WO1994012270A1

WO1994012270A1 - Chaotic convection effect mixing exchanger

Info

Publication number: WO1994012270A1
Application number: PCT/FR1993/001167
Authority: WO
Inventors: Hassan Peerhossaini; Yves Le Guer; Cathy Castelain
Original assignee: Universite De Nantes
Priority date: 1992-11-30
Filing date: 1993-11-26
Publication date: 1994-06-09
Also published as: EP0675756A1; FR2698559B1; CA2150457A1; FR2698559A1

Abstract

A chaotic convection effect mixing exchanger comprising a duct consisting of one or more members (1, 1?1, 12,..., 1n¿) arranged end to end and containing a flowing fluid to be mixed and/or subjected to a heat flow. A portion of said duct is made of a chaos-generating member including at least two curved portions defining two planes of curvature defined as the planes passing through the centre of the arc of the circle defining the curve and the tangent to said curved portion, said planes of curvature being orthogonal. Said exchanger may be used with viscous and fragile fluids.

Description

Mixer exchanger with chaotic convection effect.

The present invention relates to a mixing exchanger with chaotic convection effect for mixing and heating non tonian or non-Newtonian fluids and applies more particularly to viscous and fragile fluids.

A large number of heat exchangers or mixers are known. In some of the heat exchangers, it is necessary to heat fluids bad conductors of heat, very viscous and with complex thermorheological behavior (non-Newtonian fluids in particular) by means of tubular exchangers or plate exchangers. To treat such fluids, these techniques are not always satisfactory and often pose problems. The homogeneous heating of very viscous fluids poses difficulties which are linked to the problem of mixing. A heterogeneous mixture induces unequal heat transfers according to the zones which can cause zones of overheating where the fluid is altered or on the contrary cold zones. Consequently, to improve the heating process of viscous fluids, it is necessary to stir the fluid by means of an external agitating agent such as turbulators or turbulence generators. However, external agitators often exercise restraint considerable shear on the fluid. Therefore, the heating and mixing of delicate fluids incapable of withstanding high stresses without undergoing degradation remains a problem in particular in the food, biochemical and pharmaceutical industry.

The object of the present invention is therefore to propose a mixing exchanger which solves the problem of heating and mixing inhomogeneity and, therefore, improves the performance of the exchanger without increasing the mechanical stresses within the fluid and without having recourse to the generation of turbulence which results in considerable energy losses.

The aim of the present invention is therefore to generate a mixture by Lagrangian chaos and to obtain a stirring equivalent to stirring obtained in a conventional turbulent regime but without high mechanical stresses of the type of that encountered in a flow other than laminar. .

To this end, the invention relates to a mixing exchanger with chaotic convection effect. It should be recalled below what is called chaotic convection. Historically, the distinction between laminar and turbulent flow has been based on the Eulerian nature of velocity fields. Certain flows can be considered regular when Eulerian quantities are measured (velocity fields for several fixed points) while they appear stochastic when Lagrangian quantities are recorded (particle trajectories). It is the existence of stochastic movements of particles in laminar flow that we call chaotic convection phenomenon.

Consequently, to obtain these stochastic movements in laminar flow with a view to achieving a certain degree of mixing in a given time, it is necessary to impose on the particles of fluids to follow optimal trajectories. Through the invention, the trajectories obtained are trajectories chaotic fluid particles which are produced using vortices inherent in the flow and which act as internal agitators. It is these longitudinal vortices called "Dean rollers", appearing under the effect of the imbalance which exists between the transverse pressure gradient and the centrifugal force which is linked to the curvature of the streams of fluids obtained in curved pipes. are used to get the chaotic trajectories of the particles.

The chaos generating elements used in the context of the invention are therefore characterized by their arrangement in space in order to obtain a very specific fluid trajectory. The mixing exchanger with chaotic convection effect object of the invention comprises a duct formed in one piece or made from a series of elements arranged end to end, inside which flows a fluid in front be mixed and / or subjected to a heat flux, characterized in that a portion of the duct is produced by means of a chaos generating element comprising at least two curved portions, which delimit two planes of curvature, defined as the planes including the center of the arc defining the curve and the tangent to said curved portion, said planes of curvature being orthogonal.

According to a preferred embodiment of the invention, the conduit inside which the fluid flows is constituted in particular by a combination of one or more chaos generating elements whose curved portions are of radius and angle at identical centers, the said element or elements having in cross section a central symmetry and an angle at the center of between 90 ° and 270 °, preferably equal to 120 °.

