Experimental measurements of water mist systems and implications for modelling in CFD

The use of water mist for fire extinguishment has increased rapidly in recent years. The main reason is the abandonment of halon-based extinguishing systems in favour of environmentally friendlier systems. Furthermore the use of water mist systems has spread from mainly marine applications to also include the protection of buildings. The main problem in this regard is to verify the effectiveness of the system. At present time this can only by done by full-scale tests. This is however expensive and in some cases also unrealistic and expensive when it comes to water mist systems for buildings. The aim of this thesis is to provide experimental data that can serve as a basis for simulations of the interaction of water mist and a fire and to demonstrate that CFD can predict the performance of a water mist system. The physics of water mist systems has been studied by theoretical considerations as well as experimental work. Measurements of droplet velocities, diameters and volumetric water distribution were carried out on the spray from a high-pressure system of 100 bars. Experiments have been conducted on a hollow cone nozzle without fire and with fire, as well as a full cone nozzle without fire. Relevant measurement results were obtained with Phase Doppler Anemometry and Particle Image velocimetry as well as Laser Tomography and High Speed camera. Suggestions were made for improvement of the water density apparatus. The measurements have been the basis for simulations of water mist with CFD. Initial simulations involving the complex zone around the nozzle resulted in droplets with radial velocities and insufficient transfer of momentum to the air. A new approach has been used for the simulations with the LES model in FDS 4.07. In this approach the simulations of the water mist spray is not done in the zone close to the nozzle. Instead the boundary conditions are set further downstream, based on the conducted measurements. This approach resulted in droplets and air moving downwards at relatively high velocities as expected. However, the momentum transfer is limited, and the simulations did not give sufficient mixing. Suggestion are made of how sufficient mixing can be obtained with the new approach, with regards to implementation of spray boundary conditions and treatment of the turbulence model interacting with the movement of the droplets

Verification, validation and evaluation of FireFOAM as a tool for performance based design

The open source CFD code FireFOAM has been verified and validated against analytical solution and real fire tests. The verification showed that FireFOAM solves the three modes of heat transfer appropriately. The validation against real fire tests yielded reasonable results. FireFOAM has not been validated for a large set of real fires, which is the case for FDS. Therefore, it is the responsibility of the user to perform the validation, before using the code.One of the advantages of FireFOAM compared to the Fire Dynamic Simulator is that FireFOAM can use unstructured grid.FireFOAM is parallelised and scales reasonable well, but is in general considerably slower in computation speed than the Fire Dynamic Simulator. Further, the software is poorly documented and has a steep learning curve. At present it is more a tool for researchers than for fire consultants

Influence of Draft Curtains on Sprinkler Activation - Comparison of Three Different Models

This article investigates the importance of using draft curtains to obtain faster sprinkler activation with three different models-two computational fluid dynamics (CFD) models (CFX 4.4 and fire dynamics simulator (FDS) 4.07) and a zone model (Argos) containing a ceiling-jet formula - for an actual scenario in an entertainment center in Denmark. It is found that a draft curtain has some effect on sprinkler activation, reducing activation time from 8% to 15%, depending on the model implemented. The positions of the sprinklers with in the vertical computational grid of the CFD simulations have a greater influence on the activation of the sprinkler, where FDS is more sensitive than CFX. It is confirmed that heat transfer from the ceiling jet to the ceiling has little influence on the results. The zone model with a ceiling-jet formula gives 10—20% slower sprinkler activation than the CFD results when the sprinkler is close to the ceiling, but is still considered very useful in view of the faster calculation time

CFD modeling of UMD-SBI fire for the MaCFP-3 Workshop

The purpose of this modelling exercise is to contribute to the structured effort of the MaCFP workshop in the fire research community to make progress in fire modelling, specifically Single Burning Item( SBI) Test [1] in this case, using Computational Fluid Dynamics (CFD) code Fire Dynamics Simulator (FDS) version 6.7.9

Simulation of a liquid methanol fire for the MaCFP Project

Simulations of a liquid Methanol fire with a pool diameter of 30 cm in Fire Dynamics Simulator v6.8.0 (FDS)Experimental data provided by NIST [1] (30 cm - Methanol pool fire case with a lip of 10 mm)Prescribed and Predicted mass loss rates (MLRs)Predicted MLR based on two methods:- Using an EXTERNAL_FLUX on the liquid pool- Using a hot solid object (at 1000° C) as the ignition sourc

Simulation of a liquid methanol fire for the MaCFP Project

Computational fluid dynamics (CFD) is applied to simulate methanol pool fires with pool diameters of 30cm and 1m. The corresponding validation experiments were carried out at Waterloo University and the National Institute of Standards and Technology (NIST). Steady-state profiles of gas temperature, velocity, species concentrations, and radial and axial heat flux profiles were recorded during the experiments which provided the basis for the numerical analysis. During the experiments, the liquid layer height was maintained constant throughout.The CFD tool for fire simulations, Fire Dynamics Simulator v6.8.0 was used for performing the calculations [1]. The two-step simple chemistry combustion model was used to model the oxidation of methanol. FDS allows the user to prescribe the fuel mass flux into the domain and let FDS predict the energy release based on the evaporation of the liquid. Both methods have been incorporated in the study.Sensitivity studies have been performed and included: sensitivity to the mesh resolution for cell sizes ranging from 5 mm to 4 cm; changed temporal and spatial resolution of the radiation solver; and different turbulence models. The preliminary results from the study show that good agreement can be achieved with FDS using the default settings. However, there is still the possibility to improve the predictions from FDS, especially in the near field of the pool fire just above the surface of the liquid pool. Several modifications were done on the default models used in FDS which has led to improved predictions from the simulations.Upon comparing two different radiation solvers (grey gas model and wide band model), it was found that the predictions from the default grey gas model were superior further away from the flames. Based on the sensitivity study on the settings of the grey gas model showed that a spatial resolution of 800 angles was sufficient for good accuracy. Deardorff model and the dynamic Smagorinsky model were used to model turbulence. Upon comparison of the two models, no significant variation was found between their predictions.Furthermore, the puffing frequency of the flames during the steady burning period was estimated using the Fast-Fourier Transform algorithm on the signals of HRR, vertical velocity component and the flame height calculated within FDS

CFD modeling of UMD-SBI fire for the MaCFP-3 Workshop

The purpose of this modelling exercise is to contribute to the structured effort of the MaCFP workshop in the fire research community to make progress in fire modelling, specifically Single Burning Item (SBI) Test [1] in this case. Experiments were performed at the University of Maryland (UMD) with the same black PMMA considered in the MaCFP-2 Workshop [2]