Numerical studies on explosion hazards of vehicles using clean fuel in short vehicular tunnels

Highlights

• Heavy traffic load in short vehicular tunnels (SVT) in densely populated cities.

• Concern for fire safety of SVT due to increasing use of clean fuels in vehicles.

• Explosion hazard of a taxi using liquefied petroleum gas (LPG) in a SVT was studied.

• LPG leaking into the concealed space beneath tunnel floor level of SVT was simulated.

• Appropriate protection measures should be provided to ensure the safety of SVT.

Abstract

Short vehicular tunnels (SVT) with heavy traffic are commonly constructed in urban areas of densely populated cities of the Asia-Oceania Region. The introduction of vehicles using clean fuel poses new concerns on the fire safety of existing road tunnels. The explosion of a taxi using Liquefied Petroleum Gas (LPG) occurred in a garage, alarming the need for protecting existing road tunnels against hazards in using clean energy.

In this paper, explosion hazards of an LPG taxi in a section of SVT were studied by using Computational Fluid Dynamics (CFD) Flame Acceleration Simulator (FLACS). The scenario of LPG leaking into the concealed space beneath the tunnel floor level of SVT through vents at the road side is simulated. A full tank of LPG would completely be released in about 231 s. The leaked LPG in the concealed space is ignited 19 s after the complete release, i.e. at t = 250 s. Predicted results on explosion pressure and temperature illustrate that appropriate protection measures should be provided to ensure the safety of SVT. Results also suggest that the existing fire safety codes for SVT should be revised by including an explosion analysis of LPG taxis.

Introduction

Short vehicular tunnels (SVT) are constructed to solve the heavy traffic problem in densely populated cities of the Asia-Oceania Region (Chow, 2020). Some of them are regarded as Urban Traffic Link Tunnels (with low speed limits; say 20 km/h) or urban tunnels (with higher speed limits) (Li et al., 2019). In dense urban areas like Hong Kong, SVTs have large traffic volume with annual average daily traffic (AADT) up to 131,220 (The Annual Traffic Census, 2017) (Fig. 1). In addition to challenges on the ventilation systems (hk01.com) inside to provide better indoor air quality, big fire hazards inside SVT is a concern.

Other than collisions, engine overheating, mechanical and electrical failure, fire hazards of vehicles have been documented or have raised deep concerns:

• Vehicles burned in a tunnel (Chow and Li, 2001).

• Air-conditioned double-deck buses with good thermal insulation envelopes burned completely within 15 min once on fire (Chow, 2001). Another one occurred recently next to a footbridge (South China Morning Post, 2019).

• Burning Heavy Goods Vehicle (HGV) would give a heat release rate over 100 MW (NFPA 502 Standard, Chow, 2018). Several HGVs burning together during traffic jam would give much more hazardous consequences. Several vehicular bridges collapsed while HGVs were burning on them (The News Times, 2018, The Guardian, 2018).

• Taxis using LPG and electric cars can also burn to give big explosions (Huo and Chow, 2017, Ng et al., 2017, Chow et al., 2016, Cheng et al., 2016, Cheng et al., 2018).

• Flammable clean refrigerants in air-conditioning systems of vehicles were suspected to have explosions (Chow et al., 2017, Kujak, 2017, Chow and Ng, 2016, Ng and Chow, 2014a, Ng and Chow, 2015b, Ng and Chow, 2015c).

• Fire might spread between vehicles to give larger fire size when the SVT is under heavy traffic congestion.

Therefore, fire in SVT can be very hazardous with great challenges to firefighting. Burning of vehicles inside SVT brings hazards not only to people staying inside, but also to the firefighters. Protection of SVTs against big fires must be provided adequately. However, the detailed nature of fires in such underground or semi-enclosed spaces is not clearly understood.

Fire codes for vehicular tunnels have already been updated in some places (NFPA 502 Standard). However, there is no restriction on the conveyance of Category 2 and Category 5 dangerous goods in the SVT in Hong Kong with heavy traffic volumes as other highway tunnels (Chow, 2020). The potential fire hazards of burning HGVs inside SVT (Chow, 2018) can be very serious. The fire resistance of a tunnel is normally studied under some standard temperature–time curves, not yet deduced from burning HGV with 100 MW in heat release rate. Big problems would be encountered in firefighting and rescue in SVT, risking lives of the firefighters and rescue crew.

Taking Hong Kong as an example, the fire safety provisions in existing tunnels were designed by following the regulation and codes at the time of construction, which might not be able to cope with newly emerging risks nowadays (Tam et al., 2015).

The quick advancement of vehicle technology has induced epoch changes to the fuel system of vehicles. Coming along with the popularities of vehicles powered by alternative fuels, like hydrogen, compressed natural gas (CNG) and LPG, their safety issues have also become the focus of researchers (Van den Schoor et al., 2013, Astbury, 2008, Bauwens et al., 2011, Crow and Jo, 2007, Hansen et al., 2010, Jo and Kim, 2001, Jo and Park, 2004, Prasad and Yang, 2011, Rodionov et al., 2011, Shirvill et al., 2012, Venetsanos et al., 2008). Findings suggested that the fire behavior of vehicles powered by clean fuel would be different from an ordinary gasoline-fueled vehicle in tunnels and underground spaces. In general, the fire size involving compressed gas would be larger than a traditional car and the formation of fireball is a common phenomenon in a fire involving a ruptured compressed gas tank (Li, 2019). In some circumstances, the consequence of fire involving vehicle powered by clean fuel would be more severe than a gasoline-fueled vehicle fire inside a confined space like a tunnel (Li, 2019, Rigas and Amyotte, 2018, Swain, 2001, Wu, 2008, Gehandler et al., 2017).

In this paper, the explosion analysis of an LPG taxi inside a section of SVT is used to demonstrate that an old design may not be able to effectively handle the new fire hazards. Since Computational Fluid Dynamics (CFD) is a useful tool to simulate the fire and leakage situation of clean fuel vehicles in confined spaces like road tunnels (Houf et al., 2012, Middha and Hansen, 2009, Mukai et al., 2005, Merilo et al., 2011, Venetsanos et al., 2008, Ismail et al., 2012), the explosion hazards of LPG taxi in a section of SVT are studied by using a commercial CFD code, namely Flame Acceleration Simulator (FLACS).