Rainbow schlieren-based simultaneous mapping of single vapour bubble dynamics and temperature gradients during nucleate flow boiling in a vertical channel

Dynamic parameters of single vapor bubble and the associated thermal gradients in water-based nucleate flow boiling configuration have been simultaneously mapped using rainbow schlieren deflectometry. Flow boiling experiments (upward as well as downward flow) have been conducted in a rectangular cross-section channel for varying Reynolds numbers under subcooled conditions. Experimental results are recorded in the form of real-time schlieren images and interpreted to understand the underlying physics of various phenomena such as bubble growth, bubble departure, bubble lift-off and condensation and their dependence on process parameters. The associated sub-processes such as bubble inception inside superheated layer, microlayer formation, and depletion during departure and scavenging of the superheated layer at the time bubble lift-off have been observed. Quantitative analyses of the images provided useful information in terms of bubble dynamic parameters (equivalent bubble diameter, aspect ratio, contact angle, and bubble trajectory) along with the near wall thermal gradients as a function of varying Reynolds numbers.