Understanding high-pressure gas-liquid interface phenomena in Diesel engines

Injection of liquid fuel (or oxidizer in the case of liquid rockets) in systems where the working fluid exceeds the thermodynamic critical condition of the liquid phase is not well understood. Under some conditions when operating pressures exceed the liquid phase critical pressure, surface tension forces become diminished so that the classical low-pressure gas-liquid interface is replaced by a diffusion dominated mixing layer. These two extremes have been well recognized in the liquid rocket community for years. In other systems, however, the significance of surface tension forces is not clear. Here we focus on this topic in the context of Diesel engine fuel injection processes. We derive a coupled model to obtain a theoretical analysis that quantifies under what conditions the interfacial dynamics transition between the classical non-continuum jump conditions associated with two phase flows and a continuous gas-liquid interfacial diffusion layer. We present high-speed imaging from the Sandia non-reacting n-dodecane experiment that provides corroborating evidence that continuous gas-liquid interfacial mixing dynamics occur at high-pressure Diesel engine conditions. At constant chamber density and supercritical fuel pressures, liquid structures affected by surface tension such as ligaments and drops develop at low ambient temperatures. But at engine-relevant high-temperature conditions, a different, more diffusive mixture preparation process without measurable fuel drops is found. These two conditions are studied using our model. It applies a real-fluid model that accounts for the relevant thermodynamic non-ideal multicomponent mixture states in the system and combines this with Linear Gradient Theory, which facilitates the calculation of the detailed vapor-liquid interfacial structure. At high ambient temperature, our model shows interfaces with substantially increased thicknesses in comparison to the low temperature condition. Our analysis reveals that gas-liquid interfacial diffusion layers develop, contrary to conventional wisdom, not necessarily because of vanishing surface tension forces, but because of the combination of a reduction in mean free molecular path and broadening interfaces, which then enter the continuum length scale regime. Then, instead of inter-molecular forces, transport processes dominate. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.