摘要:

Simplified combustion modeling of nitrocellulose (NC), nitroglycerin (NG) double base propellant is considered. Two models with simple but rational chemistry are compared: the classical thermal decomposition, high gas activation energy (Eg/RT> > 1) Denison-Baum-Williams (DBW) model, and a new chain reaction, low gas activation energy (Eg/RT < < 1) model recently proposed by Ward, Son, and Brewster (WSB). Both models make the same simplifying assumptions of constant properties, Lewis number unity, single-step, second order gas phase reaction, and single-step, zero order, high activation energy condensed phase decomposition. The only difference is in the gas reaction activation energy Egwhich is asymptotically large for DBW and vanishingly small for WSB. The results show that within the same set of assumptions (those listed above plus constant heat release and radiative heat feedback) the WSB model more accurately predicts the steady-state gas phase temperature profile, burning rate sensitivity parameters and oscillatory combustion response (pressure- and radiation-driven) for NC/NG propellant than the DBW model. The oscillatory results support our earlier finding that for accurate unsteady predictions it is necessary to use the full AEA decomposition expression of Lengelle and Ibiricu/Williams, with its inherent negative Jacobian parameters (ns,q< 0), rather than the usual constant-prefactor Arrhenius pyrolysis relation (ns,q= 0). All thermophysi-cal, thermochemical, and chemical kinetic parameters are consistent with what is known about detailed chemistry of NC/NG combustion. Initiation in the condensed phase, thought to occur by CO-NO2bond homolysis, is represented by Eg= 40 kcal/mol. The primary flame zone, thought to be dominated by NO2/aldehyde reactions with an effective activation energy of Eg∼ 5 kcal/mol, is better represented by a vanishingly small value of Eg(WSB) than an asymptotically large value (DBW). The main implication of this finding is that the important (regression rate determining) gas reaction zone near the surface has more the character of chain reaction than thermal decomposition. An important consequence is that the burning rate may be more sensitive to condensed phase decomposition kinetics than previous simplified models have allowed. Also, the BDP monopropellant model is shown to be essentially equivalent to the gas kinetically controlled DBW model and a quantitative BDP-DBW link is demonstrated.

ISSN:0010-2202    

DOI:10.1080/00102200008947270

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A method for modeling the effects of finite-rate chemistry such as partial extinctions and reignitions is developed and used to compute turbulent CO/H2/N2or CH4-air-piloted jet diffusion flames close to extinction. The method is combined with a two-dimensional Large Eddy Simulation procedure employing a partial equilibrium/two-scalar exponential Probability Density Function (PDF) combustion submodel applied at the subgrid scale (SGS) level. Subgrid motions are modeled with a first-order closure utilizing an anisotropic subgrid eddy-viscosity and two equations for the subgrid scale turbulent kinetic and scalar energies. Statistical independence of the joint PDF scalars is here avoided and the required moments are obtained from an extended scale-similarity assumption. Extinction is determined by comparing the local Damkohler number against a ‘critical’ local limit related to the Gibson scalar scale and the reaction zone thickness. The post-extinction regime is treated with a reactedness progress variable computed from a Lagrangian transport equation and a two-scalar reacledness-mixture fraction presumed PDF. Comparisons with well documented turbulent flame data suggested the ability of the presented methodology to describe many experimental trends and variations of the momentum and reactive scalar mixing fields at flame conditions close to extinction.

ISSN:0010-2202    

DOI:10.1080/00102200008947271

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The objective of this investigation in a laboratory combustion system was to understand why a premixed propane flame is acoustically excited with particular acoustic boundary conditions, in particular regions of the frequency spectrum. It was demonstrated that the phase of the heat release at various heights in the flame relative to the burner exit velocity indicated a disturbance with a time-delay related to convection by the mean flow from the moment that it left the burner mouth. This gave rise to both positive and negative contributions to acoustic energy flux (the Rayleigh integral) from different heights in the flame, in some cases over as many as seven cycles. Thus, some thermo-acoustic eigen-frequencies were not strongly excited, even when local contributions were high. Conversely, the coincidence of a Helmholtz resonance (low acoustic pressure fluctuations associated with high acoustic velocity fluctuations) in the premixture supply system with a combustion chamber quarter-wave resonance (high acoustic pressures) made it impossible to stabilise the flame.

