RU2137050C1 - Method of heating liquids - Google Patents

Abstract

FIELD: storage water heaters and other heat exchange apparatus for burning gaseous fuel. SUBSTANCE: heat is transferred from combustion products to combustible mixture in tube-burner located inside heat exchanger filled with liquid being heated. Combustible mixture is fed to tube-burner from beneath at constant flow rate ensuring rate of spread of flame found from the following formula:

, where d is transversal size of tube-burner; T₀ and T are initial temperature of combustible mixture and combustion temperature, respectively. Flow of combustible mixture at constant rate ensuring spread of flame at rate "U" which is distinctive feature for free convective vorticity increases heat transfer coefficient from combustion products. EFFECT: enhanced efficiency of heat transfer. 2 dwg

Description

 The invention relates to the field of power engineering, and more specifically to a device for capacitive water heaters and other installations for heating liquids in heat exchange elements when burning gaseous fuels in domestic and industrial boilers, furnaces, furnaces.

 In well-known schemes for heating liquids in water heaters during the combustion of gaseous fuels, heat transfer is achieved through convection, diffusion, and optical radiation from the torch. An increase in the heat transfer coefficient in such water heaters is possible due to an increase in the flow rate of the combustible mixture, turbulization of the gas flow, placement of nozzles and various obstructions in the flow path of the combustion products (see, for example, the book by V.P. Mikheev and Yu.P. Mednikov The burning of natural gas. L .: Nedra, 1975).

A well-known scheme for heating liquids in DHW tanks while burning gaseous fuels (see book: Ragozin AS, Gas, Liquid, and Solid Fuel Home Appliances. L .: Nedra, 1982, p. 131), selected as a prototype, contains a cylindrical tank with a flame tube passing along its vertical axis, in which a spiral flow extension is installed. Heat transfer from a burner installed under the tank is carried out through the bottom and walls of the flame tube. In such water heaters, burners with vertically directed fire holes (or disk burners with peripherally placed fire holes) are installed. Gas burners are located at a certain distance from the bottom and the flame tube. An increase in the flow rate of the combustible mixture v on the burner and an increase in the diameter d of the burner causes an increase in the coolant flow rate in proportion to the Reynolds number of the flow Re = vd / ν, where ν is the kinematic viscosity of the gas. The increase in the heat transfer coefficient is proportional to (Re) ^1/2 (see, for example, the book of B. N. Yudaev. Technical thermodynamics. Heat transfer. M: Higher school, 1988).

Turbulization of the gas supply stream at Re> 2300, vortex formation at gas outflow in intersecting directions, placement of nozzles and various obstacles in the flow path, contributing to the turbulization of the combustion process, increase the thermal stress in the furnace device and, accordingly, the heat transfer, but this increases the fuel consumption gas, causes incompleteness of combustion, thereby reducing the efficiency of the device. For example, for heating a room with an area of 60 m ² for 6 months using an AGV-80 domestic DHW cylinder, 470 m ^{3 of} natural gas is required (Ragozin A. S. Household appliances using gas, liquid and solid fuels. L .: Nedra, 1982) .

 A further increase in the heat transfer coefficient in water heaters (heating devices) during the combustion of gaseous fuels, and, accordingly, an increase in the efficiency of the device can be achieved only by changing the combustion and heat transfer process itself.

 The aim of the present invention is to increase the coefficient of heat transfer from the combustion products to the heat exchange element and a corresponding increase in the efficiency of the device during the laminar flow of combustible gas.

 This goal is achieved due to the free convective vortex formation in the combustion products during the laminar flow of the combustible mixture from the bottom up into the burner pipe installed inside the heat exchanger, with a constant flow rate that ensures the flame propagation velocity u, which is characteristic of the free convective vortex formation mode in the combustion products.

где d - поперечный размер трубы-горелки, T_o и T - начальная температура горючей смеси и температура горения.

where d is the transverse dimension of the burner pipe, T _o and T are the initial temperature of the combustible mixture and the combustion temperature.

 The inventive method satisfies the criteria of novelty and materiality of the difference from the known technical solutions, since there is no information in the scientific literature that predicts this effect using this feature (free-convective vortex formation in the combustion products when the combustible mixture flows from the bottom up into the burner tube of the heat exchanger with a certain constant flow rate, ensuring the fulfillment of the above condition (I)), and the application of a similar solution in other areas of technology is unknown to the applicant.

 In FIG. 1 shows a General view of the proposed method for heating liquids; in FIG. Figure 2 shows a typical picture of vortex formation in combustion products.

 Combustible gas is supplied from the mixer 1 from the bottom upwards through the stabilizer 2 to the burner pipe 3 installed inside the tank with the heated medium 4. The combustible gas is ignited by the ignition burner 5 installed inside the burner pipe 3. At certain speeds of the combustible gas near the mouth of the burner pipe 3 or rather, near the stabilizer 2, a flame is installed. The flame stabilizer 2 prevents the leakage of flame into the mixer 1. The combustion products under certain conditions form a hydrodynamic field with separation of the boundary layer on the walls of the pipe-burner 3, leading to the formation of a vortex. Gradually, the vortex is carried by the flow to the open end of the burner pipe 3 and decays. After a while, a similar vortex arises on the opposite wall of the burner pipe near stabilizer 2. This forms a sequence of vortices that appear on opposite walls of the burner pipe and rotate alternately to the right and left: down near the pipe wall and up on the pipe axis. When the flame velocity is reduced to the limiting size, the vortices increase to the width (diameter) of the burner pipe channel and the flow behind the flame front becomes chaotic.

The results of the studies indicate the free-convective nature of the vortex formation mechanism in combustion products. In FIG. 2-a) shows a typical photograph of the process of separation of the boundary layer and the subsequent formation of a vortex near the wall of a square tube burner (d = 2.7 cm), obtained using a polarization shear interferometer assembled on the basis of the IAB-451 shadow device. Photographic registration of the process was carried out by a SKM-1M high-speed movie camera. In FIG. 2-b) a picture of the separation of the boundary layer and the formation of a vortex is obtained, obtained by numerical solution on a computer. The lines in the figure of FIG. 2-b) correspond to different temperatures in the field of combustion products (temperature values in Fig. 2-b) are given in relative units T / T _o ).

 During the experiments, the temperature was measured with a thermocouple embedded in the side wall so that its junction with a diameter of 50 μm protruded above the wall plane at a distance of 0.5 mm. The results of computer calculations and thermocouple measurements showed that with free-convective vortex formation, the temperature on the wall differs little from the temperature on the pipe axis equal to the combustion temperature. In the absence of vortex formation, the temperature near the wall is 4 times lower; accordingly, the heat transfer from the combustion products to the heated liquid also decreases.

The necessary and sufficient conditions for convective vortex formation near the flame front are as follows: the convection induction period must be sufficiently short, and the flow separation condition on the wall must be satisfied in the vortex nucleation region. To evaluate the first of the conditions, we introduce the parameter σ = τ / t, where τ is the characteristic propagation time of the flame, t is the convection induction period. At Pr≈ 1, the convection induction period is t = 70d ² / αRa ^2/3 , where d is the pipe cross-sectional size, α is the thermal diffusivity of the combustion products, Ra = PrGr is the Rayleigh number, Gr is the Grashof number, Pr is the Prandtl number ( see article Merzhanov A.G., Shtessel EA.Appearance and development of thermal convection in a layer of viscous fluid.- DAN USSR, 1970, v. 191, No. 4). Since τ = d / u, where u is the flame propagation velocity, then σ = 70Pe / Ra ^2/3 , where Pe = ud / a is the Peclet number. For σ> 1, the necessary vortex formation condition near the flame front is satisfied. This relation makes it possible to estimate the value of the flame propagation velocity at which the necessary condition of vortex formation is satisfied. For example, for a pipe with a diameter of 2.7 cm, the flame propagation velocity is u ≤ 10 cm / s.

и учитывая, что скорость вытекания продуктов горения из трубы вследствие теплового расширения ν_- = u_o(T/T_o-1), можно оценить скорость распространения пламени, при которой должны наблюдаться отрыв пограничного слоя и образование вихря. Следует отметить, что данные соотношения справедливы в общем случае только для плоских пламен. Однако и в данном случае со слабоискривленными пламенами эта формула дает значения скоростей, достаточно близкие к реальным. Так как нормальная скорость u_o = uS_o/S, где S_o и S - площадь поперечного сечения трубы и площадь поверхности фронта пламени, а для слабоискривленных пламен S_o ≃ S, то u_o=u и из условия ν₊ = ν_- можно получить соотношение для скорости распространения пламени:

Для трубы с поперечным размером d = 2,7 см оценка скорости распространения пламени по этому соотношению дает величину 5 см/с, что соответствует реальным значениям.A sufficient vortex formation condition is equivalent to the vanishing of the velocity gradient in the y direction, perpendicular to the wall at the separation point, i.e. (∂v / ∂y) = 0. In a vertical pipe, this condition can be realized only when the flame propagates from top to bottom, when the outflow velocity of expanding combustion products is compensated by the equal in return velocity of the descending combustion products cooled on the wall. Using the formula for the speed of free convection movement of combustion products

and taking into account that the rate of flow of combustion products from the pipe due to thermal expansion is ν _- = u _o (T / T _o -1), we can estimate the flame propagation velocity at which separation of the boundary layer and the formation of a vortex should be observed. It should be noted that these relations are valid in the general case only for plane flames. However, in this case with slightly curved flames, this formula gives values of velocities that are quite close to real ones. Since the normal velocity is u _o = uS _o / S, where S _o and S are the cross-sectional area of the pipe and the surface area of the flame front, and for weakly curved flames S _o ≃ S, then u _o = u and from the condition ν ₊ = ν _- you can get the ratio for the speed of flame propagation:

For a pipe with a transverse size d = 2.7 cm, an estimate of the flame propagation velocity from this ratio gives a value of 5 cm / s, which corresponds to real values.

 The use of the invention will make it possible to create the most energy-efficient compact heating devices with high efficiency, which is especially important, for example, when developing domestic water heating devices.

Claims (1)

A method of heating liquids by transferring heat from the products of combustion of a combustible mixture supplied from the bottom up to a burner pipe located inside a heat exchanger with a heated liquid, characterized in that the combustible mixture is fed into a burner pipe with a constant flow rate, providing a flame propagation speed u, determined by the formula

where d is the transverse dimension of the burner pipe, T _o and T are the initial temperature of the combustible mixture and the combustion temperature.

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