An elliptical tube for enhancing condensation heat exchange

Abstract

The present invention discloses an elliptical tube for enhancing condensation heat exchange, wherein the two ends of the elliptical condenser tube are connected to circular tubes, and a plurality of evenly distributed annular grooves perpendicular to the axis are arranged axially on the elliptical condenser tube. The spacing between the annular grooves arranged in the present invention is equal to the Taylor wavelength, and the convex surface between adjacent grooves is a parabola. As a result, the gas-liquid interface of the Rayleigh-Taylor wave is formed at the lower edge of the elliptical tube. Under the action of the Rayleigh-Taylor instability, the condensed liquid quickly leaves the trough, gathers toward the wave crest, and then drips, so as to achieve the purpose of rapid liquid discharge and thinning of the liquid film, thereby improving the condensation heat exchange coefficient.

Description

Technical Field

[0001] The invention relates to an elliptical tube for strengthening condensation heat exchange, belongs to the field of manufacturing heat exchange tubes for heat exchangers, and is particularly suitable for tubes for condensers. Background Art

[0002] The working principle of the condenser is that steam exchanges heat with the cold water in the heat exchange tube, condenses on the surface of the heat exchange tube and forms a liquid film. Since the liquid film prevents 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 greater the thickness of the liquid film, the higher the temperature of the liquid film surface, the smaller the heat exchange temperature difference, the lower the heat transfer coefficient, and the liquid film thermal resistance becomes the largest thermal resistance of condensation heat exchange, so thinning the liquid film thickness is an effective way to improve the condensation heat transfer coefficient. The elliptical tube is a heat exchange tube used in the condenser. Because under the condition of equal inner cross-section of the tube, the outer surface area of ​​the elliptical tube is larger than that of the circular tube. Therefore, under the same condensation heat exchange conditions outside the tube, the liquid film thickness is thinner than that of the circular tube, so the heat transfer efficiency of the elliptical tube is higher than that of the circular tube. However, the distribution of the liquid film on the surface of the elliptical tube is uneven. Under the action of the viscosity 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 flooded area. The thickness of the liquid film is greater than 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, in which the outer tube is a round tube and the inner tube is set as a single elliptical twisted tube. The elliptical twisted tube is an elliptical tube with a spiral twisted channel structure. The orientation of the long axis of the ellipse changes periodically, causing severe disturbances of the fluid in the tube and in the sleeve gap, thereby improving the heat transfer coefficient 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 condenser tube based on hydrophilic and hydrophobic intervals. The inner tube is a smooth round tube, and the wall thickness of the outer tube is periodically undulating according to the critical instability wavelength of Prato-Rayleigh, forming a sinusoidal wave-shaped outer surface heat exchange tube, and the two tubes are set to form a composite tube. In addition, vinyl triethoxysilane is used to treat the wall surface at the trough with hydrophobicity, and the peak is kept wet and hydrophilic, and is distributed in a hydrophilic and hydrophobic interval. After the droplets condense, they tend to leave the hydrophobic area, ensuring that the area is always in a state with fewer droplets and maintaining a higher condensation heat transfer coefficient. The shortcomings of this technology are that the two-layer sleeve technical solution significantly increases the thickness of the tube wall and the contact thermal resistance between the two additional tube walls, reducing the heat transfer coefficient; in addition, this technology requires hydrophilic / hydrophobic treatment, which is complex and costly. Summary of the invention

[0003] The object of the present invention is to overcome the above-mentioned deficiencies of the prior art and provide an elliptical tube for enhancing condensation heat exchange.

[0004] The technical solution provided by the present invention is as follows: an elliptical tube for enhancing condensation heat exchange, characterized in that it includes a circular tube and an elliptical condenser tube, the two ends of the elliptical condenser tube are connected to the circular tube, and the elliptical condenser tube is provided with a plurality of evenly distributed annular grooves perpendicular to the axis along the axial direction.

[0005] Furthermore, the cross-section of the annular groove is semicircular, the upper opening width is 0.5-2.0 mm, and the groove depth is 0.5-2.5 mm.

[0006] Furthermore, the spacing λ between the center lines of adjacent annular grooves is calculated according to the following formula:

[0007]

[0008] in:

[0009] λ—groove spacing, m;

[0010] σ--surface tension, N / m;

[0011] g—acceleration due to gravity, m / s2;

[0012] ρ1—Liquid density, kg / m3;

[0013] ρg—gas density, kg / m3;

[0014] a—coefficient, dimensionless; its value can be calculated according to the following formula:

[0015] a=548.2×d-5.41

[0016] Where: d is the dimensionless tube diameter, m / m, that is, the ratio of the diameter of the elliptical tube to the unit length of the circular tube.

[0017] Furthermore, the surface of the elliptical condenser tube between the circular tube and the annular groove and between adjacent annular grooves is a parabolic curved convex surface, and the parabolic shape can be calculated by the following formula:

[0018]

[0019] in:

[0020] y—coordinate value of the convexity of the parabola surface, m;

[0021] x—coordinate value along the axis of the elliptical tube, 0≤X≤λ, m;

[0022] λ—spacing between grooves (Taylor critical wavelength calculated above), m;

[0023] h—the maximum height of the parabola, m, whose value is: h=λ / d;

[0024] d—Ratio of the equivalent diameter of an elliptical tube to its unit length, dimensionless.

