Ledinegg Instability. Figure 1: Sketch illustrating the Ledinegg instability. Two- phase flows can exhibit a range of instabilities. Usually, however, the instability is . will focus on internal flow systems and the multiphase flow instabilities that occur in . Ledinegg instability (Ledinegg ) which is depicted in figure This. Ledinegg instability In fluid dynamics, the Ledinegg instability occurs in two- phase flow, especially in a boiler tube, when the boiling boundary is within the tube.
|Published (Last):||13 October 2014|
|PDF File Size:||5.87 Mb|
|ePub File Size:||16.42 Mb|
|Price:||Free* [*Free Regsitration Required]|
Certain two-phase flow phenomena can cause a major disturbance and can lead to instability or modify the instability characteristics significantly.
The characteristics of the flow excursion instability or Ledinegg type instability depend very much on the geometry as well as the system pressure, power, and channel inlet subcooling [ 22 ]. The sudden condensation results in depressurization causing the liquid water to rush in and occupy the space vacated by the condensed bubble. Geysering is a thermal nonequilibrium phenomenon. Member feedback about Index of physics articles L: Fluid dynamics Revolvy Brain revolvybrain. Examples and applications Historically, probably the most commonly studied cases of two-phase flow are in large-scale power systems.
Ledinegg instability | Revolvy
AB – The static Ledinegg instability in horizontal microchannels under different flow conditions and fluids pertinent to electronics cooling was studied experimentally and numerically. In this case, however, the instability continues with limit cycle oscillations.
The process repeats itself.
The static instabilities observed in their loop are due to the high heat flux and subcooled boiling occurring in the heated section, which are ideal instabilihy the cause of chugging-type instability.
Lee and Ishii [ 25 ] found that the nonequilibrium between the phases created flow instability in the loop.
Instability is undesirable as sustained flow oscillations may cause forced mechanical vibration of components. Usually, the homogeneous model predicts a larger void fraction than the two-fluid model for the same instabiluty quality due to the absence of slip between the water and steam in this model. Such an instability can occur at very low-power and at high-power conditions. As should be expected, the stability of a nuclear-coupled system is in instabiluty different from that given by Equation 6.
Acoustic oscillations have been observed in subcooled boiling, bulk boiling, and film boiling.
However, in certain cases depending on the geometry and operating conditions, islands of instability have been observed to occur [ 8 — 10 ]. They found that the single-phase circulation was stable. If the quality is disturbed by a small amount, the void fraction with smaller drift velocity can have larger fluctuation than the other due to larger slope of void fraction versus quality.
This depends on the relative importance of the respective components of pressure drop such as gravity or frictional losses in the system. Also inlet throttling between the surge tank and the boiling channel is found to stabilize PDO just as Ledinegg instability. However, feedback effects also are paramount in the phenomena. Hence,the flow will be disturbed larger for a smaller fluctuation in quality in this case.
Ledinegg instability in microchannels
Like Ledinegg instability, there is a danger of the occurrence of CHF during pressure drop oscillations. If we have any complex roots s having positive real lesinegg the system is unstable.
Typical flow pattern transition instability in boiling natural circulation systems. At high flow rate, the frictional and local pressure drop in the two-phase region become greater, which has a destabilizing effect. Under the circumstances, it looks relevant to classify instabilities into various categories which will help in improving our understanding and hence control of these instabilities.
To receive news and publication updates for Science and Technology of Nuclear Installations, enter your email address in the box below. Of these instability modes, the most important, and most widely studied, have been Ledinegg instabilities [Ledinegg ] and density-wave oscillations DWOs.
There are many other things that could be said about two-phase instabilities. As an unstable single-phase system progresses through single-phase NC to boiling inception and then to fully-developed two-phase NC with power change, it can encounter several unstable zones.
In view of the existence of more than two unstable zones, this method of classification could be confusing at times. With increase in pressure, the threshold power for the lower instability boundary moves to much higher power and the upper threshold boundary does not change significantly.