Elliptical tube for strengthening condensation heat exchange

Abstract

The invention discloses an elliptical tube for strengthening condensation heat exchange, which is characterized in that the two ends of an elliptical condensing tube are connected with circular tubes, and a plurality of annular grooves which are uniformly distributed and are vertical to the axis are formed in the elliptical condensing tube along the axial direction. The distance between the annular grooves is equal to the Taylor wavelength, and the surfaces of the protrusions between the adjacent grooves are paraboloids. And therefore, a gas-liquid interface shape of Rayleigh-Taylor waves is formed at the lower edge of the elliptical tube. And under the Rayleigh-Taylor instability effect, the condensed liquid quickly leaves the wave trough, gathers towards the wave crest and then drips, so that the purposes of quickly discharging the liquid and thinning the liquid film are achieved, and the condensation heat exchange coefficient is increased.

Description

[0001] This application is a divisional application of the original application titled "An Elliptical Tube for Enhancing Condensation Heat Transfer" filed with the Chinese Patent Office on December 22, 2020, with the application number 202011528305.1. The full content of the original application is incorporated herein by reference. Technical Field

[0002] The present invention relates to an elliptical tube for enhancing condensation heat transfer, belonging to the field of manufacturing heat exchange tubes for heat exchangers, and is particularly suitable for tubes used in condensers. Background Art

[0003] The working principle of a condenser is that steam exchanges heat with cold water inside the heat exchange tube, condenses on the surface of the heat exchange tube, and forms a liquid film. Since the liquid film prevents the direct contact between the steam and the surface of the heat exchange tube, the steam can only exchange heat with the surface of the liquid film. The thicker the liquid film, the higher the temperature of the liquid film surface, the smaller the heat exchange temperature difference, and the lower the heat transfer coefficient. The liquid film thermal resistance becomes a relatively large thermal resistance for condensation heat transfer. Therefore, reducing the thickness of the liquid film is an effective way to improve the condensation heat transfer coefficient. The elliptical tube is a heat exchange tube used inside a condenser. Because under the condition of equal internal cross-sectional area of the tube, the outer surface area of the elliptical tube is larger than that of the circular tube. Therefore, under the same external condensation heat transfer conditions, the liquid film thickness is thinner than that of the circular tube, so the heat exchange efficiency of the elliptical tube is higher than that of the circular tube. However, the liquid film distribution on the surface of the elliptical tube is uneven. Under the action of the viscous force and surface tension of the liquid, the liquid film often stays at the lower edge of the elliptical tube and is not easy to fall off, forming a submerged area, and its liquid film thickness is greater than that of the upper surface, resulting in a sharp drop in the heat transfer coefficient of the lower half of the elliptical tube. Chinese Patent CN201837276U discloses an elliptical twisted tube coaxial heat exchanger, with the outer tube being a circular tube and the inner tube being a single elliptical twisted tube. The elliptical twisted tube is an elliptical tube with a spiral twisted channel structure, and the azimuth of the major axis of the ellipse changes periodically, causing violent disturbance of the fluid inside the tube and in the casing gap, and improving the heat transfer coefficients inside and outside the tube. However, this type of elliptical tube heat exchanger does not involve condensation phase change heat transfer. Chinese Patent CN109059604A proposes an unstable wave enhanced condensation tube based on hydrophilic-hydrophobic intervals. The inner tube is a smooth circular tube, and the wall thickness of the outer tube fluctuates periodically according to the Plateau-Rayleigh critical unstable wavelength, forming an outer surface heat exchange tube with a sine wave shape. The two tubes are sleeved to form a composite tube. In addition, the wall surface at the wave trough is treated with hydrophobicity using vinyltriethoxysilane, and the wave peak remains wet and hydrophilic, showing a hydrophilic-hydrophobic interval distribution. After the liquid droplets condense, they tend to leave the hydrophobic area, ensuring that this area is always in a state with fewer liquid droplets and maintaining a high condensation heat transfer coefficient. The disadvantages of this technology are that the technical solution of using two layers of sleeves significantly increases the wall thickness and the additional contact thermal resistance between the two layers of tube walls, reducing the heat transfer coefficient; in addition, this technology requires hydrophilic / hydrophobic treatment, with complex processes and high costs. Summary of the Invention

[0004] The object of the present invention is to overcome the deficiencies of the above-mentioned existing technologies and provide an elliptical tube for enhancing condensation heat transfer.

[0005] The technical solution provided by the present invention is as follows: An elliptical tube for enhancing condensation heat transfer, characterized in that it includes a circular tube and an elliptical condensation tube. The two ends of the elliptical condensation tube are connected to the circular tube, and a plurality of annular grooves are arranged axially on the elliptical condensation tube, which are evenly distributed and perpendicular to the axis.

[0006] Further, the cross-section of the annular groove is semi-circular, with an upper opening width of 0.5 - 2.0 mm and a groove depth of 0.5 - 2.5 mm.

[0007] Further, the distance λ between the center lines of adjacent annular grooves is calculated according to the following formula:

[0008]

[0009] Where:

[0010] λ - groove spacing, m;

[0011] σ - surface tension, N / m;

[0012] g - acceleration due to gravity, m / s 2 ;

[0013] ρ 1 —liquid density, kg / m 3 ;

[0014] ρ g —vapor density, kg / m 3 ;

[0015] a - coefficient, dimensionless; its value can be calculated according to the following formula:

[0016] a = 548.2×d - 5.41;

[0017] Where: d - dimensionless pipe diameter, m / m, that is, the ratio of the equivalent circular pipe diameter of the elliptical pipe to the unit length.

[0018] Further, the surface of the elliptical condensation tube between the circular tube and the annular grooves and between adjacent annular grooves is a parabolic surface protrusion, and the parabolic shape can be calculated by the following formula:

[0019]

[0020] Where:

[0021] y - coordinate value of the parabolic surface protrusion, m;

[0022] x - coordinate value along the axis of the elliptical tube, 0 ≤ X ≤ λ, m;

[0023] λ—the distance between the grooves (Taylor critical wavelength calculated above), m;

[0024] h—the maximum height value of the parabola, m, and its value is: h = λ / d;

[0025] d—the ratio of the equivalent diameter of the elliptical tube to the unit length, dimensionless.

[0026] Furthermore, the length of the circular tube is 20 - 100 cm.

[0027] In the present invention, semicircular grooves perpendicular to the axis are arranged along the axis of the heat exchange tube, and the convex surface between adjacent grooves is a parabolic surface. The spacing of the annular grooves along the lower edge of the elliptical tube and the surface shape between the annular grooves are respectively equal to the vapor-liquid interface wavelength and the shape of the vapor-liquid interface in the Taylor theory, that is, the annular grooves are at the trough position, and the wave crest is at the vertex position of the parabolic surface between adjacent grooves. Thus, the lower edge of the elliptical heat exchange tube constitutes the wave surface shape of the Taylor vapor-liquid interface wave.

[0028] The principle and function of the present invention are as follows: For the condensation heat transfer process, the thickness of the liquid film is the key factor determining the heat transfer performance. Accelerating the liquid drainage speed is a method to reduce the thickness of the liquid film. According to the analysis of the Rayleigh-Taylor instability theory in fluid mechanics: In the case of two superimposed fluids with different densities, when the fluid with a higher density is on top and the fluid with a lower density is below, this state is an unstable state with high potential energy and will develop towards a stable state with low potential energy. The Rayleigh-Taylor instability has several development stages: the linear development stage, the weak linear development stage, and the non-linear development stage. Correspondingly, the gas-liquid interface shape has experienced sine and cosine shapes, the shape of the superposition of cosines with different wave numbers, and finally forms a parabolic shape with the vertex of the liquid film below, and the liquid drips from the vertex. At this time, the interface forms continuous corrugations, and the distance between each corrugation is the Rayleigh-Taylor critical wavelength length, and the gas-liquid interface shape within its critical wavelength is a parabolic shape. As a result, the liquid film at the trough of each corrugation is thinned, and the liquid quickly gathers at the peak. As the amount of liquid gathering at the peak increases, under the action of gravity, the liquid overcomes the action of viscosity and surface tension and drips downward; as the liquid droplets fall off, the interface between the liquid attached to the lower edge of the elliptical tube and the steam quickly returns to a horizontal state. Under the action of instability, the interface tends to form a wavy interface composed of multiple parabolic surfaces again, that is, the liquid at the trough gathers at the peak again, and the liquid at the peak drips. This cycle repeats to achieve the dripping of the liquid on the surface of the heat exchange tube. Obviously, the liquid on the lower surface of the elliptical heat exchange tube needs to go through several development stages from a horizontal state to the parabolic shape of the Rayleigh-Taylor interface wave. In order to shorten the formation time of the Rayleigh-Taylor interface wave, accelerate the liquid drainage speed, and make full use of the action of the Rayleigh-Taylor instability, the surface of the heat exchange tube is designed into the shape of the Taylor interface wave. The specific method: On the one hand, according to the characteristic that the distance between adjacent waves is equal to the Rayleigh-Taylor wavelength, annular grooves are arranged at both ends of the critical wavelength to thin the liquid film at that place and form troughs. On the other hand, according to the characteristic that the Taylor wave interface is parabolic, the surface is designed into a parabolic shape. Thus, once a liquid film appears on the surface of the heat exchange tube, a parabolic interface of the Rayleigh-Taylor wave will be quickly formed, promoting the condensed liquid to quickly gather at the peak and drip downward under the action of gravity. Eventually, the thickness of the liquid film on the surface of the heat exchange tube is reduced, and the condensation heat transfer coefficient is enhanced. The calculation formula for the Rayleigh-Taylor critical wavelength of a classic circular tube is as follows:

[0029]

[0030] Wherein,

[0031] This formula is applicable to the condition of circular tubes. In the case of elliptical tubes, the coefficient needs to be re-determined. Through multiple visualization experiments, it is found that the Taylor wavelength is related to the equivalent circular tube diameter of the elliptical tube. (Experimental conditions: working medium is water, condensation temperature is 40 °C). The experimental results are as follows:

[0032] Table 1 Relationship between Taylor wavelength and equivalent diameter of elliptical tube

[0033] Equivalent diameter of elliptical tube / m Taylor wavelength / m 0.016 0.009 0.019 0.013 0.025 O.022

[0034] Therefore, the coefficient a in the calculation formula of the Taylor wavelength under the condition of elliptical tubes is corrected, and it is mathematically fitted according to the experimental results. The obtained formula is as follows:

[0035] a = 548.2 × d - 5.41;

[0036] According to theoretical calculations, the Taylor wave interface is a parabola composed of multiple high-order harmonics. Therefore, the surface between the grooves of the elliptical heat exchange tube is processed into a parabolic shape. Through calculation and considering the processing conditions, the equation of its shape is:

[0037]

[0038] The beneficial effects of the present invention are:

[0039] The present invention overcomes the problem that the condensed liquid is not easily separated from the tube body under the action of surface tension and viscosity, and stays at the bottom of the heat exchange tube to form a large submerged area, increasing the liquid film thickness and reducing the condensation heat transfer coefficient of the elliptical tube.

[0040] According to the Rayleigh-Taylor instability theory, the annular grooves and parabolic surfaces provided on the elliptical tube can accelerate the gathering of the condensed liquid at the vertex of the parabolic surface at the bottom of the elliptical tube. Thereby, the dripping speed of the liquid droplets is increased, the liquid film thickness is reduced, and the purpose of improving the condensation heat transfer coefficient is achieved.

[0041] To better understand the role of Taylor instability, Figure 3 shows a schematic diagram of the Taylor wavelength and the shape of the vapor-liquid interface of a classical circular condensation tube. Where 4 - condensation circular tube; 5 - vapor-liquid interface; 6 - Taylor wavelength; 7 - liquid droplet. Brief description of the drawings

[0042] Figure 1 is a schematic structural diagram of the present invention;

[0043] Figure 2 is Figure 1 the A-A cross-sectional view of;

[0044] Figure 3It is a schematic diagram of the Taylor wavelength of a classical circular condenser tube and the shape of the vapor-liquid interface. Detailed implementation mode

[0045] The following is a detailed description of the specific implementation mode of the present invention in conjunction with the accompanying drawings:

[0046] As Figure 1 - Figure 2 shown, an elliptical tube for enhancing condensation heat transfer. The elliptical tube can be made of copper and copper alloys, carbon steel, stainless steel and their alloy materials. It includes a circular tube 3 and an elliptical condenser tube 1. The two ends of the elliptical condenser tube 1 are connected to the circular tube 3 for easy connection with the end plate, and its length is 20-100 cm.

