Combustion unsteady

Fay, J. A., and D. H. Lewis, Jr. 1977. Unsteady burning of unconfined fuel vapor clouds. 16th Symposium (International) on Combustion, pp. 1397-1405. Pittsburgh, PA The Combustion Institute. 

Andreev Chulko (Ref 7) claim that the pressure above which PETN combustion becomes unsteady and accelerates decreases with increasing PETN particle size. Other studies suggest that there is an optimum particle size for DDT (see Sect VII of Propellants, Solid in this Vol). In a recent study, Bernecker et al (Ref 16) found that at a given degree of compaction, 20-micron Tetryl had a longer run-up to detonation, 12, than 470-micron Tetryl. This is shown in Fig 3... 

Y. S. Ko and S. H. Chung, Propagation of unsteady tribrachial flames in laminar nonpremixed jets. Combust. Flame 118 151-163, 1999. 

Unsteady combustion is a strong source of acoustic noise. The emission of sound by gaseous combustion is governed by the classical set of conservation equations Mass conservation ... 

Flames submitted to convective disturbances experience geometrical variations, which can in turn give rise to heat release unsteadiness. This process can be examined by considering different types of interactions between incident velocity or equivalence ratio modulations and combustion. The flame dynamics resulting from these interactions give rise to sound radiation and... 

P. Clavin and G. Searby. Unsteady response of chainbranching premixed-flames to pressure waves. Combustion Theory and Modelling, 12(3) 545-567, 2008. 

A. Wangher, G. Searby, and J. Quinard. Experimental investigation of the unsteady response of a flame front to pressure waves. Combustion and Flame, 154(1-2) 310-318, 2008. 

The analysis of combustion dynamics is then intimately linked to an understanding of perturbed flame dynamics, the subsequent generation of unsteady rates of heat release, and the associated radiation of sound and resulting acoustic feedback. In practical configurations, the resonance loop involves the flow, the combustion process, and the acoustic modes of the system as represented schematically in Figure 5.2.2. 

These data indicate that thermal losses during unsteady flame-wall interactions constitute an intense source of combustion noise. This is exemplified in other cases where extinctions result from large coherent structures impacting on solid boundaries, or when a turbulent flame is stabilized close to a wall and impinges on the boundary. However, in many cases, the flame is stabilized away from the boundaries and this mechanism may not be operational. 

Qualitative comparison of the inclined structure of thin layers of high scalar dissipation in a piloted CH4/air jet flame as revealed by (a) mixture fraction imaging, (b) LES with a steady flamelet library (a and b are adapted from Kempf, A. Flemming, F., and Janicka, ]., Proc. Combust. Inst, 30, 557, 2005. With permission.), and (c) LES with unsteady flamelet modeling. (Adapted from Pitsch, H. and Steiner, H., Proc. Combust. Inst., 28, 41, 2000. With permission.)... 

Furthermore, we will take all other properties as constant and independent of temperature. Due to the high temperatures expected, these assumptions will not lead to accurate quantitative results unless we ultimately make some adjustments later. However, the solution to this stagnant layer with only pure conduction diffusion will display the correct features of a diffusion flame. Aspects of the solution can be taken as a guide and to give insight into the dynamics and interaction of fluid transport and combustion, even in complex turbulent unsteady flows. Incidentally, the conservation of momentum is implicitly used in the stagnant layer model since ... 

FlameMaster v3.3 A C+ + Computer Program for OD Combustion and ID Laminar Flame Calculations. FlameMaster was developed by H. Pitsch. The code includes homogeneous reactor or plug flow reactors, steady counter-flow diffusion flames with potential flow or plug flow boundary conditions, freely propagating premixed flames, and the steady and unsteady flamelet equations. More information can be obtained from http //www.stanford.edu/group/pitsch/Downloads.htm. 

J. P. Agrawal, V. K. Bapat, R. R. Mahajan, Mehilal and P. S. Makashir, A Comparative Study of Thermal and Explosive Behaviour of 5- Picrylamino-l,2,3,4-Tetrazole (PAT) and 5,5 -Styphnylamino-1,2,3,4-tetrazole (SA ) , International Work-Shop on Unsteady Combustion and Interior Ballistics, St. Petersburg, Russia, Vol. 1, June 25-30, 2000, 199. 

I, 2,3,4-tetrazole (SAI) , International Work-Shop on Unsteady Combustion and Interior Ballistics, St. Petersburg, Russia, June 25-30, 2000, Vol. 1, 199. 

