Technical Field
[0001] This invention relates to diffusers.
Background Art
[0002] Diffusers are well known in the art.
Webster's New Collegiate Dictionary (1981) defines diffusers as "a device for reducing the velocity and increasing the
static pressure of a fluid passing through a system". The present invention is concerned
with the most typical of diffusers, those having an inlet cross-sectional flow area
less than their outlet cross-sectional flow area. While a diffuser may be used specifically
for the purpose of reducing fluid velocity or increasing fluid pressure, often they
are used simply because of a physical requirement to increase the cross-sectional
flow area of a passage, such as to connect pipes of different diameters.
[0003] As hereinafter used in this specification and appended claims, "diffuser" shall mean
a fluid carrying passage which has an inlet cross-sectional flow area less than its
outlet cross-sectional flow area, and which decreases the velocity of the fluid in
the principal flow direction and increases its static pressure.
[0004] If the walls of the diffuser are too steep relative to the principal flow direction,
streamwise, two-dimensional boundary layer separation may occur. Streamwise, two-dimensional
boundary layer separation, as used in this specification and appended claims, means
the breaking loose of the bulk fluid from the surface of a body, resulting in flow
near the wall moving in a direction opposite the bulk fluid flow direction. Such separation
results in high losses, low pressure recovery, and lower velocity reduction. When
this happens the diffuser is said to have stalled. Stall occurs in diffusers when
the momentum in the boundary layer cannot overcome the increase in pressure as it
travels downstream along the wall, at which point the flow velocity near the wall
actually reverses direction. From that point on the boundary layer cannot stay attached
to the wall and a separation region downstream thereof is created.
[0005] To prevent stall a diffuser may have to be made longer so as to decrease the required
diffusion angle; however, a longer diffusion length may not be acceptable in certain
applications due to space or weight limitations, for example, and will not solve the
problem in all circumstances. It is, therefore, highly desirable to be able to diffuse
more rapidly (i.e., in a shorter distance) without stall or, conversely, to be able
to diffuse to a greater cross-sectional flow area for a given diffuser length than
is presently possible with diffusers of the prior art.
[0006] Diffusers of the prior art may be either two- or three-dimensional. Two-dimensional
diffusers are typically four sided, with two opposing sides being parallel to each
other and the other two opposing sides diverging from each other toward the diffuser
outlet. Conical and annular diffusers are also sometimes referred to as two-dimensional
diffusers. Annular diffusers are often used in gas turbine engines. A three-dimensional
diffuser can for example, be four sided, with both pairs of opposed sides diverging
from each other.
[0007] One application for a diffuser is in a catalytic converter system for automobiles,
trucks and the like. The converter is used to reduce exhaust emissions (nitrous oxides)
and to oxidize carbon monoxide and unburned hydrocarbons. The catalyst of choice is
presently platinum. Because platinum is so expensive it is important to utilize it
efficiently, which means exposing a high surface area of platinum to the gases and
having the residence time sufficiently long to do an acceptable job using the smallest
amount of catalyst possible.
[0008] Currently the exhaust gases are carried to the converter in a cylindrical pipe or
conduit having a cross sectional flow area of between about 2.5 - 5.0 square inches.
The catalyst (in the form of a platinum coated ceramic monolith or a bed of coated
ceramic pellets) is disposed within a conduit having, for example, an elliptical cross
sectional flow area two to four times that of the circular inlet conduit. The inlet
conduit and the catalyst containing conduit are joined by a diffusing section which
transitions from circular to elliptical. Due to space limitations the diffusing section
is very short; and its divergence half-angle may be as much as 45 degrees. Since flow
separates from the wall when the half-angle exceeds about 7.0 degrees, the exhaust
flow from the inlet pipe tends to remain a cylinder and, for the most part, impinges
upon only a small portion of the elliptical inlet area of the catalyst. Due to this
poor diffusion within the diffusing section there is uneven flow through the catalyst
bed. These problems are discussed in a paper titled, Visualization of Automotive Catalytic
Converter Internal Flows by Daniel W. Wendland and William R. Matthes, SAE paper No.
861554 presented at the International Fuels and Lubricants Meeting and Exposition,
Philadelphia, Pennsylvania, October 6 - 9, 1986. It is desired to be able to better
diffuse the flow within such short lengths of diffusing section in order to make more
efficient use of the platinum catalyst and thereby reduce the required amount of catalyst.
Disclosure of the Invention
[0009] One object of the present invention is a diffuser having improved operating characteristics.
[0010] Another object of the present invention is a diffuser which can accomplish the same
amount of diffusion in a shorter length then that of the prior art.
[0011] Yet another object of the present invention is a diffuser which can achieve greater
diffusion for a given length than prior art diffusers.
[0012] Accordingly, the present invention includes a thin, essentially two sided wall member
disposed within the conduit upstream of a diffuser section inlet and spaced from the
conduit wall surface, the member having a convoluted downstream portion which generates
a plurality of adjacent vortices within the diffuser section rotating in opposite
directions about respective axes extending in the direction of bulk fluid flow adjacent
the plate-like member.
