WO2004076954A1

WO2004076954A1 - Reinforced heat exchange pipe with sections of round and ellipse of alternate change

Info

Publication number: WO2004076954A1
Application number: PCT/CN2003/000490
Authority: WO
Inventors: Zengyuan Guo; Jian Meng; Weilin Hu; Zhixin Li
Original assignee: Tsinghua University
Priority date: 2003-02-28
Filing date: 2003-06-25
Publication date: 2004-09-10
Also published as: CN1190645C; AU2003303973A1; CN1447088A

Abstract

A reinforced heat exchange pipe with sections of round and ellipse of alternate change, which is composed of several round pipe portions, elliptical pipe portions and round-elliptical transition pipe portions. The round pipes and elliptical pipes are arranged alternatively and have a crossed angle of 15°-90° between the long axis and the short axis of adjacent ellipse. This invention has smaller flow resistance and good heat transfer property. The invention has various applications and can prevent accumulation of dirt. For the tube-and-shell heat exchanger of the invention, the vibration of the bundle of pipe is much lower than the bundle of the general round pipe, and thereby the service life of the heat exchanger is increased.

Description

Circular-elliptical cross-section cross-scaling enhanced heat exchange tube

The invention relates to an enhanced heat transfer element used in the field of heat transfer enhancement and heat exchangers, in particular to a circular-elliptical cross-section cross-scaling enhanced heat exchange tube. Background technique

Heat transfer enhancement technology has been widely used in engineering due to its ability to increase heat transfer rate and save energy. It can generally be divided into active and passive reinforcement technologies. The former refers to heat transfer enhancement technology that requires energy input from the outside, such as electric field strengthening, magnetic field strengthening, vibration strengthening technology, etc .; the latter refers to heat transfer enhancement technology that does not require external energy, so it is easier to be used in engineering. Passive reinforcement techniques usually include extended surfaces (fins), in-tube inserts, swirling (turbulent) generators, etc. The above heat transfer enhancement techniques are described in detail in the book "Enhanced Heat Transfer" (Science Press 1990) by Gu Weizao, Shen Jiarui, Ma Chongfang, and Zhang Yuming. Although passive strengthening technologies have been widely used, there is still a common problem, that is, while heat transfer is strengthened, the flow resistance increases more, which is not conducive to energy saving, which prevents these technologies from being widely used in various engineering fields.

Different from conventional heat transfer enhancement technology, Guo ZY, etc. (Guo ZY, Mechanism and Control of Convective Heat Transfer― Coordination of Velocity and Heat Flow Field, Chinese Science Bulletin, 46 (7): 596-599 Apr. 2001.) From the perspective of the synergy of the fluid velocity field and the temperature gradient field, a new method of heat transfer enhancement, the "field cooperation principle", is proposed, which is to increase the degree of field coordination (reducing the angle between the velocity vector and the temperature gradient vector, increasing the velocity field Uniformity, etc.) can enhance heat transfer with less increase in resistance. Convective heat transfer in tube flow, if it can The formation of radial flow can significantly improve the degree of synergy between the velocity field and the temperature gradient field, thereby enhancing heat transfer. This is the essential reason for the emergence of secondary flow or vortex in the tube to enhance heat transfer.

The Chinese invention patent (application number 00136122.8, publication number CN1361406A) with the invention name of "cross-ellipsoidal cross-section heat exchange tube" proposes a new type of heat exchange tube. This heat exchange tube is composed of several oval tube segments, and adjacent elliptical tube segments The major axes of the ellipse intersect at a certain angle (typically 60 degrees or 90 degrees). This design conforms well to the "field cooperation principle", so it has a relatively large heat transfer enhancement effect and a small increase in flow resistance. However, the patented technology still has obvious shortcomings, mainly: 1. The flow resistance can be smaller; 2. The poor pressure resistance of the oval tube restricts its application under high pressure conditions; 3. When this kind of heat transfer is used When the tube is used to manufacture a heat exchanger, the design of the baffle plate or the baffle rod in the heat exchanger is a bit cumbersome due to the orientation of the oval tube section. Summary of the Invention

The purpose of the present invention is to propose a circular-elliptical cross-section cross-scaling enhanced heat transfer tube based on the "field synergy principle" and the shortcomings of the above-mentioned invention patents. Compared with other enhanced heat transfer tubes, the same pump (fan) function Consumption, has better heat transfer performance; compared with "cross-ellipsoidal cross-section heat transfer tube", with the same heat transfer capacity, it has less flow resistance, higher pressure resistance, and can eliminate design and manufacturing Trouble with baffles and baffles.

