Patent application title: HEAT EXCHANGER ARRANGEMENT
Paul Howard Riley (Derby, GB)
THE UNIVERSITY OF NOTTINGHAM
IPC8 Class: AF28F110FI
Class name: Heat exchange side-by-side tubular structures or tube sections
Publication date: 2011-07-07
Patent application number: 20110162827
Heat exchangers can be utilised in thermo-acoustic engines to facilitate
through acoustic oscillations electrical power generation via linear
alternators and/or cooling effects. Provision of a heat exchanger
arrangement 31 which can be associated with a traditional open flame
stove utilised in third world countries would be advantageous. However,
such heat exchanger arrangements must be lightweight and robust to
withstand operational use. By creating a heat exchanger arrangement
formed from plates having apertures which develop a folded conduit to act
as a resonance tube of appropriate length it is possible to more easily
accommodate thermo-acoustic electrical power generation and cooling
effects in a traditional stove configuration.
1. A heat exchanger arrangement comprising a plurality of sheets
associated together to define a flow conduit, intermediate sheets of the
plurality of sheets having an aperture aligned and offset relative to
apertures in adjacent sheets in a stack to form an internal cavity
defined by the apertures as a conduit through the heat exchanger.
2. An arrangement as claimed in claim 1 wherein the conduit includes side-by-side passage segments with a bend formed by apertures in succession.
3. An arrangement as claimed in claim 1 wherein the conduit provides a wave resonator tube.
4. An arrangement as claimed in claim 3 wherein the wave resonator is for a standing wave.
5. An arrangement as claimed in claim 3 wherein the wave resonator is for a travelling wave or stirling engine configuration.
6. An arrangement as claimed in claim 1 wherein the sheets are formed from stainless steel or a ceramic or plastics material.
7. An arrangement as claimed in claim 1 wherein the sheets are secured together through brazing or diffusion bonding or super plastic forming or other fusion bond.
8. An arrangement as claimed in claim 1 wherein the sheets are corrugated.
9. An arrangement as claimed in claim 1 wherein the sheets are separated by a spacer.
10. An arrangement as claimed in claim 1 wherein the spacer is corrugated.
11. An arrangement as claimed in claim 1 wherein the internal cavity defines a pressure containment formed by relatively thin walls in comparison with the pressure expected in the containment and reinforced with strengthening members.
12. An arrangement as claimed in claim 11 wherein the strengthening members are corrugated.
13. An arrangement as claimed in claim 1 wherein the sheets of material define separate flow paths for heat exchange between fluids.
14. An arrangement as claimed in claim 1 wherein one part of the heat exchanger arrangement incorporates fire tubes through which flames can pass.
16. A thermo-acoustic engine formed by a stack of sheets associated together to form a conduit as a resonance path; the conduit comprising side-by-side passages with a bend formed by overlapping apertures in the sheets.
17. A thermo-acoustic engine as claimed in claim 16 incorporated within a stove.
19. A heat exchanger arrangement incorporating a conduit having a curved aspect to define a heat radiation cavity having a plurality of heat exchanger passages opposite the curved aspect, ends of the passages including end hood interrupters to prevent direct heat radiation along the heat exchanger passages.
20. An arrangement as claimed in claim 19 wherein each hood interrupter is flat and angular to a principal axis of the heat exchanger passages.
21. An arrangement as claimed in claim 19 wherein the hood interrupters are curved.
22. An arrangement as claimed in claim 19 wherein each hood interrupter extends substantially across a respective width of a heat exchanger passage.
24. A stove comprising a heat exchanger arrangement as claimed in claim 1.
25. A stove comprising a thermo-acoustic engine as claimed in claim 16.
26. A stove comprising a heat exchanger arrangement as claimed in claim 19.
 The present invention relates to heat exchanger arrangements and
more particularly to heat exchanger arrangements utilised in
thermo-acoustic constructions for heating, refrigeration and electrical
 In order to improve efficiency it is known to provide arrangements which achieve so-called combined heat and power regimes. In such circumstances typically a boiler is arranged to heat water for circulation through a central heating system as well as provide steam for turning an electrical power generator. Such arrangements are typically on a relatively large scale and arranged to heat whole buildings or housing estates. More recently it has been known to provide so-called micro heat and power arrangements. Such micro combined heat and power arrangements as described below will be utilisation with respect to individual homes and generally at remote locations. As previously a heat source such as an open hearth fire is used to provide a heat differential which can then be utilised for electrical power generation as well as for heating both of a room as well as for cooking.
