The critical condition of longitudinal emergency tunnel ventilation – Comparison of theoretical prediction with experimental data

Systematic comparison has been carried out between the experimental data with different tunnel fire configurations and the prediction by the theory presented in a previous paper by Zhang et al. The objective of such comparison is to validate the mechanism explained by the theory and its conclusions. Equally important is to identify the limitation of the theory and its applicable range in real engineering cases. The comparison has indicated that the new theory has provided a more general curve that represents the critical Froude number Fr in longitudinal tunnel fire ventilation. In terms of fire sizing, the comparison has confirmed that the two ventilation regimes, namely Thomas (cubic root) regime and Froude regime (Fr = constant), are indeed separated by the condition Q′ = 1.0. Although the original theory was based on a 2D pool fire model, the mechanism revealed is equally applicable in more general 3D tunnel fires with blockage and fire surface elevation. However significant discrepancy exists between the theoretical prediction and experimental data when both fire site aspect ratio and tunnel height/width ratio are larger than 1.0. The discrepancy is believed to be the result of energy and momentum loss through tunnel wall.

► Three sets of experimental data have been compared with the prediction from the new theory.
► The theoretical curve represents the maximum critical Fr of each fire size.
► Different ventilation regimes are identified by the condition of Q′ = 1.
► The mechanism revealed by the theory is applicable to general 3D fires with tunnel blockage and fire site elevation.
► Energy and momentum loss through tunnel wall needs further investigation.