Boiling heat transfer in rectangular channels of small gaps

An experimental investigation was conducted to elucidate boiling heat transfer and flow pattern in a narrow-flat rectangular channel of a 100 mm-heated length with R-113 as the test fluid. The cross section of the channel is 20 mm wide and its height (referred to the gap) is varied as 0.2, 0.5, 1.0 and 2.0 mm to form a narrow boiling space. It was found that heat transfer coefficient is a complex function of mass velocity, heat flux and vapor quality for a given value of the gap size. In the downstream of the onset point of nucleation there follows the bubbly flow region of nucleate boiling dominance. This region is localized rather shortly in the present narrow channels and replaced by the regions of successive slug and annular flows where convective evaporation becomes dominant. Heat transfer coefficient in the slug and the annular flow regions increases in the flow direction under some thermal and hydrodynamic conditions until dry-out qaulity is approached, while for another conditions heat transfer coefficient decreases steadily in the flow direction. This steady decrease is observed when the gap size is small, mass velocity low and heat flux is high. In the present narrow channels five flow patterns are identified: (a) bubbly flow of spherical bubbles, (b) bubbly flow of two-dimensionally flattened bubbles, (c) coexistence of slug and bubbly flows, (d) slug flow, and (e) annular flow. A smaller gap of the narrow channels gives higher heat transfer coefficients when heat fluxes are low, whereas heat transfer deterioration begins at lower quality as heat flux becomes higher.