COMPARATIVE ANALYSIS OF ASSUMPTIONS FOR NUMERICAL SIMULATION OF THE EFFECTS OF FIRE-SAFETY OF EVACUATION FROM THE BUILDING STRUCTURE

Ensuring a safe evacuation for the users of a building is a major goal when designing structures and systems protecting them against the effects of a fire. The article discusses the safety assessment for evacuation of users from a building exemplified by an analysis using computational fluid mechanics to reproduce the environmental conditions during a fire. It presents a way to evaluate the possibility of a safe evacuation of users from a facility by indicating the criteria for the assessment of conditions on the evacuation routes during emergency evacuation. In order to verify the criteria for assessing the evacuation safety, a three-dimensional model of the object under consideration has been prepared, for which a dedicated calculation solver of Fire Dynamic Simulator fluid mechanics has been used to recreate the fire conditions in the building. Prepared calculation model takes into account both the development of a fire on a given floor of the building and the simulation of the designed fire ventilation system in operation. In the paper the authors compare the assumptions used to create a calculation model and analyze their impact on the assessment of evacuation safety. Comparative analysis of the assumptions used to prepare the fire model allowed to draw conclusions particularly important for the people evaluating the evacuation safety on the basis of the analysis of the operation of fire ventilation systems using the computational fluids mechanics.


INTRODUCTION
Fire incidents in buildings are one of the most dangerous hazards when it comes to evacuation safety. In order to reduce the aftermath of fire in buildings, systems are designed to improve the fire evacuation safety. These systems include fire ventilation systems, which Inżynieria Bezpieczeństwa Obiektów Antropogenicznych 4 (2020) 212-226 DOI: 10.37105/iboa.89 -213 -nowadays are complex installations using the control of individual elements of the system depending on the fire scenario. The essence of the operation of fire ventilation system is to remove smoke and control the spread of fire to enable the evacuation and protection of people in the facility during the fire until the arrival of fire-fighting and rescue services [2][3][4][5].
According to the Polish regulations, safe evacuation is assured by fire ventilation systems able to ensure that "...neither smoke nor temperature preventing safe evacuation will occur along the protected passageways and escape routes during the time needed to evacuate people. [1].
Particularly important is the protection of escape routes during a fire on those floors of the building, where the risk of fire and its consequences are high. These floors include, among others, underground levels, especially underground garages. The design of fire ventilation systems in underground garages is a complex, multi-stage issue, which causes ventilation designers many problems. That is because of the complex nature of the physical phenomena that accompany the formation and development of a fire, against the effects of which the fire ventilation system should provide protection. A useful tool for assessing the performance of fire ventilation systems at the design stage is conducting thermal-flow analyses using the computational fluid mechanics CFD to recreate the conditions occurring in a building during a fire.

Evacuation safety assessment criteria
The main goal in assessing the performance of smoke extraction systems is to ensure the safe evacuation of users from the fire compartment by maintaining appropriate environmental conditions on the evacuation routes. According to the understanding of national legislation [5]: "The smoke ventilation system should :  -The local visibility range of signs illuminated by reflected light shall not be less than 10 m To ensure the conditions for safe evacuation, the above criteria should be fulfilled at the evacuation points leading to the emergency exits on a given floor for the time necessary to complete the evacuation.
The time necessary to evacuate users from a given space represents the time required by the occupants to recognize and interpret alarm signals, make decisions and react to an emerging threat, start an evacuation and the time to move to a safe zone. The total time required for evacuation is the sum of the above times [6], [7] (Fig.1).

Figure1.
Diagram showing the components of the total safe evacuation time, own elaboration based on [6] 2.

Subject of analysis
The analyzed object, for which a three-dimensional model has been prepared, is a one-   MW [11]. The course of the fire development curve over time assuming a development according to the square curve "αt 2 ". (α= 0.00988 kW/s 2 ) is shown in Figure 4 and is hereinafter referred to as curve 1. The second standard for modelling a fire in an underground garage uses a fire development curve derived from the Dutch standard NEN 6098:2010, which refers to a fire of 3 passenger cars with a maximum fire power of about 9.4 MW, [12], [1] (hereinafter referred to as curve 2). The course of development of both fire curves in time required for safe evacuation from the garage space is shown in Figure 4. the considered underground garage, own elaboration based on [11], [12].

Calculation solver
The analysis was carried out using a calculation solver Fire Dynamic Simulator (FDS) which is a computational fluid dynamics (CFD) model dedicated to fire-driven fluid flow.
During the calculation FDS numerically solves a form1of the Navier-Stokes equations appropriate for-low-speed (Ma < 0.3), thermally-driven flow [8]. This form of Navier-Stokes equations is refer to flow with an-emphasis on smoke and heat transport from fires [8].
Detailed information of equations and the numerical algorithm are contained in the FDS Technical Reference Guide [9]. Fire Dynamic Simulator is based on Large Eddy  [6]. In this study to performed calculation Deardorff SGS model has been used as an default model to analyze the heat transfer from fire [8]. The governing equations in Fire Dynamic Simulator refer to conservation equations of mass, momentum and energy for a Newtonian fluid [8], [9]. (1) where: density,time,flow velocity, del operator, pressure, identity matrix, where: absorption coefficient, source term, solution of the radiation transport equation (RTE) for a non-scattering grey gas

The calculation model
Based on the design and assumptions, the three-dimensional representation of the geometry of the object along with the physical properties of the materials and environmental       fig.1.). In the first minute after the fire detection by the system (60-120 s of fire), the power released from the fire modelled according to the curve 2 is between 492 and 984% higher than the power released during the development of the fire modelled according to the curve 1, which is reflected in the amount of smoke produced according to the ratio of the power released during the course of both curves [9]. As a result of the change of fire power, the temperature above the evacuation passages also changes (Fig.3, A-B; Fig.4. A-B; Fig.5.
A-B). In the further course of both curves the differences in the released power decrease with time but still have a noticeable influence on the evaluation of evacuation conditions. When the first person starts evacuation from the garage space (120-180 s of fire), the power released by curve 2 is 328 to 492 % higher than in curve 1 (Fig.3, B-C; Fig.4. B-C; Fig.5. B-C). During the evacuation of the last persons from the garage space (240 -300 s of fire), the fire power released by curve 2 is 157 to 246 % higher than that of curve 1 (Fig.3