加工了铝基微通道热沉样件并进行散热性能测试。数值仿真结果表明,相比于传统平行微通道热沉,叶脉型微通道热沉不仅强化了传热,而且使得热源温度更均匀,压力损失更小;实验结果验证了叶脉型微通道热沉优良的散热性能。研究结论可为相控阵多热源阵面的热沉设计提供依据。     

Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance

This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%.     

Comparison of the transient behavior of microchannel and fin-and-tube evaporators

The development of control algorithms for refrigeration systems requires models capable of simulating transient behavior with sensible computational time and effort. The most pronounced dynamics in these systems are found in the condenser and the evaporator, especially the transient behavior of the evaporator is of great importance when designing and tuning controllers for refrigeration systems. Various so called moving boundary models were developed for capturing these dynamics and showed to cover the important characteristics. A factor that has significant influence on the time constant and nonlinear behavior of a system is the amount of refrigerant charge in the evaporator which is considerably reduced when microchannel heat exchangers are utilized. Here a moving boundary model is used and adapted to simulate and compare the transient behavior of a microchannel evaporator with a fin-and-tube evaporator for a residential air-conditioning system. The results are validated experimentally at a test rig.     

供液结构对毛细芯微槽蒸发器流动传热的影响

为了探索大功率器件热控系统采用毛细芯微槽蒸发器的结构优化方案,特建立了基于多孔介质模型的微小槽道蒸发器数值计算单元,采用CFD研究了储液腔布置位置、供液口位置对两相流动与传热过程的影响.仿真结果得到储液腔位置对于毛细芯微槽蒸发器的换热性能、均温性和进出口压降影响显著,其中底部布置储液腔的蒸发器更具优势;在底部布置储液腔情况下,不同供液口位置影响作用不明显.以上结论为提高毛细芯微槽蒸发器的流动传热特性提供了参考数据.     

Flash gas bypass for improving the performance of transcritical R744 systems that use microchannel evaporators

The performance of transcritical R744 systems with direct expansion (DX) can be significantly improved by implementing a Flash Gas Bypass (FGB). The idea behind the concept is to bypass refrigerant vapor, created during the isenthalpic expansion process, around the evaporator. By feeding the evaporator with liquid refrigerant, pressure drop is reduced and refrigerant distribution is improved. With R744 as the working fluid, increased refrigerant-side heat transfer coefficients are expected as well. In addition, the FGB concept proves to be beneficial in terms of system design, in particular for combined air-conditioning and heat pumping applications. An experimental comparison to a conventional DX-system reveals that FGB increases the cooling capacity and COP at the same time by up to 9 and 7%, respectively. Even larger improvements are possible in case a variable speed compressor is utilized to match the performance of the conventional DX-system. A simulation model helps to separate the individual improvement mechanisms. It was found that the reduction of refrigerant-side pressure drop is the dominant improvement mechanism of FGB.     

Numerical solution to the problem of heat transfer in a bed with periodic reversals of the heat carrier flow

A numerical solution is obtained to the problem of regenerative heat transfer in a packed bed of granular material with a counterflow of heat carriers.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 2, pp. 309–316, February, 1973.     

Ammonia-water desorption heat and mass transfer in microchannel devices

This paper presents the results of an experimentally validated model for the prediction of local heat and mass transfer rates in a microchannel ammonia-water desorber. The desorber is an extremely compact 178 mm × 178 mm × 0.508 m tall component capable of transferring the required heat load (∼17.5 kW) for a residential heat pump system. The model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Previous experimental and analytical research by the authors demonstrated the performance of this same microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small-capacity heat pump market.     

平翅片传热与流动特性的数值模拟

利用计算流体力学软件Fluent研究翅片管换热器的换热和压降特性。使用流固耦合准确确定翅片及管内外侧的传热边界条件，并利用场协同理论进行分析。将计算出的结果与试验结果进行比较。结果表明，数值模拟与实验结果吻合较好，数值计算可为换热器的结构优化和产品的研究开发提供依据。     

Features of nonstationary periodic contact heat transfer

Results are presented of investigations of nonstationary periodic contact heat transfer performed on special experimental installations and under full-scale tests. The regularity of thermal hysteresis is here disclosed experimentally.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 60, No. 1, pp. 151–161, January, 1991.     

Predicting heat pump performance by thermodynamic generalizations of fluid flow and heat transfer data

Thermodynamic generalizations based on reduced pressure proposed in the 1960s are reviewed and updated to reflect the current state of the art. The application of the method is illustrated by analytical and numerical examples and an assessment made of its value in heat exchanger design practice with special emphasis on two-phase forced convection refrigeration cycle applications. It is shown that this thermodynamic approach provides the heat exchanger designer, and to some extent the system engineer with an additional tool which is simple, effective and above all more reliable, particularly in evaporator and condenser design practice, than current conventional semi-empirical correlations.     

不同空气侧风速下微通道蒸发器换热特性实验研究

搭建微通道蒸发器性能实验台,采用控制变量法研究不同空气侧风速下微通道蒸发器表面温度分布、制冷剂进出口压力的变化规律,计算换热量和换热系数,从而分析空气侧风速对微通道蒸发器的流量分配特性和换热效果的影响。结果表明,随着风速增大,微通道蒸发器制冷剂流量分配不均匀性增大,进出口压力波动振幅和周期增加,压降增大,风速2 m/s时微通道蒸发器换热效果最佳。     

Effect of heat transfer on the performance of thermoelectric generator-driven thermoelectric refrigerator system

A model of thermoelectric generator-driven thermoelectric refrigerator with external heat transfer is proposed. The performance of the combined thermoelectric refrigerator device obeying Newton’s heat transfer law is analyzed using the combination of finite time thermodynamics and non-equilibrium thermodynamics. Two analytical formulae for cooling load vs. working electrical current, and the coefficient of performance (COP) vs. working electrical current, are derived. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the cooling load and the coefficient of performance (COP) of the combined thermoelectric refrigerator device. For a fixed total number of thermoelectric elements, the ratio of number of thermoelectric elements of the generator to the total number of thermoelectric elements is also optimized for maximizing both the cooling load and the COP of the combined thermoelectric refrigerator device. The influences of thermoelectric element allocation and heat transfer area allocation are analyzed by detailed numerical examples. Optimum working electrical current for maximum cooling load and COP at different total number of thermoelectric elements and different total heat transfer area are obtained, respectively.     

冰浆流动及传热特性研究进展

通过综述近年冰浆流动特性方面的研究，介绍不同的冰浆模型，并讨论冰浆在管内流动的压降及流型。然后从冰浆导热系数开始，综述冰浆的传热特性，讨论不同表面换热系数和相应的Nusselt数，为研究冰浆的流动和传热特性提供借鉴。     

Effect of thermal conductivity ratio on flow features and convective heat transfer

In this paper, we numerically investigate the steady laminar natural convection in a water-filled 2D enclosure containing a rectangular conducting body. Computations are performed for a wide range of dimensionless parameters including the Rayleigh number Ra, the thermal conductivity ratio k, and the location of the inner body δ, while the value of the Prandtl number is maintained constant at 6.8. This simulation spans four decades of Rayleigh number, Ra, from 103 to 106. The influence of these various dimensionless parameters on the flow behavior is investigated. Correlations of the averaged Nusselt numbers are obtained as a function of two parameters (Ra and δ) for each working thermal conductivity ratio. The results indicate that the heat transfer rate through the enclosure can be controlled by the position of the rectangular body. A comparative study was also carried out between a horizontal and a vertical conducting shape inside the enclosure. It is proved that a vertical position leads to a better heat transfer compared to the horizontal case.