Shock-stabilized supersonic flame-jet method and apparatus

Abstract: A supersonic flame jet device includes a body having an entry portion (22) of relatively small cross-sectional area, an expanding supersonic nozzle section (23) and a cylindrical duct (21) of extended length connected in series with each other. In using the device, an oxidant (24) at high pressure is introduced into the entry passage (22) wherein the flow is increased to sonic velocity. The sonic velocity flow of the oxidant (24) is then introduced into the passage (23) of expanding cross-section in the direction of the gas flow while introducing a fuel (25) to be burned into the flow of the oxidant (24). The velocity of flow of the oxidant, or the oxidant and the fuel, is then increased to supersonic velocity prior to entry into the extended duct (23) of constant cross-sectional area where a shock (40) is produced to stabilize flame reactions along the extended duct length (21) whereby a supersonic flame jet will exit the extended duct.

Description: The invention relates to a method for producing a supersonic jet stream of high temperature extending beyond the exit of an internal burner continuously initiating combustion. The invention also relates to a flame jet apparatus for carrying out such a method. Such flame jet apparatus is used in practice for creating a high temperature and high velocity flame jet suitable for spraying high melting point materials. Flame jets are utilized for general heating purposes as well as specific uses including cutting and drilling of granite and the thermal spraying of metallic or other materials to form coatings on a base material. Where high heat transfer rates and/or supersonic velocity flame jets are required, certain types of flame-producing device have been available. These devices reduce to two basic modes of operation -- the chamber-stabilized mode and the duct-stabilized mode. The earliest description of both the duct and chamber modes is given in the G.H. Smith et al. patent (US-A-2,861,900). Figure 1a of the present application is a simplified sketch of a "duct stabilized" device of the type described by Smith et al. The burner 10 consists of two bores of different diameter. Oxygen enters the burner 10 through a relatively small diameter bore 12. Fuel, entering bore 12 through passage 13, mixes with the oxygen flow and the combined flow is discharged from bore 12 into the larger duct 11. The oxy-fuel mixture is ignited upon its entry to duct 11 with nearly complete combustion occurring prior to exit of the flame products from duct 11. Supersonic flame 14 extends as a flame-jet beyond duct 11 and is characterized by shock diamonds 16. Metallic powder is injected through duct 16. In this conventional "duct mode" geometry (Fig. 1a) the gas flow is "choked". That is, the walls of duct 11 prevent the rapid expansion of the gas required to reach supersonic velocity. Supersonic velocity only occurs beyond the exit of duct 11 in the open atmosphere. In "choked flow" the gas pressure over the entire duct length remains above atmosphere (see Fig. 1b). In "choked flow" the exit gas velocity has reached sonic velocity (see Fig. 1c) which for the hot products of combustion is about 3,000 feet per second. Figure 3a of the present application is a simplified sketch of a "chamber-stabilized mode" of the type described by Smith et al. The "chamber stebilized mode" of Figure 3a utilizes a relatively large volume chamber 31 to stabilize and contain the combustion reactions. Oxygen and fuel are fed under pressure into chamber 31 in burner 30 through ports 32 and 33. A very small nozzle throat 34 with an expanding conical bore 35 expands the hot gas exiting from chamber 31 to extremely high velocity. For an inlet oxygen pressure of 3.45 x 10 6 Pa (500 psig) (Figs. 1b and 1c) the exit gas velocity is over 2.44 x 10 3 m/s (8,000 ft/sec). Where high particle impact velocities are required for thermal spray process optimization, the "chamber mode" is superior to the "duct mode". However, as the oxygen pressure is raised to produce favorable particle velocities, unacceptable heat losses to the cooling water (not shown) occur. Higher melting point materials such as aluminum oxide remain solid and will not form a coating. The "duct mode", with a much smaller "wetted surface" available for heat transfer from the flame to the cooling water (not shown) has much higher flame-jet temperatures than for the "chamber mode". Thus, even though particle velocities are much lower, it may have to be selected for certain types of thermal spraying. Another form of duct-stabilized device for limiting particle build-up on the inner duct walls is disclosed in the Browning patent (US-A-4,836,447). In this patent, the expanding section acts as a diffuser and at no point along the path of the gas stream is the flow supersonic. The document EP-A-0 163 776 discloses a supersonic flame spray method and a corresponding apparatus with improved material feed. The method corresponds to the "chamber-stabilized mode" as explained above. In this