Volumetric heat release, fuel-air mixing and turbulent dissipation of vertically-downward turbulent nonpremixed jet flames under sub-atmospheric pressures

In this study, volumetric heat release rate (related to flame radiation characteristics), fuel-air mixing, and turbulent dissipation rate of vertically-downward nonpremixed jet flames under standard- and various sub-atmospheric pressures are investigated, which have not been quantified yet. The interaction of downward jet momentum and upward buoyancy influences flow turbulence and fuel-air mixing. Experiments are conducted in a reduced-pressure chamber with controlled ambient pressures from 40 to 101 kPa. The flame volume and volumetric heat release rate are experimentally determined through image processing. Three-dimensional large eddy simulations are performed to further understand fuel-air mixing and flame behaviors. A quantitative agreement is obtained between the measured and calculated flame volumes. Results show that the turbulent dissipation rate $\epsilon$ of vertically-downward jet flame is stronger and distributes more broadly than that of the upward jet flame because of the rapid deceleration of jet momentum by the buoyancy. As the pressure decreases, more intense turbulent kinetic energy and turbulence dissipation rate are observed. The flame volume V_f for the vertically-downward jet flame is found to increase as heat release rate $\dot{Q}$ increases and decreases with the increase of ambient pressure. The flame volume has a power relation as $V_f\sim {\dot{Q}}^{10/7}$ and on ambient pressure P_c as $V_f\sim P_c^{-4/7}$. Moreover, the volumetric heat release rate scales satisfactorily with P_c as $\dot{Q}{}^{″\prime }\sim P_c^{4/7}$. The differences of V_f (or $\dot{Q}{}^{″\prime }$) among pool-type flame (purely buoyancy driven), upward jet flame and downward jet flame are revealed and explained by turbulent dissipation rate and fuel-air mixing characteristics. A general global dimensionless model for flame volume of downward jet flame is proposed by taking into account the initial jet momentum, flame buoyancy, and ambient air pressure, in which a new dimensionless heat release rate is defined based on two derived length scales (momentum-buoyancy competition length scale and total plume buoyancy strength length scale).