Patent Description:
Exhaust systems that emit high temperature gases, especially exhaust systems for combustion engines, typically have silencers. Silencers generally do not provide strong noise reductions at low frequencies and in some applications low frequency noise can be problematic.

Conventional silencers only provide good noise attenuation when the wavelength of the sound is comparable with the thickness of the absorptive material in the silencer. For example, at low frequencies, such as the <NUM> octave band, which has a lower frequency limit of <NUM>, and the <NUM> octave band, and with a speed of sound around <NUM>/s in a hot exhaust gas, the acoustic wavelength can be as large as <NUM>. For large exhaust systems, such as those used on gas-fired power plants, industrial silencers are used. These silencers are lined with sound absorptive material that is typically about <NUM> thick (only about <NUM>/45th of the wavelength), and hence do not provide good sound attenuation at low frequencies. This can result in loud 'rumbling' or other low frequency sound effects being perceived at distances remote from the power plant.

A further challenge with controlling noise emanating from hot exhaust systems is that high temperature gases have reduced viscosity, making sound absorption more difficult.

A still further challenge with controlling noise emanating from hot exhaust systems is that high temperature gases have increased flow rates which has a big impact on self-noise (sound generated by the flow of the high speed gas through the exhaust system).

In outside environments, which are subject to variable winds, sound pressure levels (SPLs) at low frequencies do not always reduce at 6dB with a doubling in distance in a direction away from an exhaust gas noise source. This is especially so where wind interacts with exhaust gases. Certain atmospheric conditions can cause or contribute to sound refraction over long distances. As a result, a challenge arises in adequately controlling noise emitted from hot exhaust systems.

Examples of problematic noise control situations as described above include gas-fired power plants, especially single-cycle plants. This is because the higher exhaust temperatures in single-cycle plants (<NUM>-<NUM> compared with <NUM>-<NUM> for combined-cycles) reduces the efficacy of traditional silencers. This presents a challenge for acoustic control.

Other examples of problematic noise control situations include internal combustion engine powered generators used for commercial power generation, and calciners used in cement production.

<CIT> discloses a device for attenuating the noise generated by the expansion of gases into the atmosphere. The device includes sections having a sound absorbing coating.

<CIT> discloses a vacuum creating exhaust muffler for internal combustion engines by reducing the back pressure at the exhaust of the internal combustion engine below the atmospheric pressure and keeping it at a constant value independently of the engine operating conditions.

<CIT> discloses a device for attenuating the noise radiated by gas jets by causing the sound to pass from the annular section of the jet to a narrow annular section of a nozzle slot. Sound wave diffraction takes place on emergence from the slot by using a baffle and reflection of the sound onto a sound absorption lining.

<CIT> discloses a ground exhaust noise suppressor that reduces noise by discharging the jet engine exhaust through a multiplicity of small holes or nozzles.

<CIT> discloses a vehicular exhaust resonator includes an exhaust inlet conduit which narrows down in a manner so as to define a venturi effect region. Air entrainment apertures are provided in the housing of the resonator. Due to the venturi effect inside the inlet conduit, a pressure drop is created that pulls air from the outside into the air inlet chamber and then into an outlet conduit of the resonator.

<CIT> discloses an internal combustion engine noise reduction apparatus that includes sound absorbing layers disposed within the apparatus housing.

<CIT> discloses an acoustic device including a chamber that has a wall partitioning an expansion chamber into a central chamber and a peripheral chamber. The internal partition wall has a plurality of apertures defined through the same.

It is an object of the disclosure to address at least some of the problems described above or to at least provide a useful choice.

According to a first aspect of the present disclosure, there is provided an exhaust duct assembly for conveying exhaust gases emanating from a combustion zone to atmosphere, the assembly including: an exhaust gas outlet for exhausting exhaust gas into the atmosphere; and an acoustic duct portion located upstream of the exhaust gas outlet, the acoustic duct portion having a peripheral wall defining a through-passage and including an acoustically porous structure, and the acoustic duct portion has a length (L) in a flow direction that is at least <NUM>% of an average hydraulic diameter (DH) of the through-passage and characterised in that said peripheral wall is constructed to promote propagation of sound there-through to atmosphere.

