Vapor quality distribution in a microchannel heat exchanger Header: Impact of downward and horizontal distributor spraying

Abstract

The vapor quality distribution in the header of a microchannel heat exchanger significantly affects the refrigerant charge, distribution, pressure drop, and heat transfer efficiency. However, the distribution differs markedly with and without a distributor. This study examines the use of a 2 mm nozzle in a 5 mm inner diameter circular tube as a distributor in an inlet header, spraying R134a into the header. Capacitance measurements are analyzed to characterize the distribution under two orientations: downward and horizontal, and compare with a no-distributor configuration. The test conditions cover inlet vapor qualities from 0.1 to 0.6 and mass flow rates of 5, 7, and 10 g-s⁻¹. The local vapor qualities are calculated based on the capacitance. For the downward configuration, distribution is relatively uniform at lower vapor qualities as the flow rate increases. The local vapor quality on the left side of the nozzle typically exceeds that on the right. For the horizontal configuration, a bimodal distribution is observed, except at high flow rates and vapor qualities (${\dot{m}}_{\mathrm{in}}$ = 10 g-s⁻¹ and$x_{\mathrm{in}}$ > 0.4). For both configurations, vapor phase superficial velocity is the key parameter influencing distribution. A comparison shows that the downward configuration improves uniformity at the inlet vapor qualities below 0.4.