Convective Combustion Research Articles

High-rate combustion of cellulose nitrate in filled polymer systems

It was found that, at a certain external pressure, the conventional slow flameless combustion of nitrocellulose in filled compositions can go over into a high-rate low-temperature mode, with a burning velocity increasing by more than 40-fold. It was shown that the critical external pressure at which the changeover of the combustion modes occurs decreases with the sample porosity. It was assumed that the high-rate combustion of nitrocellulose in filled systems proceeds by the well-known mechanism of convective combustion of energetic condensed systems with some specific features.

Nonideal regimes of deflagration and detonation of black powder

The explosive and deflagration properties of black powder differ significantly from those of modern propellants and compositions based on ammonium nitrate or ammonium perchlorate. Possessing a high combustibility, black powder is capable of maintaining stable combustion at high velocities in various shells, be it steel shells or thin-walled plastic tubes, without experiencing deflagration-to-detonation transition. It is extremely difficult to detonate black powder, even using a powerful booster detonator. The results of numerical simulations of a number of key experiments on the convective combustion and shock initiation of black powder described in the literature are presented. The calculations were performed within the framework of a model developed previously for describing the convective combustion of granulated pyroxylin powders, with small modifications being introduced to allow for the specific properties of black powder. The thermophysical properties of the products of combustion and detonation and the parameters of the equation of state of black powder were determined from thermodynamic calculations. The calculation results were found to be in close agreement with the experimental data. The simulation results were used to analyze the regularities of the wave processes in the system and their relation to the properties of black powder and the experimental conditions. It was demonstrated that the effects observed could be explained by a weak dependence of the burning rate of black powder on the pressure.

Combustion Characteristics of a Novel Grain‐Binding High Burning Rate Propellant

AbstractThe novel grain‐binding high burning rate propellant (NGHP) is prepared via a solventless extrusion process of binder and spherical propellant grains. Compared with the traditional grain‐binding porous propellants, NGHP is compact and has no interior micropores. During the combustion of NGHP, there appear honeycomb‐like burning layers, which increase the burning surface and the burning rate of the propellant. The combustion of NGHP is a limited convective combustion process and apt to achieve stable state. The larger the difference between the burning rate of the binder and that of the spherical granular propellants exists, the higher burning rate NGHP has. The smaller the mass ratio of the binder to the spherical granular propellants is, the higher the burning rate of NGHP is. It shows that the addition of 3 wt.‐% composite catalyst (the mixture of lead/copper complex and copper/chrome oxides at a mass ratio of 1 : 1) into NGHP can enhance the burning rate from 48.78 mm⋅s−1 in the absence of catalyst to 56.66 mm⋅s−1 at P=9.81 MPa and decrease the pressure exponent from 0.686 to 0.576 in the pressure range from 9.81 to 19.62 MPa.

Combustion of damaged PBX 9501 explosive

Impact or thermal ignition of high explosives (HE) results in deformation that can lead to damage. Fractures or defects, combined with sufficient pressure, dramatically increase the available surface area and potentially changes even the mode of combustion. Recent impact and cookoff experiments on PBX 9501, (HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, with a plasticized, Estane-based binder), have shown complex cracking patterns caused by impact or pressurization. Fast reactive waves have been observed to propagate through the cracks at hundreds of meters per second. We present experiments that examine the combustion of mechanically and thermally damaged samples of PBX 9501. Mechanically damaged samples, damaged by quasi-static compression, exhibit large, ∼200 μm stress fracture accompanied by extensive rubblization. Combustion experiments determine a 1.4±0.6 MPa critical pressure for the onset of violent convective combustion, consistent with connected porosity of 25 μm. Thermally damaged samples, damaged by heating in a 180 °C oven for 30 min, exhibit 2–20 μm cracks distributed throughout the sample. Combustion experiments indicate a 9.2±0.4 MPa critical pressure for the onset of violent convective combustion in the thermally damaged sample, consistent with connected porosity of 4 μm. Below the critical pressure, the burn rate and pressure exponent of thermally damaged PBX 9501 are close to those of the pristine material.

Convective Combustion of “Gasless” Systems

A convective mechanism of combustion is proposed to explain the anomalously high burning rate of titanium–soot and titanium–soot–polystyrene “gasless” systems under conditions of one‐dimensional filtration of impurity gases. An analysis of experimental and theoretical works shows that the convective mechanism of combustion under conditions of impurity gas release may be ensured by the motion of the melted layer of one of the reagents under the action of the pressure difference of impurity gases. Physical and mathematical models of the process are formulated. It is found that a free volume, which is not occupied by the burden, is necessary for an accelerating regime of combustion. It is shows that the front velocity and the gas pressure increase exponentially at the initial stage of combustion and also if the free volume is greater than the specimen volume. Analytical expressions for the mean convective burning rate are derived. Consideration of the model formulated in the present paper offers an explanation of the differences in the burning laws of “gasless” systems, which are observed in experiments under conditions of counterflow, cocurrent, and two‐sided filtration of impurity gases.

Effect of force interphase interaction on the characteristics of convective combustion of porous media

The force interphase interaction in a low-porosity media with high-velocity gas filtration has been studied experimentally. New approximation dependences are proposed for the resistance coefficients. The dependences are valid over a wide range of parameters of the flow and the porous layer. Parametric studies of characteristics of transient combustion regimes were performed using various formulas for the interphase interaction force.

