EP0575499B1 - Mixing device and method for gaseous, liquid or pulverised solid substances - Google Patents

Abstract

Device for mixing gas, liquids or pulverised solid substances with a gas flow (1), whirling (vortex) around an axis in the flow direction. The whirling gas flow is guided through a converging passage (4) and, during or after being charged with the substance, abruptly widens in cylindrical space (11), as a result of which vortex break down occurs and an exceptionally thorough mixing and/or atomizing of the substance is obtained. Application in a burner improves the combustion result, keeps the NOX-values low and prevents the flame from being blown off.

Description

• Burner provided with means for mixing a combustion air flow with a fuel and method for realizing said mixing.
• The invention relates to a burner provided with means for mixing a combustion air flow with a fuel, comprising
• (i) an axially symmetrical vortex chamber having in the axial flow direction at the exit side a tapering down portion with a central opening followed by a flame room
• (ii) a body enforcing rotation of the combustion air arranged at the periphery of said vortex chamber and provided with exit openings for the combustion air adjacent to the beginning of said tapering down portion,
• (iii) said vortex chamber having at its beginning a greater cross-sectional area in the axial flow direction than that exit openings, and
• (iv) the flow section past the central opening increasing abruptly, the diameter of said flame room being at least 2,5 times that of the said central opening and to a method for mixing a combustion air flow with a fuel.
• A burner as described in the above is known from the EP-A-0028025, fig. 7. In this known device the tapering down reduces the diameter of the air flow to less than 30%, which means a reduction of cross-section to less than 9%. This means a high velocity increase and a very substantial friction, so that a relative high pressure drop for the combustion air is necessary. Part of the combustion air is fed immediately into the flame room and the flow pattern obtained in said room is helicoidal.
• The invention aims to obtain a flow pattern giving a very intensive mixing of air and fuel, so that NOx values are very low.
• Accordingly the invention provides that the tapering down reduces the diameter of the vortex chamber to 0,9 - 0,7 of its largest diameter.
• The invention is based on the understanding that a gas flow with a strong rotation around the flow axis compared to the axial component, can, with a sudden widening of the flow diameter, create a flow pattern giving a sudden and very strong turbulence in the flow, in the following referred to as 'vortex break down'. This vortex break down manifests itself in a shattering or explosion of the jet while forming very strong local turbulence which leads to an extremely thorough mixing of the substances in the flow.
• In addition a very stable vortex is created in which a very thoroughly mixed fuel-air mixture can be burnt in a short period of time so that exceptionally low NOx values occur. The same thorough mixing allows complete combustion with virtually no excess of air. As the tangential component increases to the same extent as the axial component with the increase of the gas velocity, blowing off the flame is virtually impossible. Another property of vortex break down is that an axial counter flow is induced, which in the invention flows back through the tapering down portion and, by doing so, forces the substance that is to be mixed towards the outer side and preferably even against the wall of the tapering down.
• It is pointed out that the tapering down portion can consist of a material tapering down of the chamber as defined in the above as well as of air injecting or directing means giving the combustion air flow an inwardly directed radial component.
• Accordingly the invention provides a method for mixing a combustion air flow with a gaseous, liquid or pulverized fuel in order to burn it, in which an air flow is fed to an axially symmetrical vortex chamber at the periphery of said chamber with a rotational component, said vortex chamber having an exit debouching in a flame room having a diameter of at least 2,5 times that of the contracted air flow, which is characterized in that a gaseous flow containing air with in inwardly directed radial component causes the combustion air flow to contract its cross-section in the axial direction to less than the axial cross-sectional area of the vortex room at its beginning, where it receives the combustion air flow and to 0.9 - 0.7 of its largest diameter before said contraction.
• In order to avoid big differences in flow velocity at the confrontation of this gas flow and the rotating combustion air flow, with this embodiment it will, of course, be ensured that the gas flow which causes the narrowing has a rotation motion and possibly an axial movement as well.
• When tapering down a jet rotating around its axis, the energy fed to the jet is being converted into rotation energy by means of the Coriolis forces. As a result the ratio between the rotation component, in particular on the outside of the jet, and the translation component increases. This results in a fall of pressure in the centre of the jet, which can lead to underpressure, as a result of which in principle a flow can be created that is directed against the axial direction of the flow.
