Effect of Tube Diameter on Flow Pattern and Flow Boiling Characteristic of R32 in Horizontal Small-Diameter Smooth Tubes

Abstract

This study was conducted to investigate the effect of tube diameter on flow pattern, boiling heat transfer, and frictional pressure drop characteristics of R32 in horizontal smooth small-diameter tubes with 3.5 and 2.2 mm inner diameter. The boiling heat transfer coefficients were measured over a range of mass velocities from 50 to 400 kg/(m ²s), over a range of heat fluxes from 5 to 20 kW/m ², and at a saturation temperature of 15°C. The frictional pressure drop for adiabatic two-phase flow was measured over a range of mass velocities from 100 to 400 kg/(m ²s) and over a range of vapor qualities from 0.1 to 0.9. The flow patterns for adiabatic two-phase flow were observed at the exit of the test tubes for plug, wavy, and annular flows. Plug flow was observed over a wide range of vapor qualities with decreasing the tube diameter. The effect of tube diameter on the heat transfer and frictional pressure drop under low mass velocity conditions was larger than that under high mass velocity conditions.