[0013] In one embodiment, the convoluted member is disposed upstream of a diffuser section
inlet and is supported in closely spaced relation to the surface of the diffuser inlet
conduit. The convoluted portion of the member comprises a plurality of adjoining,
alternating, U-shaped lobes and troughs extending in the direction of bulk fluid flow
near the member and terminating at a downstream edge, which is wave shaped. The trough
depth and lobe height increase in the downstream direction, and the troughs and lobes
are contoured and dimensioned such that each trough generates a pair of adjacent vortices
downstream of the downstream edge of the member. The vortices energize the boundary
layer adjacent the diffuser section wall and delay or eliminate its separation therefrom.
Thus, a diffuser can accomplish the same amount of diffusion in a shorter length (i.e.,
can operate effectively with greater diffusion angles) or can achieve a greater amount
of diffusion for a given length than was possible with prior art diffusers. The proper
orientation of the member and its troughs and lobes can also result in the fluid leaving
the troughs with a direction of momentum that carries it toward the diffuser wall
surface.
[0014] It is believed that the vortices generated from each side wall surface at the trough
outlet are large-scale vortices. (By "large-scale" it is meant the vortices have a
diameter about the size of the overall trough depth.) These two vortices (one from
each sidewall) rotate in opposite directions and create a flow field which tends to
cause fluid from the trough and also from the nearby bulk fluid to mix with the fluid
near the diffuser wall surface immediately downstream of the member. The net affect
of these phenomenon is to direct bulk fluid outwardly toward the diffuser wall surfaces
and also to create bulk fluid mixing within the diffuser section, all of which energize
the boundary layer along the diffuser wall thereby improving diffuser performance.
Even if the convoluted member is not close enough to the diffuser wall to energize
the boundary layer and delay separation, or if the diffuser wall is much too steep
to avoid separation, the mixing of bulk fluid within the diffuser caused by the large-scale
vortices may still improve overall diffuser performance.
[0015] The troughs and lobes of the present invention are preferably contoured such that
they flow full (i.e., no streamwise, two-dimensional boundary layer separation occurs
within the troughs). Thus, it is important there is no streamwise, two-dimensional
boundary layer separation of the flow over the member immediately upstream of the
troughs thereof as this would result in separated flow entering the troughs, which
would inhibit the formation of strong vortices. The prevention of streamwise, two-dimensional
boundary layer separation within the troughs is an important consideration in their
design. For example, two-dimensional boundary layer separation may occur if the slope
of the bottom of a trough is too steep relative to the nearby bulk fluid flow direction.
[0016] Preferably the troughs and lobes are U-shaped in cross section taken perpendicular
to the downstream direction and are preferably smoothly curved (e.g., no sharp angles
where trough sidewall surfaces meet the trough floor) to minimize losses. Most preferably
the troughs and lobes form a smoothly undulating surface which is wave-shaped in cross
section perpendicular to the downstream direction.
[0017] According to another aspect of the present invention, it is preferred that the fluid
exiting from each trough have a lateral component of velocity as small as possible
to minimize secondary flow losses. For this reason the trough sidewalls, for a significant
distance upstream of the trough outlet, are preferably parallel to the direction of
bulk fluid flow entering the trough.
[0018] One important advantage of the present invention is its ability to improve diffuser
performance without introducing substantial flow losses as a result of its own presence
in the flow field.
[0019] In accordance with another aspect of the present invention, it is preferred that
the trough sidewalls at the outlet be steep. This is believed to increase the intensity
of the vortex generated by the sidewall. The word "steep" as used herein and in the
claims means that, in cross section perpendicular to the direction of trough length,
lines tangent to the steepest point on each sidewall intersect to form an included
angle of no more than about 120°. Most preferably the walls are parallel to each other.
For purposes of this application, when the walls are parallel the included angle shall
be considered to be 0°.
[0020] Commonly owned U.S. patent application serial number 857,907 filed on April 30, 1986
titled,
Airfoil Shaped Body, by Walter M. Presz, Jr. et al. describes an airfoil trailing edge region with streamwise
troughs and ridges (convolutions) formed therein defining a wave-like, thin trailing
edge. The troughs in one surface define the ridges in the opposing surface. The troughs
and ridges help delay or prevent the catastrophic effects of two-dimensional boundary
layer separation on the airfoil suction surface, by providing three-dimensional relief
for the low momentum boundary layer flow. The present invention, however, is directed
to improving the performance of a diffuser located just downstream of a convoluted
member.
[0021] The foregoing and other objects, features and advantages of the present invention
will be come more apparent in the light of the following detailed description of preferred
embodiments thereof as shown in the accompanying drawings.
Brief Description of the Drawing
[0022]
Fig. 1 is a perspective view which illustrates the use of a convoluted plate to reduce
base drag in accordance with the teachings of related, commonly owned U.S. patent
application serial number 117,770.
