Faculty Profile

Pega Hrnjak

Mechanical Science and Engineering
 Pega Hrnjak
Pega Hrnjak
Research Professor, Director Air Conditioning and Refrigeration Center
2121 Mechanical Engineering Lab
105 S. Mathews
Urbana Illinois 61801
(217) 244-6377

Primary Research Area

  • Fluid Mechanics

Academic Positions

  • Director, Air Conditioning and Refrigeration Center (ACRC), Department of Mechanical Science and Engineering, UIUC, Jan. 2015-date

Other Professional Employment

  • Creative Thermal Solutions. Inc., President 2003-date

Research Areas

  • Energy
  • Environment
  • Fluid Mechanics
  • Manufacturing
  • Security and Defense
  • Thermo Heat and Transfer
  • Transportation

Selected Articles in Journals

  • Yang, C-M. and P. Hrnjak. "Effect of Straight Micro Fins on Heat Transfer and Pressure Drop of R410A during Evaporation in Round Tubes." International Journal of Heat and Mass Transfer, vol. 117, February 2018, pp. 924-939.
  • Li, H. and P. Hrnjak. "Effect of Refrigerant Thermophysical Properties on Flow Reversal in Microchannel Evaporators." International Journal of Heat and Mass Transfer, vol. 117, February 2018, pp. 1135-1146.
  • Li, Y. and P. Hrnjak. "Control of Flash Gas Bypass MAC System with Emphasis on Start-ups and Transients." International Journal of Refrigeration, vol. 84, December 2017, pp. 1-12.
  • Li, H. and P. Hrnjak. "Effect of Channel Geometry on Flow Reversal in Microchannel Evaporators." International Journal of Heat and Mass Transfer, vol. 115, no. A, December 2017, pp. 1-10.
  • Li, H. and P. Hrnjak. "Effect of Periodic Reverse Flow on the Heat Transfer Performance of Microchannel Evaporators." International Journal of Refrigeration, vol. 84, December 2017, pp. 309-320.
  • Hrnjak, P., P. Zhang, and C. Rennels. "Effect of Louver Angle on Performance of Heat Exchanger with Serpentine Fins and Flat Tubes in Frosting: Importance of Experiments in Periodic Frosting." International Journal of Refrigeration, vol. 84, December 2017, pp. 321-335.
  • Li, H. and P. Hrnjak. "Measurement of Heat Transfer Coefficient and Pressure Drop during Evaporation of R134a in New Type Facility with One Pass Flow-through Microchannel Tube." International Journal of Heat and Mass Transfer, vol. 115, December 2017, pp. 502-512.
  • Meyer, M. and P. Hrnjak. "Flow Regimes during Condensation in Superheated Zone." International Journal of Refrigeration, vol. 84, December 2017, pp. 336-343.
  • Li, J. and P. Hrnjak. "Visualization and Quantification of Separation of Liquid-vapor Two-phase Flow in a Vertical Header at Low Inlet Quality." International Journal of Refrigeration, November 2017, doi: 10.1016/j.ijrefrig.2017.09.018.
  • Jin, S. and P. Hrnjak. "A New Method to Simultaneously Measure Local Heat Transfer and Visualize Flow Boiling in Plate Heat Exchanger." International Journal of Heat and Mass Transfer, vol. 113, October 2017, pp. 635-646.
  • Kang, Y., W. A. Davies, P. Hrnjak, and A. Jacobi. "Effect of Inclination on Pressure Drop and Flow Regimes in Large Flattened-tube Steam Condensers." Applied Thermal Engineering, vol. 123, August 2017, pp. 498-513.
  • Xu, J. and P. Hrnjak. "Quantification of Flow and Retention of Oil in Compressor Discharge Pipe." International Journal of Refrigeration, vol. 80, August 2017, pp. 252-263.
  • Xiao, J. and P. Hrnjak. "A Heat Transfer Model for Condensation Accounting for Non-equilibrium Effects," International Journal of Heat and Mass Transfer, vol. 111, August 2017, pp. 201-210.
  • Li, J. and P. Hrnjak. "Separation in Condensers as a Way to Improve Efficiency." International Journal of Refrigeration, vol. 79, July 2017, pp. 1-9.
  • Zhu, L. and P. Hrnjak. "Thermosiphon with Vapor Separation: Experimental Comparison to Conventional Type." Applied Thermal Engineering, vol. 121, July 2017, pp. 879-886.
  • Li, J. and P. Hrnjak. "Improvement of Condenser Performance by Phase Separation Confirmed Experimentally and by Modeling." International Journal of Refrigeration, vol. 78, June 2017, pp. 60-69.