 There is a desire to provide stoves which can not only provide cooking but also electrical power generation and refrigeration in developing and rural areas. It will be appreciated that a significant number of people still cook upon three stone hearths upon which cooking pots are located upon the stones or similar supports and a wood or similar material fire utilised for heating the pot etc. Such stoves although rudimentary are convenient, use local materials and are cheap to construct. Essentially, these stoves are used as indicated in developing and rural areas with limited resources instead of more expensive forms of cooking and heating possibly even where mains or local generator electrical supplies are available or there are gas supplies. Such developing and rural communities typically have access to biofuels in the form of wood or animal dung but hydrocarbon fuels such as diesel, gas and petrol are too expensive for cooking or utilisation with regard to electrical power generators.
 In the above circumstances delivery of even rudimentary electrical power generation and refrigeration capabilities is difficult due to cost and remoteness in particular.
 In view of the above it would be advantageous to provide a stove which utilises a thermo-acoustic engine or stirling engine construction in order to allow a linear alternator to generate electrical power. Such thermo-acoustic engines have reduced moving parts and therefore have low requirements with regard to maintenance whilst achieving high reliability. In order to provide a thermo-acoustic engine it is necessary to have a heat exchanger arrangement. FIG. 1 provides a schematic illustration of a thermo-acoustic arrangement operating as a refrigerator as a result of acoustic power provided by an electromagnetic transducer 6. In the arrangement 1 a thermo-acoustic stack is provided by a series of plates presented in parallel channels. Gas pressure in the arrangement 1 oscillates acoustically at a frequency set by the resonance between the gas in the closed volume of the arrangement 1 as a resonator and a moving mass of the transducer 6. In such circumstances the enclosed gas oscillates in the direction of arrowheads 7 in response to oscillations in the moving mass of the transducer 6. The oscillating gas communicates heat with the stack 2 and through the heat exchangers and the acoustics of the arrangement 1 ensures heat is pumped out of the ambient heat exchanger 3 to the hot heat exchanger 4 using a hydrodynamic energy transfer cascade enabled by compressing and expanding gas parcels. In order to create a heat pump or thermo-acoustic engine a reversal of the operation can be performed in that a high temperature gradient along the stack leads to a spontaneous generation of acoustic power which can be converted to electricity by utilising the electro-dynamic transducer 6 as a linear alternator.
 A further refinement has a thermo-acoustic engine combined with a linear alternator and a thermo-acoustic cooler in an aligned configuration. In such circumstances, as previously the arrangement has a thermo-acoustic stack 12 between a hot heat exchanger 14 and a cold heat exchanger 13. The stack 12 as previously comprises a series of tubes or plates such that the disparity across the heat exchangers 13, 14 creates acoustic gas oscillations 17 which act upon a linear alternator 16 in order to create electrical energy 18 through a moving mass. Upon a thermo-acoustic cooler side further heat exchangers 13, 14 are provided such that again acoustic oscillations 27 act to create a cool heat exchanger and an ambient heat exchanger 14 for an appropriate cooling effect. It will be noted that in the arrangement depicted ambient air 20 is presented initially through the heat exchanger 14 to gain some heat and then through the heat exchanger 13 before being utilised in a combustor 10 where that air acts with a fuel such as wood to heat the heat exchanger 14 prior to being utilised in a hob 21 and exhausting through a chimney 22. In such circumstances again the arrangement 11 depicted in FIG. 2 comprises a half wavelength standing wave resonator which combines a quarter wavelength engine part with a linear actuator and quarter wavelength refrigerator cooler part. In such circumstances the only moving mechanical part is the linear actuator. It will be understood that the standing wave as illustrated in FIG. 1 and FIG. 2 may be replaced by a travelling wave system similar to a stirling engine which may result in higher efficiency devices but with a higher geometric complexity. The working gas within which the oscillations 17, 27 are provided is sealed and for simplicity is generally pressurised air at up to 10 bar although other gases such as Helium could be used. Pressurisation may be achieved through a simple hand or foot operated pump. Although other gases such as Helium would add to initial cost in terms of the expense of the gas it may be possible to reduce overall costs by having a reduced size heat engine having a higher frequency of operation.
