Research on Optical Diagnostic Method of PDE Working Status Based on Visible and Near-Infrared Radiation Characteristics

Fill fraction not only has a profound impact on the process of deflagration to detonation in pulsed detonation engine, but also affects the propulsion performance in both flight and ground tests. In this paper, a novel optical diagnostic method based on detonation exhaust radiation in visible and near-infrared region within 300–2600 nm is developed to determine the current working state in the gas–liquid two-phase pulsed detonation cycle. The results show that the radiation characteristic in each stage of detonation cycle is unique and can be a good indicator to infer the fill fraction. This is verified experimentally by comparison with the laser absorption spectroscopy method, which utilizes a DFB laser driven by ramp injection current to scan H2O transition of 1391.67 nm at a frequency of 20 kHz. Due to concentrated radiation intensity, time duration reaching accumulated radiant energy ratio of 50% in detonation status would be smaller than 1.2 ms, and detonation status would be easily distinguished from deflagration with this critical condition. In addition, the variation of important intermediates OH, CH, and C2 radicals during detonation combustion are obtained according to the analysis of detonation spectrum, which can also be proposed as a helpful optical diagnostics method for the combustion condition based on C radical concentration. The study demonstrates the feasibility of optical diagnostics based on radiation in visible and near-infrared regions, which could provide an alternative means to diagnose and improve pulsed detonation engine performance.

Adjoint-based optimisation of detonation initiation by a focusing shock wave

An optimisation study of a shock-wave-focusing geometry is presented in this work. The configuration serves as a reliable and deterministic detonation initiator in a pulsed detonation engine. The combustion chamber consists of a circular pipe with one convergent–divergent axisymmetric nozzle, acting as a focusing device for an incoming shock wave. Geometrical changes are proposed to reduce the minimum shock wave strength necessary for a successful detonation initiation. For that purpose, the adjoint approach is applied. The sensitivity of the initiation to flow variations delivered by this method is used to reshape the obstacle’s form. The thermodynamics is described by a higher-order temperature-dependent polynomial, avoiding the large errors of the constant adiabatic exponent assumption. The chemical reaction of stoichiometric premixed hydrogen-air is modelled by means of a one-step kinetics with a variable pre-exponential factor. This factor is adapted to reproduce the induction time of a complex kinetics model. The optimisation results in a 5% decrease of the incident shock wave threshold for the successful detonation initiation.

TWO ENGINEERING SOLUTIONS FOR PULSED DETONATION ENGINE DESIGN

The idea of realization of the detonative combustion cycle in pulse jet engines appeared many years ago but in fact, it has not been realized in practice yet. Most investigations Óarried out both in our country and abroad have not been brought up to practical use because of various challenges. In most cases, technology demonstrator are described where the process of cyclic detonation is studied. But there are no engines which could realize detonative combustion of regular fuel in incoming air. As a rule, well known constructions need the controlled system of high-pressure fuel supply and fuels of special type.

NUMERICAL SIMULATION OF DEFLAGRATION-TO-DETONATION TRANSITION IN A PULSED DETONATION ENGINE

The air-breathing pulsed detonation engine (PDE) for an aircraft designed for a subsonic flight when operating on the products of pyrolysis of polypropylene was developed using the analytical estimates and parametric multivariant threedimensional (3D) calculations. The PDE consists of an air intake with a check valve, a fuel supply system, a prechamber-jet ignition system, and a combustion chamber with an attached detonation tube. Parametric 3D calculations allowed choosing the best length of the PDE combustor, which provides an efficient mixing of air with fuel, the best way to ignite the mixture (prechamber-jet ignition), the best location of the prechamber, the minimum length of the section with turbulizing obstacles for flame acceleration and deflagration-to-detonation transition (DDT), and the best degree of filling the detonation tube with the fuel mixture to achieve the maximum completeness of combustion.

Investigation on the Time Error of Detonation Acoustic in Process of Formation and Propagation

The time error of detonation acoustic in process of detonation formation and propagation in a multi-cycle gas-liquid two-phase pulsed detonation engine is experimentally investigated. Results from the tests show that before the detonation wave escapes through the open-end of PDE tube, the maximum average arrival time error of detonation acoustic is achieved in the process of overdriven detonation. After detonation wave exists of PDE tube, arrival time error at 0 deg is greater than the other directivity angles in all distances and increases dramatically first and then almost stays at a certain value. The filling fraction has a major impact on the time error of detonation acoustic. With filling fraction increasing, there are increases in arrival time error and interval time error. Arrival time error with the highest filling fraction at 30 deg is much greater than other filling fraction. The convergent nozzle exhibits a marked suppression in the time error of detonation acoustic, where the maximum reductions of 62.02 percent and 56.13 percent are obtained in arrival time error and interval time error respectively.

Intake Flow Analysis of a Pulsed Detonation Engine

Unsteady flow within the intake system of a hydrogen–air pulse detonation engine (PDE) has been analyzed using a quasi-one-dimensional (Q1D) computational fluid dynamic (CFD) code. The analysis provides insight into the unsteady nature of localized equivalence ratios and their effects on PDE performance. For this purpose, a code originally configured to model the PDE tube proper was modified to include a 6.1 m long intake with a single fuel injector located approximately 3.05 m upstream of the primary intake valve. The results show that constant fuel mass flow rate injection from the injector creates large local variations in equivalence ratio throughout the PDE within a cycle. The effect of fill fraction on the engine performance is better described with the presence of the inlet model. However, the effect of ignition delay is shown to be better predicted with a model without the inlet.

Constrained Repetitive Model Predictive Control Applied to an Unsteady Compressor Stator Vane Flow

This paper presents a constrained Repetitive Model Predictive Controller (RMPC) implemented as closed-loop flow controller for an experimental compressor stator cascade. The objective of the controller is to decrease the impact of periodic disturbances on the passage flow. The disturbances are generated by movable flaps that are located downstream of the trailing edges of the stator vanes. The flaps emulate the throttling effect of periodically closed combustion tubes in a pulsed detonation engine on the flow over the stator vanes. The RMPC adjusts the actuation amplitude of fluidic sidewall actuators according to the present state of the passage flow. The current flow situation is monitored by pressure sensors that are mounted flush to the surface of one of the stator vanes. This data is fed back in real-time to the RMPC which thereupon modifies the actuation amplitude. By learning from period to period, a control command trajectory is computed that reduces detrimental effects of the periodic disturbance in an optimal manner while respecting the input constraints of the physical system. Five-hole-probe measurements in the wake of the passage are utilized to compare the optimized, transient actuation trajectories to the case of constant amplitude actuation.