Fig. 1A is a view taken generally in the direction 1A - 1A of Fig. 1.
Fig. 2 is a cross sectional view of a two dimensional diffuser incorporating the features
of the present invention.
Fig. 3 is a view taken generally in the direction 3 - 3 of Fig. 2.
Fig. 4 is a graph displaying the results of tests for the embodiment of the present
invention shown in Figs. 2 and 3, as well as the prior art.
Fig. 5 is sectional view of an axisymmetric diffuser incorporating the features of
the present invention.
Fig. 6 is a view taken generally in the direction 6 - 6 of Fig. 5.
Fig. 7 is a sectional view of a two-dimensional "stepped" diffuser incorporating the
features of the present invention.
Fig. 8 is a view taken generally in the direction 8 - 8 of Fig. 7.
Fig. 9 is a sectional view illustrating the use of convoluted plates in a heat exchanger
application in accordance with the teachings of related, commonly owned U.S. patent
application serial number 947,349.
Fig. 10 is a sectional view taken generally along the line 10 - 10 of Fig. 9.
Fig. 11 is a sectional view of a catalytic converter system incorporating the features
of the present invention.
Fig. 12 is a view taken generally in the direction 12 - 12 of Fig. 11.
Fig. 13 is a sectional view showing another embodiment of a catalytic converter system
incorporating the features of the present invention.
Fig. 14 is a view taken generally along the line 14 - 14 in Fig. 13.
Best Mode for Carrying Out the Invention
[0023] As will be more fully described hereinafter, the convoluted wall member of the present
invention is used immediately upstream of or at a diffuser inlet to create fluid flow
downstream of the member which help diffuse the fluid and also energize the boundary
layer along the diffuser wall, whereby the diffuser performance is improved. The fluid
dynamics is similar to the fluid dynamics involved in commonly owned U.S. patent application
serial number 117,770 entitled
Convoluted Plate to Reduce Base Drag, by Robert W. Paterson et al. filed on November 5, 1987 and of which this application
is a continuation-in-part. Figs. 14 and 14A of U.S.S.N. 117,770 are reproduced in
this application as Figs. 1 and 1A. In that application a convoluted plate was disposed
upstream of a blunt end surface of a moving body to reduce base drag by generating
certain fluid dynamic flow patterns downstream of the plate. As described therein,
and with reference to Figs. 1 and 1A, a blunt based article is generally represented
by the reference numeral 200. The article 200 has a smooth, relatively flat upper
surface 202 over which fluid flows in the generally downstream direction represented
by the arrows 204. The article 200 has a blunt base or end surface 206. Without the
convoluted plate the flow along the surface 202 is assumed to separate from the article
along the line 208. For purposes of the present discussion the separation line 208
shall be considered the beginning or upstream edge of the blunt end surface 206.
[0024] A convoluted wall member or plate 210 is mounted on and spaced from the surface 202
by means of support members or standoffs 212, only one of which is shown in the drawing.
The plate 210 has an upstream or leading edge 214 and a downstream or trailing edge
216. While the plate may be fairly thin, the leading edge 214 should be rounded, like
the leading edge of an airfoil, to assure that attached uniform flow is initiated
on both the upper surface 218 and lower surface 220 of the plate. The plate may then
taper to a smaller thickness, if desired, toward the trailing edge 216, such as to
save weight or to minimize base drag of the plate itself.
[0025] A plurality of U-shaped troughs 222 and lobes 224 are formed in the plate. Adjacent
troughs and lobes blend smoothly into each other forming an undulating or convoluted
downstream portion of the plate which terminates as a wave-shape at its trailing edge
216. For vortices to be generated trough depth must increase in the downstream direction,
although trough depth could reach its maximum upstream of the trough outlet and thereafter
remain constant to the outlet. In Fig. 1, the plate leading edge 214 is straight and
the plate is initially flat for a short distance. The troughs and lobes blend smoothly
into the flat portion. Preferably, and as shown, trough depth (and lobe height) are
zero at their upstream ends and are maximum at the downstream edge 216; however, the
plate leading edge 214 could have a low amplitude wave shape, and the trough depth
would increase from that initial amplitude. The contour and shape of the troughs and
lobes is selected such that the troughs flow full throughout their length.
[0026] Since the plate 210 is attached to the article 200, the plate itself creates losses
(i.e. drag) which should be minimized. If one initially considers an imaginary, smooth
plate without convolutions and which is generally parallel, locally, to the surface
above which it is disposed, the peaks and valleys of the troughs and lobes preferably
extend an equal distance above and below such "imaginary" plate.
