Abstract

On studies of flame stabilization in GTE combustion chambers

B.G.Mingazov

KSTU of A.N.Tupolev's name, Department of Aerojet Engine

K.Marx str., 10, Kazan, Russia

As is seen from literature, the problems that deal with stabilization of flames, are discussed in a many a number of papers. Nonetheless, the mechanism of flame stabilization, in the main combustion chambers in which the process of non-homogeneous mixture combustion takes place, remains unclear and calls for further investigations.

According to the statements of numerous researchers, it is the mixture composition in the reverse-flow zone (RFZ) that is responsible for flame-out in non-homogeneous mixtures (a _RFZ). The analytical treatment of the experimentally-obtained data shows that flame stabilization is influenced also by the process of fuel evaporation during its stay in RFZ. We established the fact that flame-out in non-homogeneous mixtures is specified by conditions of ignition in the layer where the forward flow is mixed with reverse flow in the forward part of circulation zone. Based on this statement we make an attempt to obtain the generalizing relations.

From the standpoint of construction of flame stabilization model in the non-homogeneous flow of mixture, most suitable and convenient for this purposes seems to be the heat-transfer William's theory which considers individually the conditions for fresh mixture ignition and heat addition to mixture from RFZ. Based on the equation of balance of heats required and transferred from the reverse-flow zone, we can obtain the following expression:

where U_n is the normal rate of burning; d_RFZ is the RFZ diameter; W is the flow velocity; and a_m is the coefficient of thermal conductivity.

In combustion of non-homogeneous fuels, primary ignition of the fresh mixtures takes place when the local compositions of fuels are near in composition to the stoichiometric mixtures, i.e., at a _air= 1,0. We can therefore assume that T_ig = T_ig(a = 1,0) and U_n = U_n(a = 1,0).

The basic thermophysical parameters depend, in their turn, on pressure p and temperature T at combustion chamber inlet and can be determined from the following relations:

Here a_m0 and U_n0 correspond to the conditions when T₀ = 500K, p₀ = 0,1 MPa. The values of indexes k = 0,25, m = 1,8, n = 1,75 correspond to the conditions of hydrocarbon fuel combustion.

The variable magnitude in this relation is the difference of combustion products temperatures in the reverse-flow zone and in the fresh mixture that is dependent of mixture composition and efficiency of combustion in RFZ:

where H_u is the lowest calorific value of fuel; C_p is the specific heat capacity of combustion products; L₀ is stoichiometric mixture ratio.

If we unite all constants into one coefficient C and separate the variables, we shall obtain the following expression:

where the mixture composition in the reverse-flow zone is determined from the portion of fuel evaporated:

a _{RFZ n} = (a _RFZ)/ (Z_RFZ).

Here Z_RFZ is the degree of fuel evaporation in RFZ.

In order to estimate the limits of flame-out in the combustion chambers, the working expression can be conveniently written as the functional relation:

Here G_pz is the air flowrate through the primary zone; V_pz is the primary zone volume, and Z_RFZ , h _RFZare the degree of fuel evaporation and combustion efficiency in RFZ respectively.

The herein-presented calculations show that the given approach to simulation of flame stabilization process makes it possible to generalize the experimental data with high degree of convergence.

B.G.Mingazov, Head of Department, Ph.D., KSTU-KAI; scientific interests area is the combustion theory, the modelling processes in combustion chambers of GTD.

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