Heat feedback

The fire in the chair grows proportional to the heat feedback from all sources, which change with time. The flames and hot gases rise to the... 

The hottest fires may be associated with those cases where the fire is big enough to give flames to fill at least half the structure volume, cases where it is stoichiometric or just under ventilated, and cases where the hot gas layer is 10 ft (3 m) or more deep. Heavier fuels would be less likely to give the hottest fires, asthey may not receive enough heat feedback to vaporize the liquid and therefore they may be self limiting in terms of the burn rate. Where these conditions may be encountered, heat fluxes of 1320-1584 BTU/ft (250 to 300 kW/m ) may be experienced. In certain circumstances, (which are not yet fully understood) highly efficient combustion can occur with fluxes of 1848-2112 BTU/ft (350-400 kW/m2) and temperatures of 2,500°F (1,400°C). 

Fig. 3.10 Thermal structure of a combustion wave and heat feedback processes therein.

Fig.s.n Schematic representation of heat feedback processes in a combustion wave. 

As is evident from experimental measurements, most kinds of nitrate esters appear to decompose to NOj and C,H,0 species with the breaking of the O-NOj bond as the initial step. A strong heat release occurs in the gas phase near the decomposing surface due to the reduction of NO2 to NO accompanied by the oxidation of C,H,0 species to HjO, CO, and COj. NO reduction, however, is slow and this reaction is not observed in the decomposition of some nitrate ester systems. Even when the reaction occurs, the heat release does not contribute to the heat feedback to the surface because the reaction occurs at a distance far from the surface. 

Model for Heat Feedback from the Cas Phase to the Condensed Phase... 

Most importantly, the presence of lead compounds results in a strong acceleration of the fizz zone reactions, i. e., those in the gas phase close to the burning surface. Acceleration of the reactions in the subsequent dark zone or in the luminous flame zone is not significant. The net result of the fizz zone reaction rate acceleration is an increased heat feedback to the surface (e. g., by as much as 100 %), which produces super-rate burning. 

The temperature in the condensed phase increases from the initial propellant temperature, Tq, to the burning surface temperature, Tj, through conductive heat feedback from the burning surface. Then, the temperature increases in the gas phase because of the exothermic reaction above the burning surface and reaches the final combustion temperature, Tg. Since the physical structure of AP composite propellants is highly heterogeneous, the temperature fluctuates from time to time and also from location to location. The temperature profile shown in Fig. 7.2 thus illustrates a time-averaged profile. This is in a clear contrast to the combustion wave... 

Physicochemical Model. In constructing the GDF model, it is assumed that gasification at the solid regressing surface is driven by conductive heat feedback from a two-stage flame occurring in the gas phase (see Figure 1). This solid-to-gas phase step is generally endo-... 

The burning surface is much smoother and n-Al enhances the heat feedback to the unburnt solid propellant [113-116]. 4... 

A recent study of the deflagration of RDX (Ref 107) presents the following model for the deflagration process (1) partial decompn in the liq phase (2) vaporization and gas phase decompn (3) oxidation of products (particularly HCHO) by N02. As system press increases, (1) and (3) become progressively more prominent. Although the reacting liq layer at high pressures is thin, its heat feedback into the still unreacted material increases... 

Openloop Response The openloop response of a three-stage adiabatic reactor system with interstage cooling to a 20% increase in recycle flow FR is shown in Figure 6.14. The reactor inlet and exit temperatures of each reactor are shown. This system with heat feedback and large reactor gains is openloop unstable. 

As indicated in Figure 3.3, the heat feedback (q") is used for vaporization (q") and to compensate for in-depth conduction (q") and heat losses (q"). The heat lost from the fuel surface includes convection and re-radiation, and can be described by any of the following formulations ... 

Silverstein. J. L.. and Shinnar. R. Effect of Design on the Stability and Control of Fixed Bed Catalytic Reactors with Heat Feedback. 1. Concepts, Ind. Eng. Chem. Proc. Des. Dev., 21, 241-256 <1982). 

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