WO2013054343A2

WO2013054343A2 - Method and injector for controlling gas flow in a duct

Info

Publication number: WO2013054343A2
Application number: PCT/IN2012/000487
Authority: WO
Inventors: Bhaskar Roy; Madathil Pradeep AMBOOR; Dinesh Kanhaiya BHATIA
Original assignee: Indian Institute Of Technology
Priority date: 2011-08-03
Filing date: 2012-07-10
Publication date: 2013-04-18
Also published as: WO2013054343A3

Abstract

Method and injector for controlling gas flow in a duct. The gas flow in the duct is energised by injecting an auxiliary gas into the duct at predetermined locations at the surface of the gas flow in the duct in the direction of the gas flow at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct to prevent and/or reconstitute flow separation and to ensure smooth gas flow in the duct. The injector (1) comprises an annular member (2) having an inlet end (5) and an outlet end (6) and being disposed between and fixed to two duct elements (3, 4) forming a duct in a leak tight manner. One duct element (3) is at the inlet end of the annular member and the other duct element (4) is at the outlet end of the annular member. The inlet end of the annular member describes an inner diameter marginally smaller than the inner diameter of the duct element (3) at the inlet end thereof and the outlet end of the annular member describes an inner diameter matching with the inner diameter of the other duct element (4) at the outlet end thereof. The annular member further comprises a plurality of orifices (8) formed therein in a predetermined pattern. Each of the orifices defines a wide outer end (9) and a narrow inner end (10). The narrow ends of the orifices open into the duct in the direction of the gas flow in the duct at an injection angle of 0 to 3° with respect to the axis (A) of the duct and at a skew angle of 0 to 3° with respect to the axial and radial plane of the duct. A plenum chamber (11) is mounted to the annular member in a leak tight manner and communicates with the wide outer ends of the orifices (Fig 3).

Description

TITLE OF THE INVENTION

Method and injector for controlling gas flow in a duct

FIELD OF THE INVENTION

This invention relates to a method and injector for controlling gas flow in a duct.

BACKGROUND OF THE INVENTION

When a fluid (liquid or gas) flows over a surface, there is a certain amount of friction at the interface between the fluid and the surface, primarily due to the viscosity of the fluid. Very close to the surface there is a layer of fluid that is affected by this viscous effect, known as the boundary layer. At the surface, the fluid velocity is zero and within the boundary layer there is a gradient in the velocity, that is to say, fluid velocity changes continuously till it reaches the fluid velocity at the extreme fluid layer away from the surface, often referred to as free stream flow. This velocity gradient from the surface to the freestream is expressed in the form of a velocity profile.

Besides the viscosity of the fluid, there are other parameters that affect the boundary layer, like the fluid density or surface roughness. Reynolds number is a non-dimensional parameter that is used to characterise fluid flows. Reynolds number is the ratio of inertial force to the viscous force and is expressed as Re= pVd/μ, where p is the density of the fluid, V is the velocity of the fluid, d is the characteristic dimension of the surface on which the fluid flows and μ is the fluid viscosity.

Fluid flows are often classified as laminar or turbulent. In a laminar fluid flow, layers of fluid flow smoothly over the surface without mixing with one another. In a turbulent fluid flow, on the other hand, the various layers of the fluid mix with one another and flow over the surface^' with a random motion. Hence, in a turbulent flow, the fluid near the surface will have greater energy because of the mixing between the boundary layer fluid and the fluid away from the surface. Whether or not a fluid flow is laminar or turbulent is determined by the Reynolds number. For example in a fluid flow in a pipe, if the Reynolds number is about 2300 or lower, the flow is said to be laminar. For Reynolds number greater than 2300, the flow is said to be turbulent. The velocity profile is flatter in a turbulent boundary layer on account of the increased mixing between the fluid layers.

