The Experts below are selected from a list of 4533 Experts worldwide ranked by ideXlab platform
Predrag Stojan Hrnjak - One of the best experts on this subject based on the ideXlab platform.
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quantification of Liquid Refrigerant distribution in parallel flow microchannel heat exchanger using infrared thermography
Applied Thermal Engineering, 2015Co-Authors: Huize Li, Predrag Stojan HrnjakAbstract:Abstract This paper presents a method to quantify the distribution of Liquid Refrigerant mass flow rate in a parallel flow microchannel heat exchanger from the infrared images. Quantification is achieved by building the relationship between the Liquid mass flow rate through each microchannel tube and the air-side capacity calculated from the infrared measurement of the wall temperature. After being implemented in a heat exchanger model, the quantification method is validated against experimental data. This method can be used for several types of heat exchangers: evaporators, condensers, gas-coolers, etc., also it can be applied to various heat exchanger designs: different inlet/outlet locations, different flow configurations, etc.
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effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance
International Journal of Refrigeration-revue Internationale Du Froid, 2013Co-Authors: Predrag Stojan HrnjakAbstract: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%.
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flash gas bypass for improving the performance of transcritical r744 systems that use microchannel evaporators
International Journal of Refrigeration-revue Internationale Du Froid, 2004Co-Authors: Stefan Elbel, Predrag Stojan HrnjakAbstract: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.
Pega Hrnjak - One of the best experts on this subject based on the ideXlab platform.
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separation in condensers as a way to improve efficiency
International Journal of Refrigeration-revue Internationale Du Froid, 2017Co-Authors: Jun Li, Pega HrnjakAbstract:Abstract This paper introduces the concept of separation of two-phase flow in condensers and discusses its possible application of enhancing the heat transfer performance by capitalizing on the high local heat transfer coefficient of vapor flow. The benefit of vapor–Liquid Refrigerant separation and the reason why it will improve the condenser performance are explained. Numerical studies are performed on an R-134a microchannel condenser. Model predicts that at the same mass flow rate, the exit temperature is lower by 1.3 K in the separation condenser than in the baseline condenser while the difference of pressure drop remains within 2%. 6.1% more flow rate of condensate is predicted in the separation condenser as another comparison criterion. In addition, the trade-off between high quality and low mass flux for the vapor path downstream of the separation header is investigated by the model and results are presented. Modeling is conducted with pre-assumed separation efficiency in the header. The real value requires further investigation.
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numerical simulation of three dimensional two phase flow and prediction of oil retention in an evaporator of the automotive air conditioning system
Applied Thermal Engineering, 2017Co-Authors: Vladimir Stevanovic, Pega HrnjakAbstract:Abstract This paper presents a three-dimensional model of the Refrigerant-oil two phase flow developed with the aim of predicting the oil retention in evaporators of air-conditioning systems. The developed model is based on the two-fluid model approach. The governing mass, momentum and energy balance equations are written for each phase. The gas phase is the Refrigerant vapor, while the Liquid phase is the mixture of Liquid Refrigerant and oil. The balance equation for the oil mass fraction in the mixture with Liquid Refrigerant is included. Transfer processes at the vapor-Liquid interfaces and on the flow channel walls are predicted with closure laws. The model is solved by the in-house computer code based on the SIMPLE type numerical procedure. The model is validated by comparing numerically predicted Refrigerant mass and oil retention data in a brazed plate and fin evaporator, typically used in automotive applications, against measured values. Two sets of experiments performed with Refrigerants R134a and R1234yf in the mixture with PAG oil are simulated. The numerical results provide a complete picture of the two-phase flow structure in the evaporator. Model predicts that oil is mainly retained in the bottom header and in smaller amounts in the top header and in parallel evaporating channels with upward Refrigerant flow. The developed three-dimensional modeling and numerical approach has the advantage of being more reliable for the prediction of oil retention than existing one-dimensional models.
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vapor Liquid separation in a vertical impact t junction for vapor compression systems with flash gas bypass
International Journal of Refrigeration-revue Internationale Du Froid, 2014Co-Authors: Hanfei Tuo, Pega HrnjakAbstract:Abstract This paper presents results from experimental study of vapor and Liquid Refrigerant separation in vertical impact T-junctions using R134a and R410A for application in but not limited to vapor compression systems. Inlet flow rate and quality are varied in the range of 10–35 g s−1 and 10–25%. Flow patterns in the T-junction separator are identified and characterized. It is found that Liquid separation efficiency strongly depends on the flow pattern right above the impact region (junction). The efficiency deteriorates dramatically when mist turns into churn flow regime, with increasing inlet flow rate and/or quality. An empirical correlation to predict churn flow transition is proposed as a function of Fr and Xtt. That correlation is a bases for good design of T-junction separators.
G Venkatarathnam - One of the best experts on this subject based on the ideXlab platform.
