Fundamentals ofConvection and RadiationCH EN 3453 – Heat Transfer Reminders• Homework #1 due Friday at 4:00 p.m.– Minus 50% if not in by 4:00– Solutions to 2-5 are at www.chen3453.com• Help session today– 4:30 p.m. in MEB 2325 • Conflicts with that time?– Come with questionsConvection• Heat transfer due to the combined influence of bulk and random motion for fluid flow over a surface.• Requires the presence of temperature variations in the material mediumFigure 1.1(b) ConvectionRelation of convection to flow over a surface and developmentof velocity and thermal boundary layers:Convective Heat Transfer RateEq. 1.3aNewton’s law of cooling:h = Convection heat transfer coefficient (W/m2·K)Types of Convective Heat TransferHeat Transfer Coefficient hExample – ConvectionThe case of a power transistor, which is of length L = 10 mm and diameter D = 12 mm, is cooled by an air stream of temperature T∞ = 25°C. Under conditions for which the air maintains an average convection coefficient of h = 100 W/m2·K on the surface of the case, what is the maximum allowable power dissipation if the surface temp is not to exceed 85°C?Radiation• Energy that is emitted by matter due to changes in the electron configurations of its atoms or molecules and is transported as electromagnetic waves (or photons).• Transport does not require a material medium and occurs most efficiently in a vacuum.Figure 1.1(c) RadiationRadiationHeat transfer at a gas/surface interface involves radiation emission from the surface and may also involve the absorption of radiation incident from the surroundings (irradiation, G), as well as convection (if Ts ≠ T∞)Energy outflow due to emission:Radiative Heat Transfer RateE : Emissive power (W/m2)ε : Surface emissivity (0 ≤ ε ≤ 1)Eb : Emissive power of a blackbody (the perfect emitter)σ : Stefan-Boltzmann constant (5.67 x 10–8 W/m2·K4)Energy absorption due to irradiation:Gabs : Absorbed incident radiation (W/m2)α : Surface absorptivity (0 ≤ α ≤ 1)G : Irradiation (W/m2)Example – RadiationA spherical interplanetary probe of 0.5-m diameter contains electronics that dissipate 150 W. If the probe surface has an emissivity of 0.8 and the probe does not receive radiation from other surfaces as, for example, from the sun, what is its surface temperature? Example – Convection + RadiationAluminum plate 4 mm thick mounted horizontally with bottom well insulated. Thin coating on top absorbs 80% of incident solar radiation while having an emissivity of 0.25. Density and specific heat of Al are 2700 kg/m3 and 900 J/kg·K.(a) If plate, initially at 25°C is suddenly exposed to air at 20°C (h = 20 W/m2·K) and radiation of 900 W/m2, what is the initial rate of temperature change?(b) What will equilibrium temperature of plate