GB2564888A

GB2564888A - Combustion apparatus incorporating a resonance attenuation device

Info

Publication number: GB2564888A
Application number: GB1712047.8A
Authority: GB
Inventors: W T Parsons Robert
Original assignee: Ideal Boilers Ltd
Current assignee: Ideal Boilers Ltd
Priority date: 2017-07-26
Filing date: 2017-07-26
Publication date: 2019-01-30
Anticipated expiration: 2037-07-26
Also published as: GB2564888B; GB201712047D0

Abstract

A combustion apparatus 20, for example a gas boiler, includes a resonance attenuation device 3, a burner assembly 4 and a heat exchanger 11. The resonance attenuation device is located at a position upstream of the point of combustion, which may be a burner plaque 5, and the resonance attenuation device may be a Helmholtz device 3 formed integrally with a burner hood 13. The resonance attenuation device may be configured to attenuate a predetermined frequency, between the range 0.6 KHz and 1.8 KHz, and the device may include a neck (15 Fig. 6) and a cavity 14 (Fig. 6). The burner hood may include an inlet and an outlet, and the resonance attenuation device may be arranged between the inlet and outlet. The arrangement may reduce or inhibit combustion oscillations, sound/noise, resonance and vibrations within the boiler and associated ducting. The provision of the Helmholtz resonator within the burner hood, i.e. pre-combustion zone, instead of within the post-combustion flue zone, significantly reduces the influence of temperature changes and combustion products, such as water vapor on the tuned resonant frequency.

Description

Figure 15 - Frequency range; hot condition and cold condition

Combustion Apparatus Incorporating a Resonance Attenuation Device

Technical Field of the Invention

The present invention relates to resonance attenuation in a combustion apparatus, for example in a gas boiler, in particular a domestic gas boiler.

Background to the Invention

It is not uncommon during the development of new heating appliances to encounter resonance problems. Resonance is produced in combustion apparatus, such as gas boilers, due to oscillations set up in the gas stream and associated components that share a natural frequency or its harmonics, and consequent vibration of ducting and other parts, as well as the directly generated sound.

Resonance can occur at one fixed frequency, or across a range of frequencies, in the boiler’s operation. Particular resonance frequencies can be identified as characteristic of a given combustion apparatus. The resonance frequency is often related to the port geometry when operating at a low flow rate. This issue has until now required a specific gas management control to avoid the critical region of resonance and as a consequence this has restricted the turn-down capability of the burner.

Resonance management solutions have generally been added to the appliance downstream of the point of combustion to manage resonance occurring in the flue products after the combustion process has occurred.

Efforts in the art to manage resonance problems have concentrated on noise reduction through the absorption of generated sound, or the use of baffle systems to break up the resonant gas column. In EP2187124 Bl (Ideal Boilers Limited) a standing wave arm connected to a flue gas outlet of a burner is provided in a combustion apparatus. The standing wave arm establishes a standing wave which approximates to one half of the wavelength of a resonant frequency to be attenuated.

It is known to incorporate a Helmholtz device in a position downstream of the burner in a combustion apparatus. For example, the invention disclosed in EP2187124 Bl also includes a Helmholtz resonator mounted to a position in the flue system, such as to the sump of the boiler or integrated into the standing wave arm. The Helmholtz resonator is configured to attenuate a first resonance frequency and the standing wave arm is configured to generate a standing wave in proportion to one half of the wavelength of a second resonant frequency.

Incorporating a Helmholtz device downstream of the burner (i.e. after the combustion process has occurred) can be disadvantageous. For example, the Helmholtz device will be subject to the extreme temperature changes created by the combustion process. The temperature in a boiler apparatus at the point of combustion can be approximately 1700°C. Thus, downstream of the point of combustion the combustion apparatus can experience a change in temperature from an ambient temperature (prior to combustion) to around 1700°C (during combustion). The extreme temperature changes can alter acoustic properties of the combustion apparatus and, therefore, compound the attenuation problem. Further, the combustion process produces water vapour which condenses to liquid with the potential to prevent the correct operation of a Helmholtz device.

