Background of the Invention Pulse combustors comprise a combustion chamber having an inlet for the fluid to be burned and an outlet for the combusted fluid. A pressure sensitive valve, for example a simple flapper valve, is provided in the fluid flow path of the fuel/combustion air mixture to be burnt. The valve opens under pressure of the fuel/combustion air mixture to admit into the combustion chamber a charge of the fuel for combustion. The valve is then closed under back pressure of the combusted fuel in the chamber until, on reduction of back pressure due to exhaust of combusting fluid via the outlet, the pressure of the inlet fluid is sufficient to again open the valve. This cycle is repetitively executed so that combustion occurs in a continuous pulsating fashion.

Properly designed pulse combustors are highly efficient, exhibiting full and effective burning of the combustion fluid, with low levels of generated pollutants.

However, the operation is dependent on matching acoustic properties of the combustion chamber to related physical properties of the combustion fluid. For example, the combustion chamber is commonly formed as a box-like or tube-like structure in any event exhibiting one relevant acoustic resonant frequency at which operation will optimally be possible. On the other hand, the properties of the fluid may vary considerably in use of the combustor. For example, the density of the fluid at cold start will normally be substantially different to that prevailing during stable operation. The speed of propagation of sound in the fluid will likewise vary, and may so affect the matching of combustion chamber acoustic characteristics to those of the fluid that improper operation will occur. This may exhibit itself as a certain difficulty in effecting cold start or in less than optimum operation occurring under at least some operating conditions.

Furthermore, because of the nature of the essentially tuned acoustic output of combustors (arising from cyclic acoustic pressure waves), it might be considered that the noise level produced in operation could be reduced by so positioning matched combustors that a significant cancellation of noise from both is achieved. Practically speaking, however, it is extremely difficult to achieve such finely adjusted and controlled operation as to make significant noise reduction possible.

Summary of the Invention In one broad aspect, there is provided a pulse combustion chamber having an anharmonic internal surface. The anharmonic internal surface results in the chamber itself having no particular resonant frequency. Resonance can instead be established as a function of other combustor structure to which the chamber is coupled.

In another aspect, the invention provides a pulse combustor having a combustion chamber which exhibits, over a substantial part of the length of the chamber from an inlet to an outlet, a varying cross sectional dimension measured in at least one plane substantially containing the mean flow path of combustion fluid through the chamber, at least one internal boundary of the combustion chamber, in said plane, being non-linear and having variable curvature. Preferably both internal boundaries, in said plane, are non-linear and have variable curvature. The or each said boundary may have a portion closer to the inlet which continuously varies in curvature with the centres of curvature being internal to the combustion chamber and another portion, closer to the outlet, which continuously varies in curvature with the centres of curvature being external to the combustion chamber.

Preferably, opposed first wall portions defining respective sides of the chamber or side edges of the chamber may be spaced apart a distance which varies over said substantial part of the length of the chamber. For example, the opposed first wall portions or side edges may diverge in the direction away from the inlet towards the outlet, from locations adjacent the inlet. These first wall portions or side edges may converge at locations towards the outlet. The wall portions or side edges may be curved, such as concavely, or concavely and, closer to the outlet, convexly, when viewed from the interior of the combustion chamber.

The combustion chamber may also exhibit, over a substantial part of the length of the chamber from the inlet to the outlet, varying cross sectional dimensions measured in more than a single plane substantially containing the mean flow path of combustion fluid through the chamber. The internal boundaries of the chamber in each said plane being non-linear and of variable curvature.

For example, there may be further opposed wall portions, or side edges, of the combustion chamber, additional to the first wall portions or side edges, which are spaced apart a distance which varies over said substantial part of the length of the chamber.

Wall portions or side edges defining any part of, or all of, the combustion chamber may be curved, such as concavely, when viewed from the interior of the combustion chamber.

According to yet another aspect of the invention there is provided a pulse combustion chamber, an interior surface of which is substantially in the form of a surface of revolution about an axis, the ratio of the axial length of the combustion chamber to the maximum transverse width thereof being less than one and the combustion chamber having an inlet for substantially tangential inlet of working fluid to be combusted in the chamber and a substantially axially positioned outlet for exhaust of combusted working fluid, the outlet communicating with an exhaust pipe.

