Kailas Kailasanath

Numerical Investigation of Rotating Detonation Engines

Rotating detonation engines (RDE’s) represent an alternative to the extensively studied pulse detonation engines (PDE’s) for obtaining propulsion from the high efficiency detonation cycle. Since it has received considerably less attention, the general flow-field and effect of parameters such as stagnation conditions, combustion chamber length, and fuel mixture on specific impulse are less well understood than for PDE’s. In this paper we develop a model for doing time-accurate calculations of RDE’s in two and three dimensions, using algorithms that have successfully been used for PDE simulations previously. Results are shown for a stoichiometric hydrogen-air RDE operating at 10 atm, 300 K stagnation premixture conditions and 1 atm back pressure. Conditions within the chamber are described as well as inlet and outlet conditions and integrated quantities such as total mass flow, force, and specific impulse. Further computations examined the role of back pressure and inlet stagnation pressure on performance. It was found that the specific impulse was dependent on pressure ratio, whereas the mass flow and propulsive force were primarily dependent on the stagnation properties of the inlet micro-nozzles. The specific impulse varied from 3845 sec to 5560 sec over a pressure ratio of 5 to 30. The specific impulse for the 10 atm stagnation pressure, 1 atm back pressure was 5130 sec.

Feedback into Mixture Plenums in Rotating Detonation Engines

Rotating detonation engines (RDE’s) represent a natural extension of the pulse detonation engine (PDE) concept to a continuous detonation for obtaining propulsion and power from the high efficiency detonation cycle. Although extensively studied for the last few years, RDE’s are still less understood and developed than the PDE concept. In particular, interaction between the detonation wave in the combustion chamber and the upstream mixture plenum is critical to the stability and performance of an RDE, and has not been studied in detail. In this paper, we extend our well-tested RDE models to examine the effect of pressure feedback into the mixture plenum with two and three-dimensional simulations. We examine three main injector configurations to determine their effect on stability, performance, and feedback into the mixture plenum. Results indicate that although the flow-field is considerably different, the performance of the RDE with all three modeled injection systems is close to that of assuming an ideal injection process, especially for a injector throat to injector face area ratio of 0.2. For an area ratio of 0.4, the drop in performance is more noticable, varying between 2.4% at 10 atm plenum pressure to 11.4% at 4 atm. Specific impulses for all the modeled injector cases varied from 3975 s to 5263 s. The stability at high and low pressures becomes much more of an issue when modeling specific injectors, due to an increase in flow unsteadiness from the injection. Pressure feedback into the mixture plenum appears to be relatively low for area ratios of 0.2 (less than 10% of stagnation pressure), but becomes very large for even moderate area ratios of 0.4. The cylindrical micro-injectors had the worst pressure feedback, with underpressures between 1 and 1.77 atm and overpressures between 3.09 and 3.83 atm with the large area ratio.

Numerical Study of the Effects of Engine Size on Rotating Detonation Engines

Rotating detonation engines (RDE’s) represent an alternative to the extensively studied pulse detonation engines (PDE’s) for obtaining propulsion from the high efficiency detonation cycle. Since it has received considerably less attention, the general flow-field and effect of parameters such as stagnation conditions, combustion chamber sizing, and fuel mixture on specific impulse are less well understood than for PDE’s. In this paper we use a model developed previously for doing time-accurate calculations of RDE’s in two and three dimensions to examine the effect of different engine sizing parameters on mass flow rate, performance, and thrust. Specific sizing parameters that are discussed are area ratio of micro-injectors to head-end wall, combustion chamber diameter and length. Additionally, several three-dimensional simulations for small combustion chambers with different thicknesses are shown. Results indicate that for many of these parameters, the characteristics of the engine scale in predictable ways for high plenum pressures. At lower plenum pressures, the results are more difficult to interpret. Specific impulses varied from 3300 s (low-pressure, large chamber length) to 5500 s.