  • Li, H. and P. Hrnjak. "Visualization and Measurement of Reverse Flow in an Actual Channel of a Microchannel Evaporator." International Journal of Heat and Mass Transfer, vol. 108, no. B, May 2017, pp. 2346-2354.
  • Xiao, J. and P. Hrnjak. "A New Flow Regime Map and Void Fraction Model Based on the Flow Characterization of Condensation." International Journal of Heat and Mass Transfer, vol. 108, May 2017, pp. 443-452.
  • Stevanovic, V. D. and P. Hrnjak. "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, vol. 117, May 2017, pp. 468-480.
  • Jin, S. and P. Hrnjak. "Effect of End Plates on Heat Transfer of Plate Heat Exchanger." International Journal of Heat and Mass Transfer, vol. 108 no. A, May 2017, pp. 740-748.
  • Li, H. and P. Hrnjak. "Modeling of Bubble Dynamics in Single Diabatic Microchannel." International Journal of Heat and Mass Transfer, vol. 107, April 2017, pp. 96-104.
  • Xiao, J. and P. Hrnjak. "Heat Transfer and Pressure Drop of Condensation from Superheated Vapor to Subcooled Liquid." International Journal of Heat and Mass Transfer, vol. 103, December 2016, pp. 1327-1334.
  • Wang, X., K. Tang, and P. Hrnjak. "Evaporator Performance Enhancement by Pulsation width Modulation (PWM)," Applied Thermal Engineering, vol. 99, April 2016, pp. 825-833.
  • Jin, S. and P. Hrnjak. "Refrigerant and Lubricant Charge in Air-condition Heat Exchangers: Experimentally Validated Model." International Journal of Refrigeration, vol. 67, April 2016, pp. 395-407.
  • Feng, L. and P. Hrnjak. "Modeling of a Reversible Air-conditioning, Heat Pump System for Electric Vehicles," SAE International Journal of Passenger Cars - Mechanical Systems, vol. 9, no. 1, April 2016, pp. 68-74.
  • Khovalyg, D., P. Hrnjak, and A. M. Jacobi. "Interactions between Parallel Unevenly Heated Minichannels during Flow Boiling of R134a.." Heat Transfer Engineering, vol. 37, no. 11, January 2016, pp. 921-937.
  • Li, H. and P. Hrnjak. "An Experimentally Validated Model for Microchannel Heat Exchanger Incorporating Lubricant Effect." International Journal of Refrigeration vol. 59, November 2015, pp. 259-268.
  • Zou, Y. and P. Hrnjak. "Comparison and Generalization of R410A and R134a Distribution in the Microchannel Heat Exchanger with the Vertical Header." Science and Technology for the Built Environment, vol. 21, no. 5, July 2015, pp. 555-563.
  • Sethi, A. and P. Hrnjak. "Modeling of Oil Retention and Pressure Drop in Vertical Suction Risers." Science and Technology for the Built Environment, vol. 21, no. 7, April 2015, pp. 1033-1046.
  • Khovalyg, D., P. Hrnjak, and A. M. Jacobi. "Transient Pressure Drop Cross-correlation During Flow Boiling of R134a in Parallel Minichannels." Science and Technology for the Built Environment, vol. 21, no. 5, April 2015, pp. 545-554.
  • Li, H. and P. Hrnjak. "An Infrared Thermography-based Method for Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger." SAE International Journal of Materials and Manufacturing, vol. 8, no. 3, April 2015, pp. 603-608.
  • Li, H. and P. Hrnjak. "Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger using Infrared Thermography." Applied Thermal Engineering, vol. 78, March 2015, pp. 410-418.
  • Pottker, G. and P. Hrnjak. "Experimental Investigation of the Effect of Condenser Subcooling in R134a and R1234yf Air-conditioning Systems with and without Internal Heat Exchanger." International Journal of Refrigeration, vol. 50, February 2015, pp. 104-113.
  • Pottker, G. and P. Hrnjak. "Effect of the Condenser Subcooling on the Performance of Vapor Compression Systems." International Journal of Refrigeration, vol. 50, February 2015, pp. 156-164.
  • Agarwal, R. and P. Hrnjak. "Condensation in Two-phase and Desuperheating Zone for R1234ze(E), R134a and R32 in Horizontal Smooth Tubes." International Journal of Refrigeration, vol. 50, February 2015, pp. 172-183.
  • Pottker, G. and P. Hrnjak. "Ejector in R410A Vapor Compression Systems with Experimental Quantification of Two Major Mechanisms of Performance Improvement: Work Recovery and Liquid Feeding." International Journal of Refrigeration, vol. 50, February 2015, pp. 184-192.
  • Ren, T. and P. Hrnjak. "Pressure Drop in Round Cylindrical Headers of Parallel Flow MCHXs: Pressure Loss Coefficients for Single-phase Flow." International Journal of Refrigeration, vol. 49, January 2015, pp. 119-134.