The above object of the present invention can be achieved by the technical solution described below: A circular-elliptical cross-section cross-strengthening and strengthening heat-exchanging tube is composed of a plurality of circular tube segments, an elliptical tube segment and a circular-elliptical transition segment, and the circular tube segment and the The pipe segments are arranged periodically in sequence, and the ellipse long axes of adjacent elliptical pipe segments intersect at an angle of 15 to 90 degrees. The cross-sectional shape of the elliptical pipe section according to the present invention adopts a standard ellipse or an approximate ellipse Or an oblate circle composed of two straight lines and two arcs, or a symmetrical and approximately symmetrical oblate circle fitted by multiple arcs. In the above technical solution, each circular pipe section may be of equal or unequal length; each elliptical pipe section may be of equal or unequal length, and each circle-ellipse transition section may be of equal or unequal length. The ratio of the length of the inner diameter of its respective cylindrical section is _0.5~10; ratio of length to inner diameter of each segment of the circular pipe section is oval tube _1~10; ratio of major and minor axes of the ellipse of the tube outer diameter is 1.1 ~ 2.5, preferably 1.3 ~ 1.7. The technical feature of the present invention is also that the inner surface or the outer surface or the inner and outer surfaces of the heat exchange tube may be a smooth surface, or a low-rib surface or a threaded surface. The invention has the following advantages and outstanding progress: ① The obtained large-scale secondary flow (eddy current) is not generated by external inserts, but is caused by the change in cross-sectional shape of the pipe along the way, so the flow resistance is small; ② The obtained The secondary flow is a longitudinal vortex (the direction of the vortex is parallel to the flow direction). Compared with the lateral vortex in the tube (the direction of the vortex is perpendicular to the flow direction), the velocity field and the temperature gradient field have a better degree of synergy. Slow decay, so it can have better heat transfer enhancement performance. Compared with other reinforced heat exchange tubes, the present invention has better heat transfer performance under the same power consumption of the pump (fan); compared with "cross-ellipsoidal cross section heat transfer tubes", it has the same heat transfer capacity The flow resistance is smaller, the flow resistance is reduced by more than 10%; the pressure resistance is higher, the internal stress of the pipe wall is smaller, and the pressure bearing capacity is increased by 30%; and it eliminates the problems encountered in the design and manufacture of baffles and baffles Trouble, design, processing, and assembly are more convenient. The invention has a wide range of applications, and can be used for both single-phase fluid heat transfer enhancement and phase boiling heat transfer enhancement such as flow boiling and flow condensation. It can be used for both in-pipe heat transfer enhancement and outer-pipe heat transfer enhancement. In addition, the reinforced heat exchange tube of the present invention is very beneficial to prevent the formation of dirt due to the frequent changes in the shape and size of the tube cross section. The vibration of the tube bundle is much less than that of ordinary round tubes, which greatly improves the service life of the heat exchanger. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic structural diagram of a circular-elliptical cross-section cross-scaling enhanced heat exchange tube according to the present invention.

FIG. 2 is an E-E view of FIG. 1. Figure 3 shows the flow structure in the elliptical tube segment. The near-wall region presents eight longitudinal vortices. DETAILED DESCRIPTION The specific structure of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic structural diagram of a circular-elliptical cross-section cross-scaling enhanced heat exchange tube according to the present invention. The enhanced heat exchange tube is composed of a plurality of circular tube sections 1, an elliptical tube section 2, a circular-elliptical transition section 3, and inlet circular tube sections 4 at both ends. The length Lc of the round pipe section is taken according to the requirements of pressure resistance, flow resistance and the installation of a baffle or a baffle rod. Generally, Lc / d = 0.5 ~ 10, where d is the inner diameter of the round pipe. The length of each round pipe section can be equal or different. The length Lo of the round pipe sections at both ends can be 2 to 4 times the thickness of the tube sheet, which depends on the needs of the assembly with the tube sheet, and a small value on the premise that the assembly needs are met. The length Le of the elliptical tube segment is taken according to the requirements for enhanced heat transfer, and generally Le / d = 1 to 10. The length of each ellipse segment can be the same or different. The cross-sectional shape of the elliptical tube section is a standard oval or an approximately ellipse or an oblate or multi-segment arc fitted with two straight lines and two arcs. Ellipse (oblate) The ratio of the long outer diameter and short axis of the pipe is A / B = 11 ~ 2.5, preferably A / B = 1.3 ~ 1.7. There is an included angle C between the long axes of the ellipse (oblate circle) of two adjacent elliptical pipe sections (see Figure 2, E-E view), take the angle of C = 15 ~ 90 degrees, preferably C is equal to 60 degrees and 90 degrees Two kinds; if the heat exchanger tube in the heat exchanger is pressed according to four For a rectangular arrangement, take C = 90 degrees. If the heat exchanger tubes are arranged in a triangle, take O60 degrees. The length Lt of the circle-ellipse transition section is Le / 2 ^ Lt ^ 0.1d, and a small value is recommended. The length of each transition section may be the same or different.

