Diffuser space for a turbine of a turbomachine

Abstract

A turbine housing defining a pair of volutes with respective outlets divided by a divider wall, includes a diffuser space in the gas flow path between the volutes and the turbine wheel. The diffuser space has an upstream portion having a smaller axial extent than a downstream portion of the diffuser space. The widening of the diffuser space tends to direct exhaust gas entering the diffusion space from at least one side of the divider wall towards the corresponding axial end of the diffuser space. Thus reduces the tendency of this gas to interrupt the flow into the diffuser space of exhaust gas from the other inlet volute.

Claims

1. A turbine, comprising: a turbine wheel having an axis; and a turbine housing defining two volutes, the volutes having respective radially inner openings separated by a radially extending divider wall, the turbine wheel being positioned within the turbine housing for rotation about the axis, and the turbine housing including a diffuser space defined between two shroud surfaces and providing a gas flow path between the volutes and the turbine wheel; wherein the diffuser space includes a first portion at a first radial position, the first portion of the diffuser space having a circumferential-average axial extent which is lower than the circumferential-average axial extent of a second portion of the diffuser space at a second radial position, the second portion of the diffuser space being closer to the rotational axis than the first portion of the diffuser space; and wherein: a circumferential-average distance from the rotational axis to a radially-inner tip of the divider wall is denoted by D.sub.divider; a radius of the turbine wheel is denoted by D.sub.wheel; and the value $\frac{D_{\mathrm{divider}}}{D_{\mathrm{wheel}}}$  is in the range 1.05 to 1.2.

2. The turbine of claim 1, wherein the volutes are symmetrical with respect to each other in a mirror plane perpendicular to an axial direction.

3. A turbocharger including the turbine of claim 1.

4. A turbine, comprising: a turbine wheel having an axis; and a turbine housing defining two volutes, the volutes having respective radially inner openings separated by a radially extending divider wall, the turbine wheel being positioned within the turbine housing for rotation about the axis, and the turbine housing including a diffuser space defined between two shroud surfaces and providing a gas flow path between the volutes and the turbine wheel; wherein the diffuser space includes a first portion at a first radial position, the first portion of the diffuser space having a circumferential-average axial extent which is lower than the circumferential-average axial extent of a second portion of the diffuser space at a second radial position, the second portion of the diffuser space being closer to the rotational axis than the first portion of the diffuser sapce; and wherein a circumferential-average distance from the rotational axis to a radially-inner tip of the divider wall is denoted by D.sub.divider, a radius of the turbine wheel is denoted by D.sub.wheel, a distance from the axis to a radial position within the diffuser space in which the circumferential-average axial extent of the diffuser space is minimal is denoted D.sub.min, and D.sub.min greater than D.sub.wheel+(0.5*(D.sub.divider−D.sub.wheel).

5. A turbocharger including the turbine of claim 4.

6. A turbine, comprising: a turbine wheel having an axis; and a turbine housing defining two volutes, the volutes having respective radially inner openings separated by a radially extending divider wall, the turbine wheel being positioned within the turbine housing for rotation about the axis, and the turbine housing including a diffuser space defined between two shroud surfaces and providing a gas flow path between the volutes and the turbine wheel; wherein the diffuser space includes a first portion at a first radial position, the first portion of the diffuser space having a circumferential-average axial extent which is lower than the circumferential-average axial extent of a second portion of the diffuser space at a second radial position, the second portion of the diffuser space being closer to the rotational axis than the first portion of the diffuser space; and wherein a sum of respective circumferential-average axial widths of the volutes measured at a radially-inner tip of the divider wall is denoted by W.sub.inlet, an axial spacing of the shroud surfaces at the radially-outermost portion of the turbine wheel is denoted by W.sub.throat, and a throat-to-inlet ratio defined as $\frac{W_{\mathrm{throat}}}{W_{\mathrm{inlet}}}$  is in the range of 1.05 to 1.3.

7. The turbine of claim 6, wherein the throat-to-inlet ratio is at least 1.1.

8. The turbine of claim 6, wherein the throat-to-inlet ratio is at most 1.2.

9. The turbine of claim 6, wherein a circumferential-average axial extent of the diffuser space at a radial position for which the circumferential-average axial extent of the diffuser space is minimal is denoted by W.sub.min, and W.sub.min is at least equal to W.sub.inlet*0.7.

