The visual experiment of boiling crisis triggering process during subcooled flow boiling

Boiling crisis is a very interesting and complex phenomenon, which has attracted wide attention. But even so, the boiling mechanism remains unclear, and there is still further work that should be carried out. In this paper, a visual experiment is carried out to investigate the vapor-liquid interfacial features near the heating wall in subcooled flow boiling for revealing the mechanism of boiling crisis. The test section is a square channel with a cross section of 19 x 19 mm2, and the outer diameter of the heating rod is 9.5 mm. Based on high-speed photography, combined with images processing techniques, characteristics of bubble behaviors near the heating wall region during the critical heat flux (CHF) transient are detailedly analyzed under different working conditions, including the local bubble coalescence, large vapor agglomeration, local rewetting, periodic wavy vapor layer, and liquid film. In particular, through the qualitative and quantitative analysis of the vapor-liquid interfacial features close to the CHF, a different understanding of boiling crisis triggering is presented in this paper. The experimental results show that the thickness of the vapor layer has an indirect effect on the boiling crisis, and the wavelength of the vapor layer and the liquid film are the key parameters for the developing boiling crisis trigger mechanism.