Fire Safety Science Digital Archive

The rate at which a fire burns in a compartment is determined by the feedback of heat from the flame and hot walls to the fuel bed. In order to measure the heat transfer to the fuel bed an instrument was required which could measure total heat fluxes up to 3-4 cal cm^-2s^-1 with an accuracy no greater than 5%, had a receiving angle of 180 degrees, and was reasonably simple to operate. Descriptions of a number of instruments for measuring heat fluxes in this range and higher have been published, particularly by furnace and rocket engineers; Appendix I gives a broad classification and references. The heat flow meter of Baulk and Thring alone possessed all the desired characteristics and an instrument based largely on this was constructed and is described in this report. The principle of operation is that heat absorbed on the front face of a receiver is transferred to water flowing at a known rate through the receiver and the temperature rise of the water measured by thermocouples. The instrument of Baulk and Thring was used to measure heat flows up to at least 30 cal cm^-2s^-1 and it was found that the water velocity through the receiver had to be high to avoid fluctuations in water flow due to steam formation. When the performance of the present instrument was being investigated it was found that high water flows were required, not however to prevent steam formation, but to preserve the simple relation between heat flow and water flow and temperature rise. This is described further in Section 2.