摘要

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 geometries (twist ratio, tube diameter and length) and fluid thermal-hydraulic conditions (mass flux, exit pressure, and inlet temperature) on both CHF and pressure drop have been analyzed and presented. Experiments were performed using stainless steel tubes with diameters ranging from 2.44 to 6.54 mm and aluminum twisted tapes (δ = 0.42 mm) with twist ratios from 1.9 to ∞. The gap between the twisted-tape edge and the tube wall was between 0.05–0.1 mm. The experimental results show significant enhancement of stable CHF by using twisted tapes with small twist ratios. Under high-mass-flux conditions, swirl-flow CHF is shown to be inversely proportional to twjst 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 empirical subcooled swirl-flow CHF correlation, covering all six parameters, is proposed for a wide range of parametric values. Pressure drop is strongly coupled to the conditions of CHF. The studies have revealed that the heat transfer enhancement achieved by means of twisted-tape 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 affecting both single- and two-phase pressure drop are twist ratio, tube diameter, length-to-diameter ratio, and mass flux.