Refrigerant distribution effects on the performance of microchannel evaporators.
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant maldistribution as they typically have many parallel channels fed by a common header or manifold. This paper outlines general trends in refrigerant flow distribution in such evaporators taking into account various heat exchanger orientations. In addition, links between heat exchanger performance and refrigerant flow distribution characteristics in microchannel evaporators are made using both analytical and experimental methods. These results show that unification of the liquid refrigerant flow distribution in the evaporator can lead to quantifiable improvements in evaporator performance.
Improvement of Two-Phase Flow Distribution in Compact Heat Exchangers by Using Ultrasound
In this communication we present a new active method to modify two-phase flow distribution in a heat exchanger, when it is in operation, by using ultrasound generators which can be activated when necessary. An experimental study has been carried out to validate the concept and to evaluate the effects of ultrasound on the flow distribution. An experimental test rig was built to measure the flow distribution in realistic manifold and parallel channel geometry. The test section is composed of a manifold feeding 10 channels with air-water mixture. In front of each channel a piezoelectric generator is placed to emit ultrasonic waves which can interact with liquid flows. Comparison was made between two-phase flows with and without ultrasound. It was demonstrated that, in most cases, uneven distribution was improved by using ultrasound.
Mass Flowrate Distribution and Phase Separation of R-22 in Multi-Microchannel Tubes Under Adiabatic Condition
The present study investigated mass flowrate distribution and phase separation of R-22 in multimicrochannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and 15 parallel multi-microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.32 mm. The key experimental parameters were the orientation of the header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow) and inlet quality (0.1, 0.2 and 0.3). The effect of inlet quality on the mass flowrate distribution and phase separation in the microchannel tubes was negligible. The effect of the orientation of the header on the flow mass flowrate distribution and phase separation was the largest among the test parameters. Horizontal header showed better mass flowrate distribution and phase separation characteristics than vertical header. Both parallel and cross flow conditions showed better mass flowrate distribution and phase separation than in-line flow condition.Copyright © 2003 by ASME