PULSE-OPTIMIZED FLOW CONTROL

Abstract

A flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; and a flow-guidance element in fluid communication with the turbine and comprising a flow-guiding vane and configured to guide a flow of fluid 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 the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid.

Claims

1. A flow-control assembly for guiding a flow of fluid having a variable mass flow rate onto a turbine comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; and a flow-guidance element in fluid communication with the turbine and comprising a flow-guiding vane and configured to guide a flow of fluid 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 the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid.

2. A flow-control assembly according to claim 1, wherein the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid is reduced.

3. A flow-control assembly according to claim 1, 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.

4. A flow-control assembly according to any of claim 1, wherein the rotation of the turbine and the flow-guidance element is in the same direction about the axis of rotation.

5. A flow-control assembly according to any of claim 1, wherein the rotation of the turbine and the flow-guidance element is in different directions about the axis of rotation.

6. A flow-control assembly according to claim 1, wherein the rotation of the flow-guidance element is controlled by an actuator.

7. A flow-control assembly according to claim 6, wherein the actuator is configured to vary the speed of rotation of the flow-guidance element based upon the mass flow rate of the flow of fluid.

8. A flow-control assembly according to claim 6, wherein the actuator is configured to rotate the flow-guidance element at a higher speed at peak mass rate flow than at trough mass flow rate.

9. A flow-control assembly according to claim 6, wherein the actuator is configured to rotate the flow-guidance element at a lower speed at peak mass flow rate than at trough mass flow rate.

10. A flow-control assembly according to claim 6, wherein the actuator is configured to rotate the flow-guidance element at a fixed speed.

11. A flow-control assembly according to claim 10, wherein the fixed speed is less than or equal to the rotation speed of the turbine.

12. A flow-control assembly according to claim 10, wherein the fixed speed is or equal to 150 revolutions per second.

13. A flow-control assembly according to any of claim 1, wherein the rotation of the flow-guidance element is driven by the flow of fluid.

14. A flow-control assembly according to claim 1, wherein the flow-guidance element is in the form of a ring and is positioned around the circumference of the turbine.

15. A flow-control assembly according to any of claim 1, wherein the flow-guidance element is axially displaced with respect to the turbine.

16. A turbocharger comprising: a turbine comprising a blade and configured to rotate about an axis of rotation; and a flow-guidance element in fluid communication with the turbine and comprising a flow-guiding vane and configured to guide a flow of fluid 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 the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid, wherein the flow of fluid is pulsed exhaust gas.

17. An engine comprising: a turbocharger comprising a turbine comprising a blade and configured to rotate about an axis of rotation; and a flow-guidance element in fluid communication with the turbine and comprising a flow-guiding vane and configured to guide a flow of fluid 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 the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid.

18. A vehicle comprising: an engine comprising a turbine comprising a blade and configured to rotate about an axis of rotation; and a flow-guidance element in fluid communication with the turbine and comprising a flow-guiding vane and configured to guide a flow of fluid 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 the variation of the relative fluid flow angle at turbine ingress arising from varying mass flow rate in the flow of fluid.

19.-33. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments will be described below, by way of example only, with reference to the accompanying drawings, in which:

[0029] FIG. 1 is a graph illustrating exhaust pressure traces in an internal combustion engine manifold with automotive-type valve timing;

[0030] FIG. 2 is a cross-sectional view of a flow-control assembly according to an example;

[0031] FIG. 3a is a velocity triangle diagram illustrating fluid velocity through a stationary prior art nozzle ring at trough mass flow rate;

[0032] FIG. 3b is a velocity triangle diagram illustrating fluid velocity at a rotating turbine having been passed through the stationary prior art nozzle ring of FIG. 3a at trough mass flow rate;

[0033] FIG. 4a is a velocity triangle diagram illustrating fluid velocity through a stationary prior art nozzle ring at peak mass flow rate;

[0034] FIG. 4b is a velocity triangle diagram illustrating fluid velocity at a rotating turbine having been passed through the stationary prior art nozzle ring of FIG. 4a at peak mass flow rate;

[0035] FIG. 5a is a velocity triangle diagram illustrating fluid velocity through a rotating flow-guidance element according to an example of the present disclosure at trough mass flow rate;

[0036] FIG. 5b is a velocity triangle diagram illustrating fluid velocity at a rotating turbine having been passed through the rotating flow-guidance element of FIG. 5a at trough mass flow rate;

[0037] FIG. 6a is a velocity triangle diagram illustrating fluid velocity through a rotating flow-guidance element according to an example of the present disclosure at peak mass flow rate;

