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Abstract
An energy balance at the exterior surface of a roof is an essential boundary condition for mathermatically describing the heat flow through a multilayered insulated roof. The dominant parameters affecting the heat flow during the daylight hours are convection and the surface properties of reflectance and emittance. At night and during the twilight hours, the infrared emittance and mass transfer balance against the roof's heat flow. The mass-transfer coefficient is calculated directly from a dimensionless term called the Lewis number and the air-side convective-heat transfer coefficient. The approach is aplicable to turbulent and laminar flow regimes. However, research has shown the calculation procedure to be questionable at both low velocities and small air-to-water temperature differences, conditions prevalent during the eveniung hours on a low-slope roof. Hence,a correlation ws formulated and validated against experimental data to better estimate the mass-transfer coefficient and, therefore, the heat transfer through the roof when condensation or evaporation occurs.
An energy balance at the exterior surface of a roof is an essential boundary condition for mathermatically describing the heat flow through a multilayered insulated roof. The dominant parameters affecting the heat flow during the daylight hours are convection and the surface properties of reflectance and emittance. At night and during the twilight hours, the infrared emittance and mass transfer balance against the roof's heat flow. The mass-transfer coefficient is calculated directly from a dimensionless term called the Lewis number and the air-side convective-heat transfer coefficient. The approach is aplicable to turbulent and laminar flow regimes. However, research has shown the calculation procedure to be questionable at both low velocities and small air-to-water temperature differences, conditions prevalent during the eveniung hours on a low-slope roof. Hence,a correlation ws formulated and validated against experimental data to better estimate the mass-transfer coefficient and, therefore, the heat transfer through the roof when condensation or evaporation occurs.
Date
12/2001
12/2001
Author(s)
William Miller; Jerald Atchley
William Miller; Jerald Atchley
Page(s)
Keyword(s)
heat transfer; turbulent flow; energy balance; heat flow; laminar flow; heat transfer; ORNL; ASHRAE
heat transfer; turbulent flow; energy balance; heat flow; laminar flow; heat transfer; ORNL; ASHRAE