Grouping methods for MPS soot transport model and its application in large-scale enclosure fires

A soot transport model called Multi-Particle-Size model (MPS model) was developed to improve the prediction of soot movement by considering the uneven mass size distribution of soot particles and the influence of particle size on the gravitational settling. The model requires a sophisticated grouping method to divide the soot particles into several groups and determine the representative size for each group. In this paper, several soot particle grouping methods and the approach to calculate the representative sizes are developed with the aim of balancing the computational efficiency and the prediction accuracy of the model. The performance of the MPS model when different grouping methods are applied is investigated through the comparison of the predicted movement of soot particles generated from several materials. Based on this analysis a grouping method that results in the identification of three groups is shown to be sufficient to represent the influence of particle size on the gravitational settling for a variety of combustible materials and the computational cost of the extra governing equations for the transport of soot particles in the groups is acceptable. Furthermore, the efficiency of the model is demonstrated by simulating soot movement in a large-scale industrial building with a high ceiling.     

Numerical model and validation experiments of atrium enclosure fire in a new fire test facility

The use of computational fluid dynamics (CFD) as a tool for buildings, warehouses or factories design requirements fulfilling about fire safety is becoming more common and reliable. Performance-based fire safety assurance procedures make use of the CFD fire modelling to anticipate the evolution of fire, but they need always to be validated. This is especially difficult for big structures, with great clear volumes, where effects of natural and forced ventilation can be very scale dependent. A good opportunity to check the prediction capability of CFD codes to establish temperatures and velocities fields is the new full-scale fire test facility of the Technological Metal Centre in Murcia, Spain. It is an aluminium prismatic squared base building of 19.5 m×19.5 m×20 m, with several vents arranged in its walls and four exhaust fans at the roof. Series of experimental tests have been carried out using several heptane normalized pool-fires placed at the centre of the atrium. The data obtained from these experiments have been later used in a validation study of two CFD simulations implemented for temperature wall, ambient temperature prediction and exhaust fan assessment. The results show good agreement between experimental and numerical predictions and allow concluding that for a fire test of 1.6 MW of average heat release power, the exhaust and ventilation system is not enough to extract the hot combustion products. There is an excessive and dangerous accumulation of hot gases at the upper part of the atrium and the exhaust capacity of the roof fans must be increased. The CFD models can give the answer to that question.     

Experimental and numerical evaluation of the influence of the soot yield on the visibility in smoke in CFD analysis

Experimental and numerical analysis have been performed to evaluate the influence of the soot yield parameter on the results of advanced engineering analysis, in regards to visibility. After identifying soot yield as the most influential factor on the results, fuels with various values of Ys have been analysed in a fire chamber and then compared to numerical data. The numerical analysis has been performed using two different CFD packages, ANSYS® Fluent®, and Fire Dynamics Simulator. The numerical analysis itself show an apparent hyperbolic trend of the visibility when changing the soot yield with clear consequences on the ASET (Available Safe Egress Time). Below a cut-off point, that exists at a soot yield value close to Ys =0,10 g/g, a small change in the parameter causes a substantial shift in the results (visibility or ASET time), while above this value an increase to soot yield does barely influence the results. Qualitative assessment of the results shows a need for use of conservative values of Ys in engineering analysis if detailed and precise material data is not available. Additionally to the full-scale experiments, a real case study has been included to show how this research can be translated into the Fire Safety Engineering design process. In this study, change of Ys value below 0,10 g/g caused a significant change of the qualitative assessment of the results of CFD.     

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72 m×1.5 m×1.3 m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.     

Sensitivity of air change rates in a naturally ventilated atrium space subject to variations in external wind speed and direction

Design guidelines for natural ventilation (NV) in buildings focus on the potential hourly air change (ACH) rates based on the building space parameters. Critically, external airflow data is often assumed on the basis of a single mean wind speed and an associated prevailing wind direction. This can result in significant variation in ventilation rates and comfort conditions when non-design external wind conditions prevail. This paper describes a CFD study aimed at examining the influence of variations in external wind speed and direction on the air change rate for the atrium space of a two-storey naturally ventilated building. The building atrium is ventilated by a series of entry vents on one wall of the building in conjunction with roof vents. External wind speeds from 25 to 250% of the mean site wind speed (5.7 m/s) were examined and found to result in an almost linear increase in the ACH rate. For a single wind speed, the relationship between wind direction and the ACH rate was also found to be approximately linear for wind directions between 0° and 90° (orthogonal and parallel) to the wall vent openings, but non-linear for other wind directions (90–135°). More generally, the significant variation in the atrium ACH rate with changes in external wind conditions, evident in this particular building model, illustrates the importance of considering non-design wind conditions when designing NV buildings.     

