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Compact evaporators like plate heat exchangers can play a significant role in reducing the investment cost of low cooling power sorption systems. If water is used as refrigerant, their design remains mainly empirical. The objective of this paper is thus to investigate the specific characteristics of water pool boiling in narrow channel at subatmospheric pressure in order to acquire the fundamental knowledge needed to improve the design of compact evaporators in these sorption systems. An experimental test setup was thus designed and built to study water pool boiling in narrow channel at subatmospheric pressure (from 5 to 1.2 kPa) on a vertical heated copper disk. The influence of the thickness of the narrow channel and of the pressure on the heat transfer is discussed. As the pressure and the channel thickness decrease the occurrence of a specific subatmospheric pool boiling regime is observed, degrading heat transfer coefficient. Nevertheless, the general trends of evolution are in agreement with those generally observed in the literature: heat transfer is enhanced as the thickness of the narrow channel decreases but, depending on the pressure, decreasing too much the channel thickness could lead to a deterioration of the heat transfer coefficient. A particle image velocity (PIV) device was implemented to the experimental setup in order to highlight the effect of the wake-induced flow on the heat transfer. [ABSTRACT FROM AUTHOR]

The objective of this paper is to underline the specific characteristics of water pool boiling at subatmospheric pressure compared to atmospheric pressure in order to optimize the design of evaporators dedicated to sorption systems. An experimental test set up was built to characterize the pool boiling of water at pressure down to 0.85 kPa. The major specificity of the pool boiling at low pressure lies in the non homogeneity of the boiling environment, both in terms of pressure and subcooling degree. This non-homogeneity makes really different boiling phenomena than those usually observed. That is why a particular shape and size of bubbles at low pressure is observed. Bubbles have centimeter size (up to 15 cm at 1.2 kPa) and are often followed by a liquid jet at the departure that confer them a “mushroom” shape. Boiling curves from 100 kPa to 0.85 kPa were plotted and related to specific subatmospheric pressure pool boiling regime. This regime is characterized by significant wall temperature fluctuations (up to 20 K for our experimental conditions) affecting the heat transfer coefficient and causing material fatigue. [ABSTRACT FROM AUTHOR]