EP1711769A1

EP1711769A1 - Heat exchanger

Info

Publication number: EP1711769A1
Application number: EP05702138A
Authority: EP
Inventors: Youming Calsonic Kansei UK Limited YUAN
Original assignee: Calsonic Kansei UK Ltd
Current assignee: Marelli Automotive Systems UK Ltd
Priority date: 2004-02-05
Filing date: 2005-02-07
Publication date: 2006-10-18
Also published as: WO2005075917A1

Abstract

A gas side heat transfer surface of the heat exchanger (10) includes spaced fins having respective louvers (13). The louvers (13) are formed in a plurality of adjacent banks (13a, 13b) of likewise extending louver elements (13), louver elements in adjacent banks (13a, 13b) being offset in a staggered relationship. Typically the louver elements (13) are inclined to the general plane of the fin (12) and adjacent partner louver elements in adjacent banks (13a, 13b) are staggered out of alignment with one another.

Description

Heat Exchanger

The present invention relates to a heat exchanger and in particular to an automotive heat exchanger having a louvered fin gas side arrangement which facilitates improved thermal exchange between a gas side and another side of gas, liquid or a two phase flow.

Particularly for automotive applications, engine compartment space is at a premium and compact designs of automotive heat exchangers (radiators, HVAC condensers, oil coolers etc) are desired.

This invention applies particularly to heat exchangers involving heat exchange between a gas side and another side of gas/liquid/two-phase flow. To make such heat exchanger compact, the gas side heat transfer surface is often finned with enhanced louvered surface. Examples are automotive radiators, condensers, evaporators, charge air coolers etc. The present inventions aims to provide means for further performance enhancement of such heat transfer surface which will lead to heat exchanger performance improvement and cost reduction.

The compact high performance heat exchanger requires overall low thermal resistance over unit volume. The dominant thermal resistance of many automotive gas-liquid or gas and two-phase fluid heat exchangers lies with the gas side, which usually accounts for 80% and over of the total heat exchanger thermal resistance. Therefore it is industrial practice to enhance the gas-side heat transfer with secondary heat exchange surface. It is also common practice to further enhance the gas-side heat transfer by adding louvers to the gas-side secondary heat transfer surface which will increase the heat transfer coefficient of such surfaces.

It is well known that increasing the surface area between two media will improve the thermal exchange from one to the other. It is for this reason that heat exchangers such as car radiators comprise fins (typically folded serpentine- form fins) . It is also well known that perforated louvers formed along each fin further enhance the heat transfer between the two media. The louvers act to steer the air flow close to the surface of the fins allowing a more effective exchange of heat from the gas to the fin or vice versa.

A general prior art of louvered fin arrangement is shown in Figures 1 and 2. The mechanism of the heat transfer enhancement of such louvered fins is through the repetitive interruption and reformation of aerodynamic and thermal boundary layers. The heat transfer coefficient at the start of the freshly formed boundary layers is significantly higher compared to that of a fully developed boundary layer. This heat transfer coefficient drops rapidly as flow goes further downstream along the boundary layer.

The augmentation of heat transfer along the boundary is found to occur when

1. There are many louvers - the greater the number of louvers the better in order to take advantage of the much higher heat transfer offered by the leading edge of the freshly formed boundary layers;

2. There is good alignment of the gas flow with the louver; and when 3. The interaction of the thermal wake of the upstream louvers with the downstream louvers is avoided or kept to a minimum.

An Improved arrangement has now been devised. According to a first aspect, the present invention provides a heat exchanger having a gas side heat transfer surface system including spaced fin elements provided with respective louver arrangements, wherein a said respective louver arrangement comprises a plurality of adjacent banks of likewise extending louver elements, louver elements in adjacent banks being offset in a staggered relationship.

It is preferred that the louvers are inclined, preferably at a common angle of inclination.

Louver elements in adjacent banks are preferably offset to opposed respective sides of a median plane of the respective fin element. In one embodiment the fin element may comprise two banks only of offset louvers. In other embodiments more louvers may be provided although it is proposed that five and possibly six may be the maximum desirable number of louver banks (or groups) of offset louvers .

The louver elements in adjacent banks are preferably offset in staggered relationship such that most closely adjacent partner louver elements in respective banks are staggered out of alignment. Typically the louvers are inclined and the partner louver elements in respective banks are staggered out of inclined alignment.

