Analysis of Evaporating Fuel Films Using Shadowgraph and Schlieren Imaging Techniques 2008-01-2443

Fuel spray impingement and the formation of a fuel film in the intake port of port-injected engines or on the piston crown of direct-injected engines have been shown to result in increased pollutant emissions and reduced engine performance. While models that predict the nature of fuel film evaporation exist, little experimental data on the evaporation characteristics of fuel films have been published. This paper discusses the use of two closely related imaging techniques, shadowgraphy and schlieren photography, to measure the transient evaporation rate, surface area, and thickness of evaporating films, as well as the thickness and concentration distribution of the vapor layer that forms directly above the films. The measurements were conducted in order to better understand the interdependent transport phenomena that control the evaporation process. Although this work is motivated by the problems caused by fuel films in engines, the experiments were conducted outside of an engine under conditions that are easy to control. Two advantages of using imaging techniques to study film evaporation are that such methods are non-intrusive and they can have high temporal resolution through the use of a high-speed camera. The results of these imaging experiments indicate that under the conditions studied, the gravitational force has a strong influence on the evaporation rate through its effect on the vapor layer thickness.

The shadowgraph method was applied to obtain images of the profile of the film during the course of film evaporation. For each recorded image, the mass, surface area, and thickness of the film were computed. The evaporation rate and the transient changes in surface area and film thickness were computed from the images recorded over time at a rate of 60 images/second. To validate the mass and evaporation rate results of the shadowgraph experiments, simultaneous measurements of the film mass were made using an analytical balance.

The schlieren technique utilizes gradients in indices of refraction of transparent media (e.g. the vapor from an evaporating film) to construct images whereby these media may be observed visually. This technique was used to obtain qualitative and semi-quantitative information about the vapor layer that quickly forms above the evaporating film. The vapor layer thickness and the spatial distribution of the vapor concentration were estimated quantitatively for each schlieren image and their changes were computed from images recorded over time.

Because of the large number of images acquired per experiment, automation of the image analysis was essential. Computer programs were written using Matlab to identify the liquid and vapor boundaries and to compute the various characteristics of the evaporating film.