Abstract:
The present invention is concerned with combustion and heat transfer processes and apparatus. The invention has general applicability in the fields of combustion and heat transfer and is applicable to industrial and non-industrial processes as well as residential use. Practical industrial application of the invention may be found in the field of steam generation for heating and for electrical power generation. In addition, non-industrial applications of the invention include cooking appliances, stoves, water heaters, furnaces and the like.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefits of U.S. Provisional Application Serial No. 60/138,009 filed Jun. 8, 1999 and the subsequent PCT filing of PCT/CA00/00658 filed Jun. 1, 2000 under the provisions of 35 U.S.C. §119(e). 
    
    
     FIELD OF THE INVENTION 
     The present invention is concerned with combustion and heat transfer processes and apparatus. The invention has general applicability in the fields of combustion and heat transfer and is applicable to industrial and non-industrial processes as well as residential use. Practical industrial application of the invention may be found in the field of steam generation for heating and for electrical power generation. In addition, non-industrial applications of the invention include cooking appliances, stoves, water heaters, furnaces and the like. 
     BACKGROUND OF THE INVENTION 
     Efficient use of heat generated from a fuel involves two fundamental steps. This first is the combustion of the fuel, and the second is the heat transfer from the products of combustion to the desired heat sink. Combustion processes are carried out so that the ambient temperature in the combustion area is extremely high, i.e., typically greater than 1500° C. It is well known that at high temperatures, nitrogen present in fuel and air reacts with oxygen to forms various oxides, commonly referred to as NO x . The generation of NO x  increases with the temperature, especially when an excess of oxygen is present. It is therefore desirable, when dealing with combustion of fuel, to maintain temperatures as low as possible to inhibit the formation of pollutants like NO x . An alternative is to reduce the concentration of oxygen below the stoichiometric requirement. 
     In many areas of the world, wood is still used as the main fuel for cooking. This is particularly true for so-called lesser-developed countries where access to other fuels may not be readily available, or affordable. 
     To inhibit the formation of pollutants during combustion, and to efficiently utilize available fuels, it is desirable to develop appliances in which there is efficient combustion of the fuel and simultaneous efficient heat transfer of the heat generated during the combustion process to an appropriate heat sink. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a heating apparatus comprising a housing having a general axis. The apparatus further comprises a fuel support surface. The apparatus comprises a plurality of air injectors arranged on the support surface. The air injectors have a plurality of apertures to deliver air in a first direction substantially parallel to the axis of the housing and in a second direction substantially normal to the axis of the housing. Fuel is burned adjacent the fuel support surface. Air is injected with a fan from an air inlet chamber to the air injectors. In addition, the heating apparatus preferably comprises a restrictor ring placed within the housing above the fuel support surface to restrict the cross-sectional area of the housing adjacent the restrictor ring. Further, the apparatus comprises a support means for supporting a heat sink adjacent the upper portion of the combustion chamber. There is a thermal transfer gap between the upper edge of the combustion chamber and the lower edge of the heat sink so that gases passing upwardly through the housing impinge upon the heat sink and pass through the thermal transfer gap after transferring the heat contained therein to the heat sink. 
    
    
     IN THE DRAWINGS 
     FIG. 1 is a vertical cross section of a heating device according to a preferred embodiment of the invention; 
     FIG. 2 is a plan view of the air injection system of the apparatus shown in FIG. 1; 
     FIG. 3 is a vertical, sectional view of an air injector; 
     FIG. 4 is an plan enlarged view similar to FIG. 2 showing the air flow patterns from the injectors of FIG. 2; 
     FIG. 5 is a vertical, sectional view similar to FIG. 1 showing the air flow patterns within the apparatus of FIG. 1; 
     FIG. 6 is a cross-sectional view along the line A—A of FIG. 5; 
     FIG. 7 is a cross-sectional view along the line B—B of FIG. 5; 
     FIG. 8 is a vertical, sectional view of a further preferred embodiment according to the present invention. 
