An Experimental and Analytical Study of Flash-Boiling Fuel Injection

Flash-boiling fuel injection is obtained by preheating liquid fuel within the injector so that the sudden decrease of pressure during injection causes rapid boiling of the fuel. Rapid expansion of the vapor phase may then atomize the liquid jet. Two distinct types of flash atomization were observed experimentally. First, at relatively low fuel temperatures the liquid jet remains intact for a short period after leaving the nozzle, then suddenly breaks up when rapid bubble growth begins. Second, at higher fuel temperatures, flash boiling within the injector orifice produces an underexpanded, compressible, two-phase flow. Sudden expansion immediately upon leaving the orifice yields large spray cone angles. An analytical model of vapor-bubble growth was applied to enhance understanding of the flash-injection process. Calculated time scales for bubble growth and delay times were found to be consistent with the above two flashing regimes at both low and high fuel preheat temperatures. Also an unfavorable trend toward suppressed bubble growth with increasing injection pressure was calculated.

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