 It will be understood that the external ambient air stream handles all external heat transfer functions. After removing the heat from the refrigerator's ambient side the air flow passes through an ambient side of the engine stack 12 where it absorbs more heat and into the combustion chamber where it oxidises the fuel. The hot flue gases flow through the engine's heat exchanger and cooking element providing necessary heat for both and then out of the chimney as illustrated. As indicated above aspects of the present invention may be utilised in a number of situations in addition to those with regard to rudimentary open flames. For example, aspects of the present invention may be utilised with a domestic or industrial boiler which also creates heat as well as electrical power. Such installations may be referred to as micro combined heat and power or MCHP. In such circumstances heat is supplied from the usual heat source which could typically be natural gas, oil propane etc. The ambient heat exchanger in this case is water cooled, and the water is then pumped around the house to heat the radiators by the usual method. The stove hob is replaced by a second heat exchanger that condenses the flue gases in the same way as a conventional condensing boiler. (i.e. the stove function is removed) In this way, more energy is taken out of the combustion gases and fed into the circulating water making the thermal efficiency very high. The electricity generated is typically converted to mains voltage. The electricity can then be used within the building, sold back to the electricity provider or a combination of both.
 Essentially aspects of the present invention require a heat source to provide operation in order to generate electrical power. In such circumstances electrical energy may be generated utilizing industrial waste heat from a number of processes. It will be understood that many industrial processes require heat for an operation. The heat is then either lost to the environment or passed through heat exchangers for re-use. A typical example is the brewing industry, where water needs to be heated for sterilisation and then cooled to a temperature required for fermentation.
 In this embodiment, the cold water required for the process is passed through the ambient heat exchanger and heats up, and the waste (hot) water passes through the hot heat exchanger and cools down. (Neither the stove or radiant heat exchanger functions are used) the difference in temperature again causes an acoustic resonance that is converted to electricity using a transducer. In this case the transducer could be a linear alternator, or other rotating generator and turbine assembly.
 In order to achieve the necessary heat transfer for the flue gas the heat transfer to the acoustically oscillating air in the duct must be understood. There is a potential to use an external heat transfer function by utilising self circulating thermo-acoustic heat exchangers. It will be understood that the functional parts as described above may be constructed separately that is to say an electrical generation function as depicted in FIG. 1 or refrigeration on its own or both. In either respect it is necessary to provide a heat exchanger arrangement. This heat exchanger should ideally allow a simple light weight construction for transportation within rural and developing communities with limited road or other transport infrastructure and also enable a compact construction which avoids some of the in line linearity of the acoustic engine arrangements described above with regard to FIG. 1 and FIG. 2.
 In accordance with aspects of the present invention there is provided a heat exchanger arrangement comprising a plurality of sheets associated together to define a flow conduit, intermediate sheets of the plurality of sheets having an aperture aligned and offset relative to apertures in adjacent sheets in a stack to form an internal cavity defined by the apertures as the conduit through the heat exchanger.
 Typically, the conduit includes side-by-side passage segments with a bend formed by apertures in succession.
 Generally, the conduit provides a wave resonator tube. Typically, the wave resonator is for a standing wave. Alternatively, the wave resonator is for a travelling wave or stirling engine configuration.
 Typically, the sheets are formed from stainless steel or a ceramic or plastics material. Generally, the sheets are secured together through brazing or diffusion bonding or super plastic forming or other fusion bond. Possibly, the sheets are corrugated. Generally, the sheets are separated by a spacer. Generally, the spacer is corrugated.
 Generally, the sheets of material define separate flow paths for heat exchange between fluids.
 Generally, one part of the heat exchanger arrangement incorporates fire tubes through which flames can pass.
 Possibly, the cavity defines a pressure containment which may be reinforced with strengthening members possibly of a corrugated form.
 Also in accordance with aspects of the present invention there is provided a thermo-acoustic engine formed by a stack of sheets associated together to form a conduit as a resonance path; the conduit comprising side-by-side passages with a bend formed by overlapping apertures in the sheets.
 Typically, a heat exchanger in accordance with aspects of the present invention is incorporated within a stove.