• It is preferably provided that the tapering down portion includes at its end an angle with the axis of over 50 degrees. Herewith it is pointed out that also in case of acute angles with the axis at the end of the narrowing good results are to be obtained, but that with angles of 50 degrees to about 60 degrees an adequate rapid compression of the jet can be combined with a short transit time and, therefore, little thrust losses and formation of micro-turbulence in the rotating flow itself.
• With the invention the rotation enforcing body is connected only to the outside shell of the vortex chamber. This means that the inside area of the jet, which is the area within the outside shell, is also made available to the axial flow of the jet. As a result of which the total section becomes larger because the cross-sectional area of the outside shell is smaller than the total sectional area in the tapering down. This means a decrease of the axial velocity of the flow and, therefore, an increase of the ratio between the rotational component and the axial component of the flow.
• In accordance with a further elaboration of the invention, it is provided that in the centre of a surface closing the central region of the rotation enforcing body an inlet is present for a liquid or pulverized fuel, which can move along said surface unto the exit openings of the rotation enforcing body. Important is that in the centre of the jet an underpressure is created with a counterflow near the axis. This throws the mixing substance towards the back surface. In the process the strong rotation will contribute to the fact that, when the mixing substance is a fluid such as oil, it will move along said surface. By this it is achieved that the fluid can be carried along to the areas of the vortex which are located more to the outside, where the velocity of the vortex flow is high so that the liquid can be atomized.
• The liquid or pulverized fuel, however, is subject to the strongest atomization and mixing when it is thrown off the edge of the tapering down portion and enters the vortex break down area. As a result it is possible to obtain a very good atomization of oil, which is introduced under a very low pressure, for instance 5 cm of water.
• A further refinement of this is that said surface is conically widened in the direction of the flow. As a result the gravity component, which has its effect on the mixing substance, is partially compensated by the inclination of the back surface, which ensures a better symmetrical discharge of the fuel.
• As has been mentioned above, the vortex break down will occur when the flow section is widened. It is to be recommended that the flow section for small burners (up to circa 50 kW) will preferably be enlarged at least five times in relation to that of the tapering down, and for large ones circa 2.5 to 3.5 times.
• The above has been found favourable for the operation of an inventive burner. Due to the vortex break down, such a burner has an exceptionally thorough mixing of fuel with combustion air in a very short range. In addition, it has been found that in the area direct behind the widening a vortex occurs, which has not only a rotation component around the axis of the flow but also a rotation perpendicular to it, which means that gas is fed back to the back wall of the widening and from there back again to the base of the flame. This means that the base of the flame also receives an already completely or partially burned and cooled off gas mixture, as a result of which the combustion temperature remains lower and, consequently, the formation of nitrogen oxides is countered.
• When with a burner vortex break down occurs, it is possible to ensure that the diameter of the flame room has such a taper that, past a underpressured space caused by the explosion of the jet, a stable gas body comes into being, which prevents gas flowing back from the end area of the burner cone to the underpressured space. Though the vortex already plays an important part in the prevention of the blowing off of the flame direct behind the opening, this above indicated taper of the flame room, which causes sufficient outflowing gas to be bent inwards, ensures even more the prevention of the blowing off of the flame. It is pointed out that, due to the application of the invention, a large part of the flow energy has the form of turbulence and, as a result, blowing off is actually already countered. In practice, a stable burner of relatively small dimensions can be obtained wherein blowing off the flame is impossible.
• Moreover, the formation of nitrogen oxides can be countered by providing that the back wall of the flame room is cooled.
• Furthermore, the created vortex at the widening beyond the central opening can be employed by providing that an air slot in the flame room is present near the rear wall for introducing air, burnt gas and/or waste gas that is to be destroyed by combustion. This slot pulls the gases towards the centre, where cool gas ensures a reduction in temperature of the flame base.
• In order to provide a burner wherein the invention is applied and that is controllable, it will be clear that the air velocity, also in a lower setting and resulting, therefore, in a limited air supply, has to meet minimum requirements. Accordingly, an embodiment of the invention provides that a controllable air tap is present, for air that has entirely or partially passed through the rotation enforcing body.
• As will be further discussed below, an analytical examination of the flow before and past the tapering down, indicates that no solutions exist for a continuous flow in case of an adequate high vortex intensity and a widening of the flow section. The result of this examination is that, when the equation:

