Pega Hrnjak

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Papers by Pega Hrnjak

Separation in condensers as a way to improve efficiency

by Jun Li and Pega Hrnjak

International Journal of Refrigeration, 2017

This paper introduces the concept of separation of two-phase flow in condensers and discusses its... more 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.

Improvement of condenser performance by phase separation confirmed experimentally and by modeling

by Jun Li and Pega Hrnjak

International Journal of Refrigeration, 2017

This paper presents results of an experimental study to determine the effect of vapor–liquid sepa... more This paper presents results of an experimental study to determine the effect of vapor–liquid separation in a header of microchannel condenser for a MAC system. R134a is used as the working fluid. A condenser with separation and a baseline condenser identical on the air side have been tested to evaluate the difference in the performance due to separation. Two categories of experiments have been conducted: the heat exchanger-level test and the system-level test. In the heat exchanger-level test, it is found that at the same inlet and outlet temperatures the separation condenser generates 1.6% to 7.4% more condensate flow rate than the baseline. The separation condenser also lowers the refrigerant exit temperature compared with the baseline at the same condensate flow rate. The improvement in the separation condenser confirms the results by a model. In the system-level test, COP is compared under the same superheat, subcooling and refrigerating capacity. System with separation condenser shows up to 6.6% a higher COP than the system with baseline condenser.

The Analysis of Phase Separation in Vertical Headers of Microchannel HEs

by Jun Li and Pega Hrnjak

SAE 2016 World Congress and Exhibition, 2016

Separation of Liquid and Vapor in Header of MCHE

by Jun Li and Pega Hrnjak

16TH INTERNATIONAL REFRIGERATION AND AIR CONDITIONING CONFERENCE AT PURDUE

This paper presents the experimental study of separation of two-phase flow in a vertical header o... more This paper presents the experimental study of separation of two-phase flow in a vertical header of microchannel heat exchanger (MCHE) based on quantified visualization using fast camera, modelling analysis and experimental evaluation. A condenser model is developed to explore separation effects on heat exchanger. The modeling results show the benefits that a separation condenser has over a conventional condenser is affected by the separation results in the header. A header prototype is made that has an inlet in the longitudinal center part. Two sub-passes downstream are incorporated, lower for liquid and upper vapor flow. The header for experiment is clear to provide visual access. R-134a is used as the fluid of interest and mass flux through the inlet microchannels is controlled between 55 kg/(m2s)-195 kg/(m2s). The experiment results indicate that ideal separation in that header can happen at low mass flux up to 70 kg/(m2s). Results are presented in function of liquid and vapor separation efficiencies (ηl, ηv). Two phase flow inside the header is analyzed to study the mechanisms for liquid-vapor separation.

Developing Adiabatic Two Phase Flow in Headers—Distribution Issue in Parallel Flow Microchannel Heat Exchangers

Http Dx Doi Org 10 1080 01457630490280128, Aug 17, 2010

Heat exchangers with microchannels (hydraulic diameter less than 1 mm) are attracting significant... more Heat exchangers with microchannels (hydraulic diameter less than 1 mm) are attracting significant attention lately. Performance of these heat exchangers mostly evaporators is greatly affected by imperfect distribution of a two-phase fluid to each channel. This paper presents an overview of the issue, showing basic options to evenly distribute single and two-phase fluid and the methods to determine maldistribution. Special

Effects of fluid properties on two-phase flow and refrigerant distribution in the vertical header of a reversible microchannel heat exchanger – Comparing R245fa, R134a, R410A, and R32

Applied Thermal Engineering, 2014

Effects of fluid properties on distribution in the vertical header are explored. Churn and semi-a... more Effects of fluid properties on distribution in the vertical header are explored. Churn and semi-annular flow are observed for all four refrigerants. R245fa distribution is better than R134a, R410A, and R32 in both flow regimes. Fluid with low vapor density and high density ratio may be have good distribution. Method to improve distribution is different for different fluids.

