Road Tunnels Operation: Effectiveness of Emergency Teams as a Risk Mitigation Measure

Managing a major event in a road tunnel requires more resources than an open-air event. In the case of fire, the confined environment of road tunnels can represent a critical situation for both users and rescuers. The safety level of a tunnel can be estimated by using dedicated risk models that consider, on the one hand, the traffic (type, quantity and distribution) of a tunnel and, on the other hand, the structural and plant safety measures. According to the European Directive, road tunnel managers can adopt additional safety measures aimed at increasing the level of safety for users exposed to the consequences of an accidental event. One of these measures is the rapid intervention of emergency teams located in the proximity of the tunnel. These teams use pick-up and scooter vehicles properly equipped to cope with a fire event and have detailed knowledge of the specific tunnel system. A further advantage of the emergency teams is the possibility of supporting the evacuation of tunnel users by providing indications on emergency exits, bypasses and safe places considering the evolution of the specific event. In this perspective, the present research contributes to the evaluation of the emergency teams’ effectiveness. Thus, the emergency team was included as a safety measure within a risk analysis model for road tunnels developed by the authors in previous works. After an analysis of the technical and scientific literature, we focused on 15 interventions carried out on some highway tunnels in Italy between the year 2019 and the year 2021. The intervention times of the teams were analyzed using data provided by Strada dei Parchi S.p.A., a company that manages 14 highway tunnels in Italy. These 14 tunnels range in length from 589 m to 10,121 m and are subject to the European Directive. The observed intervention times of the emergency teams range between 2 min and 10 min with an average value of 5.9 min. Such a short intervention time is possible because emergency teams are in the proximity of the different tunnels. Because of the short intervention time and the training of the personnel, all the fires were properly managed by the teams. Furthermore, considering the results of the scientific literature and the data presented in this work, it was possible to estimate and validate an effectiveness value (higher than 90%) of the emergency teams to be used within the risk analysis model developed by the authors and which can also be used in other risk analysis models

A CFD model for biomass flame-combustion analysis

The present work addresses the study of the combustion of individual biomass particle surrounded by a gas stream of N2/O2 under the operating conditions encountered in a drop tube reactor. The aim of this analysis is to give a better insight into the chemical and physical processes that occur both at particle and reactor scale where the volatiles, generated by the biomass pyrolysis, burn in a fuel particle enveloped flame. A comprehensive CFD model was developed within the open-source OpenFOAM® framework in order to properly handle the computational mesh and the discretization of the characteristic governing equations. At the reactor scale, the reactive flow was described by the equations for continuous, multicomponent, compressible and thermally-perfect mixtures of gases. At the particle scale, instead, the solid particle was considered as a porous media with isotropic and uniform morphological properties

Towards a better understanding of the combustion of oxygenated aromatic hydrocarbons. Comparing benzene, toluene, phenol and anisole with ignition delay times in a rapid compression machine

Ignition delay times (IDTs) of the oxygenated aromatic hydrocarbons (OAHCs) anisole (C6H5OCH3) and phenol (C6H5OH) and the analogues non-oxygenated aromatic hydrocarbons (AHCs) toluene (C6H5CH3) and benzene (C6H6) have been measured in the PCFC rapid compression machine (RCM) at stoichiometric, fuel-in-air conditions. With the two targeted compression pressures () of 1 and 2 MPa a temperature range of 870 to 1100 K was covered. The IDTs of all four molecules revealed an Arrhenius behavior. The different reactivity can be ranked as the following, starting with the lowest reactivity: benzene < toluene < phenol < anisole. Literature available models containing anisole and phenol have been used to simulate the IDTs of this study highlighting discrepancies in both, model to experiment and model to model accordance. Finally, the CRECK mechanism was used to conduct rate-of-production (ROP) and sensitivity analysis to gain insight into the combustion of OAHCs and highlight interconnections and shortcomings of OAHCs

DropletSMOKE++: A comprehensive multiphase CFD framework for the evaporation of multidimensional fuel droplets

This paper aims at presenting the DropletSMOKE++ solver, a comprehensive multidimensional computational framework for the evaporation of fuel droplets, under the influence of a gravity field and an external fluid flow. The Volume Of Fluid (VOF) methodology is adopted to dynamically track the interface, coupled with the solution of energy and species equations. The evaporation rate is directly evaluated based on the vapor concentration gradient at the phase boundary, with no need of semi-empirical evaporation sub-models. The strong surface tension forces often prevent to model small droplets evaporation, because of the presence of parasitic currents. In this work we by-pass the problem, eliminating surface tension and introducing a centripetal force toward the center of the droplet. This expedient represents a major novelty of this work, which allows to numerically hang a droplet on a fiber in normal gravity conditions without modeling surface tension. Parasitic currents are completely suppressed, allowing to accurately model the evaporation process whatever the droplet size. DropletSMOKE++ shows an excellent agreement with the experimental data in a wide range of operating conditions, for various fuels and initial droplet diameters, both in natural and forced convection. The comparison with the same cases modeled in microgravity conditions highlights the impact of an external fluid flow on the evaporation mechanism, especially at high pressures. Non-ideal thermodynamics for phase-equilibrium is included to correctly capture evaporation rates at high pressures, otherwise not well predicted by an ideal gas assumption. Finally, the presence of flow circulation in the liquid phase is discussed, as well as its influence on the internal temperature field. DropletSMOKE++ will be released as an open-source code, open to contributions from the scientific community

