LAUSR

Abstract This study involves experimental investigation of key parameters influencing CHF for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Close examination of the data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressures combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions.