Heat exchange – Non-communicating coaxial enclosures
Patent
1997-02-27
2000-02-01
Flanigan, Allen
Heat exchange
Non-communicating coaxial enclosures
165155, F28D 710
Patent
active
060191686
DESCRIPTION:
BRIEF SUMMARY
The invention relates to heat exchangers and heat exchanger elements and in particular but not exclusively to such heat exchanger elements for use in Stirling engines.
Stirling engines require small heat exchangers with high rates of heat transfer and may also require high strength so that they can operate reliably under high pressures. It is also important for them to have a small volume for the working fluid of the engine to help minimise the engine dead space. High heat transfer rates to a small volume of fluid lead to a requirement for a high heat transfer surface to volume ratio within the heat exchanger. These requirements apply to the heater normally employed to transfer heat from combustion gases to a working fluid and to a cooler to transfer heat from the working fluid in a different phase of the Stirling engine cycle. For the heater, there is also a requirement to operate at high temperatures.
It is known from GB A 2 261 280 to provide a heat exchanger element comprising an outer tube, an inner tube within the outer tube, a first fluid flow path for a first heat exchange fluid formed between the inner and outer tubes and means for providing a second heat exchange fluid in heat transfer relation to the outer surface of the outer tube and/or the inner surface of the inner tube.
According to the present invention a heat exchanger element of this kind is characterised by a sleeve within the first fluid flow path between the inner and outer tubes defining an outer interface with the inner surface of the outer tube and an inner interface with the outer surface of the inner tube and by generally longitudinal grooves at each interface to provide together the first fluid flow path.
The known heat exchanger provides a greater area for heat transfer than an annular gap by means of longitudinal ribs on the tubes within the first fluid flow path. The prior proposal also provides breaks in the ribs to break up laminar flow within the first fluid flow path and further improve heat transfer. There is a practical limit to the extent that heat transfer characteristics can be improved in this way. For example, increasing the number of ribs requires a reduction in their thickness which reduces heat conduction to the tubes themselves along the ribs and also leads to fragility and manufacturing difficulties. The volume for the first fluid also remains relatively high.
By providing an additional sleeve and grooves in accordance with the present invention, a large and effective heat transfer surface can be achieved with a small internal fluid volume, resulting in a high heat transfer surface to volume ratio.
The sleeve may be in intimate heat exchange contact with at least one of the tubes. With this arrangement, an effective heat flow path exists as: first fluid; sleeve; tube; second fluid or vice versa. For this purpose, the sleeve may be shrunk on to or in to a tube. Alternatively, differential expansion may be such that contact between sleeve and tube is most effective only at operating temperatures when effective heat transfer is most important. Electron beam welding may be used to provide even more intimate contact. Good contact in part depends on precision manufacture, both as regards surface finish and dimensions.
Alternatively, the sleeve may be provided primarily as a spacer, to direct fluid through the grooves and provide most or all of the heat transfer directly between the fluid and the tubes.
In addition to the grooves described above, which are referred to as main grooves, secondary grooves in the tubes and or sleeve may be provided at an inclination to the main grooves. These secondary grooves may be provided in either surface forming the interface between tube and sleeve. On assembly, these secondary grooves form slots down which a relatively small degree of fluid flow from one main groove to the next can be induced. This fluid flow can be controlled so as to create a degree of spiral flow in a desired direction down the main grooves. This in turn allows control of the relationship between laminar and t
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Flanigan Allen
Sustainable Engine Systems Limited
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