Assessment of Fire Engineering Design Correlations Used to Describe the Geometry and Thermal Characteristics of Externally Venting Flames

Abstract

Externally venting flames (EVF) may emerge through openings in fully developed under-ventilated compartment fires, significantly increasing the risk of fire spreading to higher floors or adjacent buildings. Several fire engineering correlations have been developed, aiming to describe the main characteristics of EVF that affect the fire safety design aspects of a building, such as EVF geometry, EVF centreline temperature and EVF-induced heat flux to the fac¸ade elements. This work is motivated by recent literature reports suggesting that existing correlations, proposed in fire safety design guidelines (e.g. Eurocodes), cannot describe with sufficient accuracy the characteristics of EVF under realistic fire conditions. In this context, a wide range of EVF correlations are comparatively assessed and evaluated. Quantification of their predictive capabilities is achieved by means of comparison with measurements obtained in 30 different large-scale compartment-fac¸ade fire experiments, covering a broad range of heat release rates (2.8 MW to 10.3 MW), ventilation factor values (2.6 m 5/2 to 11.53 m 5/2 ) and ventilation conditions (no forced draught, forced draught). A detailed analysis of the obtained results and the respective errors corroborates the fact that many correlations significantly under-predict critical physical parameters, thus resulting in reduced (non-conservative) fire safety levels. The effect of commonly used assumptions (e.g. EVF envelope shape or model parameters for convective and radiative heat transfer calculations) on the accuracy of the predicted values is determined, aiming to highlight the potential to improve the fire engineering design correlations currently available.Total area of vertical openings on all walls of the compartment c (4.67)Empirical factor (Eq. 19) C p (1005 J/kg K) Specific heat of air at ambient conditions D v (m)Effective diameter of the opening d eq (m)Characteristic length scale of an external structural element E b (kW/m 2 ) Black body emissive power g (9.81 m/s 2 )Gravitational acceleration H 0 (m)Opening height H u (13,100 kJ/kg O 2 )Heat release of cellulosic fuels for each kilogram of oxygen consumed h eq (m)Weighted average of openings heights on all walls k (m -1 ) Extinction coefficient k fuel (m -1 ) Extinction coefficient for the combustion products of a specific fuel L L_0.05 (m) Flame height at the ''continuous flame'' (5% flame intermittency limit) L L_0.50 (m) Flame height at the ''intermittent flame'' (50% flame intermittency limit) L L_0.95 (m) Flame height at the ''far-field flame'' (95% flame intermittency limit) L L (m) Height of EVF L H (m) Projection of EVF L f (m) Flame length l (-) Characteristic length scale (Eq. 9) l x (m) Length along the EVF centerline, originating at the opening _ m a (kg/s) Air mass flow rate (entering the fire compartment) _ m f (kg/s) Fuel mass flow rate _ m O2 (kg/s) Oxygen mass flow rate _ m g (kg) Mass flow rate of unburnt gases venting outside the fire compartment _ Q (MW) Heat Release Rate _ Q ex (MW) Excess Heat Rele...

Thermal characteristics of externally venting flames and their effect on the exposed façade surface

“…Two main parameters are investigated, namely the effect of the fuel type on the extinction coefficient (k) and the the convective heat transfer coefficient (αc). A key parameter analysis, presented in a previous work [5], revealed that the EVF shape does not significantly influence the estimated heat flux levels.…”

“…A detailed review and comparative assessment of the most widely used fire engineering design correlations, capable of describing the main characteristics of EVF that affect the fire safety of a building, has been performed previously by the authors [5]. These correlations can be used to estimate the geometrical and thermal characteristics of EVF, such as height, projection, width and centreline temperature, as well as EVF-induced heat flux on the façade.…”

“…Building fires involving EVF appear to predominately occur in countries with poor regulatory controls concerning facades. The main fire safety aspects of such fire events concern façade heat flux and EVF plume characteristics [5], fire resistance of the façade assembly for load and non-load bearing structures and fire spread on the external surface or at the interior of a façade assembly [4]. Under-ventilated fire events generate the larger hazard in a structure regarding the heat flux impact and EVF development.…”

“…In this context, the main aim of this work is to investigate the fundamental thermal phenomena governing EVF development and their impact on façade systems. Motivated by an increasing number of recent reports [4,5,6] suggesting that existing engineering design methodologies cannot describe with sufficient accuracy the thermal characteristics of EVF, this work emphasizes on the assessment of empirical correlations and design methodologies used to describe EVF and their impact on façade systems. Focusing on the thermal characteristics of EVF that affect the fire safety of a façade system, namely the EVF centreline temperature (Tz) and the EVF-induced heat flux to the façade (q"), the predictive accuracy of various design correlations is evaluated by means of comparison with medium-and large-scale fire tests conducted by the authors and found in literature.…”

Critical Performance Aspects of Retrofitting Apartment Buildings Using a Multifunctional Façade System

“…Fire safety regulations Requirements differ for ventilated and non-ventilated façades [52] At least limited ventilation seems to be necessary for the system to have safe moisture performance. Careful selection of thermal insulation materials The fire capacity and other fire properties of the technical units needs to be assessed prior to installation.…”