  
• wherein U₀ = axial velocity in the tapering down;
• U₁ = axial velocity in the burner cone;
• k = 2 Ω / U₀ with Ω = the angular velocity and
• J₀ and J₁ are Bessel functions of the zeroth and first order, has no real solution, vortex break down is to be expected.
• The formulas, however, are developed based on a flow free of turbulence and of dissipation, which, of course, is not entirely consistent with reality so that these formulas give only an indication whether vortex break down will occur.
• In the following, the invention is further explained by means of the drawing wherein:
• fig.1 shows schematically a burner provided with the invention and the flows occurring within;
• fig.2 illustrates the flow picture to prevent a counterflow, details of the burner stream up of the tapering down portion being deleted;
• fig.3 shows schematically a cross-section of a vortex chamber used with the invention;
• fig. 4 shows schematically a cross-section of a vortex device in accordance with the invention;
• fig.5 shows a schematic cross-section of another embodiment, and
• fig.6 shows a graph to illustrate the analytical method for determining vortex break down.
• In fig.1 an air supply for a burner is indicated by 1 where the air has undergone pressure-increase up to 5 cm of water column or 500 N/m. This air is introduced through axially and tangentially directed slots 2 to a vortex chamber 3. This vortex chamber has on its exit side a tapering down portion 4, which causes the air vortex to be even stronger before flowing out. The strong vortex leads to underpressure in the axial area and, therefore, to a counterflow, as is schematically indicated with the flow lines 5.
• By means of a central oil feeding line 6 , oil is introduced to the conical back surface 7 of the vortex chamber 3. By means of the counterflow and vorticity of the air in the vortex chamber 3, oil is forced out along the cone 7 to reach, via the wall parts between the passages 2, the surface, which tapers towards the opening 4, where the vortex air flow 8 ensures that the oil in a thin film moves along this surface at a relatively high speed. In the tapering down portion 4 delamination of the oil film takes place, which atomizes directly. Due to the vortex break down, which occurs right after the tapering down 4 in the flame room 9, an extremely fine atomization takes place. This flame room has a back surface 10 and a cone wall 11, drawn as a cylinder.
• The flow that leaves the vortex chamber 3, explodes while forming a very strong turbulence, as a result of which axially an underpressure is created and a counterflow vortex 12 that flows along the back wall 10 and attaches itself in a stable way to the back wall, partly due to the underpressure created by the local flow velocity.
• When the flame is ignited, a very concentrated combustion takes place in the area 13, indicated by a dotted line, the counterflow 14 from the vortex 12, however, provides cooling of the flame. In the central part in front of the discharge area of the flow from the vortex chamber 3, an underpressure occurs and as a result a vortex can occur, as is indicated by 15. This vortex, too, is stable and impossible to be blown off. Because the main flow, as is indicated at 16, moves again to the axis of the flame room, it is impossible for gas coming from the exhaust area of the burner or even the middle area, to flow back to the area of the flame.
• The slot 17 between the wall 11 and the back surface 10 may provide a secondary-air supply, if so desired. Moreover, the back surface 10 may be cooled, for instance by water in case the burner is used for the heating of water in, for example, a central heating boiler. In stead of secondary air, exhaust gas or a gaseous product that is to be burnt may be introduced, in which case the very thorough mixing by the vortex break down ensures a most efficient combustion.
• As the rotation velocity is only allowed to decrease a little or not at all, in order to obtain vortex break down by means of controlling the burner, a control may be obtained by bringing the combustion air at full speed and subsequently feeding-back part of this air, as is schematically indicated by the slots 18 that give access to a space 19 that has an air exhaust through a control cock 20.
• The shown burner has not only a high stability in order to prevent blowing off and an exceptionally thorough mixing of combustion air and fuel and, therefore, a short flame, it also ensures that a mixture containing oxygen and nitrogen is at a high temperature for a short while only. This is an additional reason why this burner emits few nitrogen oxides.
