Transient from natural convection or nucleate boiling to nucleate boiling or film boiling on a horizontal cylinder in pool subcooled wetting liquid by depressurization

Processes of the transient heat flux, q, versus surface superheat, ΔTsat, from an initial heat flux, qin, with Natural Convection (NC) or Nucleate Boiling (NB) including the transition to terminal nucleate boiling or film boiling, on a 1.2 mm-diam. horizontal cylinder in a pool of ethanol with the initial subcooling of 76 K at an initial pressure, Pin, of 1.08 MPa caused by an exponentially decreasing pressure, P=Pinexp(-t/τp), with a pressure reduction period, τp, to atmospheric pressure were measured for the wide ranges of the qin and the τp. The obtained processes of q for ΔTsat from various qin values with NC regime caused by the pressure reductions for the τp ranging from 3.8 to 127ms, were direct transitions from NC regime to film boiling one: namely first the transitions of q occurred at the transient CHF, qcr, being almost the same as the qin at a certain ΔTsat value with sharply decreasing of the q to the heat flux of around minimum stable film boiling one for corresponding pressure and subcooling, and finally the q rapidly increased up to terminal stable film boiling regime at atmospheric pressure. On the other hand, the processes of the q versus ΔTsat from qin values with the NB regime for the pressure reductions with the shorter τp of about 4 and 11 ms were such that first the ΔTsat increased up to a certain value with the q being almost equal to the qin with nucleate boiling regime, next the q rapidly increased up to qcr, and then sharply decreased down to the film boiling heat flux around steady minimum film boiling one for corresponding pressure and subcooling, and finally the q increased up to terminal steady film boiling at atmospheric pressure. While the process for the longer τp of 26 ms was such that first the ΔTsat increased up to a certain one without changing of q with nucleate boiling regime, then the q sharply increased up to the maximum CHF, qmax, and finally the q sharply decreases down to steady nucleate boiling heat flux at atmospheric pressure with an increase in ΔTsat. The measured qcr data for the qin with NC and NB regimes at certain pressures with subcoolings, were significantly lower than the corresponding q values derived from steady subcooled pool boiling CHF correlation based on hydrodynamic instability model. The transient qcr mechanisms for the qin with NC and NB regimes were suggested