Towards Efficient, Unsteady, Three-Dimensional Rotating Detonation Engine Simulations

Rotating detonation engines (RDEs) represent an alternative to the extensively studied pulse detonation engines (PDEs) for obtaining propulsion from the high efficiency detonation cycle. Numerical simulations play an important role in understanding the basic physics of the RDE and will be important in optimizing the geometry and flow-field conditions for an RDE, but are necessarily unsteady and three-dimensional. The current paper describes efforts to develop a new code, Propel, for simulating complex engine designs. Propel has support for structured and unstructured meshes, several different numerical algorithms and limiters, and can be run on both CPUs and GPUs in HPC to laptop environments. This paper compares two and three dimensional solutions using Propel for a detonation tube and baseline RDE with our current RDE simulation tool. As an example of the capabilities of the new Propel code, we examine some preliminary calculations of the expansion flow region of a Rotating Detonation Engine as the expansion geometry of the combustion chamber is modified.

Modeling Exhaust Effects in Rotating Detonation Engines

Rotating detonation engines (RDEs) represent an alternative to the extensively studied pulse detonation engines (PDEs) for obtaining propulsion from the high efficiency detonation cycle. The current work focuses on the effect of adding an exhaust plenum to the RDE simulation. The addition of an exhaust plenum causes only minimal changes in the RDE flow field, most significantly at the exhaust plane. The specific impulse varied by less than 1% between modeling with and without an exhaust plenum, showing that using a mixed boundary condition for the exhaust instead of an exhaust plenum is valid. The exhaust plume for three different RDEs is also described. The highest plume temperatures are found after the circulation zone behind the centerbody, although the temperature profiles vary greatly depending on the RDE size. Finally, the effect of a simple conical nozzle extending from the centerbody on the exhaust is investigated. This nozzle reduces the size of the recirculation zone and reduces the temperature in the plume, however, it has very little effect on the flow field inside the RDE or at the exhaust plane. For the hydrogen/air RDE with 10 atm injection pressure and 1 atm back pressure, the specific impulse varied from 4930 s to 4980 s for all cases examined in this paper.

Physics of Heat-Release in Rotating Detonation Engines

Rotating detonation engines (RDEs), along with the well-studied Pulsed Detonation Engine (PDEs), represent an alternative for obtaining propulsion and power generation using the high efficiency detonation cycle. In the last half decade, the RDE has been studied aggressively internationally due to its advantages over conventional PDEs. Many previous studies have focused on the main characteristics and performance of RDEs in various configurations and conditions. The performance of any combustor is tied closely to the local heat-release conditions in the combustor. Unlike more conventional combustors or even the closely related Pulsed Detonation Engines, heat-release occurs under a wide range of conditions within RDEs from detonation to deflagration to even non-premixed burning, and so describing it solely as a “detonation cycle engine” is not completely accurate. In this paper we use a deflagration model in simulations with different configurations to try to more accurately pinpoint where heat is being released, and how this affects the performance of the resulting engine. In particular, we examine how changing the pressure, the injector configuration, and the mixedness of the reactants may change the different heat-release regimes and ultimately the performance of the RDE. Results show that for cases using the ideal injection model, low pressure reactions are insignificant and have only a small effect on performance. When computing finite-sized injectors, a separate low pressure reaction model has to be used due to overprediction of deflagration and recovery zone reactions that eventually cause the detonation wave to fail. In these cases, less than half of the reactants are burned within the detonation wave, although the detonation wave is still the dominant feature of the flow-field. Detonation wave speeds varied from 1637 m/s to 1844 m/s, and specific impulses varied from 4650 s to 4980 s for 10 atm feed stagnation pressure.

