WO2018051127A1

WO2018051127A1 - Pulse optimized flow control

Info

Publication number: WO2018051127A1
Application number: PCT/GB2017/052754
Authority: WO
Inventors: Allen Kun CAO; Ricardo Martinez-Botas
Original assignee: Imperial Innovations Ltd
Priority date: 2016-09-16
Filing date: 2017-09-18
Publication date: 2018-03-22
Also published as: GB201615856D0

Abstract

There is provided a system and method for guiding a flow of fluid having a variable mass flow rate onto a turbine (110), the turbine comprising a blade (115) and configured to rotate about an axis of rotation (150), the method using a flow-guidance element (120) in fluid communication with the turbine, the flow-guidance element comprising a flow-guiding vane (125), wherein a trailing edge of the flow-guiding vane is arranged at an angle of β 4 relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane and wherein the angle β 4 is less than or equal to 80 degrees, the flow-guidance element configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation. The method comprises rotating the flow-guidance element about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid.

Description

Pulse Optimized Flow Control

Field

The present disclosure relates to a method and a flow control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine. In embodiments, it also relates to a turbocharger comprising the flow control assembly and to an engine comprising the turbocharger.

Background

Turbochargers for gasoline and diesel internal combustion engines utilise the heat and volumetric flow of exhaust gas exiting the engine to pressurise an intake air stream that is routed to a combustion chamber of the engine. Specifically, the exhaust gas exiting the engine is routed into a turbocharger in a manner that drives a turbine within the

turbocharger to spin within a housing. The turbine is mounted on one end of a shaft, and a radial air compressor is mounted on the other end of the shaft. Rotary action of the turbine is transmitted via the shaft to cause the air compressor to spin within a

compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and to be pressurized; the air is then mixed with fuel and combusted within the engine combustion chamber.

Turbocharger technology is used to enhance power output in various applications, for example in vehicles, marine crafts, and in manufacturing or power plants. In the example of a reciprocating internal combustion engine, using a turbocharger to harness the energy in the exhaust gas may increase the engine output by 40% or more. Turbochargers can thus be used in combination with smaller, downsized, engines, enabling a reduction in exhaust emissions without a corresponding decrease in the power output. Consequently, as legislation on vehicle emissions becomes ever more stringent, turbochargers are becoming ever more prevalent in engine design.

A reciprocating internal combustion engine generates an unsteady, pulsating, exhaust flow with significant variations in the exhaust mass flow rate, pressure and temperature throughout the engine cycle. When a turbocharger turbine is fed with such a variable flow, the performance of the turbine deteriorates because, critically, current turbocharger designs cannot harness the full energy potential of the pulsating, unsteady, exhaust flow. This leads to poor turbocharger performance, and higher overall environmental impact. As such, there is a necessity for new technology to be developed to improve the performance and efficiency of turbochargers within combustion engines, and provide benefits more generally for other applications utilising variable fluid flow.

Summary

According to a first aspect of the present invention there is provided a flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine, the assembly comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation, wherein the flow-guidance element is configured to rotate about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid, the flow-guidance element comprising a flow-guiding vane, wherein a trailing edge of the flow-guiding vane is arranged at an angle of β₄ relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane, and wherein the angle β₄ is less than or equal to 80 degrees.

According to a second aspect of the present invention, there is provided a method for guiding a flow of fluid having a variable mass flow rate onto a turbine, the turbine comprising a blade and configured to rotate about an axis of rotation, the method using a flow-guidance element in fluid communication with the turbine, the flow-guidance element comprising a flow-guiding vane, wherein a trailing edge of the flow-guiding vane is arranged at an angle of β₄ relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane and wherein the angle β₄ is less than or equal to 80 degrees, the flow-guidance element configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation, and the method comprising rotating the flow-guidance element about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid.

Turbochargers are typically designed using component maps, which describe the steady state aerodynamic and thermodynamic behaviour of each component; turbochargers are thus designed and optimised using quasi-steady fluid flow rates rather than the highly dynamic fluid flow such systems or assemblies are subjected to in reality. For example, the exhaust flow of a reciprocating internal combustion engine has a variable mass flow rate which varies throughout the engine cycle due to the reciprocating movement of the engine pistons. A practical consequence of this varying mass flow rate onto the turbine of a turbocharger is that the flow angle of the fluid relative to the rotating blades of the turbine will deviate as the mass flow rate varies. Accordingly, the flow angle onto the turbine does not remain steady at an optimum design angle, but instead varies, leading to inefficiencies in the turbine as a result of the fluctuations in the turbine rotation speed and turbine torque. Put another way, when the relative flow angle of the fluid onto the turbine (or the relative turbine inlet flow angle) varies, the difference between the relative turbine inlet flow angle and the angle of the leading edge of the turbine blade also changes; this difference is known as the angle of incidence. Any deviation in the angle of incidence from the optimum range (about -20 degrees to -40 degrees) causes the fluid flow to impinge the solid turbine blade; the subsequent flow separation and recirculation effects results in a less efficient conversion of the fluid flow into rotational energy by the turbine. The invention of the first and second aspects is able to control the variation of the relative flow angle in order to address this. It is difficult to control the relative flow directly, since the turbine blade geometry is fixed, but a reduction in the variation of the relative flow angle at turbine blade ingress can be achieved by regulating the absolute flow angle of the exhaust gas onto the turbine; this regulation can be achieved by rotating a flow- guidance element, which is in fluid communication with the turbine, in order to guide the fluid flow (such as the exhaust) onto the turbine.