[0038] FIG. 6b is a velocity triangle diagram illustrating fluid velocity at a rotating turbine having been passed through the rotating flow-guidance element of FIG. 6a at peak mass flow rate;

[0039] FIG. 7 is a combined velocity triangle diagram illustrating fluid velocities at a rotating turbine according to FIGS. 5b and 6b;

[0040] FIG. 8a illustrates turbine stage efficiency as a function of the rotation speed of the flow-guidance element in both turbo and compressor modes at trough mass flow rates;

[0041] FIG. 8b illustrates turbine stage efficiency as a function of the rotation speed of the flow-guidance element in both turbo and compressor modes at peak mass flow rates;

[0042] FIG. 9a illustrates power output as a function of the rotation speed of the flow-guidance element in both turbo and compressor modes at trough mass flow rates; and

[0043] FIG. 9b illustrates power output as a function of the rotation speed of the flow-guidance element in both turbo and compressor modes at peak mass flow rates.

DETAILED DESCRIPTION

[0044] The following embodiments relate generally to a flow-control assembly for guiding a flow of fluid onto a turbine so as to rotate the turbine.

[0045] A cross-sectional view of a flow-control assembly 100 according to an example of the present disclosure is illustrated in FIG. 2. The flow-control assembly 100 comprises a turbine 110 which is configured to rotate about an axis of rotation 150. The turbine 110 comprises at least one blade 115 configured to cause the turbine 110 to rotate about the axis 150 in response to a flow of fluid across the blades 115.

[0046] Flow-control assembly 100 further comprises a flow-guidance element 120 in fluid communication with the turbine 110 and comprising at least one flow-guiding vane 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 115 of the turbine 110. The flow-guidance element 120 may take the form of a nozzle ring having one or more nozzles which act to guide the fluid flow to turbine ingress.

[0047] The vanes 125 and the blades 115 may comprise pressure and suction surfaces so as to act as aerofoils.

[0048] The flow-guidance element 120 is arranged upstream of the turbine 110 and is configured to guide a flow of fluid onto the blades 115 of the turbine in order to rotate the turbine about the axis of rotation 150.

[0049] The flow-guidance element 120 is configured to rotate about the same axis of rotation as the turbine 110 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.

[0050] In the example of FIG. 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 110 onto the blades 115 of the turbine 110. For example, a turbocharger for an internal combustion engine may include a flow-control assembly arranged as illustrated in FIG. 2.

[0051] As discussed above, inefficient operation of the turbine may occur where the flow angle .sub.3 of the fluid relative to the rotation of the blades is sub-optimal. This variation in the relative flow angle is demonstrated in further detail with respect to FIGS. 3a, 3b, 3c, and 3d.

[0052] In prior art arrangements a stationary nozzle ring 140 may be placed around the circumference of a turbine. FIG. 3a illustrates a velocity triangle for such a prior art nozzle ring 140, where the nozzle ring is located around the circumference of a turbine. Unlike the arrangement of FIG. 2, the prior art nozzle ring 140 of FIG. 3a is fixed with respect to the axis of rotation and is therefore unable to rotate.

[0053] In the arrangement of FIG. 3a, the absolute flow velocity of fluid flowing into the stationary nozzle ring 140 at trough mass flow rate is defined as C1.sub.min. As also illustrated in FIG. 3a, the absolute flow velocity of fluid flowing out of the stationary nozzle ring 140 at trough mass flow rate is defined as C2.sub.min. The flow angle relative to the nozzle ring 140 is not shown since the relative flow angles into and out of the nozzle ring 140 are the same as the corresponding absolute values, as the nozzle ring 140 is stationary.

[0054] In FIG. 3b, the fluid having passed through the nozzle ring of FIG. 3a arrives at turbine ingress. The absolute flow velocity of fluid flowing onto the rotating turbine 110, i.e. at turbine ingress, is defined as C3.sub.min. The flow velocity (m/s) of the fluid flowing onto the rotating turbine 110 relative to the speed of rotation U of the turbine 110 is defined by W3.sub.min. As defined by FIG. 3b, the absolute flow angle and relative flow angle is determined based upon the speed of rotation of the turbine and the absolute flow velocity C3.sub.min.

[0055] The arrangement of FIGS. 3a and 3b show the relative flow angle of the fluid at trough mass flow rate of the fluid. FIGS. 4a and 4b illustrate velocity triangles for an arrangement which corresponds to that of FIGS. 3a and 3b, apart from the mass flow rate of the fluid being at peak mass flow rate rather than at trough mass flow rate.

