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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For the two-phase flow density-wave instability, the unstable region below the lower threshold occurs at a low power and hence at low quality and is named as type I instability by Fukuda and Kobori [ 5 ]. Subscribe to Table of Contents Alerts. Hence thermal oscillations are characterized by large amplitude surface temperature oscillations due to the large variation in the heat transfer coefficient. However, in certain cases depending on the geometry and operating conditions, islands of instability have been observed to occur [ 8 — 10 ].
The existence of multiple flow rates as a particular operating power and subcooling makes the system unstable. Instead, the system swings from one solution to the other. 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.
Boiling inception is a static phenomenon that can lead to instability in low-pressure systems. Instabillty system pressure and channel diameter, reducing parallel channel number and channel length, and including an inlet restrictor can enhance the flow stability in microchannels.
International Journal of Heat and Mass Transfer. Moreover, from these results it is interpreted that the homogeneous model for void fraction, which leinegg a zero drift velocity and unity void distribution parameter, predicts the most unstable region as compared to the slip models.
Different modes of two-phase flows. In natural circulation loops, flow direction can also change during oscillations. Pressure-drop oscillations are associated with operation in the negative sloping portion of the pressure drop-flow curve of the system. This depends on the relative importance instabipity the respective components of pressure drop such as gravity or frictional losses in the system.
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The necessary condition for flashing is that the fluid temperature at the inlet of the instabiliity is greater than the saturated one at the exit [ 17 ].
Tubular heat exchanger A heat exchanger is a device used to transfer heat between two or more fluids.
That is the reason for the reduction of type I instability with increase in drift velocity Figure Numerical predictions from the developed pressure gradient model agree well with results from the flow boiling experiments.
International Journal of Heat and Mass Transfer52 Besides, many NCSs with only a unique steady-state solution can also become unstable during the approach to the steady state due to the appearance of competing multiple solutions due to the inertia and feedback effects pure dynamic instability.
It can be observed from Figure 3 that there can be multiple steady-state flow rates point at which the driving head intersects the internal loss curve at this operating condition. However, natural circulation systems are more unstable due to the regenerative feedback inherent in the mechanism causing the flow. 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 causing the system to swing toward it.
The flow is found to reverse even before boiling ledinsgg initiated Figure 4. Again, during the process of settling down on this solution, another feedback of opposite sign favoring the original solution is self-generated and the leeinegg swings back to it. Abstract The static Ledinegg instability in horizontal microchannels under different flow conditions and fluids pertinent to electronics cooling was studied experimentally and numerically.
Ledinegg instability in microchannels. Fluid dynamics Revolvy Brain revolvybrain. 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. The process repeats itself resulting in perpetual oscillatory behavior if the operating conditions are maintained ledijegg.
Experimentally, both inphase and out of phase oscillations are observed in parallel channels. Besides, classifications based on the oscillatory characteristics are sometimes reported for dynamic instability. Some of these flow regimes are stable while others are unstable. As the flow area is increased, the flow rate increases, which gives rise to small frequency oscillations, typical of low quality type I density-wave instability Figure 19 due instabiility reduction in void fraction.
This is true both for type I and type II instabilities. The instability is characterized by oscillatory behavior and gets suppressed with rise in instabklity [ 1819 ]. Another type instability which can occur in systems with a compressible volume a pressurizer for example at the inlet of the heated channel is the pressure-drop-type instability. Typical flow pattern transition instability in boiling natural circulation systems.
Such switching is often accompanied by period doubling, tripling, or n-tupling. Further, even in steady state case, especially for two-phase systems with slug flow, small amplitude oscillations are visible. Instability is undesirable as sustained flow oscillations may cause forced mechanical vibration of components.
Such an instability can occur at very low-power and at high-power conditions. In general, instabilities can be classified according to various bases as follows: These models are based on kinematic formulation which considers the problem of mechanical nonequilibrium between the phases by having a relationship between the quality and void fraction through superficial velocities of liquid and vapor phases, vapor drift velocity, and void distribution parameter.
Introduction Natural circulation NC systems are susceptible to several kinds of instabilities.
Ledinegg instability in microchannels — Arizona State University
Larger the riser height, larger is the channel flow rate which may cause larger two-phase pressure drop due to larger riser length. Shell and tube heat exchang In these cases, more than two zones of instability were oserved.
Acoustic oscillations are also observed during ledimegg experiments with pressurized hot-water systems possibly due to multiple wave reflections. 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.
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. Advantage of homogeneous flow assumption is that it is easier to apply and the model is also found to instabioity the stability boundary or the threshold of instability with reasonable accuracy.
In any two-phase system, both types of waves are present, however, their velocities differ by one or two orders of magnitude allowing us to distinguish between the two. 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.
Cold water injection can also cause a major disturbance and instability in natural circulation systems.