US3559729A

US3559729A - Thermodynamic circulatory system apparatus

Info

Publication number: US3559729A
Application number: US779587A
Authority: US
Inventors: Reinhart Radebold
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1967-11-27
Filing date: 1968-11-27
Publication date: 1971-02-02
Anticipated expiration: 1988-02-02
Also published as: GB1242750A; FR1592997A

Abstract

Thermodynamic circulatory system with a heat exchanger of high heat flux density which receives a liquid metal stream and delivers a substantially homogeneous, two-phase metallic stream. The heat exchanger includes a plurality of parallel tubes for receiving the liquid metal stream; each tube having a constant cross section and a constriction located at a point beyond the inlet end of said tube which corresponds to no less than the length of the tube through which the liquid stream must flow in order to be preheated to at least its initial boiling temperature. The liquid stream is converted into a low vapor quality, two-phase stream as it passes through such constriction.

Description

United States Patent THERMODYNAMIC CIRCULATORY SYSTEM APPARATUS 6 Claims, 4 Drawing Figs.

U.S. Cl 165/107,

Int. Cl ..F28d 15/00, H02b 45/00 Field ofSearch 165/174,

ABSTRACT: Thermodynamic circulatory system with a heat exchanger of high heat flux density which receives a liquid metal stream and delivers a substantially homogeneous, twophase metallic stream. The heat exchanger includes a plurality of parallel tubes for receiving the liquid metal stream; each tube having a constant cross section and a constriction located at a point beyond the inlet end of said tube which corresponds to no less than the length of the tube through which the liquid stream must flow in order to be preheated to at least its initial boiling temperature. The liquid stream is converted into a low vapor quality, two-phase stream as it passes through such constriction.

THERMODYNAMIC CIRCULATORY SYSTEM APPARATUS BACKGROUND OF THE lNV ENTlON The present invention relates to apparatus for forming substantially homogeneous metallic two-phase streams of low vapor content in thermodynamic circulatory systems and, particularly, to such apparatus employing tubes into which high heat flux density streams are conducted.

In thermodynamic circulatory systems, particularly in those wherein a liquid metal is circulated, homogeneous or at least substantially homogeneous two-phase streams are used more and more often.

These two-phase streams serve either to directly convert energy, as in HD (Magneto-Hydrodynamic) systems, or to transfer heat from heat sources of high power density to other systems. In MHD systems, both tasks can be performed together, i.e., transfering heat from a heat source of high power density to a liquid metal and converting energy by changing a liquid metal into a substantially homogeneous twophase stream.

The dynamic stability of such a system causes some difficulties. For example, one approach would try to completely evaporate a liquid metal in some type of boiler and to convert it into a two-phase stream by subsequent expansion. The term "two-phase stream as used herein, is always intended to mean a substantially homogeneous mixture of the gaseous and liquid phases of the metal. The liquid phase of such mixture is present in the form of droplets of the material to be used. This method of forming a two-phase stream, however, does not lead to the desired goal because the vapor content in the mixture is not sufi'rciently low and, moreover, the ability of the stream to transfer heat decreases with increasing evaporation so that streams of high heat flux density will also require great differences in temperature.

The vapor quality of a two-phase flow which is formed by expanding a gas is about 0.9. The desired two-phase flow should have a quality of no more than 0.1 however.

The heat transfer coefficient for a gas is about 0.05 W/deg.cm in contrast, the same coefficient for low vapor quality two-phase flow is about 20 W/deg.cm=.

In order to solve this problem, heretofore it has been necessary to begin with the liquid phase of a particular material and to convert a portion of the same into vapor by, for example, the addition of heat thereto. Experience with metal-cooled reactors has shown that the process of boiling can be unstable. It has been observed that alkali metals, particularly, can be superheated to several hundred degrees above their boiling temperature without being changed into a vapor. At such temperatures, however, if the vapor forms, it will occur explosively. This, of course, is not desirable since damage can be caused to the apparatus which in turn will lead to malfunctions thereof.

SUMMARY OF THE INVENTION It is therefore an object the present invention to provide an improved thermodynamic circulatory system for liquid metal in which the above-described difficulties are eliminated.

The present invention provides a solution to the abovedescribed difficulties by teaching a heat exchanger having a secondary side formed of a plurality of parallel tubes with constant cross sections. Such tubes, however, are constricted at a defined throttle point, the liquid metallic stream being heated to at least the initial boiling temperature before reaching this constriction.

