K. Kailasanath

Review of Propulsion Applications of Detonation Waves

Applications of detonations to propulsion are reviewed. First, the advantages of the detonation cycle over the constant pressure combustion cycle, typical of conventional propulsion engines, are discussed. Then the early studies of standing normal detonations, intermittent (or pulsed) detonations, rotating detonations, and oblique shock-induced detonations are reviewed. This is followed by a brief discussion of detonation thrusters, lasersupported detonations and oblique detonation wave engines. Finally, a more detailed review of research during the past decade on ram accelerators and pulsed detonation engines is presented. The impact of the early work on these recent developments and some of the outstanding issues are also discussed.

Determination of detonation cell size and the role of transverse waves in two-dimensional detonations☆

Abstract Two-dimensional time-dependent numerical simulations have been performed to study the structure and propagation of self-sustained detonations. The simulations are first used to develop a systematic approach for determining the detonation cell size. This approach involves simulating systems with channel widths both larger and smaller than the transverse cell spacing. The cell size estimated using this approach is compared with experimental data. The simulations also provide insight into some aspects of the mechanism by which a two-dimensional, self-sustained detonation propagates. The evolution of the curvature of the transverse wave appears to be a crucial feature. It is shown that depending on the curvature of the transverse wave at the time of its reflection from either a neighboring transverse wave or a wall, flattened cells or pockets of unreacted gas can be formed.

Detonation structures behind oblique shocks

Detonation structures generated by wedge‐induced, oblique shocks in hydrogen–oxygen–nitrogen mixtures were investigated by time‐dependent numerical simulations. The simulations show a multidimensional detonation structure consisting of the following elements: (1) a nonreactive, oblique shock, (2) an induction zone, (3) a set of deflagration waves, and (4) a ‘‘reactive shock,’’ in which the shock front is closely coupled with the energy release. In a wide range of flow and mixture conditions, this structure is stable and very resilient to disturbances in the flow. The entire detonation structure is steady on the wedge when the flow behind the structure is completely supersonic. If a part of the flow behind the structure is subsonic, the entire structure may become detached from the wedge and move upstream continuously.

Large-Eddy Simulations of a Supersonic Jet and Its Near-Field Acoustic Properties

Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield and near-field acoustics for various total pressure ratios from overexpanded to underexpanded jet flow conditions have been investigated. The location and spacing of the shock cells are in good agreement with experimental data and previous theoretical results. The velocity profiles are also in good agreement with data from experimental measurements. A Mach disk is observed immediately downstream of the nozzle exit for overexpanded jet conditions with nozzle pressure ratios much lower than the fully expanded value. It is found that this type of nozzle with a sharp turning throat does not have a shock-free condition for supersonic jet flows. The near-field intensities of pressure fluctuations show wavy structures for cases in which screech tones are observed. The large-eddy simulations predictions of the near-field noise intensities show good agreement with those obtained from experimental measurements. This good agreement shows that large-eddy simulations and measurements can play complementary roles in the investigation of the noise generation from supersonic jet flows.

Flow-field effects on soot formation in normal and inverse methane-air diffusion flames

Abstract We investigate the effects of the flow-field configuration on the sooting characteristics of normal and inverse coflowing diffusion flames. The numerical model solves the time-dependent, compressible, reactive-flow, Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. A benchmark calculation is conducted and compared with experimental data, and shows that computed peak temperatures and species concentrations differ from the experimental values by less than 10%, while the computed peak soot volume fraction differs from the experimental values by 10–40%, depending on height. Simulations are conducted for three normal diffusion flames in which the fuel/air velocities (cm/s) are 5/10, 10/10, and 10/5, and for an inverse diffusion flame (where the fuel and air ports have been reversed) with a fuel/air velocity of 10/10. The results show significant differences in the sooting characteristics of normal and inverse diffusion flames. This work supports previous conclusions from the experimental work of others. However, in addition, we use the ability of the simulations to numerically track soot parcels along pathlines to further explain the experimentally observed phenomena. In normal diffusion flames, both the peak soot volume fraction and the total mass of soot generated is several orders of magnitude greater than for inverse diffusion flames with the same fuel and air velocities. In normal diffusion flames, soot forms in the annular region on the fuel-rich side of the flame sheet, while in inverse flames, the soot forms in a fuel-rich region on top of the flame sheet. Surface growth is the dominant soot formation mechanism (compared to nucleation) for both types of flames; however, surface growth rates are much faster for normal diffusion flames compared to inverse flames. Soot oxidation rates are also much faster in normal flames, where the dominant soot-oxidizing species is OH, compared to inverse flames, where the dominant soot-oxidizing species is O 2 . In the inverse flames, surface growth continues after oxidation has ceased, causing the peak soot volume fraction to be sustained for a long period of time, and causing the emission of soot, even though the quantity of soot is small. Comparison of soot formation among the three normal diffusion flames shows that the peak soot volume fraction and total mass of soot generated increases as the fuel-to-air velocity ratio increases. A larger fuel–air velocity ratio results in a longer residence time from the nucleation to the oxidation stage, allowing for more soot particle growth. When the fuel-to-oxidizer ratio decreases, there is less time for surface growth, and the particles cross the flame sheet (where they are oxidized) earlier, resulting in decreased soot volume fraction.

