Catalytic Converter

Abstract: Downstream extending convolutions (118,120) in the wall (110, 112) of a diffuser(100) energize the boundary layer and delay separation or permit an increase in the diffusion angle (Y). Such convolutions (118,120) are parti­ cularly useful when rapid diffusion is required in a short distance, such as in automotive catalytic converter systems.

Description: The invention relates to a catalytic conversion system. Catalytic converter system are used in automobiles, trucks and the like 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. 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 Internal 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. To achieve that there is already known 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, wherein said diffuser flow surface includes a plurality of downstream extending, alternating, adjoining, U-shaped troughs and ridges forming a smoothly undulating portion of said flow surface, said undulating portion terminating as a wave-shaped outlet edge, said troughs and ridges initiating with zero depth and height at said delivery conduit outlet and increasing in depth and height to a maximum at said wave-shaped edge, wherein said troughs and ridges are sized and contoured such that each trough generates a pair of large-scale counterrotating vortices, each vortex rotating about an axis extending substantially in the downstream direction, and wherein at said wave-shaped edge there is a step-wise increase in cross-sectional flow area and said wave-shaped outlet edge is spaced upstream from said catalyst bed. A catalytic conversion system of that type is disclosed in EP-patent applications 0 244 335 and 0 318 413. In each of these prior publications the ridges each have a downstream extending peak that is parallel to the downstream direction. As described in the above recited EP-patent applications 0 244 335 and 0 318 413 the provision of the troughs and ridges delay or prevent the catastrophic effect of streamwise two-dimensional boundary layer separation by providing three-dimensional relief for the low momentum boundary layer flow. The local flow area variations created by the troughs and ridges produce local control of pressure gradients and allow the boundary layer approaching an adverse pressure gradient region to move laterally instead of separating from the wall surface. It is believed that as the boundary layer flows downstream and encounters a ridge, it thins out along the top of the ridge and picks up lateral momentum on either side of the peak of the ridge toward the troughs. In corresponding fashion, the boundary layer flowing into the trough is able to pick up lateral momentum and move laterally on the walls of the trough on either side thereof. The net result is the delaying of two-dimensional boundary layer separation because the boundary layer is able to run around the pressure rise as it moves downstream. By delaying the boundary layer separation the same amount of diffusion can be accomplished in a shorter length diffuser by increasing the diffuser angle, or greater diffusion can be achieved for a given length diffuser for a more efficient use of the catalyst. The object of the invention is to provide a catalytic conversion system capable of further delaying boundary layer separation to achieve additional improvements regarding the efficient use of the catalyst. In accordance with the invention this is achieved in that each of said ridges has a downstream extending peak which slopes inwardly toward the central flow area within said intermediate conduit creating a blockage of flow parallel to the downstream direction. By having the peaks sloping inwardly toward the central flow area even better flow distribution results are achieved. The ridges create blockage to the straight through flow (i.e., flow parallel to the downstream direction) and force such flow outwardly away from the center of the duct, toward the bottoms of the troughs. This permits even greater angles of inclination of the trough bottoms without separation occurring. More rapid mixing and a more uniform velocity profile across the duct a short distance downstream of the troughs is possible using such a configuration. A streamlined centerbody can be disposed within the intermediate conduit to create a flow blockage in the downstream direction and to force a portion of the flow toward the troughs and ridges. Preferably each of the troughs has a downstream extending bottom which slopes outwardly away from the central flow area forming an angle ϑ of at least 30° with the downstream direction and the peaks of the ridges form an angle α of at least 30° with the downstream direction. The catalytic conversion system will now be described in greater detail with reference to the drawings, wherein: Fig. 1 is a perspective view of a prior art catalytic converter system. Fig. 2 is a sectional view taken generally in the direction 2-2 