Beatrice Pulvirenti

Fire Suppression by Water-Mist Sprays: Experimental and Numerical Analysis

Water-mist systems have become a promising technology in the fire-fighting field over the last twenty years. The present work is aimed at employing the available knowledge on water-mist sprays in an experimental and numerical analysis of the suppression mechanism. Therefore, a water-mist system has been operated within a typical fire case. Most notably, this latter is constituted by a heptane pool fire: experiments have been carried out inside a test chamber, where a set of thermocouples has conveniently been placed to evaluate the thermal transient at different locations of interest. Some free-combustion tests have been run as a benchmark to validate combustion models. Then, a typical water-mist nozzle has been inserted and activated to realize control, suppression and potential extinction of the generated fire. The recognized FDS (Fire Dynamics Simulator) and Fluent® codes have been challenged in reproducing the test case: thermal transient and suppression time have been considered as parameters for validation. Therefore, the water-mist spray has been modeled and the already mentioned results about its characterization have been implemented as initial or boundary conditions. Moreover, the fire scenario has been modeled as well. A good agreement between experimental and numerical results has been obtained, even under some approximations, with specific reference to combustion mechanisms.© 2010 ASME

Use of the μPIV technique for an indirect determination of the microchannel cross-section passage geometry

In this work the possible use of the μPIV technique for the experimental determination of the microchannel cross-section geometry has been investigated by means of a blind test in which a series of experimental measurements obtained using glass microchannels having a declared rectangular cross-section with a depth of 100 μm and width of 300 μm and a square microchannel with a 300 μm side have been compared with the direct SEM visualisation of the real cross section of the microchannels. For the (oPIV measurements water is used as working fluid. The laminar fully developed 2D velocity profile has been reconstructed by moving the focal plane of the microscope objective from the bottom to the top of the microchannel. The results shown in this paper demonstrate that the real cross section geometry of the microchannel can be predicted by minimizing the difference between the theoretical and the experimental 2D velocity profiles. When the right passage geometry is determined, the average difference between the theoretical and the experimental velocity is within 4-6%.

Height function interface reconstruction algorithm for the simulation of boiling flows

In this paper we present our results on numerical study of vapor bubbles growing in quiescent superheated liquid, as effect of liquid evaporation at the interface. Height Function interface reconstruction algorithm is coupled with an evaporation model based on continuum field representation of source terms. The flow solver is a finitevolume CFD code. Interface is tracked within a Volume-Of-Fluid framework. Continuum-Surface-Force method accounts for surface tension effects. Vapor bubble heat-transfer-controlled growth is simulated for three different working fluids: water, HFE-7100 and R134a. Accuracy of interface reconstruction algorithm is of maximum importance. Unbalance between pressure gradients and surface tension forces at interface leads to the growth of an unphysical velocity field which switches original only diffusive heat transfer mechanism to combined diffusive-convective one. Height Function algorithm reduces the magnitude of this unreal velocity field. Standard test cases are considered to assess the performances of implemented version, through comparison with the widely

Chapter 1.1 Application and validation of FLUENT flow and dispersion modelling within complex geometries

Abstract Flow patterns around buildings have a strong influence on pollutant dispersion derived from sources placed within the urban area. Computational fluid dynamics (CFD) codes are used to provide solutions to the fundamental fluid dynamics equations at spatial scales smaller than the typical urban ones. In this work, dispersion of pollutant from sources near buildings forming various street canyons is studied by means of the general purpose CFD code FLUENT to investigate the influence of small geometric features on pollutant concentration distributions. Firstly, we study the effects of a complex geometry on the flow near the ground by considering a finite array of rectangular and square-shaped rings of buildings with different aspect ratios. Secondly, we study transport and diffusion of pollutant within a finite array of rectangular buildings. FLUENT concentration results are validated against wind tunnel data (CEDVAL, 2002). Numerical simulations are performed using the Reynolds Averaged Nervier–Stokes (RANS) k–e turbulence model and the advection–diffusion model. The paper documents the potential of a general purpose CFD model for the simulation of pollutant dispersion close to emission sources and within complex building arrangements in an operational context.

Determination of droplet contours in liquid-liquid flows within microchannels

An experimental analysis of the droplet regime with a silicone oil-water two-phase flow within a micro cross-junction, varying the average velocity of the fluids, has been carried out. The micro cross-junction is made as intersection of two glass microchannels with a stadium-shaped cross-section with height H=190 μm and width Wj=195 μm within the junction and W=390 μm before and after the junction. The water flow rate is broken in droplets having spherical shape with dimensions and velocity that depend on the average velocity ratio imposed. Different kinds of intermittent droplets have been observed, in the ranges of average velocity (0.0105-0.042) m/s and (0.0004-0.0050) m/s for oil and water, respectively. The droplet velocity has been calculated starting from the detection of the shape of the droplets and then by the evaluation of the displacement of the droplets in the unit of time. The images of the droplets have been obtained from a high-speed camera, connected to an inverted microscope. The procedure of water phase contour detection is based on Matlab Image Toolbox scripts.

Numerical study of wind-driven natural ventilation in a greenhouse with screens

Abstract This paper is devoted to the CFD study of wind-driven ventilation in a greenhouse, with particular focus to the effect of screens on the inner airflow distribution. Although the use of shading screens to cover agricultural crops has been constantly increased to reduce high radiation loads, their effect on airflow distribution within the greenhouse is still not fully understood. In this paper, CFD simulations of the ventilation in a greenhouse with and without screens are performed, by means of a finite volume CFD code (Ansys-Fluent 17.2), with a standard k- e turbulence model, together with proper user defined functions (UDF) for the inlet velocity and turbulent profiles. The screens have been modeled as porous surfaces and the porosity and the permeability have been obtained experimentally and set into the model. The code has been validated by a comparison with velocity measurements performed in a greenhouse owned by the University of Bologna. Comparisons between the airflow velocity patterns obtained within the greenhouse with screens and without screens have been obtained for different external airflow velocities. The cases with screens show a more uniform distribution of velocity field inside the greenhouse than the cases without screens, especially near the crops. All the cases show that screens strongly affect the airflow velocity distribution inside the greenhouse and the distribution of volume flow rates through the vents. This work shows how the characteristics of the screens and their positioning near the vents are critical for the ventilation within a greenhouse.