WO2008136697A1

WO2008136697A1 - Method and apparatus for flow control of a gas

Info

Publication number: WO2008136697A1
Application number: PCT/RU2007/000224
Authority: WO
Inventors: Andrey Mikhailovich Bartenev; Thomas Hammer
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-05-04
Filing date: 2007-05-04
Publication date: 2008-11-13

Abstract

In a method for flow control of a gas with means of Electrohydrodynamic (EHD) actuating the flow near some dimple-like structures at walls the dimple-like structure in the walls is used for generating a turbulance flow of the gases and the electrohydrodynamic (EHD) actuation of the flow is effected by means of an electrical gas discharge generated between at least two electrodes placed near to or overlapping the dimple-like structure. In an Apparatus for the method, with at least two electrodes (13, 14) for EHD-actuation one of the electrodes (13, 14) is located flow-down of the dimple (12) and the other one is located around the bottom of the dimple (12).

Description

Method and apparatus for flow control of a gas

The invention is related to a method for flow control of a gas and to an apparatus therefore, whereby the gas is stream- ing on a dimple structure. Dimples are regular organized caverns along a surface of a wall with streaming fluids, e.g in a turbine, an exhaust channel of it or a wing of a plane respectively.

Technical Problem:

Dimples geometry are well known approaches for control of flow characteristics. In DE 10 2004 038 930 Al the dimples were placed at gas turbine nozzle vanes and in EP 1 426 5558 A3 the dimpled structure was arranged in gas turbine combus- tion chamber, thus providing prospective solutions for power generation systems. The essence of the dimples operation is that the surface of definite depth can cause the flow separation, which results in eddy generation. The increased vortic- ity in the region of dimple leads to the intensification of the heat exchange between surface and flowing gas. However, additional surface roughness causes momentum losses due to friction forces. Thus, it is desirable to intensify the vortex generation without significant air drag losses.

Current solutions of the problem:

In US 2006/0099073 Al different shapes of dimples were proposed, and in WO 2004/083651 Al it was claimed that for the caverns with height to diameter ratio h/d < 0.1 the reduced friction mode could be realized (State of art: Figure 1) and for h/d >> 0.1 the enhanced heat transfer mode is achieved (State of art: Figure 2). The general approach of finding a solution which intensifies heat transfer and reduces friction losses is a compromise found by means of pure gasdynamics, namely by variation of the dimples depth, diameter and shape. The disadvantage of such a compromise solution is that it works for a very limited range of flow parameters, only.

The aim of the invention is therefore a new technology for getting better flows of gases. These gases could be especially combustion and/or exhausting gases.

The inventive solution is defined by the steps of claim 1. An apparatus is defined in claim 11. Further realiziations of the inventive method and the related apparatus are given by the dependent claims .

Object of the invention is an electrohydrodynamic actuation of devices with dimpled structures on the walls.

The current invention is based on electro-hydrodynamic (EHD) actuation of the flow by means of an electrical gas discharge generated between at least two electrodes placed near to or overlapping with a dimple-like structure (Figure 3) . The electrical gas discharge causes weak ionization of the flowing gas by generating pairs of negatively charged electrons and positively charged ions. These electron-ion pairs are separated by means of electric field induced drift. Due to collisions both electrons and ions transfer momentum to the neutral gas molecules. However, because of their larger mass ions can transfer momentum much more efficiently to the neutral gas than electrons. Thus even in regions where the electron concentration balances the ions concentration a net force density or a pressure gradient acting on the neutral gas flow is induced, which is proportional to the product of ion charge density and local electric field value and has the direction of the local electric field vector (Figure 5) . In the case of a surface discharge electrical charges being necessary for generation of the EHD effect are generated near to the surface, only. Provided that the electrode geometry is designed properly and the properties of the applied voltage such as amplitude, frequency, general shape of the voltage trace, and so on fit to the electrode design, vortex formation can either be promoted or suppressed.

In a preferred embodiment the electrical gas discharge is generated by an electrode set-up consisting of a first electrode having direct contact to the flowing gas and a second electrode being separated from the flowing gas by a dielectric barrier. One of the electrodes is located flow-down of the dimple. The other one is located around the bottom of the dimple.