[0047] A number of annular grooves 2 are arranged axially on the elliptical condenser tube 1 and are evenly distributed and perpendicular to the axis; the cross-section of the annular groove 2 is semi-circular, the upper opening width is 0.5-2.0 mm, and the groove depth is 0.5-2.5 mm. The distance λ (Taylor critical wavelength) between the center lines of adjacent annular grooves 2 is calculated according to the following formula:

[0048]

[0049] Where:

[0050] λ - groove spacing, m;

[0051] σ - surface tension, N / m;

[0052] g - acceleration due to gravity, m / s 2 ;

[0053] ρ 1 — liquid density, kg / m 3 ;

[0054] ρ g — gas density, kg / m 3 ;

[0055] a - coefficient, dimensionless; its value can be calculated according to the following formula:

[0056] a = 548.2 × d - 5.41;

[0057] Where: d - dimensionless pipe diameter, m / m, that is, the ratio of the equivalent circular pipe diameter of the elliptical pipe to the unit length.

[0058] The surface of the elliptical condenser tube 1 between the circular tube 3 and the annular groove 2 and between adjacent annular grooves 2 is a parabolic surface bulge, and the parabolic shape can be calculated by the following formula:

[0059]

[0060] Where:

[0061] y—the coordinate value of the convexity of the parabolic surface, m;

[0062] x—the coordinate value along the axis of the elliptical tube, 0 ≤ X ≤ λ, m;

[0063] λ—the spacing between the grooves (the Taylor critical wavelength calculated above), m;

[0064] h—the maximum height value of the parabola, m, and its value is: h = λ / d;

[0065] d—the ratio of the equivalent diameter of the elliptical tube to the unit length, dimensionless.

[0066] Using a circular tube 3 with d = 16 mm, an annular groove perpendicular to the axis is set. The water vapor outside the tube is condensed, and the condensation temperature is 40°C. The density of water vapor ρ g = 0.051 kg / m3, the density of water ρ 1 = 992.18 kg / m3, the surface tension σ = 0.069 N / m, the dimensionless coefficient a = 548.2×0.016 - 5.41 = 3.36, and the acceleration due to gravity g = 9.80 m / s 2 , according to the formula Calculated, the distance between adjacent annular grooves is λ = 8.95 mm. Calculated by the profile formula h = λ / d, the vertex height is h = 0.56 mm. The mold is processed according to the described shape, and it is processed into the elliptical tube of the described shape by the method of cooperating with the mold by a hydraulic press. In the actual working condition, cold water passes through the elliptical tube, and the water vapor outside the tube is condensed, and the condensation temperature is 40°C. The condensation heat transfer coefficient of the elliptical tube of the present invention is increased by about 12% compared with that of the conventional elliptical tube.

[0067] It should be understood that the parts not elaborated in detail in this specification all belong to the prior art. The above embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solution of the present invention should all fall within the protection scope determined by the claims of the present invention.

Claims

1. An elliptical tube for enhancing condensation heat transfer, characterized in that, it includes a circular tube (3) and an elliptical condensation tube (1). The two ends of the elliptical condensation tube (1) are connected to the circular tube (3). A number of annular grooves (2) are arranged along the axial direction of the elliptical condensation tube (1) and are evenly distributed and perpendicular to the axis. The surfaces of the elliptical condensation tube (1) between the circular tube (3) and the annular grooves (2) and between adjacent annular grooves (2) are parabolic surface protrusions. The shape of the parabola is calculated by the following formula: where: λ—the distance between the center lines of adjacent annular grooves, m; y—the coordinate value of the parabolic surface protrusion, m; x—the coordinate value along the axis of the elliptical tube, 0≤X≤λ, m; h—the maximum height value of the parabola, m, and its value is: h = λ / d; d—the ratio of the equivalent diameter of the elliptical tube to the unit length, dimensionless.

2. The elliptical tube for enhancing condensation heat transfer according to claim 1, characterized in that, the cross-section of the annular groove (2) is semi-circular, and the groove depth is 0.5 - 2.5 mm.

3. The elliptical tube for enhancing condensation heat transfer according to claim 2, characterized in that, where: σ—surface tension, N / m; g — acceleration due to gravity, m / s 2 ; ρ 1 — liquid density, kg / m 3 ; ρ g — Gas density, kg / m 3 ; a—coefficient, dimensionless, and a = 548.2×d - 5.41; where: d—dimensionless pipe diameter, that is, the ratio of the equivalent circular tube diameter of the elliptical tube to the unit length.

4. The elliptical tube for enhancing condensation heat transfer according to claim 1 or 2 or 3, characterized in that, the length of the circular tube (3) is 20 - 100 cm.

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