LEDINEGG INSTABILITY PDF
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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Ledinegg instability in microchannels
Shell and tube heat exchang The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. 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.
Also inlet throttling between the surge tank and the boiling channel is found to stabilize PDO just as Ledinegg instability. In two-phase flow, the disturbances can be transported by two different kinds of waves: 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.
Introduction Natural circulation NC systems are susceptible to several kinds of instabilities. In some cases, the occurrence of multiple solutions and the instability threshold itself can be predicted from the steady-state equations governing the process instabiluty or fundamental static instability.
In a natural circulation system, the low-power type I instability increases with increase in riser height. The large variations in the heat transfer coefficient and the surface temperature causes significant variation in the heat transfer rate to the fluid even if the wall heat generation rate is constant. Of these instability modes, the most important, and most widely studied, have been Ledinegg instabilities [Ledinegg ] and density-wave oscillations DWOs.
A stable single-phase NCS can become unstable with the inception of boiling. An increase in drift velocity is also found to reduce the unstable region in the type II instability observed in Figures 14 to This may reduce the threshold power for instability for that channel which may cause the other channel to be unstable.
Moreover, from these results it is interpreted that the homogeneous model for void fraction, which considers a zero drift velocity and unity void distribution parameter, predicts the most unstable region as compared to the slip models.
The increased driving force generated by the vaporization, increases the flow rate leading to reduced exit temperature and suppression of flashing. In this case, feedback effects are important in predicting the threshold compound static instability. There are limited studies on the excursive instability behavior of a parallel downward flow system Babelli and Ishii Increasing the exit enthalpy and eventually leading to subcooled boiling again and repetition of the process.
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. Almost all the theoretical and experimental studies agree well that the DWI can be suppressed in boiling two-phase NC systems by increasing the system pressure. Very long test sections may have sufficient internal compressibility to initiate pressure drop oscillations. The characteristics of oscillation were similar as previous cases i.
Sometimes, dual oscillations also are possible. However, in certain cases depending on the geometry and operating conditions, islands of instability have been observed to occur [ 8 — 10 ].
The mechanism of instabilities occurring in two-phase natural circulation systems have been explained based on these classifications.
For a given mass flux J through the tube, the pressure drop per unit length which typically varies as the square of the mass flux and inversely as the density i. The mechanism as proposed by Aritomi et al. Thus we find that the analysis to arrive at the instability threshold can be based on different sets of governing equations for different instabilities.
Fukuda and Kobori [ 5 ] observed two modes of oscillations in a natural circulation loop with parallel heated instabipity. According to him, geysering instabiligy 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.
There are a number of instabilities that may occur in two-phase systems. International Journal of Heat and Mass Transfer52 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. We note that Equation 4 implies that a flow excursion will occur if. AB – The static Ledinegg instability in horizontal microchannels under different flow conditions and fluids pertinent to electronics cooling was studied experimentally and numerically.
Similarly, any slight disturbance causing the flow rate to decrease will shift the operating point to B and then to point A.