고려대학교

초록/요약

The objective of this study is to investigate the heat and mass transfer characteristics for spirally-coiled circular fin-tube heat exchangers used as evaporators for the refrigeration system under both frosting and non-frosting conditions. Spirally-coiled circular fin-tube heat exchangers were designed and tested by varying the fin pitch and number of tube rows under both frosting and non-frosting conditions. The heat and mass transfer characteristics of the spirally-coiled circular fin-tube heat exchangers were analyzed and then compared with those of the flat plate fin-tube heat exchangers with discrete fins. Empirical correlations were developed under both frosting and non-frosting conditions. This thesis consists of three parts. The first part was on the air-side heat transfer characteristics of a spirally-coiled circular fin-tube heat exchanger under non-frosting conditions. The second part was on the frost growth characteristics of a spirally-coiled circular fin-tube heat exchanger with time. The last part was on the heat and mass transfer characteristics of a spirally-coiled circular fin-tube heat exchanger under frosting conditions. Under non-frosting conditions, the j-factors of the tested heat exchangers were measured by varying the fin pitch, number of tube rows, and fin alignment. The j-factors of the spirally-coiled circular fin-tube heat exchangers were analyzed as a function of heat exchanger geometries and then compared with those of the flat plate fin-tube heat exchangers with discrete fins. Two empirical correlations of the j-factors were developed separately for the inline and the staggered fin alignment as a function of the Reynolds number, number of tube rows, and dimensionless fin pitch normalized by the hydraulic diameter. Under frosting conditions, the frost growth characteristics of a spirally-coiled circular fin-tube heat exchanger of the type used as evaporators in household refrigerators were investigated by varying the relative humidity (or humidity ratio), air flow rate (or Reynolds number), inlet air temperature, and fin pitch. Frost thickness and growth were observed in the transverse and circumferential directions. Frost thickness was affected primarily by relative humidity (humidity ratio), inlet air temperature, and air flow rate. Relative humidity had a major effect among all parameters tested. In addition, fin pitch also was shown to be critically related to boundary layer interruption due to frost growth and air flow rate reduction over time. The heat transfer rate, pressure drop, frost thickness, and Nusselt number of the heat exchanger were measured and analyzed by varying the fin pitch and number of tube rows under frosting conditions. In addition, the Nusselt number of the spirally-coiled circular fin-tube exchanger was compared with that of the flat plate fin-tube heat exchangers having discrete fins. An empirical correlation of the Nusselt number was developed as a function of the Reynolds number, dimensionless hydraulic diameter normalized by the fin pitch, Dh/Fp, Fourier number, and number of tube rows. The measured Nusselt number was consistent with the predicted value within mean and average deviations of 3.6 and -0.3%, respectively.