Such an apparatus, particularly when suitably dimensioned, assures that the process of forming the two-phase streams remains stable. Physically this can be explained as follows:

The speed of sound in liquid metals lies in the range of several thousand meters per second. The speed of sound in metallic, substantially homogeneous two-phase streams of high vapor content, however, is much lower. The speed of sound ranges from just a few meters per second at very low vapor quality to the speed of sound in the pure gaseous phase of the metal. which is in the order of some hundred meters per second.

This speed of sound which presupposes a thermodynamic equilibrium between the two phases suggests a solution to the problem at hand.

It is known from gas dynamics that, only during the expansion of a compressible stream of matter through a constricted cross section, the speed of sound can be attained no matter what the pressure conditions are downstream of the constric tion in the direction of the channel stream. Thus, only when the cross section widens as in a Laval nozzle can the speed of sound be exceeded. The parallel channels or tubes constructed according to the present invention are provided with constant cross sections in order that the two-phase stream will reach top speed, which is the speed of sound associated with its vapor content, independently of the pressure level in the system downstream of the heat source. This is of great significance because the temperature at which the two-phase stream absorbs heat and continues to evaporate is related to the pressure available within the stream via the steam pressure characteristic. If the pressure drops significantly, this indicates a strong reduction in temperature during evaporation and, thus, a high thermodynamic loss with reference to the utilization of the two-phase stream.

The further feature taught by the present invention, i.e. the defined constriction, forces the conversion of the liquid phase of the stream into a two-phase stream. This is accomplished at a precisely defined point along the tubes within the heat exchanger. Moreover, the two-phase stream is initially provided with very little steam content so that dynamic instabilities can be excluded. The use of, for example, a baffle or nozzle at the point of the constriction has the purpose of initiating evaporation by slightly reducing the pressure within liquid already heated to the boiling temperature or, perhaps, even superheated. The constriction point thus also represents the point at which the high speed of sound associated with liquids is changed to the lower equilibrium speed of sound in twophase streams. The speed of sound might be somewhat exceeded under certain circumstances at the constriction of the tube, but this is practically of no significance. This is due to further subsequent evaporation since the two-phase stream again passes into the subsonic range as a result of pressure shock waves downstream of the constriction. The two-phase stream can then attain no more than the speed of sound during further evaporation and acceleration in tubes with constant cross section.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of the heat exchanger according to the present invention.

FIG. 2 illustrates the operating parameters associated with the heat exchanger according to FIG. 1.

FIG. 3 is a detail view of one embodiment of the constriction of the apparatus according to the present invention.

FIG. 4 is a detail view of another embodiment of the constriction of the apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, the heat exchanger 1 receives preheated liquid metal from the input 2. This liquid is distributed to parallel tubes 3 provided on the secondary side of the heat exchanger 1. At the point along each tube 3, indicated by the broken line 4, a constriction in the form of a nozzle or baffle is provided to throttle the liquid stream. The constriction is located at such a point that the liquid is heated to at least its initial boiling temperature before reaching the same. As shown in FIG. 1, the liquid is heated to the initial boiling temperature in zone 5 of heat exchanger 1. When the liquid thus heated through the construction at 4, it is converted into a two-phase stream. Downstream of the constriction formed at point 4, the two-phase stream having a vapor content which is still considered rather low, as can be seen in FIG. 2. is evaporated further until at the end 6 of each tube 3 the desired vapor content in the stream of tube 3 is achieved. The desired vapor content is achieved by continued addition of heat which arrives. for example, in the direction of the arrows 7. In the area of the collector 8, the two-phase streams coming out of tubes 3 at their ends 6 are merged into a single tube. The merged two-phase streams are conducted further through the single channel in the direction of arrow 9 away from the region of the heat exchanger 1.

Referring to FIG. 3, this is a detail view of one parallel tube 3 being part of the secondary side of the heat exchanger. A constriction in the form of a baffle is provided at 4 to throttle the liquid stream after being heated in zone of the heat exchanger. 5

Referring to FIG. 4 the constriction at 4 within the tube 3 is embodied by a nozzle. The liquid heated to the initial boiling temperature within zone 5 passes through the nozzle at 4, thus being converted into a two-phase flow.

