Measurement of heat flux in dense air-mist cooling: Part II - The influence of mist characteristics on steady-state heat transfer

The boiling convection heat flux, −q, taking place during the impingement of a water air-mist upon the surface of a Pt-disk, held at steady-state surface temperatures Tw ranging between 550 and 1200 °C, has been measured under different conditions of water impact density, w, droplet velocity, u, and droplet size, dd. The new steady-state measurement method controls induction heating to balance the heat extracted from the sample, as described in detail in Part I. Local mist characteristics were determined at room temperature in free non-impinging mists using a patternator for w and a particle/droplet image analyzer (PDIA) for dd and u at positions equivalent to those of the Pt-disk. Three different air-mist nozzles of fan discharge type are characterized over their full range of water flow rates and air inlet pressures and using different positions of the hot surface with respect to the nozzle, to cover the following ranges of local spray characteristics: w from 2 to 106 L/m2 s; normal volume weighted mean velocities, uz,v, from 9.3 to 45.8 m/s and volume mean diameters, d30, from 19 to 119 μm. Increasing the air nozzle pressure at constant water flow rate generates mists with finer and faster drops that lead to a higher frequency of drops with large impinging Weber numbers, suggesting a higher probability of wet contact with the surface and an enhanced heat extraction. Heat fluxes as large as ∼12 and ∼10 MW/m2 were found in the transition and stable film boiling regimes, respectively. The boiling convection heat flux in the range of 750-1200 °C, which corresponded to stable film boiling, was found to correlate very well with the mist characteristics and temperature. The order of importance of the four parameters influencing −q was: d30 ≪ Tw < w ≈ uz,v. For given local water flux and surface temperature, the correlation indicates that spray cooling becomes more intense as the velocity of the drops increases; the droplet size plays a very minor role. Compared with previous results using a transient method, the steady-state heat transfer coefficients increase faster with w in the range of 5-20 L/m2 s, reaching much higher values. This suggests that within this range, the steady-state heat flux is controlled by the local water flux, while the transient heat flux must be controlled by the supply of heat conducted to the surface.