Reconstruction of a Multi-item Room Fire in an ISO 9705 Room

Abstract

A series of fire experiments were conducted in an ISO 9705 room in an attempt to reconstruct a multi-item room fire produced by individual items. Fuel items represented typical shapes and sizes of furniture in a bedroom including a double bed, bedside tables, wardrobe, dressing table and chair. As all of the fuel items were made of seasoned pine timber, these are termed as “mock furniture”. Double bed and bedside tables also had cotton sheets above them. The ignition was attained by using a steel tray of methylated spirit. This study is motivated by the fact that multi-item solid fuel-burning experimental results are rarely available. First, a number of standalone items within the ISO room were burnt separately. Then, all of the items were placed within the ISO room, and the fire was allowed to spread throughout the room from one item to another. An attempt was made to reconstruct the full room burning from individual item burning. The heat release rate of the reconstructed fire is different from that of the full room multi-item furniture burn experiment. A list of possible causes has been explained.

Appendices

Appendix 1

In this work, an oxygen calorimeter was used to calculate the HRR. The products of combustion coming out of door were collected and directed to an oxygen analyser by an exhaust hood. The details of the calculation procedure of HRR [15] are presented below.

Prior to the testing, baseline O₂ ($Base_{{O_{2} }}$) and baseline CO₂ ($Base_{{CO_{2} }}$) values were measured. During the fire, combustion gas was drawn through the hood, and smoke duct and sampling gas from the duct were collected for O₂ and CO₂ analysis. The following basic equations were followed to calculate the HRR:

$$HRR = E_{C3H8} \times \dot{m}_{e} \times Base_{{O_{2} }} \times \left( {1 - F_{{H_{2} O}} } \right) \times \frac{{\phi}}{{\left( {1 + \phi\left( {\alpha - 1} \right)} \right)}}$$

(A.1)

where $E_{C3H8}$ is the energy released per unit mass of propane gas burned (16,800 kJ/kg), $\alpha$ is the expansion factor due to combustion = 1.105. $F_{{H_{2} O}}$ is the molar fraction of water vapour which can be calculated as:

$$F_{{H_{2} O}} = \frac{{RH \times p_{s} \left( {T_{air} } \right) }}{{100 \times P_{air} }}$$

(A.2)

where RH is the relative humidity, $p_{s}$ is the saturation pressure as a function of air temperature, $T_{air}$, and $P_{air}$ is the air pressure. In this study, $F_{{H_{2} O}}$ was calculated to a value of ~ 0.014.

Flow rate, $\dot{m}_{e} = 22.4 \frac{{A_{t} }}{{K_{p} }}\sqrt {\frac{\Delta p}{{T_{e} }}}$, where $K_{t}$ is a calibration constant = 0.82 and $K_{p}$ is a friction factor = 1.08, A is the area of the duct, $\Delta p$ is the pressure difference measured by a bi-directional probe, T_e is the gas temperature.

φ, is defined by:

$$\varphi = \frac{{Base_{{O_{2} }} \times \left( {1 - \overline{X}_{{CO_{2} }} } \right) - \left( {1 - Base_{{CO_{2} }} } \right) \times \overline{X}_{{O_{2} }} }}{{Base_{{O_{2} }} \times \left( {1 - \overline{X}_{{CO_{2} }} - \overline{X}_{{O_{2} }} } \right)}}$$

(A.3)

$\overline{X}_{{O_{2} }} , \overline{X}_{{CO_{2} }}$ are the molar fractions of O₂ and CO₂ of exhaust gas sampled at the exhaust.

Appendix 2

2.1 Flashover Time Calculation for Experiment 1: Full Room Burn Test

In 1980’s Babrauskas [35] defined a criterion for flashover as a temperature rise of 575 °C based on a simplified combustion model and the results of 33 experiments on compartmental fire. The authors formulated a simple correlation for the prediction of minimum HRR required for flashover:

$$\dot{Q} = 750A\sqrt H$$

(B.1)

where $\dot{Q}$ is the required minimum heat release rate for flashover in kW, $A$ is the area of the opening in m², and $H$ is the height of the opening in m.