[0027] The vortices generated by the troughs and lobes on each side of the plate are shown
schematically in the drawing. A vortex, having its axis in the bulk fluid flow direction,
is generated off of each sidewall of each trough. Thus, the trough 226 generates a
clockwise rotating vortex 228 from its right sidewall (as viewed in Fig. 1) and a
counter clockwise rotating vortex 230 from its left sidewall. An adjacent trough 232
on the opposite side of the plate to the left of the trough 226 also generates a counter
clockwise rotating vortex 234 from its right wall which combines with and reinforces
the counter clockwise rotating vortex 230 to form what is effectively a single, stronger
vortex. Similarly, the left sidewall of the trough 236 generates a clockwise rotating
vortex 238 which combines with the clockwise rotating vortex 228 from the trough 226.
[0028] If the plate 210 is properly spaced and oriented relative to both the surface 202
and the blunt end surface 206, then the vortices generated therefrom will energize
the boundary layer flow on the surface 202 downstream of the plate thereby resulting
in the flow remaining attached to the article surface beyond the imaginary separation
line 208. Furthermore, it is believed the bulk fluid flowing from the troughs and
over the surface 202 is directed downwardly (in Fig. 1) into the space behind the
blunt end surface 206 to further reduce the separation bubble which would otherwise
be formed and thereby further reduce base drag.
[0029] For purposes of the following discussion, and still referring to Fig. 1, P is the
peak to peak wave length at the plate trailing edge 216; A is the peak to peak wave
height or amplitude (and may also be referred to as the trough depth); H is the distance
between the surface 202 and the closest wave peaks of the trailing edge 216; and D
is the distance between the trailing edge 216 and the upstream edge of the blunt end
surface which is the separation line 208 as discussed above. Preferably the peak to
peak wave length P is constant over the full length of the troughs.
[0030] Since the vortices do not become fully developed for a distance downstream of the
plate edge 216, and because it is desired to have the vortices energize the boundary
layer upstream of the line 208, it is preferred that the trailing edge 216 be located
a distance D equal to one to two wave amplitudes A upstream of the blunt end surface
206. This does not mean that no benefit would be achieved if D were less than A or
even zero; however, it is believed the advantages would be lessened. Similarly, if
the plate is situated too far upstream from the end surface 206 the vortices might
significantly or completely dampen out before reaching the end surface 206 and thereby
provide little or no benefit.
[0031] The distance H should be sufficiently great to avoid creating secondary flow fields
or blockage adjacent the surface 202 which might disrupt and cause separation of the
boundary layer on the surface 202 before it reaches the line 208. It is believed that
H should be at least about the thickness of the boundary layer. Concurrently, the
distance H should be small to keep the vortices as close to the surface 202 as possible.
The slope ϑ of the trough bottom relative to the bulk fluid flow direction adjacent
the plate cannot be too shallow or too steep. If the slope is too shallow, the strength
of the vortices generated will be too weak or they may not be generated at all as
a result of losses from surface friction. It is believed that ϑ should be at least
about 5°. On the other hand, if the slope is too steep the troughs will not flow full
(i.e., there will be two-dimensional streamwise boundary layer separation within the
troughs). This will hinder the formation of the vortices. It is likely that the greater
the slope, the greater the intensity of the vortices, as long as the troughs flow
full. It is believed that slopes greater than about 30° will not flow full. The optimum
angle for any particular application will need to be determined by experimentation.
[0032] As far as the steepness of the sidewalls of each trough is concerned, substantially
parallel sidewalls at the trailing edge 216 and for a distance upstream thereof are
preferred. The steepness of the sidewalls may be represented by the included angle
C (depicted in Fig. 1), which is the angle between lines tangent to the steepest points
along the opposed sidewalls of a trough. As stated above, the closer the angle C is
to 0°, the better; however, the angle C should be no greater than about 120° at the
trough outlet.
[0033] Preferably the overall length of the plate from its leading edge 214 to its trailing
edge 216 is equal to or slightly greater than the length L of the troughs and ridges.
Excessive length, while not devastating, will also not provide any advantage and will
simply add unnecessary surface drag, cost and weight. As mentioned above, the leading
edge 214 should be rounded and the troughs and lobes should be shaped and sized along
their entire length to assure that the troughs flow full throughout their length and
generate vortices which are sufficiently strong to provide a benefit (i.e., drag reduction)
deemed to be worthwhile considering the needs of the particular application.
[0034] In general, it is believed that the wave length P should be no less than about half
and no more than about four times the wave amplitude A in order to assure the formation
of strong vortices without inducing excessive pressure losses. The sum of the downstream
projected cross-sectional flow areas of the trough outlets should be large enough,
relative to the total downstream projected area of the blunt end surface to have a
worthwhile impact on base drag. Practical considerations such as physical constraints,
cost and weight, and even aesthetics will also have various degrees of impact upon
the final configuration selected.
[0035] Figs. 2 - 3 show another application for the convoluted wall member or plate described
in Figs. 1 and 1A; and that application is the subject of the present invention. With
reference to Figs. 2 - 3, a conduit is generally represented by the reference numeral
300. The conduit 300 includes a fluid delivery section 302 and a diffuser section
304. Both are rectangular in cross section taken perpendicular to the flow direction,
which is in the direction of the arrows 306. The delivery section 302 and the diffuser
section 304 both include flat, parallel sidewalls 308. Thus, the diffuser section
304 provides only two-dimensional diffusion. The plane 310 is at the interface between
the delivery section 302 and the diffuser section 304 and is coextensive with the
diffusion section inlet 312.
