Ulf Wickström

Temperature analysis of heavily-insulated steel structures exposed to fire

Abstract An exact analytical one-dimensional solution of the temperature response of insulated steel structures is derived and a closed form solution is given for a structure exposed to the ISO 834 standard curve. Alternative approximate solutions are also given where all the heat capacity is assumed lumped in the steel core; only one third of the insulation heat capacity is then considered. It is also shown that approximate solution schemes given elsewhere strongly underestimate the temperature rise in structures protected with heavy insulations system. In addition the theoretical background of a proposed NORDTEST test method on how to obtain the thermal properties of an insulation system is outlined. Finally a numerical calculation scheme is recommeded.

Temperature calculation of insulated steel columns exposed to natural fire

Abstract A method suitable for design purposes has been developed which allows the approximate post-flashover compartment fire temperature to be plotted versus time in one curve, the general natural fire curve; time is then modified or scaled to take into consideration ventilation conditions and wall properties. In the analysis the common assumptions of constant and ventilation controlled combustion, uniform temperature, and wall losses proportional to the thermal inertia are made. The general natural fire curve is given an analytical expression, which is then used to calculate temperature in fire exposed insulated columns by a simple integration procedure. The results are plotted in handy diagrams, and temperatures obtained in columns exposed to natural fires and standard fires according to ISO 834 are compared.

Measurement and calculation of adiabatic surface temperature in a full-scale compartment fire experiment

Adiabatic surface temperature is an efficient way of expressing thermal exposure. It can be used for bridging the gap between fire models and temperature models, as well as between fire testing and temperature models. In this study, a full-scale compartment fire experiment with wood crib fuel was carried out in a concrete building. Temperatures were measured with plate thermometers and ordinary thermocouples. Five plate thermometers and five thermocouples with a diameter of 0.25 mm were installed at different positions. These two different temperature devices recorded different temperatures, especially near the floor surface. The adiabatic surface temperature was derived by a heat balance analysis from the plate thermometer measurements. The thermal inertia of the plate thermometer was taken into account to correct the measured results. In addition, the fire experiment scenario was also simulated with fire dynamics simulator. The fire source was specified as a given heat release rate, which was calculated from the measured mass loss rate of the wood fuel. The adiabatic surface temperatures at these measuring positions were simulated by the fire dynamics simulator model and compared with the experimental adiabatic surface temperatures. The comparative results showed that fire dynamics simulator predicted the adiabatic surface temperature accurately during the steady-state period.

Equivalent concrete layer thickness of a fire protection insulation layer

Abstract The reinforcement bars in a concrete structure are usually protected against fire only by the concrete cover layer. In some cases an additional protection is needed and hence protective layers are applied, which consist of light-weight insulation materials. The theoretical analysis outlined in this paper shows that: (a) the thermal protection capacity of a protection layer can indeed be expressed as an equivalent concrete layer; and (b) a simple relation can be established between the thermal resistance ( R = δ k ) of the protection layer and the thickness of an equivalent concrete protection. The equivalent concrete thickness yields the same time for reinforcement bars to reach 500°C as the corresponding thermal protection layer when the structure is exposed to the standard ISO 834 fire. All the analyses have been carried out with the finite element temperature analysis computer program TASEF.

Use of plate thermometers for better estimate of fire development

The concept of Adiabatic Surface Temperature (AST) opens possibilities to calculate heat transfer to a solid surface based on one temperature instead of two as is needed when heat transfer by both radiation and convection must be considered. The Adiabatic Surface Temperature is defined as the temperature of a surface which cannot absorb or lose heat to the environment, i.e. a perfect insulator. Accordingly, the AST is a weighted mean temperature of the radiation temperature and the gas temperature depending on the heat transfer coefficients. A determining factor for introducing the concept of AST is that it can be measured with a cheap and robust method called the plate thermometer (PT), even under harsh fire conditions. Alternative methods for measuring thermal exposure under similar conditions involve water cooled heat flux meters that are in most realistic situations difficult to use and very costly and impractical. This paper presents examples concerning how the concept of AST can be used in practice both in reaction-to-fire tests and in large scale scenarios where structures are exposed to high and inhomogeneous temperature conditions.

Large Scale Test on a Steel Column Exposed to Localized Fire

A localized fire is a fire which in a compartment is unlikely to reach flash-over and uniform temperature distribution. Designing for localized fires is generally more difficult than for flash-over compartment fires because of the complexity of the problem. There is also a lack of experimental data. We report here on a full scale test series on a steel column exposed to localized fires. The setup is a 6 meters tall hollow circular column, ϕ = 200 mm with a steel thickness of 10 mm. The unloaded column was hanging centrally above different pool fires. Temperatures of gas and steel were measured by thermocouples, and adiabatic surface temperatures at the steel surface were measured by plate thermometers of various designs. The results are compared with estimates based on Eurocode 1991-1-2 which in all cases studied overestimate the thermal impact for this setup. The input from plate thermometers was used to compute the steel temperatures using finite element methods. Excellent agreement was found if the rad...

Timber under real fire conditions – the influence of oxygen content and gas velocity on the charring behavior

PurposeThe purpose of this study is to investigate the influencing factors on the charring behaviour of timber, the char layer and the charring depth in non-standard fires.Design/methodology/approa ...

Compartment fire temperature - a new simple calculation method

In this paper a new simple calculation method for compartment temperatures is derived. The method is applicable to post-flashover ventilation controlled fires. A parameter termed the ultimate compartment fire temperature is defined as the temperature obtained when thermal equilibrium is reached and thick compartment boundaries cannot absorb any more heat from the fire gases. This temperature depends only on the product of the heat of combustion and the combustion efficiency over the specific heat capacity of air. It is, however, independent of the air mass flow rate, and of the fire compartment geometry and the thermal properties of the compartment boundary materials. These parameters on the other hand govern the rate at which the fire temperature is increasing towards the ultimate temperature. It is shown how the fire temperature development as a function of time in some idealized cases may be calculated by a simple analytical closed form formula. The fire temperature developments of two types of compartment boundaries are presented, semi-infinitely thick and thin structures. It is also shown that for the semi-infinite case, the solution resembles the standard ISO 834/EN 1363-1 curve and the parametric fire curves according to Eurocode 1, EN 1991-1-2.

Travelling Fires for CFD

There are numerous methods in structural fire safety engineering to assess a time-temperature input for structural calculations in fire enclosures but there is very little on design fires for CFD calculations. This study is an attempt to explore a simpler form of design fire. The simplified approach consists of two main features, a travelling behaviour and a heat release rate specified by the user.

Unsteady-State Conduction

When a body is exposed to unsteady or transient thermal conditions, its temperature changes gradually, and if the exposure conditions remain constant it will eventually come to a new steady state or equilibrium. The rate of this process depends on the mass and thermal properties of the exposed body, and on the heat transfer conditions. As a general rule the lighter a body is (i.e. the less mass) and the larger its surface is, the quicker it adjusts to a new temperature level, and vice versa. The temperature development is governed by the heat conduction equation (Eq. 1.29) with the assigned boundary conditions. It can be solved analytically in some cases, see textbooks such as [1, 2], but usually numerical methods are needed. This is particular the case in fire protection engineering problems where temperature generally varies over a wide range, often several hundred degrees.