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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Two-Phase Instabilities Lahey, Jr. Besides, many NCSs with only a unique steady-state solution can also become unstable during the approach to the steady state due to the appearance of competing multiple solutions due to the inertia and feedback effects pure dynamic instability.

An increase in power suppresses the type I instabilities, while enhances the type II instabilities according to the basic classification of these leedinegg.

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. Increasing the exit enthalpy and eventually leading to subcooled boiling again and repetition of the process. The parameter is a hypothetical concept called Superficial velocity.

## Ledinegg instability in microchannels

So increase in orificing at channel inlet does not always increase the stability of a natural circulation system with multiple parallel channels Figure Linear stability analyses with homogeneous flow assumption and empirical model for the slip to calculate void fraction as a function of mixture quality have been carried out by Fukuda et al.

Thus, the flow rate can jump from one value to the other even though the operating power and pressure are constant. According to Boure et al.

This variable heat transfer rate modifies the pure DWO. Interaction of parallel channels with DWO can give rise to interesting stability behaviors as in single-phase NC. For smaller riser flow area, the flow rate is smaller due to larger resistance in small riser pipes.

Shell and tube heat exchang The next flow excursion occurs due to rise in pressure drop when the flow pattern changes from annular to slug flow in the vertical portion of the riser pipes.

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.

In a natural circulation system, the instabiligy rate in the channel depends on the heating power and the channel resistance. It is felt to have a review and summarize the state-of-the-art research carried out in this area, which would be quite useful to design and safety of current and future light water reactors with natural circulation core cooling.

### LEDINEGG INSTABILITY | HTRI

Fukuda and Kobori [ 5 ] have classified the density-wave instability as type I and type II for the low power and high-power instabilities, respectively. However, with power increase, the flow became unstable as soon as boiling was initiated in the heated section. Finally, it should be noted that time domain evaluations may be performed with the nonlinear conservation equations leading to Hopf bifurcations e. For example, based on the periodicity sometimes ledihegg are characterized as periodic and chaotic.

However, there are many situations with multiple steady-state solutions where the threshold of instability cannot be predicted from the steady-state laws alone or the predicted threshold is modified by other effects.

When the heat flux is such that boiling is initiated at the heater exit and as the bubbles begin to move up the riser they experience sudden enlargement due to the decrease in static pressure and the accompanying vapor generation, eventually resulting in vapor expulsion from the channel. Divergent oscillations can occur depending on the magnitude of the pressure-loss fluctuation in the two-phase and single-phase regions and the propagation time delay.

The phase shift of out-of-phase oscillations OPO is known to depend on the number of parallel channels. The index of physics articles is split into multiple pages due to its size. We note that case 3 is unstable while case 4 is stable. Commonly observed, static instabilities are flow excursion and boiling crisis. Similar to the type I and type II density-wave oscillations, two types of Ledinegg instabilities are observed at any subcooling depending on the operating power.

Typical examples are boiling inception, flashing, flow pattern transition, or the occurrence of CHF. Table of Contents Alerts. Instability Classification Mathematically, the fundamental cause of all instabilities is due to the existence of competing multiple solutions so that the system is not able to settle down to anyone of them permanently.

There are limited studies on the excursive instability behavior of a parallel downward flow system Babelli and Ishii The last flow excursion occurs when the flow becomes single phase and the pressure drop increases with increase in flow rate.

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