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comparison of performance of a vapor absorption refrigeration system operating with some hydrofluorocarbons and hydrofluoroolefins as Refrigerants along with ionic Liquid hmim tf2n as the absorbent
International Journal of Refrigeration-revue Internationale Du Froid, 2018Co-Authors: I Sujatha, G VenkatarathnamAbstract:Abstract The performance of a vapor absorption refrigeration system (VARS) operating with ionic Liquid [hmim][TF2N] and HFC Refrigerants R32, R152a and R125 and HFO Refrigerants R1234ze(E) and R1234yz has been analysed. The PC-SAFT equation of state is used to estimate the properties of ionic Liquid-Refrigerant solutions as well as that of pure Refrigerants. The effect of absorber, generator, evaporating and condensing temperatures on the coefficient of performance and circulation ratio has been studied. A COP value of 0.5 and above can be obtained with the combination R32-[hmim][TF2N] in typical air-conditioning applications. An exergy analysis has been carried out to understand the major sources of losses in different components. The results show that the performance of a VARS with [hmim]TF2N] and HFC Refrigerants R32 and R152a is better than that with [hmim]TF2N] and HFO Refrigerants R1234yf and R1234ze(E). The results of this study show that ionic Liquid-low GWP Refrigerants R32/R152a combinations are suitable for use in vapor absorption refrigeration systems for air conditioning applications.
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relationship between composition of mixture charged and that in circulation in an auto Refrigerant cascade and a j t refrigerator operating in Liquid Refrigerant supply mode
Cryogenics, 2017Co-Authors: Bura Sreenivas, Gurudath H Nayak, G VenkatarathnamAbstract:Abstract The composition of the Refrigerant mixture in circulation during steady state operation of J-T and allied refrigerators is not the same as that charged due to Liquid hold up in the heat exchangers and phase separators, as well as the differential solubility of different Refrigerant components in the compressor lubricating oil. The performance of refrigerators/liquefiers operating on mixed Refrigerant cycles is dependent on the mixture composition. It is therefore important to charge the right mixture that results in an optimum composition in circulation during steady state operation. The relationship between the charged and circulating composition has been experimentally studied in a J-T refrigerator operating in the Liquid Refrigerant supply (LRS) mode and an auto Refrigerant cascade refrigerator (with a phase separator) operating in the gas Refrigerant supply (GRS) mode. The results of the study are presented in this work. The results show that the method presented earlier for J-T refrigerators operating in GRS mode is also applicable in the case of refrigerators studied in this work.
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performance of an auto Refrigerant cascade refrigerator operating in Liquid Refrigerant supply lrs mode with different cascade heat exchangers
Cryogenics, 2010Co-Authors: Gurudath H Nayak, G VenkatarathnamAbstract:Abstract Auto Refrigerant cascade (ARC) refrigerators operating with zeotropic mixtures provide refrigeration at temperatures less than 173 K (−100 °C) using a single compressor. Different authors have suggested different cascade heat exchangers for ARC refrigerators. There is no study in literature that suggests at what temperature ranges one, two or three cascade heat exchangers are necessary. In this paper the performance of an ARC refrigerator operating in the Liquid Refrigerant supply mode and operating with optimized hydrocarbon mixtures and different cascade heat exchangers is studied. The optimum number of cascade heat exchangers (stages) to be used for different operating temperatures is suggested.
Yongcha Kim - One of the best experts on this subject based on the ideXlab platform.
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influence of Liquid Refrigerant injection on the performance of an inverter driven scroll compressor
International Journal of Refrigeration-revue Internationale Du Froid, 2003Co-Authors: Honghyu Cho, Jin Taek Chung, Yongcha KimAbstract:The operation of a scroll compressor at high compression ratios can cause excessively high discharge temperatures, which can be detrimental to the reliability and efficiency of the compressor. In the present study, the performance of an inverter-driven scroll compressor with Liquid Refrigerant injection was measured with a variation of compressor frequency, injection pressure, and injection location. The influence of the Liquid injection on the performance is presented as a function of operating parameters and injection location by comparing the results with those for the non-injection case. It was found that Liquid injection under high frequency was very effective at attaining higher performance and reliability of the compressor, but injection under low frequency showed some disadvantages. For high frequency at a given injection ratio, the injection at α=180°, for an injection angle at an injection port, yielded slightly better performance of the compressor as compared to that at α=90°.
Mitsuhiro Fukuta - One of the best experts on this subject based on the ideXlab platform.
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an investigation of the performance of a scroll compressor under Liquid Refrigerant injection
International Journal of Refrigeration-revue Internationale Du Froid, 2001Co-Authors: Asi K Dutta, Tadashi Yanagisawa, Mitsuhiro FukutaAbstract:Abstract In this study, fundamental and practical influence of Liquid Refrigerant injection on the performance of a Refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/Liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection Refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the Liquid Refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection Refrigerant. And furthermore, the performance of the Liquid Refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the Refrigerant injection on the oil viscosity and Refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.