ISSN:0010-2202    

DOI:10.1080/00102200008947272

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

To study turbulent combustion, experiments with expanding, statistically spherical flames ignited by a spark are widely used. The goal of the work is to show that certain trends in the behavior of turbulent flame speed 5, observed in such experiments, are substantially affected by the curvature of the mean flame brush and by the ignition conditions. For this purpose, simulations of expanding, spherical, premised flames were performed using the k - ϵ turbulence model and the Turbulent Flame Speed Closure of the balance equation for a progress variable. Three major trends have been observed in the simulations. First, the analysis of various physical mechanisms controlling the increase of St, has shown that the time-dependence of the mean heat release rate, invoked by the model, is of substantial importance for small kernels only. For moderately large flames, the development of St, is mainly controlled by the relaxation of the reduction effect of the mean flame curvature on the flame speed. The second manifestation of the mean curvature mechanism is the opposite effects of the turbulent length scale L on the speed of asymptotically stationary, planar flames and of moderately large, statistically spherical flames. In the spherical case, a stronger reduction of the flame speed of small kernels is observed in turbulence with a larger scale. As the kernel grows, the reduction effect relaxes and the dependence of St, on L reverses. Third, when the ignition energy is close to the critical value igniting the turbulent mixture, a regime of kernel expansion characterized by substantially reduced flame speed and burning velocity can occur even in relatively large, statistically spherical turbulent flames. The physical cause of this memory effect consists in the formation of a highly dispersed kernel followed by slow after-burning, When the spark energy is kept constant, the increase in turbulent velocity u′ increases the critical ignition energy and the transformation to the aforementioned regime occurs. This mechanism can contribute to the decrease of St, with u′, observed in many experiments. Finally, the suppression of counter-gradient diffusion in spherical flames is discussed at the end of the paper.

ISSN:0010-2202    

DOI:10.1080/00102200008947273

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A numerical study is conducted to investigate the fire structure, heat transfer and pyrolysis rate between vertical parallel burning surfaces with a fire-induced flow. The strong coupling of the two initially unknown important parameters, such as the burning and fire-induced mass flow rates, is modeled using a parubolized numerical technique which takes account the effects of the streamwise pressure gradient in parallel configuration. Transport equations for mass, momentum, gas-phase chemical species, enthalpy are solved using a finite volume method. The turbulent flow field is solved using a standard k - φ turbulence model in conjunction with a wall function. A two-dimensional adaptation of the discrete ordinates method is used for estimating the flame radiation energy to the burning wall. Soot model is also included in order to permit application to radiative heal transfer within a flame. The results indicate that with decrease of the wall spacing/height ratio (L/H), convection flux decreases slightly, whereas, contribution by radiation increases considerably from 70 to 90 percent of the total heat feedback to the pyrolyzing surface. Of particular interest is a maximum local burning rate for a wall spacing/height ratio (UH≈0.1) due to enhanced convection and radiation fluxes.

ISSN:0010-2202    

DOI:10.1080/00102200008947274

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

An attempt has been made to examine the flame-generated turbulence in a large-scale and low-intensity turbulent premixed flame by measuring change of the local gas-velocity vector across the flame front in relation with local movement of the flame front. In order to achieve this measurements, specially arranged diagnostics, composed of an electrostatic probe with three identical sensors and a two-color four beam LDV (laser-doppler velocimeter) system, have been adopted. From simultaneous measurements by these two instruments at the same point, local movement of the flame front in a vertical plane above a vertically oriented burner can be measured by the electrostatic probe, and simultaneously, the instantaneous axial and radial components of the local gas velocity in the same plane can be measured by the suitably oriented LDV system. Thus, change of the power spectrum density function and individual change of the local gas-velocity vector can be examined in relation with local movement of the flame front.

