RU2767237C1 - Method for organizing diffusion combustion of a microjet of gaseous fuel - Google Patents

Abstract

FIELD: thermal power engineering.

SUBSTANCE: invention relates to the field of thermal power engineering and can be used in the chemical industry and metallurgy to increase the degree of mixing during diffusion combustion of gaseous fuel in an impact microjet with a subsonic flow rate of a laminar microjet from a nozzle. A method for organizing diffusion combustion of a microjet of gaseous fuel includes a laminar outflow of a microjet of gaseous fuel at a subsonic speed from a nozzle, igniting the microjet and impacting the torch with an external acoustic source. The acoustic effect is carried out on a raised torch that is not in contact with the nozzle, and the further process of diffusion combustion of a jet of gaseous fuel is carried out in an impact microjet flowing onto an obstacle located from the nozzle exit at a distance of the torch height or within the existence of the torch.

EFFECT: invention allows to provide a stable mixing zone of a microjet of gaseous fuel with air and the completeness of its combustion.

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Description

The invention relates to the field of thermal power engineering and can be used in the chemical industry and metallurgy to increase the degree of mixing during diffusion combustion of gaseous fuel in an impact microjet with a subsonic laminar microjet outflow velocity from a nozzle.

Impact jets are used in various industries for both cooling and heating surfaces. The wide distribution of impact jets is primarily due to the constructive simplicity of the solution and, at the same time, significant efficiency in heat transfer processes.

From the field of combustion of gaseous fuels, an invention is known in which a method and a burner are claimed to improve the efficiency of fuel combustion by improving the quality of mixing fuel with air (patent RU 2152559, F23D 14/22, 1996). Air is supplied to a certain small volume in which the gas collector is located. Through holes made in a certain way, the gas enters the same volume. As a result, an optimal composition and quality mixture of gas and air is formed, which ignites and continues to burn out beyond the specified volume.

The disadvantages of such a burner include: significant technological costs due to an increase in the number of combustion zones, the fragility of the burner due to the presence of the combustion process in the internal cavity of the burner and insufficient completeness of fuel combustion.

There is a known method for stabilizing the diffusion combustion of hydrogen in a gas micro-burner (options), described in patent RU 2677322, which includes the generation of a microjet of hydrogen in the conical nozzle of the burner with a subsonic flow rate, a hydrogen jet is generated in the nozzle, then the jet is ignited directly at the nozzle exit, thereby the most stable zone of hydrogen jet mixing with air and combustion in the form of an attached torch. The nozzle exit is temporarily heated, providing stable subsonic laminar combustion, both at the nozzle exit and along the entire length of the working area of the torch.

In the described microburner, an extended flame is formed during laminar diffusion combustion of hydrogen, which implies the presence of a corresponding area for combustion. In the case of an impact jet, the parameters of diffusion combustion deteriorate significantly. In this case, it should be noted that laminar diffusion combustion in this case is feasible at low gas outflow velocities, which will ensure the implementation of an attached torch (the torch is in contact with the nozzle). And attempts to increase the degree of mixing through the action of an external acoustic signal in this case will not lead to success, since this requires the implementation of a raised torch (distanced from the nozzle exit).

An increase in the degree of mixing of hydrocarbon fuels can be provided by burners with the generation of acoustic oscillations in the internal cavity of the nozzle, where acoustic waves propagate through nozzle holes along with steam jets.

There is a known method of combustion of gaseous fuel (patent RU 2262039), which includes a separate supply of fuel and air to the burner, preferential supply of fuel to the central region of the air flow and combustion of fuel on the periphery of the torch in excess air mode, and in the central region of the torch in excess fuel mode , steam is supplied to the central region of the torch and a field of acoustic oscillations is superimposed on this region. EFFECT: invention makes it possible to reduce emissions of nitrogen oxides from power boilers without worsening fuel burnup. However, this method of fuel combustion does not allow reducing emissions of nitrogen oxides without a significant deterioration in combustion and an increase in the formation of other oxides. Also, the disadvantages of the method include large hydraulic losses and, accordingly, a significant fuel supply pressure in front of the nozzle.

