GB2508164A

GB2508164A - Diffuser

Info

Publication number: GB2508164A
Application number: GB201220948A
Authority: GB
Inventors: Paul Eifion Roach
Original assignee: Napier Turbochargers Ltd
Current assignee: Napier Turbochargers Ltd
Priority date: 2012-11-21
Filing date: 2012-11-21
Publication date: 2014-05-28
Also published as: GB201220948D0

Abstract

A diffuser, especially for an exhaust gas turbocharger, which has a turbine rotor with a row of circumferentially arranged blades 20 and is rotatable about a rotation axis to extract energy from a flow of working gas passing between the blades. The diffuser 22 has an inlet which receives the gas from the rotor, and an exit through which the gas is discharged. The flow area of the exit is greater than the flow area of the inlet such that some of the kinetic energy of the received gas is recovered in the diffuser. The diffuser further has a row of circumferentially arranged fences 24 which each extend longitudinally in the streamwise direction of flow through the diffuser and transversely across a portion of the span of the diffuser to redirect the gas flow through the diffuser. Preferably the fences are aligned to within 200 of the axis of the rotor.

Description

D F F Li S ER

Field of the Invention

The present inventon relates to a diffuser, and parflcularly, but not exclusively to a diffuser for an exhaust gas turbine of a turbocharger.

S Background of the Invention

In conventional, non-supercharged internal combustion engines, a negative pressure is formed in the air intake which increases with increasing engine speed and limits the theoreticafly achievable performance of the engine. An exhaust gas turbocharger (also known simply as a turbocharger) is a supercharging system for an internal combustion engine, by means of which the cynders of the internal combustion engine can he charged with higher pressure air.

A turbocharger has a turbine rotor in the engine exhaust stream, which is connected via a shaft to a compressor in the air intake of the engine. The rotor is rotated by the exhaust gas flow and thus drives the compressor, which increases the pressure in the air intake of the engine,. larger quantity of air is introduced into the cylinders of the internal combustion engine, providing more oxygen For combustion, and increasing the mean pressure of the engine. In this way, the engines torque and the power output can he augmented.

The charging process uses energy that would otherwise be lost through the exhaust system, so that a turbocharger can improve the overall efficiency of an internal combustion engine.

A diffuser is commonly located downstream of the rotor to raise the static pressure of the exhaust gas at the expense of its kinetic energy. In this way, the static pressure at the entry to the diffuser is lower than the static pressure at its exit, and so the static pressure at the exit of the rotor is lower than the static pressure at the exit of the diffuser. As the exit of the diffuser is generally at atmospheric conditions, the static pressure at the rotor exit can thus be subatmospheric. This allows the pressure ratio across the turbine to be increased relative to a turbine without a diffuser, and therefore more work can be extracted from the exhaust flow entering the turbine. Th.e consequence is an increase in total-to-static turbine efficiency of typically around three to four percent.

The diffuser may have an annuiar or frustoconical flow passage, and may extend along the axis of the turbocharger and/or radiaily outwardly, employing either straight or curved walls.

With reference to FIgure 1, an axial flow turbine of a turbocharger Is generally IndIcated at 10 and has a principal and rotational axis X-X. The turbine comprises, in axial flow series, an exhaust gas intake 11, a row of guide vanes 12, a rotor 13 having a central disc and a row of blades 14 at the outer side of the disc, and an annular exhaust discharge diffuser 15. An outer casIng 16 generally surrounds the rotor and also defines the outer wall of the diffuser, and a hub casing 17 defines the radially inner wall of the diffuser.

During operation, engine exhaust gas enters the intake 11, is swirled by the guide vanes 12, drives rotation of the rotor 13 and then exits via the diffuser 15. The rotor is connected via a shaft 18 to a compressor wheel (not shown) at the other side of the turbocharger. The compressor is used to compress air entering the engine.

A key to the effectiveness of a diffuser is its area ratio, defined as the ratio of the diffuser exit flow area dMded by Its Inlet area. To maximlse the recovery of energy from the turbine, thIs area ratio should be as large as possible, within the overall geometric constraints of the turbocharger. Designed well, typically 50% of the kinetic energy of the exhaust flow can be recovered by the diffuser. Designed badly, however, flow separation can occur in the diffuser, and it is possible to have lithe or no recovery. Moreover, flow separation is usually an unsteady phenomenon, leading to pressure pulses in both upstream and downstream components of the turbocharger, adversely affecting both the aerodynamic and mechanical performance of those components.

Diffuser performance is a function of a number of geometric properties, including diffuser area ratio, streamwIse length and surface curvature. The fluid properties are also important, and these include diffuser inlet boundary layer blockage, turbulence and velocity profiles, Mach and Reynolds numbers and swirl angle. There remain significant issues in the design of efficient turbine diffusers.