TRAM: a New Quantitative Methodology for Tunnel Risk Analysis

The paper illustrates and describes the structure of a new quantitative model of risk analysis for road tunnels named TRAM (Tunnel Risk Analysis Model). The result of the model, in accordance with the European Directive and the Italian Legislative Decree, returns the F-N curves of societal risk, in other words functions that relate the frequency of occurrence of an accidental scenario (F) with the expected consequences in terms of potential victims (N). Starting from two types of initial events, a fire and a Dangerous Goods (DG) release, a total of 18 accidental scenarios was defined. The frequencies of occurrence of each accidental scenario is obtained using the Event Tree Analysis (ETA) technique. For each scenario, the number of fatalities, expressed in terms of deaths, is obtained by simulating the formation dynamics of the queue of vehicles, using a model able to calculate the queue length, depending on traffic, the vehicle type, as well as the closure time of the tunnel. Then, a distribution model of the potentially exposed users has been defined and coupled with an egress model. The users’ tenability is estimated on the basis of the egress model and the evolution of each accidental scenario, which is evaluated using a zone model. The proposed model can simulate each of the 18 accidental scenarios in several different positions along the tunnel, considering the impact that different tunnel infrastructure measures, equipment and management procedures can have on the users egress and on the propagation of the effects of the accidental scenarios. The model is able to consider the interdependence between these measures and their reliability in terms of their availability in an emergency situation. Finally, to validate the model, comparisons are made with the QRAM software developed by PIARC for some representative case studies. Through this model, it is possible to perform the risk analysis of a tunnel in an actual configuration and compare the expected value of damage with the corresponding one of the tunnel in a virtual configuration, as prescribed by the Italian decree compliant with the European Directive 2004/54/EC

Fully-resolved simulations of coal particle combustion using a detailed multi-step approach for heterogeneous kinetics

Fully-resolved simulations of the heating, ignition, volatile flame combustion and char conversion of single coal particles in convective gas environments are conducted and compared to experimental data (Molina and Shaddix, 2007). This work extends a previous computational study (Tufano et al., 2016) by adding a significant level of model fidelity and generality, in particular with regard to the particle interior description and hetero- geneous kinetics. The model considers the elemental analysis of the given coal and interpolates its properties by linear superposition of a set of reference coals. The improved model description alleviates previously made assumptions of single-step pyrolysis, fixed volatile composition and simplified particle interior properties, and it allows for the consideration of char conversion. The results show that the burning behavior is affected by the oxygen concentration, i.e. for enhanced oxygen levels ignition occurs in a single step, whereas decreasing the oxygen content leads to a two-stage ignition process. Char conversion becomes dominant once the volatiles have been depleted, but also causes noticeable deviations of temperature, released mass, and overall particle con- version during devolatilization already, indicating an overlap of the two stages of coal conversion which are usually considered to be consecutive. The complex pyrolysis model leads to non-monotonous profiles of the combustion quantities which introduce a minor dependency of the ignition delay time $τ_{ign}$ on its definition. Regardless of the chosen extraction method, the simulations capture the measured values of $τ_{ign}$ very well

Prediction of flammable range for pure fuels and mixtures using detailed kinetics

In this work, the flammable range of several hydrocarbons was predicted using a freely-propagating flame method for pure hydrocarbons and their mixtures, investigating the effects of operating conditions, in terms of temperature, pressure, fuel/oxidizer composition. The model showed accurate agreement with a wide set of experimental data. The average deviation between the experiments and the model was reduced to ∼20% for the UFL of methanol, methane, ethane, propane, n-butane, n-heptane, ethylene, benzene and two different mixtures, methane/ethylene and methanol/benzene. Model performance was improved for the upper flammability limit by including the effect of soot radiation, modeled using an optically-thin approximation. A comprehensive kinetic mechanism was adopted, and a skeletal kinetic mechanism including a soot sectional model was used to predict soot formation in rich flames. Comparison with Calculated Adiabatic Flame Temperature (CAFT) and Le Chatelier models was also carried out, discussing the advantages of a model including the effects of chemical kinetics. Sensitivity analysis was performed to point out the major role of chemical kinetics especially at the UFL, where chemistry drives the process. This methodology showed that the chemical interaction between two different fuels at the rich limit is the reason for the deviation from the thermally controlled behavior. Finally, chemistry was found to be relevant even for the lean flammability limits predictions of lower alkanes, when pure N2O is used as oxidizer