ISSN:0010-2202    

DOI:10.1080/00102200008947275

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The combustion and soot processes in a fluidized-bed (FB) - like furnace are studied numerically. Fuel considered is methane. The simulation results obtained show that the most important sooting region in the furnace is the lower circulating region formed between the secondary air jets and the primary inlet of the furnace (referred to here as the FB-region), The most important factor affecting the soot formation in this FB-region are local temperatures in the stagnation point - like flow region formed in the middle of the furnace between the two opposing secondary air jets. These temperatures affect soot inception in this region, as well as local temperatures in the FB-region. Turbulent temperature fluctuations are high enough, so that they have important favoring effects on soot formation in the important sooting regions indicated above, By accounting for radiation heat transfer in the simulation, temperatures in the hottest sites along the central symmetry plane of the furnace are reduced, roughly, by about 100 K. This has significant adverse consequences on total soot amounts generated; soot amounts generated in this case are about 10 times lower than those predicted when radiation effects are ignored. Nevertheless, soot levels in the furnace, even under radiating conditions, are still clearly higher than those typical for normal methane diffusion flames under atmospheric pressure. In contrast to the case when radiation is not simulated, under radiating conditions decoupling soot from the gas phase is found to have almost no noticeable impact on local temperatures and species concentrations. In this case, however, less heat will be radiated to the walls and to colder regions in the furnace.

ISSN:0010-2202    

DOI:10.1080/00102200008947276

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The turbulent micro mixing in the Eddy Dissipation Concept (EDC) has been investigated. The EDC is a model for turbulence-chemistry interaction modeling that allows for the use of multi-step reactions. The EDC was originally developed assuming that the reaction kinetics is fast compared to the turbulent mixing. With a more detailed description of the chemistry the interaction between turbulence and chemistry becomes more complex which may call for a revision of the model. Three ways to estimate the turbulent mixing times have been investigated. The first is the time scale proposed in the EDC, i.e., the lime scale related to the Kolinogorov time scale. The second is a time scale based on the geometrical mean of the Kolinogorov time scale and the time scale given by k/ϵ. The third time scale is the time scale given by 4k/epsiv;, The fuel chemistry was modeled with a three-step simplified mechanism A separate model based on calculations with a comprehensive reaction mechanism was used for estimating the chemical time scale at which extinction occurs. In the study a natural gas-fired sudden expansion reactor was modeled. The use of the Kolmogorov related time scale resulted in an overestimation of the extinction effects, whereas best result was obtained when the mixing time was modeled with the time scale given by 4kϵ.

ISSN:0010-2202    

DOI:10.1080/00102200008947277

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The multiple-path on combustion hysteresis and vaporization interactive factor of two droplets in a high-temperature flow stream with mutual interaction is numerically analyzed, to recognize the droplets burning situation in sprays. The conservation equations of mass, momentum, energy, and species in the gas-phase are solved by utilizing a body-fitted technique in association with curvilinear coordinate system. By increasing the ambient temperature from lower to higher value and then reversing, the obtained results illustrate that the hysteresis effect of the two-droplet system can be classified into four different types, depending intrinsically upon the droplet size ratio. As a whole, as the droplet size ratio decreases, the hysteresis effect will be contributed from the leading droplet to the trailing one. Within this transition, the effect can be induced by both of the leading and trailing droplets such as the droplet size ratio of 0.5. This results in a double-hysteresis effect being characterized, Accordingly, an extensive demonstration on combustion hysteresis effect is conducted. On the other hand, the droplet interactive factor is taken into consideration to account for the vaporization rate of a con-vective droplet influenced by droplet mutual interaction. The results elucidate that, no matter what the upper- or lower-branch is the droplet's vaporization rate is always suppressed by the interaction in the lower and the higher ambient temperatures. However, under certain conditions with moderate ambient temperature, such as a flame is stabilized between droplets and the droplet size ratio is not very large, unlike the stationary many-droplet systems, the existence of the leading droplet is conducive to vaporization of the trailing one, Therefore, a more realistic insight for burning droplet in sprays is outlined.

ISSN:0010-2202    

DOI:10.1080/00102200008947278

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Measurements with different diesel engines and fuel additives permit a characterisation of the aerosol of a diesel engine under the influence of fuel additives. Combined with chemical analysis and gravimetry a clear picture of the size range and composition of the emitted particles is oblained. Emission factors computed by gravimetry and coulometry correlate well. A correlation is found between the minimum additive concentration needed to induce exhaust particles consisting of additive products, and the elemental carbon emission factor of the engines. It provides informalion for optimising the exhaust cleaning system.

ISSN:0010-2202    

DOI:10.1080/00102200008947279

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor