Combustors cross-sectional area

The elevated pressure of PFBC (1000-1500 kPa) means that even the bubbling bed combustor cross-sectional area is reasonable for shop fabrication at relatively large capacities. 

Cross-sectionai area. The combustor cross section can be determined by dividing the volumetric flow at the combustor inlet by a reference velocity which has been selected as being appropriate for the particular turbine conditions on the basis of proven performance in a similar engine. Another basis for selecting a combustor cross section comes from correlations of thermal loading per unit cross section. Thermal loading is proportional to the primary zone air flow because fuel and air mixtures are near stoichiometric in all combustors. 

In the model equations, A represents the cross sectional area of reactor, a is the mole fraction of combustor fuel gas, C is the molar concentration of component gas, Cp the heat capacity of insulation and F is the molar flow rate of feed. The AH denotes the heat of reaction, L is the reactor length, P is the reactor pressure, R is the gas constant, T represents the temperature of gas, U is the overall heat transfer coefficient, v represents velocity of gas, W is the reactor width, and z denotes the reactor distance from the inlet. The Greek letters, e is the void fraction of catalyst bed, p the molar density of gas, and rj is the stoichiometric coefficient of reaction. The subscript, c, cat, r, b and a represent the combustor, catalyst, reformer, the insulation, and ambient, respectively. The obtained PDE model is solved using Finite Difference Method (FDM). 

Primary zone size is important with regard to efficiency and limits also. Within practical limits, a larger primary zone cross-sectional area will provide the best performance 138). Possible reasons arc lower velocities, less wall impingement by fuel, larger zone of low velocity, and less wall quenching of chemical reactions. The best axial distribution of open area of a combustor will depend on required operating conditions, the pressure loss characteristics, and the shape of the air entry ports. It will also depend on fuel-injection and fuel-volatility characteristics, as these factors will affect the amount of vapor fuel present at any location. If proper burning environment is to be obtained, these factors must be matched, and compromises in performance must be expected. 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The Helmholtz pulse combustor operates under the principle of the standard acoustic Helmholtz resonator in which a short, small-diameter stub (tailpipe) is attached to one of the walls of a large cavity (combustion chamber) and valves are placed at the wall opposite the tailpipe. A Helmholtz resonator operates at a frequency determined by both the volume of the combustion chamber and the length and cross-sectional area of the tailpipe. It is important to note that the pressure within the Helmholtz combustion chamber is considered to be uniform in space while the pressure oscillations become space-dependent once within the tailpipe. 

With fitted reflection factors, this model does an excellent job of predicting axial density profiles in the upper part of a range of different columns with secondary air addition (Senior and Brereton, 1992), not only for a small-scale pilot plant CFB combustor but also for a prototype boiler of cross-sectional area 0.43 m operated at high temperature (845°C). 

The reed valves normally used in heavy-duty pulse combustors are made from thin-sheet spring-sted (Fig. 2.6b and c), and the spring action of reed valves is such that, when normally shut, the valves are sprung lightly. In order to ensure a vigorous mixing of the fuel and air, the fully open flow area of the inlet reed valves must be considerably smaller than the cross-sectional area of the combustion zone. The major problem often encountered with reed-type mechanical valves is fatigue-based failure. 

Aerodynamic valves employ the properties of the fluid entering a specially designed inlet to the combustor to create an artificial barrier to the backflow of combustion products out of the combustor through its inlet section. The main advantage of aerodynamic valves is a lack of mechanical parts that are prone to failure. The design concept of aerodynamic valves exploits the principle of a fluid diode which are, of course, inferior in performance when compared to mechanical valves as the backflow of combustion products cannot be fully eliminated. One mechanism that is known to limit the amount of backflow is to include a nonuniform cross-sectional area. A tapered inlet that diverges gradually towards the combustion chamber initially accelerates the inlet stream of air and then diffuses it before... 

Figure 19. Variation of the instantaneous local heat transfer coefficient and the point voidage with time on the wall of the cold model circulating fluidised bed combustor. The signals are seen to be strongly cross-correlated. The cross sectionally area averaged suspension densities are from top to bottom, 46.7 kg/m3, 32.0 kg/m3,15.3 kg/m3, (Wu et al, 1991).

A quantitative validation of the SURE model was carried out (31) with application of steady state retention data that were obtained in the 4MWth coal fired fluidized bed boiler of TNO (30) in The Netheilands (bed height about 1 m bed cross sectional area 2.25 m ). The results are shown in Fig. 10, where the calculated values of the Ca/S ratio (without any fitting to the combustor data) are compared with the actual values as used in the TNO experiments. A good agreement is obtained. 

Visual inspection of the subscale unit revealed an overall good cross-section uniformity for the catalytic channels (see Fig. 2.4). The unit tested in this study differed from the proposed mesoscale catalytic combustor only in its inner diameter (all other geometric parameters were kept constant), with the former unit having a radial dimension 42% smaller than the latter. With the number of catalyst-coated channels being proportional to the honeycomb cross sectional area, the power output of the subscale unit was reduced nearly threefold compared to the mesoscale unit heat losses from the outer combustor surface were accordingly impacted due to the reduced surface area of the subscale combustor. 

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