     FIG. 9 is a vertical, section view of another embodiment of the device of the present invention; 
     FIG. 10 is a second vertical sectional view of an air injector; and 
     FIG. 11 is a plan view of an air injection system of the device illustrated in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order that the invention may be more clearly understood, reference is made by way of example to a preferred embodiment of the invention which is illustrated in the accompanying drawings. 
     The apparatus  10  illustrated in FIG. 1 is a heating apparatus transferring heat to a heat sink. The heating apparatus  10  is a wood fired cook stove transferring heat to a heat sink  12 . In this case, the heat sink  12  is in the form of a cooking pan such as fry pan. The particular configuration of heat sink  12  to be heated by the apparatus does not form part of the invention, and can take any shape, whether flat, concave or otherwise. However, the relationship between the heat sink and the relevant portions of the heating apparatus  10  are important in the heat transfer process which will be discussed more fully below. 
     Heating apparatus  10  comprises an air inlet chamber  14 , a housing  16  and a heat sink support  18 . The air inlet chamber  14  comprises a fan  20  driven by a conventional electric motor (not shown) that may be battery driven, powered by an alternate electric source or by a winding mechanism supplying the required electric energy. Fan  20  draws air axially into the lower portion of air inlet chamber  14  and directs the air to flow axially upwardly from air inlet chamber  14  into housing  16 . Air inlet chamber  14  further comprises an adjustable air flow valve (not shown). 
     Housing  16  comprises a fuel support surface  30 . Steps to reduce heat losses through the housing  16  obviously increase the efficiency of the heating unit. Accordingly, it is preferred that the inner wall  36  reflects radiated heat back to the combustion gases. A further tubular member  120  is provided around housing  16 , defining an area  122  therebetween extending throughout the length of housing  16 . Support plate  30  is divided into  2  separate plates  124  and  125  inside housing  16  between the combustion chamber and the air inlet chamber  14 . Plate  124  comprises a central opening  126  and a plurality of openings  128  around its circumference. Plate  125  is also provided with a central opening  130 . Plates  124  and  125  are coupled by a tubular member  129  that forms a narrow gap  127 . Air is forced to pass through aperture  126  and impinges on plate  125 , making an air stream  101  passing through gap  127  and mixes with air coming through openings  128 , thereby forming air stream  132  before ultimately exiting through circular gap  134 . Air also passes through aperture  130  and flows upwardly along with stream  100  (see FIGS. 1 and 5.) Injectors  34  extend throughout both plates  124  and  125 . 
     In a preferred embodiment, the fuel is a piece of wood that can be placed on the support surface  30  which may be a flat generally circular plate (FIG. 2) or a generally flat square or rectangular plate (FIG.  11 ), coupled to or sitting upon the air inlet chamber  14 . Housing  16  rests upon the fuel support surface  30 . To assist in positioning the housing  16  and tubular member  120 , heat sink support  18  and the upper part of tubular member  120  comprise each a slot for engagement therein, and an annular flat ring  37  sitting on structure supporting plate  39  is provided to ensure proper positioning. Alternately, the fuel support surface  30  may include two or more bosses  32 . The housing  16  is positioned over the bosses  32  and lowered onto the fuel support surface  30  where it is then aligned over the air inlet chamber  14 . It should be noted that although wood is used as an example for fuel, other solid fuels like particulate fuels, powder fuels, liquid and gaseous fuels can also be employed. More specific examples include coal, natural gas, gasoline, kerosene etc. 
     Referring to the embodiment illustrated in FIGS. 1,  5  and  9 , the support surface  30  further comprises a plurality of air injectors  34  located in a substantially circular array. The diameter of the array is slightly smaller than the interior diameter of housing  16  so that air injectors  34  are located substantially adjacent to the inner surface  36  of housing  16 . 
     FIG. 3 illustrates a vertical cross-sectional view through an air injector  34 . The air injector comprises a body  40  comprising itself a bore  42  extending longitudinally through the body  40  from an inlet end  44  to a first outlet  46  and a second outlet  48 . First and second outlets  46  and  48  discharge air in directions that are substantially perpendicular to one another. First outlet  46  discharges air in a direction substantially parallel to axis  17  of housing  16 . The second outlet  48  discharges air in a direction substantially perpendicular to axis  17  of housing  16 . Preferably, the diameter of second outlet  48  is larger than that of first outlet  46 . First outlet  46  is considerably smaller in cross-sectional area than bore  42  of injector  34 , thereby ensuring that the air discharged from the first outlet  46  is speeded up to exit at a relatively high velocity compared to the is velocity at the inlet  44 . 
