US3363682A

US3363682A - Heat exchangers having vortex producing vanes

Info

Publication number: US3363682A
Application number: US469610A
Authority: US
Inventors: Hartley Donald Edward
Original assignee: International Combustion Holdings Ltd
Current assignee: International Combustion Holdings Ltd
Priority date: 1964-07-09
Filing date: 1965-07-06
Publication date: 1968-01-16
Anticipated expiration: 1985-01-16

Description

Jan. 16, 1968 D. E. HARTLEY HEAT EXCHANGERS HAVING VORTEX PRODUCING VANES 8 Sheets-Sheet 1 Filed July 6, 1965 Jan. 16, 1968 D. E. HARTLEY 3,363,682

HEAT EXCHANGERS HAVING VORTEX PRODUCING VANES 8 Sheets-Sheet 2 Filed July 6, 1965 Jan. 16, 1968 D. E. HARTLEY 3,

HEAT EXCUANGERS HAVING VORTEX PRODUCING VANES Filed July 6, 1965 8 Sheets-Sheet 5 Jan. 16, 1968 D. E. HARTLEY 3,363,582

HEAT EXCHANGERS HAVING VORTEX PRODUCING VANES Filed July 6, 1965 8 Sheets-Sheet 4 v e PRfSSU/M TAPP/IVG Pas/710A! Jan. 16,1968 I D. E. HARTLEY I 3,363,632

IIEAT EXCHANGERS HAVING VORTEX PRODUCING VANES Filed July as, 1965 s Sheets-Sheet av 7154/2 K/JT 147 f. Iii/4D W6 Jan. 16, 1968 D. E. HARTLEY 3,363,582

HEAT EXCUANGERS HAVING VORTEX PRODUCING VANES Filed July 6, 1965 8 Sheets-Sheet (5 V6.2. V6. 3. V6.4

Jan. 16, 1968 D. E. HARTLEY 3,363,682

HEAT EXCHANGERS HAVING VORTEX I'PODUCING VANES Filed July 6, 1965 8 SheetsSheet '7 Jan. 16, 1968 D. E. HARTLEY HEAT EXCHANGERS HAVING VORTEX PRODUCING VANES 8 Sheets-Sheet 8 Filed July 0, 1965 (/KMfE w/m v 51w 0 AR[ W17 #[AT TKAMYFFK WIT/l V6 f/[AT TKANSFK WIT/I007 VG United States Patent 3,363,682 HEAT EXCHANGERS HAVING VORTEX PRODUCING VANES Donald Edward Hartley, London, England, assignor to International Combustion (Holdings) Limited, London, England Filed July 6, 1965, Ser. No. 469,610 Claims priority, application Great Britain, July 9, 1964, 28,424/64 10 Claims. (Cl. 165-181) The invention relates to heat exchangers and is concerned with the promotion of increased rates of heat transfer in heat exchangers of a wide variety.

It is generally accepted in heat transfer engineering that, other conditions being unchanged, the rate of heat transfer increases if the degree of mixing within a fluid increases. The devices and methods suggested in the past and in use at the present time to enhance fluid mixing possess the common disadvantage that they also cause wakes and stagnant pockets which result in disproportionate increase in fluid resistance and consequent pressure losses.

It is an object of the present invention to provide means of good fluid mixing with only small increases in the pressure drops. In theory the fluid will be mixed without creating wakes, pockets, etc., if the fluid particles describe helical paths with the axes of the helices parallel to the heat transfer surface and to the flow direction.

The invention provides a heat exchanger comprising a heat exchange surface over which fluid flows in use and means in advance of at least part of the surface to generate close to the said part of the surface and without substantially altering the general direction of fluid flow, a trailing vortex with its axis substantially parallel to the general direction of flow.

Preferably the generating means generates a succession of trailing vortices side by side across the surface which vortices have axes parallel to the general direction of fluid flow.

It is proferred that the generating means comprises a fin or fins inclined at an angle to the direction of fluid flow.

In one embodiment the fin is short in relation to the length of the said part of the surface in the flow direction. The length of the fin in the flow direction may be between A of an inch and an inch.

It is further preferred that the fin is upstanding from the surface and the height of the fin may be between of an inch and an inch and preferably between a A of an inch and /2 an inch.

In a further embodiment there is at least one row of fins transverse to the direction of fluid flow and adjacent fins or groups of fins may be oppositely inclined with respect to the said direction.