[0056] FIG. 4a illustrates the absolute flow velocities into C1.sub.max and out of C2.sub.max the fixed nozzle ring 140. FIG. 4b also illustrates the absolute flow velocity C3.sub.max flowing onto the rotating turbine 110.

[0057] FIG. 4b also shows the flow velocity W3.sub.max of the fluid flowing onto the rotating turbine 110 relative to the speed of rotation U of the turbine 110. As can be seen from FIGS. 3a and 4a, the absolute flow velocity at peak mass flow rate C2.sub.max is larger than the absolute flow velocity at trough mass flow rate C2.sub.min. Accordingly, the relative flow velocity W3.sub.max and W3.sub.min at turbine ingress differs depending upon the absolute flow velocity of the fluid at turbine ingress. Accordingly, the relative flow velocity at any given point is between W3.sub.max and W3.sub.min.

[0058] As can be seen from the arrangements of FIGS. 3b and 4b, the relative flow angle at turbine ingress varies depending upon the mass flow rate. Any deviation from the optimal relative flow angle onto the blades 115 of the turbine 110 will result in inefficient operation of the turbine due to incidence loss (see Equation 1). Accordingly, it is desirable to reduce the variation in so as to increase the efficiency of the turbine 110.

[0059] FIG. 5 illustrates 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 FIG. 5, a velocity triangle diagram is shown for a flow-control assembly 120 configured to rotate at speed U.sub.n1.

[0060] FIG. 5a illustrates an arrangement in which fluid enters the rotating flow-guidance element 120 at trough mass flow rate and at an absolute flow velocity C1.sub.min and relative flow velocity W1.sub.min. As will be appreciated, the relative flow angle W1.sub.min entering the rotating flow-guidance element 120 of FIG. 5a is different to that experienced by the stationary nozzle 140 of FIG. 3a since the flow-guidance element 120 is rotating. The absolute flow leaving the flow-guidance element 120 is demonstrated in FIG. 5a as C2.sub.min and the flow of fluid relative to the flow-guidance element 120 is defined as W2.sub.min. FIG. 5b illustrates the resultant absolute C3.sub.min and relative W3.sub.min flow velocities at turbine ingress. As can be seen from FIG. 5b, the relative flow angle of the fluid at trough mass flow rate with respect to the turbine 110 is altered when compared with the corresponding flow angle arriving at the turbine blades where the nozzle ring 140 of FIG. 3 is used in place of the illustrative flow-control assembly of FIG. 5.

[0061] The corresponding arrangement for peak mass flow rate is illustrated in FIG. 6a, which defines the absolute C1.sub.max and relative W1.sub.max flow velocities into and absolute C2.sub.max and relative W2.sub.max flow velocities out of the flow-guidance element. Similarly, due to the rotation of the flow-control assembly 120, the relative flow angle experienced by the turbine 110 at peak mass flow rate is altered when compared with the corresponding flow angle at turbine ingress where the prior art nozzle ring 140 of FIG. 3 is used in place of the flow-guidance element 120 of FIG. 5.

[0062] The relative and absolute flow angles at both peak and trough mass flow rates illustrated in FIGS. 5b and 6b have been superimposed in FIG. 7 for illustrative purposes.

[0063] In FIG. 7, the variation of the relative flow angle for the arrangement in which the stationary nozzle ring 140 is used (see FIG. 3) is illustrated by . Similarly, the variation in relative flow angle between peak and trough mass flow rate for the arrangement using the flow-control assembly 120 is illustrated by .

[0064] As can be seen, from FIG. 7, is less than . 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 140. Accordingly, the efficiency of the turbine is increased.

[0065] The absolute flow of fluid out of the flow-guidance element 120 and at turbine ingress is more tangential at trough mass flow rate than the case with a stationary nozzle ring 140. Similarly, the absolute flow at of fluid out of the flow-guidance element 120 and at turbine ingress is more radial at peak mass flow rate than the case with a stationary nozzle ring 140. Accordingly, the variation in relative flow angle at turbine ingress is reduced.

[0066] Rotation Mode

[0067] As set out in the above examples, the flow-control assembly 120 is configured to rotate about the same axis of rotation as the turbine 100 so as to guide the inbound fluid onto the blades 115 of the turbine 110. Different approaches to rotating the flow-control assembly 120 about the axis of rotation are envisaged and are set out below in further detail.

[0068] In a first mode of rotation, an external actuator is used to drive the rotation of the flow-guidance element 120 about the axis of rotation. In this first mode, the layout of the pressure and suction surfaces of the flow-guidance vanes 125 is opposed to that of the blades 115 of the turbine 110. As fluid flows over the flow-guidance vanes 125, the direction of torque imposed on the flow-guidance element 120 by the pressure difference between the pressure and suctions surfaces of the flow-guiding vanes 115 is opposite to that of the turbine 110. Accordingly, the external actuator is used to overcome the negative torque and to enable the flow-guidance element 120 to rotate favourably to the turbine. This arrangement is referred herein as the Compressor Mode.

