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CFD Analysis on Critical Velocity of Longitudinal Ventilation for Super Large Horizontal Tunnel

XU Lin,ZHANG Xu(HVAC & GAS Engineering Institute,Tongji University,Shanghai 200092,P.R.China)  
The longitudinal air velocity required to prevent back-layering is often called the "critical velocity".So far its values for various tunnels have been obtained mainly from sets of equations derived by applying Froude number preservation,combined with some experimental data.In virtue of the discrepancies between the Kennedy model and Atkinson experimental data, the study addressed the problem of critical ventilation in large cross sectional horizontal tunnel.Especially the leveling-off of critical velocity for large values of heat release rate was analyzed.The CFD results implied that within the variation of the ambient temperature studied,the ambient temperature has negligible effect on the critical velocity.It was also shown clearly that there are two regimes of variation of critical velocity against fire heat release rate.At low heat release rates,the critical velocity varies just like Kennedy model,however once the heat release rate surpasses 40MW,the critical velocity becomes independent of fire heat release rate.Furthermore the study used dimensionless velocity and dimensionless heat release rate with the tunnel hydraulic height as the characteristic length in the data analysis.And a good agreement has been obtained with the results of both reduced-scale tunnel and the full-scale colliery tunnel.
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