In yet another embodiment there are at least two rows of fins and the rows are separated by between one and fifty times and preferably by thirty times the height of a fin.

The inclination of the fins to the direction of fluid flow may be less than 25 and preferably about 13 Some specific examples of heat exchangers according to the invention will now be described with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 shows a trailing or free vortex,

FIGURE 2 shows a vortex generator,

FIGURE 3 shows three rows of vortex generators,

FIGURE 4 shows a finned tube heat exchanger,

FIGURES 5 and 6 are a plan and a perspective view respectively of one of the fins in FIGURE 4,

FIGURE 7 shows a plate type heat exchanger,

FIGURE 8 shows a strip of metal out ready for bending to form vortex generators,

3,363,682 Patented Jan. 16, 1968 FIGURE 9 is an end view of two plates with the vortex generators in position,

FIGURE 10 is a plan of a strip of metal shaped to provide a vortex generator for a tubular heat exchanger,

FIGURES 11 and 12 are an end view and a longitudinal section respectively of a circular tube: heat exchanger,

FIGURE 13 is a longitudinal section through a testing apparatus for a vortex generator,

FIGURES 14 and 15 are sections along the lines 14 14 and 15-15 respectively in FIGURE 13,

FIGURES 16a and 16b, 17a and 17b, 18a and 18b, 19a and 1%, are plan and perspective views respectively of four alternative vortex generators and FIGURES 20 to 24 are graphs showing test results using the apparatus of FIGURES 13-15 in combination with the vortex generators shown in FIGURES 16 to 19.

FIGURES 1 to 3 show diagrammatically the principle on which the invention is based.

FIGURE 1 shows a free or trailing vortex. The line 1 represents a surface across which heat is being transferred to or from a fluid stream 2. The dotted line 3 represents the axis of the vortex and the helical path 5 is typical of any fluid particle. The vortex is produced by a generator 4 consisting of a projecting fin inclined at an angle to the flow. In practice the vortex generators may have a very wide variety of shapes, sizes, materials, positions and spacings. Essentially, the appropriate longitudinal vortices are produced by any object with asymmetry relative to planes in the stream direction, perpendicular to the heat transfer surface. The object is preferably of relatively short length along the surface to avoid excessive pressure losses.

A simple form is a rectangular sheet of material perpendicular to the surface, but inclined to the flow direction. FIGURE 2 shows such a generator. The heat transfer surface is shown as 1, the vortex generator 4 is a rectangular sheet of material, and the lines 2 are lines showing the direction of the fluid stream. The generator is perpendicular to the surface 1.

So far the description has been confined to a single vortex generator. In practice the single: generator described, or any other, would be one of a number in a transverse row or grid so that a series of parallel vortices are formed transversely across the stream. Further, owing to the diffusion or decay of vorticity, similar transverse grids may be required at one or more downstream positions.

FIGURE 3 shows diagrammatically a plan view of a rectangular heat transfer surface 1 with

rows

6, 7 and 8 of vortex generators. In the first two

rows

6 and 7 the generators are all inclined in the same way to the stream. In the last row 8 alternate generators are inclined asymmetrically which avoids diversion of the fluid from its original path.

FIGURES 4 to 6 show the invention applied to a finned tube heat exchanger, with specific reference to a circular tube with square fins as used for heat transfer between liquids or condensing vapours and a gas.

FIGURE 4 shows a circular tube 11 with thin metal fins 12. Gas flows in the direction 13 and a liquid or condensing vapour inside the tube flows in the direction 14. The heat transfer of the finned surface may be increased by forming vortex generators at the leading edges 15 of the fins.

FIGURE 5 shows one way of making suitable generators. abcd is the plan view of a thin sheet of metal and j is a circular hole for the tube 11. ab is the leading edge 15 of FIGURE 4. The fin is marked out as shown in FIGURE 5 with the distances ge, eh and hrg equal to one another and to half the distance between adjacent fins in the assembled unit. Cuts are made along the lines e7 and the metal is folded upwardly by bending along the dotted lines gf, fh. The triangles ghf are such that the lines gf and fh are at less than 25 and preferably 13 to the oncoming gas flow. The formed fin is shown isometrically in FIGURE 6 where the triangular generators hef and gef are vertical. The generators are arranged to be between V inch and one inch and preferably between inch and /2 inch in height and to have a length in the direction of flow (the dimension cf) of between inch and one inch. Gas flow will be in the direction 13 relative to the fins, that is parallel to the edges be and ad. When a series of fins formed as in FIGURE 6 are assembled one above the other and attached to the circular tube 11, possibly by a brazing process, the vortex generators will provide Vortices midway between the fins thereby increasing the heat transfer rate on both surfaces.