  • Agarwal, R. and P. Hrnjak. "Effect of Sensible Heat, Condensation in Superheated and Subcooled Region Incorporated in Unified Model for Heat Rejection in Condensers in Horizontal Round Smooth Tubes." Applied Thermal Engineering, vol. 71, no. 1, October 2014, pp. 378-388.
  • Zou, Y. and P. Hrnjak. "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, vol. 70, no. 1, September 2014, pp. 966-976.
  • Zou, Y. and P. Hrnjak. "Single-phase and Two-phase Flow Pressure Drop in the Vertical Header of Microchannel Heat Exchanger." International Journal of Refrigeration, vol. 44, August 2014, pp. 12-22.
  • Sethi, A. and P. Hrnjak. "Oil Retention and Pressure Drop of R1234yf and R134a with POE ISO 32 in Suction Lines." HVAC and R Research, vol. 20, no. 6, May 2014, pp. 703-720.
  • Jin, S. and P. Hrnjak. "An Experimentally Validated Model for Predicting Refrigerant and Lubricant Inventory in MAC Heat Exchangers." SAE International Journal of Passenger Cars -- Mechanical Systems, vol. 7, no. 2, April 2014, pp. 769-780.
  • Zou, Y. and P. Hrnjak. "Effect of Oil on R134a Distribution in the Microchannel Heat Exchanger with Vertical Header." International Journal of Refrigeration, vol. 40, April 2014, pp. 201-210.
  • Tuo, H. and P. Hrnjak. "Vapor-liquid Separation in a Vertical Impact T-junction for Vapor Compression Systems with Flash Gas Bypass." International Journal of Refrigeration, vol. 40, April 2014, pp. 189-200.
  • Tuo, H. and P. Hrnjak. "Visualization and Measurement of Periodic Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an Air-conditioning System." International Journal of Heat and Mass Transfer, vol. 71, April 2014, pp. 639-652.
  • Ren, T. and P. Hrnjak. "Pressure Drop in D-shaped Cylindrical Headers of Parallel Flow MCHXs: Pressure Loss Coefficients for Single-phase Flow." Applied Thermal Engineering, vol. 65, no. 1-2, April 2014, pp. 111-123.
  • Tuo, H. and P. Hrnjak. "Enhancement of Vapor-liquid Separation in Vertical Impact T-junctions for Vapor Compression Systems with Flash Gas Bypass." International Journal of Refrigeration, vol. 40, April 2014, pp. 43-50.
  • Zou, Y., H. Tuo, and P. Hrnjak. "Modeling Refrigerant Maldistribution in Microchannel Heat Exchangers with Vertical Headers Based on Experimentally Developed Distribution Results." Applied Thermal Engineering, vol. 64, no. 1-2, March 2014, pp. 172-181.
  • Tuo, H. and P. Hrnjak. "Effect of Venting the Periodic Reverse Vapor Flow on the Performance of a Microchannel Evaporator in Air-conditioning Systems." International Journal of Heat and Mass Transfer, vol. 69, February 2014, pp. 66-76.
  • Zou, Y., H. Tuo, and P. Hrnjak. "R-410A Maldistribution Impact on the Performance of Microchannel Evaporator." ASHRAE Transactions, vol. 120, no. 1, January 2014, pp. 406-415.
  • Zou, Y. and P. Hrnjak. "Refrigerant Distribution in the Vertical Header of the Microchannel Heat Exchanger - Measurement and Visualization of R410A Flow." International Journal of Refrigeration, vol. 36, no. 8, December 2013, pp. 2196-2208.
  • Tuo, H. and P. Hrnjak. "New Approach to Improve Performance by Venting Periodic Reverse Vapor Flow in Microchannel Evaporator." International Journal of Refrigeration, vol. 36, no. 8, December 2013, pp. 2187-2195.
  • Tuo, H. and P. Hrnjak. "Effect of the Header Pressure Drop Induced Flow Maldistribution on the Microchannel Evaporator Performance." International Journal of Refrigeration, vol. 36, December 2013, pp. 2176-2186.
  • Petersen, M., C. Bowers, S. Elbel, and P. Hrnjak. "Development of High-efficiency Carbon Dioxide Commercial Heat Pump Water Heater." HVAC&R Research, vol. 19, no. 7, August 2013, pp. 823-835.
  • Zou, Y. and P. Hrnjak. "Experiment and Visualization on R134a Upward Flow in the Vertical Header of Microchannel Heat Exchanger and its Effect on Distribution." International Journal of Heat and Mass Transfer, vol. 62, no. 1, July 2013, pp. 124-134.