It is therefore an object of the present invention to produce a combustion apparatus incorporating a resonance attenuation device which can ameliorate the abovementioned problems.

Summary of the Invention

According to a first aspect of the invention, there is provided a combustion apparatus incorporating a resonance attenuation device, the apparatus comprising a burner assembly and a heat exchanger, and a point of combustion (or ignition) being provided in the combustion apparatus, characterised in that the resonance attenuation device is located at a position upstream of the point of combustion (or ignition).

The resonance attenuation device may be mounted to the burner assembly. The combustion apparatus may comprise a burner plaque. Typically, the burner plaque comprises an inner surface and an outer surface. Typically, the inner surface is the surface of the burner plaque which is facing into the burner assembly. Typically, the outer surface is the surface of the burner plaque which is facing towards the heat exchanger. The burner plaque may define the point of combustion. Preferably, the outer surface of the burner plaque may define the point of combustion. The point of combustion (or ignition) may be defined by the position of an ignition electrode which triggers ignition and hence combustion.

Advantageously, when located at a position upstream of the point of combustion, the problematic resonance frequency can be attenuated prior to the point of combustion. Beneficially, the resonance can be attenuated prior to the combustion process altering the acoustic properties of the combustion apparatus and compounding the resonance problem.

Advantageously, the resonance attenuation device is not subject to the extreme temperature changes created by the combustion process. The resonance attenuation device is therefore operable effectively at any thermal condition resulting from the combustion process.

Advantageously, the resonance attenuation device permits a reduction in the minimum operating rate of the burner. Preferably, the resonance attenuation device is configured to attenuate a predetermined frequency.

Preferably, the physical dimensions of the resonance attenuation device are calculated based on the predetermined frequency.

Preferably, the predetermined frequency is between 0.6 KHz and 1.8 KHz, more preferably between 0.8 KHz and 1.6 KHz, even more preferably between 1.0 KHz and 1.4 KHz, and most preferably 1.2 KHz.

Preferably, the resonance attenuation device comprises a neck and a cavity.

Combustion apparatus generate resonance at particular frequencies. The resonance attenuation device of the present invention can be configured to attenuate a particular resonant frequency of the apparatus. The frequency, in particular the resonant frequency, of the resonance attenuation device is dependent on the physical dimensions of the resonance attenuation device, as is shown in the following formulae:

Equation 1 -= ? —_. -------------§

L^2s

Where: V2 = Volume of cavity

Γ2 = Radius of neck ln2 = Length of neck f2 = Frequency of the resonance attenuation device

C = Speed of sound constant

Equation2 ( s d/ := ί --------- 4

Where:	V2 = Volume of cavity
10	d2 = Diameter of cavity 1_C2 = Length of cavity

Taking into account the above-described formulae, by amending the physical dimensions π, L2, d2 and 1_C2, the frequency of the resonance attenuation device may be 15 adjusted to the resonant frequency of the acoustical waves generated by the gas stream of the combustion apparatus.

Further, the speed of sound constant (C) is temperature dependent. Thus, the temperature of the gas stream and resonance attenuation device need to be taken into account, in addition to the physical dimensions of the resonance attenuation device, so 20 that the physical dimensions of the resonance attenuation device can be accurately calculated. Taking into account the temperature dependency, the speed of sound can be calculated using the following formula:

Equation 3

Where: k = adiabatic index

R = Molar gas constant

T = Temperature of the gas stream

M = Molar mass (kg/mol)

A combustion process may undergo significant temperature changes, therefore, altering the acoustic properties of the gas stream downstream of the point of combustion.

