GB2055595A - Mixing device for flowing fluids - Google Patents

Abstract

A mixing device for fluids of different and/or varying temperatures, particularly liquid metals, such as sodium, comprising a compression-proof housing 4 in which a mixing tube 5 open on both ends is arranged, the mixing tube having numerous radial openings 7 and being mounted at one end at least to be displaceable longitudinally with respect to the housing 4. A pipe stub 9 communicates with the radial openings of the mixing tube, and a shock tube 6 is positioned between said mixing tube 5 and said housing 4, at least one portion of said shock tube 6 being displaceable longitudinally in respect of said mixing tube and said housing. The pipe stub 9 terminates in an annular space 10 between said shock tube 6 and said mixing tube 5 and is surrounded by a housing union 2 tightly connected to said housing, said annular space 10 between said shock tube and said mixing tube being closed at least on the outflow side of the latter mentioned tube. <IMAGE>

Description

SPECIFICATION Mixing device The present invention relates to a mixing device for fluids of different and/or varying temperatures, more particularly for mixing liquid metals, such as sodium or the like, comprising a compression-proof housing in which a mixing tube open on both ends is arranged, the mixing tube having numerous radial openings and being mounted at one end at least to be displaceable longitudinally with respect to the housing, and further comprising a pipe stub communicating with the radial openings of the mixing tube.

Mixing devices for liquid media of different temp- eratures in pipelines are usually made in the form of T-pieces. Particularly in the case of high temperature levels of the media, e.g. 550 C, if sudden (shock-like) changes in media temperature occur and if the temperature differences of the media to be mixed are relatively big, the pressure-carrying structural members of the mixing device are exposed to considerable thermal stresses. Already during stationary operation, i.e. during nonvarying existence of a given temperature difference between the incoming flows, assymetric temperature stressing is produced in T-shaped mixers. Further stress results from internal pressure, piping forces and moments and changes in media temperatures.Particularly critical stressing in respect of the life span is produced by temperature oscillations on the walls wetted by the fluid, which are created by turbulences in the mixing zone. Dependent on the mixing principle and construction there result different stresses in respect of the amplitude- and frequency spectrum of the temperature oscillations on the one side and different structural lengths in which the mixing process decays, on the other side. In principle, a high degree of turbulence in the mixture also results in a highfrequency spectrum of the oscillations in the fluid and at the wall of the structural parts. The boundary layer has a dampening effect on the amplitudes only.

What is essential for the stressing is the difference between the surface temperature at the wall and the simultaneous average temperature in the wall. This difference and the stressing, accordingly, decrease with increasing frequency of the oscillation in the fluid, and simultaneously also the depth of material affected by the stressing. On the other hand, higher frequencies directly produce a greater number of load cycles within the same operating period of the mixer and, thus, effect a reduction in the life span thereof. These two effects oppose one another and result in a minimum of fatigue, dependent upon the wall thickness and heat transfer for a given frequency spectrum.

In the US-Patent 3 409 274, a mixing device for high-pressure fluids at different temperatures for a steam generator is described. In a thick-walled, cylindrical housing, a likewise thick-walled pipe stub terminates in a T-shaped manner within a space concentric with the housing and defined by the thick-walled housing and a thin-walled tube. This thin-walled tube is formed, in vicinity of the inlet, with numerous holes which are distributed about the periphery and over part of the length of the tube. In the flow direction upstream and downstream of these numerous holes, circular ring-shaped plates having small perforations formed therein are disposed, the perforations permitting only a very limited flow.

Tests and calculations have established that this construction is unsuited for great temperature differences and transients, i.e. temperature change rates, particularly in liquid-metal installations, even if the wall thicknesses of the housing are reduced considerably in accordance with the low pressure of the hot liquid metal. The medium entering through the T-shaped inlet strikes the inner wall of the housing directly and produces excessive thermal stresses thereat, which cause fatigue of the material, when there are many temperature changes.It has furthermore been found experimentally that, in such a mixer of about 100 mm inner diameter, the length of the tube required downstream of the numerous holes, in the direction of the flow, for the purpose of equalizing the temperature differences in the mixed medium to an extent permissible for the outer housing is at least seven times the inner diameter thereof.

This required tube length, however, results in considerably large dimensions of the mixing device according to the embodiment described in the hereinaforementioned US-Patent. In addition, a non-uniform pressure and temperature distribution in the cross section of the inner tube results.

A similar mixing device is described in the DE-AS 1 937 735, according to which an excessive stressing of the pressurecarrying walls and of a mixing tube in the form of a sieve should be prevented by contracting the hotter media flow in the mixing zone to a smaller diameter and by surrounding it in a jacketlike manner with the cooler media flow.

