An experimental investigation of two-phase pressure drop in small diameter horizontal channels

In recent years research has increasingly focused on two-phase heat transfer and fluidynamics inside mini and microchannels. Experience of macroscale convective heat transfer inside channels has shown that, at a certain mass velocity, higher heat transfer coefficient can be obtained by reducing the hydraulic diameter at the expense of increasing the frictional pressure drop. Although this simple mechanism suggests that reducing the channel diameter is advantageous, it is not clear to what extent the know-how on macroscale heat transfer and fluidynamics remains valid in the microscale setting. In particular, there is a need to validate design correlation for two-phase pressure drop in order to facilitate the design and optimisation of compact heat exchangers for use with refrigerants. The present study has therefore two objectives: (a) the collection of several pressure drop correlations suitable for two-phase flow in minichannels; (b) the experimental analysis of two-phase pressure drop of a dielectric fluid flowing in a horizontal small diameter channel for different vapour qualities and mass flow rates. As a working fluid we used HFE-7100 (C4F9OCH3), a dielectric fluid that has been recently proposed to replace FC-72 (C6F14), as it has slightly better thermophysical properties and better environmental characteristics (lower GWP). The experimental apparatus consists of a two-phase loop feeding the test section with an assigned mass flow rate (G = 100 ¸ 500 kg/m2s) at fixed saturation pressure and vapour quality. The test section consists of a horizontal small diameter channel, 760 mm long, inside which the dielectric fluid flows adiabatically. The cross-section of the channel is circular with a ID of 4 mm. Two-phase pressure drop along the test section was measured by means of 4 pressure taps placed at various distances from the inlet (distance = 30; 250; 570; 730 mm), which connect the channel to a pressure acquisition circuit. To visualise the flow, the channel is made of Plexiglas. The experimental data show that, as is well known, the pressure drop increases with quality and mass flow rate. For each mass flow rate, the slope of the pressure drop/quality curve changes at a certain x value, probably because the flow pattern inside the channel changes according to the vapour quality. The experimental frictional pressure drop data were compared with 18 different correlations and models for predicting the two-phase frictional pressure gradient available in literature. Generalised two-phase pressure drop correlations were utilised for comparison, including correlations developed for both macro and minichannels. Experimental data were also analysed by means of a model based on flow regime, i.e. by means of a phenomenological frictional pressure drop model based on the two-phase flow structure of all various flow regimes. In general, correlations underpredict experimental pressure drop and, as reported in literature by several authors, lead to predictions that significantly differ from one another. For experimental data presented in this study, the predictive methods were evaluated according to two criteria: the fraction of data predicted to within 30% and the mean absolute error. Various correlations and predictive methods were so ranked.