Abstract:
A porous material inserted into a fluid-containing vessel reduces turbulence, heat transfer, and mass transfer in the fluid. The material may be used in a cryostat to reduce turbulence in a boiling cryogenic fluid. The cryostat may be used in an energy dispersive x-ray analysis unit to cool an x-ray detector.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The invention relates to the field of reducing turbulence in a fluid. 
     B. Related Art 
     In the field of energy dispersive x-ray analysis, vessels known as Dewars or cryostats are commonly used to cool the x-ray detectors to cryogenic temperatures. The cryostats are commonly filled with liquid nitrogen, but can be filled with any cryogenic liquid. Due to imperfections in the insulation of the cryostats, the cryogenic liquid may boil violently. The boiling results turbulence, which leads to vibration, which in turn can cause deterioration in the resolution of the x-ray detector. 
     Even when the boiling is of the nucleate type, from “hot” walls of the vessel, significant turbulence may occur. “Hot” in this context is of course relative to the temperature of the cryogenic liquid. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to reduce turbulence in a fluid. 
     The object is achieved by using a porous material in the fluid. 
     The invention can also be used to distribute heat transfer throughout a fluid or reduce mass transfer throughout a fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described by way of non-limitative example with reference to the following drawings. 
     FIG. 1 shows a prior art cryostat. 
     FIG. 2 shows a cryostat with hard porous material 
     FIG. 3 Shows a cryostat with soft porous material 
     FIG. 4 shows an energy dispersive x-ray analysis unit cooled with a cryostat in accordance with FIG. 2 or FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a prior art cryostat. The cryostat may have any shape. The cryostat commonly has a vacuum vessel  101 , insulation  102 , and an inner vessel  103 . There is an opening at the top called a neck  104  for filling the vessel  103  with cryogenic liquid. The cryostat is closed by a non-hermetic cap  105 , which allows for continuous venting of the inner vessel. 
     FIG. 2 shows implementation of the invention in a cryostat. The vessel  103  is filled with a hard, porous material  206 . The material is porous in the sense that it is filled with passages for the cryogenic liquid to flow through. The majority of passages must communicate with each other throughout the vessel  103  so that the fluid can access them. The passages restrict the natural circulation of the cryogenic liquid into narrow channels, changing turbulent flow to laminar or transition flow. 
     The material preferably occupies 20-30% of the volume of the vessel  103 , with the rest of the space occupied by passages defined by the material. Conceivably the material might occupy as much as 50% of the volume of the vessel  103 . The hard porous material might be of a foamed and/or sintered type. Some appropriate materials could be metals, silica compounds, ceramics or polymers, e.g. aluminum, stainless steel, or quartz. An example of a suitable foamed material would be Duocel® metal/ceramic foam available from ERG Materials &amp; Aerospace, 900 Stanford Ave, Oakland, Calif. 94608. 
     Since the passages should communicate, they might be embodied in just one passage with some turns, angles and/or forks or a spiral with one long, continuous curve. The term “a plurality of passages” as used herein therefore includes the situation of one passage with such a curve, turns, angles, and/or forks. 
     The material  206  is preferably secured to all walls of the vessel  103  at the time the vessel is built. 
     FIG. 3 shows an alternative embodiment of the invention. In this embodiment, a soft, porous material  306  is inserted in the vessel  103 . The soft, porous material is preferably fibrous such as metal wool or silica wool. Suitable metal wools are GSS-90 Stainless Steel Fibers or GCU-340 copper fibers, both available from Global Material Technologies, Inc., 1540 E. Dundeet Road, Suite 210, Palatine, Ill. 60067, tel. 1-847-202-7000. The metal wool can be added after manufacturing of the cryostat, by simple insertion through the neck  104 . After insertion, the metal wool expands to fill the vessel  103 . The soft, porous material  306  is preferably not secured to the walls of the vessel  103 . 
     Those of ordinary skill in the art will be able to devise other materials in line with the inventive concept explained herein to accomplish the function of reducing turbulence in the fluid. Also, the invention can be applied to vessels of other shapes and functions. 
     FIG. 4 shows an energy dispersive x-ray analysis unit provided with the cryostat  405  of FIG. 2 or FIG.  3 . The unit also includes an x-ray detector  402  cooled by the cryostat  405 , cold finger  401 , and processing apparatus  403 . The x-ray detector may be a lithium-drifted silicon crystal. The cold finger  401  is intended to provide good thermal contact between the detector  402  and cryostat  405 . The cold finger may also have means to attenuate vibrations.