A major problem with diffuser operation is that the gas flow downstream of the turbine rotor can be highly swirling, particularly when operating away from design conditions. A limited amount of swirl (typically less than 20°) in the gas flow can often improve the diffuser performance by energising the boundary layers near its walls. But high swirl (typically greater than 200) can generate a centrifugal pressure field which may cause the diffusion process to break down, leading to reverse flow, unsteadiness and high losses.

Summary oF the nventron

An aim of the present invenUon is to address the issue of high swirl into a diffuser, with a view to improving its performance.

Accordingly, in a first aspect, the present invention provides a diffuser for a turbine rotor which has a row of circumferentiafly arranged blades and is rotatable about a rotation axis to extract energy from allow of working gas passing between the blades, wherein the diffuser has: an inlet which receives the gas from the rotor; an exit through which the gas is discharged, the flow area of the exit being greater than the flow area of the inlet such that some of the kinetic energy of the received gas is recovered in the diffusen and a row of circumferentially arranged fences which each extend longitudinally in the streamwise direction of flow through the diffuser and transversely to the streamwise direction across a portion of a span of the diffuser to redirect the gas flow through the diffuser.

Particularly at conditions ftr removed irorn the design point, the fences can reduce the amount of swirl in the diffuser, and thereby improve the diffuser performance, e.g. by reducing or eliminating flow reversal. The diffuser may be retro-fitted to a turbine which has no diffuser or has a diffuser without such fences.

In a second aspect, the present invention provides an exhaust gas turbine having in flow series a turbine rotor and Ihe diffuser of the first asoect, the rotor having a row of cwcumferentally arranged blades and heng rotatable about a rotabon axs to extract energy from a flow of exhaust gas passing between the blades.

In a third aspect, the present invention provides a turbocharger having the exhaust gas turbine of the second aspect, and further having a compressor and a drive shaft which connects the rotor to the compressor, whereby the rotor drives rotation of the compressor.

Optional features of the invention will now he set out. These are applicable singly or in any combination with any aspect of the invention.

The diffuser may form an annular or frustoconical passage between its inlet and its exit. For example, an annular diffuser may be used with an axial flow turbine, and a frustoconical diffuser may be used with a radial flow turbine.

The fences may extend transversely froni a wall of the diffuser. In a frustoconicat difFuser, the fences may thus extend inwardly from the wail of the diffuser, However, in an annular diffuser, the fences may extend outwardly from the huh side wail and/or inwardly from the shroud side wail of the diffuser. Advantageously, the fences can re-energise the near-wail flow. e.g. by generaflng axiaily-ailgned vorfices at theft traffing edges.

In their Iongitud!nafly extending dftection, the fences may he ailgned to within 20° of the axial dh'ection of the rotor. Preferably, they are ahgned to within 10° or 5° of the axial dfrecflon of the rotor, and more preferably they are ahgned paraflel with the axial direction of the rotor.

The number of fences in the row of fences may be in the range from (N-b) to (N÷10), where N is the number of blades in the row of blades. Preferably the number of fences in the row of fences is greater than (N+6) or less than (N-6).

The axial location of the upstream ends of the fences may be a distance from the trailing edges of the blades of the rotor which distance is less than the traiilng edge span of the blades, and preferably less than one half the trailing edge span.. The axial location of the downstream ends of the Fences may be a distance from the trailing edges of the blades of the rotor which distance is greater than the trailing edge span of the blades, and preferably greater than two times the trailing edge span.

The fences may extend transversely to the streamwise direction of flow across up to 20% of the span of the diffuser. The fences may extend transversely to the streamwise direction of flow across at least 10% of the span of the diffuser.

Each fence may be oF substantially unil rin thickness, and/or each fence may be planar. For example, each fence can be in the form of a flat plate. However, another option is for the fences to have. e.g. aerofoil sections.

Further optional features of the invention are set out below.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically a longitudinal cross-section through an axial fiow turbine of a turbocharger; Figure 2 shows schematically a longitudinal cross-section through an axial flow turbine of a turbocharger having a diffuser according to the present invention; Figure 3 is a three-dimensional view showing wall streamlines derived from a computational fluid dynamics (CFD) analysis of the flow through a diffuser according to the present InventIon; Figure 4 is a plan view showing streamlines in the near wall region oF the diffuser derived from the CFD analysis; and Figure 5 shows flow vectors on longitudinal cross-sections through the diffuser from further CFD analyses (a) with and (b) without fences In the diffuser.

Detailed Descriotion and Further Cotional Features of the Invention Figure 2 shows schematically a longitudinal cross-section through an axial flow turbine of a turbocharger having a diffuser according to the present invention. The turbine has a rotor whIch canles a circumferentlally arranged row of blades 20. The blades extract energy from a flow of exhaust gas. After passing between the blades, the exhaust gas is received is through the inlet of an annular diffuser 22. The flow area of the diffuser increases from its Inlet to its outlet, allowing some of the kinetic energy of the gas to be recovered.