Unlike a traditional approach, in which a flow-guidance element in the form of a stationary nozzle ring is located around the circumference of a turbine, the flow-control element of the present disclosure is configured to rotate about the same axis of rotation as the turbine. This rotation of the flow-guidance element varies the absolute flow angle at the exit of the flow-guidance element which, when combined with the effect of the rotation of the turbine itself, acts to change the relative flow angle of the fluid flow at turbine ingress. Advantageously, it is therefore possible to reduce the variation in the relative flow angle at turbine ingress arising from a varying mass flow rate of fluid, and thereby improve the efficiency of the turbine.

In particular, it has been recognised that the variation in the relative flow angle at turbine ingress is advantageously reduced when an angle of a trailing edge of a fluid-guiding vane of the rotating flow-guidance element is orientated at an angle of β₄ relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow- guiding vane. For a flow guidance element of the size typically used in industry, the angle β₄ is less than or equal to 80 degrees. Preferably, the angle β₄ is less than or equal to 70 degrees. More preferably, the angle β₄ is greater than or equal to 60 degrees and less than or equal to 70 degrees. More preferably, the angle β₄ is about 65 degrees. Optionally, the flow-guidance element comprises a ring. Preferably, when the flow- guidance element is a ring the axis of rotation passes through a centre of the ring and, optionally, the axis of rotation is normal to a plane in which the ring lies. Preferably, when the flow-guidance element is a ring, the flow guidance element is positioned around a circumference of the turbine. Optionally, the flow-guidance element is axially displaced with respect to the turbine.

Optionally, the flow control assembly of the first aspect comprises a volute configured to guide the flow of fluid onto a leading edge of the flow-guiding vane. Preferably, the volute has an inlet radius of r_x and an outlet radius of r₂. The volute is configured to guide the exhaust onto the flow guidance element. Preferably, the volute is scroll-like in shape. Alternatively, the volute can be any other design or shape of housing configured to guide the exhaust onto the flow guidance element with an inlet radius of r_t and an outlet radius of r₂. Optionally, the method of the second aspect comprises guiding the flow of fluid onto a leading edge of the flow-guiding vane using a volute.

Optionally, the angle β₄ is defined as

where a₃ is the absolute angle of incoming air flow at the nozzle inlet, as measured relative to a reference line extending perpendicular from the axis of rotation to the leading edge of the flow-guiding vane, a½ is the cycle averaged speed of rotation of the flow- guidance element, is the cycle averaged mass flow rate, r₃ is the radius at flow-guiding vane ingress and r₄ is the radius at flow-guiding vane egress.

Optionally, the cycle averaged speed of rotation of the flow-guidance element is between about 90 and 150 revolutions per second. Preferably, the cycle averaged speed of rotation is between about 1 10 and 130 revolutions per second. More preferably, the cycle averaged speed of rotation is about 120 revolutions per second. Optionally, the method of the second aspect comprises rotating the flow-guidance element at a cycle averaged speed of rotation of between 90 and 150 revolutions per second, preferably between 1 10 and 130 revolutions per second, more preferably at about 120 revolutions per second. Optionally, the turbine comprises a plurality of blades. This is advantageous as it facilitates an improved turbocharger performance with increased efficiency as compared to a turbine with only one blade. Optionally, the flow-guidance element comprises a plurality of flow-guiding vanes displaced from one another. Optionally, each of the plurality of flow-guiding vanes are arranged at an equal angle relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of each flow-guiding vane. Preferably, each of the plurality of flow-guiding vanes are arranged at an angle of β₄, where β₄ is less than or equal to 80 degrees. More preferably, the angle β₄ is greater than or equal to 60 degrees and less than or equal to 70 degrees. More preferably, the angle β₄ is about 65 degrees.

Optionally, the rotation of the turbine and the flow-guidance element is in the same direction about the axis of rotation. Optionally, the method of the second aspect comprises rotating the turbine and the flow-guidance element in the same direction around the axis of rotation.

Preferably, the flow-guidance element is driven by the flow of fluid. This arrangement facilitates the passive rotation of the flow-guidance element at a speed determined by the mass flow rate of the fluid. Optionally, the flow-control assembly comprises a controller configured to adjust a speed of rotation of the flow-guidance element about the axis of rotation. This latter, optional, arrangement can facilitate maintenance of the speed of rotation of the flow-guidance element at a rotation speed which provides for optimum turbocharger efficiency. Alternatively, the flow-guidance element is actively driven by a controller or actuator.

According to a third aspect of the present invention there is provided a flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine, the assembly comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation, wherein the flow-guidance element is configured to rotate about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid, the flow-guidance element comprising a flow-guiding vane, wherein a trailing edge of the flow-guiding vane is arranged at an angle of β₄. relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane, wherein the angle β₄ is defined as:

and where a₃ is the absolute flow angle of incoming air, as measured relative to a reference line extending perpendicular from the axis of rotation to the leading edge of the flow-guiding vane, a½ is the cycle averaged speed of rotation of the flow-guidance element, is the cycle averaged mass flow rate and r₃ and r₄ are the inlet and outlet radii of the flow guidance element. Preferably, the angle β₄ is in a range from about 0 degrees to about 60 degrees. More preferably, the angle β₄ is less than or equal to about 80 degrees. More preferably, the angle β₄ is less than or equal to about 70 degrees. More preferably, the angle β₄ is greater than or equal to 60 degrees and less than or equal to 70 degrees. More preferably, the angle β₄ is about 65 degrees.

