EXPERIMENTAL STUDY OF BUBBLE DYNAMICS DURING NUCLEATE FLOW BOILING ON HORIZONTAL AND VERTICAL SURFACES

Abstract

Knowledge of the physical mechanisms governing bubble dynamics and two-phase heat transfer is required to develop mechanistic models and to accurately predict and scale the performance of two-phase systems in reduced gravity environments. To better understand the effect of the magnitude of gravity on flow boiling, especially the dynamics of single bubbles under different levels of bulk liquid velocity, surface orientation at earth normal gravity and contact angle effects were studied in this work. A microfabricated nucleation site (10 mm in diameter) at the center of a flat heating surface was used to generate bubbles. Silicon and aluminum were used as substrate materials, with static contact angles of 56° and 19°, respectively, and water as the test liquid. Bulk liquid velocities ranged from 0 to 0.25 m/s, for horizontal and vertical surface orientations, in upflow conditions. Bulk liquid temperature was set close to saturation temperature, with bulk liquid subcooling less than 1°C, and wall superheat was maintained between 5.0°C and 6.0°C. The experimental results for bubble growth, departure diameter and time for departure, bubble sliding distance and velocity, bubble lift-off diameter and time for lift-off, and bubble base diameter are reported for vertical and horizontal orientations, as the system variables were varied parametrically. It is found that irrespective of orientation and contact angle, departure and lift-off diameters decrease with increase in bulk liquid velocity. At a given velocity, departure diameter decreases with increase in the component of gravity parallel to the heater surface. For a given bulk liquid velocity, increase in contact angle results in increase in bubble departure and lift-off diameters, for all orientations. For given conditions, bubble lift-off diameter increases with decrease in the component of gravity normal to the heater surface. The effect of gravity on lift-off diameter is reduced at high liquid bulk velocities. For given orientation and flow velocity, sliding distance decreases monotonically with decrease in contact angle. The bubble velocity at lift-off increases with increase in flow velocity and increase in the component of gravity parallel to heater surface.