     Second outlet  48  is substantially adjacent to fuel support surface  30  so that air discharged from that outlet travels across the fuel support surface  30  toward the fuel. In the embodiment of FIGS. 1 and 5, it will be seen that injector  34  extends through plates  124  and  125  and that second outlet  48  is substantially adjacent to plate  125  (see also FIG.  10 ). 
     FIG. 4 illustrates the flow pattern from the second outlets  48  of air injectors  34  arranged in a circular array. The air stream exiting the second outlet  48  extends in a plane generally perpendicular to axis  17 . Each second outlet  48  is arranged to direct the exiting air to flow across fuel support surface  30  or plate  125  substantially along dotted line  35  as shown on FIG.  4 . However, the direction of each exiting air stream is slightly shifted so that the stream is not directed to pass over axis  17 . Air injector  34  identified  34 - 1  in FIG. 4 is substantially diametrically opposite to air injector  34 - 6 . The direction of the air exiting second outlet  48  of injector  34 - 1  is directed to impinge on inner wall  36  midway between air injectors  34 - 5  and  34 - 6 . Similarly, the direction of flow from the second outlet  48  of air injector  34 - 2  is across the fuel support surface to a point midway between injectors  34 - 6  and  34 - 7 . Thus, the air flow of each injector is directed to the left of central axis  17 , thereby creating a swirl within the combustion chamber. The flow pattern developed by the plurality of exiting air streams from the second outlets  48  thus develops a high pressure zone indicated generally by the circle  60 . 
     Arrangements of the parts of the combustion chamber may be more clearly understood from reference to FIGS. 1 and 9. Housing  16  is generally cylindrical and has a central axis  17 . The primary combustion zone is located immediately above fuel support surface  30  (FIG. 9) or plate  125  (FIGS.  1  and  5 ). Combustion takes place within the volume  70  defined by the tubular housing  16  between fuel support surface  30  or plate  125  and heat sink  12 . Housing  16  comprises an annular restriction ring  72  coupled to inner surface  36  of housing  16 . Annular ring  72  comprises a central aperture  74  preferably concentric with axis  17 . 
     Housing  16  has an upper edge  80 . Thus, the axial length of the combustion chamber contained within by the housing  16  is the length between fuel support surface  30  (FIG. 9) or plate  125  (FIGS. 1 and 5) and heat sink  12 . The location of restriction ring  72  within housing  16  is such that optimum flame height and heat transfer to the heat sink are achieved. 
     Above ring  72 , housing  16  comprises a thermal break at  82  that may be in the form of an air gap with the portion of the housing above the air gap being separated from the portion below the air gap by relatively narrow metallic components. Alternatively, the thermal break may be in the form of a ceramic or other material that would inhibit the flow of heat from the upper portion of the combustion chamber to the lower portion thereof below the thermal break. 
     Housing  16  may further comprise a plurality of pressure release apertures  84 , preferably holes, provided circumferentially through the wall of housing  16 . These are located above thermal break  82  but below upper edge  80  of housing  16 . 
     As shown in the drawings, apparatus  10  comprises a heat sink support  18  located on the housing  16  adjacent the upper edge  80 . In a preferred embodiment, heat sink support  18  comprises a plurality of metallic rails projecting slightly above upper edge  80  and coupled to the outer surface  45  of housing  16 . Furthermore, a flat annular ring  86  is coupled to upper part  80  of housing  16 . The beforementioned rails are arranged circumferentially around housing  16  and serve to support heat sink  12 . When heat sink  12 , in this case a cooking utensil, is placed over support  18 , a heat transfer gap  90  is defined. Combustion products which travel upwardly within housing  16  impinge directly upon heat sink  12  and then pass through heat transfer gap  90  to exit from heating apparatus  10 . As the combustion gases pass through heat transfer gap  90 , they are forced to travel along a portion of the periphery of heat sink  12  moving radially outwardly along the bottom surface of heat sink  12 . To facilitate this heat transfer process, heat sink  12  is preferably larger than the diameter of housing  16 . Thus, heat transfer gap  90  is effectively toroidal in shape. 
