Radiative-convective boundary

The relationship between heat transfer and the boundary layer species distribution should be emphasized. As vaporization occurs, chemical species are transported to the boundary layer and act to cool by transpiration. These gaseous products may undergo additional thermochemical reactions with the boundary-layer gas, further impacting heat transfer. Thus species concentrations are needed for accurate calculation of transport properties, as well as for calculations of convective heating and radiative transport. 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

If we can define the radiative terms a priori, we are simply adding a constant to the RHS. An expedient transformation reduces the boundary condition to its original convective form ... 

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. 

Radiative heat transfer is often coupled wim convective heat transfer to form me boundary conditions for conduction problems. A radiation heat transfer coefficient may be defined by 7rad = A rad (T) ), where /trad = iO(7i + )(7i + Tsur) is a strong function of me temperatures... 

With newer computational tools such as finite element and boundary element methods, conduction and convection in arbitrary three-dimensional geometries can be handled in a relatively straightforward manner. Some recent efforts have been made to link the FEM method (for ease in handling complex geometries) with alternative methods to evaluate view factors in order to predict thermal processing of complex shapes with specular surface radiative properties or specular reflection [177]. 

The boundary condition at the inner (top) surface of the load is exposed to the radiative and convective heat q"ot such that... 

Heat was introduced into the model at the heated drift and at the inner and outer wing heaters. The heated drift cavity was not explicitly included in the model to avoid difficulties associated with representing the air space within the drift, radiative and convective heat transfer between the heater canisters and the drift wall, and the physics of heat and mass transfer at the drift-cavity/drift-wall boundary. The disadvantage to this simplification is that coupled thermal-hydrologic processes at the drift wall cannot be directly or easily investigated using this model. 

As the battery exchanges heat with the environment, radiative and convective heat flows are apphed to the outermost discretization element. These two boundary conditions depend on... 

The first term on the right hand side represents heat transfer due to conduction and second term represents the heat released due to heterogeneous reactions within the electrodes which vanishes in the case of cathode. Two source terms, the radiative heat source term Qr, and the convective heat source term enters Eq. 4.48 as boundary condition at the interface between electrode and the flow channel, and the electrochemical heat source term enters as boundary condition at the interface between the anode and the electrolyte. The radiative heat transfer between the interconnect and the outer most discretised cell in the porous electrode is given by... 

The first boundary condition, Equation 8.9a, implies that we are assuming the wall and bed temperatures are equal at the initial point of contact. Also, the mean free path for the gas at the contact wall is sufficiently large that convective heat exchange by the gas is not in local equilibrium with the conduction through the bed. However, radiative heat transfer can play a vifal role within the penetration layer (Perron and Singh, 1991). As we did for the freeboard, the most practical approach is not only to solve the differential equation but to establish a heat transfer coefficient that can be used for practical calculations. The heat transfer coefficient per unit contact area may be written in terms of the overall heat balance using Newton s law of cooling. 

At fuel manifold inlets, gaseous species concentrations are specified as equilibrium compositions of the town gas reformate at 650°C. Steam-to-carbon ratio is kept as 3.06 for this particular steady-state analysis. Both fuel and air gas manifold inlet conditions are summarized in Table 9.5. Mixed convective and radiative heat transfer boundary conditions are applied to the side surfaces of the stack to accurately model the heat exchange with the balance of plant components. Top and bottom surfaces, on the other hand, are assigned with... 

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