PHOTOGRAPHIC STUDY OF SUBCOOLED POOL BOILING CRITICAL HEAT FLUX ON HORIZONTAL CYLINDERS WITH COMMERCIAL AND ROUGH SURFACES

The subcooled pool boiling critical heat flux (CHF) using a 1.2-mm diameter horizontal commercial surface cylinder in water at pressures ranging from atmospheric pressure up to 2063 kPa for a wide ranges of liquid subcoolings from zero to 80 K have been measured to investigate for the mechanisms of subcooled boiling CHF depending on the effects of subcoolings and pressures on the steady and transient CHF. The steady-state CHFs for subcooling were divided into two groups for low and high subcoolings with pressure as a parameter with two different CHF mechanisms resulting from hydrodynamics instability (HI) and heterogeneous spontaneous nucleation (HSN). It was also observed that the measured CHFs resulting from the HI at low pressures in liquid subcoolings as well as the measured CHFs due to HSN at high pressures in high subcoolings respectively on a 1.2-mm diameter of commercial and rough (CS and RS) surface cylinders were approximately same with one another independently of surface conditions. The purpose of the present work is to investigate and compare the heat transfer processes and the vapor behaviors for two mechanisms of subcooled CHFs for the liquid subcooling of 20 K at 101.3 kPa and for the liquid subcooling of 80 K at 690 kPa by mean of photographically. The vapor behavior such as periodic bubble growths and detachments corresponding to HI mechanism at CHF for the liquid subcooling of 20 K at 101.3 kPa and small vapor bubbles existing for some periods on the cylinder corresponding to the HSN mechanism at CHF for the liquid subcooling of 80 K at 690 kPa were investigated.