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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The occurrence of this type of instability can be ascertained by investigating the steady-state behavior alone.
Ledinegg instability – Wikipedia
References  Ruspini, Two-phase flow instabilities: Besides, due to the presence of tall risers in natural circulation BWRs, the frequency of density-wave oscillation can be much lower due to longer traveling period of the two-phase mixture in the risers.
The Ledinegg-type instability decreases with an increase in pressure. Instability is considered compound when more than one elementary mechanisms interact in the process and cannot be studied separately.
View at Google Scholar K. There are a number of instabilities that may occur in two-phase systems. Lee [ 22 ], Ledinebg et al.
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To analyze this type instability, it is required to predict the pressure drop characteristics of the system against the flow rate similar to the Ledinegg-type instability [ 3 ]. Member feedback about Two-phase flow: In case of a natural circulation BWR, the existence of a tall riser or chimney over the core plays a different role in inducing the instability. For example, when a compression pressure wave consists of compression and rarefaction wave passes, the vapor film is compressed enhancing its thermal conductance resulting in increased vapor generation.
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.
Abstract The static Ledinegg instability in horizontal microchannels under different flow conditions and fluids pertinent to electronics cooling was studied experimentally and numerically. The quantification instabiluty density-wave oscillations requires an analysis of the mass, momentum and energy conservation equations of the boiling system. This variable heat transfer rate modifies the pure DWO.
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.
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.
Instability can also disturb control systems and cause operational problems in nuclear reactors. Moreover, it is found that these instabilities do not occur in isolated manner in NCSs, however, many times, they occur together which are known as compound instabilities. Following a perturbation, if the system returns back to the original steady state, then the system is considered to be stable.
Thus, as the boiling boundary moves up the tube, the total pressure drop falls, potentially increasing the flow in an unstable manner. However, for sake of completeness, only a brief discussion is presented below.
For example, [ 13 ] observed seven different types of flow modes in a boiling two-phase natural circulation loop with increase in heater power such as i surface evaporation; ii a static instability characterized by periodic exit large bubble formation; iii a steady flow with continuous exit of small bubbles; iv a static instability characterized by periodic exit of small bubbles; v another static instability characterized by periodic extensive small bubble formation; vi a steady natural circulation; and vii the density-wave oscillation dynamic instability.
A dynamic force balance on the boiling loop yields:. Certain two-phase flow phenomena can cause a major disturbance and can lead to instability or modify the instability characteristics significantly.
Also the amplitude of oscillation was found to be larger than that under single-phase conditions. Neither the cause nor the threshold of instability of such systems can be predicted purely from the steady state equations alone.
Differences also exist in their transport mechanism, oscillatory mode, and analysis methods. Venkat Raj, and M. View at Google Scholar. It may be noted that in case of forced circulation BWRs, instabilities observed under natural circulation conditions are due to pump trip transients when the core exit quality is high due to low flow and high power. So increase in orificing at channel inlet does not always increase the stability of a natural circulation system with multiple parallel channels Figure Acoustic oscillations have been observed in subcooled boiling, bulk boiling, and film boiling.
On the other hand, with increase in local losses in the single-phase region such as orificing at the inlet of channelsthe improvement in stability has been found to be conditional [ 240 ] unlike in forced circulation systems wherein it has been observed that with increase in local losses in the single-phase region always improves the flow stability.
In general, instabilities can be classified according to various bases as follows:. Usually, there exists a low-power and a high-power unstable zone for density wave instability in forced as well as NC two-phase flows Figure 3 a.
Hence, the two-phase frictional pressure loss may be high owing to the smaller two-phase mixture density. 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.
They found that the single-phase circulation was stable.
Flow Instabilities in Boiling Two-Phase Natural Circulation Systems: A Review
Interaction of parallel channels with DWO can give rise to interesting stability behaviors as in single-phase NC. However, with initiation of boiling, no flow reversal is observed Figure 5. Lee and Ishii [ 25 ] found that the nonequilibrium between the phases created flow instability in the loop.
The effects of subcooling on these instabilities are always debatable. Experimental investigations in two-phase natural circulation loops having single boiling heated channel have been carried out by [ 131724 — 27 ]. The thermal response of the vapor film to passing pressure wave is suggested as a mechanism for the oscillations during film boiling. Many numerical codes in time domain as well as in frequency domain have been developed using various mathematical modelling techniques to simulate the flow instabilities occurring in the NCSs.
The effects inwtability negative void reactivity feedback are found to stabilize the very low frequency type I instabilities [ 4344 ]. 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.
Even among two-phase systems, the NC systems are more unstable than forced circulation systems due to the above reasons. 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. In other words, a regenerative feedback is inherent in the mechanism causing NC flow due to the strong coupling between the flow and the driving force.
The film itself is not stable causing repetitive wetting and dewetting of the heating surface resulting in an oscillatory surface temperature. Science and Technology of Nuclear Installations.
With that purpose, a review of flow instabilities in boiling natural circulation systems has been carried out. Besides, classifications based on the oscillatory characteristics are sometimes reported for dynamic instability. Hence, with increase in riser flow area, the type I instability appears [ 38 ].