Other characteristics and advantages of the invention will become apparent on reading the following description and the accompanying drawings, which description and drawings are given mainly by way of examples. In these drawings: FIG. 1 represents a chaos generating element consisting of a series of four curved elements of square section, and

FIG. 2 represents a perspective view of a mixing exchanger made up of a series of chaos generating elements made up of curved elements of circular section.

According to the invention, the mixer exchanger with chaotic convection effect comprises a series of hollow elements 1, 1, 1, etc. open at each of their ends and arranged end to end. These elements can be of two types, straight or curved, that is to say curved. In FIG. 1, rectilinear elements such as element 1 are associated with curved elements 1, 1, 1. One can also imagine that a single element has several curved portions, this element being in this case formed in one piece by bending.

The chaos generating elements, objects of the invention, consist of at least two curved portions which delimit two planes of curvature orthogonal to each other. It is recalled in accordance with what has been said above that by plane of curvature is meant the plane containing the center of curvature and the tangent of the arcuate portion of the curved part. This means, for example in FIG. 1 to simplify, that the element 1 and the element 1 ′ are assembled in the same plane then the element 1 ′ is subjected to a rotation of 90 °. Therefore, by introducing a geometric discontinuity in the flow of the fluid by variation in the space of the planes of curvature from one curved element to another, one modifies the trajectories within the flow at the entry of each curved portion. In fact, we generate a type of trajectory in an element, we then destroy it and we regenerate another type of trajectory in the next element. So very complex trajectories can be produced by this medium and a particle of fluid subjected to such a system will follow a chaotic movement if the rotation of a plane of curvature with respect to another is chosen appropriately. The curved portion of each of the elements can vary from one element to another. Thus, the angle at the center Θ of each curved portion can be between 90 and 270 °. It will preferably be chosen equal to 120 °. Conversely, it can be decided, in order to standardize the production and manufacture of such an installation, that the chaos generating element is produced from a combination of curved portions whose planes of curvature are orthogonal between them and whose radii, center angles, and straight lengths are identical. Therefore, one can mathematically define the coordinates of a point M-_ arranged on the central fiber at the angle © i in a curved element of rank i or at the length L ^ in a straight element of rank i . By central fiber is meant the line shown in dotted lines in FIG. 1 which joins the successive centers of section of a curved portion. This definition constitutes an aid to the realization of the conduit by allowing on the one hand an optimization of the layout of the conduit, on the other hand an automated design and manufacture of said conduit.

The curved elements have an average radius of curvature R and an angle at the center Θ.

- ^• > We will choose an orthonormal reference of origin 0- ^* designated by (i- |, ^* > r jj, k- |) in which we will determine the coordinates of the point M_ in the element i. To do this, we will use the matrices and vectors defined as follows:

0 - CosΘ Sin © 0 CosΘ Sin © ₉₀ ° = 1 0 0 M-90 ° = -1 0 0

0 Sin © Cos © 0 -Sin © CosΘ R90 = R_ ₉₀

R (1-CosΘi)

0 Di =

Either a heat exchanger mixer with chaotic convection effect with n elements.

The coordinates of the point M-] at the angle Θ- \ if the element 1 is curved or at the length L- ^ if it is straight are given in the ->.—> - ^* > coordinate system (0- * , i- _| , j- *, k- *), by the coordinates of the vector V-- which is equal to C- * if the element is curved or D- ^* if it is straight.

The coordinates of point 2 at angle © 2 if element 2 is curved or at length L2 if it is straight are given in the ^* > »coordinate system (0- ^* , i- *, j- *, k - _| ), by the coordinates of the vector T2:

T2 = (V- ^* ) L Θ ^{+ M} 2-1 ^v 2 with (V- \) _LΘ = V- ^* with Θ- ^* = Θ and L- ^* = L

with V2 which is equal to C2 if the element 2 is curved and in this case with M2_ι which is equal to:

- Mgg if element 1 is curved and if the angle of rotation of element 2 with respect to element L is + 90 °

- M_9 _Q if element 1 is curved and if the angle of rotation of element 2 with respect to element 1 is -90 °

- Rgg if element 1 is straight and if the angle of rotation of element 2 with respect to element 1 is + 90 ° - R_9o if element 1 is straight and if the angle of rotation of element 2 compared to element 1 is -90 °

- H _D if element 1 is straight and if the angle of rotation of element 2 with respect to element 1 is 0 °

or with V2 which is equal to D if the element 2 is straight and in this case with M2- 1 ^< 3 ^u; '- ^es, equal to H _c if the element - ^• is curved and to H _D if element 1 is straight.

