Technical

Elliptical finned tubes

An elliptical finned tube is a heat exchanger element consisting of a base tube, an elliptical tube, and an outer fin. The common ones are elliptical rectangular finned tubes, oval oval finned tubes, oval round finned tubes, spiral elliptical flat tubes, oval H-shaped finned tubes, etc. Elliptical finned tubes are being valued because of their superior performance compared to round-finned tubes, which have been widely used in industrial fields such as ethylene and oil refining. Elliptical finned tubes are used as high-efficiency heat exchange elements in heat exchange equipment. The outflow resistance of the tube is small and the heat exchange efficiency is high, which makes the heat exchange equipment tend to be compact, lightweight, efficient and miniaturized. While much has been achieved on them, much remains to be done.

Brief introduction

With the development of science and technology and industry, heat exchanger equipment tends to be compact, lightweight, efficient and miniaturized, but the general heat exchanger can not meet the above requirements, which prompts people to study high-efficiency heat exchanger. Therefore, tube-fin heat exchanger is favored by scholars as a high-efficiency heat exchanger. Tube-fin heat exchanger has been widely used in refrigeration, air conditioning and other industrial fields, the core element is the tube bundle inside the heat exchanger, in order to improve the heat transfer performance in the base tube surface fin is a very effective way to strengthen heat transfer, and compared with the plain tube, the finned tube has the advantages of compact structure, flexible and reasonable material selection (the material selection of base tube and fin can be different), and high heat transfer efficiency.

Types of finned tubes

According to the different fin installation positions, there are two kinds of fin tubes: inner fin tube and outer fin tube, among which the outer fin tube is more commonly used. According to the different fin arrangement, there are two types of finned tubes: longitudinal finned tubes and transverse finned tubes. According to the different shape of the base tube, finned tubes include round finned tubes, oval finned tubes, and flat finned tubes. The round-finned tube heat exchanger occupies a dominant role in the market, but a large number of tests show that compared with the round-finned tube, the return area and windward area of the elliptical tube are much smaller, which effectively reduces the flow resistance and energy consumption on the air side, and when the number of tube bundles is the same, the elliptical tube and the flat tube are more compact than the round tube, and the volume of the heat exchanger is smaller, which reduces the cost. Therefore, the research and development of elliptical tube-fin heat exchangers has attracted more and more attention from scholars.

Research status of elliptical finned tubes

The exothermic and resistance properties of ten kinds of elliptical rectangular finned tube bundles were experimentally studied. Linear regression analysis and F-level significance test were used to analyze the experimental data, and the correlation formula between the air-side heat release and resistance performance of the elliptical rectangular fin tube bundle was obtained. The optimal transverse and longitudinal pipe spacing corresponding to the minimum volume and minimum windward area of the elliptical rectangular finned tube bundles were determined. The experimental data were analyzed to obtain the interaction between the number of pipe rows, the spacing between horizontal and longitudinal pipes and the friction coefficient. Yang Jinbao et al. studied the effect of elliptical tube fin spacing with different ratios of the long and short axes of the ellipse (a/b) on the heat release, and studied the heat release of a rectangular fin elliptical tube with four spoiler holes in the traversing gas. A simple formula that can be used as the basis for engineering calculations is derived. Tu Shan et al. used the steady-state constant-wall temperature method to study the heat transfer and resistance characteristics of three elliptical finned tube air coolers and one circular finned tube air cooler. Through the study and analysis of the experiments, the correlation formula of Nu and Re under different working conditions of the two finned tubes was obtained. The results also show that when the flow velocity of the windward side is equal, the heat transfer coefficient of the elliptical finned tube is about 3~7 times greater than that of the round finned tube, and when the heat transfer coefficient is equal, the pressure drop of the elliptical finned tube is lower than that of the round finned tube, and the elliptical finned tube heat exchanger needs less energy consumption and heat exchange surface. Chen Yaping tested the heat transfer and flow resistance characteristics of the rolled sheet elliptical aluminum-finned steel tube heat exchanger. The test data show that with the increase of the onward wind speed on the side of the finned tube, the pressure drop gradually increases, and the pressure drop of the 4-row tube test piece is significantly greater than that of the 3-row tube, so the increase of the number of tube rows weakens the heat transfer of the finned tube. The test results show that the rolled sheet elliptical aluminum-finned steel tube manufactured by the unique process has good heat transfer, resistance performance and sufficient structural strength, so it has a wide application prospect. Duan Rui et al. experimentally studied the heat transfer and resistance performance of finned tube radiators with unequal fin spacing in the arrangement of tube bundle forks. The steel tube steel fin elliptical tube radiator used in the experiment has two rows of tubes in the heat exchanger, and the arrangement of the fin spacing of the second row is greater than that of the first row to reduce the air resistance outside the second row and increase the heat exchange.

