Heat flux, external radiative

Experiments can be performed where chemical, convective and radiative heat release rates can be measured at various external heat flux values. Linear relationships should be found for the experimental data, where the slope is equal to xj (AH /L). 

Note that if this net flux is for a heat flux meter cooled at 7 ,XJ, the ambient temperature, the gage directly measures the external incident radiative and flame heat fluxes. 

As reported in Ref. , the spread rate of a flame moving up a vertical surface of a sufficiently thick PMMA sheet increases under the effect of an external heat radiation. Depending on the heat radiation intensity and exposure time, various effects on the flame spread rate are observed. Additional heating of the polymer surface by a radiative flux results, first of all, in a decrease of the temperature dilTerence (T — Tp) and, in accordance with Eq. (2.19), in an increase of v. The experimental relationship v (T — To)" at T = 363 °C is close to that predicted by theory. According to Femandez-Pello , an increase of the initial polymer surface temperature, Tp, cause a parallel enhancement of the natural convection in the boundary heat layer and heat radiation by the surface, leading to its partial cooling. Therefore, when the intensity of the external radiative heat flux is low, the flame spread rate increases with time, but only up to a certain constant value. 

ABSTRACT The radiative pyrolysis of wood (thick cylinders and chip beds) has been investigated experimentally for external radiative heat fluxes in the range 28-80kW/m, resulting in maximum sample temperatures of 600-950K. Radial temperature profiles, product yields and composition, and devolatilization rates have been measured. The influences of wood variety (hardwoods and softwoods) on the pyrolysis characteristics are discussed and comparisons are made with biomass (agricultural residues). 

In this equation, q",.dA is the net radiative heat flux at the moving material surface imposed by external sources such as radiant burners/heaters or electric resistance heaters. Both parabolic, boundary layer [80], and full, elliptic [61,81] problem solutions have been reported. Because of the nature of the problem, the heat transfer results can t be given in terms of correlations. The interested reader is referred to Refs. 62 and 79 for citation of relevant references. 

An adiabatic refractory surface of area Ar and emissivity er, for which Qr = 0, proves quite important in practice. A nearly radiatively adiabatic refractory surface occurs when differences between internal conduction and convection and external heat losses through the refractory wall are small compared with the magnitude of the incident and leaving radiation fluxes. For any surface zone, the radiant flux is given by Q = A(W - H) = tA(E - H) and Q = eA/p( - W) (if p 0). These equations then lead to the result that if Qr = 0,Er = Hr = Wrfor all 0 < er< 1. Sufficient conditions for modeling an adiabatic refractory zone are thus either to put , = 0 or to specify directly that Q, = 0 with , 0. If er = 0, SrSj = 0 for all 1 < j < M which leads directly by definition to Qr = 0. For er = 0, the refractory emissive power Er never enters the zoning calculations. For the special case of 0 and Mr = 1, a sin-... 

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