Experimental simulations of fire-induced smoke control in tunnels using an “air–helium reduced scale model”: Principle, limitations, results and future

When a fire occurs in a long tunnel, smoke control is crucial for obvious reasons of safety. Ventilation and extraction systems have to be designed with accuracy in order to control the longitudinal motion of the fire-induced smoke and to extract it efficiently in a zone close to the fire source. This paper presents experimental investigations carried out on a small scale tunnel model (scale reduction is 1:20) to study the fire-induced smoke control by longitudinal and transverse ventilation systems. The experimental model is non-thermal and a buoyant release (a mixing of air and helium) is used to represent the fire smoke plume. The main objective of this model is to represent, as well as make possible, the duality between inertial forces (due to ventilation) and buoyant forces. Radiation and heat losses at the walls are not taken into account in this model. At first, the principle of the simulation is widely described. Then, some results are presented for both longitudinal and transverse smoke control by a mechanical ventilation. Finally, perspectives for future investigations are proposed.     

Experimental study and numerical investigation of particle penetration and deposition in 90° bent ventilation ducts

Particle deposition in a ventilation duct is a severe problem as it can pose health hazards. This is more evident in 90° bent ventilation ducts. The exposure of particle deposition location in bend section is important and useful in understanding and dispelling particle contamination. This paper investigated particle penetration and deposition in 90° bent ventilation ducts numerically and validated using experimental and previous research data. In the numerical study, particle penetration and deposition in a 2D 90° bend turbulent flow were analyzed. The Renormalized Group (RNG) k-<epsilon > model and Lagrangian particle tracking model were utilized to characterize turbulent gas flow and particle behavior, respectively. Particle turbulent dispersion was introduced by adopting the eddy lifetime model with the near wall fluctuating velocity corrected to take turbulence anisotropy into account. In the experimental validation, particle pollution collected from an actual ventilation duct was observed. The particle penetration rates in a test duct at 6 different Stokes numbers were measured for validation. The numerical results were consistent with both the experimental study and the data obtained from previous research.     

Experimental and numerical analyses of an opening in a jointed rock mass under biaxial compression

Tunnel construction in a rock mass produces damage around the tunnel by concentration of in situ stress and by construction activity such as blasting. The generated damage changes the mechanical and hydraulic properties of the rock mass. In this study, the rock fracture and joint sliding behaviors of jointed rock masses with an opening under biaxial compression are investigated through experimental and numerical analyses. The tested rock models have a persistent joint set with dip angles of 30°, 45°, and 60° to the horizontal. Under the applied biaxial compression, tensile crack initiation and propagation are the dominant fracture behaviors around the hole in a low joint dip angle rock model (i.e., 30° to the horizontal). The propagation direction of the tensile cracks is roughly normal to the joint surface, and with propagation of tensile cracks, removable rock blocks are generated. The experimental results are simulated using a discrete element code. The numerical analysis simulates several aspects of rock mass cracking and the joint sliding processes around an opening: progressive fracture behaviors in a low joint angle rock model, abrupt initiation and propagation of tensile cracks and joint sliding in a high joint angle rock model (i.e., 60° to the horizontal), propagation of tensile cracks normal to the joint surface, generation of removable blocks in rock segments, an increase of lower hoop stress threshold inducing tensile fractures with a decrease in the joint angle, and an increase of the damage zone around the hole with a decrease in the joint angle.     

Mean age of air in a naturally ventilated office: Experimental data and simulations

Indoor air quality has an important role as health determinant; it relates to the health and comfort of building occupants.The present paper investigates the wind-forced natural ventilation rates of an office as observed with tracer decay, according to ISO 16000-8:2007 Standard, both with an experimental and numerical procedure; experimental data were used to validate the numerical method. Moreover the only infiltration rate case was analyzed.Natural ventilation was created by opening the window and the door of the office. A simulation model of the room was carried out by using the computational fluid dynamics (CFD) code Fluent. The experimental set up of the procedure was developed by a portable gas chromatograph. In both the numerical and experimental approach the mean age of air linking to ventilation efficiency was calculated. The defined experimental procedure can be applied to different situations, in order to evaluate the efficiency of both natural and forced ventilation existing systems. When the ventilation is due to the opening of the window and the door, the simulation results are in a good agreement with the experimental ones, therefore the model could be applied to different situations, in order to reduce costs and time in the evaluation of indoor air quality.     

Validation of the computational fluid-structure interaction simulation at real-scale tests of a flexible 29 m umbrella in natural wind flow

The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. Particularly transient wind loads could lead to dynamic amplification of the structural response. The assessment of the dynamic response of membrane structures is complex due to their special load carrying behavior, their material properties, and their distinct structural interaction with flow induced effects. Computationally intensive fluid-structure interaction simulation could overcome simplifications and limitations of existing approaches, especially small scale wind tunnel tests, and allow the assessment of all relevant structural and fluid phenomena. This paper outlines a virtual design methodology for lightweight flexible membrane structures under the impact of fluctuating wind loads and provides results on the unique validation of the method at real-scale tests of a highly flexible 29 m umbrella.