Device for damping thermoacoustic pressure vibrations

A device for damping thermoacoustic pressure vibrations in a combustion chamber (1), in particular in the combustion chamber of a gas turbine, includes a pressure sensor (6) which is connected to the input of a regulating device (10), the regulating device (10) is connected at its output to a device for electrically controlling the flame in the combustion chamber (1). This device for electrically controlling the flame includes a voltage source (11) and an electrode (14). The electrode (14) is connected to a heat shield (12) which surrounds the outflow side of a burner (3) in an annular manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a device for damping thermoacoustic pressure 
vibrations. 
2. Discussion of Background 
During the combustion of fuels in a combustion chamber, pressure 
fluctuations may occur on account of the combustion processes, which 
pressure fluctuations excite thermoacoustic vibrations under suitable 
conditions. These vibrations encourage the increase of pollutant emissions 
on account of combustion inhomogeneities. At vibration resonance, the 
pressure vibrations constitute an undesirable material stress for the 
combustion chamber and impair the flame to the point of extinction. In 
order to dampen such thermoacoustic vibrations, various devices and 
methods have already been proposed in which the combustion chamber is 
influenced in its vibration properties for example. A periodic variation 
in the flow quantities of fuels has likewise been proposed for the 
reduction of vibrations. A feature common to these devices and the 
regulating methods implemented with them to reduce vibrations is that they 
detune the resonant frequency of a burner/combustion-chamber arrangement 
and thus dampen thermoacoustic vibrations. Thus devices are proposed here 
which bring about an indirect reduction of the pressure vibrations with a 
comparatively slow regulating compensation behavior. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention is to reduce and/or dampen 
thermoacoustic pressure vibrations by means of direct control of the 
flame, which thermoacoustic pressure vibrations develop during the 
combustion of inflowing fuel in a combustion chamber. 
The essence of the invention therefore consists in designing the device in 
such a way that, upon a change in the vibration to be damped, the flame is 
correspondingly influenced electrically via a regulating circuit having a 
connected voltage source. 
The essential advantage of the invention may be seen in the fact that the 
vibration-damping measures proposed here act directly on the flame front, 
and thus comparatively quick compensation of the regulating circuit is 
effected.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, only the 
elements essential for understanding the invention are shown, and the 
direction of flow of the fuel and the supplied combustion air and the 
effective direction of the regulating circuit are shown by arrows, FIG. 1 
shows in cross-section a combustion chamber 1 into a which a burner 3 
projects. The outflow side of the burner 3 is provided by the opening of a 
combustion-chamber front plate 13 and by an opening 15 in a heat shield 12 
without the burner 3 coming into contact with the heat shield 12. The heat 
shield 12 is made of a heat-resistant, electrically conductive metal alloy 
and is screwed to the combustion-chamber front plate 13 by a number of 
insulating screw connections 2 in both an electrically and thermally 
insulated manner. The burner 3 is supplied with fuel via a fuel line 5 and 
with combustion air via air inlets 4. The thermoacoustic pressure 
vibrations occurring due to inhomogeneous combustion of a flame 16 are 
detected by a pressure sensor 6 installed in the combustion chamber 1. The 
pressure sensor 6 is connected via a regulating device 10 to a voltage 
source 11 and an electrode 14, and the electrode 14 is electrically 
connected to the heat shield 12 installed in an insulated manner. 
The series connection comprises the regulating device 10, a signal 
conditioner 7 which is connected on the input side to the pressure sensor 
6, a signal processor 8 and an activating means 9 which is connected on 
the output side to the voltage source 11. 
FIG. 2 shows a plan view of the heat shield 12 in the opposite direction to 
the fuel flow. Here, the heat shield 12 is designed as a ring segment of 
an annular gas-turbine combustion chamber and has a circular opening 15. 
The insulating screw connections 2 are arranged around the opening 15, and 
the heat shield 12 is electrically connected to the electrode 14. There is 
an electrically and thermally insulating, annular air gap 18 between the 
burner 3 and the opening 15 of the heat shield 12. Furthermore, the heat 
shield 12 is insulated from adjacent heat shields 12a by means of air gap 
19 and, as shown in FIG. 1, is likewise insulated from the walls of the 
combustion chamber 1 by an air gap 17. 
On account of its electrically and thermally insulated arrangement, the 
heat shield 12 can be loaded as electric field electrode by an electric 
potential generated by the voltage source 11. The aim of using the 
electric potential is to control the combustion properties of the flame 16 
in a regulated manner. 
At this point, it is stressed by way of explanation that the flame 16 is 
considered below as a highly ionized, electrically conductive plasma and 
can therefore be controlled in its combustion properties by loading with 
an electric potential. Only a few thousand volts of an electrode arranged 
near the flame are sufficient, for example, to control the combustion. The 
comparatively small energy loss of the loading voltage source occurring as 
a result is about 0.01% of the controlled combustion energy. The electric 
field causes electric forces to act on the ions contained in the flame. In 
this way, a type of electric wind develops within the flame 16, which 
electric wind has a striking effect on the combustion velocity of the 
flame 16 and stabilizes it. While utilizing this phenomenon by means of 
the device according to the invention, the combustion in the flame 16 is 
regulated in such a way that the load-dependent, thermoacoustic pressure 
vibrations caused by it are reduced and/or damped. It is especially 
advantageous here that no masses have to be moved for the action on the 
flame 16 and that the regulating compensation is effected comparatively 
quickly by the direct electrical control of the flame. 
The most suitable regulated variable for the regulating device 10 is the 
pressure in the combustion chamber 1, which pressure is detected by the 
pressure sensor 6. The measured pressure values are transmitted to the 
signal conditioner 7 and subsequently further processed in the signal 
processor 8. The contiguously installed control unit 9 generates 
corresponding signals for the voltage source 11. In accordance with the 
load-dependent pressure vibrations, the voltage source 11 then loads the 
heat shield 12 via the electrode 14 with a positive direct-current voltage 
in the range up to a few thousand volts. 
Since the outlet of the burner 3 and thus the flame front of the flame 16 
are surrounded by the opening 15 of the heat shield 12, this heat shield 
12 acts on the flame 16 like a positively charged annular electrode, and 
the flame 16 is controlled by the regulating method described above, it 
being especially advantageous that no moving mass is required for the 
regulating device. 
Of course, the invention is not restricted to the exemplary embodiment 
shown and described. It is also conceivable within the scope of the 
invention to load the heat shield 12 with a negative or alternating 
voltage. The arrangement of a different geometric shape of electrode in 
the region of the flame 16 is also conceivable according to the invention. 
A rod electrode, for example, could also be used here. Parallel voltage 
loading of all the heat shields 12 of a combustion chamber 1 which are 
arranged in a ring is likewise conceivable within the scope of the 
invention. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.