The angle of the flow-guiding vane egress, β₄, can affect the performance of the fluid control assembly, since the variation in the absolute angle of the fluid flow leaving the rotating flow-guidance element during an engine cycle is negatively correlated with the angle β₄. A bigger variation in the absolute flow angle at the egress of the flow-guiding vane indicates that there will be less deviation of the relative flow angle at turbine ingress from the optimum angle. As such, an angle β₄ in accordance with the ranges provided above can facilitate an improvement in the efficiency of a turbocharger of the present invention. This will be described in more detail below with reference to the Figures. According to a fourth aspect of the present invention there is provided a flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine, the assembly comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative angle to rotate the turbine about the axis of rotation, wherein the flow-guidance element is configured to rotate about the same axis of rotation as the turbine and wherein said rotation is driven by the flow of fluid; and a controller configured to adjust a speed of rotation of the flow-guidance element about the axis of rotation. According to a fifth aspect of the present invention there is provided a method for guiding a flow of fluid having a variable mass flow rate onto a turbine, the turbine comprising a blade and configured to rotate about an axis of rotation, the method using a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative angle to rotate the turbine about the axis of rotation, and the method comprising: rotating the flow-guidance element about the same axis of rotation as the turbine, wherein said rotation is driven by the flow of fluid; and adjusting a speed of rotation of the flow-guidance element about the axis of rotation.

Optionally, the method of the fifth aspect comprises sensing the speed of rotation of the flow-guidance element, and adjusting the speed of rotation of the flow-guidance element according to the sensing result. Optionally, the method further comprises adjusting the speed of rotation of the flow-guidance element to a cycle averaged speed of 120 revolutions per second.

Over time, the performance of components of a turbocharger can deteriorate, due to changes in component alignment from general wear or environmental contamination. Whilst the flow-guidance element of the fourth and fifth aspects is configured to be driven by a flow of fluid, for example, an engine exhaust (and therefore to be passively driven), it can be beneficial to modulate or adjust the speed of rotation of the flow-guidance element over time. For example, power can be added to the system to increase the speed of rotation of the flow-guidance element, or the speed of rotation of the flow-guidance element can be decreased by the controller, for example by increasing the friction of the system and thus slowing the rotation of the flow-guidance element.

Optionally, the assembly further comprises at least one sensor configured to sense the speed of rotation of the flow-guidance element, wherein the controller is configured to adjust the speed of rotation of the flow-guidance element according to an output of the at least one sensor. Similarly, the method of the fifth aspect may optionally comprise sensing the speed of rotation of the flow guidance element. By measuring or sensing the speed of rotation of the flow-guidance element directly, and adjusting the speed of said flow-guidance element accordingly, the speed of rotation of the flow-guidance element can be maintained at or around a pre-determined, optimum, rotational speed. In this way, any deterioration of the components that form the flow control assembly can be compensated for, thereby facilitating the continued performance of the flow control assembly over time. Optionally, the controller is configured to adjust the speed of rotation of the flow-guidance element based upon a mass flow rate of the flow of fluid. Optionally, the assembly further comprises at least one sensor configured to sense a mass flow rate of the flow of fluid, wherein the controller is configured to adjust the speed of rotation of the flow-guidance element according to an output of the at least one sensor. By measuring or sensing the speed of the fluid flow which drives the flow-guidance element, the speed of rotation of the flow guidance element can be adjusted to ensure that the speed of rotation remains at or around a pre-determined optimum speed during any given engine cycle. This act as control facilitates an improvement in the efficiency of the turbine over the exhaust cycle.

Optionally, the controller is configured to adjust the speed of rotation of the flow-guidance element to a cycle averaged speed of between about 90 and 150 revolutions per second. Preferably, the cycle averaged speed of rotation is between about 110 and 130 revolutions per second. More preferably, the controller is configured to adjust the speed of rotation of the flow-guidance element to a cycle averaged speed of about 120 revolutions per second.

Any of the optional features described above with reference to any of the aspects can be combined with any of the other aspects described herein, either alone or in combination with any of the other features described. As such, those features described with reference to a system or assembly may be incorporated into a method, or vice versa.

Brief Description of the Drawings

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 provides experimental results illustrating the change in mass flow rate, turbine torque and turbine rotation speed during a pulsating exhaust cycle of 60 Hz;

Figure 2 is a cross-sectional view of a flow control assembly according to an example Figure 3A shows a velocity triangle diagram illustrating fluid velocity through a rotating nozzle ring and Figure 3B shows a velocity triangle diagram illustrating fluid velocity onto a rotating turbine;

Figure 4 is a combined velocity triangle diagram comparing fluid velocities at a rotating turbine for a stationary and rotating flow-guidance element;

Figure 5 is a cross sectional view of a flow guidance element according to an example illustrating the orientation of the flow-guiding vanes; Figure 6 is a cross sectional view of a flow guidance element according to an example illustrating the absolute relative flow angles and velocities at peak and trough mass flow rate of the fluid flow;

Figure 7 is a detail of the cross sectional view of Figure 6 illustrating the relative and absolute flow angles;

Figure 8 provides computational results showing the variation in relative angle of fluid flow at turbine ingress over time for a prior art stationary nozzle ring and a flow guidance element according to an example;

Figure 9 provides computational results showing the variation in turbocharger efficiency as a function of the flow-guidance element speed of rotation for peak (Figure 9a) and trough (Figure 9b) mass flow rate of the fluid flow;

Figures 10A to 10C show the cycle averaged pressure and velocity ratio between a turbocharger with a passively rotating nozzle ring (rotating vane), a variable geometry turbocharger with a fixed vane set at the optimum angle (fixed vane) and no nozzle ring (vaneless) for a turbine rotation speed of 48,000 (48k) rotations per minute (rpm) and 30k rpm: Figure 10A shows results for an exhaust with a pulse frequency of 80Hz, Figure 10B shows results for an exhaust with a pulse frequency of 60 Hz, and Figure 10C shows results for an exhaust with a pulse frequency of 20 Hz; and

Figure 11 shows a cross section of an example flow-guidance element.