The louver elements are typically flat substantially planar elements providing a substantially flat or planar surface for fluid flow thereover. Other Preferred features of the invention are presented in the appended claims.

It has been found that louver elements of length (or pitch) in the range of 0.25-2mm produce optimal performance. However, a small pitch alone may be insufficient for improving the heat transfer performance since as the ratio of louver pitch to fin separation (or fin pitch) is reduced to a threshold value, the air flow is found to by-pass the louver elements.

Thus, in a first embodiment of this invention, the heat transfer along the fin may be improved by splitting consecutive louvers in half, lengthwise, and offsetting the centre of each louver by translating respective halves in opposite directions, perpendicular to the plane of the fin. In a second embodiment of this invention, a central bridge section is asymmetric with the adjoining half louvers or margins having a different pitch and angle of tilt.

In a third embodiment of the invention, and in conjunction with the second embodiment, the first one, two or more louvers arranged on the downstream side of the bridge section are offset through a distance greater than the offset associated with the other louvers in the downstream section.

The invention will now be further described, in specific embodiments, by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a prior art airway fin;

Figure 2 shows a plan view of the cross-sectional plane A-A of figure 1; Figure 3 highlights the problem when the louver pitch-to- fin pitch ratio falls below a threshold value;

Figure 4 shows an arrangement in accordance with the invention, the louvers in banks to overcome the problem of figure 3;

Figure 5 shows the arrangement whereby the central bridge zone is flat;

Figure 6 shows the arrangement whereby the central bridge zone is asymmetrical;

Figure 7 shows the increased offsetting of the first two louvers on the downstream side of the symmetric bridge zone;

Figure 8 shows the arrangement of figure 7 with an asymmetrical bridge zone; and

Figure 9 is plot representative of the heat transfer coefficients for the various embodiments disclosed in this document as a function of the air flow speed;

Figure 10 is a sectional view of a further alternative louver arrangement in accordance with the invention;

Figure 11 is a detailed view of the arrangement of figure 10; and

Figures 12 to 15 are plots showing performance for varying louver number, pitch, fin pitch, louver angle and designs.

The configuration of a typical heat exchanger (10) is shown figure 1. Heat exchanger (10) is representative of an automotive heat exchanger such as an engine coolant radiator, HVAC condenser or the like. In its simplest form the heat exchanger (10) consists of a series of fins (12) situated between and in contact with tubes (11) containing a gas or liquid or other medium.

With reference to figure 2, each fin (12) contains a flat entrance region (15) followed by an upstream zone of louvers (13) arranged at a specific angle to the plane of the fin, a bridge zone (14), a second (downstream) louver zone in which the louvers (13) are the mirror image of the first louvered section in a plane normal to the bridge zone (14), and finally a flat exit region (16) . It should be emphasised that in its broadest aspect the invention encompasses fin arrangements with or without such bridge turn around zones (14), and mirror image upstream/downstream louver zones. The fin may simply comprise a louvered zone intermediate an inlet and an exit zone.

Air flowing from the left as shown in figures 1 and 2 will be influenced by the louvers (13) which serve to improve the thermal exchange between the fin (12) and the flowing gas. In this manner the heat associated with the air flow and the medium contained in the tubes (11) are brought into thermal equilibrium.

Fig. 3 shows a louvered fin airway design in which the number of the louvers (13) is increased by halving the louver pitch compared to the prior art shown in Fig. 2.

However, the air flow, shown schematically in Fig. 3, reveals that such short louvers are ineffective as the air flow is simply bypassing them. The fin airway (12a/12b) consists of an inlet straight zone (15), and zones of louvers, (an example of only two zones of louvers is shown in Fig. 4), spaced by a turnaround bridge zone (14) between adjacent louver zones. A downstream outlet straight zone is provided. The widths of the inlet straight zone, exit straight zone and the turnaround bridge zone can be the same or different from each other.

The inlet straight zone (15) has its downstream end (15b) turned at a certain angle equal to the angle of the other louvers, in the upstream louver zone, the width of the turned part is made equal to the louver pitch. The exit straight zone (16) is a mirror image of the inlet straight zone (15) . Each turn-around bridge zone (14) between upstream and downstream louver zones consists of an upstream and downstream angled margin (18), with angles equal to the louver angle, and width of the margin (18) equal to the louver pitch (Lp) . While they are shown to be the same and equal to the louver pitch of the rest of the louvers in Fig. 4, they can be made greater or less than the louver pitch of the rest of the louvers. This embodiment is shown by simulation analysis to increase surface heat transfer coefficient by 15-30 percent or more across a range of air flow velocities, compared to the prior art.

