Analytical solution, experimental data and CFD simulation for longitudinal tunnel fire ventilation

• An integral solution for tunnel fire ventilation has been presented.
• The solution shows good agreement with experimental data.
• CFD simulation may predict the trend inconsistent with both theory and experiment.
• The effect of tunnel wall on fire flame requires further study.
• Unverified CFD simulation can be a safety concern.

A new integral theory for tunnel fire under longitudinal ventilation has been presented. Its solution on critical velocity has been compared with experimental data and the results of CFD simulation from two different computer programs. The exercise of cross examination is not only aimed at further verification of the new theory but also to reveal any problem in all three kinds of data being compared, particularly the pitfalls that may exist in CFD simulation. The comparison has shown that the general agreement among all three kinds of data is satisfactory. Both theoretical and CFD predictions have confirmed the trend of variation for critical velocity versus fire size shown in the experimental data. However the CFD prediction from FDS program for a narrow tunnel has failed to conform to the same trend as that in the theory and experimental data. Considering similar FDS result in comparable condition previously published by another researcher, the authors of the current article believe that CFD simulation results for tunnel fire need to be more closely scrutinized. The simulated result may not only contain numerical error but also go way out of trend and difficult to be physically interpreted. Discrepancy between the current theory and experimental data in some cases is believed due to flame heat loss that has not been accurately predicted by the theory.