In one form, the acoustic duct portion has a length in the flow direction that is at least <NUM>% of the average hydraulic diameter of the through-passage. In one form, the acoustic duct portion has a length in the flow direction that is at least <NUM>% of the average hydraulic diameter of the through-passage.

Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:.

Referring now to <FIG>, an exhaust duct assembly for conveying exhaust gases emanating from a combustion zone to atmosphere, according to a first embodiment of the disclosure, is shown diagrammatically in an elevational view. The assembly <NUM> includes an exhaust gas outlet <NUM> for exhausting exhaust gas into the atmosphere and an acoustic duct portion <NUM> located upstream of the exhaust gas outlet <NUM>. The acoustic duct portion <NUM> has a peripheral wall <NUM> as can be seen more clearly in the diagrammatic cross-sectional view of <FIG>. The peripheral wall <NUM> defines a through-passage arranged and constructed to promote propagation of sound there-through.

With the embodiment of the disclosure illustrated in <FIG>, there is a plant utilising a gas turbine <NUM>, to which the exhaust duct assembly <NUM> is connected, as diagrammatically shown in <FIG>. Within the gas turbine <NUM> is a combustion zone <NUM>. Exhaust diffuser ducting <NUM> links the turbine <NUM> to a silencer <NUM>. High temperature exhaust gases flow from the combustion zone <NUM> through the exhaust gas diffuser <NUM> and then through the silencer <NUM> before entering the above described acoustic duct portion <NUM>, in this case, linked by further intermediate ducting <NUM>.

Referring now to <FIG>, a generalized schematic drawing of an exhaust duct assembly shown in <FIG> is illustrated. Like the exhaust duct assembly of <FIG>, the exhaust duct assembly of <FIG> includes an exhaust gas outlet <NUM> for exhausting exhaust gas into the atmosphere and an acoustic duct portion <NUM> located upstream of the exhaust gas outlet <NUM>. The acoustic duct portion <NUM> has a peripheral wall that defines a through-passage arranged and constructed to promote propagation of sound there-through. This is illustrated by the dashed semi-circular lines that illustrate hemi-spherical sound waves <NUM>. The exhaust duct assembly shown in <FIG> could be fitted to any hot exhaust system.

In <FIG> and <FIG>, the acoustic duct portion <NUM> has a length "L" in the flow direction. The internal diameter of the acoustic duct portion <NUM> is "DH". Acoustic duct portions <NUM> where the ratio L/DH is higher will provide greater noise reduction benefits at locations remote from the plant, such as the downwind location <NUM> illustrated in <FIG>, than acoustic duct portions <NUM> where the ratio L/DH is lower. While a range of L/DH may be used, with the embodiment illustrated, the acoustic duct portion has a length, L, in a flow direction, that is at <NUM>% of average hydraulic diameter, DH, of the through-passage. Or in other words, the ratio L:DH is approximately <NUM>:<NUM>.

In <FIG>, sound waves <NUM> are shown. At length L, the sound waves <NUM> have a surface area of: <MAT>.

In the embodiment shown in <FIG> where L = <NUM> DH, this becomes: <MAT> The inlet <NUM> to the acoustic duct portion <NUM> has a cross-sectional area = π DH<NUM>/<NUM>. The sound intensity ratio for the length L of acoustic duct portion <NUM>, having a hydraulic diameter DH as compared to a hard duct that is not porous to sound is as follows: <MAT>.

This is equivalent to a <NUM> dB reduction in sound intensity. The longer the porous acoustic duct portion <NUM>, the greater the reduction in sound intensity at the outlet <NUM> of the acoustic duct portion <NUM>, which reduces the amount of sound which can interact with the hot plume <NUM>, thereby reducing sound refraction downstream (of any cross-flowing wind).