[0025] Furthermore, the length of the circular tube is 20-100 cm.

[0026] The present invention provides a semicircular groove perpendicular to the axis along the axis of the heat exchange tube and a convex surface between adjacent grooves in a parabolic shape. The spacing between the annular grooves on the lower edge of the elliptical tube and the surface shape between the annular grooves are respectively equal to the wavelength of the vapor-liquid interface and the shape of the gas-liquid interface in Taylor theory, that is, the annular groove is at the trough position, and the wave crest is at the vertex position of the parabola on the surface between adjacent grooves. As a result, the lower edge of the elliptical heat exchange tube is formed into the wave surface shape of the Taylor vapor-liquid interface wave.

[0027] The principle and function of the present invention are as follows: For the condensation heat exchange process, the thickness of the liquid film is the key factor that determines the heat exchange performance. Accelerating the liquid discharge speed is a method of reducing the thickness of the liquid film. According to the analysis of the Rayleigh-Taylor instability theory of fluid mechanics: When two fluids with different densities are superimposed, when the fluid with high density is on the top and the fluid with low density is on the bottom, this state is an unstable state with high potential energy and will develop towards a stable state with low potential energy. Rayleigh-Taylor instability will have several development stages: linear development stage, weak linear development stage, nonlinear development stage, and the corresponding gas-liquid interface shape undergoes sine and cosine shapes, and superimposed shapes of cosines with different wave numbers, and finally forms a parabolic shape with the vertex of the liquid film at the bottom, and the liquid drips from the vertex. At this time, the interface forms a continuous corrugated shape, and the spacing between each corrugation is the Rayleigh-Taylor critical wavelength length, and the gas-liquid surface shape within its critical wavelength is a parabolic shape. As a result, the liquid film at the trough of each corrugation becomes thinner, and the liquid quickly gathers at the crest. As the amount of liquid gathered at the crest increases, under the action of gravity, the liquid overcomes the effects of viscosity and surface tension and drips downward; as the 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 once again tends to form a wavy interface composed of multiple parabolic surfaces, that is, the liquid at the trough once again gathers at the crest, and the liquid at the crest drips. Repeatedly, the dripping of the liquid on the surface of the heat exchange tube is achieved. Obviously, the liquid on the lower edge of the elliptical heat exchange tube needs to go through several development stages from the 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, speed up the drainage speed, and make full use of the effect of Rayleigh-Taylor instability, the surface of the heat exchange tube is designed to be in the shape of the Taylor interface wave. Specific approach: On the one hand, according to the characteristic that the distance between adjacent waves is equal to the Rayleigh-Taylor wavelength, annular grooves are set at both ends of the critical wavelength to thin the liquid film there and form a trough. On the other hand, according to the parabolic characteristic of the Taylor wave interface, the surface is designed to be parabolic. Therefore, once a liquid film appears on the surface of the heat exchange tube, a Rayleigh-Taylor wave parabolic interface will be quickly formed, prompting the condensed liquid to quickly gather at the wave crest and drip downward under the action of gravity. Ultimately, the liquid film thickness on the surface of the heat exchange tube is thinned, and the condensation heat transfer coefficient is enhanced. The Rayleigh-Taylor critical wavelength calculation formula for the classic circular tube is as follows:

[0028]

[0029] This formula is applicable to circular tube conditions. In the case of elliptical tubes, the coefficient needs to be re-determined. After multiple visualization tests, it is found that the Taylor wavelength is related to the equivalent circular tube diameter of the elliptical tube (experimental conditions: working medium water, condensation temperature is 40°C). The experimental results are as follows:

[0030] Table 1 Relationship between Taylor wavelength and equivalent diameter of elliptical tube

[0031] Equivalent diameter of elliptical tube / m Taylor wavelength / m 0.016 0.009 0.019 0.013 0.025 0.022

[0032] To this end, the coefficient a in the calculation formula of Taylor wavelength under elliptical tube conditions was corrected and mathematically fitted according to the experimental results. The resulting formula is as follows:

[0033] a=548.2×d-5.41 According to theoretical calculation, the Taylor wave interface is a parabola composed of multiple high-order harmonics. For this reason, the surface between the grooves of the elliptical heat exchange tube is processed into a parabola. Through calculation and considering the processing conditions, the equation of its shape is:

[0034]

[0035] The beneficial effects of the present invention are:

[0036] The invention overcomes the problem that the condensed liquid is difficult to separate from the tube body under the action of surface tension and viscosity, and is retained at the bottom of the heat exchange tube to form a larger submerged area, thereby increasing the liquid film thickness and reducing the condensation heat transfer coefficient of the elliptical tube.

