Regimes of unsteady exhaustion and diffusion combustion of a hydrocarbon fuel jet

Results of an experimental study of hydrodynamics and diffusion combustion of hydrocarbon jets are presented. Various regimes of instability development both in the jet flame proper and inside the source of the fuel jet are considered. The experiments are performed for the case of subsonic gas jet exhaustion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13500. The fuel is the propane-butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert diluter (CO$_2$ or He) for the jet flame. The mean velocity and velocity fluctuations in the near field of the jet without combustion are measured. Among four possible regimes of cold jet exhaustion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. Schlieren visualization of the reacting flow is performed. The temperature profiles in the near field of the jet are measured by a Pt/Pt-Rh thermocouple. An attached laminar flame is observed in the transitional regime of propane exhaustion from the tube. In the case of combustion of C$_3$H$_8$ mixtures with CO$_2$ or with He in the range of Reynolds numbers from 1900 to 3500, the transitional regime is detected in the detached flame. Turbulent spots formed in the tube in the transitional regime exert a significant effect on the flame front position: they can either initiate a transition to a turbulent flame or lead to its laminarization.