In <FIG>, the exhaust gas duct <NUM> is shown to include insulation <NUM>. In practice, where the exhaust gas duct is used for a plant utilising a gas turbine, the diameter may be in the range of <NUM> to <NUM> metres for instance. The insulation may be about <NUM> metre in thickness and internal baffle silencers may be provided. These baffle silencers can be complimentary to the acoustic duct portion <NUM> and maybe useful in many applications.

With the embodiments described so far and shown in <FIG>, the ducting has a circular cross-section. In such cases, measurement of the internal diameter, D is straightforward and will generally equal DH, where DH is the hydraulic diameter of the duct. In other embodiments not shown, ducts having oval or rectangular cross-sections may be used. For such ducts, DH can be calculated and the same principal that acoustic duct portions <NUM> where the ratio L/DH is higher will provide greater noise reduction benefits will also apply.

The general arrangement illustrated in <FIG> is also illustrated in <FIG> which is a diagrammatic cross-sectional view showing a locality in which an exhaust duct assembly <NUM> is located. <FIG> illustrates the effect of wind and sound propagation over distance. <FIG> illustrates a high temperature exhaust gas plume <NUM> being deflected by a crosswind. This will be described in more detail below. Returning to <FIG>, and in particular <FIG>, it can be seen that the acoustic duct portion <NUM> is located between the exhaust gas duct <NUM> and the exhaust gas outlet <NUM>. The acoustic duct portion <NUM> includes an acoustic duct wall <NUM> having a structural portion and a non-structural portion. The structural portion, which in this embodiment is a perforated metal duct, is arranged and constructed to hold the shape of the acoustic duct portion <NUM> against loads from gravity, wind, exhaust flow, thermal expansion and others. Generally, the structural portion is also arranged to cope with wind loads. The acoustic duct wall <NUM> also has a non-structural portion which is arranged and constructed to allow at least a range of low frequency sounds to pass there-through. While various materials can be used, in the embodiment of the disclosure illustrated, the non-structural portion is made from any acoustically non-reflective structure, including thin sheet, wire mesh, sheeting with perforated holes and/or woven cloth or glass/mineral fibre batts.

<FIG> illustrate in more detail the structure of the acoustic duct wall <NUM>. In particular, <FIG> shows a portion of the structural wall portion <NUM> which defines a plurality of apertures <NUM>. <FIG> also shows the structural wall portion <NUM> and its apertures <NUM>, but also shows the non-structural wall portion <NUM>, in the form of wire mesh.

Turning now to <FIG>, the exhaust duct assembly <NUM> shown in <FIG> is similar to that of the first embodiment of the disclosure shown in <FIG>. However, with the second embodiment of the disclosure, a terminal duct portion <NUM> is provided after the acoustic duct portion <NUM>. Attached to the terminal duct portion <NUM> are guy-wires <NUM> that are secured to the ground so as to provide additional lateral stability to the exhaust duct assembly <NUM>.

Now turning to <FIG>, a third embodiment of the disclosure is shown. This embodiment includes an air aspirating portion. With this embodiment of the disclosure, the exhaust gas duct <NUM> terminates within a mouth <NUM> of the acoustic duct portion <NUM>. The acoustic duct portion <NUM> is shaped to aspirate or entrain air into the exhaust gas plume as is indicated by arrows A. The entrainment of air into the exhaust gas plume lowers its temperature. This assists in reducing the tendency of the plume <NUM> to refract sound. The acoustic duct wall <NUM> shown in <FIG> can be constructed in the same way as was described above with reference to <FIG>. Alternatively, a different construction may be used.