of Fig. 1. Fig. 3 is a view taken generally in the direction 3-3 of Fig. 2. Fig. 4 is a cross-sectional illustrative view of a catalytic converter system incorporating an embodiment of the present invention. Fig. 5 is a sectional view taken generally in the direction 5-5 of Fig. 4. Fig. 6 is a sectional view taken generally in the direction 6-6 of Fig. 4. A prior art catalytic converter system, such as for an automobile, is shown in Figs. 1-3. The converter system is generally represented by the reference numeral 800. The converter system 800 comprises a cylindrical gas delivery conduit 802, an elliptical gas receiving conduit 804, and a diffuser 806 providing a transition duct or conduit between them. The diffuser 806 extends from the circular outlet 808 of the delivery conduit to the elliptical inlet 810 of the receiving conduit. The receiving conduit holds the catalyst bed. The catalyst bed is a honeycomb monolith with the honeycomb cells being parallel to the downstream direction. The inlet face of the monolith is at the inlet 810; however, it could be moved further downstream to allow additional diffusion distance between the trough outlets and the catalyst. Catalysts for catalytic converters are well known in the art. The configuration of the catalyst bed is not considered to be a part of the present invention. As best seen in Fig. 3, diffusion occurs only in the direction of the major axis 820 of the ellipse. The minor axis of the ellipse remains a constant length equivalent to the diameter of the delivery conduit outlet 808. In a sense, the diffuser 806 is effectively a two-dimensional diffuser. There is a step change in the diffuser cross-sectional area at the plane 812. The diffuser wall 814 upstream of the plane 812 includes a plurality of U-shaped, downstream extending, adjoining alternating troughs 816 and ridges 818 formed therein defining a smoothly undulating surface. The troughs initiate in the plane of the outlet 808 with zero depth and increase in depth gradually to a maximum depth at their outlets at the plane 812, thereby forming a wave-shaped edge in the plane 812, as shown in Fig. 3. The peaks of the ridges 818 are parallel to the downstream direction and substantially aligned with the inside surface of the delivery conduit. The bottom 822 of the troughs 816 forms an angle ϑ with the downstream direction or the peaks of the ridges 818. Since diffusion takes place only in the direction of the major axis 820 of the elliptical inlet 810, the depth dimension of the troughs is made substantially parallel to that axis. The contour and size of the troughs and peaks are selected to avoid any two-dimensional boundary layer separation on their surface. The stepwise increase in cross-sectional area at the trough outlet plane 812 provides volume for the exhaust flow to diffuse into prior to reaching the face of the catalyst, which is at the outlet 810. The external wall 824 of the diffuser downstream of the trough outlets has an increasing elliptical cross-sectional flow area. It would probably make little difference if the wall 824 had a constant elliptical cross-sectional flow area equivalent to its maximum outlet cross-sectional flow area since, near the major axis of the ellipse, there is not likely to be any reattachment of the flow to the wall surface even in the configuration shown. Such a constant cross-section wall configuration is represented by the phantom lines 826. In that case, the diffuser 806 would be considered to have terminated immediately downstream of the plane of the trough outlets 812; however, the catalyst face is still spaced downstream of the trough outlets to permit the exhaust gases to further diffuse before they enter the catalyst bed. A catalytic conversion system according to one embodiment of the invention is shown in Figs 4-6. The distinction between the diffuser of Figs. 1-3 and that of Figs. 4-6 is that in Figs. 4-6 the ridge peaks 918, rather than being parallel to the downstream direction, are inclined or sloped inwardly toward the center of the duct and present a blockage to flow parallel to the downstream direction. A solid insert 910 disposed within the duct 912 forms the troughs 914 and ridges 916. The outwardly sloped troughs 914 more than compensate for the blockage such that the actual duct cross-sectional flow area increases gradually from the trough inlets to the trough outlets at the plane 920. The cross-sectional flow area then expands suddenly (i.e., stepwise) and continues to increase to the plane 922. The flow area remains constant for a short distance thereafter before it reaches the catalyst bed 924. In tests of a configuration like that shown in Figs. 4-6, the cylindrical inlet conduit 923 was 2.0 inches in diameter. At the plane 922 the cross-sectional area was essentially elliptical, with a minor axis length of about two inches and a major axis length of about four inches. The distance between the trough outlets (the plane 920) and the catalyst face 925 was about 1.4 inches to provide a mixing region. While actual catalyst was not used in the test, the catalyst bed was represented by a honeycomb structure comprised of axially extending open channels of hexagonal cross-section. For each test configuration, at approximately the plane of the catalyst bed outlet, the flow velocity was measured at points over the entire elliptical flow cross-section. An overall velocity "non-uniformity" parameter, V, was calculated as the velocity standard deviation divided by the mean velocity. The lower the value of V for a test configuration, the less variations in flow velocity over the cross-section. V=0.0 means the same flow velocity at every point. In a base-line configuration like that shown in Fig. 4, but without an insert 910 (i.e., without lobes in the diffusing section) the variance V was 2.665. In another test an insert was used, wherein the angle ϑ between the bottom 926 of the troughs 914 and the downstream direction and the angle α between the downstream direction and the peaks 918 of the ridges 916 were both 30°. The axial length L of the troughs was about 1.06 inches; and their depth D at the outlet plane was 1.2 inches. The trough width T was about 0.2 inch, and the ridge width R was about 0.35 inch. Unlike in the drawing Figs. 4-6, bottoms 926 of the troughs and the peaks 918 of the ridges were squared off. And the surfaces 928 were flat. Thus the insert was formed of many relatively sharp internal and external corners. The variance V for that configuration was 2.723, actually worse than the base-line, non-lobed configuration. Another test configuration had the same sharp edges, the same trough and ridge widths, and the same trough axial length as the preceding configuration; however, the angle ϑ was 35° and α was 40°. This increased the trough depth D at the outlet to about 1.6 inches. The variance for that configuration improved to 2.455. The insert was then removed and all the sharp edges and corners were rounded such that it appeared as shown in Figs. 4 and 5. It was retested and the variance dropped significantly to 2.008. The insert was removed again and the width T of the troughs was increased to about 0.28 inch, which decreased the width of the ridges to 0.28 inch. All corners remained rounded. A test of that configuration produced another significant improvement in variance, dropping it to 1.624. Evidently, the previous slots were too narrow relative to their depth at the outlet. It is believed that by having the lobes or ridges extend into the path of the inlet flow stream, a portion of the flow is projected outwardly away from the central flow area or axis of the duct. The adverse pressure gradient within the troughs is reduced, allowing very steep trough angles ϑ. The result is more rapid and more even flow distribution across the conduit downstream of the lobes, particularly near the outer wall. Sharp corners appear to limit any improvement which would otherwise occur. Trough and ridge width also plays an important role. A streamlined centerbody within the lobed section of the duct should produce a similar effect, and could be used in conjunction with the lobes. Thus, the centerbody would present a blockage to the flow parallel to the downstream direction and force a portion of the flow outwardly toward the upper and lower walls. Although not actually tested, one such centerbody 930 is shown in phantom in Fig. 4 and would extend between the sidewalls of the duct (perpendicular to the plane of the drawing), Whether or not a centerbody is used, experimentation with various trough and lobe angles would need to be conducted for each application to determine the best configuration for the application at hand. What is "best" will be different for each application, since the variance V is only one of several parameters which may be important to the operation of the device. For example, the configuration described above with a variance of 1.624 resulted in a 12% increase in back pressure, which is not desirable, although it may be acceptable. For example, it may be better to have a configuration with a higher variance and lower back pressure. Space constraints may also play an important role in configuring the device. As described previously, the exhaust gas delivery conduit 928 is circular in cross-section and the receiving conduit is elliptical because this is what is currently used in the automotive industry. Clearly they could both be circular in cross-section. To have the desired effect of preventing boundary layer separation, it is believed the maximum depth D of the troughs (the peak-to-peak wave amplitude D) will need to be at least twice the 99 % boundary layer thickness immediately forward of the upstream ends of the troughs. It is believed that best results will be obtained when the maximum wave amplitude D is about the size of the thickness (perpendicular to the