 Further in accordance with aspects of the present invention there is provided a heat exchanger arrangement incorporating a conduit having a curved aspect to define a heat radiation cavity having a plurality of heat exchanger passages opposite the curved aspect, ends of the passages including end hood interrupters to prevent direct heat radiation along the heat exchanger passages.
 Typically, each hood interrupter is flat and angular to a principal axis of the heat exchanger passages. Alternatively, the hood interrupters are curved. Generally, each hood interrupter extends substantially across a respective width of a heat exchanger passage.
 Also in accordance with aspects of the present invention there is provided a stove comprising a heat exchanger as described above and a hearth including a stove ring or similar support for a cooking receptacle, and a source of heat whereby the heating is in thermal association with the heat exchanger arrangement.
 Embodiments of aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which:
 FIG. 3 is a schematic illustration of a cross section through a heat exchanger arrangement associated with a linear actuator in accordance with aspects of the present invention;
 FIG. 4 is a plan view of the heat exchanger as illustrated in FIG. 3;
 FIG. 5 provides a schematic illustration of a first intermediate sheet in accordance with aspects of the present invention;
 FIG. 6 is a schematic illustration of a second intermediate sheet in accordance with aspects of the present invention;
 FIG. 7 is a schematic illustration of a bottom sheet in accordance with aspects of the present invention;
 FIG. 8 is a schematic isometric view of two sheets in accordance with aspects of the present invention utilised to define a conduit;
 FIG. 9 is a schematic illustration of a heat exchanger arrangement in which heat is radiated towards passages of a heat exchanger;
 FIG. 10 provides one embodiment with regard to avoiding straight through heat radiance into passages in accordance with aspects of the present invention; and,
 FIG. 11 is a schematic illustration of an alternative arrangement to avoid straight through heat radiance to heat exchanger passages in accordance with aspects of the present invention.
 As indicated above in order to create a thermo-acoustic engine with a resonator it is necessary to provide effectively a closed duct with a thermo-acoustic stack arranged to have heat exchangers to develop expansion and compression alternately which can be picked up as acoustic oscillations by a linear alternator. As indicated above what is required is generally a closed pressurised volume of fluid which can be acted upon by heat exchangers and in particular a hot side and a cold side in order to create the acoustic oscillations which can be picked up by the linear alternator.
 Provision of a suitable heat exchanger arrangement that includes a conduit as resonance tube but which is lightweight and robust enough for utilisation in rural environments would be desirable. It is known to create heat exchangers in the form of stacks of plates with generally correlations between the plates in order to define exchanger pathways along which fluids to undertake heat exchange are presented. In such circumstances, along one set of pathways a relatively hot or relatively cold fluid passes for heat exchange with an alternate cool or hot fluid or simply a fluid to be utilised for heat release in a radiator configuration or which requires heating. If an appropriate heat exchanger arrangement can be provided it will be understood the heat exchanger arrangement may be located within a basic stove configuration in order to provide a basis for electrical power generation and/or a refrigeration effect. The stove will simply provide a fire associated with a hot heat exchanger end of the heat exchanger arrangement including fire tubes which then extend up to a burner ring upon which a pot or kettle can be presented. The flames can be generated through wood or another biofuel or utilising if available natural gas or stored gas such as butane or propane. The burner ring may simply be a mud stove with a ring of stones to support a pot or kettle.
 Previous problems with heat exchangers related to the relative weight of the heat exchanger and with regard to incorporating acoustic resonators to form thermo-acoustic as such resonators have a generally linear in line configuration for the resonator tube filled with the working fluid such as pressurised air. In other aspects of the present invention a heat exchanger arrangement is provided which can incorporate a conduit to act as the resonator tube but which can also create the desired heat exchanger effects for utilisation in the configurations as depicted in FIG. 1 or FIG. 2 above.
 The cavity defining the resonator is a pressure confinement for the pressurised fluid (air/helium etc) formed from relatively thin wall materials at least relative to the pressure to be contained. Thus, strengthening members are used which may be corrugated to improve their effectiveness.
 Referring to FIG. 3 and to FIG. 4 in which a heat exchanger arrangement 31 is illustrated as a schematic cross section. As can be seen the heat exchanger arrangement creates a conduit 35 which extends in general hairpin loops with curved ends 30. In such circumstances the length or width of the heat exchanger 31 with an integral half wave resonator is reduced allowing easier operation and configuration within a primitive stove. It will be understood the length does not need to be 1/4 or 1/2 wavelength. (although it can be this length) In a normal Thermo-acoustic engine, there does need to be a resonator. In this invention, the mass and compliance of the alternator form the resonant component, thus reducing the duct length required.
