Experimental study on liquid-vapor two-phase pressure drop of pulsating flow in an evaporator

Pulsating liquid-vapor, two-phase, flows, characterized primarily by periodic oscillations in the mass flux, can occur naturally due to thermal-hydraulic conditions imposed on a flow, and they can be created by design in pursuit of improved thermal performance in heat pumping, air-conditioning, and refrigeration systems. Such flows are not fully understood, especially if the flow oscillations give rise to significant changes in liquid-vapor interface morphology (changes in two-phase flow regime). In this work, the pressure drop of R134a liquid-vapor, two-phase pulsating flows is measured under varying pulsation periods (2s to 24s), mass fluxes (100 to 200 kg.m(-2).s(-1)), and inlet/outlet vapor qualities (0.1-0.4/0.6-0.8). The flow regimes and liquid-vapor interface evolution are also observed. Experimental results show the pressure drop increases sharply at the start of flow and drops quickly as the flow ceases in a pulsation cycle. The peak of pressure drop as the pulsation starts is larger at a higher inlet vapor quality, but the valley as flow ceases is lower. With an increasing mass flux, the rate of pressure drop increase is closely related to the pulsation period and vapor quality. A higher rate of increasing pressure drop is observed in flows with a shorter pulsation period and higher inlet vapor quality. Fluctuations in the pressure drop are also observed, and are more pronounces in low mass flux and low inlet vapor quality flows, a result attributed to fluctuations in the liquid-vapor interface and changes in two-phase flow regimes. (C) 2020 Elsevier Ltd. All rights reserved.