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.
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Venkat Raj, and M.
The steep variation in heat transfer coefficient typical of transition boiling conditions in a post CHF scenario can get coupled with the DWO. Figures 1 a and 1 b show an example of occurrence of Ledinegg-type instability at different powers [ 2 ] in a boiling two-phase NC system.
The change in power required from the first to the last stage is quite significant and it may not be reached in low-power loops. When the power is in between the above specified range, the internal pressure loss curve intersects the driving buoyancy curve at three points i.
It may be noted that the amplitude of oscillations cannot go on increasing indefinitely even for unstable flow. While the mechanism of instability is same for upward- and downward-flow systems, however, one instabikity finding is that the flow excursion can be the dominant mode of instability as compared to the density-wave instability in boiling NCs. The oscillatory mode during boiling inception can also be significantly affected by the presence of parallel channels.
Ledinegg instability | Revolvy
The process repeats itself resulting in perpetual oscillatory behavior if the operating conditions are maintained constant. Type I Instability For this type of instability to occur, the presence of a long riser plays an important role such as in a boiling two-phase natural circulation loop.
Two-phase flow can occur in various forms, such as flows transitioning from pure liquid to vapor as a result of external heating, separated flows, and dispersed two-phase flows where one phase is present in the form of particles, droplets, or bubbles in a continuous carrier phase i.
Almost all the theoretical and experimental studies agree well that the DWI can be suppressed in boiling two-phase NC systems by increasing the system pressure. 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 for the cause of chugging-type instability. System stability is determined by solving for the roots, s, of Equation 6.
Depending on the operating conditions, the oscillations can be periodic or chaotic. Boiler tubes normally overcome this which is effectively a ‘negative resistance’ regime by incorporating a narrow orifice at the entry, to give a stabilising pressure drop on entry.
Advantage of homogeneous flow assumption is that it is easier to apply ledlnegg the model is also found to predict the stability boundary or the threshold of instability with reasonable accuracy. A density-wave instability is the typical dynamic instability which may occur due to the multiple regenerative feedback between the flow rate, enthalpy, density, and pressure drop in the boiling knstability.
The process repeats itself. This will not be confused with the premature occurrence of CHF during an oscillating flow, in which case the oscillations occur first followed by CHF see Figure 9 b. However, regular flow stagnation is observed, which is of concern for the safety of nuclear reactors.
Fukuda and Kobori [ 5 ] observed two modes of ledonegg in a natural circulation loop with parallel heated channels.
Fukuda and Kobori [ 5 ] have classified the density-wave instability as type I and type II insability the low power and high-power instabilities, respectively. Moreover, it instabilihy found that these instabilities do not occur in isolated manner in NCSs, however, many times, they occur together which are known as compound instabilities.
In addition, pressure-drop oscillations and the parallel-channel instability are also characteristic of the loop geometry. In general, instabilities can be classified according to various bases as follows:. Under low quality conditions, leinegg slight change in quality due to any disturbance can cause a large change in void fraction and insgability in the driving head.
Linear analyses of boiling flow instabilities in natural circulation systems with homogeneous flow assumptions have been carried out by Furutera [ 29 ], S. 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.
In such cases, pressurised water is passed through heated pipes and it changes to steam as it moves through the pipe. 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.
While Ledinegg instability is known to be a problem in low pressure boiling systems, an increase of the system pressure, or an increase in the inlet orificing in the channel, can stabilize the system.
This may reduce the threshold power for instability for that channel which may cause the other instabilitj to be unstable.
According to Boure et al. Similarly, any slight disturbance causing the flow rate to decrease will shift the operating point to B and then to point A. The Ledinegg-type instability is found to occur at a lower power as compared to the flow-pattern transition instability at any subcooling.
In some cases, the occurrence of multiple solutions and the instability threshold itself can be predicted from the steady-state equations governing the process pure or fundamental static instability.
Thus, there can be five different flow rates for a particular operating condition of power and subcooling as indicated in Figure 2 by points A—E. The other flow excursion occurs when the flow pattern changes from the annular to dispersed bubbly flow in the horizontal portion of riser pipes due to ibstability of pressure drop with flow rate.
That is the reason for the reduction of type I instability with increase in drift velocity Figure A dynamic force balance on the boiling loop yields:.
There are limited studies on the excursive instability behavior of a parallel downward flow system Babelli and Ishii The effect of riser geometry such as riser height and area on flow stability is important. Finally, it should be noted that time domain evaluations may be performed with the nonlinear conservation equations leading ledinebg Hopf bifurcations e.
Access to Document Both these instabilities are observed during low-pressure conditions only. Pressure-drop oscillations are associated with operation in the negative sloping portion of the pressure drop-flow curve of the system.