EP3173698B1 - Adaptive vibration damper - Google Patents

Description

The invention relates to an adaptive vibration damper.

In the combustion of fuel gas in a fully premixing combustion system undesirable self-excited combustion oscillations may occur at certain operating points. These are usually associated with unpleasant noises for the operator and can in certain circumstances lead to damage to the system.

The phenomenon is in DE 102004013584 A1 and the VDI progress report no. 364 of the series 6 for energy technology. Self-excited vibration (SES) triggers interactions between the temporal release of the flame and the combustion chamber acoustics. Condition for the emergence of the described phenomenon is the temporal power release of the flame at the time of a positive sound pressure amplitude (Rayleigh criterion). Remedies are off DE 19730254 C2 known.

According to the prior art, the problem is usually solved by designing the geometry of the air and fuel gas-air mixture-carrying parts so that the system acoustically is detuned and therefore as possible no self-excited vibrations occur. In this case, for example, additional components are used in the air or exhaust gas path, which have the function to detune the system and thus to prevent or dampen self-excited vibrations. For this purpose, for example, intake pipes which increase the pressure loss upstream of the flame or a Helmholtz resonator are used. Depending on the geometrical characteristics of the Helmholtz resonator, it has a certain natural frequency at which it acts. Vibrations of the combustion system are damped or completely prevented.

Another possibility is to change the excess air or the heat load in an electronic fuel gas-air composite when vibrations occur.

Out DE 102005052881 A1 a sound damping device and a method for a heater with a Helmholtz resonator according to the preamble of claims 1 and 4 is known. A transducer picks up the frequency of the vibration; a temperature-dependent bimetal material realizes the volume change of the resonator space.

The object of the invention is a device and a method for operating the same, with which self-excited vibrations can be turned off in vollvormischenden combustion systems.

According to the invention this is achieved by an individually adaptable Helmholtz resonator according to claim 1 and a method according to claim 4. Advantageous embodiments result from the features of the dependent claims.

The invention is preferably used in electronic fuel gas / air systems in which a mass flow sensor measuring a pressure difference between airway and fuel gas path is used. Self-excited vibrations can not only be detected with the aid of the mass flow sensor, but can also be determined at a sufficiently high sampling rate with respect to the frequency of the oscillation. By means of a Fourier transformation, the measured sensor signal is converted into the frequency domain.

The invention also relates to a Helmholtz resonator which can be adapted in terms of its frequency of action. The frequency at which a Helmholtz resonator acts is defined by its geometric dimensioning. Influencing factors are, on the one hand, the damping volume in the actual body, and, on the other hand, the oscillating volume in the connection channel.

Thus, for example, in the case of a closed cylinder, which is connected via a correspondingly designed connection channel to the combustion system at the corresponding previously determined active site or integrated into the combustion system at this position, the change in a side length of the Helmholtz resonator can be adjusted by the position the bottom of the cylinder along the symmetry axis by means of suitable electromechanics (eg Stellmotor) can be changed. From the determined Frequency of the oscillation occurred, the required side length of the cylinder is calculated and changed so that adjusts this page length.

The invention protects a control system that uses a sensor to determine the frequency of a self-excited oscillation and adjusts an adaptive Helmholtz resonator so that the oscillation is turned off. The system requires a gas control valve, a controllable blower and a burner.

• Figur 1 einen Helmholtz-Resonator,
• Figur 2 eine erste Ausführungsform eines Verbrennungssystems mit einem erfindungsgemäßen Helmholtz-Resonator und
• Figur 3 eine zweite Ausführungsform eines Verbrennungssystems mit einem erfindungsgemäßen Helmholtz-Resonator.

The invention will now be explained with reference to the figures. Hereby show:

• FIG. 1 a Helmholtz resonator,
• FIG. 2 a first embodiment of a combustion system with a Helmholtz resonator according to the invention and
• FIG. 3 a second embodiment of a combustion system with a Helmholtz resonator according to the invention.

FIG. 1 shows a cylindrical Helmholtz resonator 2 with two cylindrical volumes. The first cylindrical Helmholz resonator volume 17 has a length l ₁ and a radius r ₁ . The first cylindrical Helmholz resonator volume 17 is open on both sides and opens on one side in the second cylindrical Helmholz resonator volume 18, which has a length l ₂ and a radius r ₂ .

The natural frequency f _{0 of} this Helmholtz resonator can be determined as follows. $$f_0=\frac{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">c</figure-callout>}}{2\pi }\sqrt{\frac{A_1}{V_2\left({\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_1+2\cdot \mathrm{<figure-callout\; id="1"\; label="\Delta \; l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{l}_{}{}_1\right)}}$$

).Here, c is the speed of sound, A _{1 is} the cross-sectional opening of the entrance of the Helmholtz resonator (here: A ₁ = r ₁ ² * π), V _{2 is} the volume of the second Helmholtz resonator volume 18 (here: V2 = r ₂ ² * π * l ₂ ) and 2 * Δl ₁ the orifice correction (here: $\mathrm{<figure-callout\; id="1"\; label="\Delta \; l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{l}_{}{}_1\approx \frac{\mathrm{<figure-callout\; id="4"\; label="\pi "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{4}\cdot r_1$

).

