Heat model calculation

The SCREEN model calculates plume rise for flares based on an effective buoyancy flux parameter. An ambient temperature of 293° K is assumed in this calculation and therefore none is input by the user. It is assumed that 55 percent of the total heat is lost due to radiation. 

Using fire models, locations of equipment, heat transfer calculations, and environmental qualifications of the equipment, it is possible to estimate the time to failure. Fragility cuives that relate fire durations and equipment damage while considering the probability of fire suppre.ssion are produced to relate to the overall PSA. These fragility curves and their use is simitar the methods ised for seismic analysis. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

The subject of flash fires is a highly underdeveloped area in the literature. Only one mathematical model describing the dynamics of a flash fire has been published. This model, which relates flame height to burning velocity, dependent on cloud depth and composition, is the basis for heat-radiation calculations. Consequently, the calculation of heat radiation from flash fires consists of determination of the flash-fire dynamics, then calculation of heat radiation. 

Miedema model calculated heats vs. experimental heats of formation,... 

Fig, 7. Laser-induced heating model. The solid line represents the temperature transient calculated from Eq. (3) for a S ns FWHM laser pulse (dott trace). The instantaneous desorption rate calculated from Eq. (4) is represented by the... 

If the heat capacity of a chemically complex melt can be obtained by a linear summation of the specific heat of the dissolved oxide constituents at all T (i.e., Stebbins-Carmichael model), the melt is by definition ideal. The addition of excess Gibbs free energy terms thus implies that the Stebbins-Carmichael model calculates only the ideal contribution to the Gibbs free energy of mixing. 

An interesting application of this approach in another field has been described by Keller (K2). In the design of steam turbines rather complicated heat-balance calculations are required. While each particular installation is different, and therefore requires a different mathematical model, the components of each turbine are always similar. A large-scale computer program was developed, therefore, which would through suitable instructions combine the calculations required for each component into an over-all heat balance for the turbine. 

The model atmosphere is driven by net heating rates calculated with the radiactive module considering the distribution of H2 O, C02,02,03 (solar heating) and H2 0,... 

The mathematical model developed to describe the nitric acid absorption process isdiscussed in detail in Section G.2. It uses a tray-by-tray approach that incorporates reaction-mass balance corrections and heat balance calculations. Tray efficiency calculations are also included in the model, the efficiency being a function of the tray geometry and gas velocity. Rate equations and other data specific to the nitric acid/nitrous gas system are applied. 

FIGURE 16.3 Heat release calculated by the additive model versus heat release measured in the MCC for... 

Typically, a fire growth model is evaluated by comparing its calculations (predictions) of large-scale behavior to experimental HRR measurements, thermocouple temperatures, or pyrolysis front position. The overall predictive capabilities of fire growth models depend on the pyrolysis model, treatment of gas-phase fluid mechanics, turbulence, combustion chemistry, and convective/radiative heat transfer. Unless simulations are truly blind, some model calibration (adjusting various input parameters to improve agreement between model calculations and experimental data) is usually inherent in published results, so model calculations may not truly be predictions. 

FIGURE 20.4 Comparison of measured and modeled HRR in room corner test on particle board. Model calculations are for total HRR calculated with heat transfer limited pyrolysis model. (Adapted from Yan, Z. and Holmstedt, G Fire Saf../., 27, 201, 1996.)... 

Figure 4. Effect of carbon on the wall heat flux with diffuse walls. Results of coupled radiation-aerodynamic model calculations by Toor and Boni (1974).

This last item is important because it leads to an easy way to accommodate the molar contraction of the gas as the reaction proceeds. The program calculates steady-state profiles of each of these down the length of the tubular reactor, given the reaction kinetics models, a description of the reactor and catalyst geometries, and suitable inlet gas flow-rate, pressure and composition information. Reactor performance is calculated from the flow-rate and composition data at the reactor outlet. Other data, such as the calculated pressure drop across the reactor and the heat of reaction recovered as steam, are used in economic calculations. The methods of Dixon and Cresswell (7) are recommended for heat-transfer calculations. 

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