• The drawn vortex chamber 3 receives its rotating gas through the slots 2 forming a rotation enforcing device. The axial velocity of the air flowing out, is now inversely proportional to the quotient of the annular slot zone 2 and the circular opening in the tapering down portion. It is very well possible that the latter may be larger than the section of the annular slot, in which case the axial velocity is lower when flowing out of the vortex chamber than when entering it, which increases even further the ratio between the rotation velocity and the axial velocity.
• Even in the situation in which a rotation enforcing body causes a vortex with everywhere the same angular velocity around the axis (solid body rotation) and this vortex is carried via the tapering down to a more spacious flow tube; vortex break down occurs again and also an annular vortex. This causes an exceptionally intensive intermixing of the gas flow, for instance, when it contains a mixing gas, a mixing fluid or pulverized particles. In addition, it is pointed out that the invention is most suitable for the combustion of pulverized fuel such as coal particles, but also of aluminium that can be burnt to aluminium oxide, which can possibly be of importance in obtaining solar energy when, by means of solar energy, aluminium oxide can be reduced and the aluminium can later be burnt again as a source of energy.
• Fig. 2 schematically shows the situation in which a rotation enforcing body 21 causes a vortex with everywhere the same angular velocity around the axis (solid body rotation). This vortex is carried via the tapering down 22, which is not on scale, to a more spacious flow tube 23, in the process of which vortex break down occurs again and also the annular vortex 24, This device, too, causes an exceptionally intensive intermixing of the gas flow, for instance, when it contains a mixing gas, a mixing fluid or pulverized particles.
• In the drawing in fig.3 the tapering down of which is not on scale, at point 25, an underpressure is created by the vortex break down, as a result of which the flow, indicated by the arrow 26, threatens to develop. The flame room 27, however, forces the outflowing gases, the volume of which has considerably increased by the combustion, back to the axis of the room, as is indicated by the arrows 29. This ensures that the flow 26 remains slight or is even interrupted, while the flow body 30, due to the underpressure at 25 and the underpressure created by the rapid movement of the gases in its immediate vicinity, remains stabilized and is not blown off.
• Fig. 4 shows a schematic cross-section that represents an advantageous form of the tapering down. It has been found that when the tapering down is too steep it causes a certain thrust and that when it is too flat it takes up too great an axial length and consequently causes too much friction. In the example of fig. 4 the angle made by the tapering down with the axis at the end of the tapering down is a little smaller than 60 degrees.
• In the case of an annular slot with a width of 1/8 to 1/4 of the outside diameter D (see fig.1 and 4), at an axial velocity in the slot equal to the rotational velocity in situ, a steadily burning burner was obtained with an outside diameter of the slot of 17.5 mm, an inside diameter of the central opening of 12 mm and a diameter of the burner cone of 90 mm. Such a burner can stand a pressure of introduced combustion air of 1000 N/m without running the risk of blowing off.
• In fig. 5 a further example of embodiment is schematically represented. Here, the air-supply slot 2 is shown again, by which axially whirling air enters the space 31, as is indicated by the arrow 32. This arrow bends inwards, because from a ring or annular slot 33 radially inflowing and tangentially whirling gas is introduced, which preferably has an axial velocity as well. This, however, is not shown in fig. 5. This air forces the whirling air coming out of the annular slot 2 inwards, as a result of which the latter is narrowed and this causes an expansion of the vortex.
• The invention not only provides a compact and most steady burner, it may also serve to manufacture a burner-spray-nozzle with a wide adjusting range. Compared to conventional pressure spray nozzles, such a burner-spray-nozzle has two advantages:
• 1) At a low oil through-flow, the atomization is better than at a high oil through-flow. As, however, at a high through-flow the flame is longer and therefore takes up more room in which mixing can occur, a constant combustion quality is obtained at a higher and lower oil through-flow.
• 2) A good air cooling, which prevents the burner from getting dirty and blocked at high temperatures.
• For this reason the invention is suitable as a spray nozzle for any type of burner that is to mix fuel with combustion air, for any application with a wide adjusting range.
• To elucidate the phenomenon of vortex break down, the following remarks should be noted.
• In a rotation-symmetrical two-dimensional continuous flow it is possible to deduce from the equation of continuity $$\overrightarrow{\text{∇}}\text{.}\overrightarrow{\mathit{\text{u}}}\text{=0}$$

