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.
Effect of Louver Angle on Performance of Heat Exchanger With Serpentine Fins and Flat Tubes in Periodic Frosting
This paper presents the results of an experimental study on the air-side pressure drop and overall heat transfer coefficient characteristics for serpentine-louvered-fin, microchannel heat exchanger in periodic frosting. It focuses on quantification of the effects of louver angle on heat transfer and pressure drop and on defrost and refrost times. Nine heat exchangers differing in louver angle and fin pitch (i.e. louver angle 15o to 39o and fin pitch 15 to 18 fpi) are studied. The face velocity was 3.5 ms/ and inlet air relative humidity of 70% and 80%. Effect of fin pitch and louver pitch on initial Colburn j0 factor and Fanning friction f0 factor during the start of the first frosting cycle are reported, and compared to the prediction by the Chang and Wang (1997).
Analysis of a coaxial-to-waveguide adaptor including a discended probe and a tuning post
A field matching analysis of a coaxial-to-waveguide adaptor incorporating a disc-ended probe and a tuning post is presented. In this analysis, the tuning post is considered as a short-circuited second probe. A computer algorithm for calculating the input impedance of this structure is developed. This algorithm is used to investigate and improve the operation of a commercially available coaxial-to-waveguide adaptor. >