Measurement of species concentration and estimation of temperature in the wake of evaporating n-heptane droplets at trans-critical conditions

Rising pressures in modern combustion systems lead to conditions that can be supercritical for the injected liquid fuels. Quantitative measurement data is required for a better understanding of this complex process and for validation of corresponding simulations. The present study focusses on the evaporation of single free-falling droplets to reduce the complexity of the problem. The species concentration field in the wake of n-heptane droplets is measured with 2D Raman imaging. The atmosphere consists of nitrogen at conditions that are sub- to super-critical with regards to pure n-heptane. The concentration fields of all droplets at one set of operation conditions are overlaid to allow statistical interpretation. Additionally, data points for the correlation between n-heptane concentration and the relative number density are determined from the Raman signals. These data points are fitted by a model based on the adiabatic mixing assumption and a Virial equation of state for mixtures. The fitted model is used to calculate the correlation between fuel concentration and temperature. As suggested by the temperature results the droplet is cooled due to evaporation rather than being heated up towards its critical temperature. The data is available for the validation of numerical simulations.