  

  the existence of a flow function φ : $${\mathit{\text{u}}}_{\mathit{\text{z}}}\text{=}\frac{\text{1}}{\mathit{\text{r}}}\text{.}\frac{\text{δφ}}{\text{δ}\mathit{\text{r}}}$$

  

  and $${\mathit{\text{u}}}_{\mathit{\text{r}}}\text{=-}\frac{\text{1}}{\mathit{\text{r}}}\text{.}\frac{\text{δφ}}{\text{δ}\mathit{\text{z}}}\text{.}$$

  
• If no external forces are present, and the influence of the viscosity is neglected, Navier-Stokes becomes: $$\frac{\mathit{\text{D}}\overrightarrow{\mathit{\text{u}}}}{\mathit{\text{Dt}}}\mathit{\text{=}}\overrightarrow{\mathit{\text{u}}}\text{.}\overrightarrow{\text{∇}}\overrightarrow{\mathit{\text{u}}}\text{=-}\frac{\text{1}}{\text{ρ}}\overrightarrow{\text{∇}}\mathit{\text{p}}$$

  

  with $$\overrightarrow{\text{ω}}\text{=}\overrightarrow{\text{∇}}\mathit{\text{x}}\overrightarrow{\mathit{\text{u}}}$$

  

  for the vorticity, this results in:

  
• The ϕ-component of (1) gives now Uϕ.r = constant = f(φ) if we assume the area around the axis to be an area with 'solid body' rotation, hence $$\mathit{\text{U}}\text{ϕ=Ω.}\mathit{\text{r}}$$

  

  and $$\text{Ω.}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}\text{=}\mathit{\text{f}}\text{(φ).}$$

  
• The components of $\overrightarrow{\text{ω}}$

  

  now become: $${\text{ω}}_{\mathit{\text{z}}}\text{=}{\mathit{\text{u}}}_{\mathit{\text{z}}}\text{.}\frac{\mathit{\text{df}}}{\mathit{\text{d}}\text{φ}}{\text{, ω}}_{\mathit{\text{r}}}\text{=}{\mathit{\text{u}}}_{\mathit{\text{r}}}\text{.}\frac{\mathit{\text{df}}}{\mathit{\text{d}}\text{φ}}$$

  

  and

  

  Solving ω_ϕ from (1) and (2) gives

  

  so the flow function φ is given by

  

  with f=Ωr.
• Now we want to examine what happens if a flow in a cylinder passes to another cylinder with a larger diameter.
• Upstream, in the smallest cylinder, we assume in the vicinity of the axis a velocity U₀ in the z-direction, so $${\mathit{\text{u}}}_{\mathit{\text{z}}}\text{=}\frac{\text{1}}{\mathit{\text{r}}}\text{.}\frac{\text{δφ}}{\text{δ}\mathit{\text{r}}}\text{=}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}\text{⇒φ=}\frac{\text{1}}{\text{2}}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}\text{.}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}$$

