Assessment of the potential for pressure transients due to trapped air compression in rapidly filling CSO tunnels

Steven J. Wright, University of Michigan, Ann Arbor, MI, USA, Peter Klaver, LimnoTech Inc., Ann Arbor, MI, USA and Jose G. Vasconcelos, Auburn University, Auburn, AL, USA


Classical research in the form of numerical modeling has shown that quite high pressures can be expected when air that is trapped in a pipeline is compressed as it stops a moving water column.  In particular, the model results show that larger pressures are to be expected when air volumes are small.  However, the model  formulation essentially treats the moving water column as a vertical front with a trapped volume of air in front of it, an assumption that cannot be expected to hold for large diameter stormwater tunnels.  The authors have been involved in the numerical modeling of rapidly filling flows in CSO storage systems.  The Two-Component Pressure Approach can predict the location where air can become entrapped and the associated volume, but the model framework is a single phase flow simulation and so the air is not explicitly modeled.  This leads to conceptual errors in the modeling of flow processes once the air becomes entrapped.  Laboratory experiments to resolve expected flow behavior involved an experiment where an advancing pipe filling bore front was brought to rest by the compression of a trapped air pocket.  These experiments also confirm that small air volumes lead to the highest pressure rises.  The potential consequences led to a reformulation of the numerical model that allowed the inclusion of the trapped air volume in the simulation although in a simplified fashion.  Simulations with the modified model  for a specific application suggest that there are two sets of flow conditions that can lead to trapped air pockets that are subsequently compressed although future investigations may define additional conditions.  The simulation results suggest that only modest pressure rises should be expected in the particular application investigated and the physical explanation for this outcome is described.

Permanent link: