Experimental investigations of laminar and turbulent burning velocities of thermally cracked hydrocarbon fuels

Abstract

A comprehensive understanding of the flame propagation characteristics of thermally cracked fuels is crucial for developing advanced scramjets using regenerative cooling techniques. In the present study, both the laminar and turbulent burning velocities of thermally cracked hydrocarbon fuels at typical pyrolysis temperatures were systematically measured using a newly developed, fan-stirred, constant-volume bomb. The results show that variations in fuel composition have no significant influence on the laminar burning velocities of pyrolysis gases and liquids. In addition, the promotional effects of pyrolysis gas on flame propagation of thermally cracked fuel depend on the gas-production rate and equivalence ratio. The normalized turbulent burning velocities of typical thermally cracked fuels can be well scaled using a power law with regard to the turbulent Reynolds number (ReT, f). The fitting exponents gradually increase with increasing equivalence ratio, indicating a stronger promotional effect by turbulence for fuel-rich flames. For premixed turbulent flames in the wrinkled/corrugated flamelet regimes, turbulence and preferential diffusion simultaneously affect the flame propagation process. The wrinkling effects of turbulence are weakened by thermal-diffusional stability when the combustible mixture has an effective Lewis number (Leeff) greater than one. By contrast, the wrinkling process is promoted by thermal-diffusional instability, and the flame propagation is simultaneously enhanced by turbulence and thermal-diffusional instability. A unified scaling law for normalized turbulent burning velocities of thermally cracked fuels is proposed based on ReT,f Leeff–0.6, which accounts for the impacts of preferential diffusion. The effective Lewis number corrected scaling law can be applied to thermally cracked fuels at different pyrolysis temperatures, with a maximum relative error not exceeding 15%.