Experimental evidence of a new regime for boiling of water at subatmospheric pressure

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.