Using a modified single-phase model to predict microgravity flow boiling heat transfer in the bubbly flow regime

It is hypothesized that the heat transfer in microgravity bubbly flow boiling can be computed through a single-flow simulation that accounts for the acceleration of the liquid as bubbles form since the slip velocity in microgravity is negligible. Measurements within the bubbly flow regime were obtained in a 6 mm ID sapphire tube in microgravity using HFE-7000 at four mass fluxes, six heat fluxes, and two subcoolings at atmospheric pressure. Flow visualization was performed and time and space resolved temperature and heat transfer distributions at the wall–fluid interface were measured using a Temperature Sensitive Paint (TSP) applied to the inside of the tube. The local liquid velocity was determined from the movement of small bubbles in the flow. The local, time-averaged heat transfer data were compared to numerical simulations of single-phase flow in a tube whose diameter was varied to match the experimentally obtained local liquid velocity. When the flow within the tube was laminar (low heat flux and mass flux cases), the measured heat transfer agreed well with the numerical results. For cases where the flow became transitional/turbulent and significant bubble coalescence was present, the measured heat transfer was higher, but was bounded by numerical solutions assuming laminar and turbulent flow.