Detailed Description

The following notation will be used throughout the following disclosure:

Nomenclature:

C Absolute flow velocity (m/s)

r Radius (m)

U Blade tip speed (m/s)

W Relative flow velocity (m/s)

a Absolute flow angle (°)

β Relative flow angle (°)

ω Angular velocity (rad/s)

Superscripts:

Stationary flow-guidance element

Cycle averaged

Subscripts:

1 Volute inlet

2 Volute exit 3 Flow-guidance element inlet

4 Flow-guidance element exit

5 Turbine inlet

6 Turbine exit

n flow guidance element

trough

P peak

As previously described, the exhaust flow of a reciprocating internal combustion engine has a variable mass flow rate which varies throughout the engine cycle due to the reciprocating movement of the engine pistons. This variation in mass flow rate is illustrated in Figure 1. In particular, it can be seen from Figure 1 that the mass flow rate of gas entering the turbocharger is not fixed and oscillates between a peak and a trough mass flow rate. This change in mass flow rate leads to a variation in the absolute flow velocity at turbine ingress and therefore in the relative flow angle of the gas at the turbine ingress; the relative flow angle and velocity is defined in the reference frame of the rotating turbine. Since the relative flow angle of the exhaust gas at turbine ingress varies according to the mass flow rate, the efficiency of the turbine is reduced because the angle of incidence of the fluid flow onto the turbine deviates from the optimal range.

A cross-sectional view of a flow-control assembly 100 according to an example of the present disclosure is illustrated in Figure 2. The flow-control assembly 100 comprises a turbine 1 10 which is configured to rotate about an axis of rotation 150. The turbine 1 10 comprises a plurality of blades 1 15 configured to cause the turbine 1 10 to rotate about the axis 150 in response to a flow of fluid across the blades 1 15. Flow-control assembly 100 further comprises a flow-guidance element 120 in fluid communication with the turbine 1 10. The flow guidance element comprises a plurality of flow-guiding vanes 125 separated about the circumference of the flow-guidance element 120. The flow-guiding vanes 125 are shaped elements, such as nozzles, which guide the fluid on to the blades 1 15 of the turbine 110.

In other embodiments, the flow-guidance element 120 comprises only one flow-guiding vane 125 and/or the turbine 1 10 comprises only one blade 1 15.

The flow-guidance element 120 is in the form of a nozzle ring having one or more nozzles which act to guide the fluid flow to turbine ingress and the vanes 125 and the blades 1 15 may comprise pressure and suction surfaces so as to act as aerofoils. In particular, positions of a pressure surface and a suction surface of the flow-guiding vanes 125 are arranged such that a pressure difference between the pressure and suction surfaces imposes a torque on the flow-guiding vane when the flow-guidance element is exposed to the flow of fluid and enables the flow guidance element to be driven by the flow of fluid. However, the flow guidance element and flow guiding vanes may have any other form suitable for guiding fluid onto the turbine 1 10.

The flow-guidance element 120 is arranged upstream of the turbine 1 10 and is configured to guide a flow of fluid onto the blades 1 15 of the turbine in order to rotate the turbine about the axis of rotation 150. The flow-guidance element 120 is configured to rotate about the same axis of rotation as the turbine 1 10 so as to alter or reduce the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid. In the example of Figure 2, the flow-guidance element 120 is positioned about the external circumference of the turbine 110 in order to guide fluid arriving at the circumference of the turbine 1 10 onto the blades 1 15 of the turbine 110. For example, a turbocharger for an internal combustion engine may include a flow-control assembly arranged as illustrated in Figure 2.

Figures 3A and 3B illustrate an example of a flow-control assembly according to the present disclosure in which the flow-guidance element 120 comprises at least one vane 125 configured to guide fluid flow onto the blades 115 of the turbine 110. In the arrangement of Figure 3, a velocity triangle diagram is shown for a flow-control element 120 configured to rotate at a linear speed U_n3 at the leading edge of the flow guiding element.

Figure 3A illustrates an arrangement in which fluid enters the rotating flow-guidance element 120 at a mass flow rate with an absolute flow velocity C3 and a relative flow velocity of W3; the relative flow velocity is in the reference frame of the flow-guidance element. As will be appreciated, the relative flow angle at the ingress of the flow-guiding vane 125 is different to that of the absolute flow angle because the flow-guidance element is rotating with a linear speed of U_n3 at the leading edge of the flow guidance element. The absolute flow leaving the flow-guidance element 120 is defined as C4 and the flow of fluid relative to the flow-guidance element 120 at the egress of the flow-guiding vane is defined as W4. The flow guidance element is rotating with a linear speed of U_n4 at the trailing edge of the flow guidance element. Figure 3B illustrates the resultant absolute flow velocity C5 at the turbine ingress (which is equal to the absolute flow velocity at flow-guidance element egress, C4) and the relative flow velocity, W5, at turbine ingress. The turbine is rotating with a linear speed of U at turbine ingress.

