UCLA

Knowledge of the physical mechanisms governing bubble dynamics and two-phase heat transfer is critical in order to accurately predict and scale the performance of two-phase systems, most importantly in low-g environments. To better understand flow boiling, especially under microgravity conditions, the dynamics of single and multiple bubbles under different levels of bulk liquid velocity, surface orientation, contact angle, and substrate materials are studied in this work. Microfabricated cavities at the center of a flat heating surface are used to generate bubbles. Silicon and aluminum are used as substrate materials, with contact angles of 56o and 19o, respectively, with water as test liquid. The investigated bulk liquid velocities ranged from 0 m/s to 0.25 m/s, while surface orientation varies from horizontal to vertical, through 30^o, 45^o and 60^o, and cavity spacing from 0.4 mm to 1.2 mm, in upflow conditions. Bulk liquid temperature was set close to saturation temperature, with bulk liquid subcooling less than 1 ^oC, and wall superheat was maintained between 5.0 ^oC and 6.0 ^oC.

Based on the experimental data, a simple force balance model was developed, and is used to develop a model to predict bubble lift off. These forces are the lift force (F_l), the buoyancy force (F_b), the surface tension force (F_s), the contact pressure force (F_cp), and the inertia of both the vapor and the liquid displaced by the growing bubble. It is showed that at the instant when bubble lift off is initiated, the sum of forces acting on the bubble is equal to zero (and then becomes positive in the direction normal to the heater). This force balance is used to develop an expression for bubble lift off diameter. It also is found that for single and merged bubbles, when lift off occurs, buoyancy and lift forces are the only forces acting on the bubble, regardless of orientation, contact angle and flow velocity, and that for all cases, the ratio (F_b + F_l) / A_l is constant and equal to 2.25 N/m², where A_l is the bubble surface area at lift off.