Ignition piloted temperature

Calculate the time to ignite (piloted) for the materials listed below if the irradiance is 30 kW/ m2 and the initial temperature is 25 °C. The materials are thick and the convective heat transfer coefficient is 15 W/m2 K. Compute the critical flux for ignition as well. 

Flares ideally bum waste gas completely and smokelessly. Two types of flares are normally employed. The first is called the open flare, the second is called the enclosed flare. The major components of a flare consist of the burner, stack, water seal, controls, pilot burner, and ignition system. Flares required to process variable air volumes and concentrations are equipped with automatic pilot ignition systems, temperature sensors, and air and combustion controls. 

This method is based on the Ohio State University Release Rate Apparatus. The specimen is injected into the environmental chamber through which a constant flow of air passes. The exposure of the specimen is determined by a radiant-heat source adjusted to produce the desired total flux on the specimen, which may be tested horizontally or vertically. Combustion may be initiated by non-piloted ignition, piloted ignition of evolved gases, or by point ignition of the surface. The changes in temperature and optical density of the gas leaving the chamber are monitored, and from the resultant data the release rate of heat and visible smoke are calculated. This aj aratus is also used to measure the rate of toxic gas release and consumption. 

A square sample of 100 x 100 mm is exposed to the radiant flux of an electric heater. The heater has the shape of a truncated cone (hence the name of the instrument) and is capable of providing heat fluxes to the specimen up to 100 kW/m. An electric spark plug is used for piloted ignition. Heater temperature is measured as an average of the readings of three thermoconples in contact with the coil. It is set and maintained at a certain level by a three-term controller. Calibration of heat flnx as a function of heater temperatnre is performed with a total heat flux meter of the Schmidt-Boelter type. Prior to testing, the heater temperatnre is set at the appropriate value resulting in the desired heat flux. 

On proving the pilot ignition, the main gas valve opens and the burner ignites. Once alight, the main burner will modulate to the temperature set by the room thermostat. 

Often combustion is initiated in a mixture of fuel and oxidizer by a localized source of energy. This source might be an electric arc (or spark) (moving charged particles in a fluid or a plasma) or a small flame itself. Because the spark or small flame would locally raise the temperature of the mixture (as 7 X did in the autoignition case), this case is defined as piloted ignition. The bulk of the mixture remains at T, well below the AIT. 

I. A preheat region in which the heat transfer from the flame brings the unbumed mixture to its critical temperature for ignition, T[g. This is much like what occurred in describing auto and piloted ignition, except that the the heat is supplied from the flame itself. 

Example 6.1 Estimate the minimum piloted ignition temperature for methanol in air... 

The minimum temperature for piloted ignition is given by A) = Xg(7L)- From Equation (6.18),... 

Let us just consider the piloted ignition case. Then, at Tpy a sufficient fuel mass flux is released at the surface. Under typical fire conditions, the fuel vapor will diffuse by turbulent natural convection to meet incoming air within the boundary layer. This will take some increment of time to reach the pilot, whereby the surface temperature has continued to rise. 

Table 7.4 clearly shows that the ignition temperature for autoignition is considerably higher than for piloted ignition. In conformance with this behavior, Boonmee [15] indicates that the corresponding fuel mass fractions needed are about 0.10 and 0.45 0.15 for the pilot and autoignition of redwood respectively. 

A flame radiates 40 % of its energy. The fuel supply is 100 g/s and its heat of combustion is 30kJ/g. A thin drapery is 3 m from the flame. Assume piloted ignition. When will the drapery ignite The ambient temperature is 20 °C and the heat transfer coefficient of the drapery is... 

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