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

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 u(c)(x), which allows modelling in terms of the local flow width delta(x). In both the momentum-dominated jet limit and buoyancy-dominated plume limit, dimensional arguments show delta(x) similar to x, and experimental data show the proportionality factor c(delta) to remain constant between these limits. The entrainment modelling required in traditional integral methods is thus replaced by the observed constant c(delta) value in the present method. In non-reacting buoyant jets, this new integral approach provides an exact solution for u(c)(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 c(delta) value gives an expression for the buoyancy flux B(x) that allows the integral equation for u(c)(x) to be solved for arbitrary exit conditions. The resulting u(c)(x) determines the local mass, momentum and buoyancy fluxes throughout the flow, as well as the centreline mixture fraction zeta(c)(x)and thus the flame length L. The latter provides the proper parameters Omega and Lambda 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.