Mechanisms of Subcooled Pool Boiling CHFS Depending on Subcooling, Pressure, and Test Heater Configurations and Surface Conditions in Liquids
Authors: Sakurai Akira , Fukuda Katsuya 
Published: May 2008

The measured subcooled pool boiling CHFs on a horizontal cylinder heater for subcoolings with pressures as a parameter in water, liquid nitrogen, liquid helium etc. were previously well described by the subcooled pool boiling CHF correlations newly derived assuming two different CHF mechanisms resulting from hydrodynamic instability (HI) and heterogeneous spontaneous nucleation in originally flooded cavities (HSN): the CHFs for lower subcoolings which depend on pressure, and the CHFs for higher subcoolings which were almost independent of pressure in water extremely disagreed with corresponding values derived from existing subcooled pool boiling CHF correlations only based on hydrodynamic instability except the CHFs near saturated condition. In the present paper, to begin with the effect of heater configuration on the CHFs resulting from the HI and the HSN were clarified using the existing CHF data measured for the horizontal cylinders with diameters ranging from 0.2 to 5.84 mm, and the test ribbon heaters with different shapes for subcoolings ranging from zero to about 160 K at pressures ranging from 0.013 to 2 MPa in water and ethanol etc., and next the effects of horizontal cylinder surface conditions on the CHFs resulting from the HI and the HSN were clarified using the CHF data measured by the authors for 1.2 mm-diam. platinum horizontal cylinders with commercial, rough and mirror surfaces (CS, RS and MS) in water for the wide ranges of subcooling and pressure. The CHFs for subcoolings at a pressure measured using a horizontal cylinder, first increased up to the maximum CHF, secondly decreased down to the minimum CHF, and finally again increased monotonously with an increase in subcooling as a typical trend; namely the CHFs were divided into three groups of first, second and third ones for low, middle (transition) and high subcoolings respectively. However, the CHFs for middle subcoolings became insignificant, and the CHFs resulting from the HI or the HSN for all subcoolings including zero occurred depending on pressure, heater configuration, its surface conditions and a kind of liquid. The effect of cylinder diameters on the coefficients in the two CHF correlations representing CHFs resulting from the HI and the HSN previously presented were clarified in the liquids. It was confirmed that there existed the CHFs resulting from the HI measured for the horizontal cylinders with CS, RS and MS at lower pressures for lower subcoolings; the CHFs almost agreed with one another at the same subcooling independently of the surface conditions, though the CHFs resulting from the HSN for the MS cylinder were extremely lower corresponding CHFs for CS and RS cylinder at higher pressures. It should be noted that the CHFs resulting from the HSN predominated and on the other hand the CHF resulting from the HI existed in extremely narrow regions in a large number of the CHF data measured for the wide ranges of the conditions not only in wetting liquids but also in water.