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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Thermal oscillations are considered as a regular feature of dryout of steam-water mixtures at high pressure [ 4 ]. Introduction Natural circulation NC systems are susceptible to several kinds of instabilities. Advantage of homogeneous flow assumption is that it is easier to apply and the model is also found to predict the stability boundary or the threshold unstability instability with reasonable accuracy.
Based on the Analysis Method or Governing Equations Used 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 number of flow excursions is seen to be five, unlike that of the Ledinegg-type instability.
With increase in power, subcooled boiling begins in an unstable single-phase system leading to the switching of flow between single-phase and two-phase regimes. The fundamental cause of this instability is that the hot liquid from the heater outlet experiences insatbility pressure decrease as it flows up and may reach its saturation value in the riser causing it to vaporize.
We note that case 3 is unstable while case 4 is stable. Abstract Several decades have been spent on the study of flow instabilities instanility boiling two-phase natural circulation systems.
For smaller riser flow area, the flow rate is smaller due to larger resistance in small riser pipes. Saha and Zuber [ 33 ] considered subcooled boiling in the drift flux model and applied the model to the stability of a natural circulation system.
Instability due to boiling inception usually disappears with increase in system pressure due to the strong influence of pressure on the void fraction and hence the density Figure 6. This may be due to the fact that with an increase in pressure, the void fraction decreases with quality ijstability in the two-phase region, which can reduce the S-shaped variation of the irreversible losses i.
Flow Instabilities in Boiling Two-Phase Natural Circulation Systems: A Review
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. To receive news and publication updates for Science and Technology of Nuclear Installations, enter your email address in the ledinegh below.
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. 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 feedback appears making another solution more attractive instablity the system to swing toward it.
Fukuda and Kobori [ 5 ] have classified the density-wave instability as type I and type II for the low lecinegg and high-power instabilities, respectively. According to Boure et al. This gives larger two-phase pressure drop insfability to large channel exit quality. 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.
The criterion for this type of instability is given by where is the instabilith pressure loss in the system and is the driving head due to buoyancy. However, both instabilities increase with rise in subcooling. View at Google Scholar M. We note that Equation 4 implies that a flow excursion will occur if.
These may be classified into static and dynamic instabilities [Lahey andPodowski ]. In a natural circulation system, the low-power type I instability increases with increase in riser height. View at Google Nistability A.
LEDINEGG INSTABILITY | HTRI
Coal and gas-fired power stations used very large boilers to produce steam for use in turbines. Thus instabilit find that the analysis to arrive at the instability threshold can be based on different sets of governing equations for different instabilities.
Thus, as the boiling boundary moves up the tube, the total pressure drop falls, potentially ledijegg the flow in an unstable manner. This instability is driven by the interaction between the single and two-phase frictional component of pressure losses, mass flow, void formation, and propagation in the ledinegv region. 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.
Differences also exist in their transport mechanism, oscillatory mode, and analysis methods. 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. 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.
However, regular flow stagnation is observed, which is of concern for the safety of nuclear reactors. Similarly, the unstable region beyond the upper threshold occurs at a high power and hence at high qualities and is named as type II instability.
The characteristics of oscillation were similar as previous cases i. Linear analyses of boiling flow instabilities in natural circulation systems with homogeneous flow assumptions have been carried out by Furutera [ 29 ], S. A typical boiling loop includes a heated channel or channelsan unheated riser, a condenser, a downcomer in which a pump may be installed and a lower plenum. The phase shift of out-of-phase oscillations OPO is known to depend on the number of parallel channels.