  

  hence $$\mathit{\text{f}}\text{=Ω.}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}\text{=}\frac{\text{<figure-callout id="0" label="2Ω U" filenames="imgf0003.png" state="{{state}}">2Ω</figure-callout>}}{{\mathit{\text{U}}}_{}}\text{.φ}$$

  

  and $$\text{-}\mathit{\text{f}}\frac{\mathit{\text{df}}}{\mathit{\text{d}}\text{φ}}\text{=-}{\left(\frac{\text{<figure-callout id="0" label="2Ω U" filenames="imgf0003.png" state="{{state}}">2Ω</figure-callout>}}{{\mathit{\text{U}}}_{}}\right)}^2\text{.φ .}$$

  
• The Bernoulli surfaces in this flow are cylinders, hence the pressure, except for a constant term, is given by $$\mathit{\text{p(r)}}\text{=}\frac{\text{1}}{\text{2}}\text{ρ(}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}{}^{\text{\hspace{0.05em}2}}\text{+}{\mathit{\text{u}}}_{\text{ϕ}}{}^{\text{\hspace{0.05em}2}}\text{) ∼}\frac{\text{1}}{\text{2}}\text{<figure-callout id="2" label="ρ u φ" filenames="imgf0002.png,imgf0003.png" state="{{state}}">ρ</figure-callout>}{\mathit{\text{u}}}_{\text{ϕ}}{}_{}{}^{\text{\hspace{0.05em}2}}\text{.}$$

  

  Then $$\frac{\mathit{\text{p}}}{\text{ρ}}\text{+}\frac{\text{1}}{\text{2}}{\left|\overrightarrow{\mathit{\text{u}}}\right|}^{\text{2}}$$

  

  except for a constant, is represented by $$\frac{\text{1}}{\text{2}}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}{}^{\text{\hspace{0.05em}2}}\text{+}{\mathit{\text{<figure-callout id="2" label="u φ" filenames="imgf0002.png,imgf0003.png" state="{{state}}">u</figure-callout>}}}_{\text{ϕ}}{}^{\text{\hspace{0.05em}2}}\text{=}\frac{\text{1}}{\text{2}}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}{}^{\text{\hspace{0.05em}2}}{\text{+<figure-callout id="2" label="Ω" filenames="imgf0002.png,imgf0003.png" state="{{state}}">Ω</figure-callout>}}^{\text{2}}\text{.}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}\text{=}\frac{\text{1}}{\text{2}}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}{}^{\text{\hspace{0.05em}2}}\text{+}\frac{{\text{2Ω}}^{\text{2}}}{{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}}\text{.φ}$$

  
• The expression above gives for

  
• The flow function $$\text{φ(}\mathit{\text{r,z}}\text{)=Ψ(}\mathit{\text{r}}\text{).}\mathit{\text{r}}\text{+}\frac{\text{1}}{\text{2}}{\mathit{\text{U}}}_{\text{0}}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}$$

  

  of a rotating flow in a cylinder is therefore determined by

  

  a Bessel equation of the order 1.
• So for the solution, regular on the axis, we find $$\begin{gathered} \text{Ψ=}{\mathit{\text{A.J}}}_{\text{1}}\text{(}\mathit{\text{kr}}\text{)} \\ \text{φ=}\frac{\text{1}}{\text{2}}{\mathit{\text{U}}}_{\text{0}}{\mathit{\text{<figure-callout id="2" label="r" filenames="imgf0002.png,imgf0003.png" state="{{state}}">r</figure-callout>}}}^{\text{2}}\text{+}{\mathit{\text{A.r.J}}}_{\text{1}}\text{(}\mathit{\text{kr}}\text{)} \\ {\mathit{\text{u}}}_{\mathit{\text{z}}}\text{=}\frac{\text{1}}{\mathit{\text{r}}}\text{.}\frac{\text{δφ}}{\text{δ}\mathit{\text{r}}}\text{=}{\mathit{\text{<figure-callout id="0" label="U" filenames="imgf0003.png" state="{{state}}">U</figure-callout>}}}_{\text{0}}\text{+}{\mathit{\text{A.k.J}}}_{\text{0}}\text{(}\mathit{\text{kr}}\text{)} \\ {\mathit{\text{u}}}_{\text{ϕ}}\text{=}\frac{\mathit{\text{f}}}{\mathit{\text{r}}}\text{=}\frac{\text{<figure-callout id="0" label="2Ω U" filenames="imgf0003.png" state="{{state}}">2Ω</figure-callout>}}{{\mathit{\text{U}}}_{}}\text{.}\frac{\text{φ}}{\mathit{\text{r}}}\text{=}\mathit{\text{k.}}\frac{\text{φ}}{\mathit{\text{r}}} \\ {\mathit{\text{u}}}_{\text{ϕ}}\text{=Ω}{\mathit{\text{r+k.A.J}}}_{\text{1}}\text{(}\mathit{\text{kr}}\text{).} \end{gathered}$$

