Asymmetric flow effect in a horizontal natural ventilated tunnel with different aspect ratios under the influence of lon
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Asymmetric flow effect in a horizontal natural ventilated tunnel with different aspect ratios under the influence of longitudinal fire locations
1. State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, Anhui 230026, China 2.College of Environment and Resources, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China
Abstract
Keywords
This paper has analyzed the asymmetric flow effect of fire-induced thermal flow in a horizontal tunnel under the natural ventilation condition by conducting large eddy simulations (LES). The key objective is to reveal and to have a better understanding of the asymmetric flow effect caused by the upstream and downstream tunnel length difference. The mechanism behind it can be explained based on the conservation of mass and dynamic force analysis on the smoke and fresh air. The strength of the asymmetric flow effect is characterized by the mass flow rate of the induced longitudinal flow (net mass flow rate of a cross-section). An empirical correlation to predict the induced longitudinal mass flow rate is proposed. Furthermore, the law of smoke and air flow distribution within a horizontal tunnel is established. The proportion of smoke (or air) flowing out (or coming in) through the opening increase (or decrease) linearly with the increasing distance between that opening to the fire location. The variation of the air flow with the longitudinal fire location in a tunnel is more sensitive than the smoke flow. Results have shown that as the fire approaches the tunnel exit from the middle of the tunnel, the smoke spilling out through this opening is reduced from 50% to 40%, while the fresh air incoming from this opening is increased from 50% to 100% and vice versa.
asymmetric flow effect,
1
Introduction
E-mail: [email protected]
fire location, thermal flow, smoke, tunnel fire, CFD
Article History Received: 19 June 2020 Revised: 25 September 2020 Accepted: 29 October 2020 © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
smoke flow transport characteristics. Extensive researches on transportation of smoke flow have been reported and some quantitative models were proposed to estimate the smoke production rate, which are aimed to provide methods in calculating the smoke extraction rate. Based on the ideal plume theory, Zukoski et al. (1981) carried out experiments to measure the smoke mass flow rate of a fire plume in the open space and proposed a classical model to predict the smoke mass flow rate. In view of this idea, Li et al. (2011) conducted a theoretical analysis and proposed a model for the mass flow rate of the fire plume in a ventilated tunnel. In addition, Yang et al. (2011) conducted small-scale experiments in a reduced scale corridor to measure the velocity and the smoke layer thickness and obtained the volume flow rate of smoke at different heat release rates (HRRs). More recently, Ji et al. (2018) studied the effect of ambient pressure on transport characteristics
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