Experimental study and theoretical analysis on the criterion of boiling of wall film for different fuels

In direct-injection engines, the formation and vaporization of wall-attached fuel film after the spray impingement is the main factor affecting the emissions of hydrocarbon (HC), soot, and other pollutants. In response to this situation, the formation and evolution characteristics of the fuel film formed by a high-pressure spray impingement onto a heated wall were investigated. The change in the fuel film thickness during the evolution process was measured with the refractive index matching (RIM) method for three different fuels. This paper also quantitatively studied the boiling criterion of the micrometer-scale wall film at various film temperatures. The results indicate that there exists a critical film thickness to trigger the boiling phenomenon, which is closely related to the film temperature. According to the theoretical analysis, it is concluded that the critical thickness is essential for the fuel film boiling at the corresponding fuel film temperature. At the same fuel superheat degree, the critical thickness of different fuels highly relies on their physical properties, among which surface tension is the dominant factor, while the effect of saturation temperature and enthalpy of vaporization is relatively weak.