Abstract
The turbulent flowfield in a backmixed combustor is modeled analytically by simultaneously solving the governing partial differential conservation equations. An algebraic and a two-equation eddy viscosity model are employed in the numerical simulation to account for the turbulence transport processes. Predicted distributions of isothermal flow properties are systematically compared to experimentally obtained data to appraise the eddy viscosity models. In addition, predicted distributions of reacting flow properties are presented to illustrate the applicability of current numerical methods to the prediction of continuous combustion flows. The turbulent momentum and mass transport properties are evaluated in isothermal flow for a range of mixing conditions. Both eddy viscosity models qualitatively describe the system hydrodynamics, but the detailed flow structure is inadequately represented. The mass transport predictions from the algebraic viscosity model agree favorably with experiment. The inferior performance of the two-equation viscosity model is remedied by refining the boundary condition specification for the turbulence energy dissipation rate. It is shown that the turbulence energy dissipation rate adjacent to critical solid walls strongly influences the overall mixing characteristics of the two-equation model. The isothermal flow results indicate that the algebraic eddy viscosity model provides cost-effective predictions of the general fluid flow patterns and mass transport trends in confined flows exhibiting strong recirculation. The two-equation eddy viscosity model provides better resolution of the small-scale turbulence processes but requires careful testing to ensure realistic predictions. The hot flow calculations accentuate the inadequate transport characteristics identified in the isothermal flow analysis and verify that considerable testing of the numerical model is required before proceeding to the complicating conditions of combustion.
Highlights
The design and operation of continuous combustion devices such as gas turbines, |)oilers, and furnaces may be assisted by the development and application of suitable predictive models that account for pollutant production, combustion efficiency, and heat release behavior
The present study addresses an essential element of continuum flow modeling--the use of eddy viscosity submodels to account for turbulence transport processes
The relative merits of an algebraic and a two-equation eddy viscosity submodel for simulating turbulence transport properties in recirculating flows have been assessed by comparing continuum model predictions of the turbulent, backmixed flowfield in an opposed-jet combustor to experimental measurements
Summary
The design and operation of continuous combustion devices such as gas turbines, |)oilers, and furnaces may be assisted by the development and application of suitable predictive models that account for pollutant production, combustion efficiency, and heat release behavior. ~. To effectively test the submodels of eddy viscosity over a range of turbulent conditions, experimental data were obtained for approach velocities of 15.24 and 7.62 m/see and jet velocities of 30.5, 61 and 130 m/see. This result is especially evident near the plane of the jet exit where the redistribution of the bulk flow with the jet discharge is restricted This effect is conveyed downstream from the recirculation zone where the two-equation velocity profiles correlate well with experiment near the chamber wall but deviate near the jet wall. A two-step global reaction mechanism for methane oxidation was adopted for the present study to illustrate the applicability of current numerical methods to the prediction of continuous combustion flows, and to briefly explore the suitability of the eddy viscosity submodels for the case of hot flow: ca 4 + 0 2 ~ CO + H 20. The measurement of species susceptible to sample transformations (e.g. NO, NO2) must be regarded as qualitative estimates of the actual local concentrations
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