Although several different system configurations have been simulated, the focus of this paper will be on the unsteady, compressible, multiphase flow in an axisymmetric ramjet combustor. After a brief discussion of the details of the geometry and the numerical model in the next section, a series of numerical simulations in which the physical complexity of the problem solved has been systematically increased are presented. For each case, the significance of the results for the combustion of high-energy fuels is elucidated. Finally, the overall accomplishments and the potential impact of the research for the simulation of other advanced chemical propulsion systems are discussed. 

Kailasanath, K., J.H. Gardner, E. S. Oran, and J. P. Boris. 1991. Nnmerical simulations of unsteady reactive flows in a combustion chamber. Combustion Flame 86 115-34. 

Kaplan, C. R., S. W. Back, E. S. Oran, and J. L. Ellzey. 1994. Dynamics of strongly radiating unsteady ethylene jet diffusion flame. Combustion Flame 96 1-22. Kennedy, C.A., and M. H. Carpenter. 1994. Several new numerical methods for compressible shear-layer simulations. Applied Numerical Methods 14 397-433. Baum, M., T. Poinsot, and D. Thevenin. 1994. Accurate boundary conditions for multicomponent reactive flows. J. Comput. Phys. 116 247-61. 

Figure 13.5 Unsteady nonpremixed combustion and fluid dynamics (a) contours of the vorticity magnitude O in planes indicated to the right (6) cross-sectional averaged measures of instantaneous chemical product and product formation (left frame), instantaneous unconstrained and vorticity-bearing (ff > 5% peak-value) streamwise mass flux Q (right frame). 1 — product, 2 — instantaneous production, 3 — Oo = 0, and 4 flo/f peak — 0.05...

The goal of this work has been to characterize the effects of the unsteady vor-ticity dynamics on jet entrainment and nonpremixed combustion. The main focus of the numerical simulations of rectangular jets has been on the vortic-ity dynamics underlying axis switching when the initial conditions at the jet exit involve laminar conditions, negligible streamwise vorticity, and negligible azimuthal nonuniformities of the momentum thickness. 

Grinstein, F.F., and K. Kailasanath. 1995. Three-dimensional numerical simulations of unsteady reactive square jets. Combustion Flame 100 2 101 192. 

Gulati, A., and R. Mani. 1992. Active control of unsteady combustion-induced oscillations. J. Propulsion Power 8(5) 1109-15. 

The outline of this paper is as follows. First, a theoretical model of unsteady motions in a combustion chamber with feedback control is constructed. The formulation is based on a generalized wave equation which accommodates all influences of acoustic wave motions and combustion responses. Control actions are achieved by injecting secondary fuel into the chamber, with its instantaneous mass flow rate determined by a robust controller. Physically, the reaction of the injected fuel with the primary combustion flow produces a modulated distribution of external forcing to the oscillatory flowfield, and it can be modeled conveniently by an assembly of point actuators. After a procedure equivalent to the Galerkin method, the governing wave equation reduces to a system of ordinary differential equations with time-delayed inputs for the amplitude of each acoustic mode, serving as the basis for the controller design. 

The formulation of combustion dynamics can be constructed using the same approach as that employed in the previous work for state-feedback control with distributed actuators [1, 4]. In brief, the medium in the chamber is treated as a two-phase mixture. The gas phase contains inert species, reactants, and combustion products. The liquid phase is comprised of fuel and/or oxidizer droplets, and its unsteady behavior can be correctly modeled as a distribution of time-varying mass, momentum, and energy perturbations to the gas-phase flowfield. If the droplets are taken to be dispersed, the conservation equations for a two-phase mixture can be written in the following form, involving the mass-averaged properties of the flow ... 

The precursor has initially a higher velocity than the flame front. An intermediate phase of the DDT, in which the shock front is still advancing faster than the flame front behind it, and each front can be considered separately, has been called a pseudo-detonation , an "unsteady double discontinuity , or a "latent combustion phase ... 

For this study mixts of CeH6 O and H O were detonated in a tube either by a shock wave or by a spark. The arrival of the pressure step was detd by a thin-film, heat-transfer probe with a rise time of 0.5 microsecs. The spectrograph viewed the passing deton wave thru a window slit and lens arrangement. Recording was accomplished by photomultiplier tubes. The deton waves observed consisted of a shock front followed by a combustion front and were classed as "strong , which is equiv to "unsteady or "decelerating detonation. Detailed structure of the detonations could not be resolved... 

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