Film boiling heat transfer for saturated drops impinging on a heating surface

Abstract

FILM boiling of liquid drops in contact with a heating surface can be found in combustion inside a fuel injection engine, spray cooling of a heating surface, mist flow inside a heating tube and vapor explosion at liquid-liquid contact. In general, heat transfer performance in the liquid-drop system is superior to that in pool boiling, and is thus characterized by broad applications. However, its mechanism is not well understood, since the dynamic behavior and transient heat transfer of drops are clearly interrelated. For example, to determine the heat transfer rate between a drop and a heating surface, it is necessary to determine the timewise variations of both the drop spreading area and vapor layer thickness, the vapor flow velocity and the effects of the impinging Weber number, drop diameter and wall superheat on the heat transfer rate. The experimental studies on film boiling of drops include Wn,.ktor. ot n, 111 Pe.&.r.,=n 131 T1za-t~ nt n/ 171 snrl Phki ot .V‘.I,..U.YCI U,. t.,, IU.zI.Y..I. [LJ, “IUUCL “.. L-‘, u.... “.,“,.C. al. [4]. They determined drop heat transfer effectiveness, not from the measurement of heat transfer coefficients, but from the timewise variations in both the temperature and heat capacity of the heating surface following drop impingement. By approximating drop shape as a circular cylinder and a hemisphere, Kendall and Rohsenow [5] conducted heat transfer analysis taking into account both the motion due to drop deformation and the motion of the drop’s gravitational center. They determined the radius of the spreading drop, .,“..__ I-.,-_ +L:^L-~rr t;,,...;,, ..,..:,t:,.., ;n r.n..t a,.,,, ..“A vap,vr ray=, L,,,clL,I~ss, L‘III~VVIJC Y‘aLlall”Ll?, 111 ,,&a1 I‘unc-J ‘Il‘Ll the effects of the drop diameter, wall superheat and impinging Weber number on heat transfer effectivenes. The assumed drop geometry produced little difference in analytical results. No attempt was made to derive a dimensionless correlation equation indicating the effects of each governing parameter on heat transfer performance (i.e. Nusselt number). Hence, results for film boiling heat transfer of an impinging drop system (Kendall and Rohsenow [5]) are not applicable to broad ranges of the governing parameters. The present study derives a dimensionless correlation equation for film boiling heat transfer of an impinging drop system, as a function of the Weber number related to the impinging velocity of a drop, Bond number, Prandtl number and a new parameter, that is, the Weber number related to the vapor stream, through an analytical investigation of five