Burning of condensed phases

The rate at which the droplet evaporates and bums is generally considered to be determined by the rate of heat transfer from the flame front to the fuel surface. Here, as in the case of gaseous diffusion flames, chemical processes are assumed to occur so rapidly that the burning rates are determined solely by mass and heat transfer rates. 

Many of the early analytical models of this burning process considered a double-film model for the combustion of the liquid fuel. One film separated the droplet surface from the flame front and the other separated the flame front from the surrounding oxidizer atmosphere, as depicted in Fig. 6.11. 

In some analytical developments the liquid surface was assumed to be at the normal boiling point of the fuel. Surveys of the temperature field in burning liquids by Khudyakov [15] indicated that the temperature is just a few degrees below the boiling point. In the approach to be employed here, the only requirement is that the droplet be at a uniform temperature at or below the 

FIGURE 6.11 Characteristic parametric variations of dimensionless temperature T and mass fraction m of fuel, oxygen, and products along a radius of a droplet diffusion flame in a quiescent atmosphere. j is the adiabatic, stoichiometric flame temperature, pA is the partial density of species A, and p is the total mass density. The estimated values derived for benzene are given in Section 2b. 

In the film oxygen diffuses to the flame front, and combustion products and heat are transported to the surrounding atmosphere. The position of the boundary designated by °o is determined by convection. A stagnant atmosphere places the boundary at an infinite distance from the fuel surface. 

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Phosphorus-containing compounds are a family of condensed-phased FR, which are able to increase the conversion of organic matter to char during burning, and, thus... 

For the RDX flame, we can also use sensitivity analysis to determine the importance of condensed phase physical properties in affecting the bum velocity. The results are shown in Fig. 12. As expected, one sees that the burn rate is very sensitive to the initial temperature of the propellant. Similarly, one also sees a strong dependence on pressure. 

Molybdenum trioxide is a condensed-phase flame retardant (26). Its decomposition products ate nonvolatile and tend to increase chat yields. Two parts of molybdic oxide added to flexible poly(vinyl chloride) that contains 30 parts of plasticizer have been shown to increase the chat yield from 9.9 to 23.5%. Ninety percent of the molybdenum was recovered from the chat after the sample was burned. A reaction between the flame retardant and the chlorine to form M0O2 012 H20, a nonvolatile compound, was assumed. This compound was assumed to promote chat formation (26,27). 

Studies of the incineration of liquid and solid wastes must determine the rates at which hazardous compounds are released into the vapor phase or are transformed in the condensed phase, particularly when the hazardous materials make up a small fraction of the liquid burned. We must be particularly concerned with understanding the effects of the major composition and property variations that might be encountered in waste incinerator operations—for example, fluctuations in heating value and water content, as well as phase separations. Evidence of the importance of variations in waste properties on incinerator performance has been demonstrated by the observation of major smges in emissions from rotary-kiln incinerators as a consequence of the rapid release of volatiles during the feeding of unstable materials into the incinerator. 

Figure 3. Content of oxygen in condensed phase of burning polypropylene at varying distances below the surface.

Burning rate is strictly defined as the mass rate of fuel consumed by the chemical reaction, usually but not exclusively in the gas-phase. For the flaming combustion of solids and liquids, the burning rate is loosely used to mean the mass loss rate of the condensed phase fuel. However, these two quantities - mass loss rate and burning rate - are not necessarily equal. In general,... 

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