TITLE : Fire in a wind-exposed building

BY : Dr S V Zhubrin, CHAM Ltd

DATE : November, 2000

FOR : Demonstration case for V3.3.1

INTRODUCTION

A Simple Chemical Reaction System, SCRS, model application is presented aimed at the demonstration of the method for the simulation of the fire in a 3D building structure exposed to the cross wind when the external air flow over a structure and the convection followed by chemical reactions inside it have to be considered simultaneously.

PHYSICAL SITUATION

The demonstration case considered consists of rectangular building structure having two window openings and near-ceiling slot exposed to the 30 degree external wind.

It is supposed that a fuel (methane) leak from the communication system enters the building structure through an internal aperture.

The fuel is ignited on entry and steady combustion is in progress producing the high temperarture combustion products. Their movement is influenced by buoyancy and the air coming in from outside.

The task is to calculate the temperatures and composition of combustion gases along with all related field distributions.

COMPUTATIONAL DETAILS

Conservation equations

The independent variables of the problem are the three components of cartesian coordinate system, namely X, Y and Z.

The main dependent (solved for) variables are:

• Pressure, P1
• Three components of velocity, U1, V1, W1
• Turbulence energy and its dissipation rate, KE, EP
• Gas total enthalpy, H1 and
• Gas mixture fraction, MIXF.

Turbulence and combustion models

The K-epsilon model, KEMODL, closed by wall functions is used to calculate the distribution of turbulence energy and its dissipation rate from which the turbulence viscosity is deduced.

Combustion is treated as a single-step irreversible diffusion-controlled chemical reaction with a infinitely fast rate between fuel and oxygen. The gas composition and its enthalpy are related to the mixture fraction according to the Simple Chemical Reaction Scheme, SCRS, concept.

Properties and auxiliary relations

The gas density is computed from the local pressures, gas temperatures and local mixture molecular masses.

The specific enthalpies are related to gas temperatures, fuel mass fraction and the heat of combustion.

THE RESULTS

The plots show the distribution of temperatures, velocities and other related fields within and outside the building structure.

Pictures are as follows :

• Building structure geometry
• Flow streamlines
• Flow velocity vectors
• Velocity vectors at the window plane
• Temperature contours at the window plane
• Velocity vectors at the ceiling plane
• Temperature contours at the ceiling plane
• Velocity vectors in the building middle plane
• Temperature contours at the building middle plane
• Fuel mass fraction at the building middle plane
• Air mass fraction at the building middle plane
• Combustion products mass fraction at the same plane
• Turbulent viscosity at the building middle plane
• Gas density at the same plane
• Pressure at the building middle plane
• Turbulence energy at the same plane
• Reciprocal of turbulence time scale at the same plane

THE IMPLEMENTATION

All model settings have been made in VR-Editor of PHOENICS 3.3.1

The relevant Q1 file can be inspected by clicking here.

svz