FIG. 2 shows a combustion system according to the invention with a burner 11 and a blower 10 in a fresh gas guide 14. The blower 10 sucks combustion air and supplies it to the burner 11. Upstream of the blower 10, a fuel gas duct 15 opens into a venturi 3 in the fresh gas duct 14, in which a fuel gas control valve 9 is arranged in front of the mouth. A mass flow sensor 1 is arranged between the fuel gas guide 15 and the fresh gas guide 14 and serves to adapt the fuel gas-air mixture. Upstream of the Venturi 3 is an off FIG. 1 known cylindrical Helmholtz resonator 2 connected to the fresh gas guide 14. The connection of the Helmholtz resonator 2 must take place at a previously determined active site / active position. The Helmholtz resonator 2 has a displaceable resonator cylinder 4 in the second cylindrical Helmholz resonator volume 18, so that its volume can be continuously adjusted. To move the resonator cylinder 4 is a linear stepping motor 5, which can move via a drive shaft 6 in the form of a threaded rod and an internal thread 7 in the resonator cylinder 4 latter. To prevent rotation of the resonator 4, a rotation is provided. An ultrasonic or Laserwegmesser 8 serves to detect the current length of the second cylindrical Helmholz resonator volume 18. The resonator 4 closes gas-tight to the side walls of the second cylindrical Helmholz resonator volume 18 from. The necessary seal can be performed for example as a lip seal or ring seal. A controller 16 is connected to the blower 10, the fuel gas control valve 9, the mass flow sensor 1, the linear stepping motor 5 and the ultrasonic or Laserwegmesser 8.

If self-excited combustion oscillations occur during operation of the system, this leads to a feedback to the combustion and fuel gas flow, so that the mass flow sensor 1 detects the vibrations. Since in practice the signal of the mass flow sensor 1 is always superimposed by various frequencies, threshold values for the intensity of the vibrations must be exceeded for the necessity of carrying out remedial measures.

First, N values of the sensor signal are recorded at a sampling rate f. The sampling rate must be twice as high as the expected maximum frequency. In order to be able to apply a fast Fourier transform analysis, the number N of the values must correspond to a power of two.

$$\mathrm{mit}k=\mathrm{0,1,}\dots \dots ,N-1$$

The Fast Fourier Transform (FFT) algorithm is based on the Discrete Fourier Transform. For N real samples ^N / ₂ frequency or spectral lines F ( iω _k ) are calculated: $$F\left(i{\omega }_k\right)=\frac{1}{N}{\displaystyle \underset{n=0}{\overset{N-1}{\Sigma }}f\left(t_n\right)\cdot e^{-\mathrm{<figure-callout\; id="2"\; label="i"\; filenames="imgf0002.png"\; state="\{\{state\}\}">i</figure-callout>}2\mathit{\pi kn}/N}}$$

$$\mathrm{With}k=0.1.......N-1$$

The frequency with the highest deflection is set as the frequency of the self-excited oscillation and further processed. Subsequently, the required height of the resonator cylinder is determined from this determined frequency of the combustion oscillation. For the natural frequency f _{0 of} the Helmholtz resonator applies (with reference to FIG. 1 , see above): $$f_0=\frac{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">c</figure-callout>}}{2\pi }\sqrt{\frac{A_1}{V_2\left({\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_1+2\cdot \mathrm{<figure-callout\; id="1"\; label="\Delta \; l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{l}_{}{}_1\right)}}$$

It follows for l ₂ : $${\mathrm{<figure-callout\; id="2"\; label="l"\; filenames="imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_2=\frac{c^2{A}_1}{f_0{}^2\cdot 4{\pi }^3{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0002.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^2\left({\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_1+2\cdot \mathrm{<figure-callout\; id="1"\; label="\Delta \; l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{l}_{}{}_1\right)}$$

The controller 16 controls the linear stepper motor 5 such that the resonator cylinder 4 is shifted to the target position until the second cylindrical Helmholtz resonator volume 18 has this calculated length l ₂ . The ultrasonic or Laserwegmesser 8 here serves the control 16 for control. Alternatively, starting from a stop position, a previously calculated number of steps can be moved. This results from the required path, the pitch of the thread and the number of steps per revolution of the motor.

In another embodiment, an incremental encoder can be used to determine the position of the stepper motor and thus the height of the cylinder. This step losses can be compensated.

FIG. 3 shows an alternative embodiment. The Helmholtz resonator 2 is arranged between the blower 10 and burner 11. A motor 12, which is connected to the control 16, moves via a threaded rod 13 the resonator cylinder 4 within the second cylindrical Helmholz resonator volume 18. The threaded rod 13 is connected to the resonator cylinder 4 rigid or via a movable bearing. In this case, a determination of the length of the second cylindrical Helmholz resonator volume 18 can be dispensed with by starting from an extreme position of the resonator cylinder 4 while the combustion oscillations occur, which is still detected by the mass flow sensor 1, in the direction of the other extreme position is until the self-excited combustion oscillations are turned off, and thus the correct side length is reached.