The relative and absolute flow angles of the fluid flow, at both peak (max) and trough (min) mass flow rates, are illustrated in Figure 4, which shows the velocity triangles for the peak and trough cases as superimposed triangles. The variation in the relative flow angles at turbine ingress between the peak and trough mass flow situations is given by Ab. The dotted lines of Figure 4 illustrate the situation in which the flow-guidance element is stationary, rather than rotating, for comparative purposes. When the flow-guidance element is stationary, the absolute flow velocity is equal at flow-guidance element ingress and egress, and only the rotation of the turbine affects the relative flow angle at turbine ingress. The variation of the relative flow angle at turbine ingress for the arrangement in which a stationary flow-guidance element, such as a stationary nozzle ring, is given by Ab'.

As can be seen from Figure 4, Ab is less than Ab'. Put another way, the overall variation in the relative flow angle at turbine ingress is reduced when the flow-guidance element 120 is used in place of a stationary nozzle ring. Accordingly, the efficiency of the turbine is increased because the relative flow angle deviates less, and thus the angle of incidence deviates less from the optimum angle range. Figure 5 illustrates the orientation of the flow-guiding vanes 125 of the flow-guidance element 120. The orientation of the trailing edge of the flow-guiding vane can be defined with respect to a reference line 160 extending perpendicular from the axis of rotation 150 to the flow-guiding vane exit. The trailing edge is the point on the flow-guidance element at which the fluid leaves the flow-guidance element, that is, the flow-guiding vane egress, or exit. The angle of the flow-guiding vane exit measured relative to reference line 160 is β₄. Similarly, the absolute flow angle of air flow onto the flow-guidance element can be defined with respect to a reference line 170 extending perpendicular from the axis of rotation 150 to the leading edge of the flow-guiding vane entrance. The leading edge is the point on the flow-guidance element at which the fluid enters the flow-guidance element, that is, the flow-guiding vane ingress, or entrance. The absolute flow angle of the air flow at the flow-guiding vane entrance measured relative to reference line 170 is a₄. In this example, the flow-guidance element is in the form of a ring. The ring lies in a plane normal to the axis of rotation and the reference lines 160 and 170 lie in the plane of the ring. Angle a₄ is the angle of the absolute flow velocity C3 at flow-guidance element ingress. Angle β₄ is the angle of the relative flow velocity W4 at flow-guidance element egress, i.e. the angle the flow follows when it leaves the flow-guiding vane, in the reference frame of the flow guidance element.

Figure 6 illustrates the absolute and relative flow angles of the fluid upon exit of the flow- guidance element in the case of peak and trough mass flow rates. The 'balance speed' (illustrated by the dotted line of Figure 6) is the rotational speed of the flow-guidance element at which torque on the flow-guidance element is zero. A rotational speed greater than the balance speed indicates the torque on the flow-guidance element is less than zero; the flow-guidance element acts as a flywheel, or a compressor, and transfers energy to the fluid. A rotational speed less than the balance speed indicates that the torque on the flow-guidance element is greater than zero; the flow-guidance element acts as a turbine and energy is transferred to the flow-guidance element from the fluid.

At the balance speed, the absolute flow angle at the exit of the flow-guiding vanes is identical to that at the inlet of the flow-guiding vane, i.e. the absolute flow angle at flow- guidance element egress is a₄ (based on the principles of mass conservation and energy conservation). The angular velocity of the flow guidance element at the peak balance speed can therefore be defined as:

U_n4 r₃ C₃cosa₃ (tana₃— ίαηβ₄)

7¹ 2nr₄ 2nr₄ ²

Equation 1 where r₃ and r₄ are the inlet and outlet radii of the flow-guidance element, i.e. the radii at the leading and trailing edges of the flow-guiding vanes of the ring shaped flow guidance element, respectively. That is to say, r₃ is the radial distance from the axis of rotation of the flow guidance element to the leading edge of the flow guiding vane and r₄ is the radial distance from the axis of rotation of the flow guidance element to the trailing edge of the flow guiding vane.

Since the radii r₃ and r₄, and the angles a₃ and β₄, are fixed by the geometry of the flow- guidance element, the balance speed is dependent only on C3, the absolute velocity of the fluid flow onto the flow-guidance element. Since C3 is determined by the mass flow rate, which varies throughout the exhaust cycle (as illustrated by Figure 1), the balance speed will vary with the mass flow rate. Though the real rotation speed of the flow- guidance element will also fluctuate during the exhaust cycle, due to the pulsing of the exhaust flow, these fluctuations have been found to be less than 0.3% of the mean speed of rotation (based upon calculations of the moment of inertia of the flow-guidance element which assume a carbon steel nozzle ring assembly). As such, the speed of rotation of the flow guidance element can be considered to be constant during the exhaust cycle.

