Transition Delay in Hypervelocity Boundary Layers By Means of CO₂/Acoustic Instability Interaction

The potential for hypervelocity boundary layer stabilization was investigated using the concept of damping Mack’s second mode disturbances by vibrational relaxation of carbon dioxide (CO₂) within the boundary layer. Experiments were carried out in the Caltech T5 hypervelocity shock tunnel and the Caltech Mach 4 Ludwieg tube. The tests used 5-degree half-angle cones (at zero angle of attack) equipped near the front of the cone with an injector consisting of either discrete holes or a porous section. Gaseous CO₂, argon (Ar) and air were injected into the boundary layer and the effect on boundary layer stability was evaluated by optical visualization, heat flux measurements and numerical simulation. In T5, tests were carried out with CO₂ in the free stream as well as injection. Injection experiments in T5 were inconclusive; however, experiments with mixtures of air/CO₂ in the free stream demonstrated a clear stabilizing effect, limiting the predicted amplification N-factors to be less than 13. During the testing activities in T5, significant improvements were made in experimental technique and data analysis. Testing in the Ludwieg tube enabled optical visualization and the identification of a shear-layer like instability downstream of the injector. Experiments showed and numerical simulation confirmed that injection has a destabilizing influence beyond a critical level of injection mass flow rate. A modified injection geometry was tested in the Ludwieg tube and we demonstrated that it was possible to cancel the shock wave created by injection under carefully selected conditions. However, computations indicate and experiments demonstrate that shear-layer like flow downstream of the porous wall injector is unstable and can transition to turbulence while the injected gas is mixing with the free stream. We conclude that the idea of using vibrational relaxation to delay boundary layer transition is a sound concept but there are significant practical issues to be resolved to minimize the flow disturbance associated with introducing the vibrationally-active gas into the boundary layer

Stagnation Hugoniot Analysis for Steady Combustion Waves in Propulsion Systems

The combustion mode in a steady-flow propulsion system has a strong influence on the overall efficiency of the system. To evaluate the relative merits of different modes, we propose that it is most appropriate to keep the upstream stagnation state fixed and the wave stationary within the combustor. Because of the variable wave speed and upstream stagnation state, the conventional Hugoniot analysis of combustion waves is inappropriate for this purpose. To remedy this situation, we propose a new formulation of the analysis of stationary combustion waves for a fixed initial stagnation state, which we call the stagnation Hugoniot. For a given stagnation enthalpy, we find that stationary detonation waves generate a higher entropy rise than deflagration waves. The combustion process generating the lowest entropy increment is found to be constant-pressure combustion. These results clearly demonstrate that the minimum entropy property of detonations derived from the conventional Hugoniot analysis does not imply superior performance in all propulsion systems. This finding reconciles previous analysis of flowpath performance analysis of detonation-based ramjets with the thermodynamic cycle analysis of detonation-based propulsion systems. We conclude that the thermodynamic analysis of propulsion systems based on stationary detonation waves must be formulated differently than for propagating waves, and the two situations lead to very different results

Detonation Diffraction Through a Mixture Gradient

A simple one-dimensional model of a self-propagating gaseous detonation consists of a shock wave tightly coupled to a reaction zone, propagating through a combustible gas mixture as shown in Fig. 1 (Strehlow 1984). A feedback mechanism exists in that the shock wave generates the thermodynamic conditions under which the gas combusts, and the energy release from the reaction zone maintains the strength of the shock This is in contrast to a flame, or deflagrative combustion, in which thermal and species transport processes dominate. Given a particular set of initial conditions, a self-propagating detonation wave travels at a constant Chapman-Jouguet velocity (VCJ) on the order of a few thousand meters per second, with associated pressures and temperatures of tens of bar and several thousand degrees, respectively. A detonation is actually a three-dimensional shock-reaction zone complex with a dynamic wavefront composed of curved incident, mach stem, and transverse shock waves as depicted in Fig. 2 (Strehlow 1970). The transverse shocks sweep across the wavefront and the triple-point paths form a diamond-shaped cellular pattern. The cell width [Greek lambda] is a characteristic length scale of detonations, indicative of the coupling between gasdynamic and chemical processes

Mixing and Combustion of Rich Fireballs

A series of experiments was carried out to investigate the effect of fireball composition on secondary combustion. The fireball was created from a 1.5 liter balloon filled with a propane-oxygen mixture (1 1, the incompletely oxidized products from the primary burn mix with the surrounding air and may be oxidized in a secondary combustion process. The unique feature of the present experiments was a repeatable secondary pressure pulse for sufficiently rich mixtures. The secondary pressure rise was observed repeatably for all initiation configurations. The nature of the secondary pressure pulse is a strong function of the initial equivalence ratio. For [Greek Phi] = 1 and 1.5, no secondary pressure waves are observed. An acoustic analysis of the measured pressure histories has been carried out to infer the rate of volume displacement and the total volume displaced by the secondary combustion. The results of the acoustic analysis are in reasonable agreement with both a simplified thermodynamic model predicting the total volume displacement assuming constant-pressure combustion for the secondary burn and the analysis of the fireball luminosity of the high-speed images. For nearly stoichiometric mixtures, [Greek Phi] = 1 and 1.5, the leading blast wave peak pressures and impulses are comparable with the previously-measured gaseous and high explosive blasts when the energy content of the balloon only is used to formulate Sachs scaling variables. Due to a much slower combustion process than detonation for [Greek Phi] >2 the peak pressure of the leading wave rapidly decreases below the energy-equivalent reference blast values as the equivalence ratio is increased. The Sachs-scaled impulse agrees well with the predictions on the basis of the energy in the balloon alone for 2.75 > [Greek Phi] > 1. One of the key results of the present study has been the documentation of the existence of the secondary pressure wave. The present study has emphasized the acoustic nature of the secondary pressure waves and the origin of these pressure waves due to the processes at the interface between the fireball and the atmosphere. The presence of the secondary pressure peak and the higher impulses indicate that there is the potential for significant enhancement of the blast through secondary combustion