In spite of efforts made to improve the mixing efficiency within a short distance by means of twist formation by the stub tangentially running into the main tube, measurements which were made behind the mixer in stationary operation with sodium and 530C C for flow rates of 0.5, 1 and 2 and temperature differences of 150 and 200 K between the entering fluids have established, that the amplitudes of the temperature oscillations, measured after a length of 6.5-times the mixing tube diameter, still amounted to about 50-70% of the difference between the entering media.

This means, that this embodiment is not suited to eliminate most of the afore-mentioned disadvantages, particularly the stressing on adjoining pipelines and components. Particularly high stressing occurs, when the media to be mixed change their temperatures in a shock-like manner.

It is, therefore, the object of the present invention to provide a compact mixing device of the abovedescribed kind, which avoids excessive thermal stress, major temperature oscillations in the pressure-carrying housing as well as temperature striping in the adjoining pipelines and fatigue problems over long periods of operation. The device should be of minimum length and diameter so that it can fit into a piping system with a minimum of occupying space. It is a further object of the invention to provide such a mixing device which, in special cases, is operable with different flow directions at two of the three connections thereof.

According to the invention there is provided a mixing device for fluids of different and/or varying temperatures, suitable for mixing liquid metals such as sodium or the like, comprising a compressionproof housing in which a mixing tube open on both ends is arranged, the mixing tube having numerous radial openings and being mounted at one end at least to be displaceable longitudinally with respect to the housing, and further comprising a pipe stub communicating with the radial openings of the mixing tube, wherein a shock tube is arranged between said mixing tube and said housing, at least one portion of said shock tube being displaceable longitudinally in respect of said mixing tube and said housing, and wherein said pipe stub terminates in an annular space between said shock tube and said mixing tube and is surrounded by a housing union tightly connected to said housing, said annular space between said shock tube and said mixing tube being closed at least on the outflow side of the latter mentioned tube.

In this arrangement, a direct contact between the entering media and an endangered housing part of different temperature is avoided by disposing a concentric tube in orderto obtain a shock chamber, both between the mixing tube and the housing as well as within the housing union. The dampening effect of the stagnant fluid in the shock chamber on the wall of the pressure-carrying structure in the case of sudden changes in the fluid temperature can also be obtained by a laminated structure of said chamber made of a material with comparatively poor conducting properties.

Thermal stresses are restricted by means of thinwalled, elastic connections between the tubes resp.

between the tubes and the housing by conical sleeves.

By disposing the tubes so that they are displaceable in longitudinal direction, stresses due to impeded thermal expansion between the mixing tube, the inner pipe stub and the housing are avoided. Since these tubes are not subjected to any appreciable pressure difference, they need not be made absolutely tight and can have an annular gap at one end thereof. The fluid flowing through the gap reduces the temperature gradient in the wall of the mixing tube. It should be noted, however, that no gap between the mixing tube and the shock tube is provided on the outflow side because, otherwise, temperature strings can endanger adjacent structural parts.If the direction of flow in the mixing tube can vary, both ends of the mixing tube must be connected tightly to the housing and a gap for separating the mixing tube must be provided therebetween, which gap should be adequately distant from the ends of the mixing tube. The gap can also be replaced by a tight elastic corrugated tube.

In order to obtain good mixing, the flows in the innertube must be turbulent and one or both incom ing flows must be split into partial flows by means of numerous openings. If the inflow through the annular space between the shock tube and the mixing tube is effected into the mixing tube, the inlet veloc itythrough the openings should then be so high that the partial jets will still just penetrate to the axis of the mixing tube.In tests with water it has been found, for example, that for an inner diameter of the mixing tube of about 100 mm and radial holes perpendicularto the axis of the inner tube, the inflow velocity into the holes VB should be at least equal to and, preferably higher than the inflow velocity VA in - the mixing tube, but not more than twice that value, in order to obtain favourable mixing conditions within a short distance. VA was varied from 0.5 to 3.5 m/s. With VEIVA = 1 to 2, the mean values of the amplitudes of the temperature oscillations decay satisfactorily after a distance of 7-times the inner diameter. With VB/VA = 2, somewhat larger amplitudes are produced in the vicinity of the holes than with VB(VA = 1.If VB/VA becomes too small, mixing becomes very poor, as a ring of liquid is formed at the wall of the inner tube and mixes with the core, which is at a differenttemperature, only after a considerably great distance has been traversed. In the case of flow instabilities at low velocities such interm ittent strings stress the wall of the structural parts generally more than brief turbulence eddies do. If, on the other hand, VS/VA becomes too large, the probability of large temperature oscillations in the immediate region of influence of the holes arises.