10. A turbocharger including the turbine of claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A non-limiting embodiment of the invention will now be described, for the sake of example only, with reference to the following figures, in which:

(2) FIG. 1 is a cross-sectional drawing of a known turbocharger;

(3) FIG. 2 shows the variation with time of the pressure in two exhaust gas volutes of the turbocharger of FIG. 1;

(4) FIG. 3 is composed of FIG. 3(a) and FIG. 3(b) and shows schematically gas flow from the volutes of the turbocharger of FIG. 1 past the turbine wheel;

(5) FIG. 4 is a schematic cross-sectional view of a first embodiment of the invention;

(6) FIG. 5 is a schematic cross-sectional view of a second embodiment of the invention;

(7) FIG. 6 is a schematic cross-sectional view of a third embodiment of the invention; and

(8) FIG. 7 is a cross-section of an element which can be used to define a diffuser space in an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(9) An embodiment of the invention will now be described with reference to FIG. 4. In this figure elements corresponding to respective elements of FIG. 1 are denoted by reference numerals 100 higher. The embodiment is a turbine housing with two symmetrical exhaust gas inlets, and a turbine wheel. The gas inlets receive exhaust gas from respective outlet ports of an exhaust manifold of an engine, such as from respective sets of one or more cylinders of the engine. FIG. 4 is a cross sectional view of a portion of the turbine. The turbine wheel is arranged for rotation about an axis 101 which lies in the plane of the cross-section, and is the horizontal direction in FIG. 4. This is referred to as the “axial direction”. The turbine wheel includes blades 109. The inlets feed exhaust gas to two respective volutes 119a, 119b arranged in a twin-volute configuration. The volutes have respective openings into a diffuser space 140 defined between two shroud surfaces 123, 124. The volutes 119a, 119b and the diffuser space 140 may be substantially rotationally symmetric about the axis 101.

(10) The diffuser space 140 extends radially inwardly towards the axis 101, and defines a gas flow path from the volutes 119a, 119b to the turbine wheel. In the following description the gas flow direction is assumed to be radially inward in the diffuser space 140, but in reality it may also include a circumferential component in a direction transverse to the plane of FIG. 4.

(11) The volutes 119a, 119b are divided by a divider wall 120 having a radially-inward tip 122. The tip 122 is at a distance D.sub.divider from the axis 101. The axial extent of the diffuser space 140 at this radial position is denoted as W.sub.inlet. This axial extent is the gap between the shroud surfaces 123, 124 at this radial position. In fact, the radial position D.sub.divider happens to be the radial position at which the axial extent of the diffuser space 140 is minimal. The shroud surfaces 123, 124 define respective corners 123a, 123b (as viewed in cross-section) which are in radial register with the tip 122 of the divider wall 120.

(12) Each blade 109 of the turbine wheel extends to a respective tip which has a maximum distance from the axis 101 denoted by D.sub.wheel. The axial extent of the gap between the shroud surfaces 123, 124 at this radial position is denoted by W.sub.throat. The axial extent of the tip of the blade is denoted by W.sub.tip (note that the tip of the blade may also have a longitudinal component in the circumferential direction and/or the radial direction). This is illustrated in FIG. 4 as being significantly less than W.sub.throat, but in other embodiments W.sub.throat and W.sub.tip may be substantially equal. For example, W.sub.tip may be less than W.sub.throat by an amount (e.g. less than 10 microns) selected to ensure that there is clearance between the blade 109 and the surfaces 123, 124 despite machining and casting tolerances in the formation of the turbine housing and the turbine wheel.

(13) The value of W.sub.throat is greater than W.sub.inlet, preferably by a factor in the range 1.05 to 1.3, and more preferably in the range 1.1 to 1.2. The value of the value

(14) $\frac{D_{\mathrm{divider}}}{D_{\mathrm{wheel}}}$
is in the range 1.05 to 1.2. In numerical simulations it has been found that, particularly when this condition is met, the gas flows entering the diffuser space 140 from the respective volutes 119a, 119b are drawn to the axial ends of the diffuser space 140, and have a reduced risk of interfering with each other compared to known turbine housings in which there is no diffuser space, or the diffuser space has substantially the same axial extent at all radial positions between the turbine wheel and the volutes.

(15) Note that in this embodiment the shroud surfaces 123, 124 are both substantially flat, as viewed in cross-section, in the range of radial positions between the respective corners 123a, 124a and the radial position D.sub.wheel from the axis 101. In this range of radial positions, the shroud surfaces 123, 124 diverge in the radially-inward direction, that is in the direction of radial gas flow. Both the shroud surfaces 123, 124 are inclined to the radial direction (i.e. at an angle to the radial direction which is greater than zero degrees (such as at least 2 degrees)).

(16) Turning to FIG. 5, a second embodiment of the invention is shown. Elements corresponding to those of FIG. 4 are denoted by reference numerals 100 higher. In contrast to the first embodiment, the shroud surface 224 is radial (i.e. includes no component transverse to the rotational axis 201) between the corner 224a and the radial position which is D.sub.wheel from the axis 201. Another difference between the embodiments of FIGS. 4 and 5 is that in the embodiment of FIG. 5, the tip of the blade 209 extends across the whole axial extent of the diffuser space 240. That is W.sub.tip is substantially equal to W.sub.throat, less a small clearance space. Although in this embodiment only the shroud surface 223 is inclined to the radial direction, this diffuser space 240 tends to reduce interaction between the gas flow paths of gas entering the diffuser space from the respective volutes 219a, 219b to either side of the tip 222 of the divider wall 220, compared to known arrangements in which there is no diffuser space, or the diffuser space has unvarying axial extent at different radial positions.