Three-dimensional numerical simulations of unsteady reactive square jets☆☆☆

Abstract Results of finite-difference, time-dependent numerical studies of the near field of subsonic, reactive square jets were presented. The simulations model space/time-developing compressible (subsonic) jets, using species- and temperature-dependent diffusive transport, and finite-rate chemistry appropriate for H2 combustion. Comparative measurements of entrainment for square jets were obtained based on evaluations of streamwise mass-flux to obtain an assessment on how the jet development is affected by chemical exothermicity and density differences between the jet and the surroundings. Depending on initial conditions (i.e., on the chemical exothermicity level implied by the initial reactant concentration), chemical energy release and expansion effects can be significant in determining reduced entrainment and initial jet growth relative to corresponding nonreactive jets. The instantaneous product formation rates are closely correlated with the local entrainment rates controlled by the vorticity bearing fluid. Instantaneous entrainment rates—based on the rate of increase of mass flux of rotational fluid—were found to be significant in the regions of roll-up and initial self-deformation of vortex rings, and then farther downstream, in the vortex merging region, where fluid and momentum transport between the jet and its surroundings are considerably enhanced by the presence of hairpin vortices aligned with the corners. Analysis of the combustion dynamics in terms of scalar mixing fraction diagnostics previously used in laboratory reactive turbulent jet experiments, was shown to be also potentially useful in characterizing their transitional regime by bringing out the relation between product formation rates and underlying fluid dynamical events.

Numerical modeling of water mist suppression of methane-air diffusion flames

Abstract This paper describes a numerical model for studying the suppression of co-flow diffusion flames by fine water mist. A two-continuum formulation is used in which the gas phase and the water mist are both described by equations of the eulerian form. The model is used to obtain a detail understanding of the physical processes involved during the interaction of water mist and flames. The relative contribution of various mist suppression mechanisms is studied. The effect of droplet diameter, spray injection density and velocity on water mist entrainment into the flame and flame suppression is quantified. Droplet trajectories are used to identify the regions of the flame where the droplets evaporate and absorb energy Finally, the model is used to determine the water required for extinction, and this is reported in terms of the ratio of the water supply rate to the fuel flow rate.

Flow structure and acoustics of supersonic jets from conical convergent-divergent nozzles

Conical convergent-divergent (CCD) nozzles represent an important category of supersonic jet-engine nozzles which require variable throat areas and variable exit areas to adapt to a range of operating conditions. CCD nozzles with design Mach numbers of 1.3, 1.5, and 1.65 are examined experimentally over a range of fully expanded Mach numbers from 1.22 to 1.71. The characteristics of the flow and acoustic fields from these nozzles are explored. Shadowgraph, Particle Image Velocimetry, far-field and near-field acoustic surveys are presented. Results of a Monotonically Integrated Large Eddy Simulation are presented for the Mach 1.5 nozzle at an underexpanded condition. The agreement between simulations and measurements is excellent. It is shown that these nozzles differ from traditional smoothly contoured method-of-characteristics nozzles in that they never achieve a shock free condition. Furthermore it is shown that these nozzles produce a “double diamond” pattern in which two sets of shock diamonds are generated with an axial displacement between them. The cause of this phenomenon is explored. It is further shown that as a consequence they are never free from shock-associated noise even when operated at perfect expansion. In spite of this difference, it is found that CCD nozzles behave like traditional convergent-divergent nozzles in that they produce the same shock-cell size, broadband shock-associated noise peak frequency, and screech frequency as traditional convergent-divergent nozzles. The apparent source regions for mixing noise, broadband shock associated noise and screech are all similar to those from traditional convergent-divergent nozzles.

Effect of Inlet on Fill Region and Performance 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. Unlike the PDE, RDE’s require fuel and oxidizer under high pressure to be injected through micro-nozzles from mixture plenums. This injection process is critically important to the stability and performance of the RDE. In previous papers, this injection process has been idealized as a mass addition source term at the injection wall. This has allowed us to do a variety of parametric studies on the effect of plenum pressure, back pressure, and engine geometric parameters. Due to the importance of the injection process, it is important to more closely focus on different approximations to this process; from idealized representations of the micro-nozzles to actually modeling individual micro-injectors. The results show that the RDE simulation is very sensitive to how these injectors are modeled; however, the more stable ideal injection approximation still provides valuable information on the influence of different parameters on overall performance. Values for specific impulse vary from 5331 s to 4918 s.

Numerical Simulations of the Cellular Structure of Detonations in Liquid Nitromethane-Regularity of the Cell Structure.

Abstract The detailed structure of planar detonation waves in liquid nitromethane was studied using time-dependent two-dimensional numerical simulations. The walls are assumed to confine heavily the liquid explosive and boundary layer effects are neglected. The solution thus simulates the detonation structure near the center of a wide channel. Chemical decomposition of nitromethane is described by a two-step model composed of an induction time followed by energy release. A simplified equation of state based on the Walsh and Christian technique for condensed phases and the BKW equation of state for gas phases us used. When mixtures of both phases are present, pressure and temperature equilibrium between them is assumed. The simulations show a cellular pattern traced by a system of triple points dividing the detonation front into sections. However, a substructure of weaker triple points also traces out a nonuniform pattern within the main pattern, resulting in an irregular cellular structure. A correlation exists between the regularity of the cellular pattern and both the curvature of the front and the change in induction zone thickness at the triple points. If the induction time is a stronger function of temperature, the weaker triple points disappear and a more regular structure is produced. When the structures are regular, the detonation front is more curved and there is a larger change in induction zone thickness at the triple points. However, the large change in induction zone thickness also leads to the formation of unburned pockets that eventually disturb the symmetry and uniformity of the structure. We conclude that the regularity of the cellular pattern is strongly influenced by the temperature-dependence of the induction time.