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FOCUS


Modelling applications


Figure 3: FDS predictions for the time averaged received radiative heat flux at 1.6m above the floor for the three cases.


the received heat flux for the ‘2 walls’ case compared to the other cases. It also clearly indicates that for the apartment arrangement and design fi re chosen, the hob (when adjacent to two walls) should be located approximately 2m from the corner of the wall to reduce the significance of the corner effect when compared to the ‘1 wall’ arrangement. If the open plan design includes a corner positioned hob, the increased reflection of radiative heat should be considered in the calculations, and any radiative losses amended accordingly. With the above results obtained from the FDS modelling, the ‘Fractional Effective Dose’ (FED) methodology outlined in the SFPE Handbook of Fire Protection Engineering8


can


be utilised. This methodology calculates the thermal


radiation exposure, in terms of the received heat fl ux q (kW/m2


) from the source of the fi re. From


this, the cumulative FED, ∑FED, can be derived from the summation of all the FEDs of thermal radiation received while a person is making their escape. The point at which the FED limit is exceeded by the person making their escape


is when the escape route is deemed to be unacceptable. In terms of an acceptable upper limit of


FED to thermal radiation, guidance can be taken from BS 85249


pain threshold of 75(kW/m2


which suggested a lower )4/3


)4/3 . Where a large


proportion of particularly vulnerable persons (eg elderly persons in a residential care home) may be present, the use of 56.25(kW/m2


may


be more appropriate. The FDS modelling data can then be used


to inform a numerical calculation spreadsheet. This allows for the calculation of the thermal radiation received within a specific design arrangement while an occupant makes their escape, in addition to the cumulative FED, based on the location of the hob from the escape route. Figure 4 illustrates an example of the


spreadsheet interface, which calculates the heat fl ux, thermal does and FED received, taking into account the hob location with respect to all the factors discussed previously, ie travel speed, distance and time. The location of the most intense heat fl ux levels received can be easily


Figure 4: Example snapshot of numerical analysis interface.


26 DECEMBER 2018/JANUARY 2019 www.frmjournal.com


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