  • Wang, X., P. Hrnjak, S. Elbel, A. M. Jacobi, and M. G. He. "Heat Transfer Performance for a Falling-film on Horizontal Flat Tubes." Journal of Heat Transfer, vol. 135, no. 7, June 2013, pp. 1-12.
  • Tuo, H. and P. Hrnjak. "Periodical Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an Air-conditioning System." International Journal of Refrigeration, vol. 36, no. 4, June 2013, pp. 1263-1275.
  • Tuo, H. and P. Hrnjak. "Periodic Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an R134a Mobile Air-conditioning System." SAE International Journal of Materials and Manufacturing, vol. 6, no. 3, April 2013, pp. 540-548.
  • Li, H. and P. Hrnjak. "Effect of Lubricant on Two-phase Refrigerant Distribution in Microchannel Evaporator." SAE International Journal of Materials and Manufacturing, vol. 6, no. 3, April 2013, pp. 567-575.
  • Jin, S. and P. Hrnjak. "Refrigerant and Lubricant Distribution in MAC System." SAE International Journal of Passenger Cars -- Mechanical Systems, vol. 6, no. 2, April 2013, pp. 1013-1020.
  • Zhao, H., S. Elbel, and P. Hrnjak. "Controlled Atmosphere Brazing of Aluminum Heat Exchangers." Welding Journal, vol. 92, no. 2, February 2013, pp. 44-46.
  • Kondou, C. and P. Hrnjak. "Heat Rejection in Condensers Close to Critical Point-de-superheating, Condensation in Superheated Region, and Condensation of Two-phase Fluid." Heat Transfer Engineering, vol. 34, no. 14, February 2013, pp. 1180-1190.
  • Wang, X., M. A. Rahman, A. M. Jacobi, and P. Hrnjak. "Dynamic Wetting Behavior and Water Drops on Microgrooved Surfaces." Heat Transfer Engineering, vol. 34, no. 13, January 2013, pp. 1088-1098.
  • Wang, X. F., P. Hrnjak, S. Elbel, A. M. Jacobi, and M. G. He. "Heat Transfer Performance for a Falling-film on Horizontal Flat Tubes." Journal of Heat Transfer, vol. 135, no. 7, January 2013, pp. 1-12.
  • Tuo, H. and P. Hrnjak. ￿Flash Gas Bypass in Mobile Air-conditioning System with R134a.￿ International Journal of Refrigeration, vol. 35, no. 7, November 2012, pp. 1869-1877.
  • Kondou, C. and P. Hrnjak. "Condensation From Superheated Vapor Flow of R744 And R410A at Subcritical Pressures in a Horizontal Smooth Tube." International Journal of Heat and Mass Transfer, vol. 55. no. 11-12, May 2012, pp. 2779-2791.
  • Tuo, H., A. Bielskus, and P. Hrnjak. ￿Experimentally Validated Model of Refrigerant Distribution in a Parallel Microchannel Evaporator.￿ SAE International Journal of Materials and Manufacturing, vol. 5, no. 2, April 2012, pp. 365-374.
  • Wang, X., P. Hrnjak, S. Elbel, A. M. Jacobi, and M. He. "Flow Modes and Mode Transitions for Falling Films on Flat Tubes." Journal of Heat Transfer, vol. 134, no. 2, February 2012, 9 pages.
  • Tuo, H., and P. Hrnjak. "Experimental Study of Refrigerant Two-phase Separation in a Compact Vertical T-Junction." ASHRAE Transactions, vol. 118, no. 1, January 2012, pp. 672-680.
  • Tuo, H., A. Bielskus, and P. Hrnjak. "An Experimentally Validated Model of Refrigerant Distribution in a Parallel Microchannel Evaporator." ASHRAE Transactions, vol. 118, no. 1, January 2012, pp. 375-383.
  • Li, B., S. Peuker, P. Hrnjak, and A. G. Alleyne. "Refrigerant Mass Migration Modeling and Simulation for Air Conditioning Systems." Applied Thermal Engineering, vol. 31, no. 10, July 2011, pp. 1770-1779.
  • Kondou, C. and P. Hrnjak. ￿Heat Rejection from R744 Flow Under Uniform Temperature Cooling in a Horizontal Smooth Tube Around the Critical Point.￿ International Journal of Refrigeration, vol. 34, no. 3, May 2011, pp. 719-731.
  • Tuo, H., A. Bielskus, and P. Hrnjak. ￿Effect of Flash Gas Bypass on the Performance of R134a Mobile Air-Conditioning System with Microchannel Evaporator.￿ SAE International Journal of Materials and Manufacturing, vol. 4, no. 1, April 2011, pp. 231-239.
  • Li, B., S. Peuker, P. Hrnjak, and A. Alleyne. ￿Evaluation of Transient Refrigerant Migration Modeling Approach on Automotive Air-Conditioning Systems.￿ SAE International Journal of Materials and Manufacturing, vol. 4, no. 1, April 2011, pp. 864-874.
