Droplet Cluster Behatior in Dense and Dilute Regions of a Spray

concern regarding the ef%ciency, stability and safety margins of hi-propellant combustion in rocket engines has prompted the investigation of nlany specific aspects of spray evaporation, ignition and combustion previoudy not studied. Thus, early s!,ucties of combustion in liquid rocket engines were based upon the results of the classsid single-component, isolated drop combustion at atmospheric pressure ] . Although the resulti from th~e studies provided a baseline for understanding some of the phenonlella occurring in liquid rocket engines, they fail to explain important observations and facts obtained horn examining rocket performance after many !lig}lts. Exanlples are the loss of about 370 of the liquid oxygen (LOZ, one of the propellants) which exits unburncd2 , and the existence of striations on the inner wall of engines examined after a flight3 . It then becomes apparent that many significant issues of liquid rocket spray combustion were not addressed by the early models and that in order to mitigate existing problenls, it is necessary to ‘b understand aspects pxevlo sly unexplored. Since characterization of combustion in liquid rocket chambers is extremely difficult due to the lack of diagnostics operating under such harsh condition , the strategy has been to infer the relevant aspects to be studied from laboratory experiments bearing similarity to liquid rocket combustion chambers. Such experiments have focussed generally on a single poorly-understood aspect of liquid rocket chamber combustion so as to isolate the fundamental effects of this aspect before coupling it to the other phenomena. The phenomenobgy of hi-propellant spray evapoI ation, ignition and combustion in a highly turbulent environment at elevated pressure (supercritical with respect to the fuel) is as follows: When oxygen and hydrogen entel the combusticm chamber, due to the higher critical temperature of oxygen with respect to that of hydrogen, the drops of hydrogen quickly become a fluid whereas the drols of oxygen remain liquid. Thus, the relevant phenomena to be studied are the evaporation of l.O= dlops in surroundings initially composed of fluid H2, the eventual transitioil of LOX drops< to a fluid, ignition of the fluid mixture and the subsequent combustion. I he to the supercritical conditions, volubility of 112 into LOZ becomes important, so that although the two component fuels are initially separated, very rapidly the liquid drops l,ecome compm$cd of a binary-fuel. Since the concentration of the drops varies in time, the critical point c)f the fuel inside the drops is aLw a function of time. Therefore, during eva~)oration and solvation, the critical point may be crossed back and forth as the composition of the fuel changes. The same comments apply to tri-propellant liquid rocket engine phenomenology where an additional