Volumetric heat release rate

Given the mechanisms and temperatures, waste combustion systems typically employ higher percentages of excess air, and typically also have lower cross-sectional and volumetric heat release rates than those associated with fossil fuels. Representative combustion conditions are shown in Table 11 for wet wood waste with 50—60% moisture total basis, municipal soHd waste, and RDF. 

Volumetric heat-release rate. The heat-release rate is proportional to the fuel-to-air ratio and the combustor pressure, and it is a function of... 

Volumetric heat release rates The rates of volumetric heat release from shell boiler furnaces fired by oil and gas are typically 175,000 to 235,000 Btu/ft3/hr. (Heat releases from the various tube passes are significantly lower than from the furnace, thus reducing the overall heat-flux rating.)... 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

The decrease in temperature predicted by the analysis is relatively small and has not been observed experimentally. Experiments with higher precision and accuracy are warranted for checking if this is an artifact of the present chemistry that does not include the effects of higher hydrocarbon formation and radiative heat loss. The peak temperature was found to decrease because of a decrease in the peak volumetric heat release rate caused by a broadening of the reaction zone. 

Using GRI-Mech (GRIM30. mec), determine the temperature and species distributions for a freely propagating, stoichiometric, methane-air, mixture at a pressure of 10 Torr. From the solution determine the volumetric heat-release-rate profile through the thickness of the flame. 

Attempts have been made to estimate the volumetric heat release rate during autoignition, from pressure records and CARS temperature measurements [106]. It was estimated that the activation temperature for this was in the region of 20,000 K under knocking conditions. Paraffinic fuels gave a higher volumetric heat release rate than aromatic fuels and were more prone to knock severely. 

This one-step expression for the reaction rate is adequate for a demonstration of some of the general principles involved in the generation of pressure pulses. The equations are used to show the effects of the temperature gradient around a hot spot and the volumetric heat release rate there upon the propagation of autoignition. 

Fig. 7.26. Computed dependence of mode of autoignition on the kinetic parameters, A and E, for heat release rate in equation (7.17). E/R is the activation temperature, Ta , and To = 1000 K. Contours show initial volumetric heat release rate at the initial temperature of 1000 K in GW m . Regimes of different modes of propagation are indicated by the thick lines. P(c) is the probability of an autoignition centre becoming critical. Ap is an initial hot-spot overpressure. Temperature gradient of -23.5 K/mm. From [180].

There are a few drawbacks associated with catalytic combustion. First, as noted by Pfefferle, the power density is low. The volumetric heat release rates of catalytic combustors (without a homogenous flame downstream) are much lower than those found in conventional flame combustors because catalytic combustors are mass transfer limited, and mass transfer coefficients are relatively low. 

Fig. 2 Non-dimensional temperature profiles and volumetric heat release rates for benchmark case (Table 1). Condensed-phase heat release is concentrated near surface due to E 1 assumption. Large- Eg case concentrates gas-phase heat release in a flame sheet at Xg = 0.27. Small- Eg case distributes gas-phase heat release broadly over a region with characteristic (1/e) length scale of x = 1.4.

Figure 11 also shows the calculated volumetric heat release rates due to chemical reaction (Q( c and QgQg). For Eg 1 the effect of large Eg is to... 

Incinerator volumetric heat release rates (l3 l U/ft /h) exceed design limits ... 

Volumetric heat release rate, power per unit volume ... 

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