[0069] The actuator may be any externally powered means of rotating the flow-guidance element about the axis of rotation, such as an electric motor.

[0070] The compressor mode is advantageous since it is possible to control, using the actuator, the speed of rotation of the flow-guidance element 120 about the axis of rotation. However, the flow-guidance element 120 powered in this way can be considered to be an energy consumer since external power is needed to rotate the flow-guidance element 120.

[0071] The flow-guiding vanes 125 of the flow-guidance element 120 may be configured as a forward vane or a backward vane when used in the externally powered compressor mode. Specifically, the forward vane is configured to rotate the flow-guidance element 120 favourably to the upstream exhaust flow whilst the backward vane is configured to rotate the flow-guidance element 120 towards the exhaust flow.

[0072] In a different, second mode of operation, it is not necessary to provide external power to cause rotation of the flow-guidance element 120. Instead, the flow-guiding vanes 125 are configured such that the positions of the pressure and suction surfaces differ from the above-described compressor mode so that the direction of the torque imposed on the vanes 125 by the pressure difference between the pressure and suction surfaces is the same as the turbine 110 torque. Accordingly, the flow-guidance element 120 is able to rotate favourably to the turbine 110 without the need for an external actuator. The fluid flow passing over the flow-guiding vanes 125 causes the flow-guidance element 120 to rotate. This arrangement is referred to herein as Turbo Mode,

[0073] Rotation Direction

[0074] It is also possible to select the direction of rotation of the flow-guidance element 120 relative to the direction of rotation of the turbine 110 so as to adapt the relative flow angle at turbine 110 ingress.

[0075] Specifically, there are four possible configurations based upon the above-described forward vane and backward vane.

[0076] A first configuration is to use a forward vane on a flow-guidance element 120 rotating in the same rotational direction as the turbine; a second configuration is to use a forward vane on a flow-guidance element 120 rotating in an opposing rotational direction to the turbine; a third configuration is a backward vane on a flow-guidance element 120 rotating in the same rotational direction as the turbine; and a fourth configuration is to use a backward vane on a flow-guidance element 120 rotating in an opposing rotational direction as the turbine.

[0077] All of these four configurations are able to adjust the flow angle adaptively according to the varying mass flow rate. The difference between the configurations is the direction of the flow angle adjustment. With the first and second configurations, the flow angle out of the flow-guidance element 120 will be bigger in low mass flow rate than in high mass flow rate.

[0078] With the third and fourth configurations, the flow angle out of the flow-guidance element 120 is smaller in low mass flow rate than in high mass flow rate, which may not be suitable for turbocharger turbine, but may have suitability for other applications.

[0079] The skilled person will recognize that external power sources may be used to actuate the movement of the flow guidance element 120 according to design requirements.

[0080] Rotation Speed

[0081] In some arrangements, the rotation speed of the flow-guidance element 120 may be constant. For example, the flow-guidance element 120 may be rotated by an actuator at any rotation speed greater than zero revolutions per second and up to the rotation speed of the turbine 110.

[0082] As indicated by the velocity triangle analysis of FIGS. 5 to 9, higher rotation speed will generally increase the advantageous reduction in the variation of relative angle flow. However, the incidence loss on the flow-guidance element will also increase along with the increasing flow guidance element rotation speed, and the friction loss in a real application will also increase. These losses will counterbalance the benefits of a flow-guidance element. Therefore, it is preferable that the rotation speed of the nozzle is less than or equal to 150 revolutions per second. However, other rotations speeds are envisaged.

[0083] It is also possible to control the variation in relative flow angle at turbine ingress using a variable rotational speed.

[0084] A first approach for controlling the deviation in relative flow angle .sub.3 is to rotate the flow-guidance element 120 at a lower rotational speed when the mass flow rate into the flow-guidance element 120 is at its peak compared with the rotational speed of the flow-guidance element 120 at trough mass flow rate.

[0085] A second approach for controlling the deviation in relative flow angle .sub.3 is to rotate the flow-guidance element 120 at a higher rotational speed when the mass flow rate into the flow-guidance element 120 is at its peak compared with the rotational speed of the flow-guidance element 120 at trough mass flow rate.

[0086] With the first approach, the absolute flow angle out of the flow-guidance element 120 will be larger at trough mass flow rate and smaller at high mass flow rate, compared with a fixed nozzle ring or a flow-guidance element 120 at a constant rotational speed.