  • Oh, J. T., A. S. Pamitran, K. I. Choi, and P. Hrnjak. ￿Experimental Investigation on Two-phase Flow Boiling Heat Transfer of Five Refrigerants in Horizontal Small Tubes of 0.5, 1.5 and 3.0mm Inner Diameters.￿ International Journal of Heat and Mass Transfer, vol. 54, no. 9-10, April 2011, pp. 2080-2088.
  • Park, C. Y. and P. Hrnjak. ￿Pure NH3 and CO2 Heat Transfer and Pressure Drop in Horizontal Smooth Tubes for NH3 Two-stage and NH3/CO2 Cascade Refrigeration Systems: A Review.￿ International Journal of Air-conditioning and Refrigeration, vol. 18, no. 2, 2010, pp. 85-100.
  • Zhang, P. and P. Hrnjak. "Air-side Performance of a Parallel-flow Parallel-fin (PF2) Heat Exchanger in Sequential Frosting." International Journal of Refrigeration, vol. 33, no. 6, September 2010, pp. 1118-1128.
  • Hrnjak, P. and S. H. Hong. ￿Effect of Return Bend and Entrance on Heat Transfer in Thermally Developing Laminar Flow in Round Pipes of Some Heat Transfer Fluids with High Prandtl Numbers.￿ Journal of Heat Transfer, vol. 132, no. 6, June 2010, pp. 1-12.
  • Pamitran, A. S., K. I. Choi, J. T. Oh, and P. Hrnjak. "Characteristics of Two-phase Flow Pattern Transitions and Pressure Drop of Five Refrigerants in Horizontal Circular Small Tubes." International Journal of Refrigeration, vol. 33, no. 3, May 2010, pp. 578-588.
  • Hrnjak, P., J. Manzione, D. Adams, D. Garski, and S. Collier. "EXITOSA CONVERSIÓN A R744 EN UNA UNIDAD DE CONTROL MEDIOAMBIENTAL." Frio-Calor-Aire Acondicionado, (Based on #125 conference publication), September 2009.
  • Zhang, P. and P. Hrnjak. ￿Effect of Some Geometric Parameters on Performance of PF2 Heat Exchangers in Periodic Frosting.￿ International Journal of Refrigeration, vol. 33, no. 2, March 2010, pp. 334-346.
  • Hrnjak, P. "Issues in Two-phase Flow Distribution in Microchannel Heat Exchangers, KGH - Air-conditioning, Heating and Refrigeration." (in Serbian: Problematika raspodele dvofaznih tokova u mikrokanalnim razmenjivacima toplote), vol. 39, no. 1, 2010, pp. 63-74.
  • Park, C. Y. and P. Hrnjak. "CO2 Flow Condensation Heat Transfer and Pressure Drop in Multi-port Microchannels at Low Temperatures." International Journal of Refrigeration, vol. 32, no. 6, September 2009, pp. 1129-1139.
  • Park, C. Y. and P. Hrnjak. "Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern for CO2 in a 3.5mm Horizontal Smooth Tube." Journal of Heat Transfer, vol. 131, no. 9, September 2009, 12 pages.
  • Zhang, P. and P. Hrnjak. "Air-side Performance Evaluation of Three Types of Heat Exchangers in Dry, Wet and Periodic Frosting Conditions." International Journal of Refrigeration, vol. 32, no. 5, August 2009, pp. 911-921.
  • Elbel, S. and P. Hrnjak. "REFRIGERACI￿ CON EYECTOR EN APLICACIONES DE AIRE ACONDICIONADO VISI￿ GENERAL DE LA EVOLUCI￿ DE DESARROLLOS." Frio-Calor-Aire Acondicionado, (Based on #123 conference publication), July/August 2009.
  • Peuker, S. and P. Hrnjak. "Transient Refrigerant Migration and Oil Distribution of an R134a Automotive A/C System." SAE International Journal of Passenger Cars -- Mechanical Systems, vol. 2, no. 1, April 2009, pp. 714-724.
  • Wujek, S. S. and P. Hrnjak. "Using Density to Calculate the Oil Circulation Ratio of a PAG Oil in R134A." SAE International Journal of Materials and Manufacturing, vol. 1, no. 1, April 2009, pp. 362-368.
  • Kim, Y. J., J. Jang, P. Hrnjak, and M. S. Kim. "Condensation Heat Transfer of Carbon Dioxide Inside Horizontal and Vertical Pressure Smooth and Microfin Tubes at Low Temperatures." Journal of Heat Transfer, vol. 131, issue 2, February 2009, 10 pages.