Referring now to <FIG>, a generalized schematic drawing of an exhaust duct assembly similar to that shown in <FIG> is illustrated. Like the exhaust duct assembly of <FIG>, the exhaust duct assembly of <FIG> includes an exhaust gas outlet <NUM> for exhausting exhaust gas into the atmosphere and an acoustic duct portion <NUM> located upstream of the exhaust gas outlet <NUM>. Again, the acoustic duct portion <NUM> has a peripheral wall that defines a through-passage arranged and constructed to promote propagation of sound there-through. The shape of acoustic duct portion <NUM> and its positioning relative to the exhaust gas inlet <NUM> is such that ambient air is aspirate or entrained into the exhaust gas plume as is indicated by arrows A.

Both of the acoustic duct portions <NUM> shown in <FIG> and <FIG>, have a length of "N" in the flow directions. The more relevant length for acoustic performance is "L", again measured in the flow direction. While a range of L/DH may be used, with the embodiment illustrated, the acoustic duct portion has a length, L, in a flow direction, that is at <NUM>% of the average hydraulic diameter, DH, of the through-passage. Or in other words, the ratio L:DH is approximately <NUM>:<NUM>.

<FIG> shows a fourth embodiment of the disclosure. This fourth embodiment of the disclosure is similar to the first three embodiments of the disclosure, but where previously the acoustic duct portion <NUM> was orientated vertically at or towards the terminal end of the exhaust duct assembly <NUM>, with this embodiment, the acoustic duct portion <NUM> is orientated horizontally. In other respects, this embodiment of the disclosure is similar. For instance, acoustic duct wall <NUM> may have the same construction as the acoustic duct portion <NUM> of the embodiments of the disclosure shown in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>. On the other hand, with this arrangement, it may be less necessary to have a significant structural portion to the acoustic duct wall <NUM>. The acoustic duct portion <NUM> may be placed and supported such that it is not subject to wind loads for instance.

In a variant of the embodiment illustrated in <FIG>, the entire duct downstream of the silencer <NUM> may be acoustically porous (not just the duct portion <NUM> illustrated).

A fifth embodiment of the disclosure is shown in <FIG>. With this embodiment of the disclosure, the acoustic duct portion is downstream of a combustion zone <NUM> within a generator set having an internal combustion engine.

Embodiments of the disclosure described above result in an exhaust stack that the internal wall is acoustically non-reflective (which implies the wall is either acoustically transparent or absorptive), meaning sound can readily pass into or through it, but still constrains the gas flow like, or at least somewhat like, a rigid-walled pipe. In doing so, the sound is separated from the exhaust plume and is able to radiate away from it, thereby being refracted (bent) less. Desirably, the acoustically porous exhaust duct portion will not change, or at least not substantially change, the flow characteristics or back pressure, which is important for dispersion of the gas and for turbine performance. Such embodiments can be constructed by using various materials with an appropriate flow resistance - sufficiently low so that sound can easily pass through it, but sufficiently high so that the gas flow continues along the exhaust duct until it reaches the outlet. Examples of such materials may include, but are not limited to: thin sheet, wire mesh, sheeting with perforated holes and/or woven cloth, or structures common in absorptive silencers comprising absorptive material (such as glass or mineral wool) sandwiched between structural elements such as perforated sheet. For instance, rockwool having a flow resistivity of <NUM>,<NUM> mks rayl/m, giving a flow resistance of ~<NUM>,<NUM> mks rayls (Pa/m/s) may be suitable in some applications. More generally, rockwool or other suitable materials may be sized and arranged such that they provide a flow resistance in the range of <NUM>,<NUM> mks rayls (Pa/m/s) to <NUM>,<NUM> mks rayls (Pa/m/s).

An example of a specific application where embodiments of the disclosure can be used is in single-cycle gas-fired power plants where higher exhaust temperatures (<NUM>-<NUM>) are generated. Exhaust plumes from such single-cycle gas-fired power plants produce high exhaust sound pressure levels that is strongly refracted (bent downwards) by the hot exhaust stream. When this is combined with a mild crosswind, this results in increased sound pressure levels downstream at ground level, as illustrated in <FIG>.

