Comparison of Frost and Defrost Performance Between Microchannel Coil and Fin-and-Tube Coil for Heat Pump Systems
This paper presents a comparison of frost and defrost cycling performance between a microchannel heat exchanger with louvered fin and a fin-and-tube heat exchanger with straight fins employed as outdoor coils of a 14 kW (48 000 Btu/h) heat pump system. In addition to temperature, pressure and flow rate measurements taken at various locations of the systems, the fin-base and tube wall surface temperature were also recorded by using fine-gauge precalibrated thermocouples on the coils. Further, load cells were used to measure the mass of frost accumulation during heating tests. Data showed that the frosting time of the microchannel heat exchanger is more than 50% shorter than for the fin-and-tube heat exchanger, which is chosen as the baseline system. The average heating capacity and system performance over a frost–defrost cycle are also lower for the system with microchannel heat exchangers. Higher frost growth rate was mainly due to augmented temperature difference between air and the surface of the heat exchanger, and preferential frost nucleation sites on the louvered fins and microchannel tubes. Removal of residual water in the microchannel heat exchanger did not improve the frost performance significantly. Blowing nitrogen on the microchannel coil after defrost removed any visible water retained in the coil after the defrost cycle but the cycle time increased only by 4% with respect to wet and frost conditions. The cycle time of the same microchannel coil starting with dry conditions was about 60% longer than the cycle time in wet and frost conditions.
Developing empirical correlations for frost thickness and air face velocity degradation for microchannel heat exchangers used in heat pump applications under frosting conditions
This study experimentally investigated the frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air-source heat pump systems. The effects of surface temperature and fin geometries on the performance of the microchannel heat exchangers under frosting condition were studied. Seven fin samples with various fin widths, fin heights, and fin densities and a louver angle of approximately 30° were tested in controlled laboratory conditions that replicated those of actual heat pump systems in the winter season. The fin surface temperature was experimentally measured with the novel methodology developed in the present study. Experimental data of local frost thickness, air pressure drop across the coils, time of frost–defrost cycles, and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that frosting time and frost growth rates depended mainly on the local fin surface temperature. A set of empirical correlations were developed to predict the frost thickness on the fin leading edge and the reduction of air face velocity due to air pressure drop across the frosted coil during frosting operation. The correlations were compared against experimental data of frost collected in the present study, and satisfactory agreement was achieved. The frost thickness correlation and air face velocity correlations aid in the calculation of the instantaneous air-side Reynolds numbers during frosting operation of the fin samples. These are critical for predicting the heat transfer rates of the microchannel coils in quasi-steady-state frosting operating conditions.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.