Of course one of the electrodes can be located flow-up of the dimple, whereas the other one is located around the bottom of the dimple. If the wall material is electrically insulating, the dielectric material may be the wall material itself. Otherwise an insert may be used carrying both the dimple structure and the high voltage electrode, whereas the conductive wall material is electrically connected to ground.

In another embodiment an insulating insert carrying the HV- electrode only is provided. Further both electrodes can be covered by a dielectric barrier, which increases the lifetime of the electrode structure, especially in corrosive gases. In this case according to our invention the lateral extension of the electrodes differs substantially in order to have a strong EHD effect.

In another embodiment both electrodes have direct contact to the flowing gas, so that a direct current flow is enabled. In this case short voltage pulses are applied repetitively to the electrodes, whereby the pulse duration is long enough to enable surface corona discharge formation and short enough to avoid spark breakdown. The electrode structures can e.g. be printed on the insulating surface, which allows great flexi- bility with regard to the electrode design.

The EHD-effect may be improved by superimposing a very short high-voltage pulse providing the non-thermal corona surface discharge and a long pulse having a much lower voltage amplitude below the gas discharge breakdown limit providing the force acting on the gas flow.

Advantages of the current invention:

The current invention has the advantage that it uses small - depth dimples providing decreased momentum losses due to friction in comparison with deep dimples. At the same time it uses EHD actuation for eddy generation. Combined effect of gasdynamics (dimples) and electro-hydrodynamics (DBD) provide powerful solution for heat transfer intensification.

The advantage of the surface barrier discharge is that the current is limited by the capacity provided by the dielectric barrier covering at least one of the electrodes .

The surface corona discharge allows independent control of gas discharge formation and momentum transfer to the fluid.

There is now more flexibility in respect to the working point of a machine with a dimple-structure on the walls.

More advantages and details of the invention are given in the description of preferred embodiments in respect to the draw- ing.

There is shown schematically:

Figure 1 a cross section of a dimple without flow separation in respect to the state of art,

Figure 2 a cross section of a dimple with flow separation in respect to the state of art,

Figure 3 a cross section of a dimple with flow separation caused by EHD, Figure 4 a view onto an arrangement of figure 3,

Figure 5 the EHD (Electro-HydroDynamic) - effect in an asymmetric dielectric barrier surface discharge in respect to the state of art, Figure 6 an alternative electrode geometry for the generation of electro-hydrodynamic effects in an asym metric dielectric barrier surface discharge,

.Figure 7 a second alternative electrode geometry for the generation of electro-hydrodynamic effects in an asymmetric dielectric barrier surface discharge

Figure 8 a third electrode geometry for the generation of electro-hydrodynamic effects in an asymmetric di electric barrier surface discharge, Figure 9 a forth electrode geometry for the generation of electro-hydrodynamic effects in an asymmetric dielectric barrier surface discharge,

Figure 10 an alternative set-up for generation of a direct, non-thermal surface discharge, Figure 11 a graphic representation of superimposed voltage pulses for gas discharge formation due to ionization and EHD effect due to ion drift,

Figure 12 an alternative set-up for generation of a direct, non-thermal surface discharge for ionization and momentum transfer to the neutral gas flow induced by low electric field ion drift,

Figure 13 a graphic representation of superimposed voltage pulses of an alternative set-up of figure 12.

In the figures same parts have same or likely numerals. The examples are described partly together as far as possible.

Drawings and examples :

In figure 1 and figure 2 there are shown a cross section of a wall 1 with a dimple 2 known by the state of art. Dimples are regular organized caverns along a surface of a wall with streaming fluids, e.g. in a turbine, an exhaust channel of it or a wing of a plane respectively.

The wall 1 can be an electrical conductive or insulating material. The dimple 2 has a height h and a diameter d. The ratio of height h and diameter d are responsible for turbulence flow of the gas near the dimple .

In figure 1 there is almost no flow separation because of h/d < 0.1. Similar to divergent-convergent channels, un- steady-state microseparations may exist in a dimple. For this, the extent of the convex region of the rounding-off of the recess edge must be smaller than the depth of dimple. In the presence of sharp edge, such microseparations are possible if the angle between the tangent to the recess surface at the forward stagnation point and the initially smooth surface exceeds 0.1 rad. Longitudinal microvortices like Taylor- Goertler microvortices may exist on a concave surface of recess of even such a small depth. Both unsteady microseparations behind the leading edge of a spherical recess and lon- gitudinal wall microvortices may be the cause of some enhancement of heat transfer.