The diffuser has a row of clrcumferentlally arranged fences 24 whIch extend from the Inner (hub) wall of the diffuser longitudinally In the streamwlse direction of flow through the diffuser and transversely across a portion of the span of the diffuser.

Figure 3 is a three-dimensional view of the turbine showing wall streamlines derived from a computational fluid dynamics (CFD) analysis of the flow through such a diffuser. Three neighbouring fences 24 are shown in the view, and a row of vanes 26 upstream of the blades are also illustrated. The streamlines enter the diffuser at a large angle relative to the machine axIs (Z), but the fences re-dIrect the flow more In-lIne with the Z-axls.

Figure 4 is a plan view of the turbine showing streamlines in the near wall region derived from the same CFD analysis. The fences 24 also act as vortex generators, creating strong axially-aligned vortices from the trailing edges of the fences. This also helps to energise the near-wall flow in the diffuser downstream of the fences.

At conditions far removed from the design point, the turbine blades 20 generate high swirl into the diffuser 22, which can lead to flow reversal within the thffuser. The fences 24 address this problem by re-aligning the flow in the streamwise direction, and re-energising the near-wall flow. Figure 5 demonstrates their effectiveness: the flow vectors on a longitudinal oross-section through the diffuser show strong flow reversal without the fences (Figure 5(a)), which is mostly overcome when the fences are present (Figure 5(b)).

The fences may b.c attached to the shroud (outer) and/or the hub walls of the diffuser. They are generally aligned with the machine axis, although some setting angle may be adopted (a typical vaiue might be less than 200), The number of fences is typically that of the rotor blades plus or minus up to ten, hut preferably the number of fences differs from that of the rotor blades by at east plus or minus seven. The axial location of the upstream ends of the fences may be within one rotor blade trailing edge span downstream of the rotor trailing edge. The axial iocation of the downstream ends of the lences may be between the trafling edqe span downstream of the rotor trailing edge and the exit of the diffuser (for example, the fences may extend longitudinally substantially further than shown in the drawings such that the axial location of the downstream ends is over two times the trailing edge span downstream of the rotor trailing edge). The transverse extent of the fences is typically 10 to 20% of the rotor span.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. For example, the turbine may he a radial flow turbine. The diffuser may be a frustoconical diffuser. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

C LA MS1. A diffuser (22) for a turbine rotor which has a row of ciroumferentially arranged blades (20) and is rotatable about a rotation axis to extract energy from a flow of working gas passing between the blades, wherein the diffuser has: an inlet whioh receives the gas from the rotor; an exit through which the gas is discharged, the flow area of the exit being areater than the flow area of the iniet such that some of the kinetic energy of the received gas is recovered in the diffuser; and a row of cimumferentially arranged fences (24) which each extend longitudinally in the strearnwise direction of flow through the diffuser and transversely to the streamwise direction of flow across a portion of a span of the diffuser to redirect the gas flow through the diffuser.
2. A diffuser according to claim 1, wherein the diffuser forms an annular or frustoconical passage between the inlet and the exit.
3, A diffuser according to claim 1 or 2. wherein the fences extend transversely from a wall of the diffuser.
4. A diffuser according to any one of the previous claims, wherein the fences are aligned in theft longitudinally extending direction to within 2O of the axial direction of the rotor.
5. A diffuser according to any one of the previous claims, wherein the number of fences in the row of fences is in the range from (N-b) to (N+b0), where N is the number of blades in the row of blades.
6. A diffuser according to any one of the previous claims, wherein the axial location of the upstream ends of the fences is a distance from the trailing edges of the blades of the rotor which distance is less than the span of the blades at the trailing edge thereof.
7. A diffuser according to any one of the previous claims, wherein the axial location of the downstream ends of the fences is a distance from the trailing edges of the blades of the rotor which distance is greater than the span of the blades at the trailing edge thereof.
8. A diffuser according to any one of the previous claims, wherein the fences extend transversely to the streamwise direction of flow across up to 20% of the span of the diffuser.
9. A diffuser according to any one of the previous claims, wherein the fences extend transversely to the streamwise direction of flow across at east 10% of the span of the diffuser.
10. An exhaust gas turbine having in flow series a turbine rotor and the diffuser of any one of the previous claims, the rotor having a row of drcumferentiaUy arranged blades and being rotatahe about a rotation axis to extract energy from a flow of exhaust gas passing between the Nades.
11. A turbocharger having the exhaust gas turbine of claim 10, and further having a compressor and a drive shaft which connects the rotor to the compressor, whereby the rotor drives rotation of the compressor.