Preferably, the pulse combustion chamber, as described above, is provided in a pulse combustor, which further includes: a mixing chamber for mixing air and fuel admitted thereto to form said working fluid igniting means for igniting the working fluid when in the combustion chamber; valve means for having an inlet communicating with said mixing chamber and an outlet communicating with said inlet, for controlling inflow of working fluid to the combustion chamber from the mixing chamber to cause the inflow to be pulsed; and an exhaust pipe communicating with the outlet for outlet of combusted working fluid.

It has been found, surprisingly, that by these arrangements, the same form and size of combustion chamber can be adapted to produce different outputs simply by modification of the exhaust pipe, particularly by variation of the length of the exhaust pipe.

The interior surface may be substantially spheroidal.

The exhaust pipe may be coiled or otherwise conformed so as to reduce the distance over which the pipe extends away from the combustion chamber.

The exhaust pipe may communicate with an outlet decoupler.

The inlet may comprise one of a plurality of inlets. The outlet may comprise one of a plurality of outlets. For example there may be two opposed outlets, one at each axial end of the chamber.

Preferably, said ratio is substantially less than or equal to 0.75, and preferably have a small value such as in the range 0.05 to 0.25.

A combustion chamber in accordance with the invention may be characterised by the following: where Q is the firing rate, Vis the chamber volume, f is the firing frequency, a is the pulse amplitude, D is the chamber diameter, d is the tailpipe diameter, Cd is the coefficient of discharge from the chamber to the tailpipe and is a function of D and d, P is the effective tailpipe length, w is the width of the chamber at the tailpipe entrance, and N is a real number.

By these constructions, it has been found that stable operation can be achieved, over a wide range of operating conditions, with there being a band or bands of frequencies at which such operation can be achieved. Cold starting may be more readily effected.

With particular constructions in accordance with the invention, it is also possible to obtain a higher firing rate than with conventional designs of equal volume. The turndown rate may also be greater.

Further, when two combustors are run together and appropriately positioned, acoustic output from each influences the other such that both tend to operate at the same frequency and in antiphase, so that effective noise reduction can be achieved largely automatically.

Brief Description of the Drawing The invention is further described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic lengthwise cross sectional view of a pulse combustor constructed in accordance with the invention; Figure 2 is a cross section on the line 2-2 in Figure 1; Figure 3 is a diagrammatic axial section of a pulse combustor with a modified combustion chamber, constructed in accordance with the invention; Figure 4 is a transverse section substantially on the line 4-4 in Figure 3; and Figure 5 is a side view of a further modified form of combustion chamber formed in accordance with the invention.

Detailed Description of Preferred Embodiments The pulse combustor 10 shown in Figures 1 and 2 has a combustion chamber 12 having an inlet opening 14, and an outlet opening 16. The outlet opening 16 leads to an exhaust pipe 18 and the inlet opening 14 communicates via a valve 22 with a source 20 of fluid to be burnt. The source 20, valve 22 and inlet and outlet arrangements may be in accordance with usual forms for pulse combustors.

A baffle plate 46 may be positioned close to the inlet to facilitate mixing of inlet air/fuel mixture in the combusting mixture in the chamber.

The combustion chamber 12 has two major opposed side walls 24,26 interconnected, as best seen in Figure 2 by respective opposed side walls 28 and 30. The side walls 24,26

may be flat, but it is preferred that these be of the somewhat curved configuration shown in Figure 2 where each is of somewhat concave cross sectional form, viewed in transverse section of the combustion chamber and viewed from the interior of the combustion chamber. The walls 28,30 are also of curved configuration, diverging away from each other from inlet opening 14, with reference to the mean flow direction of fluid through the combustion chamber, designated by arrow 34. This divergence continues for a certain length along the combustion chamber between the inlet and outlet openings, in this case shown to be about half way along that length, after which the side walls converge towards each other until the outlet opening is reached.