 The bent duct has two advantages:  1). It reduces cost and the physical size by both folding the tubes over so that the engine length is shorter than the tube length, and by minimising the area of tube that has to contain pressure. (The section in the centre is common to both front and back tubes so only has to contain differential front/rear pressure rather than the full mean pressure)  2). It allows for three configurations, depending on the termination between the pipe that comes from the front of the speaker and the one that comes from the rear.  a). If the termination is blocked, then the engine is a conventional standing wave engine with a cold bounce volume at the rear of the speaker.  b). If the termination is blocked, and two HHX's, Stacks and AHX's are used, one on the front tube and one on the rear, then a push pull TAE is formed.  c). If the termination is a restrictor, and the relative lengths of the tube are correctly designed, then a travelling wave design is formed.
 As indicated above generally the resonator comprises a half wavelength resonator which operates at reduced frequencies. Thus, in order to create a quarter wavelength standing wave with the resonator tube 35 a length is required which may not be easily accommodated, if in a straight line, within an acceptable stove configuration or size envelope. By providing a bent conduit 35 to act as the resonator tube it will be understood that the quarter wavelength can be achieved but in a much shorter width or length for the stove. However, it is also necessary to provide heat exchanger functions.
 Before proceeding with further descriptions with regard to other aspects of the present invention it will be appropriate to describe operation of a radiant heat exchanger. The outside face 43 is typically coated matt black so as to be a good absorber of radiant heat, and surface 34 is reflective. The heat source 28 generally provides heat through both radiation and convection, the gases pass though cavity 29 which is shaped to alter the velocity so that even heating of the face 43 occurs. Surface 43 heats up through a combination of direct heating through convection and radiation from the heat source, and indirectly from radiation reflected from surface 34. Heat passes through the end face 43 from the outside to the inside through conduction in the material. The inside face 43 is also typically coated matt black so as to be a good radiator of heat. All other internal surfaces are reflective to reduce heat flow back again to the outside. Heat exchanger matrices 81, 44 are typically coated matt black so as to be a good absorber of radiated heat. The matrices 81, 44 absorb the radiation produced from the face 43 and heat up the material. The acoustic gases pass through the heat exchanger matrix pick up heat from the material through direct conduction and convection.
 In terms of stove operation generally an end 34 includes fire tubes 29 along which flames 28 pass to heat the end 34 in order to provide the heat stimulus in accordance with aspects of the present invention. The flames extend through the fire tubes 29 upwards through a stove ring 41 to allow heating of a cooking pot 42. The end 34 heats a curved surface 43 which in turn radiates heat towards a hot side heat exchanger shown schematically as 44. A cold side of the heat exchanger 32 is also shown schematically although in reality it will be appreciated that the heat exchangers 32, 34 will be constructed either side-by-side of an acoustic stack 100 including the working fluid contained within the conduit 35 to create the acoustic oscillations which can be utilised by a linear alternator 36 to generate electrical power in use. It will be understood that typically in the order of at least 100 Watts may be created which can be sufficient to provide lighting in a rural location using heat energy from a primitive stove.
 The above configuration for the heat exchanger 31 is desirable but it is also necessary to enable easy construction of this heat exchanger 31 configuration. In accordance with aspects of the present invention such an approach is achieved by creating stacks of plates having apertures aligned and offset with respect to each other in order to create the conduit 35. As will be described later with regard to FIGS. 4 to 7 generally layers or sheets of material are arranged in a stack side-by-side with spacers in the form of corrugations to create the heat exchanger paths necessary in accordance with aspects of the present invention. In such circumstances by creating sheets of material which have apertures which progressively form a cavity it will be understood the conduit 35 can be provided.
 In such circumstances the stacks of sheets of material will be presented with spacer corrugations between and then utilising standard diffusion bonding or forming techniques a conduit 35 as a cavity within heat exchanger 31 can be created. It will be understood that where corrugations are utilised as flow pathways then where these corrugations extend to the sides of the conduit 35 it will be necessary to plug or close the corrugations if required.