Fluid flows (laminar as well as turbulent fluid flows) are heavily dependent on the downstream pressure or the pressure gradient across the boundary layer. Flow naturally prefers a decreasing pressure in the direction of the flow and therefore such a pressure gradient is known as favourable pressure gradient. However in flow situations, where the downstream pressure is greater than the upstream pressure (also known as adverse pressure gradient), the boundary layer behaviour can be drastically affected. If the adverse pressure gradient is higher than what the boundary layer can withstand, the fluid particles will detach from the surface. This is known as boundary layer separation or flow separation. Flow separation leads to loss of energy because a part of the energy of the fluid will be lost due to flow separation. Since a turbulent boundary layer has a higher energy as a result of its mixing characteristics, such boundary layers can withstand higher adverse pressure gradients as compared to laminar boundary layers. There are numerous instances or situations where boundary layer separation is likely to be encountered. A typical example is fluid flow in a pipeline. Flow separation may be experienced in a pipeline when the flow encounters a sudden expansion or a significant change in the pipe shape or geometry, for instance, a bend or joint in the pipeline. Boundary layer separation occurs due to lack of energy in the boundary layer to withstand adverse pressure gradients. When a pipe flow encounters flow separation, the amount of power or energy required to pump the fluid in the pipe increases tremendously. It is, therefore, important to prevent or delay flow separation or reconstitute flow separation in order to realise savings in terms of pumping power. The longer the distance the fluid has to be conveyed or transported, the higher the pumping power saving. Further while fluids are conveyed or transported through ducts, especially over long distances it is often necessary to reduce or eliminate the effects of external cold or hot conditions affecting the fluids flowing through the ducts. In order to avoid the effects of external hot or cold conditions influencing the fluids flowing through the ducts, the material thickness of the ducts is increased. As a result, the material cost and fabrication cost of the ducts are increased.

DETAILED DESCRIPTION OF THE INVENTION

The term gas as used in the specification in the context of the invention refers to any gas and includes air and any homogenous liquid like water but excludes highly viscous and heavy fluids.

The term duct as used in the specification in the context of the invention refers to any flow passage or channel and includes pipe, pipeline or conduit.

According to the invention there is provided a method of controlling gas flow in a duct comprising energising the gas flow in the duct by injecting an auxiliary gas into the duct at predetermined locations at the surface of the gas flow in the duct in the direction of the gas flow at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct to prevent and/or reconstitute flow separation and to ensure smooth gas flow in the duct. According to the invention there is also provided an injector for controlling gas flow in a duct comprising an annular member having an inlet end and an outlet end and being disposed between and fixed to two duct elements forming the duct in a leak tight manner, one duct element being at the inlet end of the annular member and the other duct element being at the outlet end of the annular member, the inlet end of the annular member describing an inner diameter marginally smaller than the inner diameter of the duct element at the inlet end thereof and the outlet end of the annular member describing an inner diameter matching with the inner diameter of the other duct element at the outlet end thereof, the annular member further comprising a plurality of orifices formed therein in a predetermined pattern, each of the orifices defining a wide outer end and a narrow inner end, the narrow ends of the orifices opening into the duct in the direction of the gas flow in the duct at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct and a plenum chamber mounted to the annular member in a leak tight manner and communicating with the wide outer ends of the orifices. BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying schematic drawings :

Fig ί is an inlet end view of the injector according to an embodiment of the invention; Fig 2 is a side view, of the injector of Fig 1 ;

Fig 3 is a sectional view of the injector of Fig 1 ;

Figs 4 and 5 are isometric views of the injector of Fig 1 ; Fig 6 is a partial view of the injector of Fig 1 located between two duct elements;

Figs 7a, 7b and 7c are side view, front view and top y iew of an orifice of the injector with respect to a duct axis respectively;

Fig 8 is a graphical representation of computational fluid dynamics (CFD) analysis data obtained according to an experimental example; and

Fig 9 is a graphical representation of coefficient of pressure (Cp) variation in the air flow in a duct with and without injection of auxiliary gas.

