Large eddy simulation of buoyancy effects on turbulent premixed combustion

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Large eddy simulation of buoyancy effects on turbulent premixed combustion

 

Author: Wu, Yingyan
Title: Large eddy simulation of buoyancy effects on turbulent premixed combustion
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Combustion -- Mathematical models
Turbulence -- Mathematical models
Department: Dept. of Applied Mathematics
Pages: xvi, 109 leaves : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2393017
URI: http://theses.lib.polyu.edu.hk/handle/200/5932
Abstract: In this research, numerical simulations employ large eddy simulation (LES) with a dynamic model for sub-grid scale stress. With the assumption of fast chemistry, a progress variable c-equation is applied to describe the flame front propagation. An extended progress variable model and additional mixture fraction Z are combined to consider premixed combustion with non-uniform equivalence ratio, which is caused by lateral entrainment of coflow. With low-Mach number approximation, the governing equations are solved by a projection-based fractional step method in two dimensions. The numerical method with an extended model is applied to simulate a slot Bunsen flame. Computed mean flame front is comparable to that of experiment and 3D computation using detailed chemical kinetics. Computed flame surface density profiles match with those of the experiment. The present numerical simulation also well predict the flame height and the global turbulent flame speed. Study on ignition and propagation of a turbulent premixed V-flame are carried out. The non-reacting flow field with a stabilizing rod is first obtained as the initial field and ignition happens just upstream of the stabilizing rod. The early shape of the flame after ignition is significantly affected by the velocity field formed close the flame holder. During the flame propagation, the vortices fade and move to the locations along flame front. The LES computed time-averaged velocity agrees well with experimental data. Finally, buoyancy effects on methane/air turbulent premixed V-flames are investigated by comparing the differences between numerical results with and without buoyancy force under variety conditions. Computed LES results of buoyancy effects on flame angle and .ame brush thickness are consistent with those obtained from experiments. In both +g and -g conditions, the effects of buoyancy become important with increasing Richardson number (Ri). Buoyancy force tends to close up the flame under +g, but has the opposite effect under -g. Buoyancy force also suppresses flame wrinkling in +g and enhances wrinkling in -g. The effect caused by the buoyancy force term is larger than the effect caused by the gravity force term alone. The discrepancy induced by considering only the gravity force term in the governing equations is larger than by ignoring both the gravity force term and the buoyancy force term. The differences caused by buoyancy between φ =0.7 and φ =0.8 is not significant on flame angle, flame brush thickness and mean axial velocity. This indicates that buoyancy have quantitatively identical effect on the flames for φ =0.7 and φ =0.8. The computed axial velocity is shown to be significantly affected by buoyancy downstream from the flame holder. The buoyancy effects cannot be ignored when Ri ≥ 0.06.

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