Visual investigation of boiling crisis in subcooled flow boiling

Abstract EN

The present work collects the main results obtained in experimental research carried out at the laboratories of the National Institute of Thermal–Fluid Dynamics of ENEA.

A photographic study of the burnout in highly subcooled flow boiling has been performed, in order to get a detailed description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), subcooled flow boiling, and thermal crisis.

In particular, the flow visualization is focused on the phenomena occurring on the heated wall during the thermal crisis up to the physical burnout of the heater.

Vapor bubble parameters are measured from flow images recorded (single frames were taken with a stroboscope light and an exposure time of 1 μs), while the wall temperature is measured by an indirect method, by recording the heater elongation during all flow regimes studied.

The combination of bubble parameters and wall temperature measurements as well as direct observations of the flow pattern, for all flow regimes, are collected in graphs which provide a useful global point of view of boiling phenomena, especially during boiling crisis.

Under these conditions, a detailed analysis of the mechanisms leading to the critical heat flux is reported, and the so called events sequence, from thermal crisis occurrence up to heater burnout, is illustrated.

A qualitative analysis of the behavior of three test sections with different surface finish is also presented, showing the importance of this parameter on the dynamics of the bubbles and on the critical heat flux.