 Once the conduit 35 is formed it will be understood that sides of the heat exchanger 31 will typically be sealed with an external flat plate 50 to further strengthen the heat exchanger 31 arrangement.
 In use the linear alternator 36 and its moveable armature mass 39 will be sealed within the arrangement as required. However, as with the end 34 it may be possible to construct a separable part which can be removed from a conduit part to allow access to the linear alternator 36 for maintenance and repair. However, such an arrangement will require generally seals at the surfaces between the abutting parts and such seals will need to contain the working fluid pressurisation within the conduit 35 acting as the acoustic wave resonator. Such seals may add to cost and introduce unreliability. Nevertheless, as with the whole heat exchanger arrangement, it may be possible to encase the whole within a mud or other mound which will add to the seal efficiency of a stove formed in accordance with aspects of the present invention. It will be understood that provided the fire tubes 29 at the end 34 are open within a mud mound it will be possible to create a stove arrangement in which the heat exchanger is contained within that mud mound for further operational efficiency and thermal insulation.
 FIGS. 5 to 8 provide schematic illustrations of sheets and assembly in order to create stacking for heat exchangers and an acoustic stack in accordance with aspects of the present invention. Thus, an upper sheet 51 includes an aperture 52 below and next to this sheet 51 a further sheet 61 will be located typically with a space in the form of a corrugation 63 between the sheets 51, 61. In order to create the conduit a cut-out or aperture 62 is provided for alignment with but offset from the aperture 52 in order to generate and create a cavity internal to the heat exchanger which comprises a series of overlapping but at least partially aligned apertures 52, 62 to create the conduit path internally within the heat exchanger in accordance with aspects of the present invention. Generally, a large number of sheets will be created in a stack such that the conduit is formed by the apertures within intermediate sheets of the heat exchanger. As indicated above the conduit in accordance with particular aspects of the present invention is utilised as the resonance tube and will be constructed generally with a folded over configuration, that is to say with passages generally side-by-side with a curved ends connecting the passages and an acoustic stack at an intermediate position.
 FIG. 8 illustrates sheets 71, 72, 73 arranged in a stack with spacers in the form of corrugations 74, 75 generally in a perpendicular or normal relationship to each other separating the sheets 71, 72 and 72 and 73 respectively. The corrugations allow definitions of heat exchanger tubes for utilisation in accordance with aspects of the present invention. As can be seen these sheets 71, 72, 73 through respective apertures create a conduit path 76 whilst the sheets 71 to 73 in association with the corrugation spacers 74, 75 create heat exchanger matrices for utilisation.
 By the above approach a heat exchanger arrangement in accordance with aspects of the present invention can be designed for greater suitability for combination with a linear actuator for electrical power generation. The heat exchanger is of a robust construction comprising a number of sheets of material to create the heat exchanger matrices whilst having the capability of creating a conduit for the resonance cavity for a standing wave or a travelling wave (stirling engine type arrangement) for electrical power generation through a linear actuator.
 By the above approach generally thinner sheet materials can be used which are reinforced by the spacer corrugations. Furthermore, as the construction can be achieved through diffusion bonding or similar techniques local fabrication and/or transportation will be more convenient. It will also be understood that material costs will typically be a significant factor with respect to acceptability in developing countries and utilisation of thinner materials with a reduced material content may then be more acceptable.
 As indicated above application of heat exchangers in accordance with aspects of the present invention has particular applicability with regard to provision of stoves for developing world locations. Thus, an open flame such as a wood fire or biomass burner or gas (natural, butane or propane) supply may be used through a hob or stove ring support to cook. The heat from the flames can then be utilised in the heat exchanger in accordance with aspects of the present invention to create the acoustic oscillations through thermal processes which are then utilised to generate electrical power and where appropriate possibly cooling effects as described above with regard to FIG. 2. The flames of the open fire utilised for cooking act as the driver for such oscillations creating electrical power or cooling effects through appropriate heat exchanger assemblies.