If vibrations occur again later, the vehicle will first be moved back to the start position, in order to continue to move until the correct height of the cylinder is reached.

Also possible is a combination of the two described methods: The necessary position is determined from the mass flow sensor signal as described above. Then the calculated position is approached to a defined distance, and then slowly continue to move until the vibrations are turned off.

If a critical frequency is known even before the start of the incineration plant, it can be stored in the memory and the corresponding size of the Helmholtz resonator can already be set before the start of the burner in order to prevent the vibrations from occurring. In the simplest case, the setting of the Helmholtz resonator remains after switching off the incinerator in the last set position and is available unchanged at the next start.

A system may each have a self-excited swing at two different frequencies. In this case example, for example, there is a tendency to oscillate at 40Hz and at 160Hz. According to the prior art, the problem could be solved by using two Helmholtz resonators, which are tuned in their geometric properties to one of the two frequencies. According to the invention, the Helmholtz resonator can be adapted so that it is able to cover both frequencies. If there is more than one frequency in the burner operating range, the control system can always set the resonator to the frequency that has just or previously been measured in the operating point just approached. The frequencies and the operating points are stored in a memory. When changing from one operating point to another, it is read in the memory whether there is another critical frequency to the new operating point than that to which the resonator is currently set. If this is the case, the resonator geometry is already adapted during the modulation as described above.

LIST OF REFERENCE NUMBERS

1

Mass flow sensor

2

Helmholtz resonator

3

venturi

4

Resonatorzylinder

5

Linear Stepper Motor

6

drive shaft

7

inner thread

8th

Ultrasonic or Laser Wegmessers

9

Gas control valve

10

fan

11

burner

12

engine

13

threaded rod

14

Fresh gas management

15

Fuel gas management

16

regulation

17

first cylindrical Helmholz resonator volume

18

second cylindrical Helmholz resonator volume

Claims (8)

1. Combustion system having a burner (11), a fan (10) for conveying combustion air and optionally combustion gas to this burner (11), a fresh gas guide (14) in which the fan (14) is arranged, a combustion gas guide (15) in which a gas control valve (9) is arranged, wherein the combustion gas guide (15) opens in the fresh gas guide (14) or directly in the burner (11), and a Helmholtz resonator (2) is connected to the fresh gas guide (14), wherein the Helmholtz resonator (2) has a drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2), characterised in that a volume or mass flow sensor (1) is arranged in the combustion gas guide (15) and/or fresh gas guide (14) or between the combustion gas guide (15) and fresh gas guide (14), wherein the volume or mass flow sensor (1) is connected to a controller (16), and the controller (16) is connected to the drive (5, 6, 7) for adjusting the volume of the Helmholtz resonator (2).
2. Combustion system according to claim 1, characterised in that the Helmholtz resonator (2) has means for detecting at least one parameter of the Helmholtz resonator (2).
3. Combustion system according to any of the preceding claims, characterised in that the fan (10) has a speed detection unit.
4. Method for operating a combustion system having a burner (11), a fan (10) for conveying combustion air and optionally combustion gas to this burner (11), a fresh gas guide (14) in which the fan (14) is arranged, a combustion gas guide (15) in which a gas control valve (9) is arranged, wherein the combustion gas guide (15) opens in the fresh gas guide (14) or directly in the burner (11), a volume or mass flow sensor (1) is arranged in the combustion gas guide (15) and/or fresh gas guide (14) and/or the fan (10) has a speed detection unit, a Helmholtz resonator (2) is connected to the fresh gas guide (14), wherein the Helmholtz resonator (2) has a drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2), characterised in that by means of the volume or mass flow sensor (1) self-excited combustion oscillations and optionally the frequency thereof are identified and subsequently the volume of the Helmholtz resonator (2) is adjusted in accordance with the established frequency and/or the drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2) is moved until the self-excited combustion oscillations stop.
5. Method for operating a combustion system according to claim 4, characterised in that the frequency is established by means of Fourier transformation.
6. Method for operating a combustion system according to claim 4, characterised in that when self-excited combustion oscillations occur the drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2) is continuously moved from one extreme position in the direction of another extreme position.
7. Method for operating a combustion system according to any one of claims 4 to 6, characterised in that first the drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2) roughly adjusts the calculated volume and subsequently the drive (4, 5, 6, 7) for adjusting the volume of the Helmholtz resonator (2) is moved until the self-excited combustion oscillations stop.
8. Method for operating a combustion system according to any one of claims 4 to 7, characterised in that results of earlier adjustment methods are stored in a store and, before the combustion system is operated, the volume of the Helmholtz resonator (2) is adjusted accordingly.

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