Numerical Study of Noise Characteristics in Overexpanded Jet Flows

Abstract : Noise characteristics in shock-containing jets at an overexpanded jet condition have been investigated. Total temperature ratios of 1.0 (cold), 2.0, and 3.0 are considered. The cold jet is a highly screeching jet. Frequency-wavenumber Fourier analysis is employed to examine the wave characteristics of pressure waves along the lip line and also along a near-field conical surface. It is found that the radiating portion of the pressure wave intensity increases with the jet temperature, but the hydrodynamic portion is much less sensitive to the change of the jet temperature. The near-field noise intensity associated with the Mach wave radiation is observed over a large axial distance, and the Mach wave radiation extends to much higher frequencies in heated jets. The peak radiation direction in the cold jet is dominated by the axisymmetric mode, but the directions around the sideline show a much weaker azimuthal dependence. Furthermore, the axial locations of lip-line pressure peak intensities at the screech frequency are near the axial locations of shock-cell tips. A reinforcing loop between upstream/downstream propagating waves and the induced shock-cell coherent oscillatory motion is observed in the highly screeching jet. The formation of this reinforcing loop requires a match of the peak phase velocities between upstream and downstream propagating waves. This phase velocity match exists in the highly screeching cold jet, but not in the weakly screeching heated jets. It appears that the phase velocity match that sustains the reinforcing loop is important to the screech generation, and the phase velocity mismatch in heated jets is believed to be an important cause of the screech intensity reduction.

Investigation of the Noise from a Rectangular Supersonic Jet

An experimental investigation of a low aspect ratio rectangular nozzle’s flow field was completed. Over, ideally, and under expanded operating conditions were presented for nozzle temperature ratios of between unity and three. The aim of this study was threefold, first to validate the newly constructed heated jet noise rig at the University of Cincinnati, second to provide data for future computational validation, and third to compile acoustic baselines for future noise reduction studies. Validation of the experimental results was achieved through comparison to jet noise theories and empirical relations, particularly the fine and large scale similarity spectrum, screech frequency predictions, and scaling of acoustic intensity with powers of the expansion factor β. Excellent comparisons were achieve with limited deficiencies identified, such as possible nonlinear propagation and/or rig noise contamination at high frequencies for elevated temperatures. It was found for the current experimental setup that screech was eliminated at highly elevated temperatures. This phenomena that is still currently debated and bears further investigation. Additionally, the noise source known as crackle was briefly investigated, and it was found that at elevated temperature and pressure ratios the measured acoustics met the criteria for a crackling jet.

Computational Study of Shock-Associated Noise Characteristics Using LES

Abstract : The shock-associated noise characteristics of an underexpanded jet at three jet temperatures were investigated using large-eddy simulations (LES). The impact of shock cells on the flow field and near- and far-field noise characteristics were examined. The impact of shock-associated noise is confined to one and a half jet core length in the near field. The near-field shift of the shock-associated peak frequency matches well with the inverse of the shock-cell size, indicating that the variation of the shock-cell size is largely responsible for this shift. In the far field, the variation of the shock-associated peak frequency agrees well with the available empirical model over a large range of the radiation angle, but the model under-predicts the fast increase near the end of the shock-associated noise region. In addition, the distributed nature of the shock-associated noise source impacts the far-field noise characteristic if the distance is not sufficiently large. Heating decreases the shock-cell impact on the total noise, but the heating impact on the shock-associated noise level in the upstream direction is opposite to that in the downstream direction. On the other hand, heating greatly increases the mixing noise, reducing the difference between the shock-containing jets and the shock-free jets.

A Hybrid Grid Compressible Flow Solver for Large-Scale Supersonic Jet Noise Simulations on Multi-GPU Clusters

ow simulations. Supersonic jet noise simulations require the accurate representation of complex nozzle geometry and thus the use of unstructured grids. While such a grid representation is crucial for the accurate representation of the nozzle geometry, it has the disadvantage of introducing a highly irregular memory access pattern, which violates GPU coalescing requirements resulting in an inecient use of the otherwise high memory bandwidth of GPUs and therefore a bottleneck in computational performance. In order to mitigate this performance bottleneck, a hybrid grid representation is implemented which allows for augmenting the unstructured grid representation in the vicinity of complex nozzle geometry with an ecient structured grid representation in other regions of the ow domain, which is able to fulll GPU coalescing requirements, and thus achieve a signicant improvement in computational performance, leading to a reduction in simulation turnaround time.