Keumnam Cho

Thermal characteristics of paraffin in a spherical capsule during freezing and melting processes

The objective of the present study was to investigate the thermal characteristics of paraffin in a spherical capsule during freezing and melting processes. Experiments were performed with paraffin, i.e., n-tetradecane, and a mixture of n-tetradecane (40%) and n-hexadecane (60%) and water. The parameters were the Reynolds number and the inlet temperature during the freezing process and initial temperature during the melting process for a PCM-thermal-storage system. The phase-change period for the capsule at the edge of a storage tank was shorter than that at the center of the storage tank due to smaller porosity at the center than the edge of the storage tank. Water showed a bigger of subcooling than paraffin. It took longer for water to reach the dimensionless thermal-storage capacity of 1 during the freezing process, and shorter during the melting process than that for paraffin. The average heat-transfer coefficients were affected by the inlet or initial temperature and Reynolds number more during the melting process than during the freezing process due to a natural-convention effect during the melting process. The average heat-transfer coefficients for paraffin were larger by a maximum of 40% than those for water during the freezing and melting processes.

Mass flow rate distribution and phase separation of R-22 in multi-microchannel tubes under adiabatic condition

The present study investigated mass flow rate distribution and phase separation of R-22 in multi-microchannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and 15 parallel multi-microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.32 mm. The key experimental parameters were the orientation of the header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow), and inlet quality (0.1, 0.2, and 0.3). The effect of inlet quality on the mass flow rate distribution and phase separation in the microchannel tubes was negligible. The effect of the orientation of the header on the mass flow rate distribution and phase separation was the largest among the test parameters. Horizontal header showed better mass flow rate distribution and phase separation characteristics than vertical header. Both parallel and cross-flow conditions showed better mass flow rate distribution and phase separation than in-line flow condition.

Condensation heat transfer for R-22 and R-407C refrigerant–oil mixtures in a microfin tube with a U-bend

Condensation heat transfer experiments for two refrigerants, R-407C and R-22, both mixed with polyol-ester (POE) and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters included oil concentration (varied from 0% to 5%), mass flux (varied from 100 to 400 kg/m2 s) and inlet quality (varied from 0.5 to 0.9). The enhancement factors (EFs) for both R-22 and R-407C refrigerants at the first straight section decreased continuously as the oil concentration increased. They decreased rapidly as mass flux decreased and inlet quality increased. The heat transfer coefficients had the maximum at the 90° position of the U-bend. The heat transfer coefficients in the straight section after the U-bend, within a length 48 times that of the inner tube diameter, were larger by a maximum of 33% than the average heat transfer coefficient in the straight section before the U-bend.

Effect of the aspect ratio of rectangular channels on the heat transfer and hydrodynamics of paraffin slurry flow

The objective of the present study was to investigate the effect of the aspect ratio of rectangular channels on the cooling characteristics of paraffin slurry flow with a linear array of discrete heat sources. Four key experimental parameters were the aspect ratio of the rectangular channel, the mass fraction of paraffin slurry, the heat flux of simulated VLSI chips, and the channel Reynolds number. The influence of the aspect ratio of the rectangular channel on the local heat transfer coefficients for both water and paraffin slurry at a heat flux of 40 W cm−2 was greater than at a low heat flux. The paraffin slurry with a mass fraction of 5% showed the most efficient cooling performance in a rectangular channel with an aspect ratio of 0.20 regardless of the heat flux and the channel Reynolds number.

Evaporation heat transfer for R-22 and R-407C refrigerant : oil mixture in a microfin tube with a U-bend

Abstract Evaporation heat transfer experiments for two refrigerants, R-407C and R-22, mixed with polyol ester and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters include an oil concentration varied from 0 to 5%, an inlet quality varied from 0.1 to 0.5, two mass fluxes of 219 and 400 kg m−2s−1 and two heat fluxes of 10 and 20 kW m−2. Pressure drop in the test section increased by approximately 20% as the oil concentration increased from 0 to 5%. Enhancement factors decreased as oil concentration increased under inlet quality of 0.5, mass flux of 219 kg m−2 s−1, and heat flux of 10 kW m−2, whereas they increased under inlet quality of 0.1, mass flux of 400 kg m−2 s−1, and heat flux of 20 kW m−2. The local heat transfer coefficient at the outside curvature of an U-bend was larger than that at the inside curvature of a U-bend, and the maximum value occurred at the 90° position of the U-bend. The heat transfer coefficient was larger in a region of 30 tube diameter length at the second straight section than that at the first straight section.

