CRITICAL HEAT-FLUX AND PRESSURE-DROP OF SUBCOOLED FLOW BOILING IN SMALL-DIAMETER TUBES WITH TWISTED-TAPE INSERTS

Critical heat flux(CHF) and pressure drop of small diameter tubes with twisted-tape inserts in subcooled flow boiling have been investigated experimentally. The influences of twisted-tape and test-section geome tries (twist ratio, tube diameter and length) and fluid thermal-hydrau lic conditions (mass flux, exit pressure, and inlet temperature) on bo th CHF and pressure drop have been analyzed and presented. Experiments were performed using stainless steel tubes with diameters ranging fro m 2.44 to 6.54 mm and aluminum twisted tapes (delta = 0.42 mm) with tw ist ratios from 1.9 to infinity. The gap between the twisted-tape edge and the tube wall was between 0.05-0.1mm. The experimental results sh ow significant enhancement of stable CHF by using twisted tapes with s mall twist ratios. Under high-mass-dux conditions, swirl-flow CHF is s hown to be inversely proportional to twist ratio, tube diameter, inlet temperature, and length-to-diameter ratio, but directly proportional to mass flux and exit pressure. Based upon the experimental data, an e mpirical subcooled swirl-flow CHF correlation, covering all six parame ters, is proposed for a wide range of parametric values. Pressure drop is strongly coupled to the conditions of CHF. The studies have reveal ed that the heat transfer enhancement achieved by means of twisted-tap e inserts is generally accompanied by an increase in pressure drop. In this study, the influences of the above six parameters and heat flux on pressure drop have been addressed. The principal parameters affecti ng both single- and two-phase pressure drop are twist ratio, tube diam eter, length-to-diameter ratio, and mass flux.