principal flow direction and to the surface of the diffuser) of the separation region (i. e., wake) which would be expected to occur without the use of the troughs and ridges. This guideline may not apply to all diffuser applications since other parameters and constraints may influence what is best. If X is the distance between adjacent troughs (i. e., "wavelength") at the location of their maximum amplitude D (usually at the diffuser outlet), the ratio of X to D is preferably no greater than about 4.0 and no less than about 0.2. In general, if the amplitude D is too small and or X is too large in relation thereto, stall may only be delayed, rather than eliminated. On the other hand, if D is too great relative to X and/or the troughs are too narrow, viscous losses could negate some or all of the benefits of the invention, such as excessively increasing back pressure. The troughs are designed to flow full. The flow thereby stays attached to the bottoms 926 of the troughs 914 up to the plane 920. While some losses will occur at the plane 920 and for a short distance downstream thereof due to the discontinuity, the troughs and ridges create a flow pattern immediately downstream of the plane 920 which significantly reduces such losses, probably by directing fluid radially outwardly in a more rapid manner than would otherwise occur at such a discontinuity. It is believed each trough generates a single, large-scale axial vortex from each sidewall surface thereof at the trough outlet. 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 move radially outwardly into the "corner" created by the step change in the passage cross-sectional area. The net effect of these phenomenon is to reduce the size of the low pressure region or stagnation zone in the corner. In order that the vortex generated off of the side edge of one outlet is not interfered with by a counterrotating vortex generated off the side edge of the next adjacent trough it is necessary that the side edges of adjacent trough outlets be spaced apart by a sufficient distance. In general, the downstream projection of the area of the solid material between the side edges of adjacent troughs should be at least about one quarter (1/4) of the downstream projected outlet area of the trough. It is further believed that best results are obtained when the sidewall surfaces at the outlet are steep. Preferably, in a cross-section perpendicular to the downstream direction, which is the direction of trough length, lines tangent to the steepest points along the side edges should form an included angle of no greater than about 120°. The closer angle is to zero degrees, the better. In the embodiments of Figs. 4, 5 and 6 the included angle is essentially zero degrees.

Claims: A catalytic conversion system including a gas delivery conduit (923) 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 (924) disposed therein, and an intermediate conduit (912) defining a diffuser having a flow surface connecting said outlet to said inlet, wherein said diffuser flow surface includes a plurality of downstream extending, alternating, adjoining, U-shaped troughs and ridges (914, 916) forming a smoothly undulating portion of said flow surface, said undulating portion terminating as a wave-shaped outlet edge, said troughs and ridges (914, 916) initiating with zero depth and height at said delivery conduit outlet and increasing in depth and height to a maximum at said wave-shaped edge, wherein said troughs and ridges (914, 916) are sized and contoured such that each trough generates a pair of large-scale counterrotating vortices, each vortex rotating about an axis extending substantially in the downstream direction, and wherein at said wave-shaped edge there is a step-wise increase in cross-sectional flow area and said wave-shaped outlet edge is spaced upstream from said catalyst bed (924), characterized in that each of said ridges (916) has a downstream extending peak (918) which slopes inwardly toward the central flow area within said intermediate conduit (912) creating a blockage of flow parallel to the downstream direction. The catalytic conversion system according to claim 1 including a streamlined centerbody (930) within said intermediate conduit (912). The catalytic conversion system according to claim 1 wherein each of said troughs (914) has a downstream extending bottom (926) which slopes outwardly away from the central flow area forming an angle ϑ of at least 30° with the downstream direction. The catalytic conversion system according to claim 3 wherein said peaks (918) form an angle (α) of at least 30° with the downstream direction.

Inventors: EALBA ROBERT HERMAN, PATERSON ROBERT WILLIAM, PRESZ WALTER MICHAEL JR, WERLE MICHAEL JOSEPH, EALBA, ROBERT HERMAN, PATERSON, ROBERT WILLIAM, PRESZ, WALTER MICHAEL, JR., WERLE, MICHAEL JOSEPH
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1990
Main classification: F01N3
IPC codes: F04D29, F01N3, F15D1