KNOWN: Sky, ground, and ambient air temperatures. Grape of prescribed diameter and properties. FIND: (a) General expression for rate of change of grape temperature, (b) Whether grapes will freeze in quiescent air, (c) Whether grapes will freeze for a prescribed air speed. ASSUMPTIONS: (1)
KNOWN: Dimensions and construction of truck roof. Roof interior surface temperature. Truck speed, ambient air temperature, and solar irradiation. FIND: (a) Preferred roof coating, (b) Roof surface temperature, (c) Heat load through roof, (d) Effect of velocity on surface temperature and heat load.
KNOWN: Dimensions, spectral absorptivity, and temperature of solar receiver. Solar irradiation and ambient temperature. FIND: (a) Rate of energy collection q and collector efficiency η, (b) Effect of receiver temperature on q and η. ASSUMPTIONS: (1) Steady-state, (2) Uniform irradiaton, (3) Opaque,
KNOWN: Thermal conductivity, spectral absorptivity and inner and outer surface conditions for wall of central solar receiver. FIND: Minimum wall thickness needed to prevent thermal failure. Collector efficiency. ASSUMPTIONS: (1) Steady-state conditions, (2) Outer surface is opaque and diffuse, (3)
KNOWN: Plate temperature and spectral absorptivity of coating. FIND: (a) Solar irradiation, (b) Effect of solar irradiation on plate temperature, total absorptivity, and total emissivity. ASSUMPTIONS: (1) Steady-state, (2) Opaque, diffuse surface, (3) Isothermal plate, (4) Negligible radiation from
KNOWN: Glass sheet, used on greenhouse roof, is subjected to solar flux, GS, atmospheric emission, Gatm, and interior surface emission, Gi, as well as to convection processes. FIND: (a) Appropriate energy balance for a unit area of the glass, (b) Temperature of the greenhouse ambient air, T∞,i, for
KNOWN: Flat plate exposed to night sky and in ambient air at Tair = 15°C with a relative humidity of 70%. Radiation from the atmosphere or sky estimated as a fraction of the blackbody radiation corresponding to the near-ground air temperature, Gsky = εsky σ Tair, and for a clear night, εsky = 0.741
KNOWN: Plate temperature and spectral and directional dependence of its absorptivity. Direction and magnitude of solar flux. FIND: (a) Expression for total absorptivity, (b) Expression for total emissivity, (c) Net radiant flux, (d) Effect of cut-off wavelength associated with directional
KNOWN: Shallow pan of water exposed to night desert air and sky conditions. FIND: Whether water will freeze. ASSUMPTIONS: (1) Steady-state conditions, (2) Bottom of pan is well insulated, (3) Water surface is diffuse-gray, (4) Sky provides blackbody irradiation, Gsky = σT^4sky. PROPERTIES: Table
KNOWN: Large furnace with diffuse, opaque walls (Tf, εf) and a small diffuse, spectrally selective object (To, τλ, ρλ). FIND: For points on the furnace wall and the object, find ε, α, E, G and J. ASSUMPTIONS: (1) Furnace walls are isothermal, diffuse, and gray, (2) Object is isothermal and diffuse.