A NEW MODEL FOR CHF IN POOL BOILING AT HIGHER PRESSURE

The concept of critical heat flux (CHF) is discussed based on the mechanism that the CHF is caused by the dryout of a liquid layer formed on a heating surface. It is suggested that a liquid macrolayer is formed due to the coalescence of bubbles for most boiling systems, and that the dryout of the macrolayer is controlled by the hydrodynamic behavior of coalesced bubbles on the macrolayer. Based on these considerations, a new CHF model is proposed for saturated pool boiling at higher pressures. The idea of this model comes from a close examination of the measured diameters of various bubbles and the photographic records obtained by Semeria (1963) for water boiling under higher pressures. In the model, a liquid macrolayer is formed due to coalescence of the secondary bubbles formed from the primary bubbles. The detachment of the tertiary bubbles formed from the secondary bubbles determines the frequency of the liquid macrolayer formation. The CHF occurs when the macrolayer is dried out before the departure of the tertiary bubbles from the heating surface. One of the formulations of the model gives the well-known Kutateladze or Zuber correlation for CHF in saturated pool boiling.