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Experimental Study of Sub-critical Velocity in Longitudinally Ventilated Tunnels

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Abstract

The critical velocity is the minimum velocity blowing towards the fire seat in tunnels to reduce the length of back-layering to zero, and it has been extensively adopted in the design of longitudinal smoke control in tunnels. A new concept, i.e., sub-critical velocity, is proposed in lieu of critical velocity for longitudinally ventilated tunnels. The sub-critical velocity allows the presence of back-layering upstream of the fire, provided that the back-layering is kept above the heads of motorists, thus posing no detrimental impact on them during evacuation. Experiments with propane fires with heat release rates of 3.7–25.5 kW were conducted in a 1/20 reduced-scale model tunnel. Both the lengths and the heights of back-layering at varying HRRs and longitudinal velocities were studied. Subsequently, the relationship of critical velocity and sub-critical velocity were developed. The results showed that sub-critical velocity is around 45–60% of the critical velocity. The adoption of sub-critical velocity in the design of longitudinally ventilated tunnels not only significantly reduces the cost of fans to drive the airflow, but also minimizes the impact of ventilation on heat release rates.

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Abbreviations

\(A\) :

Tunnel cross-sectional area (m2)

\(C_{p}\) :

Thermal capacity of air (kJ/kg·K)

\(g\) :

Gravitational acceleration = 9.81 (m/s2)

\(H\) :

Tunnel height (m)

\(\overline{H}\) :

Hydraulic diameter (m)

\(H_{1}\) :

The height from neutral plane of carriage opening to the ceiling

\(L_{\Delta T}\) :

Back-layering length obtained by thermocouple (m)

\(l\) :

Length (m)

\(\rho_{0}\) :

Ambient density (kg/m3)

\(q^{\prime \prime }\) :

Radiant heat flux (kW/m2)

\(Q\) :

Heat release rate, HRR (kW)

\(R_{0}\) :

Distance from the target to the fire seat (m)

\(\upzeta\) :

Sectional coefficient, \(\frac{A}{{H^{2} }}\)

\(\upvarphi\) :

Cross sectional coefficient, \(\frac{W}{H}\)

\(\chi_{r}\) :

Fraction of total energy radiated

\(T_{0}\) :

Ambient temperature (K)

\(V\) :

Longitudinal ventilation velocity (m/s)

\(V_{C}\) :

Critical velocity (m/s)

\(V_{conf}\) :

Confinement velocity (m/s)

\(V_{tran}\) :

Transition velocity (m/s)

\(V_{{\text{Sub - critical}}}^{\prime \prime }\) :

Dimensionless sub-critical velocity

\(W\) :

Tunnel width (m)

*:

Dimensionless parameter (tunnel height used as the characteristic length)

′′:

Dimensionless parameter (hydraulic diameter used as the characteristic length)

\(F\) :

Full scale

\(L\) :

Back-layering obtained by laser sheet

\(M\) :

Model scale

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Acknowledgements

The authors greatly appreciated the comments from anonymous reviewers, and the financial support from National Natural Science Foundation of China (No. 71974161).

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Authors and Affiliations

  1. Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, People’s Republic of China

    Dongli Gao

  2. Department of Fire Safety Engineering, Southwest Jiaotong University, Chengdu, 610031, People’s Republic of China

    Dongli Gao, Xingchao Chen, Xinyu Yang & Peng Lin

  3. Department of Civil Engineering, Faculty of Science and Engineering, Chu Hai College of Higher Education, Hong Kong, People’s Republic of China

    Y. O. Tong

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  1. Dongli Gao
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  2. Xingchao Chen
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  5. Peng Lin
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Correspondence to Peng Lin.

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Gao, D., Chen, X., Yang, X. et al. Experimental Study of Sub-critical Velocity in Longitudinally Ventilated Tunnels. Fire Technol 58, 571–590 (2022). https://doi.org/10.1007/s10694-021-01160-8

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  • DOI: https://doi.org/10.1007/s10694-021-01160-8

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