An article is known that describes the development of combustion instability in a round propane microjet under the influence of an external acoustic field: Litvinenko Yu.A., Balbutsky A.B., Vikhorev V.V., Kozlov G.V., Litvinenko M.V. Experimental study of the development of hydrodynamic instability in a round propane microjet under the influence of an external acoustic field with and without combustion. // Bulletin of NGU. Series: Physics. 2015. Vol. 10, no. 4. S. 21-28. According to the research results, the development of hydrodynamic instability in a gas microjet, including in the presence of convective forces (during combustion) under external acoustic influence, was established. It has been established that an elevated torch, in contrast to an attached torch, during diffusion combustion of a round microjet under the action of an external acoustic field, is subject to bifurcation (bifurcation) similarly to an air free round microjet.

This article showed the possibility of exposing an acoustic periodic signal to a gas microjet, leading to the occurrence of a microjet bifurcation, and confirmed the possibility of such an effect when an elevated diffuse combustion flame is realized.

For the prototype, the method described in the article: Yu.A. Litvinenko, M.V. Litvinenko, I.D. Zverkov Experimental study of the development of impact round macro- and microjets // Thermophysics and Aeromechanics, 2021, vol. 28, No. 1, pp. 41-56 under the influence of an external acoustic field.

The results of these studies formed the basis for the method of influencing an impact microjet (a jet with a barrier) during its diffusion combustion.

The objective of the invention is to increase the degree of mixing, the creation of stable diffusion combustion of gaseous fuel and the completeness of its combustion.

The problem is solved thanks to the proposed method for organizing diffusion combustion of a microjet of gaseous fuel, which includes the outflow of a laminar microjet of gaseous fuel at a subsonic speed from a nozzle, ignition of the fuel jet, and exposure of the torch to an external acoustic source. According to the invention, the acoustic impact is carried out on a raised torch that is not in contact with the nozzle, and the further process of diffusion combustion of gaseous fuel is carried out in an impact microjet flowing onto an obstacle installed mainly at right angles to the direction of the jet and located from the nozzle exit at a distance of the torch height or in within the existence of the torch, which provides a stable zone of mixing of the microjet of gaseous fuel with air and the completeness of its combustion.

The positive effect is achieved due to the proposed acoustic effect on the raised torch of the impact jet (jet with an obstacle), which greatly increases the efficiency of the process of mixing fuel with air and the completeness of its combustion.

In FIG. 1 shows a diagram of a burner with an impact jet and acoustic impact, where 1 is a prechamber, 2 is a Vitoshinsky nozzle, 3 is a barrier, 4 is a dynamic loudspeaker (resistance - 40 m, diameter - 235 mm, power - 25 W); L is the distance from the nozzle to the barrier;

In FIG. Figure 2 shows the visualization of the process of diffusion combustion of propane/butane mixture in an impact jet without impact (a) and with acoustic impact (b), frequency F=2100 Hz.

The method for organizing diffusion combustion of a microjet of gaseous fuel is presented on the example of an experiment conducted on a stand at the Institute of Theoretical and Applied Mechanics named after V.I. S.A. Khristianovich of the Siberian Branch of the Russian Academy of Sciences (ITAM SB RAS).

The scheme of the experimental stand for studying a jet from a mixture of propane and butane in a ratio of 70 to 30 during diffusion combustion under external acoustic conditions at a low Reynolds number is shown in Fig. 1. The studies were carried out for a jet flowing out of a round micronozzle with a diameter of the nozzle outlet d=0.4 mm.

In the article by Yu.A. Litvinenko, M.V. Litvinenko, I.D. Zverkov Experimental study of the development of impact round macro- and microjets // Thermophysics and Aeromechanics, 2021, vol. 28, No. 1, pp. 41-56 under the influence of an external acoustic field. An impact microjet is understood as a jet flow interacting with a flat barrier located predominantly at a right angle at a certain distance in front of the jet.

