Blowdown choked flow

As the gas-liquid mixture travels down the vent line, the phases will slip past each other and the fluids will accelerate. This contribution to the energy balance can be most significant for high pressure blowdown. Pressure increments are calculated and when the pressure gradient becomes infinite the flow is choked. If this occurs at the end of the pipe the assumed flowrate is the converged choked flow solution. If choked flow does not occur and the end of the line is reached at the reservoir pressure, the non-choked flow solution is obtained. 

The main steam line flow limiter, a flow restricting venturi built into the RPV MSL nozzle of each of the two main steam lines, limits the coolant blowdown rate from the reactor vessel to a (choke) flow rate equal to or less than 200% of rated steam flow at 7.07 MPa (1025 psig) upstream gauge pressure in the event a main steam-line break occurs anywhere downstream of the nozzle. 

There are several instances where choked flow rates are required to be estimated. Typical examples are overpressuring drain lines due to gas breakthrough, overpressuring flare lines during blowdown, etc. The general equation of compressible fluid flow [6] is... 

During the quasi-steady phase (about 100 ms) the base and exit pressures are nearly constant (Fig. 3). Indeed, the pressure in these experiments is much more constant than in many so-called blowdown experiments, in which nucleation and bubble growth typically occur in the bulk of the liquid (see, e,g.. Winters and Merte [8]). The difference between p ase PexU is the wave amplitude , in this case 0.53 bar. In the steady state the upstream pressure is approximately maintained by the thrust of rapid vaporization. The fact that the exit pressure is greater than the reservoir pressure (namely, 0) indicates that the flow is choked. The instantaneous pressure fluctuates with frequency components between 0 and about 2 kHz the strongest peaks are below 250 Hz. The rms pressure fluctuations are approximately 3% of the mean pressure for both and... 

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