Enhanced critical heat flux of pool boiling by vapor management

Abstract

The critical heat flux (CHF) is an important parameter in pool boiling, establishing the limit in heat flux and temperature in most applications. A higher CHF offers the system a wider operational range or an additional safety factor under normal conditions. In this work, a new vapor management approach is adopted by using a chimney-like vapor shroud during the pool boiling process. The vapor shroud footprint is equal to the area of the heated surface, and it is placed directly above the heated surface with a specified gap between its lower edge and the surface. The CHF increases for most vapor shroud arrangements (shroud length and gap), with a maximum increase of 83% compared to the baseline. The CHF shows a monotonic increase with increasing shroud length and passes through a maximum (unimodal) as the gap is changed. The maximum CHF occurs for gaps ranging from 1.8 mm to 3.1 mm, corresponding to a Bond number of approximately one, based on gap size. The increase in CHF is believed to be caused by improved liquid feeding and vapor release dynamics caused by the shroud. Analysis shows that the hydrostatic pressure and the interfacial stability are important to the nonlinearity in CHF behavior. A simple model based on scale analysis has been provided to predict the shroud effects on the CHF.