Yongchan Kim

Heat transfer and pressure drop characteristics during the in-tube cooling process of carbon dioxide in the supercritical region

This paper presents the experimental data for the heat transfer and pressure drop characteristics obtained during the gas cooling process of carbon dioxide in a horizontal tube. The tubes in which carbon dioxide flows are made of copper with an inner diameter of 7.73 mm. Experiments were conducted for various mass fluxes and inlet pressures of carbon dioxide. Mass fluxes are controlled at 225, 337 and 450 kg m � 2 s � 1 by a variable speed gear pump, and the inlet pressures are adjusted from 7.5 to 8.8 MPa. The experimental results in this study are compared with the existing correlations for the supercritical heat transfer coefficient, which generally under-predict the measured data. Pressure drop data agree very well with those calculated by Blasius’ equation. Using the measured data, a new empirical correlation to predict the near-critical heat transfer coefficient has been developed. Most of the experimental data can be predicted by the new correlation within a relative deviation of � 20%. # 2003 Elsevier Ltd and IIR. All rights reserved.

Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection

A thermodynamic model for a variable speed scroll compressor with refrigerant injection was developed using continuity, energy conservation and real gas equation. The model included energy balance in the low-pressure shell compressor, suction gas heating, motor efficiency, and volumetric efficiency considering gas leakages as a function of compressor frequency. The developed model was verified by comparing the predicted results for the no injection condition with the experimental data. The deviations of the predicted from the measured values were within 10% for approximately 90% of the experimental data. Based on the model, mass flow rate, suction gas heating, cooling capacity and power consumption of the compressor were estimated and analyzed as a function of frequency. The effects of refrigerant injection on the performance of the compressor were also discussed as a function of frequency, injection conditions, and injection geometry.

Boiling heat transfer and dryout phenomenon of CO2 in a horizontal smooth tube

Abstract Evaporation heat transfer characteristics of carbon dioxide (CO 2 ) in a horizontal tube are experimentally investigated. The test tube has an inner diameter of 6.0 mm, a wall thickness of 1.0 mm, and a length of 1.4 m. Experiments are conducted at saturation temperatures of 5 and 10 °C, mass fluxes from 170 to 320 kg/m 2 xa0s and heat fluxes from 10 to 20 kW/m 2 . Partial dryout of CO 2 occurs at a lower quality as compared to the conventional refrigerants due to a higher bubble growth within the liquid film and a higher liquid droplet entrainment, resulting a rapid decrease of heat transfer coefficients. The effects of mass flux, heat flux, and evaporating temperature are explained by introducing unique properties of CO 2 , flow patterns, and dryout phenomenon. In addition, the heat transfer coefficient of CO 2 is on average 47% higher than that of R134a at the same operating conditions. The Gungor and Winterton correlation shows poor prediction of the boiling heat transfer coefficient of CO 2 at low mass flux, while it yields good estimation at high mass flux.

Modeling of frost growth and frost properties with airflow over a flat plate

A physical model of frost layer growth and frost properties with airflow over a flat plate at subfreezing temperature was developed. Frost roughness was measured, and an empirical correlation for the average frost roughness was suggested. Heat and mass transfer coefficients were calculated using the modified Prandtl mixing-length scheme containing the effects of both frost roughness and turbulent boundary layer thickness. Frost thermal conductivity was theoretically analyzed by solving the combined equations of air equivalent conductivity and thermal conductivity of the frost inner layer. Based on the present model, heat and mass transfer coefficient, frost thermal conductivity, frost thickness, frost mass concentration and frost density with time and space were estimated. The model showed good agreement with the basic trends of the test data taken from other literature. Spatial and temporal changes of heat flux and frost surface temperature were also investigated.

Evaporation heat transfer characteristics of R-410A in 7 and 9.52 mm smooth/micro-fin tubes

Abstract In-tube evaporation heat transfer characteristics of R-410A were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux, and tube geometry. Evaporation heat transfer coefficients and pressure drops were measured for 3.0 m long smooth and micro-fin tubes with outer diameters of 9.52 and 7.0 mm, respectively. The test matrix in the present study included measurements for evaporation over a refrigerant mass flux range of 70–211 kg/m2s, a heat flux range of 5–15 kW/m2 and an evaporating temperature range of −15 to 5. The objective of this study is to evaluate the heat transfer enhancement of the micro-fin tube with R-410A as a function of mass flux, heat flux, evaporating temperature and tube diameter.

