Pressure drop with highly subcooled flow boiling in small-diameter tubes

Pressure drop may be the most important consideration in designing heat-removal systems utilizing high-heat-flu subcooled boiling. In this study, an experimental investigation was performed to identify the important parameters affecting pressure drop across small-diameter tubes in highly subcooled flour boiling. The effects of five parameters-mass flux, inlet temperature, exit pressure, tube internal diameter, and length-to-diameter ratio-on both single-and two-phase pressure drop were studied and evaluated. Experiments were carried out with tubes having inside diameters ranging from 1.05 to 2.44 mm. Mass fluxes ranged from 25,000 to 45,000 kg/(m(2) s), exit pressures from 4 to 16 bar, and inlet temperatures from 22 to 66 degrees C. Two length-to-diameter ratios were tested. These conditions resulted in critical heat flux levels of 50-80 MW/m(2). The experimental results indicate that mass flux, tube diameter, and length-to-diameter ratio are the major parameters that alter the pressure-drop curves. Both single-and two-phase pressure drops increase with increasing mass flux and length-to-diameter ratio but decrease with increasing internal diameter. Inlet temperature and exit pressure have been shown to have significant effects on two-phase pressure drop but very small effects on single-phase pressure drop. These results agree well with those from other investigations under similar conditions. As a result of this study, pressure-drop correlations are presented for predicting bath single-phase and subcooled boiling pressure drop in small-diameter tubes under different heat-flux conditions. (C) Elsevier Science Inc., 1997.