FIGURES 7 to 9 show the invention applied to a plain plate type heat exchanger such as is employed for air heaters using gases at higher temperatures, both fluids being at relatively low pressures.

The matrix of such a heater is shown in FIGURE 7 where rectangular passages such as abcd are formed by joining together a series of plates such as aefb. Alternate channels are employed for the two fluids A and B and usually a counterflow arrangement is adopted.

For such heat exchangers metal strips may be employed to form vortex generators, one type being shown in FIGURES 7 to 9. In FIGURE 8 a strip of thin metal 30 has cut away from it triangular parts such as 19, 20, 21 and 22, 23, 24 and the

flaps

20, 21, 22, 23 are bent along lines such as 20, 23 by less than 25 and preferably by about 13 upwardly or downwardly from the plane of the strip 30. The lengths of the folded flaps along the direction of flow are between inch and one inch and the heights of the generators (i.e. the lengths 19-21 and 22-24) between A inch and one inch and preferably between 4 inch and /2 inch.

Next right angled folds are made along lines such as 25, 26 and 27, 28 in the appropriate direction so as to form a corrugated shape as shown in FIGURE 9. Thus the flaps or vortex generators are in the vertical limbs and the fiat parts touch the surfaces of the parallel plates of the matrix of FIGURE 7.

Vortices are formed from the

points

21 and 22 close to both surfaces of the parallel plate elements. The strips of corrugations are inserted between tht plates and held by spot welding or brazing and the rows are repeated along the streamwise direction of the plates at distances approximately twenty times the distance between the plates.

FIGURE 7 shows approximately the positions of the vortex generating strips between the top two plates.

Vortex generators may be employed to improve heat transfer on the inside of circular tubes as shown in FIG- URES 10 to 12.

In this instance a strip of metal abcd in FIGURE 10 is formed in the way described with reference to FIG- URE to have triangular generators 43 along ab. The strip of metal will be of width about one third of the diameter of the tube 45 for which it is needed and the vortex generator height approximately one seventh of the tube diameter.

The strip abcd is folded into a circle and ab joined to jk, the generators facing inwardly towards the centre of the circle. The ring thus formed is attached by a support 42 (FIGURE 11) to a central rod 44 of diameter about one tenth of the tube diameter. The generator rings are repeated along the rod about every twenty diameters. A vortex generator ring installed in a tube is shown in FIGURE 12. The outside diameter of the ring is a little less than the bore of the tube so that the ring can be withdrawn to permit the tube to be cleaned.

In an alternative arrangement the strip is wrapped helically around the inside and the outside of the tube 45. When the strip is inside the tube it may be held in position by its own resilience and when outside it may be welded or brazed in position.

The invention has many applications other than those described in FIGURES l12.

For example the generators may be used in a similar way on the outside of tubes where the flow direction is parallel to the axes of the tubes. Many heat exchangers employ such flows parallel to the tube as also do nuclear reactors, in which case the tubes are in fact the fuel elements. One way of using the generators is to wrap strips such as that in FIGURE 10 circumferentially around the tube and attaching the strips to the tubes by means of welds, or spot welds, or by brazing.

Experiments were carried out using the apparatus shown in FIGURES 13 to 15 to demonstrate the effect of raised fins on a heat exchanger surface.

The apparatus comprised a duct with an air inlet 61 and an air outlet 62. A brass steam condenser 63 with a steam inlet 64 and a condensate outlet 65 was let in to the bottom of the duct 61 intermediate in the length of the duct to provide a constant temperature surface 66. A thermo-couple (not shown) was welded to the surface to check the temperature of the surface.

To measure the airflow through the duct a series of static pressure tappings labelled 9, 10, and 12 to 18 in FIGURE 13 were provided along the length of the duct and two sets, each of four tappings (1 to 4 and 5 to 8 respectively) across the duct in conjunction with total head probes 25 and 26. The total head probes readings were subtracted from the static head readings at

probes

22 and 24 using

manometers

19, 20. The pressure drop across the surface 66 was measured by subtraction of the readings of

probes

22 and 23 using a manometer 21. The temperature rise of the air was measured by means of two thermo-couples T and T Air was drawn through the duct by an ejector (not shown) which was driven by compressed air and connected to the duct by means of the outlet pipe 62.