The improved performance is obtained with the arrangement shown in figure 4 particularly because the louvers (13) of figure 3 are split into separate banks (13a) and (13b) , each bank of louvers (13a) , (13b) , being offset a given distance from the median centre plane of the fin, in opposite directions, perpendicular to the median plane of the fin (12) . This is believed to be because, flow is entrained to follow the louver surface and double the number of beneficial leading edge components. Further, the offset nature of the louver elements minimises wake problems to a significant degree.

5 For maximum heat transfer performance the optimal alignment of louver angle with air flow is governed by a relationship between the offsets of consecutive louvers, marked as yu and yd in figure 4, and other parameters like fin gauge (Fg) , louver pitch (Lp) , louver angle (a, typically 10-35

10 degrees) and flow angle (β) , and is given by, cosαr (1) with the optimal condition occurring when, yu + yd = 2lp - Δ sin a tan β - ™Δ cos a (2)

15 where yu is the upward offset and yd is the downward shift as shown in figure 4 .

Normally, yu =yd. 0 In the above formula, and from experimental results, optimum performance is found when the louver angle is in the range of 10 to 35 degrees, the louver pitch is in the range of 0.25 to 2 mm, the fin gauge is in the range of 5 0.05 to 0.15 mm. The length of the inlet, exit and turnaround straight parts, including the angled parts, is typically each in the range of 1.5 to 3 mm.

A further relationship controlling the magnitudes of fin 30 pitch, louver pitch, louver angle, flow angle, and fin gauge is given as below:

[2(/ι+2)-(0.5+cosα)]tan^+sinα<^≤[2(n+2)-(cosα-0.5)]tan^+sinα (3) lp where n is an integer with values in the range of 0 to 4 and fp is the fin pitch, see Fig. 4.

The best value for equation (3) is

— = [2(n + 2) - cos α]tan β + sin a (4) ip

A special case for equations (3) and (4) for the louvered airways without offsetting is given as:

π + 1.25-cosα + -^ sino; tan + sin a -^Δ, cosa ≤ — ≤ lip /zip lp H+1.75-cosα + ^°Z, sin or ta + sm -J^ i_j cos a (5)

and n + l.S -cos + J^Δ_j sina tan/5+sinα _fg/ lp L Up cos a (6)

respectively. Here n again is an integer usually in the range of 0 to 4.

In a further embodiment (shown in figure 5) and building upon that shown in figure 4, the central bridge zone (14) is flat with no marginal louvers (18) projecting along the edges as shown in figure 4.

In an alternative embodiment shown in figure 6 and building upon that shown in figure 4 the central bridge zone (14) of figure 4 is made asymmetric such that the margins (18) either side of the central flat section, and the respective angle associated with each louvers are different. The ratio of the length of the downstream angled margin (18b) to the length of upstream angled margin (18a) is in the range of 1-2. The angle of the upstream angled margin (18a) is in the range of 8-20 degrees and the angle of the downstream angled margin (18b) is in the range of 26-40 degrees. These have been found to be the optimum parameters. In yet a further embodiment (shown in figure 7) and building upon that shown in figure 4, improved thermal exchange is found to occur if the offset (yu) of the first one or two louvers (13x) downstream of the bridge zone (14) is greater than the offset associated with all the other louvers (13y) as shown in figure 7.

A further embodiment, and one in which the greatest thermal exchange is found to take place is shown in figure 8. In this embodiment the central asymmetric bridge zone (14) of figure 6 is incorporated into the embodiment of figure 7, and is found to increase fin surface heat transfer by over 40% as shown in figure 9. The remaining plots in figure 9 show the comparative performance of the various embodiments of the invention.