In operation, the pulse combustor 10 operates in generally similar fashion to known combustors in that the inlet fluid to be combusted is passed via the valve 22 into the chamber 12 at inlet opening 14, where it is burnt, and the combusting and combusted fluid is exhausted through the pipe 18. At start up, combustion may be initiated by, for example, a spark plug or the like, but generally once combustion has begun it is self sustaining. The valve 22 may comprise a simple flapper valve, reed valve, for example sensitive to fluid pressure such that initially, at start up, a certain quantity of fluid to be combusted is passed into the chamber via the valve 22 which is forced to an open condition by the inlet fluid pressure. An aero valve may also be employed, this being in the form of a passage so configured as to exhibit a low pressure drop so far as fluid flow via the valve to the combustion chamber is concerned, but a high pressure drop so far as fluid flow out of the combustion chamber via the valve is concerned. When combustion is initiated, the pressure of the combusting gas exceeds the inlet fluid pressure and the valve is forced shut under pressure of the combusting gas. This condition prevails until sufficient exhaustion of combusting fluid has occurred that the inlet pressure again exceeds the outlet pressure, after which the valve 22 opens to admit further fluid for combustion.

Thus, pulses of fluid are admitted, burnt, and exhausted from the combustion chamber, the valve 22 repetitively opening and closing as above described.

The inlet fuel/combustion air mixture may be of any conventional type, such as a mixture of the gaseous fuel and air.

The inlet fluid may be of any conventional type, such as a mixture of gas and air.

It has been found that, as a consequence of the variation of the cross sectional dimension of the combustion chamber between walls 28 and 30 along the length of the combustion chamber (with reference to the mean flow path designated by arrow 34) the combustion chamber is anharmonic in the sense that it does not have a single well defined acoustic resonant frequency, but is capable of resonant type operation over a wide range of frequencies. The range of acoustic resonant frequencies is further extended in the preferred form by additionally forming the walls 24,26 of curved configuration as described.

Unlike conventional pulse combustors, the combustor described is capable of operation over a wide range of operating frequencies and firing rates, particularly under differing conditions of density of the air/fuel mixture. For a given combustor volume, the described chamber shape enables increased firing rates (ie thermal input) compared with cubicle or tubular chambers. Furthermore, it has been observed that, if two combustors of the kind described are positioned close to each other they will, once operational, more readily than conventional combustors tend to seek a common operating frequency, operating in anti- phase so that noise generated, by for example operation of the valves 22 of each, is at least to some extent cancelled through destructive source pressure addition. It is not necessary in this case to take special steps to particularly tune the combustors any resonant frequency, since, as mentioned, the combustors automatically tend to settle at a common operating frequency while operating in anti-phase.

It has been found that the described combustor is capable of starting at a low flow rate, whereas prior devices generally must start at full flow rate.

In the particular form shown the walls 28,30 have first portions 28a, 30a which extend in substantially aligned linear fashion, normal to the mean flow path. Portions 28a, 30a then lead to portions 28b, 30b which are concave when viewed from the interior of the chamber, and these lead to further portions 28c, 30c which extend to outlet 18 and which are somewhat convex, viewed from the interior of the chamber. The portions 28a, 30a more generally exhibit a continuously changing curvature over their lengths, with the centre of

curvature being within the chamber. The portions 28c, 30c exhibit a continuously changing curvature, with the centres of curvature being external to the combustion chamber. The portions 28a, 28c, 30a, 30c are thus non-linear and of varying curvature (ie not part circular). The chamber should best exhibit a substantial breadth closer to the inlet end (ie. in the first half of the chamber) and taper away in a concave/convex manner towards the outlet, as described. The inlet end may comprise opposed portions which diverge at close to 180° adjacent the inlet with increasing concave curvature away from the inlet until the second portions are reached, at which the opposed walls are generally parallel. The second and third portions may however simply exhibit a concave curvature over the combined lengths thereof or exhibit concave/convex curvature as described above.

The cross sectional form of the combustor chamber may vary from that shown in Figure 2.

For example the side walls 24,26 may be extended as shown by broken lines 24a, 24b, 26a, 26b so that the walls 24,26 meet at side edges of the chamber in which case walls 28, 30 are not present.

In the arrangement described, the combustion chamber is of somewhat flat form exhibiting over most of its length a substantially greater cross-sectional dimension in the plane A-A shown in Figure 2, midway between walls 24,26 than the plane B-B normal to plane A-A.

In this case the internal boundaries of the chamber, in the plane A-A are, as described, configured so that they are, over most of their lengths, both non-linear and of variable curvature with substantial lengths of them exhibiting continuous variation. The cross- sections transverse to the flow direction through the chamber may be non-circular.

