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Bluff-body stabilized turbulent premixed flames represent a challenging canonical configuration for computational simulation and validation. Past simulation results from many research groups on the...

Proper orthogonal decomposition and dynamic mode decomposition are evaluated for the study of self-excited longitudinal combustion instabilities in laboratory-scaled single-element gas turbine and rocket combustors. Since each proper orthogonal decomposition mode comprises multiple frequencies, specific modes of the pressure and heat release are not related, which makes the analysis more qualitative and less efficient for identifying physical mechanisms. On the other hand, dynamic mode decomposition analysis generates a global frequency spectrum in which each mode corresponds to a specific discrete frequency so that different dynamics can be correlated. In addition, proper orthogonal decomposition results are found to be inaccurate when only a limited amount of spatial information is provided in contrast with dynamic mode decomposition results, which provide more reliable results. Overall, dynamic mode decomposition analysis proves to be a robust and systematic method that can give consistent interpretati...

Combustion instability in rocket chambers is strongly influenced by acoustic interactions at the boundaries of the configuration. Many CFD simulations employ approximate boundary conditions in order to simplify the geometry but the impact that they have on the solution is not well understood. The present study focuses on the use of detailed (exact) boundary representations and an approximate boundary condition in a given longitudinal mode test chamber. The actual inlet boundary of the injector is comprised of a series of choked slots while the approximate boundary condition is a uniform constant mass flow. Both two and three-dimensional simulations are carried out. Differences in the flowfield are evident in the combustion region away from the inlet, including the size of the recirculation region and location of the peak heat release. The amplitudes of the acoustic modes are well predicted for the first two modes especially in three-dimensional simulation, while higher modes are poorly predicted. These results suggest that such boundary condition approximations must be judiciously used and having access to more detailed treatments is important to verify accuracy.

Three-dimensional simulations of combustion instability can provide extremely valuable insight into the problem physics, but they represent an enormous investment in time and, thus, may preclude parametric studies that are practical with two-dimensional simulations. Three numerical simulations of an unstable laboratory rocket combustor were performed to assess the effect of grid resolution and dimensionality on the ability to predict and model combustion instability. Computations show that two-dimensional axisymmetric simulations using both coarse and fine grids are able to capture the amplitude of the first longitudinal mode within an order of magnitude of what was measured in the experiment. Whereas the three-dimensional calculation based on the coarse grid resolution shows excellent agreement with the experimental level of instability two-dimensional simulations are valuable in parametric evaluations in developing diagnostic methods and in increasing the efficiency of expensive three-dimensional simula...

Large Eddy Simulation (LES) is applied to study transverse combustion instability in a seven element combustor. The combustor is a self-excited linear array of seven elements. Different levels of instability are produced by changing which injectors flow fuel. Two configurations were analyzed, the most stable and the most unstable experimental configuration. The simulations showed different behavior for each case. For the unstable case, the simulated amplitudes were lower than the experimental, and there was nota well-defined transverse wave present in the combustor. Interestingly in this case the center element coupled with the first mode in the combustor and appeared to have a stabilizing effect. The amplitude of the simulated stable case was larger than the experimental, but showed evidence of a transverse instability.

near the tip of the potential core and downstream. Axial and radial variation in radiation intensity fluctuations is similar to those reported for flames. Autocorrelation coefficients of the radiation intensity are approximated reasonably well by exponential curves. Integral time and length scales increase monotonically downstream of the core region and are consistent with Taylor’s hypothesis. The break frequency and slope of the normalized power spectral density function are comparable to those reported for turbulent jet flames. These findings suggest that reacting flows can be used to predict trends in turbulent radiation properties of exhaust plumes.

Bluff body stabilized turbulent premixed flames represent a canonical configuration with relevance to reacting flows in gas turbine engines. In addition to its relevance, the geometric simplicity of this flame has prompted the collection of significant amounts of experimental data that have been used by many researchers for model validation. A review of the literature shows that many of these efforts focus on stable combustion as opposed to the more challenging phenomena of lean blowout and thermoacoustic oscillations. Using the stable combustion condition as a starting point, Detached Eddy Simulations (DES) are performed to identify the mesh resolution requirements for this flow and the selected solver. Stable combustion results show reasonable agreement with mean and second moment statistics of velocity, as well as mean temperature profiles. Simulations of lean blowout and thermoacoustic instability are also presented in order to assess and understand the level of fidelity needed to capture more complex bluff body flame phenomena.

Unstable liquid rocket engines can produce highly complex dynamic flowfields with features such as rapid changes in temperature and pressure, increased heat transfer, local flame extinction and reignition, and local partially-premixed and non-premixed combustion. This type of flowfield represents a challenge for turbulent combustion models, which are typically associated with a number of assumptions that limit regime applicability. In the present study, the linear eddy model (LEM) is applied to an unstable single element liquid rocket engine to assess its performance and to contrast it with simple laminar combustion model (LCM). Two distinct operating conditions showing different dynamic behavior are used; the first is marginally stable and has peak-to-peak amplitudes of 12% of the mean, while the second is strongly unstable and has pressure amplitudes in excess of 40% of the mean. Results show that while the LEM is able to capture the general dynamics behavior, the trends are in the wrong direction when compared with the experimental results. In other words, the stable case becomes more unstable and the unstable case becomes more stable. The paper also examines the underlying assumptions of the LEM and suggests reasons for the observed behavior.