Furthermore, since the cycle averaged absolute fluid flow lies between the peak and trough balance speed, the speed of rotation of the flow-guidance element will also be between the two balance speeds of the peak and trough mass flow rate conditions. Since the only variable which affects the balance speed is the absolute flow velocity C3, as shown in Equation 1 , and the flow-guidance element rotational speed is approximately constant, the cycle averaged angular speed of rotation of the flow-guidance element can be estimated using the cycle averaged absolute flow velocity C^as follows:

U_n r₃ C₃ cosa₃ (tana₃— ίαηβ₄)

⁷¹ 2nr₄ 2nr₄ ²

Equation 2 where r₃ and r₄ are the radii at the leading and trailing edges of the flow-guiding vane respectively (i.e. r₃ is the radius at flow-guiding vane ingress and r₄ is the radius at flow- guiding vane egress), respectively, and TJ^is the cycle averaged linear speed of the flow guidance element. The radii r₃ and r₄ can be seen in Figure 1 1. The dimensions of the radii in Figure 11 are an example only.

As can be seen from Figures 6 and 7, at peak (max) mass flow rate, the approximately constant speed of rotation of the flow-guidance element is smaller than the balance speed; the difference in the two speeds is the peak balance speed minus the flow- guidance rotation speed (peak balance speed - Un4). The absolute flow angle at flow- guiding vane inlet, a₃, is larger than the peak absolute flow angle at flow-guiding vane outlet, a_4P, with the discrepancy a₃ - a_4P being positively correlated with peak balance speed - Un4. At trough (min) mass flow rate, the approximately constant speed of rotation of the flow-guidance element is greater than the balance speed; the difference in the two speeds is the flow-guidance rotation speed minus the trough balance speed (Un4 - trough balance speed). The absolute flow angle at flow-guiding vane inlet, a₃ , is smaller than the trough absolute flow angle at flow-guiding vane outlet, a_4T, with the discrepancy a_4T - a₃ being positively correlated with Un4 - trough balance speed. All angles at flow guidance element outlet, or egress, are defined with respect to the reference line 160; all angles at flow guidance inlet, or ingress, are defined with respect to reference line 170, as can be seen in Figure 7. Since the absolute flow angle at flow-guidance element exit, a₄, varies between the extremes of a_4T and a_4P during an exhaust pulse cycle, the passively controlled flow- guidance element will act as a turbine and compressor alternatively through the exhaust cycle, with the mean torque on the nozzle equal to zero. The fluctuation range a_4T - a_4P is positively correlated with the difference in balance speed at peak and trough mass flow rate (peak balance speed - trough balance speed), which is in turn negatively correlated with angle β₄, as can be seen in Figure 7. As illustrated in Figure 4, the change in absolute flow angle at turbine inlet between peak and trough mass flow rates, Aa, leads to a reduction in the variation of the relative flow angle, Ab. Therefore, the greater the change in the absolute flow angle, Aa (or the greater the variation in a₄), the lesser the corresponding variation in relative flow angle Ab, and the smaller the deviation in the incidence angle from the optimum range. As such, the negative correlation shown in Figure 7 indicates that a suitable choice of angle β₄ can be particularly beneficial in turbocharger design.

Equation 2 can be rearranged to find the optimum value for angle β₄. In particular, the relative angle at flow-guiding arranging Equation 2) as:

Equation 3

Using Equation 3, the optimum angle β₄ for any given turbocharger geometry can be estimated. Preferably, the angle β₄ is in a range from about 50 degrees to about 80 degrees. More preferably, the angle β₄ is greater than or equal to about 60 degrees and less than or equal to about 70 degrees. More preferably, the angle β₄ is about 65 degrees. If β₄ is less than about 30 degrees, the flow-guidance element will rotate at a very high speed. The resulting speed of rotation will be much higher than the optimal rotation speed of 120 revolutions per second (see below and the description with reference to Figures 8 to 10). The friction loss of the system would be high at these high rotation speeds, and therefore such high rotation speeds are undesirable.

The inventors have found that an angle of β₄ equal to about 65 degrees adapts well to pulses in air flow, for example, the pulses in exhaust flow. In industry, flow pulses would vary significantly according to different turbocharger applications. Therefore, the angle β₄ should be calculated based on Equation 3 above in order to achieve the optimal nozzle speed, which is around 120 revolutions per second.

As discussed with reference to Equation 2, the above calculation of the angle β₄ requires use of the cycle averaged speed of rotation of the flow guidance element. Preferably, the cycle averaged speed of rotation of the flow-guidance element is between about 90 and 150 revolutions per second. Preferably, the cycle averaged speed of rotation is between about 110 and 130 revolutions per second. More preferably, the cycle averaged speed of rotation is about 120 revolutions per second (rps). Evidence for about 120 rps being the optimum cycle averaged rotational speed of the flow guidance element is presented below.

The change in absolute flow angle at flow-guidance egress due to the rotation of the flow- guidance element results in a change in the relative flow angle at turbine egress, and subsequently results in a change in the incidence angle of the fluid flow at the turbine. This change in the incidence angle is shown in Figure 8, which compares computational results for a stationary flow guidance element (or stationary nozzle ring), and a rotating flow-guidance element (or rotating nozzle ring) which is rotating at 120 rps. The conditions used to obtain the computational results are given in Table 1.

It can be seen that in the case of the stationary nozzle ring the instantaneous incidence angle of the turbine fluctuates between -28.3 degrees and -102 degrees during the exhaust cycle, with the incidence angle deviating from the optimal range of approximately -20 degrees to -40 degrees for much of the simulation time. This deviation in incidence angle is a fundamental cause of the efficiency deterioration of turbochargers. In the case of the rotating nozzle ring, the fluctuation of the incidence angle is reduced to between - 31.8 degrees and -93.8 degrees. A more significant decrease in the incidence angle as compared to the stationary nozzle ring is seen for high incidence angles; the incidence angle through an exhaust cycle is generally shifted closer to the optimal incidence range, which subsequently leads to improvements in the turbocharger performance as a result.