A resonance attenuation device located in a position downstream of the point of combustion will be subject to the significant temperature changes of the gas stream. Thus the gas stream temperature dependency will change the problematic resonant frequency which the resonance attenuation device is configured to attenuate. Thus, the resonance attenuation device, which has defined physical dimensions so as to attenuate a specific resonant frequency, may no longer absorb the resonant frequency produced by the gas stream. As such, the resonance attenuation device located in a position downstream of the point of combustion may effectively be useless because the frequency attenuated by the resonance attenuation device does not cover the frequency of the acoustical waves generated by the gas stream.

The combustion process produces water vapour which condenses to liquid. The accumulation of liquid condensate in the apparatus downstream of the point of combustion may alter the effective length of the neck of the Helmholtz device and, therefore, change the resonant frequency which is attenuated by the Helmholtz device. A resonance attenuation device located downstream of the point of combustion would thus be negatively affected by the water vapour resulting from the combustion process. Upstream of the point of combustion, the gas / air mixture is dry and therefore does not interfere with the correct operation of the resonance attenuation device.

Preferably, the neck is cylindrical. Preferably, the cavity is positioned at one end of the neck.

Preferably, the neck has a radius of between about 4 mm and 10 mm, more preferably between about 5 mm and 9 mm, even more preferably between about 6 mm and 8 mm, and most preferably about 7 mm.

Preferably the neck has a length of between about 18 mm and 38 mm, more preferably between about 21 mm and 35 mm, even more preferably between about 24 mm and 32 mm, even more preferably between about 26 mm and 30 mm, and most preferably about 28 mm.

Preferably the cavity has a radius of between about 8 mm and 18 mm, more preferably between about 10 mm and 16 mm, even more preferably between about 12 mm and 14 mm, and most preferably about 13 mm.

Preferably the cavity has a length of between about 8 mm and 18 mm, more preferably between about 10 mm and 16 mm, even more preferably between about 12 mm and 14 mm, and most preferably about 13 mm.

Preferably the cavity has a volume of between about 4.0 cm³ and 10.0 cm³, more preferably between about 5.0 cm³ and 8.5 cm³, even more preferably between about 6.0 cm³ and 7.5 cm³, even more preferably between about 6.5 cm³ and 7.2 cm³, and most preferably about 6.9 cm³.

Advantageously, the point of combustion is located at a position upstream of the heat exchanger.

Preferably, the resonance attenuation device is located at a position upstream of the heat exchanger.

Preferably, the burner assembly comprises the burner plaque.

Preferably, the burner assembly comprises a burner hood.

Preferably, the resonance attenuation device is integral with the burner hood. Advantageously, this allows the resonance attenuation device to be incorporated into the manufacturing process of the burner hood. This provides for a cost-effective solution to the problem of managing resonance in a combustion apparatus. It is envisaged that several models of combustion apparatus and, therefore, several models of burner hood, will be produced. This is typical with the development of a new combustion apparatus, in particular with the development of a new gas boiler. By incorporating the resonance attenuation device into the manufacturing process of the burner hood, several different models of combustion apparatus may be manufactured, all of which may comprise a resonance attenuation device incorporated into the burner hood of that respective model. Thus, the resonance attenuation device of the present invention provides for a cost-effective solution to the problem of managing resonance across a range of combustion apparatuses.

Preferably, the burner hood comprises an inlet and an outlet.

Preferably, the resonance attenuation device is located at a position between the inlet and the outlet.

Preferably, the resonance attenuation device is located at a position opposite to the outlet.

Preferably, the inlet is located at one side of the burner hood.

Preferably, the outlet is located at one end of the burner hood.

Preferably, the resonance attenuation device is located at the opposite end of the burner hood from where the outlet is located.

Beneficially, the combustion apparatus comprises a plurality of resonance attenuation devices. It is common for a combustion apparatus to produce multiple resonance frequencies which the operator would want to be attenuated. It is not beyond the scope of the present application that a combustion apparatus and/or burner hood is produced which comprises a plurality of resonance attenuation devices whereby each resonance attenuation device attenuates a different resonant frequency.

