Vishal Acharya

Bio: Vishal Acharya is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Premixed flame & Laminar flame speed. The author has an hindex of 13, co-authored 39 publications receiving 618 citations.

Transverse to Longitudinal Acoustic Coupling Processes in Annular Combustion Chambers

Abstract: Combustion instability is a major issue facing lean, premixed combustion approaches in modern gas turbine applications. This paper specifically focuses on instabilities that excite transverse acoustic modes of the combustion chamber. Recent simulation and experimental studies have shown that much of the flame response during transverse instabilities is due to the longitudinal fluid motions induced by the fluctuating pressure field above a nozzle. In this study, we analyze the multi-dimensional acoustic field excited by transverse acoustic disturbances interacting with an annular side branch, emulating a fuel/air mixing nozzle. Key findings of this work show that the resultant velocity fields are critically dependent upon the structure of the transverse acoustic field and the nozzle impedance. Significantly, we also show that certain cases can be understood from relatively simple quasi one-dimensional considerations, but that other cases are intrinsically three-dimensional.

Premixed flames excited by helical disturbances: flame wrinkling and heat release oscillations

Abstract: This paper describes an analysis of the response of swirling premixed flames to helical disturbances, i.e., where the flow fluctuations have an azimuthal dependence of the form u^i′∞ exp(imθ) and m denotes the helical mode number. Results elaborate the nature of local flame wrinkling and heat release fluctuations. Significantly, these results show that these different induced fluctuations exhibit very different sensitivities to helical mode number m, swirl strength, and dimensionless frequency. In addition, the degree of axisymmetry of the time averaged flame plays a crucial role in these interactions, particularly in how flow disturbances u^i′ translate into heat release oscillations. Thus, the helical mode m, with the dominant contribution to local flame wrinkling and heat release, m=m0, and spatially integrated heat release fluctuations, m=m1, is generally different. For example, helical mode m, leading to the largest amplitude of local flame wrinkling and heat release in a solid body swirl flowfield, ...

Response of non-premixed flames to bulk flow perturbations

Abstract: This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the -equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

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Prediction and control of combustion instabilities in real engines

Abstract: This paper presents recent progress in the field of thermoacoustic combustion instabilities in propulsion engines such as rockets or gas turbines. Combustion instabilities have been studied for more than a century in simple laminar configurations as well as in laboratory-scale turbulent flames. These instabilities are also encountered in real engines but new mechanisms appear in these systems because of obvious differences with academic burners: larger Reynolds numbers, higher pressures and power densities, multiple inlet systems, complex fuels. Other differences are more subtle: real engines often feature specific unstable modes such as azimuthal instabilities in gas turbines or transverse modes in rocket chambers. Hydrodynamic instability modes can also differ as well as the combustion regimes, which can require very different simulation models. The integration of chambers in real engines implies that compressor and turbine impedances control instabilities directly so that the determination of the impedances of turbomachinery elements becomes a key issue. Gathering experimental data on combustion instabilities is difficult in real engines and Large Eddy Simulation (LES) has become a major tool in this field. Recent examples, however, show that LES is not sufficient and that theory, even in these complex systems, plays a major role to understand both experimental and LES results and to identify mitigation techniques.

Dynamics of Swirling Flames

Abstract: In many continuous combustion processes, such as those found in aeroengines or gas turbines, the flame is stabilized by a swirling flow formed by aerodynamic swirlers. The dynamics of such swirling flames is of technical and fundamental interest. This article reviews progress in this field and begins with a discussion of the swirl number, a parameter that plays a central role in the definition of the flow structure and its response to incoming disturbances. Interaction between the swirler response and incoming acoustic perturbations generates a vorticity wave convected by the flow, which is accompanied by azimuthal velocity fluctuations. Axial and azimuthal velocities in turn define the flame response in terms of heat--release rate fluctuations. The nonlinear response of swirling flames to incoming disturbances is conveniently represented with a flame describing function (FDF), in other words, with a family of transfer functions depending on frequency and incident axial velocity amplitudes. The FDF, howev...