GB2253576A

GB2253576A - Catalytic Coanda combustion

Info

Publication number: GB2253576A
Application number: GB9101523A
Authority: GB
Inventors: James David Coleman; Anthony Gregory Smith; Michiel Kopmels
Original assignee: S & C Thermofluids Ltd
Current assignee: S & C Thermofluids Ltd
Priority date: 1991-01-24
Filing date: 1991-01-24
Publication date: 1992-09-16
Also published as: GB9101523D0

Abstract

A catalytic reaction is enhanced by the use of the Coanda effect to maximise contact between reactant and catalyst. A device utilising this principle comprises a Coanda surface (5) which directs the flow of fuel from slot (3) to form a primary jet (4) which entrains the surrounding ambient air (6) and forms a combustible mixture for reaction on a catalytic surface 7. The Coanda surface may have an internal or external nozzle which may be axi-symmetric or two-dimensional. <IMAGE>

Description

CATALYTIC COANDA COMBUSTION This invention relates to improvements in the field of combustion technology.

The Coanda effect is a well known phenomenon whereby a jet flow remains attached to an adjacent surface over which it is made to flow. The jet flow will remain attached to the adjacent surface even if the surface is turned away from the original direction of the jet, through an angle. A jet flow which is turned through an angle over a Coanda surface will mix more rapidly, and thus entrain more rapidly, than a jet flow which remains unturned. Jet flows which exhibit the Coanda effect are able to entrain secondary flows efficiently and can induce many times the primary flow rate. For a particular size entrainment device a Coanda effect nozzle, by virtue of its higher mixing rate, will entrain more secondary fluid than an entrainment device based on an alternative entrainment principle, such as the jet-in-tube.A Coanda surface can be embodied as an internal or external Coanda nozzle, which can be axi-symmetric or two-dimensional.

Jet flows which exit over an adjacent mechanical surface are usually quieter than an equivalent free jet. This gives a Coanda flow its noise attenuation characteristic.

The primary flow, which may be a fuel, will mix rapidly with the secondary flow, which may be air, to form a combustible mixture. This combustible mixture is formed in a short distance without the noise normally associated with free jet or jet-in-tube mixing.

Catalysis is the science of altering chemical reaction rates by the presence of a catalyst. Catalysts are an essential part of the chemical industry. If catalysts did not exist many chemical processes would go so slowly as to be uneconomic. Catalysts are divided into two main groups. The first group is known as homogeneous catalysts where the catalyst is in the same state of matter as the reactant. In the second group, known as heterogeneous catalysts, the catalyst and the reactants are in different states. For example, the catalyst may be solid, and the reactants may be gaseous or liquid. Catalysis, which takes place on the catalyst's surface, will be maximized where the contact is greatest between the reactants and catalyst. In a pollutant reducing catalytic convertor this is achieved by maximising the surface area of the catalyst.The active surface area in an automotive catalytic convertor is equivalent to about three football pitches. The convertors are designed such that the flow through them is essentially laminar.

The purpose of the present invention is to satisfy the requirement of maximising the contact between reactants and catalyst by taking advantage of the high turbulence level of a Coanda flow. On a molecular scale "fresh" reactants would be brought into contact with the catalyst more frequently than in a laminar flow. In a presently preferred construction the Coanda surface itself will be covered, either completely or in part, with a suitable catalytic material to enable combustion to take place. It may, however, be desirable to position the catalyst, or any other suitable ignition means, away from the Coanda surface. As an example, a fine wire mesh coated in suitable materials may be positioned in the combustible mixture, so as to act as a catalytic igniter.

A catalyst requires to be at a certain minimum temperature in order to function. An automotive catalytic convertor will only begin to function once the exhaust gases have heated the convertor to its light off temperature. A Catalytic Coanda Combustion surface thus requires a suitable means to effect preheating of the catalyst(s). The preheating could be achieved by a fuel powered pilot light or it may be desirable to effect the preheating by electrical means. Other methods which may be employed could include lasers or other optical means. Alternatively, initial ignition may be achieved without a catalyst, with subsequent ignition or combustion being achieved catalytically.

The Catalytic Coanda unit could be used as a burner, where the primary flow is a fuel and the Coanda flow would form a combustible mixture with an entrained oxidant.