It has been found that the development of sinusoidal instability (as a result of acoustic action) leads to an increase in the completeness of propane/butane mixture combustion in an impact microjet and a decrease in soot emission. It has been established that in the absence of acoustic exposure, the spectrum of flame radiation, including the near-wall region of the barrier, is dominated by yellow color (see Fig. 2 (a) - color photograph), which indicates a lack of an oxidizing agent (air) and the presence of soot in combustion products. It is shown that the second factor affecting the completeness of combustion during diffusion combustion in an impact jet is the diameter of the nozzle, d, and the distance from the nozzle to the barrier, L. and analyzed the emission spectrum (see Fig. 2(a) and (b)). The optimal distance to the barrier in this case is 22 - 25 mm or L / d ≈ 55-60, (where d = 0.4 mm is the nozzle diameter), this is evidenced by an increase in temperature in the near-wall layer of the barrier and, accordingly, an increase in the combustion reaction rate (without acoustic excitation t ≈ 700°С, with acoustic excitation t ≈ 800°С). As the completeness of combustion increases, the flame acquires a blue color (see Fig. 2 (b) - photograph in color). The gas outflow velocity varied from 8 m/s to 15 m/s. Within these limits, the mode of gas outflow from the nozzle remains laminar, the condition of the “raised” torch is satisfied, there is a possibility for excitation of the sinusoidal mode of instability, which ensures a stable zone of mixing of the microjet of gaseous fuel with air and the completeness of its combustion.

The method of organizing diffusion combustion of a microjet of gaseous fuel is carried out as follows:

The gas outflow velocity of 8 m/s and higher corresponded to the implementation of the "raised" torch. A "raised" torch is usually called a flame, the front of which does not come into contact with the nozzle. To influence the jet, a dynamic loudspeaker was used, to which a sinusoidal signal with a frequency F = 2.1 kHz and amplitude A = 10 V was applied, while the maximum values of the noise level in the area of jet development corresponded to 80–90 dB. The frequency of acoustic oscillations was chosen from the range of natural frequencies of instability of the gaseous fuel microjet. When these conditions are met, a sinusoidal instability develops in the jet. In FIG. Figure 2 shows a visualization of the process of diffusion combustion of a propane/butane mixture in an impact jet. In the zy plane, one can observe the deformation of the flame in the form of an "umbrella" in the near-wall region of the barrier without acoustic impact (Fig. 2a) and with acoustic impact (Fig. 2b). The diameter of the near-wall combustion zone was about 50-60 mm. In the presented photograph of combustion, a change in the color of the flame can be noted, in Fig. 2(a) yellow color prevails in the emission spectrum, this color of the diffusion flame is typical for rich mixtures with incomplete combustion of the fuel and the formation of soot in the combustion products. In FIG. 2(b) the emission spectrum of the torch is dominated by blue color, which indicates an increase in the oxidizer/fuel ratio (good mixing). During the development of hydrodynamic instability, an additional part of the oxidizer is involved in the combustion process, the temperature of the near-wall layer increases and, as a result, the combustion efficiency increases with a decrease in the soot content in the combustion products.

The positive effect is achieved due to the proposed acoustic effect on the raised torch of the impact jet (jet with an obstacle), which greatly increases the efficiency of the process of mixing fuel with air and the completeness of its combustion.

Sources of information

1. Patent RU2152559, IPC F23D 14/22, 1996

2. Patent RU2677322, IPC F23D 14/20.28.11.2017

3. Latent RU2262039, IPC F23B11/34, 04.12.2003

4. Litvinenko Yu.A., Balbutsky A.B., Vikhorev V.V., Kozlov G.V., Litvinenko M.V. Experimental study of the development of hydrodynamic instability in a round propane microjet under the influence of an external acoustic field with and without combustion. Vestnik NGU. Series: Physics. 2015. Vol. 10, no. 4. S. 21-28.

5. Yu.A. Litvinenko, M.V. Litvinenko, I.D. Zverkov Experimental study of the development of impact round macro- and microjets // Thermophysics and Aeromechanics, 2021, vol. 28, no. 1, pp. 41-56.

Claims (1)

A method for organizing diffusion combustion of a microjet of gaseous fuel, including a laminar outflow of a microjet of gaseous fuel at a subsonic speed from a nozzle, ignition of the microjet and exposure of the torch to an external acoustic source, characterized in that the acoustic effect is carried out on a raised torch that is not in contact with the nozzle, and the further diffusion process combustion of a jet of gaseous fuel is carried out in an impact microjet flowing onto an obstacle located from the nozzle exit at a distance of the height of the torch or within the existence of the torch, which ensures a stable zone of mixing of the microjet of gaseous fuel with air and the completeness of its combustion.

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