The present invention relates to a combustor for pulsating combustion, having a combustion chamber which at the inlet side is connected with a supply tube for combustion air and a supply tube for fuel and at the outlet side is provided with one or more exhaust tubes for the flue gasses, the volume of the combustion chamber being variable, the inlet tubes being arranged one into the other, thus defining an annular space between an outer tube and an inner tube, the annular space being bounded at the side facing the combustion chamber by a widening.

In a pulsating combustor, the combustion chamber and the in- and outlet tubes are dimensioned in such a way that they form a resonance tube which compares with the periodical inflammation of the inflammable mixture in the combustion chamber and the natural frequency of the gas masses in those tubes and the combustion chamber. As a consequence of the inertia of the flowing gasses in the outlet tube(s), a sub-pressure arises in the combustion chamber after the combustion, so that at the one hand fuel and air is sucked in and, at the other hand, hot flue-gasses flow back into the combustion chamber, thus igniting the newly sucked in inflammable mixture. Consequently a cyclic proces arises which pulsates with a frequency mainly depending on the dimensions of the combustion chamber and the in- and outlet tubes and on the fuel applied. Such devices are used for heating, drying, evaporation, inciting of gas-turbines and so on. They have the advantage of a high heat-transfer-rate, thus allowing a compact construction, whereas at the same time, with an excess-combustion-air coefficient of 1, a nearly complete combustion is obtained with almost no exhaust of CO and only very few NOx.

For a proper functioning of a pulscombustor it is necessary that the combustion chamber and the in- and outlet tubes are dimensioned in such a way that the desired resonance occurs.

From US-A-4 699 588 a combustor is known, in which the length of the combustion tube can be altered, whereby tuning is allowed.

It is a drawback of the known combustor that the moving parts of the combustor and/or those parts that are subjected to wear, e.g. an ignition device and optional valves for the supply of air and/or fuel, are difficult to be reached for maintenance and/or inspection.

It is an object of the present invention to provide a combustor for pulsating combustion, having a combustion chamber with a variable volume, from which combustor moving parts can be taken out all together in one movement.

According to the invention there is provided a combustor of the forsaid type, in which the widening is fixed on the inner tube and comprises a tube-shaped part extending in axial direction, said tube-shaped part being provided with a collar which abuts against the inside of the outer tube, the widening thus constituting an axially movable wall of the combustion chamber, whereby at least the tube-shaped part or the collar is provided with openings for supplying the medium present in the annular space between the tubes into the combustion chamber,
   the inner tube is movable in axial direction with respect to the outer tube, and
   the inner tube, the widening with the tube-shaped part, and the collar are adapted to be taken out of the outer tube in one movement.

When the resonance of the combustor is adjusted by means of adjusting the combustor-head also the content of the fore-space between the tubes changes. This may influence the proportion of the air/fuel ratio in the combustion chamber. In regard with this, in the case of a preferable type of the invention, a disc which is provided with supply openings is axially movable in the space between both tubes. In this way it is possible to adjust the content of this space. ( fore-chamber ) By means of this adjustment it is possible to change the proportion of the air/fuel ratio, and so accomplish a combustion with the best efficiency. In the supply openings of the disc, back pressure valves may be fitted, in this way the pressure rise, occurring in the combustion chamber during the combustion, is limited in the fore-chamber as far as the disc. Likewise an axially movable disc provided with supply openings may be installed in the widening of the inner tube with the same function as the disc named above. This disc is attached to a adjustable rod in the inner tube and may equally be provided with back pressure valves. Moreover, this disc provides a regular distribution of the medium in the combustion chamber provided for by the inner tube. The spark plug or the filament can be installed on this disc whereby the wires for the electrical current supply can be lead through the inner tube.

The invention will further be elucidated with reference to the example represented in the drawing which shows a longitudinal section of a combustor according to the invention.

The combustor comprises a combustion chamber A being a part of the air supply tube B but divided from it by the combustor head C. In the air supply tube the air valve section E is located in such a way that there is a fore-chamber D between this section and the combustor head.

The combustor head C consists of the fuel supply tube 1 which is provided with a widening 2 at the side facing the combustion chamber. The widening has a radial outward situated collar 3. This collar may slide along the inner surface of the air supply tube B. In the wall of the widening 2 openings 4 for the supply of the combustion air from the fore-chamber D to the combustion chamber A are present.

In the widening 2 an axially movable disc 5 is supported by a support not shown in the drawing. This disc is fitted with supply openings 6 for the supply of the fuel. Furthermore, for the purpose of starting the combustion, a spark plug 14 is mounted on this disc. This plug is connected by wires with a power source not shown in the drawing. These wires may be led trough tube 1. The air valve section E consists of a sleeve gland 7, a gasket 8, a gland 9, a distance piece 10, a gasket 11 and a disc 12 with the air valves 13.

The air valve section E may be fastened air tight to tube 1 by means of sleeve gland 7 and the gasket 8. The location of this fitting dictates the content of fore-chamber D. By means of gland 9 the distance piece 10 and the gasket 11, disc 12 may be fastened air tight to the air supply tube B. The location of this fitting dictates the content of the combustion chamber and also the total content of the resonance system. In this way it is possible to "tune in" the combustor from the outside, without the need to take constructive measures.

For repairs or inspection combustor head C together with disc 5, spark plug 14 and the valve section E may be taken out of the outer tube B as if it where one piece. The invention is not limited to the example described above. For example, it is possible to supply the fuel trough the outer tube and the combustion air trough the inner tube. Furthermore, in stead of back pressure valves, the discs 5 and 12 may be equipped with an amount of narrow openings. When the combustion in the combustion chamber takes place, these openings deliver sufficient resistance to limit the pressure rise more or less to the front of these discs. By increasing the width of the openings an increasing portion of the flue gasses of the preliminary combustion will be Part of the new mixture. In this way it is possible to reduce the power output, but also to create a NOx reduction.

The supply openings in the wall of the widening may be made in the collar as well as any where in the outline of the widening.

The function of the spark plug or the filament is to start the combustion. When the temperature of the back flowing- or still in the combustion chamber located gasses is high enough to ignite the inflammable mixture in this chamber, the power supply of the spark plug or filament might be shut off.

The described installation preferably works with gaseous or fluid fuels, but solid fuels ( powder ) also can be used. As an example: The content of a combustion chamber of 500 cm³ equals a power output of ~ 30 kWatt, whereas in a combustion chamber of ∅ 40 x 300 cm ~ 10 MWatt can be generated.

The frequency of the combustion cycle mostly vary between 50 and 200 Hz.