Pega S. Hrnjak

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Flow boiling heat transfer coefficient and pressure drop are investigated in the horizontal smooth tube of 6.1 mm inner diameter for CO2, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement is carried out at adiabatic conditions. This research is performed at(More)
The transitional Reynolds numbers for different patterns are available and a proposed transitional map is also provided in this paper. The transition of falling-films on flat tubes is studied in this work. Water and ethylene glycol are used as working fluids, for falling-film experiments conducted under adiabatic conditions, without an imposed vapor flow.(More)
This paper presents a method to quantify the distribution of liquid refrigerant mass flow rate in 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(More)
This article presents the results of the experimental runs of a prototype of R744 (CO2) refrigeration system operating in a both air conditioning and heat pump mode when heat rejection is done in supercritical region. The prototype system is sized for a compact car. Data presented are in the limited range of operation. Further optimization and extension of(More)
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant(More)
In refrigeration systems a small amount of compressor lubricant is entrained in the refrigerant and circulated through the system, where some is retained in each component. The suction line to the compressor has the largest potential for oil retention. This paper presents results from an experimental apparatus that has been constructed to circulate POE(More)
In this study, a microchannel heat exchanger model is developed in which the thermodynamic and transport properties of refrigerant-oil mixture are taken into account as well as their impact on boiling heat transfer and pressure drop. A newly proposed infrared thermography based method is implemented in the model to describe the quality distribution in the(More)
The increased usage of hybrid and electric vehicles where waste heat availability is limited has spurred research and development of mobile heat pump systems. Many options exist for heat pump system architectures and refrigerants to be used. Currently R134a use is prevalent in vehicle air conditioning systems but offers poor heat pump performance at low(More)
Continuous capacity modulation, for example by varying compressor speed, is only one alternative to conventional cycling of vapor compression systems. Cycling the compressor at higher frequencies (~0.01–0.1 Hz) than conventional systems (~0.001 Hz) offers a potentially simpler and less costly alternative. Experiments were conducted to quantify effects of(More)
Flow boiling heat transfer characteristics of R744 with and without oil were investigated experimentally in horizontal smooth tubes with an inner diameter of 11.2 mm. In order to investigate the effect of the miscible oil on the heat transfer of R744, POE (polyolester) RENSIO C85E oil is added to give an oil circulation rate (OCR) between 0.5% and 4%.(More)