[0036] Disposed within the conduit delivery section 302 are convoluted plates 314. One is
attached to the upper wall 316 and the other to the lower wall 318 of the delivery
section by means of legs 320. The large-scale vortices generated by the plates are
illustrated by the spirals 322 and have axes generally parallel to the downstream
direction 306.
[0037] The preceding description of Figs. 1 and 1A with respect to size, shape, contour
and location of the convoluted plate 210 applies equally as well to the convoluted
plate 314, with the plane 310 in Fig. 2 corresponding to the line 208 of Fig. 1 in
diffusers which would separate at the inlet without the presence of the convoluted
plates. Thus, all of the dimensions L, D, H, A and P, as well as the angle ϑ should
be selected in the same manner as described with respect to Figs. 1 and 1A. While
in the application of Figs. 1 and 1A the convoluted plate reduces base drag on a moving
body, the present invention improves the performance of a diffuser by energizing the
boundary layer along the diffuser walls and by causing general mixing and diffusion
of the bulk fluid flow within the diffusing section 304.
[0038] A two-dimensional diffuser like that of Figs. 2 and 3 was tested both with and without
convoluted plates. With reference to Figs. 2 and 3, the diffuser had the following
dimensions: B = 21.1 inch; F = 32.7 inch; and E = 5.4 inch. With respect to the convoluted
plates, D = 2.3 inch; L = 6.3 inch; H = 0.25 inch; A = 2.3 inch; P = 1.7 inch; W
i = 0.5 inch; W
o = 1.2 inch; ϑ₁ = 11°; and ϑ₂ = 15°. The sidewalls of each trough were parallel to
each other, as shown in Fig. 3. While Figs. 2 and 3 depict the test apparatus with
the dimensions hereinabove set forth such dimensions and the relative values of such
dimensions to each other are not intended to limit the invention. For example, the
troughs on both sides of each plate may be identical (i.e., W
i = W
o and a₁ = a₂). Optimum configurations for a particular application may only be obtained
by experimentation using the guidelines set forth herein.
[0039] In both series of tests (i.e., with and without the convoluted plates) the coefficient
of performance of the diffuser was measured for diffuser half-angles α ranging from
about 2° to 10°, which is equivalent to an outlet to inlet area ratio range of from
about 1.4 to 3.1. The results of the tests are displayed in the graph of Fig. 4 wherein
the curve A represents no convoluted plates and B represents the use of convoluted
plates. For diffuser half-angles greater than about 6° (up to at least the maximum
angle tested) the present invention outperformed (in terms of the coefficient of performance)
the same diffuser without the convoluted plates. At a 10° half-angle the present invention
had a coefficient of performance higher than the maximum coefficient of performance
(i.e., at a 6° half-angle) of the diffuser without the convoluted plates and about
25 percent greater than that of the same diffuser with a 10° half-angle and no convoluted
plates. The present invention also delayed the onset of boundary layer separation
to higher half-angles.
[0040] Figs. 5 and 6 illustrate the use of the present invention in conjunction with an
axisymmetric three-dimensional diffuser 400. In that case a convoluted thin wall
member 402 is axisymmetric, with the lobes and troughs extending axially in the downstream
direction 404 and radially (in height and depth).
[0041] The invention can also be used in conjunction with a "stepped" diffuser, as shown
in Fig. 7 and 8. A stepped diffuser may be considered one in which the diffuser half-angle
is extremely steep, such as 90°. This type of diffuser is typically a conduit which
includes a step change (i.e., sudden increase) in passage cross sectional flow area.
In Figs. 7 and 8, fluid flowing a conduit 502 in the downstream direction is represented
by the arrows 500. The conduit has an inlet section 501 of rectangular cross sectional
area and an outlet section 503, also of rectangular cross sectional area. The inlet
section has sidewalls 504 parallel to each other and the downstream direction, and
upper and lower walls 507 also parallel to the downstream direction and to each other.
A step change in the passage cross sectional area occurs at the plane 508. The discontinuity
is only in the upper and lower walls 507. The sidewalls 504 remain parallel past the
discontinuity for the entire length of the outlet section 503.
[0042] Disposed adjacent and supported from each of the upper and lower sidewalls 507 is
a convoluted plate 510 similar to that shown in Figs. 2 and 3. The distance D of the
plate upstream of the plane 508 should be anywhere from about zero to twice the trough
depth and most preferably one to two times the trough depth.
