• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.7
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• pp.79-88
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• 2006

In the present paper, the swirl flow structure and flame characteristics of turbulent premixed combustion in a model gas turbine combustor are investigated using large eddy simulation(LES). A G-equation flamelet model is employed to simulate the unsteady flame behavior. When inlet swirl number is increased, the distinct flow structures, such as the shapes of corner recirculation and center toroidal recirculation zone, are observed and the flame length is shorted gradually. Also, the phenomena of flashback are identified at strong swirl intensity. In order to get the accurate description of unsteady flame behavior, the predictive ability of the acoustic wave in a combustor is primarily evaluated. It is found that the vortex generated near the edge of step plays an important role in the flame fluctuation. Finally it is examined systematically that the flame and heat release fluctuation are coupled strongly to the vortex shedding generated by swirl flow and acoustic wave propagation from the analysis of flame-vortex interaction.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.5
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• pp.342-350
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• 2008

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle($\alpha$) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of ${\alpha}=45^{\circ}$ show smaller size and upstream location of CTRZ than those of $90^{\circ}$ and $30^{\circ}$ by the development of higher swirl velocity. The flame length in the case of ${\alpha}=45^{\circ}$ is shorter than other cases, while the case of ${\alpha}=30^{\circ}$ yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of ${\alpha}=45^{\circ}$ shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97dB, compared to that of ${\alpha}=30^{\circ}$ having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.9 s.240
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• pp.988-996
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• 2005

Flame structure and extinction mechanism of counterflow methane/air non-premixed flame diluted with nitrogen are studied by NASA 2.2 s drop tower experiments and two-dimensional numerical simulations with finite rate chemistry and transport properties. Extinction mechanism at low strain rate is examined through the comparison among results of microgravity experiment, 1D and 2D simulations with a finite burner diameter. A two-dimensional simulation in counterflow flame especially with a finite burner diameter is shown to be very important in explaining the importance of multidimensional effects and lateral heat loss in flame extinction, effects that cannot be understood using a one-dimensional flamelet model. Extinction mechanism at low strain rate is quite different from that at high strain rate. Low strain rate flame is extinguished initially at the outer flame edge, the flame shrinks inward, and finally is extinguished at the center. It is clarified from the overall fractional contribution by each term in energy equation to heat release rate that the contribution of radiation fraction with 1D and 2D simulations does not change so much and the overall fractional contribution is decisively attributed to radial conduction ('lateral heat loss'). The experiments by Maruta et at. can be only completely understood if multi-dimensional heat loss effects are considered. It is, as a result, verified that the turning point, which is caused only by pure radiation heat loss, has to be shifted towards much lower global strain rate in microgravity flame.

• Journal of the Korean Institute of Gas
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• v.7 no.3 s.20
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• pp.44-50
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• 2003

The axisymmetric methane-air counterflow flame was simulated to investigate changes in the flame structure due to the fuel concentration and to evaluate the numerical method. The global strain rates $a_g=20,\;60,\;90\;s^{-1}$ and the mole fractions of methane $x_m=20,\;50,\;80\%$ in the fuel stream were taken to be numerical parameters. The axisymmetric simulation was conducted by using the Fire Dynamics Simulator (FDS) which employed a mixture fraction combustion model, and the results were compared with those of OPPDIF, which is an one-dimensional flamelet code and includes detail chemical reactions. In all the cases tested, there was good agreement in the temperature and axial velocity profiles between the axisymmetric and one-dimensional simulations. It was shown that the flame thickness and peak flame temperature increase and the flame radius decreases as the fuel concentration increases.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.5
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• pp.79-87
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• 2002

Acoustic responses to pressure oscillations in axisymmetric combustion chamber are numerically investigated to examine the qualitative trend of acoustic instability in liquid rocket engine. Chamber operating condition and excitation frequency of oscillating pressure are selected as exciting parameters of acoustic instability. Artificial perturbation is simulated by total-pressure oscillation with sine wave at chamber inlet. Many approximations and simplifications are introduced without losing the essence of acoustic pressure response. First, steady-state solution for each operating condition is obtained and next, transient analysis is conducted. Depending on operating condition and excitation frequency, the distinct response characteristics are brought. Weak-strength flames and high-frequency excitation tend to cause sensitive acoustic pressure response leading to unstable pressure field. These results are analyzed based on the correlation with acoustic pressure responses from the previous works adopting laminar flamelet model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.7
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• pp.2386-2396
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• 1996

A numerical simulation has been performed for isothermal and reacting flows in an exisymmetric, bluff-body research combustor. The present formulation is based on the density-weighted averaged Navier-Stokes equations together with a k-epsilon. turbulence model and a modified eddy-breakup combustion model. The PISO algorithm is employed for solution of thel Navier-Stokes system. Comparison between measurements and predictions are made for a centerline axial velocities, location of stagnation points, strength of recirculation zone, and temperature profile. Even though the numerical simulation gives acceptable agreement with experimental data in many respects, the present model is defictient in predicting the recoveryt rate of a central near-wake region, the non-isotropic turbulence effects, and variation of turbulent Schmidt number. Several possible explanations for these discrepancies have been discussed.