The arrangement of figures 10 and 11 in accordance with the present invention extends the idea of the design to offset louvers in a way that further enhances heat exchanger performance. In prior art as shown in Fig. 1, in order to achieve good alignment of airflow with the louver, the louver angle has to be relatively high, which means the airflow has to turn a great angle that can be cause for high pressure loss. The louver pitch to fin pitch ratio needs to be relatively high as well, which means a limitation to the number of louvers. The main feature of -lithe design of figures 10 and 11 is to push the louvers out of the respective fin plane to align with the flow instead of forcing the flow to align with the louver and therefore the louver angle needs not to be very high. Also the configuration of the louver push-out is so arranged that minimal adverse thermal interaction between consecutive louvers is realised. Furthermore, as good flow louver alignment is not limited by low louver pitch to fin pitch ratio, a smaller louver pitch and thus a greater number of louvers can be used, which makes full use of higher heat transfer at the leading edges of louvers.

In the airway fin arrangement shown in figures 10 and 11, each of the 2 adjacent airway fins shown consists of an inlet straight part 115, an exit straight part 116 and intermediately thereof groups of lovers cut and formed in the fin in four banks of lovers, two banks pushed-out spaced either side of the medial plane of the fin. In the same manner as the embodiments shown in figures 1 to 8 include a turn-around bridge zone 14 and upstream and downstream mirror image louver zones, the embodiment of figures 10 and 11 could also include such features. It should be emphasised that in its broadest aspect the invention encompasses fin arrangements with or without such bridge turn around zones, and mirror image upstream/downstream louver zones. Fig. 11 shows a schematic detailed view of the louver arrangement of figure 10, illustrating the relevant parameters of the group of louvers. Specifically, a group of n louvers, where n=4 (i.e. 4 banks of louvers are provided), is cut from the flat thin base metal sheet of the fin, the first louver is pushed out of the base metal sheet plane laterally to one side by a distance of Y_{l r} then the second louver is pushed out by a distance of Yχ-ΔYι, and the ith louver in the group is pushed by a distance of Yi=Yl-∑ΔYj (summing over Kj<ι- 1) . When Y is negative, it means the ith louver will be pushed out to the other side of the base sheet plane by a distance of the absolute value of Y_L . The pushed out louvers are also turned by an angle of θ. The range of parameters are varied as follow: 0<Yι<0.75fp, where fp is fin pitch, -5°< θ<15°, 0.25mm < lp < 2mm. ΔYi can vary although it is generally constant with ΔYi=ΔY=fp/n.

Fig. 12 shows the comparison of the performance of the design of figure 10 with a prior art design as in figures 1 and 2 and a design of an earlier described embodiment in accordance with the invention (figures 4 to 8) with the prior art. The performance has been plotted in terms of heat transfer coefficient h/hben and pressure loss coefficient f/fben. Plot A is for the design of figure 10 f/fben compared with the prior art. Plot B is for the design of figure 10 h/hben compared with prior art. Plots C and D are corresponding plots for the design of the earlier disclosed embodiment in accordance with the invention (figures 4 to 8) compared with prior art. The results show that compared to the prior art, heat transfer coefficient can be increased by up to 65% and the pressure loss coefficient is increased by about the same amount. Compared to the airway design presented in earlier embodiments (figures 4 to 8) , the heat transfer coefficient is increased significantly more than the increase in pressure loss coefficient, as can be seen from Fig. 12.

Fig. 13 shows the effects of louver angle on the performance parameters. Good performance can be achieved for a louver angle range of 0° to 7.5° and this louver angle range is recommended, although lover angles of up to 10 degrees have been found to give beneficial results. Fig. 14 shows the effects of the louver pitch on the performance parameters. A range of louver pitch up to 0.4 to 1 mm offers good performance. Although further lower the louver pitch value will enhance heat transfer and perhaps the volume goodness factor further, as can be expected by extrapolating the curves in Fig. 14, difficulty in manufacturing and high pressure loss may prohibit using a louver pitch much lower than 0.4 mm. So a louver pitch range of 0.4 mm to 1 mm is recommended.

Fig. 15 shows the effects of number of louvers in each of the repeated offset louver groups on the performance parameters. A louver number of 3 to 5 louvers shows good performance and is therefore recommended. This equates to a preferred number of banks of louvers for a fin being in the range 3 to 5.

Claims

Clai s :

1. A heat exchanger having a gas side heat transfer surface system including spaced fin elements provided with respective louver arrangements, wherein a said respective louver arrangement comprises a plurality of adjacent banks of louver elements, either likewise inclined or extending in a generally likewise direction, louver elements in adjacent banks being offset in a staggered relationship.

