Abstract
A 2D unsteady computational fluid dynamics (CFD) model is employed to simulate buoyancy-driven turbulent ventilation in attics with different pitch values and ceiling insulation levels under summer conditions. The impacts of roof pitch and ceiling insulation on the cooling load of gable-roof residential buildings are investigated based on the simulation of turbulent air flow and natural convection heat transfer in attic spaces with roof pitches from 3/12 to 18/12 combined with ceiling insulation levels from R-1.2 to R-40. The modeling results show that the air flows in the attics are steady and exhibit a general streamline pattern that is qualitatively insensitive to the investigated variations of roof pitch and ceiling insulation. Furthermore, it is predicted that the ceiling insulation plays a control role on the attic cooling load and that an increase of roof pitch from 3/12 to 8/12 results in a decrease in the cooling load by around 9% in the investigated cases. The results suggest that the increase of roof pitch alone, without changing other design parameters, has limited impact on attics cooling load and airflow pattern. The research results also suggest both the predicted ventilating mass flow rate and attic cooling load can be satisfactorily correlated by simple relationships in terms of appropriately defined Rayleigh and Nusselt numbers.
Highlights
Ventilated attic has been a long-established practice in residential building construction [1], but how different attic designs affecting building energy performance has not been thoroughly investigated
In order to account for a wide range of roof pitch and ceiling insulation, involving several orders of magnitude variation of Rayleigh number, the buoyancy-driven air flow and natural convection heat transfer are modeled by the k-kl-ω model [12], which is a physics-based transitional turbulence model capable of modeling turbulent flows from laminar-turbulence transitional regime to fully turbulence regime
In order to facilitate a parametric study of the effects of roof pitch and ceiling insulation on the cooling load, a total of 20 cases are calculated with five typical pitch values (3/12, 5/12, 8/12, 12/12, and 18/12), combined with four ceiling insulation levels (R-40, R-20, R-10, and R-1.2)
Summary
Ventilated attic has been a long-established practice in residential building construction [1], but how different attic designs affecting building energy performance has not been thoroughly investigated. Since wind effects are not included in this study, the air flow and heat transfer in the attic spaces are purely driven by stack effects. Such buoyancy-driven cases are corresponding to a worst-case scenario, because real attic ventilation is generally enhanced by winds. In order to account for a wide range of roof pitch and ceiling insulation, involving several orders of magnitude variation of Rayleigh number, the buoyancy-driven air flow and natural convection heat transfer are modeled by the k-kl-ω model [12], which is a physics-based transitional turbulence model capable of modeling turbulent flows from laminar-turbulence transitional regime to fully turbulence regime. The numerical model will be introduced first, followed by detailed presentation and discussion of the modeling results
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