Vortex generators 70 were inserted in to the duct and were attached either to the bottom or to the top surface of the duct. Four different vortex generators VG1 to VG4 were used and these are shown in FIGURES 16-19 respectively.

Generator VG1 comprised seven individual fins bent up out of the plane of a strip of 2.16 inches in length. The length of the fins in the direction of flow (at right angles to the strip) was inch.

Generator VG2 comprised two pairs of fins bent out of a strip 2.16 inches long and each fin had a length in the flow direction of /2 inch.

Generator VG1: comprised five pairs of fins bent out of a strip of length 2.16 inches and each having a length in the flow direction of about 4 inch.

Generator VG4 comprised four pairs of fins bent out of a strip of length 2.16 inches and each having a length in the flow direction of /2 inch.

The first test which was carried out was to replace the top of the duct 60 by means of a glass sheet and to introduce smoke streams into the incoming air. Vortices of the trailing type were produced by the vortex generator 70 and they produced a distinct vortex pattern for about five times the depth of the channel and to reduced turbulence for possibly fifteen times the depth of the channel. This would indicate that the distance between two rows of fins should ideally be about thirty times the height of a fin although in practice this distance could perhaps be varied between the limits of one and fifty times the height of a fin, depending on requirements of individual heat exchangers.

The glass plate was then replaced by the top member of the duct shown in FIGURES 13 to 15 and a series of tests were carried out first with the duct empty and then with each of the four vortex generators in the duct in turn. Various measurements were made and the results of the test are shown in FIGURES 20-24.

FIGURE 20 shows the pressure drop which was produced by the addition of a vortex generator as compared with the pressure drop for an empty duct. The readings on the graph are those of the static head probes 9 to 18.

FIGURE 21 shows the variation in velocity and temperature profiles with and without vortex generator VG1. The graph indicates that the fluid flow was translated to one side of the duct because of the fins all having the same inclinations.

In both FIGURES 20 and 21 readings are shown for an average velocity head of 0.5 inch water gauge and in FIGURE 21 the search positions correspond to the probe positions 1, 2, 3, 4 and the search levels A, B, C and D in FIGURE 14.

FIGURES 22 and 23 are graphs showing the variation in heat transfer with and without vortex generators (a plot of the Nusselt Number against the Reynolds Number) and the variation in pressure drop with and without vortex generators respectively. The five lines on each graph represent the empty duct and vortex generators 1-4 which correspond to the vortex generators in FIGURES 16-19 respectively.

FIGURE 24 is a plot of heat transfer ratio against pressure drop ratio and the individual points are calculated from FIGURES 22 and 23. The lines are lines of equal surface area heat exchanges determined by calculation.

It must be mentioned that all the results are the average results for tests with the vortex generators fixed first to the heated side or bottom of the duct and then to the unheated or top side of the duct. There did not appear to be any marked difference in heat transfer from generators 1 and 2 (those of FIGURES 16 and 17) which projected into the air stream for about three quarters of the duct depth whether attached to the top or to the bottom of the duct but for generator No. 4 (that of FIGURE 19) which only projected for about one half of the duct depth there seemed to be a definite increase in heat transfer when it was attached to the bottom of the duct.

FIGURE 24 shows the greater change in heating surface brought by an increase in heat transfer coefficient compared with the change caused by an equivalent increase in pressure drop. Further FIGURE 24 shows that the vortex generator VG4 is probably the best of all the generators tested and would enable the surface area of a heat exchanger to be reduced by about 25%, while generators VGZ and VGl would not be far less efficient. Generator VG3 showed no worthwhile improvement.

I claim:

1. A heat exchanger comprising in combination a first fluid flow path, a second fluid flow path, a heat exchange surface separating the first and second fluid flow paths, and, located in at least one of the flow paths, a number of discrete, trailing vortex-producing vanes, said vanes being of equal height and disposed in at least two rows spaced apart in the direction of fluid flow along the fluid flow path containing the vanes by a distance of from -50 times the height of a vane, the vanes of each row being spaced across the fluid flow path, adjacent vanes in a row being oppositely but equally inclined to the direction of fluid flow along the path so as to produce solely trailing vortices whose axes are parallel to the direction of fluid flow and close to said surface.