It is therefore possible to express the coordinates of a point M- ^ placed on the central fiber of the ith element of the exchanger mixer.

These coordinates are given by those of the vector T ^ which is equal to:

Ti = (Vi.-,) ^* !,. © + M ₂ , 1 x _3/2 ^X "- ^X M _{i /; L} _-, Vi We can therefore express the coordinates of the point Mn by means of a series of matrix equations T-, = V-,

T2 = <V-1> L.Θ ^{+ M} 2.1 ₂

T _± = (V _i _ - _l ) _{L t Θ} + - M ₂ , 1 x _3/2 x. -. Mi ^ .-, V ± T _n = (V _n _- |) _L , _Θ + M _{2 f} ix M _3/2 .-. X M _n , n-1 ^v n where the vectors V- ^* , Vj_, V _n depend on the form of the element and the matrices M -j, M ^ i_- _| , M _nn _- | the shape of the two indexed elements and their rotation relative to each other.

This mathematical solution which will allow the equation of the central fiber to be calculated quickly and reliably can only be applied if the duct is a combination of curved portions of radius, center angle and identical straight lengths for a given installation. This case is only a particular example of the invention.

To optimize the mixing effect, it is necessary to optimize the spatial chaos generated in this geometry. To do this, it can be seen that one of the preferred solutions of the invention consists in having at least four curved portions in series as shown in FIG. 1. Obviously, these curved portions placed in series can be obtained from a single and the same element formed in one piece or from four elements assembled by appropriate connecting means. In addition, again for reasons of efficiency and ease of implementation, the constituent elements 1, 1, 1, ..., 1 ⁿ of the chaotic element present in a manner obligatory in cross section a central symmetry. It is thus possible to use elements of circular, square, hexagonal section, etc. Thus, in the example represented in FIG. 1, the chaotic element is constituted by a series of four curved elements of angle © 90 °, of internal radius 200 mm, of external radius 240 mm, of square section 40 x 40 mm ² , curvature ratio 5.5. The rectilinear elements associated with these curved elements have a length of 276 mm and of course sections identical to the curved elements.

In order to allow easy mounting and dismounting of the constituent elements of the mixing exchanger without increasing the pressure drops, it is preferable to use means for connecting the elements arranged on at least one of the external faces of one of the elements in the vicinity. of the parting line between two adjacent elements. These connecting means can for example be flanges. One can also imagine a connection method by screwing, said elements being assembled by screwing by means of a thread made on a half-height so as not to modify the internal section of said elements. It is also possible to use as connecting means so-called DUDGEONNER fittings, etc.

Obviously, any other suitable connection means can be used. Likewise, when it is desired to use this mixing exchanger as a heat exchanger with a view to heating or cooling the fluid flowing inside this mixing exchanger, the walls of the constituent elements of 1 should be heated or cooled. 'Mixing exchanger either in imposed temperature or imposed flow regime, or in mixed regime, by conventional means. In the so-called direct current heating method, it will be possible to ensure that the walls of the components of the exchanger are energized. Another heating method can be used. It is the process of volume heating of a liquid by conduction direct electric. The walls of the elements can be made to form electrodes between which a potential difference is applied by connecting them to an alternative source of electrical energy so as to create an electric field which establishes an electrical conduction current and causes the appearance of volume sources of heat. The advantage of a combination of such a heating process with the mixing process by chaotic convection makes it possible to overcome the drawbacks of the different velocities of the particles in a pipe, which makes it possible to obtain a perfectly homogeneous heating of the particles and d 'Avoid the problems of fouling resulting from the compaction of the elements along the internal walls of the hollow elements constituting one mixer exchanger. In this case, for the implementation of the heating process, if the section of the elements is square, the faces of the constituent elements of the electrodes will be the parallel faces marked 3a and 3b in FIG. 1. On the other hand, in the case of a circular section conduit, an external electrode constituted by the external wall of the conduit and a separate internal electrode disposed centrally inside the conduit will be used. It is obviously possible, in another embodiment, to use so-called conventional indirect heating means such as electrical resistors.

