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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Heat exchanger topic Tubular heat exchanger A heat exchanger is a device used to transfer heat between two or more fluids.
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. According to him, geysering is expected during subcooled boiling when the slug bubble detaches from the surface and enters the riser where the water is subcooledwhere bubble growth due to static-pressure decrease and condensation can take place. Thus, as the boiling boundary moves up the tube, the total pressure drop falls, potentially increasing the flow in an unstable innstability.
The effect of riser geometry such as leinegg height and area on flow stability is important. An essential characteristic of the unstable oscillating NC systems is that as it tries to settle down to one of the solutions, a self-generated instabiliyt appears making another solution more attractive causing the system to swing toward it.
In such cases, pressurised water is passed through heated pipes and it changes to steam as it moves through the pipe. 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. Most instabilities observed in forced circulation systems are observable in natural circulation systems.
Having a large void fraction will increase the void propagation time delay in the two-phase region of the system. View at Google Scholar M. 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 a relatively stiff system, the frequency of PDO can be comparable to DWO making it difficult to distinguish between the two.
That is the reason for the reduction of type I instability with increase in drift velocity Figure Pressure-drop oscillations are associated with operation in the negative sloping portion of the pressure drop-flow curve of the system.
Over the years, several kinds of instabilities have been observed in natural circulation systems excited by different mechanisms.
For smaller riser flow area, the flow rate is smaller due to larger resistance in small riser pipes. Because of this, any disturbance in the driving force affects the flow which in turn influences the driving force leading to an oscillatory behavior even in cases where eventually a steady state is expected.
Ledinegg instability | Revolvy
It may be noted that in case of forced circulation BWRs, instabilities observed under natural instabiligy conditions are due to pump trip transients when the core exit quality is high due to low flow and high power. In view of this, more research needs to be conducted to explore the capability of existing mathematical models for prediction of these instabilities in NCSs in future. Fukuda and Kobori [ 5 ] observed two modes of oscillations in a natural circulation loop with parallel heated channels.
The oscillations can be labeled as flow excursions, pressure drop oscillations, ijstability oscillations, temperature excursions or thermal oscillations, and so on. A common characteristic of the instabilities associated with boiling inception is that single-phase conditions occur during part of the oscillation cycle. In this case, feedback effects are important in predicting the threshold compound static instability.
For any mixture quality, the void fraction is smaller for larger drift velocity. Manera, Experimental and analytical investigations on flashing-induced instabilities in natural circulation two-phase systemsPhD. Similarly, the unstable region beyond the upper threshold isntability at a high power and insfability at high qualities and is named as type II instability.
They compared the stability maps between the Ledinegg and the flow pattern transition instability Figure 8.
Science and Technology of Nuclear Installations
Introduction Natural circulation NC systems are susceptible to several kinds of instabilities. There are a number of instabilities that may occur in two-phase knstability. With an increase in riser area,the time period of oscillation reduces due to the increase in flow rate in the system.
Although instabilities are common to both forced and natural circulation systems, the latter is inherently more unstable than forced circulation systems due to more nonlinearity of the NC process and its low driving force. The parameter is a hypothetical concept called Superficial velocity. 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.
Commonly observed, static instabilities are flow excursion and boiling crisis. Instability is undesirable as sustained flow oscillations may cause forced mechanical vibration of components. View at Google Scholar A. In these cases, more than two zones of instability were oserved.
In addition,effects of loop geometry and secondary parameters also cause complications in the observed instabilities. Indexes of physics articles Revolvy Brain revolvybrain. View at Google Scholar V. Hence, the two-phase frictional pressure loss may be high owing to the smaller two-phase mixture density.