Features of elliptical finned tubes

(1) Compared with the round tube finned tube, the elliptical finned tube is easier to achieve compact arrangement, so that the overall volume of the whole heat exchanger is reduced, thereby reducing the floor space.

(2) Due to the shape characteristics of the elliptical finned tube, the air side resistance is small, and the heat transfer coefficient between the fluids increases, and the thermal resistance in the tube is small, which increases the heat transfer of the fluid in the tube.

(3) The heat exchange area of the elliptical finned tube is larger than that of the round tube with the same cross-sectional area, because the heat transfer periphery of the elliptical tube is relatively long under the same cross-sectional area.

(4) The most commonly used in oval finned tubes is rectangular steel fins, which have high strength, and the base tube should not freeze and crack in winter, and has a long service life.

(5) Because the elliptical finned tube can be arranged more compactly, the influence of the front row tube on the rear row is relatively large, and the outflow resistance of the tube can be reduced by increasing the fin spacing of the rear tube, but the number of tube rows should not be too large.

Calculation method for elliptical finned tube finned efficiency

The elliptical finned tube consists of a base tube, an elliptical tube, and an outer fin. The common ones are elliptical rectangular finned tubes, oval oval finned tubes, oval round finned tubes, spiral elliptical flat tubes, oval H-shaped finned tubes, etc. Elliptical finned tubes are being valued because of their superior performance compared to round-finned tubes, which have been widely used in industrial fields such as ethylene and oil refining.

When calculating the fin efficiency of elliptical tube-fin heat exchanger, people often convert the elliptical finned tube equivalent to the round-fin tube, and calculate and analyze it according to the round-fin tube. There are two ways to select the equivalent round tube, one is to make the cross-sectional area of the equivalent round tube equal to the cross-sectional area of the elliptical tube, and the other is to make the circumference of the equivalent round tube equal to the circumference of the elliptical tube.

Eiro Oyaka replaced the elliptical fins with rectangular fins with elliptical tubes with the same area and eccentricity of the base tube, and the theoretical calculation of the fin efficiency was carried out. Huang Suyi et al. obtained the fin efficiency at different radius of curvature outside the tube according to the formula for calculating the efficiency of the outer round fin of the round tube, and then analyzed and calculated the rectangular fin of the elliptical tube by the method of area averaging. Zhang Chunyu et al. used the finite difference numerical analysis method, and applied the temperature update processing technology to calculate the fin efficiency of the elliptical tube rectangular fin under different turbulence holes. Compared with the method of simplifying rectangular fins with equivalent ellipticals, the results show that the efficiency of the fins is higher by using the equivalent ellipse to simplify the rectangular fins. Min Jingchun et al. verified that the fan method can be used to calculate the efficiency of elliptical tube fins. The fin efficiency of the elliptical tube straight-fin heat exchanger with a long-short axis ratio range of 1~5 under different working conditions was calculated, and the calculated results were compared with the equal circumference method and the equal area method commonly used in engineering, and the regular variation of the difference between the fan method and the equal circumference method and the equal area method was obtained. At the same time, it is obtained that when the elliptical finned tube forks are arranged, the deviation of the equal circumference method is much smaller than that of the equal area method. When the elliptical finned tubes are arranged in a row, the deviations of the equal circumference method and the equal area method are approximately equal.

Prospect

Elliptical finned tubes are used as high-efficiency heat exchange elements in heat exchange equipment. The external flow resistance of the tube is small and the heat exchange efficiency is high, which makes the heat exchange equipment tend to be compact, lightweight, efficient and miniaturized. While much has been achieved on them, much remains to be done.

(1) Elliptical finned tubes have many advantages, but they are less researched than those of the university.

(2) Fin efficiency plays a key role in heat transfer calculation. There are many ways to calculate the efficiency of elliptical finned tube fins, but basically they are all based on the equivalent method, which has some deviations, and these deviations should be corrected when guiding the actual project. Therefore, in order to obtain more accurate results and guide the project more conveniently, it is necessary to study it more systematically in the future.

(3) The research on the internal and external characteristics of elliptical finned tubes and the heat exchange law has been very comprehensive, and scholars at home and abroad have done a lot of theoretical analysis, experimental research and numerical simulation. However, there are relatively few studies on flat finned tubes, especially on the inside and outside, temperature and velocity fields of flat finned tubes. Although the air side resistance of the flat tube finned tube is relatively small and the heat exchange coefficient is high, its internal fluid resistance is relatively large, and the refrigerant charge is relatively small, which limits the application of flat tube finned tube in heat exchanger. Therefore, how to solve the problem of large internal flow resistance of flat pipes requires more and more in-depth research.