  • Park, C. Y. and P. Hrnjak. "CONDENSACI￿, TRANSMISI￿ DE CALOR Y CA￿A DE PRESI￿ DEL NH3 EN EL INTERIOR DE UN TUBO LISO." Frio-Calor-Aire Acondicionado, (Based on #40), January 2009.
  • Park, C. Y. and P. Hrnjak. "Flow Condensation Heat Transfer and Pressure Drop in Multi-Port Microchannels at Low Temperatures." International Journal of Refrigeration, vol. 32, no. 6, 2009, pp. 1129-1139.
  • Pamitran, A. S., K. I. Choi, J. T. Oh, and P. Hrnjak. "Experimental Investigation of Flow Boiling Heat Transfer of R-410A and R-134a in Horizontal Small Tubes." Korean Journal of Air-Conditioning and Refrigeration Engineering, vol. 21, no. 12, 2009, pp. 1141~1146.
  • Choi, K. I., A. S. Pamitran, J. T. Oh, P. Hrnjak, and C. Y. Park. "Effect on Flow Boiling Heat Transfer of Minichannel Diameter for R-410A." 설비공학논문집 제21권 제12호, 12, 2009, pp. 663-670.
  • Kim, Y. J., J. Jang, P. Hrnjak, and M. S. Kim. ￿Adiabatic Horizontal and Vertical Pressure Drop of Carbon Dioxide Inside Smooth and Microfin Tubes at Low Temperatures.￿ Journal of Heat Transfer, vol. 130, no. 11, November 2008, pp. 1-10.
  • Park, C. Y. and P. Hrnjak. ￿Experimental and Numerical Study on Microchannel and Round-Tube Condensers in a R410A Residential Air-Conditioning System.￿ International Journal of Refrigeration, vol. 31, no. 5, August 2008, pp. 822-831.
  • Hrnjak, P. and A. D. Litch. "Microchannel Heat Exchangers for Charge Minimization in Air-cooled Ammonia Condensers and Chillers." International Journal of Refrigeration, vol. 31, no. 4, June 2008, pp. 658-668.
  • Park, C. Y. and P. Hrnjak. ￿NH3 In-Tube Condensation Heat Transfer and Pressure Drop in a Smooth Tube.￿ International Journal of Refrigeration, vol. 31, no. 4, June 2008, pp. 643-651.
  • Hrnjak, P. and A. D. Litch. ￿Microchannel Heat Exchangers for Charge Minimization in Air-cooled Ammonia Condensers and Chillers.￿ International Journal of Refrigeration, vol. 31, no. 4, June 2008, pp. 658-668.
  • Elbel, S. W. and P. Hrnjak. ￿Experimental Validation of a Prototype Ejector Designed to Reduce Throttling Losses Encountered in Transcritical R744 System Operation.￿ International Journal of Refrigeration, vol. 31, no. 3, May 2008, pp. 411-422.
  • Field, B. S. and P. Hrnjak. "Adiabatic Two-phase Pressure Drop of Refrigerants in Small Channels." Heat Transfer Engineering, vol. 28, no. 8-9, August 2007, pp. 704-712.
  • Park, C. Y. and P. Hrnjak. ￿Effect of Heat Conduction Through the Fins of a Microchannel Serpentine Gas Cooler of Transcritical CO2 System.￿ International Journal of Refrigeration, vol. 30, no. 3, May 2007, pp. 389-397.
  • Hrnjak, P. and X. Tu. ￿Single-phase Pressure Drop in Microchannels.￿ International Journal of Heat and Fluid Flow, vol. 28, no. 1, February 2007, pp. 2-14.
  • Park, C. Y. and P. Hrnjak. "CO2 and R410A Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern at Low Temperatures in a Horizontal Smooth Tube." International Journal of Refrigeration, vol. 30, no. 1, January 2007, pp. 166-178.
  • Xia, Y, Y. Zhong, P. Hrnjak, and A. M. Jacobi. ￿Frost, Defrost, and Refrost and Its Impact on the Air-side Thermal-hydraulic Performance of Louvered-fin, Flat Tube Heat Exchangers.￿ International Journal of Refrigeration, vol. 29, no. 7, November 2006, pp. 1066-1079.
  • Ni￿V. G., E. W. Jassim, P. Hrnjak, and T. A. Newell. ￿Flow-regime-based Model for Pressure Drop Predictions in Microchannels.￿ International Journal of HVAC&R Research, vol. 12, no. 1, January 2006, pp. 17-34.
  • Hrnjak, P. "Carbon Dioxide Systems." HVAC&R Research, vol. 12, no. 1, January 2006, pp. 1-2.
  • Tu, X., P. Hrnjak, and C. W. Bullard. "Refrigerant 134a Liquid Flow through Micro-scale Short Tube Orifices with/without Phase Change." Experimental Heat Transfer and Fluid Flow, vol. 30, no. 3, January 2006, pp. 253-262.
