The objective of this study is to underline differences between pool boiling at subatmospheric pressure (i.e at low reduced pressure of 5.10~(-5)) as it might occur in absorption or adsorption systems using water as the refrigerant (LiBr/H_2O absorption or, silicagel/H_2O, zeolite/H_2O adsorption systems) and pool boiling at atmospheric pressure (i.e high reduced pressure of 4.10~(-3)). The fundamental difference between subatmospheric and atmospheric pressure pool boiling lies in the non homogeneity of the bubble environment in terms of pressure and subcooling degree. An experimental test set up was built to characterize water pool boiling on a horizontal heated surface at pressure down to 1.2 kPa. In such conditions, bubbles have centimeter size (up to 15 centimeters at 1.2 kPa) and a particular shape during their growth. Flattened spheroid bubbles are often followed by a liquid jet at the departure that confers them a "mushroom" shape. Thermal studies at atmospheric and subatmospheric pressure were also investigated. Significant fluctuations of the wall superheat were observed at subatmospheric pressure. They strongly affect the heat transfer coefficient.
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