Ledinegg instability

Simple static instability. Flow excursion (Ledinegg instability) involves a sudden change in the flow rate to a lower value. It occurs when the slope of the channel demand pressure drop-versus-flow rate curve (internal characteristic of the channel) becomes algebraically smaller than the loop supply pressure drop-versus-flow rate curve (external characteristic of the channel). The criterion for this first-order instability is... 

Analysis of flow excursion The threshold of flow excursion can be predicted by evaluating the Ledinegg instability criterion in a flow system or a loop, Eq. (6-1),... 

Bergles and Kandlikar [5] reviewed the existing studies on critical heat flux in microchannels. They concluded by saying that few single-tube CHF data were available for microchannels at the time of their review. For the case of parallel multi-microchannels, they noted that all the available CHF data at that time were taken under unstable conditions, where the critical condition was reached as the result of a compressible volume instability upstream or the excursive Ledinegg instability. As a result, the unstable CHF values reported in the literature were expected to be lower than they would be if the channel flow were kept stable by an inlet restriction. 

Static Ledinegg instability Flow undergoes sudden, large-amplitude excursion to a new stable operating condition [3,7]... 

Static instabilities induce a shift of the equilibrium point to a new steady-state point Ledinegg instability, boiling crisis, bumping, geysering, or chugging are all static instabilities since they can be analyzed using only stationary models. 

Ledinegg instability can be avoided by making the slope of the external characteristics steeper than that of the internal characteristics by providing inlet throttle valve (Boure et al., 1973). 

The onset of Ledinegg instability may sometimes cause wall dryout, but often lies very close to the dryout point because the onset may be followed by limit cycle fluctuations in the flow. 

Characteristic pressure drop vs. flow rate instabilities Ledinegg instability Flow distribution instabrbty Flow pattern transition Pressure Drop Oscillation (PDO)... 

Ruspini, L.C., Dorao, C.A., Femandino, M., 2010. Dynamic simulations of Ledinegg instability. Journal of Natural Gas Science and Engineering 2, 211—216. 

Pressure drop oscillations (Maulbetsch and Griffith, 1965) is the name given the instability mode in which Ledinegg-type stability and a compressible volume in the boiling system interact to produce a fairly low-frequency (0.1 Hz) oscillation. Although this instability is normally not a problem in modern BWRs, care frequently must be exercised to avoid its occurrence in natural-circulation loops or in downflow channels. 

Check the system (or loop) instability by using the Ledinegg criterion with an average lumped channel pressure drop. If it does not satisfy the Ledinegg stability criterion, one or more of the three remedies can be taken orifice the inlet, increase the steepness of the pump head-versus-flow curve or increase the resistance of the downcomer of a natural-circulation loop. 

Ledinegg, M., 1938, Instability of Flow during Natural and Forced Circulation, Die Warme 61 8, AEC-tr-1861 (1954). (2)... 

The static limit is the non-linear limit of conditionally instability, where departure from nucleate boiling or critical heat flux will occur at low and high qualities, respectively. There are sufficient data in the literature which show that instability in multiple channels precedes the limit of classic single channel (mass-flow controlled) dryout (Mathison, 1967) (D Arcy, 1967). This differs from the result for the zero frequency condition, which can only be written as a cubic in, (Ns / Np), and does not give a critical subcooling number. The condition of static instability in parallel channels is the Ledinegg condition (Saha et al, 1976) (Duffey and Hughes, 1991),... 

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