Advantageously, the combustion apparatus may comprise one or more additional resonance attenuation device(s) located at a position downstream of the point of combustion. A first resonance attenuation device may be configured to attenuate a first resonant frequency and the one or more additional resonance attenuation devices may be configured to attenuate a one or more further resonant frequency or frequencies. The additional resonance attenuation device, or devices, may be mounted to a sump comprised in the combustion apparatus.

Preferably, the resonance attenuation device is a Helmholtz resonator.

Preferably, the combustion apparatus is a gas boiler.

Preferably, the gas boiler is a domestic gas boiler. Where the combustion apparatus is a domestic gas boiler, the present invention will permit a reduction in the minimum operating rate of the gas boiler. A low operating rate of a gas boiler is an increasing requirement for low energy homes. Thus the present invention provides an attractive product for consumers occupying low energy homes.

According to a second aspect of the invention, there is provided a burner hood comprising a resonance attenuation device.

The resonance attenuation device of the second aspect of the present invention may comprise any of the abovementioned features that have been stated in accordance with the resonance attenuation device of the first aspect of the present invention.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a cross section view of a known gas boiler comprising a burner hood, heat exchanger, and sump;

Figure 2 is a cross section view of a burner hood comprised in the boiler of figure i;

Figure 3	is a perspective view of a known gas boiler comprising the burner hood, a heat exchanger, and a sump;
Figure 4	is a perspective view of the sump in figures 1 and 3;
Figure 5	is a cross section view of a gas boiler according to a first aspect of the present invention;
Figure 6	is a perspective view of the burner hood of the boiler of figure 5;
Figure 7	is a plan view of the burner hood of figure 6;
Figure 8	is a cross section view of the burner hood of figures 6 and 7, taken along line D-D shown in figure 7;
Figure 9	is a rear view of the burner hood of figures 6-8;
Figure 10	is a cross section view of a burner hood with integrally formed resonance attenuation device;
Figure 11	is a plan view of the burner hood of figure 10;
Figure 12	is a graph showing a high frequency resonance evaluation of three combustion apparatuses;
Figure 13	is a graph showing a problematic resonant frequency occurring in a Logic 35kW gas boiler;
Figure 14	is a graph showing attenuation of the problematic resonant frequency in figure 13; and
Figure 15	is a graph showing the change in resonant frequency when a combustion apparatus is in a hot condition and a cold condition.

With reference to figures 1-4, there is shown a known combustion apparatus 10. Gas and air enter the combustion apparatus through a gas inlet 1 and an air inlet 2, respectively. The combustion apparatus 10 comprises a burner hood 12 and a heat exchanger 11. The burner hood 12 comprises a burner assembly 4. The burner hood 12 comprises a flat base 17 and a shoulder 18 extending below the base 17 for substantially the entire perimeter of the base 17. The shoulder 18 encloses a burner plaque 5 which is rectangular when viewed in plan view. The burner plaque 5 is positioned adjacent to the underside of the base 17 of the burner hood 12. The burner plaque 5 is supported and held in its position by lip 21. The burner hood 12 extends upwardly from the base 17. One end of the burner hood 12 forms a first wall 22 which is approximately three times the length of an opposite wall 23. The first wall 22 plateaus to form a substantially flat surface 19 which extends parallel to the base 17 of the burner hood 12 for approximately one third of the length of the base 17. After this point, the flat surface 19 slopes downwardly to form a sloped surface 20. The sloped surface 20 abuts the opposing wall 23.

The burner plaque comprises two oppositely facing surfaces 5a, 5b. The burner plaque 5 defines a plurality of ports (not shown). The ports each form a channel between the inner surface 5a of the burner plaque 5 and the outer surface 5b of the burner plaque 5. The gas/air mixture enters the burner plaque 5 at the inner surface 5 a via the plurality of ports. The gas/air mixture flows through the ports in the direction of the outer surface 5b. The gas/air mixture emerges from the burner plaque 5 at the outer surface 5b. Ignition is triggered across at least a substantial portion of the outer surface 5b by a spark from an ignition electrode (not shown). The spark is triggered as part of the ignition process from the main printed circuit board control. The ignition electrode is positioned in close proximity to the outside surface 5b of the burner plaque 5. The point of combustion of the gas/air mixture is on the outer surface 5b of the burner plaque 5.

