Heat Transfer Characteristics of Horizontal Two-Phase Flow Boiling in Low-Pressure Low-Flow (LPLF) Conditions

To date, two-phase flow boiling has been extensively investigated for various working fluids and geometries, mainly under operating pressures and mass fluxes in the range of medium to high. However, very limited studies have been conducted, focusing on low-pressure low-flow (LPLF) conditions. Given insufficient experimental data available in the literature, most of the existing empirical correlations fail to properly predict boiling heat transfer coefficients (BHTCs) in LPLF conditions, highlighting the need for further experimental investigations. The present study experimentally investigates the heat transfer performance of single-phase and two-phase flow boiling of distilled water in a horizontal conventional tube at constant wall heat flux under LPLF conditions where the operating pressure is set to be subatmospheric and the mass flux ranges below 20 kg/m²-s. For the saturated flow boiling, the effects of mass flux and local vapor quality on the local BHTCs and Nusselt were evaluated, revealing that local BHTCs reach a peak at a certain range of vapor qualities between 55% and 75%, while increasing with the mass flux. It was also found that the impact of mass flux is stronger than that of vapor quality on the local BHTCs. The experimental results in the present study were then compared with several well-known empirical BHTC correlations in the literature to identify those with least deviations under the LPLF conditions. In contrast to single-phase flow, heat loss estimation and vapor quality measurement are known as one of the main error sources in characterizing heat transfer coefficients for two-phase flow boiling. Accordingly, the present study employs two approaches, in parallel, to reliably estimate heat losses, calibrate heat supplies, and measure local vapor qualities under the operating conditions investigated.