Experimental investigation on incipient boiling in narrow closed gaps with water

This research deals with characterization of the Onset of Nucleate Boiling (ONB) in an enclosed narrow gap system containing a stationary liquid. In the experimental system, subcooled water at atmospheric pressure is confined between two parallel vertical rectangular copper plates. One of the plates is heated electrically and the opposite one is cooled with an external water-cooling system, maintaining its constant temperature. The experimental gaps explored in this research are: 5 mm, 3 mm, 0.9 mm and 0.5 mm. In each gap, several experiments are performed with a different controlled cold plate temperature. Various heat transfer mechanisms are observed (convection, transition and conduction), depending on the gap size. In the convection regime, the results are in good agreement with a correlation from the literature. In the gap 0.5 mm wide, the heat transfer occurs by conduction through the water, whereas for the gap 0.9 mm wide, single-phase experiments show a “transition” heat transfer regime. The onset of nucleate boiling (ONB) is detected as the point at which the heat transfer coefficient behavior changes dramatically, and is revealed also by visualization of the experimental channel. The wall superheat and the heat flux are evaluated at ONB. An analytical model is developed, based on the [1] correlation and on the experimental values of the heat transfer coefficient. A good agreement for the ONB heat flux prediction is achieved with the model, whereas the ONB superheat is over-estimated, except for the really narrow, 0.5 mm, gap.