However, it is desirable in some instances that the boundaries of transverse cross sections are, over substantial parts thereof, non-linear, as described, satisfactory operation can be obtained with the rectangular generally cross-sectional forms, shown, with operation generally being improved by adopting the outwardly bowed side walls also described.

It is also possible to obtain satisfactory operation by having the boundaries of two or more lengthwise cross-sections exhibit substantial portions which are non-linear and non- arcuate, such as of the form exhibited by the boundaries in the plane A-A. In these cases it is less important to avoid circular or part circular transverse cross-sections.

Referring now to Figures 3 and 4, a modified pulse combustion chamber 110 is shown.

The pulse combustor 110 shown has structure 125 defining a combustion chamber 112 with an inlet opening 114 and an exhaust outlet 116. The inlet opening 114 is coupled by an inlet pipe 135 and a flow pulsing valve 122 to a suitable source of combustible working fluid 120, such as a mixing chamber for mixing liquid or gaseous fuel with air. Source 120 and valve 122 may be of conventional form.

The exhaust outlet communicates via an exhaust pipe 118 with an outlet decoupler 142.

The outlet decoupler is in the form of a chamber with an inlet 145 communicating with pipe 118 an outlet 147 open to the atmosphere. The exhaust decoupler is designed to decouple pressure waves arising in the combustion chamber when in use from the outside ambient air, and may be of conventional form.

The combustion chamber is of ovaloid form having a ratio of length in the axial direction to diameter, D, to of less than 1.

The outlet 116 is at one axial end of the chamber 112, and the inlet pipe 135 and opening 114 are arranged for substantially tangential inlet of working fluid in the plane of maximum diameter of the chamber 112.

In operation, the pulse combustor 110 again operates in generally similar fashion to known combustors in that working fluid in the form of for example air/fuel mixture is passed via the valve 122 into the combustion chamber 112 at inlet opening 114. The working fluid is burnt in the combustion chamber, and the combusting and combusted fluid is exhausted through the exhaust pipe 118. At start up, combustion may be initiated by, for example, a spark plug or the like, but generally once combustion has begun it is self sustaining. The valve 122 may comprise a simple flapper valve, reed valve, for example sensitive to fluid pressure such that initially, at start up, a certain quantity of fluid to be combusted is passed into the chamber via the valve 122 which is forced to an open condition by the inlet fluid pressure. An aero valve may also be employed, this being in the form of a passage so configured as to exhibit a low pressure drop so far as fluid flow via the valve to the combustion chamber is concerned, but a high pressure drop so far as fluid flow out of the combustion chamber via the valve is concerned. When combustion is initiated, the

pressure of the combusting gas exceeds the inlet fluid pressure and the valve is forced shut under pressure of the combusting gas. This condition prevails until sufficient exhaustion of combusting fluid has occurred that the inlet pressure again exceeds the outlet pressure, after which the valve 122 opens to admit further fluid for combustion. Thus, pulses of fluid are admitted, burnt, and exhausted from the combustion chamber, the valve 122 repetitively opening and closing as above described.

Combusted gases exhaust from chamber 112 via outlet opening 116 and pass through exhaust pipe 118 to the outlet decoupler 142.

It has been found that by simple selection of the length of the pipe 118, it is possible to arrange the combustor 110 to produce a heat output which is selectable over a wide range.

Thus it is possible to provide a range of combustors with a wide variety of heat outputs simply by arranging for different length pipes 118 in conjunction with chambers 112 of the same form.

In particular, the pipe length for any chamber 112 and desired heat output may be selected on the basis of the following relationship: where Q is the firing rate, V is the chamber volume, fis the firing frequency, a is the pulse amplitude, D is the chamber diameter, d is the tailpipe diameter, Cd is the coefficient of discharge from the chamber to the tailpipe and is a function of D and, d, I is the effective tailpipe length, w is the width of the chamber at the tailpipe entrance and N is a real number.

The structure 115a in Figure 5 is similar to the structure 115 of Figures 3 and 4, but has two opposed axial outlets 16 from the chamber 112. There may be connected to a single

exhaust pipe 118 or to respective separate exhaust pipes 118. In the latter case, the exhaust pipes 118 may communicate with a single decoupler 142 or with respective separate ones.

The described construction has been advanced merely by way of explanation, and many modifications and variations may be made thereto without departing from the spirit and scope of the invention which includes every novel feature and combination of features herein disclosed.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word"comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.