Fire Safety Science Digital Archive

A series of open-space fire experiments was conducted at the National Fire Laboratory (NFL) to validate the capabilities of the NFL room-size oxygen-consumption calorimeter, and to assess the importance of accounting for actual water vapor content in the exhaust gases in calculating heat release rates (HRR). Water spray was used to partially suppress some of the fires, and to add significantly to the humidity of the exhaust gases. The equations normally used in the fire research community for oxygen calorimeter assume unsuppressed fires, and that water vapor in the exhaust gases is due solely to the humidity of the incoming air and to combustion reactions. This paper derives the basic equations for computing heat release rates based on the principle of nitrogen balance. The general equations take into account all sources of water vapor, including incoming air, combustion reactions, and evaporation due to suppression. The equations are then simplified to i) neglect all humidity, and, ii) consider only the humidity of the incoming air. The predictions of the HRR from the three sets of equations are compared with the HRR calculated for unsuppressed fires and with the HRR obtained by measuring fuel consumption rates. As long as the water vapor content in the exhaust gases is less than 7 %, both simplified equations can be used to measure the HRR of partially suppressed fires, without significant error. For larger concentrations of water vapor, water vapor should be measured and the full equations used.