| Summary: | 博士 === 國立臺灣大學 === 機械工程學研究所 === 93 === The performance of a refrigerating system with an environment-friendly refrigerant, propane（R-290）as the refrigerant, was experimentally studied. There were two evaporators connected in series within the system. The objective of the present study is to conform to the environmental protection and to increase the efficiency of the system.
The system is mainly composed of a reciprocating compressor, a condenser, high- and low-temperature capillary tubes, and high- and low-temperature evaporators. The experimental results show that the mass flow rate of the refrigerant increases with the condensing pressure and the subcooling degree of the refrigerant, decreases with the lengths of the high- and low-temperature capillary tube, and increases slightly with the frequency of the compressor. The heat transfer coefficient of the refrigerant in the high-temperature evaporator decreases with the condensing pressure, increases slightly with the length of the high-temperature capillary tube, decreases with the length of the low-temperature capillary tube, and changes not apparently with the frequency of the compressor. The heat transfer coefficient of refrigerant in the low-temperature evaporator decreases slightly with the condensing pressure, decreases with the length of the high-temperature capillary tube, changes not apparently with the length of the low-temperature capillary tube, and increases with the frequency of the compressor. The cooling capacity of the high-temperature evaporator decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, decreases with the low-temperature capillary tube, changes not apparently with the frequency of the compressor. The cooling capacity of the low-temperature evaporator decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, changes not apparently with the length of the low-temperature capillary tube, and increases with the frequency of the compressor. The coefficient of performance（COP）of the system decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, decreases with the length of the low-temperature capillary tube, increases with the frequency of the compressor.
Based on Buckingham Pi theorem, this dissertation derives the dimensionless correlations for the characteristics of a series-connected two-evaporator refrigerating system with propane (R-290) as the refrigerant. Experimental data are substituted into the correlations to show the most relevant factors. Simplified correlations are then obtained.
The results show that the mass flow rate of refrigerant ( ) is mainly affected by the condensing pressure, the length of the high-temperature capillary tube, and the subcooling of refrigerant, while the heat transfer coefficients of refrigerant in the evaporators (hr,He and hr,Le) are affected by the condensing pressure and the logarithmic-mean temperature difference of the specific evaporator. However, hr,He and hr,Le are also affected by the lengths of the low- and high-temperature capillary tube, respectively. Additionally, the ratio of the cooling capacity of the high-temperature evaporator to the total capacity ( ) is mainly affected by the condensing pressure and the logarithmic-mean temperature difference of both evaporators. The COP of the system is mainly affected by the condensing pressure, the frequency of the compressor, and the logarithmic-mean temperature difference of both evaporators.
Compared with the results of the study, the predicted results by some correlations from literatures about the mass flow rate and the two-phase heat transfer coefficient of the refrigerant have the same consistency. However, the required variables of the correlations in this study are much fewer than those of the correlations from literatures.
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