Ignition, energy flux

Figure 2. The Radiant Panel Test was designed to measure both critical ignition energy and rate of heat release. A sample is mounted facing a controlled heat flux but at a 3CP angle to it such that the upper part of the specimen is more severely exposed. Since irradiance decreases down the specimen, the time progress of ignition down the specimen serves to measure central ignition energy. Thermocouples in the stack above the specimen serve as a measure of heat release rate.

This Equation describes the stimulation of ignition, both by a small-power source acting for a long time, and by a stronger but short heat pulse. The energy flux supplied to the material surface controls the development of the heat wave. This, in turn, has an effect on... 

At some minimum, critical energy flux the ignition time approaches infinity (the ignition of the polymer does not occur). At any energy flux above the critical value the relationship between the ignition time and heat flux is given by... 

The conservation of energy applied to CVi requires a knowledge of the ignition temperature, and the heat transferred to the preheat region, q". We assume that radiation effects are negligible and approximate this heat flux as... 

For the second item ignition to lead to flashover, the area involved must equal or exceed the total critical area needed for the second item. The time for ignition depends inversely on the exposure heat flux (Equation (11.51)). Figure 11.21 shows the behavior for ignition of the second item, where Af,i is the fixed area of the first item and AF c is the critical area needed. The energy release rate of both fuels controls the size of the jump at criticality and depends directly on AFAhc/L. No flashover will occur if the jump in energy for the second item is not sufficient to reach the critical area of fuel, AFjc- The time to achieve the jump or to attain flashover is directly related to the fuel property,... 

The net incoming heat flux on a pyrolyzing surface for the case when there is no surface layer, Mt o, can be calculated using the energy balance on the surface with the surface temperature being the ignition temperature as shown in Figure 19.27 ... 

Using given numerical values of the reaction activation energies in the condensed and gas phases, the heat flux to the surface, and other physico-chemical characteristics of a polymer system, Kashiwagi calculated x. as a function of the initial oxygen concentration. A system of complex differential equations describing the ignition process is used for this calculation. 

---