VAPORIZATION OF A DENSE SPHERICAL CLOUD OF DROPLETS AT SUBCRITICAL AND SUPERCRITICAL CONDITIONS

Droplet vaporization in a dense spherical cloud is studied numerically at subcritical and supercritical ambient conditions. The interaction among movable droplets is accounted for by adopting the concept of the variable-sized sphere of influence. Transient conservation equations for both the outer homogeneous gas-phase flow region and the inner het erogeneous two-phase flow region of the cloud are solved with high-pre ssure effects being taken into account, including ambient gas solubili ty, property variation, thermodynamic non-ideality, and transient diff usion. Effects of ambient pressure, droplet size, and cloud size on th e fuel mass ejected from the cloud are investigated. At high pressures , due to the low liquid-to-gas density ratio, the fuel mass ejection f rom the cloud is much less than that at low pressures. Since this pres sure effect is found to be much more important than the influences of droplet and cloud sizes, the external group combustion mode is less li kely for a cloud of droplets at high pressures than that at low pressu res with an equivalent spray density. Furthermore, since the droplet h eating process in the cloud is entirely transient at high pressures, t he conventional quasi-steady analyses that assume uniform droplet temp erature are inadequate for the high-pressure vaporization of a dense c loud of droplets.