In figure 2 there is a flow separation because of h/d > 0.1. This means that a turbulent flow occurs. A self-organizing tornado-like vortex flow is observed in the recess. These vortex structures move in the transverse direction from one discretely steady position to the other and back. The axes of vortex structures are inclined to the external surface subjected to flow. As the flow velocity increases, the vortex assumes a pillar-like shape with the longitudinal dimension exceeding significantly its transverse dimension, sucks off the medium from the dimple and wall layer in its neighborhood, and transfers the mass along the main flow, thereby promoting enhancement of heat transfer significantly.

So far the state of art needs an exact geometry for the function at special working conditions. It is the aim of the new apparatus to get more flexible working conditions .

In figure 3 there is wall 11 which is of an electric insulating material. In the cross section of the wall 11 is a dimple 12 with means for flow separation. This flow separation is not caused by the height-diameter-ratio h/d of the dimple 12, but by the EHD effect at h/d < 0.1.

EHD means the electrohydrodynamic effect caused by plasma discharge. Therefore a electrode design for generation of a dielectric barrier surface discharge is needed. For example there is a ground electrode 13 near the surface of the wall 11 and a HV-electrode in the insulated material under the dimple 12. Both electrodes are connected with a source 15 for an AV-voltage of high level. The plasma discharge generates plasma +-ions and free electrons with negative charge -.

Figure 4 shows the structure of a wall 11 with regular dimple 12, 12', 12 ' ' , ... . The electrodes 13, 14 are one part for all dimples 12, 12', 12 ' ^• , ... .

In figure 5 the Electro-hydrodynamic effect in an asymmetric dielectric barrier surface discharge is explained: The first electrode 13 on the wall 11 of insulating dielectric material works as anode (+) and the second electrode in the wall 11 of insulating dielectric material works as cathode (-). The AC- voltage creates forces on the .plasma parts, e.g. Fi_0n on the ions in the region of the cathode and F_ei on the electrons in the region of the anode .

Figure 6 shows an alternative electrode geometry for the generation of electro-hydrodynamic effects in an asymmetric dielectric barrier surface discharge. There is the ground electrode 14 at the bottom of the dimple 12 and the HV-electrode 13 flow-up of the dimple structure.

-Figure 7 shows an alternative electrode geometry for the generation of electro-hydrodynamic effects in an asymmetric dielectric barrier surface discharge. There is an electric conductive substrate with insulating insert for the HV electrode 13. The substrate will be the ground electrode 14. Figure 8 shows an alternative electrode geometry for the generation of a electro-hydrodynamic effect in an asymmetric dielectric barrier surface discharge. The structure is like figure 7. There is an electric conductive substrate with in- sulating insert. The electric^" conductive substrate realizes the ground electrode, whereby in the insulating insert 14' there is located the high voltage (HV) electrode 13.

-Figure 9 shows an alternative electrode design for genera- tion of a dielectric barrier surface discharge. The structure is like figure 3. Both electrodes 13 and 14 are covered by the electric insulating material as a dielectric barrier.

Figure 10 shows an alternative set-up for generation of a di- rect, non-thermal surface discharge. The structure is like figure 3 , but with the electrodes 13 and 14 on the surface of the wall 11. Both electrodes 13 and 14 are in contact to the gas flow.

Figures 11 and 13 show a superimposed voltage pulse for gas discharge formation due to ionization and EHD effect due to ion. drift. In figure 11 and figure 13 there is a graphical representation of the voltage in respect to the time: The abscissa is the time t in arbitrary units and the ordinate the voltage U in arbitrary units respectively. On the ordinate it is signed Ui_0n, U_bd and U drift. For U(t) there are in figure 11 a graph 111 and in figure 13 a graph 131 respectively.

In Figure 11 the graph 111 for U(t) goes down to a minimum of Ui_on and then up to a plateau. On the abscissa there are two significant time points: ti_on, t_drift- The point ti_on is the time where U(t) raises over zero and the point tdrift is the time where U(t) goes again to zero.

Figure 12 shows an alternative set-up for generation of a direct, non-thermal surface discharge for ionization and momentum transfer to the neutral gas flow induced by low electric field ion drift. The structure is like figure 10, but there is a third electrode 18. There is a DC-voltage source 16 and further a separate pulse power source 17. All electrodes 13, 14 and 18 are in contact to the gas flow.

