Experimental studies of critical heat flux for low flow of water in vertical annuli at near atmospheric pressure

Abstract

Experiments are performed to measure the critical heat flux (CHF) for low flow of water in vertical annuli near atmospheric pressure (0.118 MPa). CHF data are collected for water inlet subcooling ranging from 44 K (or 182 kJ kg−1) to 75 K (or 312 kJ kg−1), and mass fluxes from 0 to 260 kg m−2 s−1. The CHF values vary from 155 kW m−2 for zero inlet flow to a maximum of 1418 kW m−2 for a water mass flux of 260 kg m−2 s−1. The data are classified and correlated according to the flow transition which occurred at CHF. Two CHF correlations are developed, one for the slug-churn and the churn-annular flow transitions and the other for the annular-annular mist flow transition. The heat flux at incipient flow chugging is also measured and found to vary from a minimum of about 40% of the CHF values in the smallest annulus (annulus ratio = 1.575) to a maximum of approximately 88% of CHF in the other larger annuli (annulus ratio = 1.72 and 2.0). Results also show that the heat flux at incipient chugging increases as either the water mass flux or inlet subcooling increases. The CHF values for zero inlet flow are predicted within ±25% using Block and Wallis' flooding correlation.