Jiun-Ming Li

Deflagration to Detonation Transition by Hybrid Obstacles in Pulse Detonation Engines

A hybrid DDT enhancement device comprising obstacles of orifice plates and vortex generators is investigated to achieve reliable and repeatable detonations in Pulse Detonation Engines. The vortex generator is capable of yielding homogenous mixing while detonation transition can be achieved by it. The experimental results of deflagration to detonation transition process by a series of designed obstacles are reported. All experiments were carried out using a valveless PDE operated for multiple cycles using stoichiometric ethylene/air mixtures. The results show that initiation of DDT starting within vortex generator can enhance DDT process and achieve a strong overdriven detonation. In addition, during the transition stage, the onset of detonation can occur inside the vortex generator; while it generally takes place after the obstacles terminated. It is realized that effectiveness of the DDT enhancement device depends on the operation frequency. A hybrid obstacle of 0.4m orifice plates connecting 0.5m vortex generator can achieve reliable and repeatable transition within 15 L/D DDT run-up distance.

Excessively Fuel-Rich Conditions for Cold Starting of Liquid-Fuel Pulse Detonation Engines

Cold starting a liquid-fuel pulse detonation engine with air as the oxidizer is always an unavoidable challenge because of the inherent insensitivity of droplet–air mixtures to undergo detonation initiation. The existing strategies are essentially attempts to accelerate the vaporization rates of liquid fuels to increase the amount of prevaporized fuel before ignition. In this paper, it has incidentally been discovered from experimental studies that a Jet A-1/air pulse detonation engine is able to operate successfully in relatively cold environments (mixture temperature at 70°C) when an excessively fuel-rich mixture is used. The experimental results and theoretical phase equilibrium calculations show that the vapor-phase equivalence ratio is vital for successful detonation initiation spray detonations via a deflagration-to-detonation transition process. For a two-phase mixture containing a high overall fraction of fuel as compared with air, the amount of vaporized fuel achieves a stoichiometric or slightly...

On the Investigation of Detonation Re-initiation Mechanisms and the Influences of the Geometry Confinements and Mixture Properties

The topic of detonation re-initiation is studied through both experimental measurements and numerical simulations using a bifurcation channel and the detonation research facilities in Temasek Laboratories. The main objective is to understand the re-initiation mechanisms through shock reflections, and investigate the performance of detonation re-initiation at different test conditions. Stable and unstable detonation waves are both taken into consideration. It is found that the re-initiation through shock reflection is mainly achieved through the interactions of the multiple transverse waves. The details of the generation and evolution of the transverse waves are also clarified. The influence of the geometry confinement to detonation re-initiation is investigated. It is found that the length of the bifurcation channel can affect the re-initiation results by limiting the shock reflection times, which is discovered to be the main reason leading to the discrepancies between the previous similar studies. The width of the bifurcation channel is also critical as it can directly affect the induction length during detonation diffraction which determines the shock reflection strength. The differences of re-initiation using various mixture properties are also addressed, and a sudden transitional behavior of detonation re-initiation is found between stable and unstable detonation waves. Regarding the reason why a certain number of shock reflections are required before successful re-initiation, it can be explained using the relative relation between the shock reflection strength and the corresponding marginal solution curve of a quasi-steady detonation.

Effect of ethylene fuel/air equivalence ratio on the dynamics of deflagration-to-detonation transition and detonation propagation process

ABSTRACT During the operation, the pulse detonation engine (PDE) may have to work with different fuel/air equivalence ratios (from a lean to rich fuel mixture) as in the cold start-up operation, which would strongly affect the engine performance characteristics and the outputs of interest. For a better operational control, it is necessary to gain understanding of the effect of the equivalence ratio on the dynamics of the processes of PDE. Thus, in this study, numerical simulations are performed for different ethylene fuel/air equivalence ratios to study its effect on the dynamics of the deflagration-to-detonation transition (DDT) and detonation processes. In particular, the density-based solver with a shock-capturing scheme is employed to solve for the viscous, compressible, and reacting flows governed by reacting Navier–Stokes equations. The computed flame propagation speed, run-up distance, and Chapman–Jouguet detonation velocity are comparable to the current experimental results. In addition, the numerical results show that the minimum values of the run-up distance and run-up time, as well as the maximum value of the detonation velocity occur at the equivalence ratio of about 1.1. Analysis of the computed results associate these findings to the firm correlation of the flame speed with equivalence ratio, which is in turn function of the temperature, pressure, and mixture composition. The shifting of the outputs of interest to the richer fuel side from the stoichiometric point can be attributed to the combustion product dissociation, mixture heat capacity, and oxidizer enrichment.