Experimental comparison of the rewet phenomenon in transient and stable film boiling in low pressure and low flow conditions

An experimental dataset has been collected for vertical boiling flow in a square channel to investigate the film boiling phase following critical heat flux (CHF) in subcooled conditions. The experimental data explores a combination of atmospheric and low pressure scenarios with varying flow rate and subcooling. The conditions are a combination of transient and stable film boiling scenarios at 5 °C and 15 °C subcooled and mass flux values of 93.7 kg/m²-s and 187.4 kg/m²-s. For transient conditions, the power to the heated surface is shut off at the moment CHF is reached, allowing a cooling excursion through the transition regime, film boiling, and return to nucleate boiling. In the stability cases, the power supply to the heated surface is lowered in small increments throughout the film boiling region. This results in a full film development at stable points as the power level is stepped down until the rewet excursion is observed. The boiling curve and other system parameters for each condition are presented along with the conditions at which rewet back to nucleate boiling occurs. The study reveals the differences between transient and stable film boiling scenarios as an impact on the rewet conditions. Stable film boiling scenarios are found to rewet at a lower temperature than in a transient case. In addition, an increase in the flow rate or an increase in the subcooling of the system both correlate with an increase in the heat flux at rewet. Further investigation utilizes high-speed imaging to characterize the flow patterns along the full development of film boiling and the rewet phenomena described within each system condition.