Model of unsteady n-octane droplet burning in high-temperature streams

Unsteady burning, internal heating, and modeling comparison of an n-octane fuel droplet in high-temperature flows are studied numerically. In the developed numerical method, the governing equations of the gas and the liquid phases are fully solved and linked at the interface. In addition, a moving grid system and the particle deceleration are also considered to account for the droplet regression and impact of drag force. The obtained results show that the convective droplet is ignited with a wake flame after a heating-up period. With increasing time the relative velocity between the droplet and surrounding flow becomes smaller due to the drag. As a result, the flame changes from a wake type, through a transition type, and eventually to an envelope type. With regard to the droplet interior, internal heat transfer is always dominated by internal circulation and unsteady heating prevails over the entire droplet lifetime. The vaporization fluxes predicted in terms of various quasi-steady models are also compared with the present solution. The quasi-steady analysis, omitting the initial droplet heat-up process, fails to predict the flame configurations and vaporization flux in the initial period of the droplet lifetime. Nevertheless, the role played by the initial unsteadiness on droplet vaporization is important only in a short period relative to the whole droplet lifetime. When the envelope flame develops around the droplet, the predictions by the quasi-steady models are in good agreement with the transient solution, except for the vaporization model. Furthermore, if the droplet undergoes a sudden drop in ambient temperature, it reveals that the dual solution of the quasi-steady model enables us to predict the droplet vaporization rate and flame configuration well.