     As seen in FIG. 5, the dotted lines indicated at  100  represent the flow of air passing out through second outlets  48  of air injectors  34 . As the air exits second outlets  48 , it travels substantially parallel to the plane of plate  125 . As air impacts on the fuel or other air streams from an opposing injector  34 , it swirls and passes upwardly in the combustion chamber. This swirling or turbulent air will be mixed with the gases released by the burning fuel and will form the combustion products. 
     Dotted lines  102  illustrate the air flow pattern for the air exiting first outlet  46  of air injectors  34 . The air flow from aperture  130  provides additional air needed for a better combustion at the central zone of the combustion chamber. The air flowing in the pattern  102  comprises the air exiting first outlets  46  of air injectors  34 . The air flowing out of first outlets  46  thus forms a substantially cylindrical air envelope. That air envelope exits first outlets  46  travelling substantially parallel to axis  17  of housing  16 . The air stream  102  then impinges upon restriction ring  72 , which causes the air flow  102  to divert slightly radially inwardly to pass through a circular aperture  74  defined by the restriction ring  72 . Thereafter, the air flow bends radially outwardly and passes axially upwardly along housing  16 . 
     Air flow pattern  102  thus forms an envelope confining the combustion gases generated by air flow  100  and combustion products released from the fuel on fuel support  30 . Air flow  102  is thought to serve three purposes. Firstly, the air envelope provides an envelope for the swirling combustion gases above the fuel. Secondly, it provides a cooling effect limiting heat transfer to housing  16 . And thirdly, it assists in transferring heat to the heat sink. The air stream coming out of aperture  126  cools plate  125  and mixes with the air flow incoming from apertures  128  to produce air stream  132 . The presence of air stream  132  between housing  16  and external tubular member  120  further reduces heat transfer to the external surface thereof. 
     The air stream coming out of aperture  130 , and air streams  100  and  102 , together with the gases released during the combustion process of the fuel, travel upwardly and impinge upon heat sink  12 . Thereafter, the gases exit housing  16  by passing through heat transfer gap  90  and in addition, to a minimal extent, through the pressure release apertures  84 , if any are present. In order to favor the heat transfer to the sink, the total area available for flow through heat transfer gap  90  is preferably larger than the area of aperture  74  defined by restriction ring  72 . In addition, little gas passes through apertures  84 , if any, to relieve the pressure at the upper portion of the combustion chamber, which may thus increase the temperature in the area just below the heat sink. 
     The present invention enables extremely efficient heating of the heat sink for a number of different reasons. Fan  20  forces air into the combustion zone through air injectors  34 . This means that the fuel is burnt in an area being at a pressure higher than ambient pressure. Thus, burning of the fuel occurs under pressures slightly higher than ambient. The higher pressure in the area of the fuel appears to provide a lowered ignition and combustion temperature, which in turn means that the rate of gasification from the fuels is slowed. In addition, the cylindrical air curtain formed by flow pattern  102  cools the combustion gases as combustion continues, keeping the combustion gases below the temperature at which NO x  and other pollutants are generated. This also helps in providing a more complete combustion of the fuel. 
     Restriction ring  72  within housing  16  assists in the transfer of the combustion gases released in what may be considered to be a primary combustion zone, and in moving the heat released directly towards the heat sink. Because of the location of the restriction ring  72 , there is a tuning effect within the length of the combustion chamber that also appears to favor movement of the heat generated by combustion in the general direction of the heat sink. The uppermost portion of housing  16  will be at the lowest temperature. Any heat lost through the wall of housing  16  represents loss of heat that otherwise should be directed toward heat sink  12 . Thermal break  82  tends to minimize the transfer of heat from the upper portion of housing  16  to the lower portion thereof and air flow pattern  102  tends to move heat from the combustion gases quickly and efficiently upwardly toward the heat sink so that the amount of heat lost through the wall of housing  16  is reduced. 