[0037] According to the Rayleigh-Taylor instability theory, the annular grooves and parabolic surfaces set in the elliptical tube can accelerate the condensate to gather at the vertex of the parabolic surface at the bottom of the elliptical tube, thereby increasing the dripping speed of the droplets, reducing the thickness of the liquid film, and achieving the purpose of improving the condensation heat transfer coefficient.

[0038] To better understand the role of Taylor instability, Figure 3 The Taylor wavelength of the classic circular condenser and the shape of the vapor-liquid interface are shown. 4 is the condenser; 5 is the vapor-liquid interface; 6 is the Taylor wavelength; 7 is the liquid droplet. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a structural schematic diagram of the present invention;

[0040] Figure 2 for Figure 1 AA section view;

[0041] Figure 3 Schematic diagram of the Taylor wavelength and the shape of the vapor-liquid interface of a classic circular condenser. DETAILED DESCRIPTION

[0042] The specific implementation of the present invention is described in detail below with reference to the accompanying drawings:

[0043] like Figure 1-Figure 2 As shown, an elliptical tube for enhancing condensation heat exchange can be made of copper and copper-gold, 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. The length is 20-100cm.

[0044] A plurality of evenly distributed annular grooves 2 are arranged along the axial direction of the elliptical condenser tube 1 and are perpendicular to the axis. The cross section of the annular groove 2 is semicircular, the upper opening width is 0.5-2.0 mm, and the groove depth is 0.5-2.5 mm. The spacing λ (Taylor critical wavelength) between the center lines of adjacent annular grooves 2 is calculated according to the following formula:

[0045]

[0046] in:

[0047] λ—groove spacing, m;

[0048] σ--surface tension, N / m;

[0049] g—acceleration due to gravity, m / s2;

[0050] ρl—liquid density, kg / m3;

[0051] ρg—gas density, kg / m3;

[0052] a—coefficient, dimensionless; its value can be calculated according to the following formula:

[0053] a=548.2×d-5.41

[0054] Where: d is the dimensionless tube diameter, m / m, that is, the ratio of the diameter of the elliptical tube to the unit length of the circular tube.

[0055] 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 curved convex surface, and the parabolic shape can be calculated by the following formula:

[0056]

[0057] in:

[0058] y—coordinate value of the convexity of the parabola surface, m;

[0059] x—coordinate value along the axis of the elliptical tube, 0≤X≤λ, m;

[0060] λ—spacing between grooves (Taylor critical wavelength calculated above), m;

[0061] h—the maximum height of the parabola, m, whose value is: h=λ / d;

[0062] d—Ratio of the equivalent diameter of an elliptical tube to its unit length, dimensionless.

[0063] A circular tube 3 with a diameter of d = 16 mm is provided with an annular groove perpendicular to the axis. The water vapor outside the tube is condensed at a temperature of 40°C. The water vapor density ρg = 0.051 kg / m3, the water density ρl = 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 gravity acceleration g = 9.80 m / S 2 , according to the formula According to calculation, the distance between adjacent annular grooves is λ=8.95mm, and the shape line formula h=λ / d is used to calculate the vertex height h=0.56mm. The mold is processed according to the shape, and the mold is processed into an elliptical tube of the shape by a hydraulic press with a mold. In actual working conditions, cold water is passed through the elliptical tube to condense the water vapor outside the tube, and the condensation temperature is 40°C. The condensation heat transfer coefficient of the elliptical tube of the present invention is about 12% higher than that of the conventional elliptical tube.

[0064] It should be understood that the parts not elaborated in detail in this specification belong to the prior art. The above embodiments are only descriptions of the preferred implementation methods of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made by ordinary engineers and technicians in this field to the technical solution of the present invention should fall within the protection scope determined by the claims of the present invention.

Claims

1. An elliptical tube for enhancing condensation heat exchange, characterized in that: It comprises a circular tube (3) and an elliptical condenser tube (1), wherein both ends of the elliptical condenser tube (1) are connected to the circular tube (3), and the elliptical condenser tube (1) is provided with a plurality of evenly distributed annular grooves (2) perpendicular to the axis along the axial direction; The spacing λ between the center lines of adjacent annular grooves (2) is calculated according to the following formula: in: σ--surface tension, N / m; g—acceleration due to gravity, m / s 2 ; ρ1—Liquid density, kg / m 3 ; ρ g —Gas density, kg / m 3 ; a—coefficient, dimensionless, its value is calculated according to the following formula: a=548.2×d-5.41, Where: d is the dimensionless tube diameter, that is, the ratio of the diameter of the elliptical tube to the unit length; 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 curved convex surface, and the parabolic shape is calculated by the following formula: in: y—coordinate value of the convexity of the parabola surface, m; x—coordinate value along the axis of the elliptical tube, 0≤X≤λ, m; h—the maximum height of the parabola, m, and its value is: h=λ / d.

2. The elliptical tube for enhancing condensation heat exchange according to claim 1, characterized in that: The cross section of the annular groove (2) is semicircular, the upper opening width is 0.5-2.0 mm, and the groove depth is 0.5-2.5 mm.

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

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