2. A heat exchanger as claimed in claim 1 in which the inclination of the fins is less than 25.

3. A heat exchanger as claimed in claim 1 in which the inclination of the fins is 13.

4. A heat exchanger as claimed in claim 1 in which the height of each fin is from inch to 1 inch.

5. A heat exchanger as claimed in claim 1 in which the length of each fin measured along the direction of fluid flow is from A inch to /2 inch.

6. A heat exchanger comprising in combination a first fluid flow path, a second fluid flow path, a heat exchange surface separating the first and second fluid flow paths, and, located in at least one of the flow paths, a number of discrete, trailing vortex-producing vanes, said vanes being of equal height and disposed in at least two rows spaced apart in the direction of fluid flow along the fluid flow path containing the vanes by a distance of 30 times the height of a vane, the vanes of each row being spaced across the fluid flow, adjacent vanes in a row being oppositely but equally inclined to the direction of fluid flow along the path so as to produce solely trailing vortices whose axes are parallel to the direction of fluid flow and close to said surface.

7. A heat exchanger comprising a pair of opposed parallel walls defining a fluid flow path, in which there is upstanding from one of the walls a succession of rows of similar triangular fins, the height of the fins being about one half the spacing between the opposed walls, the rows being spaced apart in the direction of fluid flow by about thirty times the height of the fins, the fins being V inch to one inch long in the direction of flow and the fins in each row being inclined at about 13 to the direction of flow, alternate fins in each row being inclined in opposite senses.

8. A heat exchanger having a heat exchange surface in which there is upstanding from the surface a succession of rows of similar fins, the height of the fins being 4 inch to /2 inch, the rows being spaced apart in the direction of fluid flow by about thirty times the height of the fins, the fins being V inch to one inch long in the direction of flow and the fins in each row being inclined at about 13 to the direction of flow, alternate fins in each row being inclined in opposite senses.

9. A heat exchanger having a pair of opposed parallel walls, in which there is upstanding from one of the walls, a row of similar triangular fins, the height of the fins being A inch to /2 inch, the fins being inch to one inch long in the direction of flow, and the fins being inclined at about 13 to the direction of flow, alternate fins, in the row being inclined in opposite senses and the walls flowing a length beyond the fins in the direction of flow of at least thirty times the height of the fins.

10. A finned tube heat exchanger comprising in combination at least one tube, a plurality of heat transfer fins fitted to the tube and spaced along the latter, and, on each heat transfer fin a row of upstanding triangular, trailingvortex-producing fins, adjacent triangular fins in the row being oppositely inclined to fluid flow over the triangular fins at an angle of less than 25, each triangular fin being between A inch and /2 inch in height and of a length in the direction of said flow of between 4 inch and 1 inch.

References Cited UNITED STATES PATENTS 1,553,093 9/1925 Modine -151 1,940,804 12/ 1933 Karmazin 165-151 2,852,042 9/1958 Lynn 165-474 X 2,965,555 12/1960 Hall 165-481 3,068,905 12/ 1962 Millington et a1. 165179 X FOREIGN PATENTS 51,532 11/ 1941 Netherlands. 123,029 10/1948 Sweden.

OTHER REFERENCES 1,132,163, June 1962, German printed application. 1,160,975, January 1964, German printed application.

ROBERT A. OLEARY, Primary Examiner,

M. A. ANTONAKAS, Assistant Examiner,

Claims

10. A FINNED TUBE HEAT EXCHANGER COMPRISING IN COMBINATION AT LEAST ONE TUBE, A PLURALITY OF HEAT TRANSFER FINS FITTED TO THE TUBE AND SPACED ALONG THE LATTER, AND, ON EACH HEAT TRANSFER FIN A ROW OF UPSTANDING TRIANGULAR, TRAILINGVORTEX-PRODUCING FINS, ADJACENT TRIANGULAR FINS IN THE ROW BEING OPPOSITELY INCLINED TO FLUID FLOW OVER THE TRIANGULAR FINS AT AN ANGLE OF LESS THAN 25*, EACH TRIANGULAR FIN BEING BETWEEN 1/4 INCH AND 1/2 INCH IN HEIGHT AND OF A LENGTH IN THE DIRECTION OF SAID FLOW OF BETWEEN 1/10 INCH AND 1 INCH.