  • McEnaney, R. and P. Hrnjak. ￿Clutch Cycling Mode of Compressor Capacity Control of Transcritical R744 Systems Compared to R134a Systems.￿ Society of Automotive Engineering Journal of Passenger Cars -￿ Mechanical Systems.￿ May 2005, 2005-01-2033.
  • Park, C. Y. and P. Hrnjak. ￿Flow Boiling Heat Transfer of CO2 at Low Temperatures in a Horizontal Smooth Tube.￿ ASME Journal of Heat Transfer, vol. 127, no. 12, March 2005, pp. 1305-1312.
  • Xia, Y., P. Hrnjak, and A. M. Jacobi. ￿The Air-side Thermal-hydraulic Performance of Louvered-fin, Flat-tube Heat Exchangers with Sequential Frost-growth Cycles.￿ ASHRAE Transactions, vol. 111, no. 1, February 2005, pp. 487-495.
  • Elbel, S. and P. Hrnjak. ￿Flash Gas Bypass for Improving the Performance of Transcritical R744 Systems that Use Microchannel Evaporators.￿ International Journal of Refrigeration, vol. 27, no. 7, November 2004, pp. 724-735.
  • Hrnjak, P. ￿Developing Adiabatic Two-phase Flow in Headers ￿- Distribution Issue in Parallel Flow Microchannel Heat Exchangers.￿ Heat Transfer Engineering, vol. 25, no. 3, April 2004, pp. 61-68.
  • Hrnjak, P. ￿Flow Distribution Issues in Parallel Flow Heat Exchangers.￿ ASHRAE Transactions, vol. 110, no. 1, January 2004, pp. 301-306.
  • Hrnjak, P. ￿Microchannel Heat Exchangers.￿ KGH, Belgrade, Serbia (in Serbocroation), 2004.
  • Richter, M. R., S. M. Song, J. M. Yin, M. H. Kim, C. W. Bullard, and P. Hrnjak. ￿Experimental Results of Transcritical CO2 Heat Pump for Residential Application.￿ Energy, vol. 28, no. 10, August 2003, pp. 1005-1019.
  • ElSherbini, A. I., A. M. Jacobi, and P. Hrnjak. ￿Experimental Investigation of Thermal Contact Resistance in Plain-fin-and-tube Evaporators with Collarless Fins.￿ International Journal of Refrigeration, vol. 26, no. 5, August 2003, pp. 527-536.
  • Tu, X. and P. Hrnjak. "Pressure Drop and Visualization of R134a Two-phase Flow in a Rectangular Microchannel." Transactions of ASHRAE, vol. 109, no. 1, January 2003, pp. 703-710.
  • Giannavola, S. M., J. M. Yin, C. W. Bullard, and P. Hrnjak. "Contribution of Heat Exchangers in Improved Performance of New Generation Automotive Air-conditioning System with R744." Transactions of ASHRAE, vol. 109, no. 1, January 2003, pp. 542-554.
  • Nino, V. G., P. Hrnjak, and T. A. Newell. "Two-phase Flow Visualization of R134a in a Multiport Microchannel Tube." Heat Transfer Engineering, vol. 24, no. 1, January 2003, pp. 41-52.
  • Yin, J. M., C. W. Bullard, and P. Hrnjak. "Single-phase Pressure Drop Measurements in a Microchannel Heat Exchanger." Journal of Heat Transfer Engineering, vol. 23, no. 4, July 2002, pp. 3-12.
  • Song, S., C. W. Bullard, and P. Hrnjak. "Frost Deposition and Refrigerant Distribution in Microchannel Heat Exchangers." Transactions of ASHRAE, vol. 108, no. 2, June 2002, pp. 944-953.
  • Bullard, C. W., J. M. Yin, and P. Hrnjak. "Compact Counterflow Gas Cooler for R744." Transactions of ASHRAE, vol. 108, no. 1, January 2002, pp. 487-491.
  • Bullard, C. W. and P. Hrnjak. "Research in Transcritical R744 at ACRC, University of Illinois at Urbana-Champaign." J. Soc. Air Conditioning and Refrig. Engrs. of Korea, vol. 31, no. 7, 2002, pp. 32-52.
  • Hrnjak, P., M. A. Shannon, T. M. Leicht, and N. R. Miller. "Detection of Liquid Mass Fraction at the Evaporator Exit of Refrigeration Systems." International Journal of Thermal Sciences, vol. 40, no. 8, September 2001, pp. 773-786.
  • Yin, J. M., C. W. Bullard, and P. Hrnjak. "R-744 Gas Cooler Model Development and Validation." International Journal of Refrigeration, vol. 24, no. 7, July 2001, pp. 692-701.
