On the problem of decreasing self-oscillations of vibrating combustion

Abstract

It has been shown that the self-oscillation amplitude can be decreased by changing the parameters weakening its mechanisms, which can be used for controlling this nonstationary phenomenon.Introduction. The numerous experimental and theoretical studies aimed at suppressing self-excited oscillations in the combustion chambers of liquid-fuel rocket engines (LR) [1][2][3] and in the stoves of blast furnaces [4], steam boilers, and other unit heaters [5, 6] make it possible to develop a number of methods for decreasing the amplitude of the above oscillations and ensure stability of the steady-state operation of the above-mentioned devices [7][8]. In LRs, the self-oscillation amplitude of vibrating combustion [2-3] is decreased by introducing into the combustion chamber anti-impulse partitions and by dynamic damping with the help of a Helmholtz resonator.It has been established experimentally that self-oscillations do not arise in the vertical combustion chamber of the stove of a large cross-section blast furnace under small thermal loads [4]. Moreover, it has been found in experiments that the above chamber produces a stabilizing effect on the flow of combustion products, which shows up as "lengthiness" of combustion. As a result, in each of the chamber cross-sections in which fuel combustion takes place a decrease in the rate of temperature increase occurs. However, there is no theoretical justification of the effects responsible for the decrease in the amplitude of self-oscillations or their complete suppression.A distinguishing feature of vertical combustion chambers of the stoves of blast furnaces and hot-water boilers is their fairly large volume determining their acoustics flexibility C a = V ⁄ c 2 . Therefore, the wave resistance of the above chambers Z = √ ⎯⎯⎯⎯⎯ ⎯ L a ⁄ C a is comparatively small. Because of their small volume, the combustion chambers of LRs feature a rather high wave resistance. Therefore, in such chambers of various designs both harmonic and relaxation self-excited oscillations take place.In conducting full-scale tests of the stoves of blast furnaces [4], it was noticed that the same measures carried out in different combustion chambers can cause different, sometimes contradicting, effects. This is due to the fact that self-oscillations of vibrating combustion arising, as is known, from the retardation of fuel combustion τ (mechanism proposed by L. Crocco) are also excited as a result of heat supply [8].Convective heat supply in the lower segment of the Rijke tube causes through motion of air. In the case where such motion takes place under laminar-flow conditions, along the length of the tube negative viscous and thermal resistances arise [7,9]. Negative resistances that are due to the fuel combustion heat [10] generate an ascending (unstable) branch on the pressure characteristic F(G) of the combustion chamber, which is a new, previously unknown property of vibrating combustion determined by the heat supply [9]. Self-oscillations arising in the oscillatory circuit of a combustion chamber w...