Non-steady-state combustion

If the pressure in a rocket motor changes during the time interval x, the pressure transient is considered to represent a steady-state combustion when x > x and a non-steady-state combustion when x < x. ... 

The experimental techniques employed in the fundamental studies of the burning rate of a liquid droplet fall into three groups (a) The process of stationary, non-steady-statc combustion in which the combustion rate of a droplet suspended in a reacting medium is determined from the variation of droplet size with time (b) the process of stationary, steady-state combustion in which the geometric dimensions of a supported droplet are maintained constant during combustion and (c) the process of nonstation-ary, non-steady-state combustion in which a freely-moving droplet is allowed to come in contact with a gaseous reactant. 

Filonenko30 in 1975 investigated the combustion of metal/gas systems and the occurrence of non-steady state phenomena. He described the conditions leading to the occurrence of two types of non-steady state phenomena, i.e. oscillatory and spin combustion. Filonenko and Versheminkov in 197531 reported the spin burning of titanium in nitrogen, and discussed conditions leading to spin and the basic factors which control the process. 

The retarding effect of metal compounds on the flaming combustion of polymers must be primarily attributed to the decrease of the yield of flammable volatile pyrolysis products. There are two alternative paths 1) catalysis of the formation of non-combustible products (COj, HjO) 2) inhibition of polymer decomposition at the high temperatures developed during steady-state combustion (high-temperature stabilization)... 

Catalytic experiments were done with 75 ml of a catalyst and FAV=10 000 m /h/kgcat-Combustion of toluene was chosen as a model volatile organic compound and non-steady-state reaction condition (linear increase 3.5 °C/min in reaction temperature) was applied in the catalyst activity evaluation. Concentration of toluene in air was 1 g /m. Temperatures of gaseous reaction mixture entering and leaving the catalyst layer were measured by thermocouples. Catalytic activities expressed as the inlet temperatures Tso or T90, at which 50 or 90 % conversions of toluene were achieved, were taken as a measure of the catalytic activity. 

The major problem to be addressed in the present paper is how the phenomenon of the surface transformations under the non-steady-state conditions can generally be applied for the NO abatement and for the combustion of volatile organic compounds (VOC). Two phenomena must be considered here ... 

Preheating combustion air is accomplished by recuperators or regenerators, discussed in detail in chapter 5. Recuperators are steady-state heat exchangers that transmit heat from hot flue gases to cold combustion air. Regenerators are non-steady-state devices that temporarily store heat from the flue gas in many small masses of... 

The subsequent fate of the assimilated carbon depends on which biomass constituent the atom enters. Leaves, twigs, and the like enter litterfall, and decompose and recycle the carbon to the atmosphere within a few years, whereas carbon in stemwood has a turnover time counted in decades. In a steady-state ecosystem the net primary production is balanced by the total heterotrophic respiration plus other outputs. Non-respiratory outputs to be considered are fires and transport of organic material to the oceans. Fires mobilize about 5 Pg C/yr (Baes et ai, 1976 Crutzen and Andreae, 1990), most of which is converted to CO2. Since bacterial het-erotrophs are unable to oxidize elemental carbon, the production rate of pyroligneous graphite, a product of incomplete combustion (like forest fires), is an interesting quantity to assess. The inability of the biota to degrade elemental carbon puts carbon into a reservoir that is effectively isolated from the atmosphere and oceans. Seiler and Crutzen (1980) estimate the production rate of graphite to be 1 Pg C/yr. 

In the case of combustion of a condensed substance, conservation of enthalpy and similarity occur only in the gas phase and only in part of the space. In the c-phase the diffusion coefficient is much smaller than the thermal diffusivity, and we have heating of the c-phase by heat conduction without dilution by diffusion. The enthalpy of the c-phase at the boundary, for x — 0 (from the side x < 0), is larger than the enthalpy of the c-phase far from the reaction zone and larger than the enthalpy of the combustion products. The advantage of the derivation given here is that the constancy of the enthalpy in the gas phase and its equality to H0 (H0 is the enthalpy of the c-phase far from the combustion zone, at x — —oo) are obtained without regard to the state of the intermediate layers of the c-phase. We should particularly emphasize that the constancy of the enthalpy in the combustion zone occurs only for a steady process. The presence of layers of the c-phase with increased enthalpy opens the possibility in a non-steady process of a temporary change in the enthalpy of the gas and the combustion temperature (on this see 5). 

Results for oscillatory combustion assuming quasi-steady gas and condensed phase reaction zone (surface reaction approximation) are presented in two groups. First, general characteristics of oscillatory combustion are discussed in the context of the non-dimensional formulation, similar to the steady-state benehmark problem of Table 1. Second, specific results for the common materials NC/NG and HMX are presented. 

In this chapter, we will expand on the modeling of chain reactions, which are of considerable practical importance in the fields of polymer chemistty and combustion, of molecnles snch as hydrocaibons for example, and explosions. Our study will examine the chain reactions that can lead to psendo-steady state modes. The extension to non-psendo-steady state modes will be discussed in Chapter 15. 

Finally, Chapter 15 addresses non-pseudo-steady state processes that are encountered in different areas. We place particular emphasis on these modes for combustion and explosion reactions and heterogeneous reactions. 

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