A prototype construction according to the invention is more particularly described with reference to the accompanying drawings wherein: Figure 1 shows a device where fuel, which may be preheated, flows through a supply pipe 1, by means of its own vapour pressure or other suitable pressurization means, into a settling chamber 2. The fuel exits through a narrow slot 3 to form a primary jet 4. By the Coanda effect the primary fuel jet 4 becomes attached to the adjacent Coanda surface 5, and entrains the surrounding ambient air 6, such that it forms a combustible mixture by the time it reaches the catalytic material 7, which is suitably distributed on the Coanda and or diffuser surface 8. The diffuser shape may be designed to allow for expansion of flow where it combusts. A suitable preheating means 9, ensures that the catalyst 7 is above its light-off temperature. An alternative ignition means 10 may be positioned in the combustible mixture above the Coanda surface, and a further ignition means 11 may be positioned downstream.

Figure 2 shows two embodiments of a catalytic Coanda combustion unit. Figure 2A shows a possible embodiment of an internal catalytic Coanda unit and Figure 2B shows a possible embodiment of an external catalytic Coanda unit.

Various modifications may be made within the scope of this invention. A catalytic Coanda device may comprise more than one of the units described in this patent, or it may be made up from a number of different catalytic Coanda units. The diffuser section of the unit may be straight, convex, concave or it may comprise of two or more sections, which may be of a different shape.

Claims

CATALYTIC COANDA COMBUSTION:

APPLICATION NO 9101523.0 CLAIMS: 1) A device which has a primary flow arranged to pass over one or more Coanda surface(s) and a suitable catalytic material, such that in use there is contact between the primary flow and the catalytic material, which may result in the primary flow undergoing a reaction.
2) A device according to claim 1 in which the, or each, Coanda surface comprises an internal profile.
3) A device according to claim 1 in which the, or each, Coanda surface comprises an external profile.
4) A device according to claim 1 in which the, or each, Coanda surface comprises a combination of internal and external profiles.
5) A device according to claims 1 to 4 in which the primary flow, which may be a pressurised gaseous fuel or mixture of air and fuel, is caused to flow over the Coanda surface(s) thereby entraining a secondary flow, which may be air, for the purpose of creating a combustible mixture.
6) A device according to claims 1 to 5 in which a catalytic material is deposited on the Coanda surface.
7) A device according to claims 1 to 5 in which a catalytic material is deposited in the combustible mixture flow field away from the Coanda surface(s).
8) A device according to any preceding claim in which the catalytic material is a precious metal.
9) A device according to claim 8 in which the catalytic material is an oxidising catalyst.
10) A device according to any preceding claim in which the combustible mixture is formed by a primary flow of exhaust containing combustion pollutants such as carbon monoxide and or unburned hydrocarbons and a secondary flow of oxygen, which may be in the form of air.
11) A device according to claims 1-9 in which the primary flow is a fuel which may be passed through a heat exchanger.
12) A device according to claim 5 in which the secondary flow may be passed through a heat exchanger.
13) A device according to claim 11 in which the fuel is heated by the combusting flow, using vaporising coils or any other suitable means.
14) A device according to claim 13 in which the vaporising coils form its body.
15) A device according to claim 13 in which the vaporising coils may be positioned within its body.
16) A device according to claim 13 in which the combusting flow can pass through the vaporising coils.
17) A device according to claim 13 in which the combusting flow can pass outside of the vaporising coils.
18) A device according to any of the preceding claims in which one or more mechanical surface(s) are positioned so as to provide shielding of the combusting flow.
19) A device according to claim 18 in which the mechanical surface(s) are a continuation of the Coanda surface(s).
20) A device according to any of the preceding claims in which one or more mechanical surface(s) are positioned so as to provide shielding of the primary exhaust flow.
21) A device according to any of the preceding claims in which one or more mechanical surface(s) are positioned so as to provide shielding of the entrained flow(s).
22) A device according to any of the preceding claims in which a non-catalytic ignition source initiates combustion of the combustible flow.
23) A device according to any preceding claim in which the catalytic material is preheated, by suitable electrical or other means.
24) A device according to any of the preceding claims in which a pilot light is contoured to suit the Coanda surface(s) in order that the mixed flows can be ignited.
25) A device according to claims 1 to 4 where the primary flow is an exhaust flow containing combustion pollutants such as nitrous oxides, which has a reducing catalyst according to claim 8.
26) A device substantially as hereinbefore described with reference to and as shown by Figure 1.
27) A device substantially as hereinbefore described with reference to and as shown by Figure 2.