[0087] This will introduce a further reduction in the varying relative flow angle and therefore increase the efficiency of the turbine rotation. With the second approach, the absolute flow angle out of the flow-guidance element 120 will be smaller at trough mass flow rate and larger at peak mass flow rate, compared with a fixed rotational speed. Whilst this approach may not be advantageous for a turbocharger turbine, the arrangement has suitability for other applications.

[0088] In an arrangement, the flow-guidance element is static under peak mass flow, and as the mass flow rate decreases it gradually speeds up until it achieves peak rotational speed under trough mass flow rate, and then it slows down again as the mass flow rate increases. It can be observed that with this method the relative flow direction at the inner turbine inlet can be maintained exactly at the design point, which is companied by peak turbine efficiency.

[0089] Calculation Results

[0090] A computational fluid dynamics (CFD) model was used to simulate the performance of an example flow-control assembly of the present disclosure. The following parameters of the turbine were used:

TABLE-US-00001 Parameter Value Leading edge tip diameter 95.14 mm Leading edge span height 18 mm Trailing edge tip diameter 78.65 mm Leading edge span height 25.79 mm Cone angle 40 degrees Leading edge blade angle 20 degrees Root mean radius at trailing edge blade 52 degrees angle Length of axial chord 40 mm Number of rotor blades 12 Tip gap height (blade span) 5% Volute exit flow angle 68 degrees

[0091] The two main components of the CFD model, namely the flow-guidance element and the turbine, were meshed with a structure hexahedral mesh giving the following mesh statistics for each component:

TABLE-US-00002 Region Element Type Nodes Flow-guidance element Hexahedral 383742 Turbine Hexahedral 636603 Total Hexahedral 1020345

[0092] To simulate the varying mass flow rates into the flow-control assembly, the following boundary conditions and setup parameters were used:

TABLE-US-00003 Boundary Condition Value Type of analysis Steady-state Non-dimensional turbine speed 80% Fluid Air Ideal Gas Residual value of parameters 1e06 Mesh connection Frozen rotor Turbulence Model k-epsilon Cp 1004 J/kgK Non-dimensional mass flow rate 60-100%.sup. Inlet total temperature 338 K Inlet flow directions 68 degrees Exit average static pressure 1 atm

[0093] It will be appreciated that the above parameters are merely used for the purposes of simulating the performance of the flow-control assembly. The above parameters should not be taken to be limiting and many different parameters may be varied without affecting the performance of the flow-control assembly.

[0094] The model was used to evaluate the above-described compressor and turbine modes of rotation and the results of the evaluation of these modes can be seen in FIGS. 8a, 8b, 9a, and 9b.

[0095] An evaluation of efficiency of the flow-control assembly is shown in FIGS. 8a and 8b, which illustrates the efficiency of the turbine stage as a function of rotation speed of the flow-guidance element 120, in this instance a nozzle ring. FIG. 8a illustrates the turbine stage efficiency at trough mass flow rate and FIG. 8b illustrates the turbine stage efficiency at peak mass flow rate.

[0096] As can be seen from the simulation results, the flow-control assembly operating in turbo-mode provides particularly increased efficiency where the flow-guidance element rotates at 120 rps. This arrangement provides a 7.2% efficiency increase at trough mass flow and a 3.3% efficiency increase at peak mass flow. In compressor mode, the flow-control assembly provides particularly increased efficiency at 50 rps, with a 2.5% efficiency increase at trough mass flow rate and a 0.9% increase at peak mass flow rate.

[0097] FIGS. 9a and 9b show that the power output of the turbine is also increased using both the above-described turbo mode and compressor mode. In compressor mode, the power increase at 50 rps, which can be considered the best-performance point, is 13.1% at trough mass flow and 6.04% at peak mass flow. In turbo mode, the power increase at 120 rps is 34.7% at trough mass flow and 18.5% at peak mass flow.

[0098] The flow-guidance element 120 may be physically separated from the turbine 110. The flow-guidance element 120 may be configured to rotate independently of the turbine 110.

[0099] The relative physical arrangement of the turbine 110 and the flow-guidance element 120 set out in FIG. 2 is not essential and other arrangements are conceivable. Specifically, in some arrangements the fluid flow onto the blades may be substantially parallel with the axis of rotation of the turbine 110 and the flow-guidance element 120, for example in aerospace applications. In such arrangements, the flow-guidance element 120 may be axially displaced from the turbine 110. Accordingly, the flow-guidance element 120 may axially guide fluid onto the blades 115 of the turbine 110.

[0100] 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.