Explosive initiation of fast convective combustion in a closed volume

We have studied experimentally the explosive initiation of granulated nitrocellulose taking place under conditions of fast convective combustion and low-velocity deformation and marked by a high rate and stability. The fundamental characteristics of the ignition process have been determined.

Influence of the Acoustic Oscillations upon Convective Combustion of Aero-Suspensions

Abstract Convective combustion of mono-fuel aero-suspensions in an open-closed channel are treated numerically. The general approach to the problem of the simulation of the dispersed material behaviour to be exploited here is based upon incorporation of multiphase flow equations. The Lax-Wendroff two-step method is used for a solution of two-dimensional planar unsteady problems. The effect of acoustic oscillations caused by combustion upon combustion itself is studied. The different regimes (accelerated and decelerated– of convective combustion spreading are identified and the constitutive criterion of the realization of those regimes is suggested. The computational results to be obtained further are compared with available experimental data.

Initial stage of ignition process in powder explosive with a closed end

Simultaneous optical and piezometric measurements are used to obtain new data on the behavior of the layer of lightly compressed picric acid in the initial stage prior to convective combustion. The formation and development of the cavity resulting from the densification and motion of the explosive driven by the combustion products of the igniter are examined. The effects of the parameters of the igniter and explosive (length, initial density, particle sizes) and of the roughness of the walls of the shell channel on the development of the cavity are investigated. It is shown that for small heights of the explosive layer, cavity formation is accompanied by the development of a backward compression wave that propagates toward the igniter end. The parameters of this wave are determined.

Convective combustion analyzed on a viscoplastic model for a porous medium

A new model is proposed for the convective combustion of a porous fuel, which incorporates the deformation and dispersal of the porous matrix as well as the dissipative heating. Parametric studies have been made on the combustion of low-porosity and granulated charges. An experimental test agrees well with the model.

Convective combustion of porous compressible propellants

This abstract describes the combustion of solid porous propellants, focusing on burning regimes characterized by a convective mechanism of propagation. A new mathematical model of the dynamics of multiphase chemically reacting media, including interphase heat and mass transfer, is developed in the course of the investigation. Mathematical simulations of the process prove theoretically that there exists a self-sustained combustion regime, driven by convection. This regime is supersonic with respect to the gas phase and subsonic with respect to the condensed phase.

DIRECT SPECTRAL METHODS FOR THE LOW MACH NUMBER EQUATIONS

The low Mach number approximation of the Navier—Stokes equations is of similar nature to the equations for incompressible flow. A major difference, however, is the appearance of a space‐ and time‐varying density that introduces a supplementary non‐linearity. In order to solve these equations with spectral space discretization, an iterative solution method has been constructed and successfully applied in former work to two‐dimensional natural convection and isobaric combustion with one direction of periodicity. For the extension to other geometries efficiency is an important point, and it is therefore desirable to devise a direct method which would have, in the best case, the same stability properties as the iterative method. The present paper discusses in a systematic way different approaches to this aim. It turns out that direct methods avoiding the diffusive time step limit are possible, indeed. Although we focus for discussion and numerical investigation on natural convection flows, the results carry over for other problems such as variable viscosity flows, isobaric combustion, or non‐homogeneous flows.

Determination of heat transfer coefficient using transient temperature response chart

An inverse finite element computer code is developed to predict surface heat flux and surface temperature in conjunction with the measured thermocouple temperature history. Determination of convective heat-transfer coefficient and combustion gas temperature is carried out employing transient temperature response chart. Examples are illustrated which are typical of those that arise in thermal design of rocket nozzle. The results demonstrate that the method is remarkable in its ability to estimate unknown boundary conditions.

Convective transport of combustion products in the atmosphere above large fires

A nonstationary axisymmetric model of the development of a turbulent convective combustion product column above a fire in a stratified atmosphere is proposed. The model takes into account the compressibility of the gas and the diffusion of the aerosol particles and makes it possible to predict the dynamics of ascent of the convective column, the height of the cloud and the distribution of the aerosol in the atmosphere. The numerical and experimental data are compared.

Investigation of convective combustion in a combustible solid propellant with longitudinal channels

Investigation of convective combustion in a combustible solid propellant with longitudinal channels

Heat transfer and convection combustion regimes of porous systems with filtration of heat carrier

Heat transfer and convection combustion regimes of porous systems with filtration of heat carrier

A deterministic model of convective combustion of porous systems

A deterministic model of convective combustion of porous systems

Variable property analysis-is there anything to it?

This paper discusses a few situations in combusting flows and attempts to demonstrate that including variable thermodynamic and transport properties in the analysis does more than simply improve the accuracy of predictions. The qualitative behaviour of the result itself is altered. Three examples are considered-single droplet combustion, forced convective turbulent boundary layer combustion, and free convective combustion.

Toward a theory of transition of convective combustion of an air suspension into detonation

Toward a theory of transition of convective combustion of an air suspension into detonation

Convective combustion of aerosuspension of a unitary fuel in a bounded region

Convective combustion of aerosuspension of a unitary fuel in a bounded region