(17) FIG. 6 shows a third embodiment of the invention. Elements corresponding to those of FIG. 4 are denoted by reference numerals 200 higher. The shroud surfaces 323, 324 have a minimal mutual spacing (denoted by W.sub.min) which is at a distance D.sub.min from the rotational axis 301. In contrast to the embodiments of FIGS. 4 and 5, D.sub.min is different from (and in this case, less than) D.sub.divider. That is, the radial portion of the diffuser space 340 which has minimal radial extent is closer to the turbine wheel than the tip 322 of the divider wall 320.

(18) W.sub.min is at least equal to W.sub.inlet*0.7, and more preferably at least W.sub.inlet*0.8 or even at least W.sub.inlet*0.9. It is desirable that D.sub.min is greater than D.sub.wheel+(0.5*(D.sub.divider−D.sub.wheel) Indeed, in variations of the third embodiment D.sub.min may be greater than D.sub.divider.

(19) Also in contrast to FIGS. 4 and 5, the shroud surfaces 323 and 324 are curved, rather than having a sharp corner. Gas streams entering the diffuser space from the respective volutes 319a, 319b are drawn apart as they reach a radial position which is less than D.sub.min from the axis 301, and thus have reduced risk of interference.

(20) In all of embodiments of FIGS. 4 to 6, the radial position for which the diffuser space 140, 240, 340 has the maximum axial extent is the one at a distance D.sub.wheel from the rotational axes 101, 201, 301. This maximum axial extent is therefore equal to W.sub.throat. However, embodiments are possible in which this is not the case. In other words, the diffuser space may (in the radially inward direction towards the turbine wheel) become first wider and then narrower.

(21) In all of the first to third embodiments we may define: CA.sub.housing which denotes the critical area of the housing, CA.sub.wheel which denotes the critical area of the wheel; and the housing-to-wheel-area ratio which is defined as

(22) $\frac{C{A}_{\mathrm{housing}}}{C{A}_{\mathrm{wheel}}}.$
This parameter is typically in the range 0.9 to 1.3, and preferably between 1.1 and 1.3. This parameter is particularly important for applications with high pulsing.

(23) FIG. 7 depicts a portion of an element 400 which may be used to define the shroud surfaces in an embodiment of the invention. Elements corresponding to respective elements of FIG. 4 are given reference numerals 300 higher. The element 400 is toroidal (ring-like), and is circularly symmetric (excepting the support elements mentioned below) about a symmetry axis which in use is positioned co-incident with the rotational axis of the turbine housing. FIG. 7 shows a cross-section through the element 400 at one angular position (i.e. one circumferential position) about the symmetry axis of the element 400. At this angular position, the element 400 has a radial extent L.sub.P.

(24) The element 400 has two inner shroud surfaces 423, 424 which are the inner surfaces of respective walls 423b, 424b. A diffuser space 440 is defined between the shroud surfaces 423, 424. The element 400 also includes a portion 420 which constitutes a divider wall. Note that the portion 420 and the walls 423b, 424b are maintained in the relative configuration shown in FIG. 6 by support elements which are not visible in FIG. 7 because they are not in the plane of the cross-section.

(25) FIG. 7 illustrates various numerical parameters of the element 400. The openings of axial extent F.sub.P which face away from the rotational axis of the turbine, are arranged to receive exhaust gas from respective ones of the volutes. The opening of axial extent F.sub.T, which faces towards the symmetry axis of the element 400, is arranged to transmit the exhaust gas towards the turbine wheel. The distance F.sub.M corresponds to W.sub.min. The distances F.sub.E denote the distance from the tip 422 of the divider wall 420 to the respective points on the surfaces 423, 424 which are separated by a distance W.sub.min. These points are respective distances L.sub.D and L.sub.N from the radially inner and outer ends respectively of the portion of the element 400 at this angular position, where L.sub.N+L.sub.D=L.sub.P.

(26) Many variations of the embodiment are possible within the scope of the invention. For example, the volutes need not be symmetrical with each other.

(27) In another example, the diffuser space may include vanes which project through the diffuser space from one of the shroud surfaces towards the other. Optionally, in the case of the element 400 of FIG. 7, the vanes may be used as the support elements which maintain the relative axial positions of the divider wall 420 and the walls 423b, 424b. Alternatively, in all the embodiments described above, the vanes may be movable axially relatively to the shroud surfaces 123, 124, 223, 224, 323, 324, 422, 424 to selectively open and close the diffuser space under the control of an actuator mechanism.