[0043] Although with a stepped diffuser such as shown in Fig. 7 the convoluted plates cannot
keep the flow attached to the walls, they can reduce the distance downstream of the
plane 508 where reattachment of the flow to the upper and lower walls occurs. The
bottom line is that the stepped diffuser will have lower losses than would occur without
the use of the convoluted plates. Also, although in this embodiment a convoluted plate
is disposed adjacent each of the upper and lower walls, a single, perhaps larger (in
terms of wave amplitude) plate disposed in the center of the conduit would also provide
benefits.
[0044] Commonly owned U.S. patent application Serial No. 947,349, entitled
Heat Transfer Enhancing Device by Walter M. Presz, Jr. et al., filed on December 29, 1986 describes a convoluted
plate similar to that described herein but used in heat transfer apparatus to improve
heat transfer across a wall having fluids flowing on both sides thereof. Figs. 9 and
10 of this application correspond to Figs. 8 and 9, respectively, of that commonly
owned application and show a convoluted wall or plate 800 disposed within a tube or
conduit 802 which carries fluid flowing in the direction of the arrow 804. As best
shown in Fig. 10, the plate 800 extends substantially across the tube along a diameter.
The lobes and troughs in the downstream portion of the plate 800 generate adjacent
counterrotating vortices 806, 808 downstream thereof which scrub the thermal boundary
layer from the internal wall surface 810 of the tube and mix the core flow with the
fluid flowing adjacent the wall. The net effect is to increase the coefficient of
heat transfer between the fluid and the wall of the conduit 802 for the purpose of
ultimately exchanging heat energy between the fluid within the conduit and fluid surrounding
the conduit. As shown in Fig. 9, it is contemplated to dispose a plurality of convoluted
plates 800 within the conduit, spaced apart along the axis of the conduit at distances
which will ensure improvement in the heat transfer rate along the entire length of
the conduit. This is, of course, required since the vortices generated by each plate
800 eventually die out due to wall friction and viscous effects.
[0045] While the present invention is not a heat transfer device, it does utilize the same
convoluted plates disposed in a conduit for the purpose of influencing the fluid flow
dynamics in a diffusion section of the conduit downstream of the plate. we have discovered
that such plates generate large-scale vortices to 1) energize the boundary layer to
improve diffuser performance, and 2) increase mixing of the bulk fluid across the
duct in directions perpendicular to the bulk fluid flow, which also improves diffuser
performance.
[0046] A specific application for the wall members of the present invention is in a catalytic
converter system, such as for an automobile. Such a converter system is generally
represented by the reference numeral 900 in Figs. 11 and 12. The converter system
900 comprises a cylindrical gas delivery conduit 902, an elliptical gas receiving
conduit 904, and a diffuser 906 which is a transition duct or conduit between them.
The bulk fluid flow direction is represented by the arrows 905 and is parallel to
the axis 907 of the delivery conduit 902. The diffuser 906 extends from the circular
outlet 908 of the delivery conduit to the essentially elliptical inlet 910 of the
receiving conduit 904. The receiving conduit holds the catalyst bed (not shown). The
catalyst bed is preferably a honeycomb monolith with the honeycomb cells parallel
to the downstream direction. The inlet face of the monolith is at the inlet 910; however,
it could be moved further downstream to allow additional diffusion distance between
the delivery conduit outlet 908 and the catalyst. Catalysts for catalytic converters
and catalyst bed configurations are well known in the art.
[0047] As best seen in Fig. 12, diffusion occurs only in the direction of the major axis
912 of the ellipse. The minor axis of the ellipse remains a constant length equivalent
to the diameter of the delivery conduit outlet 908. Thus, the diffuser 906 of this
embodiment is effectively a two-dimensional diffuser. Disposed within the delivery
conduit 902 is a convoluted plate 914 having a plurality of parallel troughs therein.
The plate 914 extends across the conduit 902 along approximately a diametral plane
which is perpendicular to the major axis 912 of the elliptical gas receiving conduit
804. The plate 914 is attached at its side edges 920 to the conduit 902. The trough
sidewalls 924 are preferably parallel to each other at the downstream wave-shaped
edge 922 of the plate and are also preferably parallel to the ellipse major axis 912.
Although optimum plate size and configuration will need to be determined by experimentation,
using the teachings of the present invention as a guide, it is believed there should
be at least two complete troughs on one side of the plate (there are three in the
embodiment of Fig. 12) and the troughs should have a depth at their downstream edge
which is a large percentage of the available space within the conduit. Although it
is believed that the best results will be obtained when the trough depth direction
is parallel to the major direction of diffusion, it is also believed that improved
diffusion may be obtained with the plate 914 oriented in virtually any direction,
such as perpendicular to the direction shown in Fig. 12.
[0048] If the conduit 902 of Figs. 11 and 12 has a diameter of 2.0 inches, one possible
set of dimensions for the plate 914 is a trough slope ϑ of 15°; a wave amplitude A
of 1.0 inch; a trough width W of about 0.30; and a plate thickness of about 30 mils.
