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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With increase in inlet throttling coefficient for same heating power, the channel flow rate decreases, which in turn causes an knstability in channel exit quality. Manera, Experimental and analytical investigations on flashing-induced instabilities in natural circulation two-phase systemsPhD. A detailed description of the analytical procedure has been given previously by Lahey and Podowski and will not be repeated here.
The adoption of drift flux model allows to replace two separated momentum equations for liquid and vapor as used in the rigorous two-fluid models, by one momentum equation for the mixture plus a nondifferential constitutive law for the relative velocity.
Acoustic oscillations have been observed in subcooled boiling, bulk boiling, and film boiling. The characteristics of the instabilities are different from one to the other due to the differences in the physical mechanism associated with their initiations. All instabilities eventually ledinefg to some kind of oscillations.
In general, instabilities can be classified according to various bases as follows: Heat exchanger topic Tubular heat exchanger A heat exchanger is a device used to transfer heat between two or more fluids.
They found that consideration of thermal nonequilibrium condition results in a more stable system at low subcooling and a more unstable system at high subcooling as compared to the thermal equilibrium model.
Nonetheless, this introduction has given the essence of two-phase instability analysis and presented some of the key references so that the interested readers may develop a more indepth understanding. 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. Access to Document For smaller riser height, lesser is the channel flow rate and larger is the channel exit quality for same heating power.
In a linear stability analysis, the mass and energy conservation equations may be integrated in the axial direction either analytically [Lahey and Moody ] or using nodal techniques [Taleyarkhan et al. 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.
Besides, due to the presence of tall instabilitj 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.
Hence, the two-phase frictional pressure loss may be high owing to the smaller two-phase mixture density. The mechanism as proposed by Aritomi et al. A stable single-phase NCS can become unstable with the instbility of boiling. Also inlet throttling between the surge tank and the boiling channel is found to stabilize PDO just as Ledinegg instability.
This classification is actually restricted to only the dynamic instabilities. Examples and applications Historically, probably the most commonly studied cases of two-phase flow are in large-scale power instavility. The steep variation in heat transfer coefficient typical of transition boiling conditions in a post CHF scenario can get coupled with the DWO. Two fluids, water at sub-atmospheric pressures and refrigerant HFE, were examined for a range of heat fluxes, mass fluxes, and channel hydraulic diameters.
The number of flow excursions is seen to be five, unlike that of the Ledinegg-type instability. Abstract Several decades have been spent on the study of flow instabilities in boiling two-phase natural insstability systems. With increase in drift velocity, the vapor propagation time lag in two-phase region reduces, which has a stabilizing effect. The mechanism of instabilities occurring in two-phase natural circulation systems have been explained based on these classifications.
They found that with an increase in void distribution parameter, the stability of boiling channel increases. Depending on the operating conditions, the oscillations can be periodic or chaotic.
We note that Equation 4 implies that a flow excursion will occur if. Gas technologies Revolvy Brain revolvybrain Heat Exchangers jsmanser. Linear stability analysis using a four-equation drift flux model has been carried out by Ishii and Zuber [ 32 ], Saha and Zuber [ 33 ], Park et al.
Ledinegg instability | Revolvy
Usually, instabikity homogeneous model predicts a larger void fraction than the two-fluid model for the same mixture quality due to the absence of slip between the water and steam in this model. Introduction Natural circulation NC systems are susceptible to several kinds of instabilities. View at Google Scholar A.
However, feedback effects also are paramount in the phenomena.
On the other hand, during flashing instability, the vapor is in thermal equilibrium with the surrounding water and they do not condense during the process of oscillation.
This reduces the buoyancy force and the flow, increasing the exit enthalpy resulting in boiling and leading to the repetition of the process. Science and Technology of Nuclear Installations.
In boiling NC systems with multiple parallel channels, inphase and out of phase modes are present depending on the geometry of instabiilty channels and heating conditions. However, the type II instability, which occurs at high power or void fraction, disappears with increase in riser diameter [ 38 oedinegg due to reduction in void fraction or decrease in two-phase pressure drop.
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; instabilty a static instability characterized by periodic exit large bubble formation; iii a steady flow with continuous leinegg 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.
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. Experimental investigations in two-phase natural circulation loops having single boiling heated channel have been carried out by ledniegg 131724 — 27 ].