  

  Upstream, in the smallest cylinder, we assume an axial velocity U₀ in an area with vorticity with a diameter 2r₀.
• Downstream, in the biggest cylinder, we refer to the axial velocity as U₁ within the area with vorticity with diameter 2r₁.
• For the upstream and downstream flow functions therefore applies: $$\frac{\text{1}}{\text{2}}{\mathit{\text{U}}}_{\text{0}}{\mathit{\text{<figure-callout id="0" label="r" filenames="imgf0003.png" state="{{state}}">r</figure-callout>}}}_{\text{0}}{}^{\text{2}}\text{=}\frac{\text{1}}{\text{2}}{\mathit{\text{U}}}_{\text{0}}{\mathit{\text{<figure-callout id="1" label="r" filenames="imgf0001.png,imgf0002.png" state="{{state}}">r</figure-callout>}}}_{\text{1}}{}^{\text{2}}\text{+}\mathit{\text{A}}\text{.}{\mathit{\text{<figure-callout id="1" label="r" filenames="imgf0001.png,imgf0002.png" state="{{state}}">r</figure-callout>}}}_{\text{1}}\text{.}{\mathit{\text{J}}}_{\text{1}}\text{(}{\mathit{\text{kr}}}_{\text{1}}\text{)}$$

  

  so then $$\text{A=}\frac{{\mathit{\text{U}}}_{\text{0}}{\text{(<figure-callout id="0" label="r" filenames="imgf0003.png" state="{{state}}">r</figure-callout>}}_{\text{0}}^{\text{2}}{\text{-<figure-callout id="1" label="r" filenames="imgf0001.png,imgf0002.png" state="{{state}}">r</figure-callout>}}_{\text{1}}^{\text{2}}\text{)}}{\text{2}{\mathit{\text{<figure-callout id="1" label="r" filenames="imgf0001.png,imgf0002.png" state="{{state}}">r</figure-callout>}}}_{\text{1}}\text{.}{\mathit{\text{<figure-callout id="1" label="J" filenames="imgf0001.png,imgf0002.png" state="{{state}}">J</figure-callout>}}}_{\text{1}}\left({\mathit{\text{kr}}}_{\text{1}}\right)}$$

  

  and

  
• If we refer to the axial velocity on the edge of the vortex upstream as U₁, we find a relation between U₁, U₀, kr₀ and kr₁:

