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Effects of heat release on turbulent shear flows. Part 3. Buoyancy effects due to heat release in jets and plumes

Published online by Cambridge University Press:  07 March 2007

FRANCISCO J. DIEZ  and
WERNER J. A. DAHM
FRANCISCO J. DIEZ
Affiliation:
Laboratory for Turbulence & Combustion (LTC), Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI 48109-2140, USA
WERNER J. A. DAHM
Affiliation:
Laboratory for Turbulence & Combustion (LTC), Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI 48109-2140, USA
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Abstract

An integral method is presented for determining effects of buoyancy due to heat release on the properties of reacting jets and plumes. This method avoids the Morton entrainment hypothesis entirely, and thus removes the ad hoc ‘entrainment modelling’ required in most other integral approaches. We develop the integral equation for the local centreline velocity uc(x), which allows modelling in terms of the local flow width δ (x). In both the momentum-dominated jet limit and buoyancy-dominated plume limit, dimensional arguments show δ (x) ≈ x, and experimental data show the proportionality factor cδ to remain constant between these limits. The entrainment modelling required in traditional integral methods is thus replaced by the observed constant cδ value in the present method. In non-reacting buoyant jets, this new integral approach provides an exact solution for uc(x) that shows excellent agreement with experimental data, and gives simple expressions for the virtual origins of jets, plumes and buoyant jets. In the exothermically reacting case, the constant cδ value gives an expression for the buoyancy flux B(x) that allows the integral equation for uc(x) to be solved for arbitrary exit conditions. The resulting uc(x) determines the local mass, momentum and buoyancy fluxes throughout the flow, as well as the centreline mixture fraction ζc(x) and thus the flame length L. The latter provides the proper parameters Ω andΛ that determine buoyancy effects on the flame, and provides power-law scalings in the momentum-dominated and buoyancy-dominated limits. Comparisons with buoyant flame data show excellent agreement over a wide range of conditions.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 575 , March 2007 , pp. 221 - 255
Copyright
Copyright © Cambridge University Press 2007

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