The heat exchanger 11 comprises one or more gas passages 16 through which combustion or flue gas is passed from the burner assembly 4 into a sump 8. The sump 8 is positioned at the base of the heat exchanger 11.

The heat exchanger 11 may comprise features to aid heat transfer from the gas passing through the one or more gas passages 16. For example, the heat exchanger 11 may comprise pins or fins, as is known in the art.

At the base of the heat exchanger 11, adjacent to an internal volume of the sump 8, is a Helmholtz chamber 6. The Helmholtz chamber 6 is in fluid connection with the internal volume of the sump 8 by a tube 7. The Helmholtz chamber 6 may comprise a Helmholtz device configured to attenuate a resonant frequency occurring in the flue products after the combustion process has occurred. Disadvantageous^, a Helmholtz device located in the Helmholtz chamber 6 may be affected by water vapour produced by the combustion process. The water vapour produced may prevent the correct operation of the Helmholtz device. Thus, the Helmholtz device will not sufficiently attenuate the problematic resonant frequency.

Further, the combustion process occurring downstream of the point of combustion in the burner plaque 5 creates substantial temperature changes which can alter the acoustic properties of the flue gas in the sump 8.

The flue gas exits the sump 8 via a gas outlet 24. The sump 8 is configured to encourage condensate formed in the sump 8 to exit the sump 8 via a condensate trap outlet 9. However, condensate may remain in the sump 8 and/or in any part of the combustion apparatus at a location downstream of the point of combustion.

With reference to figures 5-11, there is shown a combustion apparatus according to the present invention. The combustion apparatus 20 is largely synonymous with the combustion apparatus 10 of the prior art. However, the combustion apparatus 20 comprises a burner hood 13 to which is mounted a resonance attenuation device. In the present embodiment, the resonance attenuation device is a Helmholtz device 3.

The Helmholtz device 3 is located in a position upstream of the point of combustion. Accordingly, this configuration provides for a resonance attenuation device which operates effectively under any thermal conditions which result from the combustion process carried out in the combustion apparatus.

Further, the gas and air mixture which enters the combustion apparatus via the gas inlet 1 and air inlet 2 is dry. Thus, the Helmholtz device 3, located in a position upstream of the point of combustion, is not affected by water vapour, which is only produced after the point of combustion.

In the present embodiment, the Helmholtz device 3 is mounted substantially to the flat surface 19 of the burner hood 13.

The Helmholtz device 3 may be positioned in the combustion apparatus 20 at several desired locations upstream of the point of combustion, in particular where problematic resonant frequencies occur. For example, the resonance attenuation device may be coupled to the sloped surface 20 of the burner hood 13, the wall 23 of the burner hood 13, the wall 22 of the burner hood 13, or indeed any other location upstream of the point of combustion.

The Helmholtz device 3 comprises a neck 15 and a cavity 14. The neck 15 and cavity 14 form a volume of the Helmholtz device. The neck 15 forms a passage which couples the volume of the Helmholtz device 3 to the flat surface 19 of the burner hood 13. The neck 15 is an open neck which is in fluid connection with the gas/air mixture in the burner assembly 4. The cavity 14 is closed at one end and is in fluid connection with the neck 15 at its opposite end.

Combustion apparatuses generate resonance at particular resonant frequencies. The Helmholtz device 3 is configured to attenuate a particular predetermined resonant frequency of the apparatus.

The Helmholtz device 3 is used to attenuate a resonant frequency by allowing the gas/air mixture to oscillate against the internal walls of the closed cavity 14, with the volume in the cavity 14 acting like a spring to absorb the energy of resonance.