Mass Flowrate Distribution and Phase Separation of R-22 in Multi-Microchannel Tubes Under Adiabatic Condition

The present study investigated mass flowrate distribution and phase separation of R-22 in multimicrochannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and 15 parallel multi-microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.32 mm. The key experimental parameters were the orientation of the header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow) and inlet quality (0.1, 0.2 and 0.3). The effect of inlet quality on the mass flowrate distribution and phase separation in the microchannel tubes was negligible. The effect of the orientation of the header on the flow mass flowrate distribution and phase separation was the largest among the test parameters. Horizontal header showed better mass flowrate distribution and phase separation characteristics than vertical header. Both parallel and cross flow conditions showed better mass flowrate distribution and phase separation than in-line flow condition.Copyright

Performance comparison of microchannel evaporators with refrigerant R-22

An experimental study on the performance comparison of microchannel evaporators with refrigerant R-22 was conducted. Six microchannel evaporators were designed and manufactured for a residential air-conditioner. They were tested with psychrometric calorimeter test facilities. The experiment was performed with both vapor compression system and refrigerant circulation system. Each evaporator was made up of two parallel flow heat exchangers connected with several return pipes. The parallel flow heat exchanger had 41 microchannel tubes inserted between inlet and outlet headers. The microchannel tube had 8 rectangular ports with the hydraulic diameter of 1.3 mm. For the vapor compression system, the flow area ratio and the number of return pipes had a great effect on the cooling capacity. Type 3 with a flow area ratio of 73% and 58% showed the best cooling capacity. It had 12 return pipes and 3 circuits. There is a merging manifold in it. The effect of the number of circuits and merging manifold on the cooling capacity was relatively small. For the refrigerant circulation system, the effect of the mass flow rate on the cooling capacity was slightly superior to that of inlet quality. The effect of the number of circuits on the cooling capacity was different from the result of the vapor compression system. The effect of merging manifold was negligible, which was consistent with the result of the vapor compression system. The cooling capacity proportionally increased as the vertical inclination angle of the evaporator increased due to gravity force.

Two-phase flow distribution and pressure drop in microchannel tubes under non-heating and heating conditions

The present study investigated two-phase flow distribution and pressure drop in microchannel tubes under non-heating and heating conditions using R-22. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and had 15 or 41 parallel microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.3 mm. For non-heating condition, the key experimental parameters were orientation of the header, flow direction of refrigerant into the inlet header, and inlet quality. For heating condition, the key experimental parameters were inclination angle, mass flow rate, and inlet quality. It was found that the effect of the orientation of the header on the two-phase flow distribution and pressure drop was the largest under the non-heating condition. Two-phase pressure drops through the microchannel tubes with the horizontal header were higher than those of the microchannel tubes with the vertical header due to the gravitational effect. Under the heating condition, the cooling capacities of three prototype evaporators were changed as the mass flow rate and inlet quality were increased due to the flow mal-distribution.

Effect of Flow Direction on Two-Phase Flow Distribution of Refrigerants at a T-Junction

The present study experimentally investigated the effect of flow direction and other flow parameters on two-phase flow distribution of refrigerants at a T-junction, and also suggested a prediction model for refrigerant in a T-junction by modifying previous model for air-water flow. R-22, R-134a, and R-410A were used as test refrigerants. As geometric parameters, the direction of the inlet or branch tube and the tube diameter ratio of branch to inlet tube were chosen. The measured data were compared with the values predicted by the models developed for air-water or steam-water mixture in the literature. We propose a modified model for application to the reduced T-junction and vertical tube orientation. Among the geometric parameters, the branch tube direction showed the biggest sensitivity to the mass flow rate ratio for the gas phase, while the inlet quality showed the biggest sensitivity to the mass flow rate ratio among the inlet flow parameters.

Thermal characteristics of a multichip module using PF-5060 and water

The experiments were performed by using PF-5060 and water to investigate the thermal characteristics from an in-line 6 x 1 array of discrete heat sources for simulating the multichip module which were flush mounted on the top wall of a horizontal, rectangular channel of aspect ratio 0.2. The inlet temperature was 15°C for all experiments, and the parameters were the heat flux of simulated VLSI chips with 10, 20, 30, and 40W/cm2 and the Reynolds numbers ranging from 3,000 to 20,000. The measured friction factors for PF-5060 and water gave a good agreement with the values predicted by the modified Blasius equation within ±6%. The chip surface temperatures for water were lower by 14.4-21.5°C than those for PF-5060 at the heat flux of 30W/cm2. From the boiling curve of PF-5060, the temperature overshoot at the first heater was 3.5°C and was 2.6°C at the sixth heater. The local heat transfer coefficients for water were larger by 5.5-11.2% than those for PF-5060 at the heat flux of 30W/cm2, and the local heat transfer coefficients for PF-5060 and water reached a uniform value after the fourth row. This meant that the thermally fully developed condition was reached after the fourth row. The local Nusselt number data gave the best agreement with the values predicted by the Malina and Sparrow’s correlation and the empirical correlations for Nusselt number were provided at the first, fourth and sixth rows for a channel Reynolds number over 3,000.