In this study a series of three-dimensional unsteady reacting flow simulations are used to investigate the effect of swirl on the instability amplitude of a single-element gas-gas rocket combustor. The baseline combustor of interest is unstable because of a fuel cut-off event caused by the high-pressure waves in the combustor. Previous two-dimensional simulations have shown that swirl reduces the amplitude of the pressure oscillations compared with that of the baseline configuration. The current three-dimensional simulations show that swirl is indeed able reduce the amplitude of the instabilities, albeit not to the same extent observed in the two-dimensional simulations. We further observe that the enhanced mixing due to the swirling flow leads to a reduction in the recovery time associated with the fuel cut-off event, thereby allowing the combustor to experience a more continuous heat release. Nevertheless, unlike the two-dimensional case, the three-dimensional simulations show that the flame does not stay anchored to the dump-plane, which explains the higher relative amplitudes in this case.

Computational fluid dynamics simulations are used to study spontaneous combustion instabilities in a rectangular chamber that contains seven coaxial injector elements. The simulation predicts self-excited transverse oscillations that have variable amplitudes, similar to what is seen in a companion experiment. Several processes are identified in the simulation that may promote the rise of instability by increased heat release during a local compression, including the interaction between vortices shed from adjacent injectors and interactions between flames and the combustor walls.

Combustion instability continues to be a challenge in the design of rocket engines. The use of computational fluid dynamics (CFD) simulations screen potential designs offers the ability to reduce the number of costly tests and improve understanding of the underlying instability mechanism. In this study a series of axisymmetric CFD simulations are used to investigate the instability sensitivity to four design changes. The design changes are selected in an attempt to reduce the level of instability. The parameters considered are the combustor wall temperature, the effect of adding swirl to the fuel injector and two geometric changes, namely, fuel injector area reduction and the introduction of a chamfer in the injector face. The simulations show that both the wall temperature and swirl are able to significantly lower the amplitude by 70%. The results of the geometric changes are mixed with both decreases and increases in the instability amplitude. The parametric study has enhanced the understanding of the instability mechanisms by demonstrating that the when a continuous fuel supply to the combustor is maintained the instability amplitude is decreased.

An investigation of the instability mechanism present in a laboratory rocket combustor is performed using computational fluid dynamics (CFD) simulations. Three cases are considered which show different levels of instability experimentally. Computations reveal three main aspects to the instability mechanism, the timing of the pressure pulses, increased mixing due to the baroclinic torque, and the presence of unsteady tribrachial flame. The stable configuration shows that fuel is able to flow into the combustor continuously allowing continuous heat release. The unstable configuration shows that a disruption in the fuel flow into the combustor allows the heat release to move downstream and new fuel to accumulate in the combustor without immediately burning. Once the large amounts of fuel in the combustor burn there is rapid rise in pressure which coincides with the timing of the acoustic wave in the combustor. The two unstable cases show different levels of instability and different reignition mechanism.

Combustion instability in rocket chambers is strongly influenced by acoustic interactions at the boundaries of the configuration. Many CFD simulations employ approximate boundary conditions in order to simplify the geometry but the impact that they have on the solution is not well understood. The present study focuses on the use of detailed (exact) boundary representations and an approximate boundary condition in a given longitudinal mode test chamber. The actual inlet boundary of the injector is comprised of a series of choked slots while the approximate boundary condition is a uniform constant mass flow. Both two and three-dimensional simulations are carried out. Differences in the flowfield are evident in the combustion region away from the inlet, including the size of the recirculation region and location of the peak heat release. The amplitudes of the acoustic modes are well predicted for the first two modes especially in three-dimensional simulation, while higher modes are poorly predicted. These results suggest that such boundary condition approximations must be judiciously used and having access to more detailed treatments is important to verify accuracy.

Combustion instability arises from the coupling between unsteady combustion and acoustic modes in a combustion chamber. A study comprising concurrent experiment and LES simulation of a single element rocket combustor was conducted. The goal was to evaluate the a priori predictive ability of the computational model with regards to self-excited combustion instability, and to use the detailed results from the simulation to interpret the mechanism of self-excitation. Pressure modes and chemiluminescence from CH* were measured at high sampling rates. Direct comparisons between the experimental and computational results were made on the basis of instability frequency, pressure mode shapes, and limit cycle amplitude. All indicated generally good agreement. The light emission representing heat release from the experiment was qualitatively similar to the simulation. The detailed results from the simulation provided much greater insight into the complex phenomena, and showed the likely mechanism that drove high amplitude instability was a periodic ignition in the recirculation zone just downstream of the dump plane. The ignition occurred nearly simultaneously with the arrival of a compression wave traveling from downstream.

Numerical simulations of gaseous methane and decomposed hydrogen peroxide in a laboratory rocket combustor were performed in order to determine how to best calculate the Rayleigh index. Calculations using four levels of spatial re nement show the level of re nement necessary to identify the regions of ampli cation and damping is signi cantly coarser than the mesh used for simulation. The Rayleigh index for the rst longitudinal mode is bimodal and shows an isolated area of damping. The two areas of ampli cation are the result of two di erent driving mechanisms. Mode shape, phase angle, and frequency analysis of pressure, velocity, and heat release are presented to validate the combustion modeling approach and interpret the results.