Pulse detonation propulsion : challenges, current status, and future perspective

By L I Sedov London: Cleaver-Hume Press Ltd Pp xvi + 363 Price 105s This is really two separate books within the same pair of covers First of all Chapters 1-3, some 145 pages, are devoted to the discussion of similarity and dimensional, methods and their application to a variety of problems in mechanics and fluid mechanics

http://iopscience.iop.org/article/10.1088/0031-9112/11/6/008/pdf

Similarity and Dimensional Methods in Mechanics

Fuel droplet vaporization and spray combustion theory

Modeling of nitrogen oxides formation and destruction in combustion systems

Rotating detonation engines (RDEs), also known as continuous detonation engines, have gained much worldwide interest lately. Such engines have huge potential benefits arising from their simplicity of design and manufacture, lack of moving parts, high thermodynamic efficiency and high rate of energy conversion that may be even more superior than pulse detonation engines, themselves the subject of great interest. However, due to the novelty of the concept, substantial work remains to demonstrate feasibility and bring the RDE to reality. An assessment of the challenges, ranging from understanding basic physics through utilizing rotating detonations in aerospace platforms, is provided.

https://arc.uta.edu/publications/cp_files/AIAA%202011-6043%20Rotating%20detonation%20wave%20propulsion%20experimental%20challenges,%20modeling,%20and%20engine%20concepts%20(invited).pdf

Rotating detonation wave propulsion: Experimental challenges, modeling, and engine concepts

Experimental tests of an axisymmetric jet of air impinging on both water and wet cement were performed and analyzed. Since the cavities formed on the water were unsteady and irregular, cavities were formed on wet fast-setting cement and allowed to set with the jet impinging. In this way, detailed measurements of the solidified cavity shape were made and shown to agree well with theory. This correlation of the data with the theory indicates that little gas was entrained in the liquid and that the influence of liquid viscosity and surface tension was small for the experimental conditions tested. A simplified analysis is also presented for an incompressible axisymmetric gas jet impinging normally on a liquid surface. The analysis was effected by combining the following physical conditions and assumptions: (i) the stagnation pressure corresponding to the centreline conditions of the jet at the bottom of the cavity is equal to the hydrostatic pressure, wherein an empirical turbulent jet decay law is used to predict the variation of stagnation pressure with distance from the nozzle; (ii) the force on the liquid is equal to the total change in normal momentum, which is equal to the weight of the displaced liquid; (iii) the shape of the cavity is a paraboloid.

Cavities formed on liquid surfaces by impinging gaseous jets

Experimentally, the propagation of detonation waves in methane-air mixtures has been investigated. In a tube having a large length to diameter ratio steady state wave propagation was achieved. The actual propagation velocities were close to the calculated Chapman-Jouguet velocities. The limits of detonation were ascertained to be between 8 and 14.5 percent methane. A minimum initiation energy density of 9.42MJ/m 2 was established for stoichiometric mixtures implying the need for rather substantial charges for the initiation of spherically symmetric systems. In all cases the detonation wave exhibited a spinning structure. The motion compensated and laser schlieren photographs, smoked foil traces, and pressure transducer records strongly support the currently accepted model for the structure of such a wave, which results in the creation of extremely high local overpressures.

Detonation of methane-air mixtures

Numerical simulations are used to study the diffraction, decay, and reignition that occurs when a detonation propagates past an increase in cross-sectional area in a rectangular tube. The computations solve the time-dependent two-dimensional equations describing a reactive flow in an argon-diluted stoichiometric hydrogen-oxygen mixture at atmospheric pressure. Previous studies have shown that soon after transmission to a larger area, the reaction front decouples from the leading shock and forms a decaying blast wave (“bubble”) in the larger tube. Then, depending on the initial conditions, the detonation either continues to decay or is reignited as the bubble reflects off confining surfaces. For a strongly overdriven initiating detonation, reignition occurs through an interaction between the bubble and the original contact surface. For a more weakly driven system, reignition can occur in two ways: either in the slip line and Mach stem of the Mach reflection formed when the bubble reflects off the bottom surface of the tube, or by multiple shock interactions that occur when the reflected bubble overtakes the initial detonation front. The computations show the evolution and development of the cellular structure of the steady detonation front.

Reignition of detonations by reflected shocks

The complex, dynamic shock-detonation structure formed by the glancing interaction of a primary detonation with a secondary explosive is studied by using time-dependent two-dimensional simulations and related experiments. The materials considered in the simulations are stoichiometric and lean mixtures of hydrogen and oxygen diluted with argon. Related experiments have used undiluted hydrogen and oxygen as well as other gases. For the conditions simulated: (a) the primary mixture is stoichiometric and the secondary inert; (b) both the primary and secondary mixtures are the same and stoichiometric; (c) the primary mixture is lean and the secondary is stoichiometric; and (d) the primary mixture is stoichiometric and the secondary is lean. In addition, for cases (b) and (d) , comparisons are made between simulations in which the primary mixture is overdriven and when it is a Chapman-Jouguet detonation. For the overdriven stoichiometric primary detonation interacting with the lean mixture, a complex detonation structure forms and quickly asymptotes to the detonation velocity of the primary mixture. For this same case, but when the primary detonation is initially at Chapman-Jouguet velocity, the detonation appears to die but then reignites due to a series of shock reflections and then propagates as a complex structure. The lowest velocity of the complex structure is always greater than the Chapman-Jouguet velocity of the lean mixture and it increases in time, appearing to approach the Chapman-Jouguet velocity of the stoichiometric mixture. The dynamics of this decay and reignition process are described and discussed in detail.

Numerical simulations of detonation transmission

Dust explosions pose a serious hazard in many industries. The detonability and flam inability of dust/oxidizer mixtures depend on the ignition delay of the dust particles when suddenly exposed to a high temperature environment. Consequently, the ignition delay time of dust particles behind a shock wave in the Mach number range of 4.0-5.0 has been measured using a photomultiplier tube to determine the onset of ignition. The dusts investigated included Pittsburgh Seam Coal, graphite, diamond, oats, and RDX. The experimental arrangement, consisting of a shock tube and two different dust injection devices, is described in detail, and experimental results for dusts ranging in particle size from 2 to 74 /*m are presented. In the Mach number range considered, ignition delay times varied from 2 to 100 /*s. A detailed analytical model based on a solution of the heat conduction equations for the particle interior coupled with a solution of the particle equation of motion has been developed. Heterogeneous reactions occurring on the particle surface and in the pores within the particle are used to model the chemistry. The results were in reasonable agreement with most of the data. Approximate analyses based on a comparison of characteristi c thermal and chemical times were also developed. A key conclusion is that the ignition delay is determined mainly by the heat-up time of the particle surface.

/pdf/the-shock-wave-ignition-of-dusts-3rh88it2rz.pdf

Martin Sichel

Papers

Cavities formed on liquid surfaces by impinging gaseous jets

Detonation of methane-air mixtures

Reignition of detonations by reflected shocks

Numerical simulations of detonation transmission

The shock wave ignition of dusts