In one example which has been tested by the applicant, three models of combustion apparatus, in this example, three models of a domestic gas boiler (Logic 24kW, Logic 30kW and Logic 35kW), had a common resonant frequency of 1.2KHz (figure 12).

Applying equations 1-3, and where the frequency φ) is 1.2KHz, the required physical dimensions of a Helmholtz device can be determined. In the present example, it was found that the following dimensions of a Helmholtz device would attenuate a frequency of 1.188KHz, thus being acceptable to attenuate the predetermined frequency of 1.2KHz:

7mm =	Γ2 = Radius of neck
28mm =	ln2 ~ Length of neck
26mm =	d2 = Diameter of cavity
13mm =	1_C2 = Length of cavity
6.902cm³ =	V2 = Volume of cavity

In a test using a Logic 35kW domestic gas boiler, as shown in figures 1-4, without a Helmholtz device mounted to the burner hood at a position upstream of the point of combustion, a resonant frequency is observed (figure 13).

Under the same test conditions, a burner hood 13 comprising a Helmholtz device 3 mounted at a position upstream of the point of combustion was used instead of the known burner hood 12. Figure 14 displays the attenuation of the problematic resonant frequency achieved using the burner hood 13.

In the arrangement shown in Figure 5, a burner plaque 5 is located at the base of the burner hood 13. The burner plaque 5 provides the point of combustion. In particular, the outer surface 5b of the burner plaque 5 may define the point of combustion. Advantageously, the Helmholtz device is positioned at a location upstream of the point of combustion. Thus, in consideration of the temperature dependency of the problematic resonant frequency, the Helmholtz device 3 is not subject to the extreme temperature changes created by the combustion process which alters the acoustic properties and so compounds the attenuation problem.

Consequently, the Helmholtz device 3, positioned at a location upstream from the point of combustion, can attenuate a problematic resonant frequency which, if not attenuated, may vary and/or intensify after the combustion process; additionally, the Helmholtz device is not affected by water vapour.

The Helmholtz device of the present invention is described as attached to the burner hood. However, the Helmholtz device could be integral with the burner hood in one of a plurality of locations.

Figures 10 and 11 show a burner hood 24 with a Helmholtz device 3 formed integrally with the burner hood 24. In this embodiment, the neck 15 of the Helmholtz device 3 projects downwardly into the internal volume of the burner assembly 4. The neck comprises shoulder portions 25 which abut the surface 19 of the burner hood 24. The cavity 14 is positioned so as to protrude from the surface 19 of the burner hood 24. A burner hood 24 comprising an integrally formed Helmholtz device 3 provides for an arrangement which minimises the space taken up by the combustion apparatus. This is beneficial for a number of applications of the present invention. For example with domestic gas boilers where the space to install a gas boiler may be limited so reducing the size of the gas boiler is advantageous for the owner. Thus the present invention provides acceptable levels of attenuation of problematic resonance frequencies whilst also meeting requirements in terms of the overall size of the boiler unit.

Further, positioning the resonance attenuation device on the burner hood ensures that the resonance attenuation device is incorporated into the combustion apparatus discreetly. Moreover, the layout and positioning of the remaining parts used in the combustion apparatus is not compromised.

The apparatus as so far described is typically provided as a boiler unit encased in an outer casing. The apparatus may extend vertically or horizontally, or indeed in any other direction, depending on the requirements of the installation.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A combustion apparatus incorporating a resonance attenuation device, the apparatus comprising a burner assembly and a heat exchanger, and a point of combustion being provided in the combustion apparatus, characterised in that the resonance attenuation device is located at a position upstream of the point of combustion.

2. A combustion apparatus according to claim 1, wherein the resonance attenuation device is mounted to the burner assembly.

3. A combustion apparatus according to any preceding claim, wherein the combustion apparatus further comprises a burner plaque.

