A LINEAR FREE PISTON STIRLING MACHINE

The present invention relates to a linear free piston Stirling machine having a main axis of reciprocation, the machine comprising a power piston and a displacer, the displacer being reciprocated, in use, to displace gas in alternately opposing directions through a regenerator between hot and cold spaces, the regenerator having an annular configuration centred on the main axis and surrounding the displacer. Such a machine will be referred to subsequently as "of the kind described" .

Stirling machines of the kind described are known in which the regenerator is made of a wound coil of foil. These coils are disclosed, for example, in US 6,817,221, US 6,745,822, US 5,429,177 and JP 62-190391.

There are a number of drawbacks in using an annular wound foil regenerator. The wound coil provides a plurality of direct flow passages through the coil. This creates established flow conditions in each passage which does not maximise the turbulent mixing of the fluid in each passage. In the centre of each passage, fluid temperature does not change as much as the fluid closer to the foil boundary so that the heat transfer as a whole is not maximised. Also, there is a significant temperature differential between the two ends of the regenerator creating a differential thermal expansion across the regenerator. This gives rise to two problems. Firstly, the passages through the regenerator will be constricted to different degrees upon thermal expansion. As a result of this, the thermal properties of the complete structure will not be optimised. Secondly, the

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differential thermal expansion induces stresses within the regenerator structure potentially causing degradation and reducing the life of the regenerator.

According to the present invention, a Stirling machine of the kind described characterised in that the regenerator comprises a plurality of sections arranged axially in series, each section comprising a wound coil of foil, the sections being arranged such that the flow passages of one section are off-set with respect to those of an adjacent section.

By providing a plurality of axially arranged sections, the above mentioned problems are solved.

As the flow passages to the adjacent sections are offset with respect to one another, turbulent flow occurs at the junctions between sections thereby improving the thermal transfer of the regenerator.

By splitting the regenerator into a number of sections, the temperature differential across each section is significantly less than the differential across the whole regenerator. This reduces the thermal stresses induced within the overall structure, and allows each section to be designed such that, upon thermal expansion, the flow passages have the optimum size, and no leakage occurs around the outside of the regenerator.

It is possible that each of the sections has the same winding density. This sacrifices the ability to optimise the thermal characteristics for each section. However, it does lead to a simplified structure requiring fewer different parts.

However, preferably, the winding density of each section changes progressively from one end to the other. Preferably, the winding density becomes progressively larger from the hot space to the cold space.

Adjacent sections may abut one another as this would increase turbulence as desired. However, this may also serve to obstruct some of the channels. The adjacent sections are therefore preferably spaced axially from one another. Preferably adjacent sections are separated by a diffuser ring as this maximises the heat transfer surface areas in the regenerator.

Each section may be any configuration of wound foil. However, preferably, each section is a helical winding and has an alternate corrugated and non-corrugated layers . This is a simple structure which provides good control over the size of the passages.

An example of a Stirling machine in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic cross-section through a Stirling machine;

Fig. 2 is a schematic diagram of the heat exchanging configuration; and

Fig. 3 is an idealised perspective view of the regenerator.

The example described below is a Stirling engine, the invention is equally applicable to other types of Stirling machine such as a Stirling cooler.

The engine shown in Fig. 1 is well known in the art and will only be described briefly here. The engine comprises a closed casing 1 having a main axis 2 along which a displacer 3 and power piston 4 reciprocate. The engine is provided with a plurality of external fins IA and internal fins 5 which are surrounded by a burner (not shown) to provide a heat input into the engine head. A coolant circuit 6 forms a mid portion of the engine to extract heat from the engine. The engine is set up so that the heat flow causes the displacer 3 and power piston 4 to reciprocate along the main axis 2 out of phase with one another. The displacer 3 is arranged to alternately pump hot air downwardly through the annular regenerator 7 and "cool" air upwardly through the regenerator 7. The displacer 3 is supported on a rod 8 with a pair of springs 9. As the power piston reciprocates, electricity is generated in alternator 5.

The improved design of regenerator 7 is described below with reference to Figs. 2 and 3.

The regenerator 7 is a non-condensing regenerator and comprises three separate sections, although there could be two sections or more than three sections . In the

illustrated case, there is a hot section 11, a middle section 12 and a cool section 13. Each section is an annular configuration, and the three sections are arranged in series in the direction of axis 2. Adjacent sections are separated by sintered rings 14 which act as flow diffusers between sections.

As best shown in Fig. 3, the three sections are made up of alternate flat sheet 15 and corrugated sheets 16. These are formed in an annular configuration (although Fig. 3 shows them as planar) .

The winding density of the hot section 11 is less than that of the middle section 12, which in turn is less than that of the cool section 13. In practice, this effectively means that, the size of the passages in the hot section 11 is greater than that of the middle section 12 which is greater than that of the cool section 13.

In use, the displacer 3 on its up stroke pumps hot gas, in turn, through the hot 11, middle 12 and cool 13 sections.

Then, on its return stroke, pumps cool gas in the opposite direction.