Transient pressure drop cross-correlation during flow boiling of R134a in parallel minichannels

Abstract

This work examines the flow boiling of R134a in 0.54-mm square parallel minichannels with a particular focus on analyzing the transient pressure drop of individual channels in a wide range of heat and mass flux conditions. The flow rates are in the range from 0.1 to 0.51 g/s and heat fluxes are up to 18 kW/m2. Three different types of experiments are conducted, aiming to study the interaction between flow boiling phenomena in parallel minichannels based on the transient pressure drop measurements in each channel independently. Furthermore, cross-correlation analysis is undertaken to quantify the degree of interrelationship between neighboring minichannels. Flow boiling phenomena in a single, diabatic channel influences neighboring adiabatic channels by causing out-of-phase oscillations of single-phase pressure drops. The percentage of single-phase (ΔP) data correlated with two-phase ΔP data increases from 40% to 85% as the flow rate increases from 0.1 g/s to 0.5 g/s. Experiments with nonuniformly heated parallel minichannels show the presence of strong interaction between channels having the highest heat flux difference among a set of channels at a fixed total flow rate. As the flow rate increases and pressure drop across each channel increases, flow boiling behavior in each channel tends to be more independent from other channels. Therefore, less inter-channel interaction indicates better two-phase flow distribution between parallel minichannels, and the cross-correlation coefficient can be used as an indicator of the flow maldistribution.