NUCLEAR ENERGY
of clouds when a tropical storm becomes a hurricane . The system self-organizes and becomes a giant , fearsome powerhouse , moving across the ocean surface . The situation is in many ways analogous to safety valves in which order appears in the form of self-organising supersonic flows that often violently interfere with structures . In hurricanes , as in valves , the emerging order is detrimental because it impedes the necessary dissipation of energy . Ilya Prigogine called dissipative structures those emergent constructions that are born far from equilibrium in dissipative systems ; they are planets , flowers , birds , life ... and for what concerns us today : the supersonic jets in the valves . Composed of order and disorder , they generally force our admiration . However , these ordered structures are not always welcome , especially in valves , where highly structured supersonic jets are often a source of instability , particularly during variations in the system ’ s regime . A valve subjected to a high-pressure ratio sees order appear within it . Order and disorder acting simultaneously disturb any system . Dissipative structures , by the part of order they contain , prevent the dissipation of energy from taking place completely and rapidly . The degradation of energy , inherent to valves , is done by viscous friction and by an increase of entropy in supersonic flows and more or less stable shock waves . These problems of flow instabilities in valves are of concern because they generate vibrations throughout the plant , which can lead to cracking , possibly corrosion , and sometimes failure . They are all the more worrying as the difficulties are accentuated with the commissioning of nuclear power plants feeding machines with higher unit powers . Huge valves must therefore be adapted to considerable live steam volume flows . The supersonic flows that appear are not adapted to the walls they are offered ; they interfere in a complex way with these walls .
Figure 2A . Conventional nozzle Figure 2B . Bell-shaped nozzle .
The fluid system , after the bifurcation introducing these supersonic phenomena , enters into chaos , and self-organized structures appear which , due to their lack of stability , can become dangerous in these organs and in other energy applications ( safety valves , bypass devices , etc .). The supersonic jets formed have the freedom to dissipate their own kinetic energy . Using its collective intelligence , the molecular system organises itself to degrade the kinetic energy of the flows in the valves by itself , at its own pace , without concern for our safety or environment . We lose control of the situation . A proactive solution is to intentionally destroy these dangerous dissipative structures in the case of valves . But how do we do this ?
Judicious use of physics to calm the flow in valves
In the 1970s , CETIM ( Centre Technique des Industries Mécaniques ) launched a general interest study on valves at the request of the French industrialists concerned and EDF and was able to patent , in 1984 , the following solution . Let us use a classical nozzle ( Figure 2A ), adapted , thus without shock wave ( in order to simplify the presentation ), operating in a supersonic regime ; the method of characteristics makes it possible to define the state of the fluid at each point . The number of lines drawn can be large , and thus the network is much denser than the one presented . In the expansion zone , the flow characteristics ( velocity , velocity direction , static temperature , etc .) are significantly different between any point 1 and 2 in this two-dimensional flow . Consider another adapted nozzle ( Figure 2B ), analogous to space rocket propulsion nozzles , operating between the same Mach numbers as the previous one . Between the corresponding points 1 and 1 ’ ( or 2 and 2 ’) of the two nozzles , the different properties of the fluid are very different . Finally , let ’ s build a sandwich structure from these two nozzles , giving them a certain thickness . We can imagine fictitious walls separating the different layers . In each of these layers , we can assume that the flows are perfect , i . e . without energy dissipation , which is a reasonable assumption in this accelerated flow . We therefore know all the characteristics of the flow at each point . Let ’ s suddenly remove the fictitious walls that isolated the different layers . With the proposed stratification , everything changes abruptly . The flow is divided into an immense number of configurations . Each of the innumerable particles is surrounded by other particles whose velocities , velocity directions , etc . are different from its own . In this singular use of supersonic flows , two initially reversible flows lead almost instantaneously to an enormous increase in entropy , which we can simply call disorder . This approach , which consists in rapidly degrading as much energy as possible , is called the principle of worst action . The principle of worst action , by exchanging information from our macroscopic world to the molecular world , proposes to reverse the roles : namely , to impose a great disorder in the molecular system to disintegrate the order contained in these harmful dissipative structures and thus protect our macroscopic world .
Figure 3 . Valve provided with vistemboirs ( kinetic energy degraders )
Ideal control valve
Let ’ s apply this process to a model of a control valve ( Figure 3 ). The flow in the converging inlet is identical in all layers , it follows the principle of least action in this part where the entropy is constant . Vistemboirs , or kinetic energy degraders , are tools allowing the application of the principle of worst action , and have been implemented
18 Valve World April 2023 www . valve-world . net