  • Singh, G. M., P. Hrnjak, and C. W. Bullard. "Flow of Refrigerant R134a through Orifice Tubes." International Journal of HVAC&R Research, vol. 7, no. 3, July 2001, pp. 245-262.
  • Hrnjak, P. and Y. Mao. "Defrost Issues in Indirectly Refrigerated Display Cases." ASHRAE Transactions, vol. 107, no. 2, June 2001, pp. 466-472.
  • Boewe, D. E., C. W. Bullard, J. M. Yin, and P. Hrnjak. "Contribution of Internal Heat Exchanger to Transcritical R-744 Cycle Performance." International Journal of HVAC&R Research, vol. 7, no. 2, April 2001, pp. 155-168.
  • Shannon, M. A., T. M. Leicht, P. Hrnjak, N. R. Miller, and F. A. Kahn. "Thin-film Resistance Sensor for Measuring Liquid Mass Fraction in Super-heated Refrigerant." Sensors and Actuators A, vol. 88, no. 2, February 2001, pp. 164-177.
  • Fang, X., C. W. Bullard, and P. Hrnjak. "Modeling and Analysis of Gas Coolers." ASHRAE Transactions, vol. 107, no. 1, January 2001, pp. 4-13.
  • Fang, X., C. W. Bullard, and P. Hrnjak. "Heat Transfer and Pressure Drop of Gas Coolers." ASHRAE Transactions, vol. 107, no. 1, January 2001, pp. 255-266.
  • McEnaney, R. P. and P. Hrnjak. "Control Strategies for Transcritical R744 Systems." Journal of Passenger Cars, vol. 109, no. 6, 2001, pp. 1711-1720.
  • Drozdek, J., J. Chappel, C. Cusano, P. Hrnjak, N. R. Miller, and T. A. Newell. "Methods for Detection of Lubrication Failure Applied to a Swashplate Compressor." Journal of Passenger Cars, vol. 109, no. 6, 2001, pp. 1553-1557.
  • Stoecker, W. F., P. Hrnjak, and C. A. Infante Ferreira. "Application of Ammonia and CO2 in Supermarkets." Koude and Luchtbehandeling, vol. 94, no. 8, 2001, pp. 17-20.
  • Rodarte, E., G. Singh, N .R. Miller, and P. Hrnjak. "Sound Attenuation In Tubes Due to Visco-thermal Effects." Journal of Sound and Vibration, vol. 231, no. 5, April 2000, pp. 1221-1242.
  • Rodarte, E., G. Singh, N. R. Miller, and P. Hrnjak. "Refrigerant Expansion Noise Propagation through Downstream Tube Walls." Journal of Passenger Cars, vol. 108, no. 6, 1999, pp. 2224-2234.
  • Rodarte, E., N. Miller, and P. Hrnjak. "Acoustic Resonance in Plate Heat Exchangers." International Journal of Refrigeration, vol. 21, no. 8, December 1998, pp. 626-638.
  • Peixoto, R. A. and P. Hrnjak. "Elimina￿ de CFCs na Refrigera￿ de Supermercados." Abrava, Sao Paolo, Brazil, March/April 1998, pp.50-58.
  • Vujic, S., M. Markoski, and P. Hrnjak. "On Generalization of Test Results for Air Coolers with Moisture Extraction." KGH, 4, Belgrade, Yugoslavia (in Serbocroatian), 1983.

Research Honors

  • 2014 Fellow ASME, American Society of Mechanical Engineers
  • 2013 Fellow SAE, Society of Automotive Engineers
  • 2012 Academician - Academy of Engineering Sciences AINS Serbia
  • 2012 J&E Hall Medal, IoR (Institute of Refrigeration, London f. 1899) This single annual award recognizes the most noteworthy practical contribution to the science of refrigeration.
  • 2011 Gustav Lorentzen Medal, IIR (International Institute of Refrigeration, Paris f. 1908). The prize is awarded every four years to one person with outstanding and original achievements in academic or industrial research, innovation or development, in all fields of refrigeration, thus promoting creativity and renewal.
  • Fellow ASHRAE, American Society of Heating, Refrigeration and Air Conditioning Engineers
  • 2008 The Ritter von Rittinger award. The highest international award in the air conditioning, heat pump and refrigeration field by IEA. This award highlights outstanding contributions to the advancement of international collaboration in research, policy development and applications for energy-efficient heat pumping technologies. It is awarded every three years. The Rittinger award is named for Peter Ritter von Rittinger who is credited with the design and installation of the first energy-conserving heat pump system at a salt works in Upper Austria in 1855.
  • 2004-2005 Environmental Excellence in Weapon System Acquisition (EEWSA) Award - US Army. The award is the Army￿s highest honor for outstanding environmental stewardship during the acquisition of weapon systems that will support the nation￿s military objectives.