Model settings

Type of analysis Transient

Fluid Air Ideal Gas

Turbulence model k-epsilon Interface connection Transient Rotor Stator

Residual value parameters 1e^"06

Boundary conditions

Turbine speed 813 rps

Mass flow rate See Figure 1

Inlet total temperature 338 K

Exit average static pressure 1 atm

Table 1 : Unsteady model settings & boundary conditions

Figure 9 illustrates the efficiency of the flow-guidance element in both of the above described compressor and turbine modes for a varying flow-guidance element (or nozzle) rotation speed. It can be seen from Figure 9 that the maximum efficiency for peak mass flow rate is achieved with a rotational speed of about 120 rps, and is achieved at 150 rps for the trough mass flow rate. Therefore, preferably the speed of rotation used in

Equation 3 is between about 90 and 150 rps, which in the peak mass flow rate case provides the maximum efficiency gains; more preferably the speed of rotation is about 120 rps.

Figures 10A to 10C show a comparison of experimental results for a turbocharger using: a passively rotating flow-guidance element; a variable geometry turbine (configured at the optimal angle setting); and no flow-guidance element. Experimental testing was carried out using a magnetic dynamometer to replace the typical air compressor of the

turbocharger, so as to load the turbine. Inside the dynamometer there is a magnetic rotor and a stator, which are mounted in parallel. When the turbine is rotating, the

dynamometer rotor, which is coupled to the turbine wheel, also rotates. This creates eddy currents in the dynamometer stator. The turbine is therefore loaded by the magnetic resistance induced by the eddy current. A smaller gap between the dynamometer stator and rotor indicates higher loading of the turbine. In the experimental tests for which results are presented below, gaps of 2.5 mm, 3 mm and 5 mm between the dynamometer stator and rotor (dyna gaps) were used, coupled with high turbine speeds. This arrangement is of particular interest in industry, since the high speed-high loading conditions mean a high energy density.

As discussed, the testing was carried out under high loadings (2.5 mm, 3 mm and 5 mm dyna gaps), high turbine speeds (48k rpm and 30k rpm) and high pulse frequencies (80hz, 60hz and 20hz— see Figures 10A, 10B and 10C respectively); these experimental settings are representative of industrial applications. The results of Figures 10A to 10C are summarised in Table 2, which shows a marked efficiency gain for the rotating flow guidance element as compared to the variable geometry turbine.

A marked efficiency gain is found over the variable geometry turbine, as shown in Table 2.

A flow guidance element as described above facilitate a reduction of the variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid, and thereby increases the efficiency of a turbocharger comprising such a flow guidance element.

Experimental conditions (turbine Dyna gap, mm Efficiency gain from speed, x1000 rpm/ exhaust pulse variable geometry frequency, Hz) turbine

5 9.83%

30 / 80 3 5.75%

2.5 17.25%

5 8.00%

48 / 80 3 8.74%

2.5 7.77%

5 7.41 %

30 / 60 3 9.38%

2.5 11.49%

5 6.58%

48 / 60 3 8.01 %

2.5 13.07%

5 4.88%

30 / 20 3 5.56%

2.5 3.87%

5 10.47%

48 / 20 3 14.95%

Table 2: Experimental results showing efficiency gain of rotating flow-guidance element as compared to a variable geometry turbine.

Additionally or alternatively, the fluid control assembly comprises a controller arranged to adjust a speed of the flow guidance element when the flow guidance element is driven by the flow of fluid. In an example, the controller adds power to the flow guidance element, for example by way of a motor, to increase the rotational speed of the flow guidance element. Alternatively or additionally, the controller may adjust the speed of rotation by means of a friction drive; the amount of friction applied to the flow guidance element may be altered in order to increase or decrease the speed of rotation.

The flow control assembly may also comprise a sensor to measure or sense the speed of rotation of the flow guidance element. For example, the sensor may count revolutions of the flow guidance element in order to determine the speed of rotation; the results are then provided to the controller in order that the controller can adjust the speed accordingly, thereby enabling the maintenance of a predetermined speed of rotation. Preferably, the speed of rotation is maintained at about 120 rps, though it may be maintained at any other speed, for example within any of the ranges provided above.

The term 'about', when used with reference to a number or a numerical range, means the number or numbers given, plus or minus 10%.

When ranges are defined, any sub range or individual number in that range is also disclosed. For example, less than or equal of 70 degrees can be 65.5 degrees, or between 50 and 62 degrees, or between 66.95 and 69 degrees, or any other possible combination.

Other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known and which may be used instead of, or in addition to, features described herein. Features that are described in the context of separate embodiments may be provided in combination in a single embodiment. Conversely, features which are described in the context of a single embodiment may also be provided separately or in any suitable sub-combination. It should be noted that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single feature may fulfil the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present disclosure.

Claims

1. A method for guiding a flow of fluid having a variable mass flow rate onto a turbine, the turbine comprising a blade and configured to rotate about an axis of rotation, the method using a flow-guidance element in fluid communication with the turbine, the flow-guidance element comprising a flow-guiding vane, wherein a trailing edge of the flow-guiding vane is arranged at an angle of β₄ relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane and wherein the angle β₄ is less than or equal to 80 degrees, the flow-guidance element configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation, the method comprising:

rotating the flow-guidance element about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid.