2. A heat exchanger according to claim 1, wherein: i) a said respective louver arrangement includes a gas upstream louver zone comprising a plurality of likewise inclined louver elements, a gas downstream louver zone comprising a plurality of likewise inclined louver elements and a bridge zone intermediate between the gas upstream louver zone and the gas downstream louver zone, preferably wherein the louver elements in the upstream louver zone and the downstream louver zone are inclined in opposed senses but at a common inclined angle; and/or ii) the louver arrangements of spaced fin elements are configured such that louver elements of adjacent fin elements are positioned in inclined alignment with one another.

3. A heat exchanger according to any preceding claim, wherein louver elements in adjacent banks are offset to opposed respective sides of a median plane of the respective fin element.

4. A heat exchanger according to any preceding claim, wherein louver elements in adjacent banks are offset in staggered relationship such that most closely adjacent partner louver elements in respective banks are staggered out of inclined alignment.

5. A heat exchanger according to claim 4, wherein louver elements in adjacent banks are inclined in- line with one another, preferably wherein the space between in-line louver elements in adjacent banks is greater than the louver pitch distance (LP) , preferably wherein the space is substantially 2 times the louver pitch distance (2 x Lp) .

6. A heat exchanger according to any preceding claim, wherein: i) a said respective louver arrangement includes a gas upstream louver zone comprising a plurality of likewise inclined louver elements, a gas downstream louver zone comprising a plurality of likewise inclined lower elements, and a bridge zone intermediate between the gas upstream louver zone and the gas downstream louver zone, the first one, or first and further ones, of the louver elements of the downstream louver zone being offset to a greater degree than other louver elements in the downstream louver zone, and/or ii) a said respective louver arrangement includes a gas upstream louver zone comprising a plurality of likewise inclined louver elements, a gas downstream louver zone comprising a plurality of likewise inclined lower elements, and a bridge zone intermediate between the gas upstream louver zone and the gas downstream louver zone, an inlet flat zone being provided upstream of the upstream louver zone, an outlet flat zone is provided downstream of the downstream louver zone, and the bridge zone includes a flat bridge zone.

7. A heat exchanger according to claim 6, wherein the flat bridge zone includes an inclined upstream margin and a reverse sense inclined downstream margin, preferably wherein: i) the downstream margin is of greater longitudinal extent than the upstream margin; and/or ii) the angle of inclination of the upstream margin is less than the angle of inclination of the downstream margin, preferably wherein: a) the upstream margin is inclined at an angle substantially in the range 8° to 20°; and/or b) the downstream margin is inclined at an angle substantially in the range 26° to 40°; and/or iii) the ratio of the extent of the upstream margin to the downstream margin is substantially 1:2 or greater; and/or iv) the upstream margin width of the bridge zone is substantially in the range 0.2mm - 0.5 mm; and/or v) wherein the downstream margin width of the bridge zone is substantially in the range

0. mm to 0.8mm.

8. A heat exchanger according to any preceding claim, wherein: i) the louver pitch of the louver elements is substantially in the range 0.25mm to 2mm, more preferably 0.4mm to 1mm, and/or ii) the louver elements are inclined at an angle substantially in the range -5 to 35 degrees, more preferably substantially in the range -5 to 12.5 degrees, most preferably 0 to 7.5 degrees .

9. A heat exchanger according to any preceding claim, wherein the number of banks of offset louvers is in the range 3 to 5, preferably wherein the louver elements are inclined at an angle -5 to 12.5 degrees, most preferably 0 to 7.5 degrees.

10. A heat exchanger having a gas side heat transfer surface system including spaced fin elements provided with respective louver arrangements, wherein a said respective louver arrangement includes a gas upstream louver zone comprising a plurality of likewise, inclined louver elements; a gas downstream louver zone comprising a plurality of likewise inclined louver elements, and a bridge zone intermediate between the gas upstream louver zone and the gas downstream louver zone; wherein the bridge zone includes an inclined upstream margin and a reverse sense inclined downstream margin, and wherein: the angle of inclination of the upstream margin is less than the angle of inclination of the downstream margin; and/or the downstream margin is of greater longitudinal extent than the upstream margin.

11. A fin element for a gas side heat transfer surface system of a heat exchanger, the fin element having a louver arrangement comprising a plurality of adjacent banks of louver elements, either likewise inclined or extending in a generally likewise direction, louver elements in adjacent banks being offset in staggered relationship.