[0049] Figs. 13 and 14 show another embodiment of a catalytic converter system. This system
is generally represented by the reference numeral 950 and comprises a cylindrical
gas delivery conduit 952, an elliptical gas receiving conduit 954, and a diffuser
956 which is a transition duct or conduit between them. The bulk fluid flow direction
is represented by the arrows 958 and is parallel to the axis 960 of the delivery conduit.
The diffuser 956 extends from the circular outlet 962 of the delivery conduit and
smoothly transitions to the elliptical inlet 964 of the receiving conduit, which holds
the catalyst bed (not shown) whose inlet face corresponds with the plane of the inlet
964.
[0050] In this embodiment the converter system is considered to be analogous to the two-dimensional
diffuser described in Figs. 2 and 3. Thus, a pair of slightly curved convoluted plates
966 are disposed within the delivery conduit 952, both extending across the duct in
a direction substantially parallel to a diametral line which is parallel to the minor
axis 968 of the elliptical receiving conduit and perpendicular to the major axis 970.
The plates 966 are positioned close to, but are spaced from, the surfaces of the delivery
conduit which are disposed above and below the diametral line or axis 968.
[0051] The plane 972 represents the axial location along the diffuser 956 where two-dimensional
streamwise boundary layer separation would normally occur without the use of the convoluted
plates 966. The downstream wave-shaped edges 974 must be spaced upstream of the plane
972 in order to delay separation of the flow beyond the plane 972. It is believed
that best results will be obtained if the distance D is about one to two times the
maximum wave amplitude of the plates 966. This allows some downstream distance for
the large-scale vortices generated by the convoluted plates to become more fully developed
before reaching the location of the plane 972.
[0052] In this embodiment the wave forms of the upper and lower plates 966 are out of phase.
The wave forms could also be in phase as shown in the embodiment of Figs. 7 and 8,
which may produce coupling of the generated vortices, and thereby further improve
mixing. Assuming a two-inch diameter for the delivery conduit 952, it is recommended
that the maximum wave amplitude for each plate 966 be between about 0.5 and 0.75 inch.
[0053] Although this invention has been shown and described with respect to preferred embodiments
thereof, it will be understood by those skilled in the art that various changes in
the form and detail thereof may be made without departing from the spirit and scope
of the claimed invention.
1. A diffusing device including a conduit for carrying a fluid in a downstream direction
and having wall means defining the internal flow surface of said conduit, said conduit
including an upstream fluid delivery portion having an outlet end with a first cross-sectional
flow area, a downstream fluid receiving portion having an inlet end of second cross-sectional
flow area larger than said first cross-sectional flow area, said wall means interconnecting
said outlet end and said inlet end whereby fluid diffuses as it travels downstream
from said outlet end into said fluid receiving portion, a thin, vortex generating
wall member disposed within said fluid delivery conduit upstream of said outlet end
and having oppositely facing downstream extending flow surfaces, an upstream edge
and a downstream edge, said member having a convoluted portion comprising a plurality
of adjoining, alternating, U-shaped lobes and troughs extending in the direction of
bulk fluid flow adjacent thereto and spaced from said internal flow surface and terminating
at said downstream edge, said trough depth and lobe height increasing in the bulk
fluid flow direction, the contours and dimensions of said troughs and lobes being
selected to insure that each trough generates a pair of adjacent large-scale vortices
downstream of said outlet end, said pair of adjacent vortices generated by each trough
rotating in opposite directions about respective axes extending in the downstream
direction.
2. The diffusing device according to claim 1 wherein said troughs and lobes initiate
downstream of said upstream edge with substantially zero depth and height respectively.
3. The device according to claim 1 wherein each of said troughs is smoothly U-shaped
along its length in cross section perpendicular to the downstream direction and blends
smoothly with the lobes adjacent thereto to define a smoothly undulating surface which
is wave-shaped in cross section perpendicular to the downstream direction.
4. The device according to claim 3 wherein the peak-to-peak wave amplitude of said
downstream edge is A, and said downstream edge is located between about 1A and 2A
upstream of said delivery conduit outlet end.
5. The device according to claim 3 wherein said outlet end of said delivery conduit
and said inlet end of said receiving conduit are located in substantially the same
plane whereby there is a step-wise increase in cross-sectional flow area substantially
in said plane.
6. The device according to claim 3 wherein said receiving conduit inlet end is spaced
downstream from said delivery conduit outlet end, said device including a diffuser
section joining said outlet end to said inlet end, said diffuser section including
a diffuser which gradually increases in cross-sectional area from said outlet end
to said inlet end.
7. The device according to claim 6 wherein said downstream edge of said wall member
is positioned such that said large-scale vortices generated from said troughs create
mixing of the bulk fluid within said diffuser section and increases the coefficient
of performance of said diffuser.
8. The device according to claim 6 wherein said wall member is disposed sufficiently
close to said internal flow defining surface of said fluid delivery conduit that the
large-scale vortices generated by said member energize the boundary layer within said
diffuser and increase the coefficient of performance of said diffuser.