     The present invention provides particularly efficient cooking using small blocks of wood. To start the use of the apparatus, a small piece of wood or kindling is placed on fuel support surface  30  or plate  125  inside the circular array formed by injectors  34 . After initiation of fire, fan  20  is turned on, and housing  16  is placed over the support surface. If a tubular member  120  is present around housing  16 , both of these are placed over the support surface jointly. As the wood piece is consumed, more wood can be added, for example through an aperture  110  located above restriction ring  72 . The amount of air to be delivered in the chamber is adjusted with an air flow valve (not shown). 
     As shown in FIG. 3, the area of first outlet  46  of each injector  34  is preferably considerably smaller than the area of second outlet  48 . However, outlet  48  is positioned substantially perpendicular to bore  42  passing centrally through the body  40  of the injector. It is desired that the air passing through the first outlet  46  be moving relatively quickly, and that sufficient air passes through the plurality of first outlets  46  to form the air flow pattern  102 . 
     Housing  16  and air chamber  14  is preferably made of a highly thermal conductive material such as stainless steel. Fuel support surface  30  and plates  124  and  125  may also be made of stainless steel. Further, air injectors  34  should be evenly distributed about a circular array and the distance from the array to the interior surface of housing  16  is approximately 1 mm to ensure an efficient swirl within the combustion chamber. The air injectors themselves may also be made from stainless steel. Experimental evidence shows that preferred air flow and air flow patterns are achieved when the area of first outlets  46  compared to the area of second outlets  48  is between 12 and 18%. It is considered that the area of second outlets  48  compared to first outlets  46  may be as high as 20 to 1. Of course the ratio of areas could be considerably less. 
     An example of a fan  20  suitable for the purposes of the present invention is a 4715FSB30™ manufactured and sold by NMB Technologies. Preferably, fan  20  is isolated from the heat generated by the fuel burning on the support surface. In addition, to minimize heat flow conducted along the wall of air inlet chamber  14 , the housing for fan  20  may be spaced from the air inlet chamber  14  by an air gap, thereby adding to the thermal isolation of the fan. 
     One of the more interesting observations is that there does not appear to be any substantial flow through aperture  110  provided in housing  16  for addition of fuel. As shown in FIG. 5, the flow pattern  102  bends inwardly, upwardly of the restriction ring  72 . It has been observed that essentially no flame passes outwardly through the open aperture  110 . Similarly, there is been no substantial flow through pressure relief apertures  82 . The flow of gases travelling through the restriction ring, Q ring , is thus equal to the area of the ring, A ring , multiplied by the velocity at which the gases are travelling through the ring. 
     Heat sink support elements  18  support heat sink  12  so as to define a gap between the heat sink and the housing. Because of the configuration of the present device, the gases at the exit are travelling slightly slower than at the ring as they pass through aperture  74 . The exit velocity through the thermal transfer gap  90  can be reduced is further by increasing the area of the thermal transfer gap  90  while keeping the distance between the support surface  30  and the lower surface of the heat sink  12  constant. 
     In understanding the processes occurring within the combustion chamber, this might more easily be explained and understood as a fluid dynamics process. 
     The cooling air element illustrated by the flow pattern  102  desirably travels upwardly at approximately the same speed as the combination of flame and combustion gases. The exit speed of the combined gases through the thermal transfer gap  90  is reduced slightly to allow the heat to remain as long as possible adjacent the base of the heat sink. The swirl generated inside the combustion chamber causes the flame and combustion gases to remain substantially within the central portion of housing  16 , thereby concentrating the greatest portion of the heat centrally of the under side surface of the heat sink. Upon impinging the heat sink, the heat is therefore substantially uniformly diffused on the surface thereof. 
     In the device of the present invention, the maximum temperature measured inside the combustion chamber when operating with wood as the fuel, was 950° C. at the centre of support surface  30 . 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications, and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present description as come within known or customary practice within the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.