Combustors length

Length. Combustor length must be sufficient to provide for flame stabilization, combustion, and mixing with dilution air. The typical value of the length-to-diameter ratio for liners ranges from three to six. Ratios for casing range from two to four. 

Based on the measured distribution of radiation intensity along the combustor length, the average radiant heat loss rate for the entire combustor volume for the 02/Ar case is estimated at about 4.0 W/cm, corresponding to a total radiant heat loss of about 20 kW or 160 cal/g of combustion products. Heat loss of 160 cal/g, together with an estimated combustion products temperature of 3050 K, implies that the reactedness of the mixture is on the order of 60% to 70%. The lower estimate of reactedness corresponds to a case in which the unburned aluminum never ignited. The higher estimate of reactedness corresponds to a case where the unburned aluminum is at the same temperature as the rest of the mixture, which implies that it is in vapor phase. The actual reactedness should fall between these two extremes. 

To elucidate the possible causes of the decrease in suppression potential, the effects of flow residence time and relative pulsating fuel amount were examined. One possible explanation for the above trend is the reduction in flow residence time as the flow rate was increased. At these conditions some of the larger fuel droplets that persisted in the downstream may not have had enough time to react completely if the residence time became very short. When the residence time was estimated by the reference time scale which is the combustor length divided by inlet velocity (Fig. 21.12), the general trend appears to be consistent with the expectation. The scatter in the plot reflects the crudeness of the estimation larger droplets do not follow the carrier flow very well. 

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). 

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

Other processes that have increased importance at small length scales such as thermal creep (transpiration) and electrokinetic effects are also being considered for use in microcombustors. For example, transpiration effects are currently being investigated by Ochoa el al. [117] to supply fuel to the combustion chamber creating an in-situ thermally driven reactant flow at the front end of the combustor. 

Suppose fuel droplets of various sizes are formed at one end of a combustor and move with the gas through the combustor at a velocity of 30m/s. It is known that the 50-pm droplets completely vaporize in 5 ms. It is desired to vaporize completely each droplet of 100 pm and less before they exit the combustion chamber. What is the minimum length of the combustion chamber allowable in design to achieve this goal ... 

One of the basic elements of the computational algorithm is the determination of dependent variables at the inlet/outlet boundaries of a computational domain representing a finite length combustor. The essence of the problem lies in the fact that the nonstationary flow field has to be considered throughout a whole (unbounded) physical space, and only in this case the problem is mathematically well-posed. When solving a specific problem numerically, one has to consider a computational domain of a finite size, in which boundary conditions at artificial boundaries are to be imposed. 

Figure 12.3 shows some computational examples of nonreactive and reactive turbulent flows in a combustor with the bluff-body flame holder. The size of the combustor in Fig. 12.3 is 35 x 8 cm. The characteristic height and length of the bluff body is H = 2 cm. The left boundary is set as inlet, right boundary as outlet, and the upper and lower boundaries as rigid walls. 

So far, the flow patterns around bluff bodies in combustible flows are not understood completely. However, a recirculation zone in the immediate wake of the stabilizer which takes the form of a pair of eddies, similar to isothermal flows, is known to exist. The length Lrz of the recirculation zone differs for 2D and axisymmetric bluff bodies. For 2D bodies (V-gutters, rods, prisms), the measured values of Trz/H range from 3 to 6 depending on the operating conditions of combustor [11], which is considerably larger than for isothermal flows, where Lrz/H 2 [11]. For axisymmetric bluff bodies (discs, cones, cylinders), at low-blockage ratio Lrz/H 2 [32], which is similar to isothermal flows [32, 33], or Lrz/H 2.b-A [34], or even Lrz/H 10-11 [35]. 

Acoustic quarter-waves with an antinode at the upstream end of the combustor and RMS pressures up to 10 kPa have been shown to dominate the flows in the two combustors tested. The quarter-wave occupied the duct length upstream of the annular ring in the first arrangement and the entire duct length in the swirling flow. 

---