When the throughput is variable, it is advantageous to dispose the holes diametrically opposing and staggered in longitudinal direction, and to exceed the velocity ratio rather than fall short of it. Holes of square cross-section yield greater penetrations depths and jet resolution, i.e. better mixing results, than holes of round cross-section. The length of the mixing tube downstream in flow direction, as measured from the end of the openings, should be at least 7-times the diameter. When the direction of inflow and outflow varies, the numerous openings in the mixing tube should begin at least one housing diameter away from the region of the union in order to largely avoid asymmetric inflow over the periphery into the mixing tube from the union or further to avoid temperature strings and -layers in the union, in the other direction. Moreover, by dis- - posing the radial openings nearer two the side ofthe entrance, the required structural length is reduced.

In principle, it is advantageous to dispose the axis of the mixing tube vertically when such mixing tube has a very large diameter, so as to preclude temperature stratification due to differences in buoyancy and in order to better control possible degassing probiems.

Other features which are considered as characteristic for the invention are set in the appended claims.

Although the invention is illustrated and described herein as embodiment in mixing device for fluids of different and varying temperatures, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which: FIGS. 1 to 3 are diagrammatic sectional views of different embodiments of the mixing device for fluids of different and varying temperatures constructed in accordance with the invention; and FIG. 2a is a slightly enlarged fragmentary view of FIG. 2 showing a modified form of construction of the parts contained within the broken-line circle X.

FIG. 4 shows the mixing processes within the mixing tube.

Referring now to the drawing and first, particularly, to FIG. 1 thereof, there is shown an embodiment of the invention having a T-shaped mixing device for two sodium flows to be mixed in a nuclear energy installation. This mixing device is provided for a maximal temperature of about 550"C at various constant and varying temperature differences of the inflows of 50 to 100 K and, for brief periods, of 250 K (below tne maximal temperature). The planned operating pressures are considerably lowerthan in steam plants of similar temperature, and are, in fact, about 16 bar. The planned operating life is 25 years.

Three housing stubs or unions 1,2 and 3 are welded to a T-shaped housing 4, the housing union 1 being suitable only for inflow supply, and the housing unions 2 and 3 being suitable, as desired, for inflow supply or outflow (discharge). A mixing tube 5, which is guided in the vicinity of the housing union 1 so as to be displaceable in longitudinal direction and is fastened tightly at the housing union 3, extends from the housing union 1 to the housing union 3.

This mixing tube 5 is surrounded by a concentric or coaxial shock tube 6 which, in the vicinity of the housing union 1, is secured thereto and, in the vicinity of the housing union 3, is guided so as to be displaceable in longitudinal direction. In the vicinity of the housing union 1,the mixing tube 5 is formed with numerous holes 7 which are uniformly distributed over the peripheral surface thereof and through which eitherthe medium entering at the housing union 2 flows inwardly from the outside or the medium entering at the housing unions 1 and 3 flows outwardly from the inside. The housing union 2 is protected from transient temperature stresses by an inner pipe stub 9 disposed concentrically or coaxially thereto.

The pipe stub 9 is elastically or resiliently connected to the housing union 2 by a truncated cone 8 and terminates in the shock tube 6, with a small thermal expansion gap 11 provided between the wall of the pipe stub 9 and the wall of the shock tube 6. The mixing device according to the invention illustrated in FIG. 1 is provided for continuous inflow of a hot medium through the housing union 1.If another medium of different temperature is supplied or flows in through the housing union 2, it is conducted, dampingly stressing the housing union 2, through the pipe stub 9 into the concentric annular space 10 between the shock tube 6 and the mixing tube 5 and flows through the numerous holes 7 into the mixing tube 5, wherein it mixes with the medium entering through the housing union 1 up to the very outlet of the housing union 3, whereby the temperature of both media is equalized to such an extent that the housing union 3 and non-illustrated connecting pipelines are no longer endangered thereat. The pressure parts of the housing 4 are protected against excessive thermal stresses by the annular shock chamber 12 between the shock tube 6 and the housing 4.If the medium entering through the housing union 1 is to be mixed with a medium of different temperature entering through the housing union 3, then these two flows collide in counterflow in the vicinity of the holes or openings 7, which ensures good mixing although also great pressure loss. The media mixture flows outwardly through the openings 7 and is conducted from the annular space 10 between the mixing tube 5 and the shock tube 6 to the pipe stub 9 and to the housing union 2. Further mixing of the media occurs along this path. The length of the path from the end of the openings 7 to the end of the pipe stub 9 should correspond to about 7-times the diameter of the mixing tube 5.

If a medium entering through the housing union 2 is to be mixed with a medium which enters alternatinglyeitherthroughthe housing union 1 or3, itis advantageous to subdivide the openings 7 formed in the mixing tube 5 into a group above and a group below the pipe stub 9 with a distance of at least two mixing tube diameters from the middle of the pipe stub 9.