2. The method of claim 1 , wherein the flow-guidance element is in the form of a ring.

3. The method of claim 2, wherein the axis of rotation passes through a centre of the ring.

4. The method of any preceding claim, further comprising guiding the flow of fluid onto a leading edge of the flow-guiding vane with a volute.

5. The method of claim 4, wherein the angle β₄ is defined as:

where a₃ is the absolute flow angle of the flow of fluid onto the flow-guidance element measured relative to a reference line extending perpendicular from the axis of rotation to the leading edge of the flow-guiding vane, a½ is the cycle averaged speed of rotation of the flow-guidance element, is the cycle averaged mass flow rate, and r₃ is the radius at flow-guiding vane ingress and r₄ is the radius at flow-guiding vane egress.

6. The method of any preceding claim, wherein β₄ is less than or equal to 70 degrees and greater than or equal to 60 degrees.

7. The method of any preceding claim, wherein the turbine comprises a plurality of blades and wherein the flow-guidance element comprises a plurality of flow-guiding vanes displaced from one another.

8. The method of claim 7, wherein each of the plurality of flow-guiding vanes are arranged at an equal angle relative to a reference line extending between the trailing edge of each flow-guiding vane and the centre of the axis of rotation.

9. The method of any preceding claim, comprising rotating the turbine and the flow- guidance element in the same direction about the axis of rotation.

10. The method of any preceding claim, comprising positioning the flow-guidance element around a circumference of the turbine, wherein the flow-guidance element is in the form of a ring.

1 1. The method of any preceding claim, wherein the flow-guidance element is axially displaced with respect to the turbine.

12. A flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine, the assembly comprising:

a turbine comprising a blade and configured to rotate about an axis of rotation; a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative fluid flow angle to rotate the turbine about the axis of rotation, wherein the flow-guidance element is configured to rotate about the same axis of rotation as the turbine so as to alter a variation of the relative fluid flow angle at turbine ingress arising from a varying mass flow rate of the flow of fluid,

the flow-guidance element comprising a flow-guiding vane, wherein a trailing edge of the flow-guiding vane is arranged at an angle of β₄ relative to a reference line extending perpendicular from the axis of rotation to the trailing edge of the flow-guiding vane, wherein the angle β₄ is less than or equal to 80 degrees.

13. The assembly of claim 12, wherein the flow-guidance element is in the form of a ring.

14. The assembly of claim 13, wherein the axis of rotation passes through a centre of the ring.

15. The assembly of any one of claims 12 to 14, further comprising a volute configured to guide the flow of fluid onto a leading edge of the flow-guiding vane.

16. The assembly of claim 15, wherein the angle β₄ is defined as: tan(j¾) = tan(a₃) - ( ^^2m* Y

where a₃ is the absolute flow angle of the flow of fluid onto the flow-guidance element measured relative to a reference line extending perpendicular from the axis of rotation to the leading edge of the flow-guiding vane, is the cycle averaged speed of rotation of the flow-guidance element, is the cycle averaged mass flow rate, and r₃ is the radius at flow-guiding vane ingress and r₄ is the radius at flow-guiding vane egress.

17. The assembly of any one of claims 12 to 16, wherein β₄ is less than or equal to 70 degrees and greater than or equal to 60 degrees.

18. The assembly of any one of claims 12 to 17, wherein the turbine comprises a plurality of blades and wherein the flow-guidance element comprises a plurality of flow-guiding vanes displaced from one another.

19. The assembly of claim 18, wherein each of the plurality of flow-guiding vanes are arranged at an equal angle relative to a reference line extending between the trailing edge of each flow-guiding vane and the centre of the axis of rotation.

20. The assembly of any one of claims 12 to 19, wherein the rotation of the turbine and the flow-guidance element is in the same direction about the axis of rotation.

21. The assembly of any one of claims 12 to 20, wherein the flow-guidance element is in the form of a ring and is positioned around a circumference of the turbine.

22. The assembly of any one of claims 12 to 21 , wherein the flow-guidance element is axially displaced with respect to the turbine.

23. A flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine, the assembly comprising:

a turbine comprising a blade and configured to rotate about an axis of rotation; a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative angle to rotate the turbine about the axis of rotation, wherein the flow-guidance element is configured to rotate about the same axis of rotation as the turbine and wherein said rotation is driven by the flow of fluid; and

a controller configured to adjust a speed of rotation of the flow-guidance element about the axis of rotation.

24. The assembly of claim 23, further comprising at least one sensor configured to sense the speed of rotation of the flow-guidance element, wherein the controller is configured to adjust the speed of rotation of the flow-guidance element according to an output of the at least one sensor.

25. The assembly of claims 23 or 24, wherein the controller is configured to adjust the speed of rotation of the flow-guidance element to a cycle averaged speed of 120 revolutions per second.

26. A turbocharger comprising the flow-control assembly of any one of claims 12 to 25, wherein the flow of fluid is pulsed exhaust gas.

27. An engine comprising the turbocharger of claim 26.

28. A vehicle comprising the engine of claim 27.

29. A method for guiding a flow of fluid having a variable mass flow rate onto a turbine, the turbine comprising a blade and configured to rotate about an axis of rotation, the method using a flow-guidance element in fluid communication with the turbine and configured to guide a flow of fluid onto the turbine at a relative angle to rotate the turbine about the axis of rotation, the method comprising:

rotating the flow-guidance element about the same axis of rotation as the turbine, wherein said rotation is driven by the flow of fluid; and

adjusting a speed of rotation of the flow-guidance element about the axis of rotation.