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.
Frost, defrost, and refrost and its impact on the air-side thermal-hydraulic performance of louvered-fin, flat-tube heat exchangers
The thermal-hydraulic performance under conditions of an initial frost growth on the air-side surface, and for subsequent ‘refrosting’ after a defrost period is experimentally studied for folded-louvered-fin, microchannel heat exchangers. In total, five heat exchangers are considered; the thermal performances during one frost-growth cycle for four different fin geometries are compared in terms of overall heat transfer coefficient, pressure drop, and j and f factors; the defrost and refrost characteristics of two heat exchangers are compared to explore geometry effects. Typically, the performance under refrosting conditions becomes periodic and repeatable after the third or fourth refrosting cycle. The allowable frost growth period (before a defrost is required), the defrost requirement, and the thermal-hydraulic performance depend on heat exchanger geometry for the specimens used in this study.
Slotted Waveguide Antenna With Two Radiation Nulls
A design of a linear array antenna having two radiation nulls and otherwise omnidirectional radiation pattern is reported. A new configuration of a waveguide broad wall longitudinal slot doublet with positive and negative offsets with respect to the waveguide center line is used to design the array. This type of doublet produces two radiation nulls. Each of the null directions can be shifted at least up to plusmn25deg by proper choice of slot offset. Resonance and radiation characteristics of the proposed doublet are also presented. Finally, one linear array antenna consisting of seven doublets has been fabricated. Measurements show that the 15 dB null widths are 14.2deg and the null depths are at least 24.8 dB. Maximum gain ripple in the omnidirectional region is 1.75 dB.