An experimental investigation of flow boiling instability of R245fa in a horizontal tube

Abstract

Two-phase pressure drop oscillations during flow boiling of pure fluid R245fa in a horizontal microchannel with a diameter of 0.9 mm were experimentally investigated. Three mass fluxes G = 400, 630, and 1100 kg/m2 s with three different heat fluxes q = 40, 50, and 55 kW/m2 were tested under the inlet pressure of 211.72 kPa and inlet subcooling of 17 K. Flow instabilities were analyzed based on images and temporal oscillation results of two-phase mass flux (G), pressure drop (ΔP), and inlet pressure (Pin). Four types of high-amplitude/low-frequency are identified, the main reasons for which are found to be flow patterns in the test section and upstream compressible volume of vapor. The inlet pressure oscillation is attributed to the mixture between expansion vapor and subcooled liquid in the inlet plenum. Effects of mass flux and heat flux on the pressure drop oscillation are also discussed through the variation trends on ΔP vs q/G and the fast Fourier transform.