As illustrated in Figs 1 to 6 and 7a, 7b and 7c of the accompanying drawings, the injector 1 comprises an annular member 2 disposed between two duct elements 3, 4 forming a duct for a gas to flow therethrough. The annular member comprises a plurality of angular vanes 7 located in a circular configuration radially equally spaced apart from one another. The gaps between adjacent vanes form orifices 8 equally spaced apart from one another and having a wider outer end 9 and a narrow inner end 10. The narrow ends of the orifices open into the duct in the direction of the gas flow and describe an injection angle of 0 (at tangent) to 3° with respect to the axis A of the duct and a skew angle of 0 (at tangent) to 30° with respect to the axial and radial plane of the duct (Figs 7a, 7b and 7c). Preferably the vanes are adjustable to adjust the position of the narrow ends of the orifices in the direction of gas flow in the duct at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle to 0 to 30° with respect to the axial and radial plane of the duct. 1 is a plenum chamber mounted to the annular member in a leak tight manner and communicating with the wide outer ends of the orifices. The annular member 2 comprises a ring element 5 at the inlet end thereof having an inner diameter marginally smaller than the inner diameter of the duct element at the inner end thereof. The annular member 2 also comprises a ring element 6 at the outlet end thereof having an inner diameter matching with the inner diameter of the duct element at the outlet end thereof. The ring elements are fixed to the vanes. Because of the marginally smaller inner diameter of the ring element at the inlet end of the annular member, the narrow inner ends of the orifices can be designed to open into the duct at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with the axial and radial plane of the duct. The ring elements 5 and 6 each comprises a mounting flange 12 and a mounting flange 13 respectively adapted to be mounted to corresponding flanges 14 and 15 of the respective duct elements 3 and 4 in a leak tight manner. The annular member is located in the duct element 4 at the outlet end of the annular member just before the duct element 4 and hence the duct takes a conical shape marked 4a and makes a change in the duct geometry or shape.

According to the invention the gas flow in the duct is energised by injecting an auxiliary gas into the duct at predetermined locations at the surface of the gas flow in the duct in the direction of the gas flow at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0° (at tangent) to 30° with respect to the axial and radial plane of the duct. The auxiliary gas to be injected is pumped into the plenum chamber through an inlet opening (not shown) in the plenum chamber and the auxiliary gas gets evenly distributed in the plenum chamber and flows into the duct through the orifices. The auxiliary gas picks up its velocity as it is vented out through the narrow ends of the orifices. Energisation of the gas flow in the duct by injection of the auxiliary gas helps to reduce pressure gradient across the down stream flow of the gas in the duct and to prevent and /or reconstitute flow separation in the duct and to ensure smooth gas flow in the duct. As a result, considerable savings in pumping power is realised. The auxiliary gas injected may also act as a heat shield to reduce the effects of external hot or cold conditions of the duct influencing the gas flowing through the duct. Therefore, there is also the possibility of reducing the material thickness of the duct so as to reduce the material cost and fabrication cost of the duct.

The amount of auxiliary gas injected into the duct will depend upon parameters influenced by local fluid dynamics and duct geometry. Preferably the auxiliary gas is injected in 1.5 to 3% of the gas flowing in the duct. Still preferably the auxiliary gas is injected in 2% of the gas flowing in the duct. Preferably the auxiliary gas is injected at the surface of the gas flow at an injection angle of 0° with respect to the axis of the duct and at a skew angle of 0° with respect to the axial and radial plane of the duct. Preferably the auxiliary gas is injected at predetermined locations at the surface of the gas flowing in the duct just before a change in the duct geometry or shape as illustrated by the conical shape of the duct in the drawings. Preferably the auxiliary gas is injected at predetermined locations which are equally spaced apart around the surface of the gas flowing in the duct.

Preferably the auxiliary gas is tapped from the upstream of the gas flowing in the duct instead of using a different source of gas for the auxiliary gas. The auxiliary gas used for injection may be the same gas as that is flowing in the duct or a gas which is inert to the gas flowing in the duct. As the auxiliary gas is injected in the same direction as the gas flow in the duct, non-return valves are normally not required at the narrow ends of the orifices to prevent back flow. Preferably the duct is circular, rectangular or oval in cross section before the location of injection of the auxiliary¹ gas and the duct is circular, rectangular or oval in cross section or conical axially after the location of injection of the auxiliary gas. Preferably the duct element at the inlet end of the annular member is circular, rectangular or oval in cross section and the duct element at the outlet end of the annular member is circular, rectangular or oval in cross section or conical axially. The following comparative experimental example is illustrative of the invention but not limitative of the scope of the invention: Example 1