4. A combustion apparatus according to claim 3, wherein the burner plaque comprises an inner surface and an outer surface.

5. A combustion apparatus according to claim 4, wherein the outer surface of the burner plaque defines the point of combustion.

6. A combustion apparatus according to any preceding claim, wherein the resonance attenuation device is configured to attenuate a predetermined frequency.

7. A combustion apparatus according to claim 5 or 6, wherein the physical dimensions of the resonance attenuation device are calculated based on the predetermined frequency.

8. A combustion apparatus according to any of claims 6-7, wherein the predetermined frequency is between 0.6 KHz and 1.8 KHz.

9. A combustion apparatus according to claim 8, wherein the predetermined frequency is about 1.2KHz.

10. A combustion apparatus according to any preceding claim, wherein the sound attenuation device comprises a neck and a cavity.

11. A combustion apparatus according to claim 10, wherein the neck is cylindrical.

12. A combustion apparatus according to claim 10 or 11, wherein the cavity is positioned at one end of the neck.

13 . A combustion apparatus according to any of claims 10 - 12, wherein the neck has a radius between about 4 and 10 mm.

14. A combustion apparatus according to any of claims 10 - 13, wherein the neck has a length of between about 18 mm and 38 mm.

15. A combustion apparatus according to any of claim 10 - 14, wherein the cavity has a radius between about 8 and 18mm.

16. A combustion apparatus according to any of claim 10 - 15, wherein the cavity has a length between about 8 and 18mm.

17. A combustion apparatus according to any of claim 10 - 16, wherein the cavity has a volume between about 4.0 cm3 and 10.0 cm3.

18. A combustion apparatus according to any of the preceding claims, wherein the point of combustion is located at a position upstream of the heat exchanger.

19. A combustion apparatus according to any of the preceding claims, wherein the resonance attenuation device is located at a position upstream of the heat exchanger.

20. A combustion apparatus according to any preceding claim, wherein the burner assembly comprises the burner plaque.

21. A combustion apparatus according to any preceding claim, wherein the burner assembly comprises a burner hood.

22. A combustion apparatus according to claim 21, wherein the resonance attenuation device is integral with the burner hood.

23. A combustion apparatus according to claim 21, wherein the burner hood comprises an inlet and an outlet.

24. A combustion apparatus according to claim 23, wherein the resonance attenuation device is located at a position between the inlet and the outlet.

25. A combustion apparatus according to claim 23 or 24, wherein the resonance attenuation device is located at a position opposite to the outlet.

26. A combustion apparatus according to any of claims 23 - 25, wherein the inlet is located at one side of the burner hood.

27. A combustion apparatus according to any of claims 23 - 26, wherein the outlet is located at one end of the burner hood.

28. A combustion apparatus according to claim 27, wherein the resonance attenuation device is located at the opposite end of the burner hood from where the outlet is located.

29. A combustion apparatus according to any preceding claim, wherein the combustion apparatus comprises a plurality of resonance attenuation devices.

30. A combustion apparatus according to claim 29, wherein the combustion chamber comprises one or more additional resonance attenuation devices located at a position downstream of the point of combustion.

31. A combustion apparatus according to claim 30, wherein the second resonance attenuation device is mounted to a sump comprised in the combustion apparatus.

32. A combustion apparatus according to any preceding claim, wherein the resonance attenuation device is a Helmholtz resonator.

33. A combustion apparatus according to any preceding claim, wherein the combustion apparatus is a gas boiler.

34. A combustion apparatus according to claim 33, wherein the gas boiler is a domestic gas boiler.

35. A burner hood comprising a resonance attenuation device.

36. A burner hood according to claim 35, wherein the resonance attenuation device is a Helmholtz device.

37. A burner hood according to claim 36, wherein the Helmholtz device is formed integrally with the burner hood.

38. A combustion apparatus or burner hood substantially as hereinbefore described with reference to the figures 1-15.