STUDY OF POTENTIAL ENHANCEMENT OF HEAT TRANSFER BY BUBBLE ENTRAINMENT IN HORIZONTAL FLOW BOILING

摘要

The traditional approach towards explaining the experimental behavior of the heat transfer coefficient during annular flow boiling employs a forced convection evaporative process upon which the effects of nucleate boiling are superimposed whenever the applied heat flux suggests the onset of nucleate boiling. Although the traditional approach has been challenged in the past, the possibility of a significant contribution to heat transfer from the growth of entrained bubbles due to an applied heat flux has been neglected. Situations may exist in which suppression of nucleate boiling occurs, but the superheat within the liquid film is still sufficient for the growth of the larger entrained bubbles. In analyzing this possibility, knowledge of the bubble size and location within the film is fundamental. The use of backlit images obtained from clear test sections during adiabatic annular flow in an air-water loop at room temperature and pressures close to atmospheric, indicates the presence of entrained gas bubbles ranging in diameter between approximately 10 μm to 200 μm. The size distribution of these bubbles is explored at the side of the tube through high-resolution digital imaging and image processing software. The behavior of the bubble statistics indicates that the entrainment mechanism is a folding action that occurs in disturbance waves. The entrainment events are illustrated by cross sectional images of the liquid film using the pulsed laser induced fluorescence technique (PLIF). A suitable bubble growth heat transfer mechanism model will be presented with the experimental bubble information used as an input to determine the potential effect of this mechanism compared to heterogeneous nucleation at the wall.