In one experiment air was allowed to flow in a duct having an inner diameter of 0.5 m and a conical shape. Mass flow rate of the air was maintained at a constant rate of 6.75 kg/s and a volume flow rate of 5.86 m /s (12416 cfm). Power of air flow in duct was 13.3 kW. > In another experiment auxiliary air was injected into the duct through a typical injector of Figs 1 to 6 and 7a, 7b and 7c introduced in the duct just before the duct takes the conical shape. The orifices in the annular member were equally spaced apart around the annular member. The auxiliary gas was injected at the surface of the air flow in the duct at an injection angle of 0° with respect to the axis of the duct and at a skew angle of 0° with respect to the axial and radial plane of the duct and in the same direction as the gas flow in the duct. Auxiliary gas mass flow rate was 1.6% of the gas flow in the duct. Power of auxiliary air flow was 0.21 kW.

It is seen from the above experiments that pressure loss or drop in the air flow in the duct without injection of auxiliary air as measured by pitot probe inserted in the duct was 268.59 Pascals. Power loss of the air flow in the duct without injection of auxiliary air as measured by torque meter attached to the shaft of the blower blowing air through the duct was 1.57 kW. Pressure loss or drop and power loss of the air flow in the duct with injection of auxiliary air as measured as described earlier was 194.3079 Pascals and 1.13 kW, respectively. Power gain with injection of auxiliary air calculated from the above values was 0.44 kW. Power gain with injection of auxiliary air was thus found to be more than twice the power spent for injection of auxiliary air. It is reasonable to conclude from the above findings that power saving would substantially increase as the length of the duct increases.

Computational Fluid Dynamics (CFD) analysis of the effect of injection of the auxiliary gas in the duct was carried out and was as shown in Fig 8 of the accompanying drawings. Fig 8 clearly shows that the gas flow rate in the duct increased at the boundary in the conical shaped portion of the duct. The point at which the air flow in the duct takes the conical path is marked 16 in Fig 8. The coefficient of pressure (Cp) variation in the air flow in the duct with and without injection of auxiliary gas was studied and was as illustrated in Fig 9 of the accompanying drawings. Cp denotes the static pressure coefficient and it was measured along the length of the duct L, where x/L denotes the distance to a station at 0.2, 0.4, 0.6, 0.8, 0.9 or 1 axially along the duct from the beginning of the duct. Ideal Cp'" is calculated based on the equation Cp = [ 1 (AA/AB)²], where AA represents duct area at the gas inlet end and AB represents duct area at the gas outlet end of the duct. It is seen from Fig 9 that the actual Cp variation with injection of auxiliary gas is closer to the ideal Cp indicating that the duct resistance decreases at the boundary layer in the conical portion of the duct and the gas flow in the conical portion of the duct is smooth at the boundary layer.

The above embodiment is by way of example of the invention and should not be construed and understood to be limiting the scope of the invention. Several variations of the embodiment are possible without deviating from the scope of the invention. The injector construction and configuration can be different. Instead of injecting the auxiliary gas at predetermined locations equally spaced around the surface of the gas flowing in the duct, the auxiliary gas can be injected at predetermined sector or segment at the surface of the gas flowing in the duct or at predetermined sectors or segments around the surface of the gas flowing in the duct. The duct geometry and shape can be different. The gas flow in the duct can be accelerated with injection of the auxiliary gas even in a duct without any change in the geometry or shape. Such variations in the construction or configuration of the invention are obvious to a person skilled in the art and are to be construed and understood to be within the scope of the invention.

Claims

CLAIMS:

1. A method of controlling gas flow in a duct comprising energising the gas flow in the duct by injecting an auxiliary gas into the duct at predetermined locations at the surface of the gas flow in the duct in the direction of the gas flow at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct to prevent and/or reconstitute flow separation and to ensure smooth gas flow in the duct.

