| Summary: | 碩士 === 華梵大學 === 機電工程研究所 === 88 === In this study, the heat transfer and pressure drop performance of flat plate finned tube heat exchangers operating under frosting conditions was investigated by experiments. The experiment was conducted to show the effects of various geometric and environmental parameters on the thermofluid performance of the heat exchangers. The geometric parameters include the fin pitch and the row number. Environmental parameters include inlet air temperature, relative humidity of inlet air, air flow rate and inlet refrigerant temperature.
The experimental results show that, under a fixed geometric parameter condition, the heat transfer rate of the heat exchangers at the initial stage of frosting increases with the increase in the air flow rate, inlet air temperature and humidity or decrease in the inlet refrigerant temperature. The drop rate of the heat transfer increases with inlet air temperature or the air flow rate. Because the variations of the heat transfer rate, overall heat transfer coefficient U and pressure drop ΔP are function of the frost growth. Therefore, the parameters which affect the U and ΔP are the same to those which affect the variations of the heat transfer rate. If the relative humidity of the air is fixed, the variations of , U and ΔP are more significant under Ta,i=5℃. When inlet air temperature is higher or lower than 5℃, the amount of frost accumulation will be leas than that of Ta,i=5℃.
For the effects of the row number (N), the heat transfer increases with the increase in the row number. But the overall heat transfer coefficient shows the inverse trend. The drop rate of overall heat transfer coefficient decreases with the increase in the row number. Additionally, the rise rate of pressure drop decreases with the increase of row number. But under f=60% or Tr,i=-10℃ condition, the rise rate of pressure drop increases with N.
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