Understanding the Effects of Fractal Blockage Geometries on Flame Acceleration in Propane-Air Flames

Abstract

In this paper the application of fractal orifice shapes have been tested in a PDE engine in order to increase the pre-detonator exit pressure and reduce the run up distance to detonation. Fractal geometries are known to change the way fluids interact with their boundaries, generating turbulence at a wide range of length scales. In this experiment the effect of varying the fractal dimension of orifice blockages has been studied to determine whether fractal orifice geometries can be used to enhance turbulence and directly increase flame acceleration. To test this concept validity at a pilot scale a Pulse Detonation Engine (PDE) pre-detonator was fired at the University of Sheffield’s Low Carbon Combustion Centre using propane-air mixtures at 300K. The effect of fractal orifice geometries has been investigated with three test cases using a 38.1 mm diameter (D, 1.5”) 1.12m long tube with 20 orifices spaced equally at 1D. The orifices chosen in this experiment were circular, circ (standard), von Koch fractal with dimension 0, fra0 (triangular) and von Koch fractal with dimension 1, fra1 (star), Each with the same area. The results show that fractal obstacle geometries change the exit pressure from the PDE and can generate pressure up to 5% higher (fra1) than a standard orifice plate (circ) in the obstacle filled section of the tube. The maximum exit pressure after the smooth tube section of the PDE was seen to be 7.5% higher with the fra1 orifice in comparison with the circ orifice plate. It is thought that this is a direct result of smaller length scales and greater turbulence intensity generated by fractal geometries which produce greater mixing within the turbulent flame front, and a faster turbulent flame speed. This additional turbulence intensity produced greater flame acceleration rates due to enhanced flame stretch. This has direct implications for PDE pre-detonator length reduction as deflagration to detonation transition (DDT) takes place much more rapidly at higher pressure and flame speed.