2. The method as claimed in claim 1, wherein the auxiliary gas is injected in 1.5 to 3% of the gas flowing in the duet.

3. The method as claimed in claim 1, wherein the auxiliary gas is injected in 2% of the gas flowing in the duct.

4. The method as claimed in anyone of claims 1 to 3, wherein the auxiliary gas is injected at the surface of the gas flow at an injection angle of 0° with respect to the axis of the duct and at a skew angle of 0° with respect to the axial and radial plane of the duct.

5. The method as claimed in anyone of claims 1 to 4, wherein the auxiliary gas is injected at predetermined locations at the surface of the gas flowing in the duct just before a change in the duct geometry or shape.

6. The method as claimed in anyone of claims 1 to 5, wherein the auxiliary gas is injected at predetermined locations which are equally spaced apart around the surface of the gas flowing in the duct.

7. The method as claimed in any one of claims 1 to 5, wherein the auxiliary gas is injected at a predetermined sector or segment at the surface of the gas flowing in the duct or at predetermined sectors or segments around the surface of the gas flowing in the duct.

8. The method as claimed in anyone of claims 1 to 7, wherein the auxiliary gas is tapped from the upstream of the gas flowing in the duct.

9. The method as claimed in anyone of claims 1 to 8, wherein the duct is circular, rectangular or oval in cross section before the location of injection of the auxiliary gas and the duct is circular, rectangular or oval in cross section or conical axially after the location of injection of the auxiliary gas.

10. An injector for controlling gas flow in a duct comprising an annular member having an inlet end and an outlet end and being disposed between and fixed to two duct elements forming a duct in a leak tight manner, one duct element being at the inlet end of the annular member and the other duct element being at the outlet end of the annular member, the inlet end of the annular member describing an inner diameter marginally smaller than the inner diameter of the duct element at the inlet end thereof and the outlet end of the annular member describing an inner diameter matching with the inner diameter of the other duct element at the outlet end thereof, the annular member further comprising a plurality of orifices formed therein in a predetermined pattern, each of the orifices defining a wide outer end and a narrow inner end, the narrow ends of the orifices opening into the duct in the direction of the gas flow in the duct at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 3° with respect to the axial and radial plane of the duct and a plenum chamber mounted to the annular member in a leak tight manner and communicating with the wide outer ends of the orifices.

1 1. The injector as claimed in claim 10, wherein the narrow ends of the orifices open into the duct at an injection angle of 0° with respect to the axis of the duct and at a skew angle of 0° with respect to the axial and radial plane of the duct.

12. The injector as claimed in claim 10 or 1 1 , wherein the annular member is disposed between the two duct elements just before a change in the geometry or shape of the duct element at the outlet end of the annular member.

13. The injector as claimed in any one of claims 10 to 12, wherein the orifices are equally spaced apart around the annular member.

14. The injector as claimed in claims 10 to 13, wherein the orifices are provided at a predetermined sector or segment of the annular member or at predetermined sectors or segments of the annular member.

15. The injector as claimed in anyone of claims 10 to 14, wherein the annular member comprises a plurality of angular vanes located in a circular configuration radially equally spaced apart from one another, the gaps between the adjacent vanes forming the orifices which are equally spaced apart from one another, the annular member further comprising a ring element fixed to the vanes and forming the inlet end of the annular member and having an inner diameter marginally smaller than the inner diameter of the duct element at the inlet end thereof and another ring element fixed to the vanes and forming the outlet end of the annular member and having an inner diameter matching with the inner diameter of the duct element at the outlet end thereof.

16. The injector as claimed in claim 15, wherein the ring elements each comprises a mounting flange adapted to be mounted to a corresponding flange of the respective duct element.

17. The injector as claimed in claim 15 or 16, wherein the vanes are adjustable to adjust the position of the narrow ends of the orifices at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct.

18. The injector as claimed in anyone of claims 10 to 17, wherein the duct element at the inlet end of the annular member is circular, rectangular or oval in cross section and the duct element at the outlet end of the annular member is circular, rectangular or oval in cross section or conical axially.