conventional method and apparatus, combustion of a continuous flowing fuel/air mixture is confined within an essentially closed internal burner combustion chamber, and no shock waves are used to initiate the flame. Hence, when entering an extended duct, the fuel is already burnt to form products of combustion. The document US-A-4 172 558 discloses an apparatus for explosive application of coatings. The conventional apparatus provides for a cyclic batching of pulverulent coating material fed to an explosion chamber and is protected against breakthrough of gas in case of back impacts from the explosion chamber as compared with conventional apparatus of the same type. According to the explanations on column 7, lines 42 to 55, and column 9, lines 30 to 49 of US-A-4 172 558, the apparatus disclosed therein is of the detonation type, and no stationary shock waves are formed in the conventional apparatus in order to heat the reactants for combustion purposes. The present invention is an improvement in the duct-stabilized mode by providing a change in the means for continuously initiating combustion in an oxygen-fuel mixture and keeping stable flame reactions within a high-velocity flow stream of these reactants. According to the invention, a method for producing a supersonic jet stream of high temperature extending beyond the exit of an internal burner continuously initiating combustion is disclosed as specified in claim 1. Further developments of the method according to the invention are specified in the subclaims. The invention also discloses a flame jet apparatus for carrying out the method according to the invention as specified in claim 6. The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1a is a schematic cross-sectional view of a conventional device for operating in the "duct mode". Figure 1b is a plot of the pressure drop of the gas in its passage through the device of Figure 1a. Figure 1c is a plot of the gas velocity in the flow passing through the device of Figure 1a. Figure 2a is a schematic cross-sectional view of the device of this invention for operating in the shock-stabilized duct mode. Figure 2b is a plot of the pressure drop of the gas in its passage through the device of Figure 2a. Figure 2c is a plot of the gas velocity in the flow passing through the device of Figure 2a. Figure 3 is a schematic cross-sectional view of a conventional device for operating in the chamber stabilized mode. DETAILED DESCRIPTION OF THE INVENTION In an effort to keep particle velocities at relatively high values, yet reduce thermal heat losses to the coolant, the "shock-stabilized duct mode" of Figure 2a gives excellent results. The conventional "duct mode" of Figure 1a cannot operate above an inlet oxygen pressure of about 1 x 10 6 Pa (150 psig). Flame reactions are not stabilized satisfactorily and the flame is, simply, "blown-out". Although gas temperatures are satisfactorily high, flame-jet velocities are much too low. In Figure 2a, burner 20 consists of a body piece containing an entry passage 22 of relatively small cross-sectional area and an expanding supersonic nozzle section 23 connected to a cylindrical duct 21 of extended length which has larger cross-sectional area than the passage 22. Oxygen and fuel introduced to passage 22 through ports 24 and 25 mix together and reach sonic velocity prior to entering nozzle expansion 23. The powder to be coated on a substrate is injected through port 29. At an oxygen inlet pressure of 500 psig (Figures 2b and 2c) the gas pressure may become sub-atmospheric by the end of supersonic expansion with a cold gas velocity of over 609.6 m/s (2,000 ft/sec). The discontinuity formed at the wall where the expanding section 23 meets the cylindrical duct 21 forms a weak shock 40. Small pressure increases occur almost instantly across the shock front and the gas velocity is somewhat reduced. Beyond the shock 40 the reactive gases (oxygen and fuel) are nearly fully burned in duct 21. Although it is possible that a small amount of combustion may have occurred upstream of shock 40, fully stable combustion with its efficient heat release could not have occurred in the absence of the shock 40. Additional shocks 41 occur in duct 21 and shock diamonds 27 occur in the flame jet 28. The shock-stabilized duct mode can create jet velocities about double conventional duct mode devices. Jet temperatures remain high allowing ceramic spraying. This device compliments a chamber mode device where high melting point materials must be sprayed. The geometry is much simpler and length of operation is greatly extended as the small nozzle throat 34 of the chamber mode (Figure 3a) is eliminated. At high pressure, using pure oxygen as the oxidizer, throat life is limited by intense heat transfer requirements at the throat. With respect to the conventional duct mode device, both the pressure and velocity plots (Figures 1b and 1c) of the duct mode device are distinctly different from those of the shock-stabilized duct mode of the present invention. Smooth transitions exist for the duct mode. The shock