In figure 13 there is a graph 131 for U(t) and a graph 132 for U_dri_ft-_/ which is almost parallel to the abscissa. The graph 131 for U (t) goes down to a minimum of Ui_0n and then up to zero. After a time interval Ti_0n it begins again. So the graph 131 has a periodic structure. On the abscissa there is marked the time interval Ti_0n.

Claims

1. A method for flow control of a gas with means of Electro- hydrodynamic (EHD) actuating the gas flow near some dimple- like structures of walls,

- whereby the dimple-like structure in the walls is used for generating a turbulence flow of gases and

- whereby the electrohydrodynamic (EHD) actuation of the flow is effected by means of an electrical gas discharge gener- ated between at least two electrodes placed near to or overlapping the dimple-like structure.

2. The method of claim 1, whereby the electrical gas discharge causes weak ionization of the flowing gas by generating pairs of negatively charged electrons and positively charged ions.

3. The method of claim 2 , whereby the electron-ion pairs are separated by means of electric field induced drift and due to collisions both electrons and ions transfer momentum to the neutral gas molecules .

4. The method of one of the foregoing claims, whereby the ions, because of their larger mass ions transfer momentum much more efficiently to the neutral gas than electrons .

5. The method of claim 4 , whereby even in regions where the electron concentration balances the ions concentration a net force density or a pressure gradient acting^' on the neutral gas flow is induced, which is proportional to the product of ion charge density and local electric field value and has the direction of the local electric field vector.

6. The method of claim 5, whereby In the case of a surface discharge electrical charges being necessary for generation of the EHD effect are generated near to the surface, only.

7. The method of claim 6, whereby, provided that the electrode geometry is designed properly and the properties of the applied voltage such as amplitude, frequency, general shape of the voltage trace, and so on fit to the electrode design vortex formation can either be promoted or suppressed.

8. The method of one of the foregoing claims, whereby the electrical gas discharge is generated by an elec- trode set-up consisting of a first electrode having direct contact to the flowing gas and a second electrode being separated from the flowing gas by a dielectric barrier.

9. The method of one of the foregoing claims, whereby the EHD-effect may be improved by superimposing a very short high-voltage pulse providing the non-thermal corona surface discharge and a long pulse having a much lower voltage amplitude providing the force acting on the gas flow.

10. The method of claim 9, whereby the voltage amplitude is chosen below the gas discharge breakdown limit.

11. Apparatus for the method of one of the foregoing claims, with at least two electrodes (13, 14) for EHD-actuation, whereby one of the electrodes (14) is located flow-down of the dimple (12) and the other one (13) is located around the bottom of the dimple (12) .

12. Apparatus of claim 11, whereby one of the electrodes (13) can be located flow-up of the dimple (12) , whereas the other

(14) one is located around the bottom of the dimple (12) .

13. Apparatus of claim 12, whereby the wall (11) material is electrically insulating and the dielectric material may be the wall material (11) itself.

14. Apparatus of claim 12 , whereby an insert (24) may be used carrying both the dimple structure (12) and the high voltage electrode . (13) , whereas the conductive wall material (11) is electrically connected to ground.

15. Apparatus of claim 14, whereby aseparated insulating insert (24') carrying the HV- electrode ( ) is provided.

16. Apparatus of claim 15. whereby the high voltage electrode (13) is covered by a dielectric barrier, which increases the lifetime of the electrode structure, especially in corrosive gases.

17. Apparatus of claim 16, whereby according the lateral extension of the electrodes (13, 14) differs substantially in order to have a strong EHD effect .

18. Apparatus of claim 17, whereby both electrodes (13, 14) have direct contact to the flowing gas, so that a direct current flow is enabled.

19. Apparatus of claim 18, whereby short voltage pulses are applied repetitively to the electrodes (13, 14), whereby the pulse duration is long enough to enable surface corona discharge formation and short enough to avoid spark breakdown.

20. Apparatus of claim 11, whereby the electrodes (13, 14) have structures printed on a insulating surface, which allows great flexibility with re gard to the electrode design.

21. Apparatus of claim 11, whereby the EHD-effect is improved by superimposing a very short high-voltage pulse of a pulse generator (15') providing the non-thermal corona surface discharge and a long pulse having a much lower voltage amplitude providing the force acting on the gas flow.