The inner surface, the outer surface, or the inner and outer surfaces of the "circular-elliptical cross-section cross-strengthening and strengthening heat-exchanging tube" of the present invention may be both smooth surfaces (e.g., rolling or molding with ordinary round tubes or rolling seamless tubes once Forming), it can also be a low-rib surface or a threaded surface (such as a low-rib tube or a threaded tube), forming a periodic circle-vertical ellipse-circle-transverse ellipse-circle ... structure (see Figure 1, Figure 2 ). Under the condition that the ellipticity should not be too high, the flow area of the tube along the way does not change much, but due to the sharp change in the shape of the cross section, the fluid has partially contracted and the other part of the fluid has been enlarged on the same cross section, and the result is formed The secondary flow of 4 or 6 or 8 vortices is shown (see Figure 3), which greatly improves the synergy between the fluid velocity field and the temperature gradient field in the tube. The ones with a low rib surface are called "low-rib circle-ellipse cross-section strengthened heat transfer tubes"; the ones with threaded surface are called "thread circle-ellipse cross-section strengthened heat transfer tubes". The low-ribbed and threaded "circular-elliptical cross-section scaled heat-exchanging tubes" have both a field-coordinated enhanced heat transfer function and an extended surface-enhanced heat transfer function. The effect of enhanced heat transfer is better, but the flow resistance increases accordingly. Large, and strict requirements on the cleanliness of the working fluid. These two types of "circular-elliptical cross-section cross-strengthening and strengthening heat exchange tubes" are more suitable for occasions where the working fluid does not contain fibers and particles, and has a large viscosity and a small flow velocity.

Claims

1. A circular-elliptical cross-section cross-scaling enhanced heat exchange tube, characterized in that it comprises a plurality of circular tube segments, an elliptical tube segment, and a circular-elliptical transition segment, wherein the circular tube segment and the elliptical tube segment are arranged periodically in sequence, and adjacent ellipse The ellipse long axis of the pipe section crosses an angle of 15 ~ 90 degrees.

2. The circular-elliptic cross-section cross-strengthening and strengthening heat-exchanging tube according to claim 1, wherein the cross-sectional shape of the elliptical tube section is a standard ellipse or an approximate ellipse, or two straight lines and two arcs Oblate, or symmetric and approximately symmetrical oblate circles fitted by multiple arcs.

3. The circular-elliptical cross-section cross-strengthening and strengthening heat-exchanging tube according to claim 1 or 2, characterized in that each circular tube section can be of equal or unequal length, each oval tube section can be of equal or unequal length, and each circle An elliptical transition can be of equal or unequal length.

4. The circular-elliptical cross-section cross-scaling strengthened heat-exchange tube according to claim 3, wherein: the ratio of the length of each circular tube segment to its inner diameter is 0.5 to 10 _; the length of each elliptical tube segment and the circular tube The ratio of the inner diameter is 1 to 10; the ratio of the long axis to the short axis of the outer diameter of the oval tube is 1.1 to 2.5, and the preferred value is 1.3 to 1.7.

5. The round-elliptical cross-section cross-scaling and strengthening heat-exchanging tube according to claim 1, wherein the crossing angle between the major axes of adjacent elliptical tubes is preferably 60 degrees and 90 degrees.

6. The round-elliptical cross-section strengthened and strengthened heat exchange tube according to claim 1, characterized in that: the inner surface or the outer surface or the inner and outer surfaces of the heat exchange tube may be a smooth surface or a low surface. Ribbed or threaded surface.