  
• If kr₀ is big enough and/or U₁/U₀ small enough the above expression will have no solution. In that case 'vortex break down' will occur.
• In fig. 6 the value of kr₁/kr₀ is plotted as a function of kr₀ This shows that initially two solutions are available at a certain value of kr₀, subsequently just one in a maximum and finally none at all. In this last domain vortex break down will occur, but it should be taken into account that the flow has always contained smaller vortices which have not been included in the calculation so that the indicated relation can only be regarded as an approximation and cannot be construed as a limitation to the invention.
• It is easy to observe vortex break down in a burner, because in that case the combustion takes place in a fairly small torus shaped zone. This zone is calm and hardly moves. When oil constitutes the fuel, this zone will have a blue colour if the oil had already been completely gasified. As the temperature rises, this zone will become a deeper blue. In case of a less complete gasification, yellow radiant coal particles may be present in this zone, but experience shows that with a sufficient supply of oxygen, all unburnt soot particles or hydrocarbons in the residual gas are completely burnt.
• An important application of the invention is a spray nozzle or atomizer, where the obtained very fine mist, the very thoroughly mixed gas mixture or the very homogeneous suspension of solid particles will not directly be burnt in the flame room.
• It is possible that the mixing substance is brought into rotation prior to coming into contact with the air jet. This is particularly important in the case of mixing with low-calorific gas.
• The rotation enforcing body can have any shape, provided that it superimposes a rotation onto the gas flow. In addition, it may also contain moving or rotating parts such as a blade wheel.

Claims (14)

1. Burner provided with means for mixing a combustion air flow with a fuel, comprising

   (i) an axially symmetrical vortex chamber (3) having in the axial flow direction at the exit side a tapering down portion (4) with a central opening followed by a flame room (9)

   (ii) a body (2) enforcing rotation of the combustion air arranged at the periphery of said vortex chamber (3) and provided with exit openings for the combustion air adjacent to the beginning of said tapering down portion (4),

   (iii) said vortex chamber (3) having at its beginning a greater cross-sectional area in the axial flow direction than that exit openings, and

   (iv) the flow section past the central opening increasing abruptly, the diameter of said flame room (9) being at least 2,5 times that of the said central opening, characterized in that the tapering down reduces the diameter of the vortex chamber to 0,9 - 0,7 of its largest diameter.
2. Burner according to claim 1, characterized in that the equation:

   

   wherein U_o = axial velocity at the end of the tapering down;

   U₁ =axial velocity in the flame room;

   k = 2Ω / U₀ with n = the angular velocity and

   J₀ and J₁ are Bessel functions of the zeroth and first order, has no real solution.
3. Burner according to claim 1 or 2, in which the tangential component of the flow introduced into the vortex room at least equals the axial component.
4. Burner according to any of the preceding claims,
   characterized in that the tapering down portion joins immediately the exit openings of the rotation enforcing body.
5. Burner according to any of the preceding claims, characterized in that the tapering down portion includes at its end an angle with the axis of over 50 degrees.
6. Burner according to any of the preceding claims, characterized in that in the centre of a surface closing the central region of the rotation enforcing body an inlet is present for a liquid or pulverized fuel, which can move along said surface unto the exit openings of the rotation enforcing body.
7. Burner according to claim 6, characterized in that said surface is a cone directed against the main flow direction.
8. Burner according to any of the preceding claims, characterized in that the diameter of the flame room is at least five times that of the opening in the tapering down portion.
9. Burner to any of the preceding claims, characterized in that the flame room is tapered such that a stable gas body is formed, which prevents gas from flowing back from the end area of the flame room to the sub pressure area formed by the vortex break down.
10. Burner according to any of the preceding claims with a rear wall of the flame room, characterized in that means are present to cool the rear wall.
11. Burner according to any of the preceding claims with a rear wall of the flame room, characterized in that an air slot in the flame room is present near the rear wall.
12. Burner according to any of the preceding claims, characterized in that a controllable air tap is present, for air that has entirely or partially passed through the rotation enforcing body.
13. Burner to any of the preceding claims, characterized in that means are provided to admix a pulverized substance into the air inlet flow.
14. Method for mixing a combustion air flow with a gaseous, liquid or pulverized fuel in order to burn it, in which an air flow is fed to an axially symmetrical vortex chamber at the periphery of said chamber with a rotational component, said vortex chamber having an exit debouching in a flame room having a diameter of at least 2,5 times that of the contracted air flow, characterized in that a gaseous flow containing air with in inwardly directed radial component causes the combustion air flow to contract its cross-section in the axial direction to less than the axial cross-sectional area of the vortex room at its beginning, where it receives the combustion air flow and to 0.9 - 0.7 of its largest diameter before said contraction.

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