Two-phase pressure drop and void fraction in a cross-corrugated plate heat exchanger channel: Impact of flow direction and gas-liquid distribution

Abstract

Abstract Two-phase pressure drop is studied in a transparent cross-corrugated channel for uniform and non-uniform gas-liquid distribution. Both uniform and non-uniform two-phase distributions are created by injecting air and water through different numbers of nozzles directly into the channel. The frictional pressure drop clearly indicates the impact of maldistribution. The experimental data is analyzed by using previously published flow pattern visualizations done in the same test channel. The single channel is pivot-mounted to allow horizontal, vertical upward and downward flow. In general, two-phase frictional pressure drop can be directly measured only for horizontal flow, neglecting acceleration pressure drop. In vertical flow, a void-fraction model is necessary to deduce the gravitational pressure drop from the measured pressure difference. Buoyancy is known to impact void fraction and two-phase pressure drop in straight channels. However, the cross-corrugated geometry generates a swirling motion inducing centrifugal forces often much larger than buoyancy force. Therefore, buoyancy generally has less impact in cross-corrugated channels. The ratio of buoyancy to centrifugal forces is expressed by the corrugation Froude number. The friction factors of the air-water mixtures are compared to single-phase flow for the different flow directions. In this manner, homogeneous flow is distinguished from slip. For uniform two-phase distributions, the validity of the homogenous model is experimentally confirmed for volumetric gas flow ratios up to 0.7. For non-uniform gas injection, this is only the case for gas flow ratios up to 0.25. A correlation of the single-phase friction factor is given and found also to be valid for homogeneous two-phase flow. Two-phase multipliers are calculated from the experimental data and compared to correlations from literature. The differential pressure measurements are used to determine the void fraction. The results are compared to published void fraction correlations introduced or applied for cross-corrugated plate heat exchangers in the literature. In general, the homogeneous model fits best for uniform two-phase distribution. For gas-liquid maldistributions, however, the correlations of Margat et al. (1997) and of Rouhani and Axelsson (1970) predict the experimental void fractions reasonably well. None of the investigated models is able to predict the void fractions for small flow rates.