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A silencer for an auxiliary power unit of an aircraft comprises an inlet, an outlet, a housing and a flow channel with a porous wall material that is arranged in the housing. An intermediate space is formed between the housing and the flow channel and divided into outer cells that are arranged aroun

A silencer for an auxiliary power unit of an aircraft comprises an inlet, an outlet, a housing and a flow channel with a porous wall material that is arranged in the housing. An intermediate space is formed between the housing and the flow channel and divided into outer cells that are arranged around the flow channel by means of one or more partitions. This makes it possible to realize a very compact silencer with very good sound insulation properties. The sound dampening can be additionally improved by dividing the outer cells into outer regions and inner regions, by adapting the shape of the silencer to a tail section of an aircraft and by arranging an annular channel on the outlet.

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1. A silencer for an auxiliary power unit of an aircraft, comprising: an inlet;an outlet;a housing; anda flow channel arranged in the housing,wherein an intermediate space is formed between the housing and the flow channel, the intermediate space being divided into outer cells that are arranged arou

1. A silencer for an auxiliary power unit of an aircraft, comprising: an inlet;an outlet;a housing; anda flow channel arranged in the housing,wherein an intermediate space is formed between the housing and the flow channel, the intermediate space being divided into outer cells that are arranged around the flow channel by one or more partitions, wherein a wall material of the flow channel is porous,wherein the partitions are arranged relative to one another in such a way that at least first and second groups of outer cells are formed in the intermediate space between the housing and the flow channel,wherein the outer cells of the first group have a length comprised of a first interval and the outer cells of the second group have a length comprised of a second interval,wherein the first and the second intervals are adjacent to one another and respectively have a width essentially corresponding to 0.1-times the hydraulic diameter of the flow channel, andwherein the outer cells have a volume corresponding to 1.0-1.2-times the volume of inner cells enclosed by the respective outer cells and formed in the flow channel. 2. The silencer of claim 1, wherein the partitions are arranged relative to one another in such a way that at least two outer cells with a respective length corresponding to 0.2-0.6-times the hydraulic diameter of the flow channel and a respective height corresponding to 0.3-0.45-times the hydraulic diameter of the flow channel are formed in the intermediate space between the housing and the flow channel in a cylindrical section of the housing, andwherein at least one outer cell is divided into an outer region and an inner region by a porous absorber layer in the cylindrical section of the housing, wherein the outer region is not in direct communication with the flow channel. 3. The silencer of claim 2, wherein the porous absorber layer in the cylindrical section of the housing is realized in such a way that the outer regions of the outer cells helically extend around the inner regions of the outer cells. 4. The silencer of claim 2, wherein the porous absorber layer in the cylindrical section of the housing is realized in such a way that the outer regions of the outer cells annularly extend around or adjacent to the inner regions of the outer cells. 5. The silencer of claim 2, wherein the sum of the volumes of all outer regions of at least one outer cell in the cylindrical section of the housing corresponds to 0.8-1.2-times the sum of the volumes of all inner regions of the same outer cell. 6. The silencer of claim 2, wherein the partitions are arranged relative to one another in the cylindrical section of the housing in such a way that at least two groups of outer cells are formed in the intermediate space between the cylindrical section of the housing and the flow channel, wherein the outer cells of a first group respectively have a length comprised of a first interval and the outer cells of a second group respectively have a length comprised of a second interval, and wherein the first and the second intervals are adjacent to one another and respectively have a width that substantially corresponds to 0.1-times the hydraulic diameter of the flow channel. 7. The silencer of claim 1, wherein the first group of outer cells has a length corresponding to 0.3-0.4-times the hydraulic diameter of the flow channel. 8. The silencer of claim 1, wherein the second group of outer cells has a length corresponding to 0.4-0.5-times the hydraulic diameter of the flow channel. 9. The silencer of claim 1, wherein an additional third group of outer cells has a length corresponding to 0.2-0.3-times the hydraulic diameter of the flow channel. 10. The silencer of claim 1, wherein the wall material of the flow channel has a flow resistance corresponding to 0.8- to 1.2-times a basic resistance that is calculated in accordance with R=410·(293T)1,26⁢Pams wherein T is the temperature of the gas flowing through the silencer expressed in Kelvin. 11. The silencer of claim 1, wherein at least one partition respectively comprises at least one drainage hole. 12. The silencer of claim 1, wherein the housing comprises one or more conical components. 13. The silencer of claim 1, wherein the cross section of the flow channel and of at least one section of the housing surrounding the flow channel continuously changes from a first shape to a second shape, wherein each point in the first cross-sectional shape of the flow channel is spaced apart from a nearest outer contour point of the housing by a distance corresponding to no more than half the hydraulic diameter of the first shape, and wherein each point in the second cross-sectional shape of the flow channel is spaced apart from a nearest outer contour point of the housing by a distance corresponding to no more than one-fourth the hydraulic diameter of the first shape. 14. The silencer of claim 1, further comprising an annular channel placed on an elongated outlet and communicating with the outlet via at least one opening arranged on the outlet. 15. The silencer of claim 14, further comprising a plurality of openings arranged on an inner circumference of the annular channel in such a way that a sound wave entering the annular channel through an opening propagates and reaches an adjacent opening, and wherein the sound wave entering the annular channel from the outlet via the adjacent opening has an opposite phase referred to the sound wave propagating in the annular channel and the phase and the opposite phase cancel one another out at the adjacent opening. 16. The silencer of claim 14, wherein the annular channel communicates with the outlet via a plurality of openings uniformly distributed over the outlet. 17. The silencer of claim 1, wherein the flow channel is dimensioned such that radial velocity components of at least first and second transversal modes created in the silencer have a common maximum in the region of the wall of the flow channel. 18. The silencer of claim 17, further comprising additional outer cells in order to dampen high-frequency noises, and wherein the additional outer cells have a volume that corresponds to 0.75-0.85-times the volume of inner cells that are enclosed by the respective outer cells and formed in the flow channel. 19. A method for making a silencer for an auxiliary power unit of an aircraft, the method comprising: providing a silencer with an inlet, an outlet, a housing and a flow channel having a porous wall material;forming an intermediate space between the housing and the flow channel;dividing the intermediate space into outer cells arranged around the flow channel by one or more partitions; andarranging the partitions relative to one another in such a way that at least first and second groups of outer cells are formed in the intermediate space between the housing and the flow channel,wherein the first and the second intervals are adjacent to one another and respectively have a width essentially corresponding to 0.1-times the hydraulic diameter of the flow channel, and wherein the outer cells have a volume corresponding to 1.0-1.2-times the volume of inner cells enclosed by the respective outer cells and formed in the flow channel.