Referring to FIG. 2, this shows various thermodynamic parameters. These vary with the location of the heat stream along the length of the tube 3. As can be seen, the temperature 10 of a heat stream 1 l steadily rises while it flows through tube 3 upstream of the constriction at point 4 where the heat stream 111 is in the form of a liquid 12. The temperature 10, however, drops off sharply at the constriction formed at point 4 and then levels off downstream thereof to remain almost constant. The pressure 13 of the liquid 12 also remains almost constant up to the constriction at point 4, then erratically falls until it assumes a relatively constant value. The speed of sound 14 of the liquid 12, which is quite high up to the constriction at point 4, drops off drastically and falls to almost zero. It then increases rapidly again during the course of developing the twophase stream 15 up to the ends 6 of the tubes 3 of heat exchanger 1, together with the velocity of the stream 16 itself.

Indeed, as can be seen, the speed of sound 14 is almost the same as the velocity of the stream, at least, in the area of the two-phase stream 15. g g v I The vapor content of the two-phase stream 15 is reduced to the point of vanishing as it emerges from the constrictions at points 4. As the streams 15 continue to flow through the tubes 3 downstream of the constrictions at points 4, they continue to evaporate. Thus the vapor content of the streams 15 at the ends 6 of tubes 3 depends on the length of the tubes 3 downstream of the constrictions at points 4 as well as the heat supplied to the streams 15. Both the length of the tubes 3 downstream of the constrictions at points 4 and the heat supplied can be adjusted to yield any desired vapor content in the streams 15.

Due to the unavoidable reduction in pressure in the tubes 3, the evaporation temperature required falls. The ability of the streams 15 to transfer heat, however, is extremely high so that even at slight differences in temperature streams of high heat density can be transferred per unit of area. This is best illustrated by comparing the curves for temperature 10 and heat ll in F IG. 2.

According to the present invention the two-phase stream 15 is intended to be substantially homogeneous at the ends 6 of the tubes 3. Care must thus be taken that the liquid film is first removed from the walls of the tubes 3. This usually occurs automatically in the present invention at the ends of the tubes 3, since experience has shown that during evaporation at sufficiently high speeds of sound or vapor contents removal of the liquid film begins to occur without further measures being necessary. As the stream 15 swirls through the tubes 3, it pulls the liquid film from the walls thereof in the form of droplets. If, however, the swirling stream 15 should not be sufficient, a known chemical treatment can be employed on the walls of the tubes 3 just before their ends 6 (for example, a passivating coating such as a stable oxide).

This prevents wetting of the tube walls by the metal in the two-phase stream and makes possible removal from such walls of the li uid film.

lt wil be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations.

I claim:

1. In a thermodynamic circulatory system provided with a liquid metal stream and a heat exchanger of high heat flux density which receives said liquid metal stream and delivers a two-phase metallic stream, the improvement wherein said heat exchanger includes a plurality of parallel tubes each having an inlet end for receiving said liquid metal stream, each of said tubes having a constant cross section and a constriction disposed beyond said inlet end and at the closest thereto at a distance equal to the length of said tube through which the liquid metal stream must flow to be heated to its initial boiling temperature, said liquid metal stream being converted into a substantially homogeneous metallic two-phase stream as it passes through said constriction.

2. Apparatus as defined in claim 1 wherein said plurality of parallel-tubes are merged into a common conduit beyond said heat exchanger.

3. Apparatus as defined in claim 2 wherein said constriction is formed in said parallel channels, respectively, by a baffle.

4. Apparatus as defined in claim 2 wherein said constriction is formed in said parallel channels, respectively, by a nozzle.

5. Apparatus as defined in claim 4 wherein the length of each of said parallel tubes downstream of said constriction is determined by the vapor content desiredin the two-phase stream emerging from said heat exchanger.

6. Apparatus as defined in claim 5 wherein said parallel tubes are in parts defined by walls which are coated with a chemical substance to prevent wetting thereon.

Claims

2. Apparatus as defined in claim 1 wherein said plurality of parallel tubes are merged into a common conduit beyond said heat exchanger.

5. Apparatus as defined in claim 4 wherein the length of each of said parallel tubes downstream of said constriction is determined by the vapor content desired in the two-phase stream emerging from said heat exchanger.