The annular gap between the mixing pipe 5 and the wall of the shock tube 6 provided for reasons of thermal expansion should not be located directly at the end of the mixing tube 5, when the flow is conducted in this manner, so as to avoid the formation oftemperature stripes or strata in the outflow.

The embodiment shown in FIG. 2 is suitable if rapid temperature changes can occur at all three housing unions 1, 2 and 3. All three unions 1, 2 and 3 are thus protected by the connection which is shown in FIG. 1 onlyatthe housing union 2. By decoupling the structural discontinuity, i.e. the connection between mixing tube and shock tube from the location where the piping forces act, local stresses are reduced in this construction. FIG. 2a shows, on a slightly enlarged scale, the section X (expansion bearing) from FIG. 2 with an alternative construction of the sliding parts 30, 31 of an inner housing stub and the mixing tube 5 or of a housing stub and the shock tube 6. The part 30 is of cylindrical form and the part 31 is spherical or crowned. If both parts 30 and 31 slide on one another due to temperature variations, contact between both thereof is limited to a circular line contact.

In constructions formed of austenitic steel, self welding effects must be reckoned with at operating temperatures above about 400"C. In the temperature range between about 380 and 550"C under sodium conditions, it is advisable to reduce possible releas ing or loosening forces by making one of the bearing surfaces crowned or spherical so that the contact surface is kept as small as possible and, simultane ously providing a possibility for equalizing assembly inaccuracies.At even higher temperatures and contact pressures greater than about 10 N/mm2, it is recommended to make the bearing surfaces straight i.e. planar, or crowned i.e. spherical surfaces, one of the bearing surfaces being formed of a harder mater ial than the other; this can be accomplished, for example, by cladding or hardfacing, the different coefficient of thermal expansion of the cladding and the base materials, however, resulting in the formation of constraints.

Fig. 3 shows an embodiment in which the mixing tube 5 having a greater diameter in respect of the adjoining pipelines in the outflow direction. Thus, the structural length and diameter of the mixing device can be reduced.

Fig. 4 shows the mixing conditions within the mixing tube 5 by means of a diagram made in the course of thermohydraulic tests. The abscissa is the lengtk of the mixing tube standardized to the diameter, whereas the left ordinate represents the maximum values of the amplitudes ofthe temperature oscillations referred to the temperature difference of the entering media. The right ordinate shows the maximum range of oscillation in per cents, also referred to the temperature difference of the entering media.

Measurements have been made in different radial positions of the mixing tube, and it has been found that already after a length of 5xD only a maximum of 30% and after a length of 1 0xD less than 15% of the inflow temperature differences occur.

The observed frequencies lie between 1 and 60 HZ, frequencies greater than 10 HZ being correlated with small amplitudes.

Claims (8)

1. A mixing device for fluids of different and/or varying temperatures, suitable for mixing liquid metals such as sodium or the like, comprising a compression-proof housing in which a mixing tube open on both ends is arranged, the mixing tube having numerous radial openings and being mounted at one end at least to be displaceable longitudinally with respect to the housing, and further comprising a pipe stub communicating with the radial openings of the mixing tube, wherein a shock tube is arranged between said mixing tube and said housing, at least one portion of said shock tube being displaceable longitudinally in respect of said mixing tube and said housing, and wherein said pipe stub terminates in an annular space between said shock tube and said mixing tube and is surrounded by a housing union tightly connected to said housing, said annular space between said shock stube and said mixing tube being closed at least on the outflow side of the latter mentioned tube.

2. A mixing device according to claim 1, wherein a thermal expansion gap is disposed between the junction of said pipe stube and said shock tube.

3. A mixing device according to claim 1 or claim 2, wherein the radial openings in said mixing tube are staggering in the axial direction thereof relative to said pipe stub.

4. A mixing device according to any one of claims 1 to 3, wherein said shock tube and said pipe stub are resiliently connected to said pressure resistant housing and housing union by means of thin-walled conical sleeves.

5. A mixing device according to any one of claims 1 to 4, wherein said mixing tube is connected with said shock tube by means of a conical sleeve, being spaced from the ends of said mixing tube.

6. A mixing device according to any one of claims 1 to 5, wherein at least one portion of said mixing tube in the vicinity of the end of outflow has a; greater diameter in respect of the adjoining pipelines.

7. A mixing device according to claim 4, wherein said annular spaces between said mixing tube and said shock tube as well as between said shock tube and said housing are closed at their ends.

8. A mixing device for fluids of different and/or varying temperatures, substantially as hereinbefore described with reference to the accompanying drawings.