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Two-Phase Refrigerant Maldistribution in the Vertical Header and its Effect on the Heat Exchanger Performance as Evaporator in Heat Pump Mode

Volume 7: Fluids and Heat Transfer, Parts A, B, C, and D, 2012

ABSTRACT The vertical header is a usual feature of outdoor heat exchanger of the residential air-... more ABSTRACT The vertical header is a usual feature of outdoor heat exchanger of the residential air-conditioning system, typically a multipass microchannel heat exchanger. When operating in heat pump mode, it functions as an evaporator. In such a system, refrigerant maldistribution in the header can deteriorate the performance of the heat exchanger. The objective of study in this paper is the adiabatic upward flow of the refrigerant in the second pass vertical header of microchannel heat exchanger and its effect on distribution. R410A is circulated into the header through the microchannel tubes (5 or 10 tubes) in the bottom pass and exits through tubes (5 or 10 tubes) in the top pass. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The best distribution is found at high flow rate and low quality. The distribution is affected by the flow patterns in the header as well as axial momentum. A distribution correlation, obtained based on the flow rate measurement in each tube, was then incorporated with a microchannel heat exchanger model. The simulation results quantified the capacity reduction of the 2-pass microchannel condenser due to the refrigerant maldistribution.

Evaluation of Secondary Fluids for Use in Low-Temperature Supermarket Applications

ABSTRACT Printout. Thesis (M.S.)--University of Illinois at Urbana-Champaign, 1999. Includes bibl... more

Heat Exchanger Frosting Patterns with Evaporating R404A and Single Phase Secondary Refrigerant

The paper presents experimental results and a comparative analysis of frosting and defrosting of ... more The paper presents experimental results and a comparative analysis of frosting and defrosting of the same heat exchanger under different conditions. It was first used as an evaporator for R404A (in DX mode) and then as a heat exchanger with single-phase secondary refrigerants at low temperatures (-15°C to -30°C, for frozen food and ice cream) in the 8 ft single deck display case. Results reveal more uniform frosting of the heat exchanger when operated with single-phase secondary refrigerant (potassium formate). More uniform frosting results in less reduction in air-flow in refrigerating mode, faster defrost and reduced heat load due to inefficient defrost. These are just some of the reasons for better performance of the display case in indirect refrigeration mode.

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Flow of Refrigerant 134a Through Orifice Tubes

HVAC&R RESEARCH

In this paper, experimental data and a model for predicting refrigerant flow through orifice tube... more In this paper, experimental data and a model for predicting refrigerant flow through orifice tubes used as expansion devices in automotive and other air conditioning systems are presented. The results are analyzed for an extensive set (933 data points) of measurements of mass flow rate of 1,1,1,2,-tetrafluoroethane (CH2FCF3, better known as refrigerant R-134a) through orifice tubes of different diameters and lengths, with and without inlet and outlet screens, over a wide range of operating conditions, and in the range of inlet qualities from 0 to 1 and subcooling up to 40°C. The mass flow rate through orifice tubes was found to be a strong function of inlet pressure, inlet subcooling, and diameter, but a relatively weak function of length. The semi-empirical model is developed using the data set and is applicable over the complete range of operating conditions covering transients and very high vapor quality inlets that are common in automotive air conditioning applications.

Large Eddy Simulations of Particle Dispersion and Deposition in a Turbulent Square Duct Flow

In the current dissertation work, the preferential concentration and deposition of heavy solid pa... more In the current dissertation work, the preferential concentration and deposition of heavy solid particles in a downward, fully developed turbulent square duct flow are studied using large eddy simulations. A second-order accurate, finite-volume based fractional step scheme, based on an unstructured Cartesian mesh, is used to integrate the unsteady, incompressible, three-dimensional Navier-Stokes equations. An algebraic multigrid solver is used to solve the Poisson equation resulting from the fractional step method. The subgrid stresses are modeled with a dynamic subgrid kinetic energy model. The particle equation of motion includes drag, lift and gravity forces and is integrated using the fourth-order accurate Runge-Kutta method. The Reynolds number for the square duct is 360, based on average friction velocity and duct width. The grid used is 80´80´128 in the two wall-normal and streamwise directions, respectively. The preferential concentration of particles is studied assuming that...

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Thermodynamic Possibilities and Technological Opportunities for Improving the Science of Refrigeration in Search for Low Global Warming Thermal Systems

The request of new low GWP options based on either synthetic (man made) refrigerants or natural (... more The request of new low GWP options based on either synthetic (man made) refrigerants or natural (naturally occurring) materials have to offer high efficiency in addition to environmental friendliness and safety for people and property. That situation will open new possibilities for drop in refrigerants along options for natural refrigerants. The paper will present some options from the point of thermodynamic limits and opportunities for new technologies to reach those limits and/or affect some of perceived constraints and discuss experience from immediate past. It is good to remember the recent history from the point of thermodynamic limits and opportunities for new technologies to reach those limits and/or affect some of perceived constraints. This paper presents an overview of expectations as we begin to reconsider CO 2 systems versus those using R134a, HFO1234yf and other synthetic refrigerants and discusses the effects of thermodynamic properties on cycle performance and thermop...