Rapid evaporation at the superheat limit

In an experimental investigation of the transient processes that occur when a single droplet of butane at the superheat limit vaporizes explosively, short-exposure photographs and fast-response pressure measurements have been used to construct a description of the complete explosion process. It is observed that only a single bubble forms within the drop during each explosion, and that the growth proceeds on a microsecond time scale. An interfacial instability driven by rapid evaporation has been observed on the surface of the bubbles. It is suggested that the Landau mechanism of instability, originally described in connection with the instability of laminar flames, also applies to rapid evaporation at the superheat limit. The photographic evidence and the pressure data are used to estimate the evaporative mass flux across the liquid-vapour interface after the onset of instability. The ;ate of evaporation is shown to be two orders of magnitude greater than would be predicted by conventional bubble-growth theories that do not account for the effects of instability. An estimate of the mean density within the bubbles during the evaporative stage indicates that it is more than one half of the critical density of butane. Additional interesting dynamical effects that are observed include a series of toroidal waves that form on the interface between the butane vapour and the external host liquid in the bubble column apparatus after the bubble has grown large enough to contact the outer edge of the drop, and violent oscillations of the bubble that occur on a millisecond time scale, after evaporation of the liquid butane is complete, that cause the disintegration of the bubble into a cloud of tiny bubbles by Rayleigh-Taylor instability

A numerical study of detonation diffraction

An investigation of detonation diffraction through an abrupt area change has been carried out via a set of two-dimensional numerical simulations parameterized by the activation energy of the reactant. Our analysis is specialized to a reactive mixture with a perfect gas equation of state and a single-step reaction in the Arrhenius form. Lagrangian particles are injected into the flow as a diagnostic tool for identifying the dominant terms in the equation that describes the temperature rate of change of a fluid element, expressed in the shock-based reference system. When simplified, this equation provides insight into the competition between the energy release rate and the expansion rate behind the diffracting front. The mechanism of spontaneous generation of transverse waves along the diffracting front is carefully analysed and related to the sensitivity of the reaction rate to temperature. We study in detail three highly resolved cases of detonation diffraction that illustrate different types of behaviour, super-, sub- and near-critical diffraction

Single-Cycle Impulse from Detonation Tubes with Nozzles

Experiments measuring the single-cycle impulse from detonation tubes with nozzles were conducted by hanging the tubes in a ballistic pendulum arrangement within a large tank. The detonation-tube nozzle and surrounding tank were initially filled with air between 1.4 and 100 kPa in pressure simulating high-altitude conditions. A stoichiometric ethylene–oxygen mixture at an initial pressure of 80 kPa filled the constant-diameter portion of the tube. Four diverging nozzles and six converging–diverging nozzles were tested. Two regimes of nozzle operation were identified, depending on the environmental pressure. Near sea-level conditions, unsteady gas-dynamic effects associated with the mass of air contained in the nozzle increase the impulse as much as 72% for the largest nozzle tested over the baseline case of a plain tube. Near vacuum conditions, the nozzles quasi-steadily expand the flow, increasing the impulse as much as 43% for the largest nozzle tested over the baseline case of a plain tube. Competition between the unsteady and quasi-steady-flow processes in the nozzle determine the measured impulse as the environmental pressure varies

Thermal and Catalytic Cracking of JP-10 for Pulse Detonation Engine Applications

Practical air-breathing pulse detonation engines (PDE) will be based on storable liquid hydrocarbon fuels such as JP-10 or Jet A. However, such fuels are not optimal for PDE operation due to the high energy input required for direct initiation of a detonation and the long deflagration-to-detonation transition times associated with low-energy initiators. These effects increase cycle time and reduce time-averaged thrust, resulting in a significant loss of performance. In an effort to utilize such conventional liquid fuels and still maintain the performance of the lighter and more sensitive hydrocarbon fuels, various fuel modification schemes such as thermal and catalytic cracking have been investigated. We have examined the decomposition of JP-10 through thermal and catalytic cracking mechanisms at elevated temperatures using a bench-top reactor system. The system has the capability to vaporize liquid fuel at precise flowrates while maintaining the flow path at elevated temperatures and pressures for extended periods of time. The catalytic cracking tests were completed utilizing common industrial zeolite catalysts installed in the reactor. A gas chromatograph with a capillary column and flame ionization detector, connected to the reactor output, is used to speciate the reaction products. The conversion rate and product compositions were determined as functions of the fuel metering rate, reactor temperature, system backpressure, and zeolite type. An additional study was carried out to evaluate the feasibility of using pre-mixed rich combustion to partially oxidize JP-10. A mixture of partially oxidized products was initially obtained by rich combustion in JP-10 and air mixtures for equivalence ratios between 1 and 5. Following the first burn, air was added to the products, creating an equivalent stoichiometric mixture. A second burn was then carried out. Pressure histories and schlieren video images were recorded for both burns. The results were analyzed by comparing the peak and final pressures to idealized thermodynamic predictions