in the device according to the present invention causes nearly instantaneous changes in both pressure and velocity. In an earlier program to develop a duct-stabilized device to limit particle build-up on the inner duct walls, a geometry was developed which has proven quite successful (see US-A-4,836,447). This geometry is very much like that used for shock-stabilization of the present invention. However, the expanding section 12 (Figure 1 of US-A-4 836 447) acts as a diffuser. At no point along the path of the base stream is the flow supersonic. In the design of the shock stabilized duct mode unit of the present invention, the area ratio of hole 21 to hole 22 should be the correct ratio for the inlet oxygen pressure, the oxygen pressure should be above about 1.38 x 10 6 Pa (200 psig), and provision for shock attachment to the duct wall should be provided. The ratio of the cross-sectional areas of the duct-to-small passage is greater than 4 to 1. While an oxy-fuel unit has been disclosed in connection with the shock phenomenon of the invention, other oxidizers can be used. Where an expanding supersonic nozzle section is used (23 of Figure 2a) other geometries may exist. The problem of supersonic flow within a combusting system is extremely complex. The values of pressure and velocity particularly their change along the flow path, are only best estimates. In Figure 1a the exiting jet 15 expands immediately beyond the exit showing that the flow pressure just before release from the duct is above atmospheric. This is under-expanded flow. In Figure 2a, the jet 28 contracts showing an over-expansion of the gas within the duct. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those in the art that the foregoing and other changes in form and details may be made therein without departing from the scope of the invention.

Claims: A method for producing a supersonic jet stream of high temperature extending beyond the exit of an internal burner (20) continuously initiating combustion, the method comprising the following steps: introducing a mixed flow of oxidant (24) and fuel (25) directly into a first cylindrical passage (22) to flow through a nozzle passage (23) of expanding cross-section at supersonic speed in the direction of gas flow to an extended cylindrical duct (21), the first passage (22) having a small cross-sectional area relative to the cross-sectional area of the duct (21), and causing a shock (40) to form within the duct (21) by forcing a sufficient change in pressure, temperature, velocity, and turbulence of the gas flow to initiate and/or maintain combustion reactions downstream of the shock (40), wherein the combustion extends through the remaining duct (21) length. The method according to claim 1, comprising the following steps: introducing the oxidant (24) at high pressure to the first passage (22) of relatively small cross-sectional area, increasing the velocity of the oxidant (24) to sonic velocity flow within the length of the first passage (22), introducing the sonic velocity flow of the oxidant (24) to the nozzle passage (23), introducing a fuel (25) to be burnt into the flow of the oxidant (24), further increasing the velocity of flow of the oxidant (24) and fuel, (25) to supersonic velocity, introducing the flow of the oxidant (24) and fuel (25) to the extended duct (21) of essentially constant cross-section area, the diameter of which is selected, in combination with the geometry of the nozzle passage (23) and the gas flow properties, such that the shock (40) is produced in the vicinity of the entrance to the duct (21), the shock (40) acting to initiate or stabilize flame reactions along the extended duct (21) length. The method as set forth in claim 1 or 2, wherein a ratio of the cross-sectional areas of the extended duct (21) to the first passage (22) is used which is greater than 4 to 1. The method as set forth in any of claims 1 to 3, wherein the flow of the gas downstream of the shock (40) is adjusted to remain supersonic during flow through the extended duct (21). A method as set forth in any of claims 1 to 3, wherein the pressure of the gas downstream of the shock (40) is adjusted to remain sub-atmospheric during flow through the extended duct (21). A flame jet apparatus for carrying out the method of any of the preceding claims, comprising a body (20) having a first passage (22) of a relatively small cross-sectional area, an expanding nozzle passage (23) in communication with the first passage (22) and an extended cylindrical duct (21) in communication with the nozzle passage (23), wherein the ratio of cross-sectional areas of the extended duct (21) to the first passage (22) is greater than 4 to 1, and wherein means are provided for directly supplying a mixture of fuel (25) and oxidant (24) through the first passage (22) at sonic velocities into the nozzle passage (23) for acceleration to supersonic velocities.

Inventors: BROWNING JAMES A, BROWNING, JAMES A.
Applicants: DRACO AB, DRACO
Application year: 1996
Main classification: B05B7
IPC codes: B23K5, C23C4, B05B7, F23C3, F23D99