As for the flow of the fluid, it can be free or controlled. In the first case it will be carried out by gravity, in the second case, we will use flow control means such as pumps

FIG. 2 represents an exemplary embodiment of a mixing exchanger with chaotic convection effect. This mixing exchanger consists of a series of bent elements with a circular section having an internal diameter of 23 mm, an external diameter of 25 mm, a radius of curvature 126.5 mm, and a 90 ° angle between two adjacent elbow planes. In this particular mixer, there is on the one hand a high efficiency of the mixture, on the other hand a laminar flow regime, therefore a low energy dissipation, and finally a reduction in pressure losses and a minimum of mechanical stresses due to laminar flow. All of these properties make this mixing and heat exchange process efficient with low energy dissipation. Therefore, this mixing exchanger is suitable for both delicate fluids and complex fluids with long molecular chains. Furthermore, by comparing the efficiency of a chaotic exchanger compared to a spiral exchanger of the same length and of the same internal and external diameter, the fluid used is water: - an increase in efficiency d 15 to 18% heat exchange - zero or negligible increase in pressure drop.

By analogy, it can be estimated with an almost zero risk of error that an increase in mixing and heating performance would be even more significant if the fluids were more viscous.

Consequently, the increase in pressure losses is negligible compared to the increase in the exchange coefficient. This result goes against all received ideas. Indeed, until now, complex geometric constructions had been abandoned due to the significant increase generated in terms of pressure losses. It is surprisingly found here in this device that the increase in pressure drops is negligible.

The applications of such a mixing exchanger are numerous.

The performance of the mixture can be used in conventional in-line mixers, in chemical reactors or for more complex devices such as those allowing oxygenation of blood, etc.

This mixing exchanger also has the advantage of being able to operate in an open or closed circuit. In the case of an open circuit, the fluid flows from a point A of entry into the hollow elements to a point B of exit of the hollow elements. The passage of the fluid inside these elements is in this case unique. Conversely, when the circuit is closed, the fluid carries out during its flow a loop and passes several times in the same elements. This makes it possible to limit the size of such an installation. In this case, of course, at a given instant, the loop is broken to allow the fluid to escape.

Claims

1. Mixing exchanger with chaotic convection effect of the type comprising a duct formed in one piece or produced from a series of elements (1, 1 ¹ , 1 ² , ..., 1 ⁿ ) arranged end to end end, inside which flows a fluid to be mixed and / or subjected to a thermal flux, characterized in that a portion of the duct is produced by means of a chaos generating element comprising at least two curved portions , which delimit two planes of curvature defined as the planes including the center of the arc of a circle defining the curve and the tangent to said curved portion, said planes of curvature being orthogonal.

2. Mixing exchanger according to claim 1, characterized in that the curved portion of the chaos generating element has an angle at the center Θ between 90 and 270 °, preferably equal to 120 °.

3. Mixing exchanger according to claim 1, characterized in that the curved portions of the chaos generating element are of identical radius, angle and central length so that the layout and the realization of the conduit are fully automatable .

4. Mixing exchanger according to claim 1, characterized in that the chaos generating element has a central symmetry in cross section.

5. Mixing exchanger according to one of claims 1 to 4, characterized in that a chaos generating element comprises a series of at least four curved portions whose adjacent planes of curvature are orthogonal two by two.

6. Mixing exchanger according to one of claims 1 to 5, characterized in that the chaos generator is produced by assembling elements, the means for connecting two elements

'adjacent being arranged integral with at least one of the faces external of at least one of the elements in the vicinity of the joint plane between said adjacent elements.

7. Mixing exchanger according to one of claims 1 to 6, characterized in that it comprises one or more chaos generators possibly combined with one or more hollow rectilinear elements, so as to define an open circuit allowing a free flow of the fluid 'A point of entry inside the series of hollow elements to a point B of exit from the series of hollow elements.

8. Mixing exchanger according to one of claims 1 to 6, characterized in that the chaos generating elements combined or not with rectilinear hollow elements define a closed circuit, the flow of the fluid forming, during its passage to the inside the series of hollow elements, a loop.

9. Mixing exchanger according to one of claims 1 to 8, characterized in that the walls of the constituent elements of the mixing exchanger combined or not with a central electrode constitute electrodes between which a potential difference is applied by means of an alternative source of electrical energy to create an electric field which establishes an electrical conduction current and causes the appearance of voluminous sources of heat by the Joule effect.

10. Mixing exchanger according to one of claims 1 to 8, characterized in that the walls of the constituent elements of the exchanger are capable of being heated either by indirect heating means known per se, or by heating means direct constituted in particular by the tensioning of the walls so as to generate a heating of the walls by direct passage of current.