Influence of liquid refrigerant injection on the performance of an inverter-driven scroll compressor

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°.

A generalized correlation for evaporation heat transfer of refrigerants in micro-fin tubes

A generalized correlation for flow boiling heat transfer in horizontal micro-fin tubes was developed by implementing non-dimensional parameters accounting for heat transfer enhancement over smooth tubes and physical phenomena into the basic form of a smooth tube correlation. The enhancement factor in nucleate boiling consists of surface tension and turbulence effects generated by the liquid flow. A modified Reynolds number having some similarities with the roughness Reynolds number is introduced into the model to estimate heat transfer enhancement in convective boiling. The ratio of the liquid film thickness to the fin height is also employed in the correlation. The database of the present correlation includes 749 data points for five different refrigerants. The present correlation has a relatively simpler form to employ and yields closer fit to the experimental data with a mean deviation of 20.5% as compared to the existing correlations in the literature.

A generalized correlation for refrigerant mass flow rate through adiabatic capillary tubes

Abstract A capillary tube is a common expansion device used in small sized refrigeration and air-conditioning systems. A generalized correlation for refrigerant flow rate in adiabatic capillary tubes is developed by implementing dimensionless parameters based on extensive experimental data for R-22, R-290, and R-407C measured in this study. Dimensionless parameters are derived from the Buckingham Pi theorem, considering the effects of tube inlet conditions, capillary tube geometry, and refrigerant properties on mass flow rate. The generalized correlation yields good agreement with the present data for R-22, R-290, and R-407C with average and standard deviations of 0.9 and 5.0%, respectively. Approximately 97% of the present data are correlated within a relative deviation of ±10%. Further assessments of the correlation are made by comparing the predictions with measured data for R-12, R-134a, R-152a, R-410A, and R-600a in the open literature. The correlation predicts the data for those five refrigerants with average and standard deviations of −0.73 and 6.16%, respectively.

An empirical correlation and rating charts for the performance of adiabatic capillary tubes with alternative refrigerants

With the phaseout of conventional refrigerants, refrigeration and air conditioning systems must be redesigned to improve system performance and reliability with alternative refrigerants. In this study, a generalized correlation for the prediction of refrigerant flow rate through adiabatic capillary tubes is developed by generating dimensionless parameters for operating conditions, capillary tube geometry, and refrigerant properties using the Buckingham Pi theorem. The database for the present correlation includes extensive experimental data for R12, R22, R134a, R152a, R407C, and R410A obtained from the open literature. The present correlation, which has a relatively simpler form to employ, yields a mean deviation of 5.4% and a standard deviation of 6.5% from the database. In addition, rating charts for predicting refrigerant flow rate through adiabatic capillary tubes are generated for R12, R22, R134a, R152a, R407C, and R410A.

Critical quality prediction for saturated flow boiling of CO2 in horizontal small diameter tubes

Abstract The dryout for flow boiling carbon dioxide (CO 2 ) in horizontal small diameter tubes is investigated through experiment and theoretical modeling. Tests are conducted in conditions where the saturation temperature is 0, 5, and 10 °C, heat flux is 7.2–48.1 kW/m 2 and mass flux is 500–3000 kg/m 2 xa0s. The dryout phenomena of CO 2 are similar with those of water in many respects, while the effects of mass flux on dryout show differences among them. The dryout of CO 2 is predicted by a theoretical dryout model, which is developed and verified with steam–water data. Two entrainment mechanisms of interface deformation and bubble bursting are considered in the model and dryout is determined when the liquid film thickness is less than the critical liquid film thickness, the criteria film thickness of dryout. The present model well predicts the experimental critical qualities except when mass flux is relatively high, at which the deposition of liquid droplet on the liquid film and the occurrence of dryout patches become very significant.