<FIG> shows that as ratio of the velocity of the exhaust compared with the velocity of the cross-flow, R, decreases, it causes the hot plume to be projected higher angles, but the rays of sound are still refracted (bent) downwards to the ground. This can have a significant impact on neighbouring communities. This is diagrammatically illustrated in <FIG>, where <NUM> is the exhaust gas outlet of plant <NUM> and <NUM> in a downwind locality remote from the plant <NUM>. Sound is refracted within the plume <NUM> and is directed downwards towards locality <NUM>. There are many documented cases where this has resulted in the SPL from plants exceeding noise ordinance regulations.

<FIG> show schematically the acoustic interaction with the hot exhaust plume with (<FIG>) and without (<FIG>) a flow impervious, acoustically-transparent duct portion or nozzle (such as the acoustic duct portion <NUM> illustrated in <FIG> and <FIG>). The straight section nozzle <NUM> constrains the flow to the interior of the nozzle <NUM> and causes the fluid to exit the duct at a higher vertical height. The acoustically transparent nozzle or duct <NUM> allows for the acoustic centre of the duct to remain unaltered. This allows the acoustic energy (or at least a substantial proportion of the acoustic energy) to be 'released' or "leaked" from the protruding vertical exhaust stack at an earlier stage. Hence, less sound interacts with the hot plume deflected by the cooler cross-winds. The reduction in sound interacting with the hot plume leads to a reduction in the amount of sound refracted by the plume and, ultimately, a reduction in the maximum SPL observed downwind of the exhaust stack.

As has been described above, embodiments of the disclosure control the way sound interacts with the hot exhaust plume, and ultimately reduces the amount of sound being refracted (bent) down towards the ground. The proposed disclosure does not unduly change the back-pressure, and hence does not change the gas-turbine performance, and maintains the original exit gas velocity so that gas dispersion characteristics are substantially unaltered.

<FIG> is a sound directivity plot presenting test results from a small scale version of an acoustic duct portion according to the disclosure in a wind tunnel. The results shown are for the <NUM> one-third octave band which is equivalent to <NUM> at a system scaled up <NUM> times (which would be a scale typical of a gas-fired power plant). The "Nozzle" plots are for a cross wind velocity of <NUM>/s and an exhaust velocity of approximately <NUM>/s. The acoustic duct (or "nozzle") reduces the peak levels on the right-hand side lobe by <NUM>-6dB. The measurements shown in Figure <NUM> were taken at <NUM> diameters. This distance is equivalent to about <NUM> from a stack at <NUM> times scale. Diffraction continues well beyond this distance.

This disclosure helps to address the problem of high noise levels in communities near plants producing hot exhaust streams, such as single-cycle gas fired power stations. Embodiments of the disclosure can be installed in existing or new plants. In many applications it is expected that traditional silencers will still be used, however, they will be smaller with much of the decrease in sound pressure levels downstream at ground level arising from embodiments of the acoustic duct portion described herein. More generally, this disclosure helps to address noise problems associated with many, if not all, hot exhaust systems.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

Claim 1:
An exhaust duct assembly (<NUM>) for conveying exhaust gases emanating from a combustion zone (<NUM>) to atmosphere, the assembly (<NUM>) including:
an exhaust gas outlet (<NUM>) for exhausting exhaust gas into the atmosphere; and
an acoustic duct portion (<NUM>) located upstream of the exhaust gas outlet (<NUM>), the acoustic duct portion (<NUM>) having a peripheral wall (<NUM>) defining a through-passage and including an acoustically porous structure, and the acoustic duct portion (<NUM>) has a length (L) in a flow direction that is at least <NUM>% of an average hydraulic diameter (DH) of the through-passage and characterised in that said peripheral wall (<NUM>) is constructed to promote propagation of sound (<NUM>) there-through to atmosphere.