Thomas [30] developed a correlation for the heat release rate required for flashover based on a simple energy balance for the upper layer, which is

$$\dot{Q} = 7.8A_{T} + 378 A_{o} \sqrt {H_{o} }$$

(B.2)

where $\dot{Q}$ is the required minimum heat release rate for flashover in kW, $A_{T}$ is the total area of the compartment enclosing surface in m², $A_{o}$ is the area of the opening in m², and $H_{o}$ is the height of the opening in m. However, the limitations of the first expression are that it does not consider the effect of wall surface area and the thermal properties of the wall materials and the second correlation does not consider the effect of the thermal properties of the wall.

McCaffrey et al. [29] developed correlations, also known as MQH correlation, to estimate the flashover in a compartment, considering the effect of wall area and the thermal behaviour of wall material, using over 100 experimental data sets. In their analysis, the required minimum HRR for flashover in a compartment can be calculated by the following equation:

$$\dot{Q} = \left\{ {\sqrt g c_{p} \rho_{o} T_{o}^{2} \left( {\frac{\Delta T}{{480}}} \right)^{3} } \right\}^{1/2} \left\{ { h_{k} A_{T} A_{o}^{\frac{1}{1}} \sqrt {H_{o} } } \right\}^{1/2}$$

(B.3)

where $\dot{Q}$ is the heat release rate of fire (kW), $A_{o}$ is the area of the opening (m²), $H_{o}$ is the height of the opening (m), $A_{T}$ is the total area of the compartment enclosing surfaces (m²), $g$ is the acceleration due to gravity (m/s2), $c_{p}$ is the specific heat of the air (kJ/kg·K), $h_{k}$ is the effective heat transfer coefficient of ceiling and walls (kW/m²·K), $T_{o}$ is the ambient temperature (K), $\Delta T$ is the temperature difference between upper layer gas in the compartment and ambient (°C), and $\rho_{o}$ is the density of air (kg/m³).

For the above equation, $h_{k}$ must be estimated based on the time of exposure of fire $\left( t \right)$ and the time of thermal penetration $\left( {t_{p} } \right)$. The thermal penetration time $\left( {t_{p} } \right)$ is defined as:

$$t_{p} = \left( {\frac{{\rho c_{w}^{{}} }}{k}} \right)\left( {\frac{\delta }{2}} \right)^{2}$$

(B.4)

where $\rho$ refers to the density of the compartment surface (kg/m³), $c_{w}$ is the specific heat of the compartment wall surface material (kJ/kg·K), $k$ is the thermal conductivity of compartment surface (kW/m·K), and $\delta$ is the thickness of compartment surface (m).

When $t > t_{p}$, the heat transfer coefficient can be determined using a steady-state approximation as:

$$h_{k} = \frac{k}{\delta }$$

(B.5)

When $t \le t_{p}$, the heat transfer coefficient can be determined using an approximation based on conduction in a semi-infinite solid is:

$$h_{k} = \left( {\frac{{\rho c_{w} k}}{t}} \right)^{1/2}$$

(B.6)

The HRR at the time of flashover is estimated using the equations of the MQH model [29] and Thomas [30]. The material properties of walls and ceiling of the room are taken form Moinuddin et al. [9]. The details of the input parameters and the output HRR are presented in Table

Table 6 Estimation of HRR by the MQH model and Thomas correlation

6.

The estimated minimum HRR required for the flashover is 1160 kW by the MQH model [29] and 1280 kW by the Thomas [30] correlation. In the experiment, the flashover occurred at 305 s, and the corresponding temperature and HRR was about 625 °C and 1100 kW, respectively. The HRR at flashover estimated by MQH model is closer to the observed value compared to that by Thomas. It is worth to mention that the experimental set up of this study (room size, wall and ceiling construction materials, and opening area, width and height) are very much similar to the experimental data set used by the MQH model. The correlations by MQH model was formulated using the data with compartment heights from 0.3 m to 2.7 m, floor areas from 0.14 m² to 12 m², heights of the opening from 0.225 m to 2.13 m, and widths of the opening from 0.015 m to 0.991 m [36, 37]. The walls and ceilings, of the experiments used as data set for the development of the correlations, were constructed of non-combustible materials. Furthermore, only a single opening naturally ventilated fires were included in the experimental database [36].