Dibesh D. Joshi

On the Unsteady Thrust Measurement for Pulse Detonation Engines

A pulse detonation engine simulator operating at 5 and 20 Hz was used to study unsteady thrust characteristics. The natural vibration frequencies, the effective mass of the simulator and the steady thrust were first determined. The system dynamics of the simulator, deconvolved in the frequency domain, was studied. The impulse transfer function was used to reconstruct the thrust response and acceleration compensation was applied to get the thrust solely due to the pulsating jet. The thrust response of the system due to exit of the pulsed jet could be reconstructed well. The acceleration compensation technique enabled the actual thrust to be recovered from reconstructed signal. Pulse-to-pulse interference was not observed for the frequencies tested.

Acceleration Compensation for Drag Measurements in Hypersonic Shock Tunnel

Force measurements in impulse facilities are likely to be affected by inertial effects. A technique that compensated for inertial forces due to acceleration of the model-balance assembly was developed and applied. This technique included experimental determination of the transfer function to enable the drag force to be obtained by a deconvolution of the measured signal. Furthermore, the method incorporated the determination of the effective oscillating mass of the system to calculate the inertial force. This inertial force was deducted from the deconvolved signal to recover the actual drag force. Tests at different stagnation enthalpy levels were conducted in a hypersonic shock tunnel to validate the developed technique. These tests utilized an external drag balance mounted with a blunt cone at Mach 10. The results were close to modified Newtonian values.

A New Method to Predict Flow Rates of Gaseous Fuels Injected Intermittently for Pulse Detonation Engines

A new method was developed to accurately predict the mass of gaseous fuel and oxidizer injected intermittently into a Pulse Detonation Engine (PDE). The method utilized the mass flow parameter of the gas used to estimate the ideal mass flow rate. This value of ideal mass flow was multiplied by an experimentally determined discharge coefficient to account for losses due to viscous effects. The ideal mass flow rate calculation incorporated determination of the time varying injection surface area, supply pressure and temperature, specific heat ratio of the gas and Mach number. A scheme for the change in injection surface area for different types of valve openings was also discussed. The calculated mass flow rates and valve open time duration were used to calculate the total mass of propellants injected per pulse. This calculated value of mass injected per pulse was used to calculate the filling fraction of the tube. This paper also briefly described a procedure which involved the variation of duty cycle of the valves and supply pressure to attain different values of filling fraction of the tube and equivalence ratio of the mixture. Results indicate the practicality of the developed method to predict the total mass of gases injected intermittently into a PDE.

Experimental study of high-frequency fluidic valve injectors for detonation engine applications

The performance of a high-frequency fluidic valve for potential use in pulse and rotating detonation engines was studied. The valve was mounted to the side of a pulsed detonation engine and featured a plenum cavity for damping the detonation wave. The tests involved cavities of different lengths. High-pressure air at different pressures that simulated gaseous propellant injection was introduced into the valve. The injectant was repetitively blocked by pulsed detonation waves entering the cavity. Pressure histories measured within the cavity were used to determine the detonation wave residence in the cavity and the subsequent wave interactions. Preliminary results indicate complex wave behavior in the cavity that prevented the detonation products from propagating upstream.

Influence of Unsteadiness on Thrust Measurements of Pulse Detonation Engines

2-17 These methods generally require identification of the systems dynamic parameters by experiment and finite element modeling. A technique for deconvolving the dynamic response of the structure from the actual thrust that is produced is developed. The method is tested using model functions and also against a simple impulse experiment. The general approach is to first obtain the impulse transfer function of the PDE in a dynamic calibration. This impulse transfer function is then modeled. The dynamic behavior of the system subjected to a sequence of pulses, simulating a PDE operation, is next modeled. It is assumed that the system dynamics of a thrusting PDE is the same as that of the dynamic calibration. Therefore, this model can be used to determine the thrust (input) from a load cell measurement through a deconvolution procedure.

Flow Visualization of the Exhaust Jet from a Pulse Detonation Engine by Mie Scattering

A pulse detonation engine (PDE) typically consists of a straight tube through which a reactive mixture is detonated from a closed end and from which exhaust products exit from the open end. The detonations occur repetitively. To improve the understanding of PDE operation, exploratory, high-speed flow visualization experiments of the exhaust jet were conducted and reported here. The visualizations show a complex structure consisting of a rapid expulsion of a blast wave and a vortex ring. The visualizations show that the vortex ring is turbulent and breaks down. This is followed by a Mach disk and triple-shock structure, and an exhaust jet that eventually breaks down.

Flow rate measurement method for gaseous fuel injection for pulse detonation engines

A mass-flow model was developed to calculate the flow rates of a gaseous fuel and oxidizer injected intermittently into a pulse detonation engine. The procedure used the mass-flow parameter of the gas to estimate the ideal mass-flow rate. The ideal mass-flow rate calculation incorporated determination of the time-varying injection surface area, supply pressure, and temperature and specific heat ratio of the gas considering choked flow at the injecting surface. This value of ideal mass flow was multiplied by an experimentally determined discharge coefficient to account for losses. The calculated mass-flow rates and valve opening time duration were used to calculate the total mass of reactants injected per pulse for a typical pulse detonation engine. In addition, a scheme for the change in injection surface area for different types of valve openings was presented. Results indicated the practicality of the developed mass-flow model to calculate the intermittent gas mass injection into a pulse detonation engi...

Study of unsteady thrust characteristics of pulse detonation engines

A dynamical model of a pulse detonation engine was developed to study the effects of dynamic excitation and pulse-to-pulse interaction on the unsteady thrust generated. Inertial loads arise due to cyclic acceleration when the system is excited dynamically in a repetitive manner and interfere with the generated thrust. In addition, influence of system dynamics and pulse-to-pulse interaction results in the convolution of generated thrust and structural response. The convolution of structural response and the interference of inertial loads have to be accounted in order to accurately predict the actual thrust. A general approach incorporating the study of system dynamics to facilitate deconvolution procedure followed by an acceleration compensation method to account for added inertial loads was used to recover actual thrust generated. The study of system dynamics involved the establishment of system transfer function to reconstruct the thrust using known input. The acceleration compensation method required the determination of effective mass and calculation of induced acceleration of the thrust stand. The product of two quantities, effective mass and measured acceleration, yielded inertial load which was deducted from the reconstructed thrust to estimate actual thrust generated. The calculated and compensated thrust values were expressed in the form of impulse for comparison. The results confirmed the effectiveness of the approach as the inertial loads were effectively reduced and obtained results were in good agreement with published information.