 It is known that above 500° C. the principal form of heat transfer is through radiation. In the configuration as depicted in FIG. 3 it will be understood that the curved surface 43 in such circumstances will radiate heat towards the heat exchanger 44. The heat exchanger 44 will have open heat exchanger or acoustic stack tubes at an opposed surface 81. Thus, it is possible for the radiated heat to pass directly through the passages defined extending from the opposed end 81 and therefore reducing heat exchange capabilities with regard to the heat exchanger arrangement. Such a configuration is depicted in FIG. 9 in which the curved surface 43 provides heat radiation in the direction of arrowheads 82 towards open ends of heat exchanger passages or tubes 83 it will be understood that if the radiated heat in the direction of arrowheads 82 is in direct alignment or only slightly misaligned the radiated heat may pass directly through the tubes 83. In such circumstances some heat exchange to the heat exchanger matrix provided by the tubes 83 will be lost.
 In order to improve efficiency ends of heat exchanger tubes 83 incorporate heat radiance interference hoods or cap elements which extend at least partially across the open heat exchanger tubes. FIG. 10 and FIG. 11 illustrate alternative embodiments of interrupter hoods or caps in accordance with aspects of the present invention. In such circumstances heat exchanger tubes 93 as depicted in FIG. 10 has angular flat interference hoods or caps 94 which extend partially across the width 95 of the tubes 93 such that incident heat radiation in the direction of arrowheads 92 cannot pass directly along the tubes 93 without engaging the interrupter hoods or caps 94 for heat exchange.
 An alternative as depicted in FIG. 11 is to have curved interrupter hoods or caps which again extend preferably substantially across the tubes to prevent direct passage of heat radiance along the tubes without heat exchange. In such circumstances again heat exchanger tubes 103 have interrupter hoods or caps 104 to prevent direct radiance heat in the direction of arrowheads 102 passing along the tubes 103 without heat exchange.
 It will be understood that the higher the heat differentials then typically the greater the thermal acoustic oscillations which can be created within the working fluid of the thermo-acoustic engine utilised for electrical power generation through a linear actuator or to provide cooling effects driven by the heat from a wood burning or similar open flamed fire for cooking.
 The features of aspects of the present invention as described above are particularly applicable to providing a compact lightweight stove chassis for developing world situations. As indicated above the heat exchanger part may be constructed with the conduit 30 formed by stacks of sheets or plates of material with appropriate apertures in order to create the heat exchangers whilst having a resonant tube for generating thermo-acoustic oscillations. Normally, a portion of the stack creating the heat exchangers and the conduit for the resonance tube will also accommodate mountings for the linear actuator but as an alternative a further element may be secured to the side of the heat exchanger to allow access to the linear alternator when required but in such circumstances it will be necessary to provide seals which can withstand the working fluid pressure and this may be problematic in use. With regard to the fire end 34 of the stove it will be understood that this may be attached to the stack of plates or sheets through an appropriate mechanism. Clearly, incorporating fire tubes this end 34 (FIG. 3) may be subject to more intense wear and tear in use and therefore there should be the potential for replacement. The end may be formed from cast iron or clay or otherwise provided good heat exchange to the heat exchanger 31 matrix and core created in accordance with aspects of the present invention can be achieved.
 Although described above with regard to open flames it will be appreciated that other sources of heating such as for utilisation in combined heating and power situations where the heat is provided by a gas or electric boiler or through geothermal or other temperature differentials could be utilised with respect to aspects of the present invention in a heat exchanger arrangement where thermal acoustic oscillations are utilised to create electrical power. Essentially what is required to create the thermo-acoustic oscillations is a large temperature differential and in such circumstances within industries such as brewing where there are high temperature differentials between the brewing processes for certain process stages and cooling processes it will be possible to create thermo-acoustic oscillations which once generated can be utilisable within resonance tubes, whether standing wave or travelling wave (stirling engine) configurations in order to recover electrical power through a linear actuator.
 Modifications and alterations to aspects of the present invention will be appreciated by a person skilled in the technology. Thus, for example as described above through appropriate couplings respective further heat exchangers can be added to the structure as illustrated in FIG. 3 to create a cooling effect utilising the thermo-acoustic oscillations as described above in FIG. 2 with a folded conduit to act as the acoustic pathway for a cooling effect. Folding of a conduit creating the acoustic path for resonance reduces the length of the arrangement to allow provision of electrical power generation through a linear alternator.
Patent applications by THE UNIVERSITY OF NOTTINGHAM
Patent applications in class SIDE-BY-SIDE TUBULAR STRUCTURES OR TUBE SECTIONS
Patent applications in all subclasses SIDE-BY-SIDE TUBULAR STRUCTURES OR TUBE SECTIONS