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• W657197872 abstract "The present study examined design issues of microchannel evaporators associated with the flow maldistribution caused due to header pressure gradient. The effects of mass flow maldistribution on microchannel heat exchanger were quantified by using a single port simulation model. Longitudinal headers as currently used with microchannel condensers may not be suitable for the applications with microchannel evaporators. Mass flow maldistribution was investigated by using well-known pressure drop and heat transfer correlations for two-phase flow, selected after comparing with the results obtained by using different correlations. It was found that mass flow maldistribution cannot be controlled by changing either port/header diameter or the refrigerant state at the inlet to the port, but only by minimizing pressure gradients along header. At the same time the need to avoid phase separation in the header places a lower bound on mass flux. The dependence of heat exchanger capacity on the diameter and the length of the longitudinal and radial headers and was investigated in detail. Results demonstrated that the requirement for inertially dominated flow in the inlet header severely limits the set of feasible evaporator geometries. Accordingly, alternative concepts for header design were suggested. Introduction Compact cross-flow heat exchangers with flat multi-port microchannel tubes and folded louvered fins have almost completely replaced conventional round tube flat fin condensers in automotive air-conditioning applications. Nearly all have two vertical headers partitioned to accommodate 3-5 passes consisting of multiple parallel tubes. Since void fractions exceed 90% in most of the headers, achieving near-uniform vapor distribution among hundreds of parallel refrigerant parts has not presented serious problems for condensers. However in evaporators the challenge is to distribute the liquid evenly among the microchannel ports. Because of the high liquid/vapor density ratio of fluorocarbon refrigerants, void fractions are generally ~ 90% at the evaporator inlet and exceed 90% at intermediate headers in higher-quality regimes. Experimental data on microchannel evaporators are limited. Stott and Bullard [1] conducted experiments with microchannel evaporator (with port diameters ~ 0.7 mm), fed at four locations along the horizontal inlet header. They quantified maldistribution by measuring superheat on individual microchannel tubes, observing that some had nearly zero approach temperature differences while other exits were saturated, despite the TXV holding the aggregate suction superheat at 5°C. The tubes showing highest exit superheat were the ones that received most of vapor at their inlets. The experiments repeated for different refrigerant flow rates showed the same trend. In all the experiments, inlet header mass flux was less than 20% of the needed to maintain inertially-dominated fullydeveloped flow. Little is known about the flow regimes in developing two-phase flow, but it seems likely that the refrigerant would be highly susceptible to stratification at mass velocities significantly less than the 265 kg/m-s required for annular flow at these conditions [2]. Cho et al. [3] investigated flow maldistribution and phase separation in a microchannel evaporator with 15 tubes and aluminum header pipe with outer diameter of 22 mm at the mass flux of 60 kg/m-s. Mass flow rates were determined in each microchannel tube, along with the ratio of the quality in each microchannel tube to inlet quality" @default.
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• W657197872 date "2002-06-01" @default.
• W657197872 modified "2023-09-27" @default.
• W657197872 title "Refrigerant-Side Tradeoffs in Microchannel Evaporators" @default.
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