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Zou Y.,University of Illinois at Urbana - Champaign | Tuo H.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,Creative Thermal Solutions Inc.
Applied Thermal Engineering | Year: 2014

R410A and R134a upward flow in the transparent vertical header and distribution into the horizontal parallel microchannel tubes were investigated. Refrigerant entered into the header by the five tubes in the bottom pass and exited through the five tubes in the top pass, representing the flow in the outdoor coil under the heat pump mode of reversible systems. Inputting the experimental quality results into a microchannel heat exchanger model, the capacity degradation compared to the uniform distribution case was calculated. The capacity degradation is related to the coefficient of variation of refrigerant maldistribution, which is affected by header geometry and inlet conditions. The capacity degradation of the whole two-pass microchannel heat exchanger was calculated by inputting the derived empirical distribution function in to the model. It was found that the capacity was reduced by up to 30% for R410A and 5% for R134a, respectively, for the conditions examined. © 2013 Elsevier Ltd. All rights reserved. Source


Kondou C.,University of Illinois at Urbana - Champaign | Hrnjak P.,University of Illinois at Urbana - Champaign | Hrnjak P.,Creative Thermal Solutions Inc.
International Journal of Heat and Mass Transfer | Year: 2012

This paper presents experimentally determined heat transfer coefficients for condensation from a superheated vapor of CO 2 and R410A. The superheated vapor was flowed through a smooth horizontal tube with 6.1 mm ID under almost uniform temperature cooling at reduced pressures from 0.55 to 0.95, heat fluxes from 3 to 20 kW m -2, and superheats from 0 to 40 K. When the tube wall temperature reaches the saturation point, the measured results show that the heat transfer coefficient gradually starts deviating from the values predicted by a correlation valid for single-phase gas cooling. This point identifies the start of condensation from the superheated vapor. The condensation starts earlier at higher heat fluxes because the tube wall temperature reaches the saturation point earlier. The heat transfer coefficient reaches a value predicted by correlations for condensation at a thermodynamic vapor quality of 1. The measured heat transfer coefficient of CO 2 is roughly 20-70% higher than that of R410A at the same reduced pressures. This is mainly because the larger latent heat and liquid thermal conductivity of CO 2, compared to that of R410A, increase the heat transfer coefficient. © 2012 Elsevier Ltd. All rights reserved. Source


Pottker G.,University of Illinois at Urbana - Champaign | Hrnjak P.,University of Illinois at Urbana - Champaign | Hrnjak P.,Creative Thermal Solutions Inc.
International Journal of Refrigeration | Year: 2015

This paper presents experimental data and analysis comparing the performance of an R410A ejector vapor compression system to those of a liquid-fed evaporator system and a conventional expansion valve system. The objective was to quantify separately two major improvements of the ejector system: work recovery and liquid-fed evaporator. The ejector system was first compared to a system with liquid-fed evaporator at matching cooling capacities and revealed improvements from 1.9% to 8.4% solely due to the work recovery of the ejector. When compared to a conventional expansion valve system at the same cooling capacity, the ejector setup improved COP from 8.2% to 14.8% due to simultaneous benefits of liquid-fed evaporator and work recovery. Overall ejector efficiencies from 12.2% to 19.2% were achieved. Source


Zou Y.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,Creative Thermal Solutions Inc.
International Journal of Refrigeration | Year: 2013

This paper presents the R410A adiabatic upward flow in three vertical headers of microchannel heat exchangers. All microchannel tubes are inserted into the half depth. The objectives are to explore what affects R410A distribution and attempt to predict the distribution. R410A is circulated into the header through the (5 or 10) tubes in the bottom pass and exits through the (5 or 10) tubes in the top pass. It represents the flow in the outdoor heat exchanger (usually used as the condenser) when it is used as the evaporator in the heat pump mode of reversible systems. The quality was typically varied from 0.2 to 0.8 and the mass flow rate from 1.5 to 4.5 kg h-1 per tube. The best distribution was observed at high mass flux and low quality. The experiment and visualization reveals the flow pattern effects in terms of homogeneity and momentum. The churn flow had better distribution since the two-phase mixture was more homogenous and the distribution was better at high mass flux in the header because the higher momentum liquid was able to reach the top exit tube. © 2013 Elsevier Ltd and IIR. All rights reserved. Source


Zou Y.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,University of Illinois at Urbana - Champaign | Hrnjak P.S.,Creative Thermal Solutions Inc.
International Journal of Refrigeration | Year: 2014

This paper presents refrigerant R134a and oil (PAG 46) mixture upward flow in the vertical header of microchannel heat exchanger (compared to pure R134a and R410A), and its effect on distribution. The mixture enters into the header through the five tubes in the bottom pass and exits through the five tubes in the top pass representing the flow in the heat pump mode of outdoor heat exchangers in reversible systems. The quality was varied from 0.2 to 0.8 and the mass flow rate from 1.5 to 4.5 kg h-1 per tube. The oil circulation rate (OCR) was varied from 0% to 4.7%. It was found that at low OCR, e.g. 0.5%, the distribution was worse than pure R134a, whereas at high OCR, the distribution was improved and better than pure R134a in some cases. This was due to sufficient oil would create a lot of foams and enlarge the liquid region in the header based on the visualization. Source

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