Theoretical Study of the Coagulation of Ash Particles in Waste Gases during the Vibrating Combustion of Solid Biofuel
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 5, September, 2020
THEORETICAL STUDY OF THE COAGULATION OF ASH PARTICLES IN WASTE GASES DURING THE VIBRATING COMBUSTION OF SOLID BIOFUEL O. G. Stonik, V. D. Geshele, and S. A. Kovalev
UDC 536.46
Consideration has been given to the influence of acoustic oscillations on the process of coagulation of soot particles suspended in the flow of combustion products of wood and peat. It has been shown that at the characteristic parameters of vibrating combustion of these materials, the main factor of growth of the soot particles in the process of their coagulation is the deposition of small particles well entrained by the gas on large ones that are streamlined by the gas but are poorly entrained by it. Keywords: vibrating combustion, coagulation, environment. Introduction. In accordance with the plans of investigating the action of acoustic oscillations on the efficiency of combustion of a fuel, the authors have studied the possibility of reducing harmful ash emissions due to the growth of ash particles in a sound wave during the self-oscillating excitation of the combustor. This problem is critical when a radionuclidecontaminated solid fuel is burned. Growth of particles carrying harmful substances improves the possibility of filtering them. Theoretical Study of the Coagulation of Ash Particles in Waste Gases. Let us consider forces acting on a particle in a gas flow. First, there is the buoyancy force [1]
fA =
d νg πδ3 , ρg 6 dt
(1)
and second, the force of viscous resistance to the motion of the particle from the medium
f ν = 3νρgW πδ .
(2)
But under the conditions in question, the viscous resistance force is much greater than the buoyancy force for inertial particles (but not for bubbles). Indeed, in the case of harmonic perturbation of the gas flow, the condition fν >> fA is equivalent to the condition
ν
δ2 ω. 18
(3)
This inequality holds true even for particles several hundred microns in size on exposure to sound with a frequency of several hundred hertz, which is characteristic of self-excited devices. The particle in the gas flow is also acted upon by the Basset force [2], which is associated with the inertia of equalization of the pattern of flow past the particles. This force is determined by the rate of establishment of a boundary layer on the particle. For small particles the indicated rate is high and much higher than the rate of relaxation of the particle under the action of the medium′s viscous resistance [1]. The particle motion in the gas flow of a self-excited burner device is determined by both the standing sound wave and the average flow velocity, with the velocity of motion of the gas in the sound wave being much smaller than the velocity of its flow. The latter circumstance fundamentally differentiates the conditions in question from the conditions of fulfillment of the existing works [3, 4]. The velocity of motion of the particle in the gas flow is made up of the average flow velocity V0 and the velocity up determined by the
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