Microsystems for Cooling

Effect of Oil On Developing Two-Phase Flow of R134a in an Air-Conditioning System (AB-10-C031)

Heat transfer issues in laminar flow of single-phase secondary refrigerants through the pipes

The paper presents discussion of reasons for excellent performance of heat exchangers served with... more The paper presents discussion of reasons for excellent performance of heat exchangers served with secondary refrigerants when used in low temperature display case compared to direct expansion (DX) operation. Three experimental programs are described (in display case, local heat transfer measurements, and air cooler) and major findings are presented. The reason for good heat transfer on the refrigerant side is due to long and most importantly technically significant thermally developing region in laminar flow of secondary refrigerants at very high Prandtl number and positive effect of return - U bends. It is shown that the secondary flows in U bends recover heat transfer coefficients in the same way and to even slightly higher value than in thermally developing region. Result is effectively very high refrigerant side heat transfer coefficient, higher in average than evaporating two-phase refrigerants with a common superheat. This is true in particular for potassium formate that exhib...

Fractionation of R-407C in Brazed Plate Evaporators at Low Mass Flux

This investigation explored the effects of fractionation and distribution of R-407c in brazed pla... more This investigation explored the effects of fractionation and distribution of R-407c in brazed plate evaporators with an emphasis at low mass flux. Experiments were performed in parallel with R-22. A parametric study was performed with nominal and reduced mass fluxes, with and without distributor and with various orientations of the evaporator. Also, operating conditions were varied to investigate the effect of refrigerant inlet quality and superheat at the exit of the evaporator. This investigation took an evaporator as part of a system approach rather than evaluating local heat transfer. All experiments show that R-407c is a viable replacement for R-22. Overall heat transfer coefficients for R-407c were essentially equal to that of R-22 when operating at nominal conditions. U-values for R-407c at low mass flux were up to 15% less than R-22. However, R-407c U-values were approximately 10% greater than R-22 when operating with a saturated refrigerant outlet condition. Contrary to exp...

Design Tool for Display Case Heat Exchanger Frosting and Defrosting

A simulation model for predicting the performance of a display case under frosting conditions has... more A simulation model for predicting the performance of a display case under frosting conditions has been developed. An experimentally validated heat exchanger simulation model for a secondary refrigerant was extended to simulate operation in open display cases, modeling the varying frost properties and operating conditions and their effect on heat exchanger performance. A separate model based on a few simplifying assumptions has been developed to simulate the defrosting, predict the time required to melt the frost mass (existing at the end of frosting cycle) and compressor and fan power requirements for an entire day operation of a display case. Two design strategies for reducing defrost frequency and overall energy consumption of open supermarket display cases were evaluated relative to the baseline (variable air flow as a consequence of fan characteristics) operation: constant air flow operation; and fin staging. A preliminary analysis of a flat-tube-plain-fin heat exchanger with di...

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Prediction of Noise Generated by Expansion Devices Throttling Refrigerant

A method is presented for predicting the noise generated by expansion devices throttling refriger... more A method is presented for predicting the noise generated by expansion devices throttling refrigerant, based on an existing standard model for predicting the noise downstream of large, industrial control valves throttling air. The presented method can be used to predict the noise downstream of any expansion device of any size for any pure two-phase or pure vapor refrigerant flow. Comparison of the model predictions to experimental data for orifice tubes, capillary tubes, and refrigerant valves show excellent agreement over a wide range of operating conditions. Experimental results for the attenuation of sound waves in a two-phase refrigerant flow are also presented, as well as direct comparisons of the noise generated by different expansion devices over similar operating conditions. Air Conditioning and Refrigeration Project 72

Transient Tests of A/C System Performed on a Caravan 1997

ABSTRACT Tests intended to determine the variation of some parameters, mainly refrigerant mass fl... more ABSTRACT Tests intended to determine the variation of some parameters, mainly refrigerant mass flow and velocity, on the low side of the A/C system during start up conditions were performed in a &#39;94 Caravan . The tests were designed to capture the rapid changes in conditions at the evaporator outlet during air conditioning system start up. In particular we were interested in finding the range of refrigerant flow rates under transient operation to determine conditions at the plate evaporator that might produce acoustic resonance.

Continued Research on Plate Evaporator Noise Emission: Final Report

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