9. The device according to claim 6 wherein said wall member is located and oriented
within said delivery conduit such that flow separation from the wall of the diffuser
initiates at diffuser half-angles greater than it would otherwise initiate at without
the presence of said convoluted member.
10. The device according to claim 6, wherein said delivery portion of said conduit
is symmetrical about a downstream extending axis.
11. The device according to claim 10, wherein said diffuser is a three dimensional
diffuser and said wall member is symmetrical about said axis.
12. The device according to claim 10, wherein said delivery portion is cylindrical,
and said wall member extends across a diametral plane.
13. The device according to claim 6, wherein said wall member extends across a substantial
portion of the width of said delivery portion of said conduit.
14. The device according to claim 3, wherein the slope of the bottoms of said troughs
relative to the bulk fluid flow direction is between 5° and 30°, each of said troughs
including a pair of facing sidewalls, wherein lines tangent to each of said pair of
sidewalls at their steepest points at said member downstream edge form an included
angle of between 0° and 120°.
15. The device according to claim 3, wherein the slope of the bottoms of said troughs
relative to the bulk fluid flow direction is between 5° and 30°; and each trough includes
a pair of facing sidewalls which are substantially parallel to each other.
16. The device according to claim 6, wherein said device is a catalytic converter
for delivering exhaust gases from said delivery portion into and through said receiving
portion, and wherein said receiving portion has a catalyst bed disposed therein.
17. The device according to claim 16, wherein said delivery portion is cylindrical
and said diffuser and receiving conduit portion are substantially elliptical in cross-section
perpendicular to the downstream flow direction.
18. The device according to claim 17, wherein said diffuser is substantially a two-dimensional
diffuser with diffusion parallel to the major axis of the elliptical cross-section.
19. The device according to claim 17, wherein the direction of trough depth is substantially
parallel to the major axis of the elliptical cross-section.
20. The device according to claim 19, wherein said wall member is disposed substantially
along a diametral plane including the minor axis of said elliptical cross-section,
and said troughs are alternately above and below said plane.
21. A catalytic conversion system including a gas delivery conduit having an outlet
of first cross-sectional flow area, a receiving conduit having an inlet of second
cross-sectional flow area larger than said first cross-sectional flow area and spaced
downstream of said delivery conduit outlet and including a catalyst bed disposed therein,
and an intermediate conduit defining a diffuser having a flow surface connecting said
outlet to said inlet, the improvement comprising:
a thin vortex generating wall member disposed within said delivery conduit upstream
of said outlet and having oppositely facing downstream extending flow surfaces, an
upstream edge and a downstream edge, said member having a convoluted portion comprising
a plurality of adjoining, alternating, U-shaped lobes and troughs extending in the
direction of bulk fluid flow adjacent thereto and spaced from said internal flow surface
and terminating at said downstream edge, said trough depth and lobe height increasing
in the bulk fluid flow direction, the contours and dimensions of said troughs and
lobes being selected to insure that each trough generates a pair of adjacent large-scale
vortices downstream of said outlet and within said intermediate conduit, said pair
of adjacent vortices generated by each trough rotating in opposite directions about
respective axes extending in the downstream direction.
22. The catalytic conversion system according to claim 21, wherein each of said troughs
has a downstream extending floor which has a slope of no less than about 5° and no
more than about 30° relative to the downstream direction.
23. The catalytic conversion system according to claim 22, wherein each of said troughs
is smoothly U-shaped along its length in cross section perpendicular to the downstream
direction and blends smoothly with the lobes adjacent thereto to define a smoothly
undulating surface which is wave-shaped in cross section perpendicular to the downstream
direction.
24. The catalytic conversion system according to claim 23, wherein the peak-to-peak
wave amplitude of said downstream edge is A, and said downstream edge is located between
about 1A and 2A upstream of said delivery conduit outlet.
25. The catalytic conversion system according to claim 22, wherein said downstream
edge of said wall member is positioned such that said large-scale vortices generated
from said troughs create mixing of the bulk fluid within said intermediate conduit
and increases the coefficient of performance of said diffuser.
26. The catalytic conversion system according to claim 23 including a pair of said
wall members spaced apart from each other and disposed adjacent but spaced from opposed
internal surfaces of said delivery conduit.
27. The catalytic conversion system according to claim 23, wherein said delivery conduit
outlet is circular and said receiving conduit inlet is elliptical, and wherein the
direction of the depth dimension of said troughs is substantially parallel to the
major axis of the elliptical inlet.
28. The catalytic conversion system according to claim 23, wherein each of said troughs
includes a pair of parallel, facing sidewalls.
29. The catalytic conversion system according to claim 28, wherein said diffuser is
substantially a two-dimensional diffuser wherein the direction of diffusion is substantially
parallel to said trough sidewalls.
30. The catalytic conversion system according to claim 23, wherein said troughs and
ridges are sized, contoured and arranged to flow full over their length whereby two-dimensional
boundary layer separation on the surface of said troughs and lobes does not occur
during normal operation.