Thermal performance of ventilated roofs in tropical climate

• Main Author: Tong, Shanshan
• Other Authors: Li Hua
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: http://hdl.handle.net/10356/62197
• Institution: Nanyang Technological University

Ventilated roof receives great interest in recent decades due to its effectiveness in reducing the space cooling load and improving the indoor thermal comfort in summer. A common ventilated roof consists of two parallel solid slabs and an open-ended air layer between the upper and lower slabs. Due to buoyancy or mechanical force, the ventilated air carries out part of the heat accumulated in the roofing system and contributes to the reduction of heat flux transmitted into the building interior. Therefore, the present research work aims to analyze and optimize the thermal performance of ventilated roofs in tropical climate, and minimize the heat flux transferred across the roofs with different inclination angles and geometries, materials as well as ventilation modes. However, the heat transfer mechanisms across the ventilated roof from outdoors to the building interior are complicated, which include the conductive heat transfer in the upper and lower roof slabs, the convective and radiative heat transfers in the ventilated cavity, as well as the radiative and convective heat transfers between roof and the outdoor or indoor environment. Therefore, an accurate prediction of the transferred heat flux requires a complete thermo-fluid analysis of the convective airflow in the ventilated cavity, and a fundamental understanding of the thermophysial properties of roofing materials as well as the convective and radiative heat transfer coefficients at the exterior and interior roof surfaces.As the first achievement made in this thesis, a field experiment and modeling analysis are performed for the thermal performance of a flat roof subjected to the tropical climate in Singapore. A field experiment is carried out to examine the impacts of the solar-reflective cool paint on the surface temperature, transferred heat flux and indoor thermal comfort. Based on the experimentally measured data, a correlation is proposed for the coupled convective and radiative heat transfer between the exterior roof surface and the surrounding environment. In addition, the transient surface temperature of the flat ventilated roof subjected to varying indoor and outdoor conditions is modeled via the complex fast Fourier transform (CFFT) technique. Good agreements are obtained between the theoretically modeled and experimentally measured hourly roof temperatures during both sunny and rainy time. The validated model is extended to study the impacts of the solar reflectance of exterior roof surface and roof insulation on the thermal performance of flat unventilated and ventilated roofs. The second achievement of the present thesis is the development of a novel model for the fast and accurate estimation of the heat flux transferred across the naturally ventilated inclined roof. The developed model takes into account the conductive, radiative and convective heat transfers from the outdoors to the building interior. A weighted-average temperature is derived to represent the temperatures of the sol-air, outdoor air, and indoor air through the circuit transformation theory and thermal network analysis. Moreover, correlations are proposed for the convective thermal resistances at the upper and lower cavity surfaces, based on the two- dimensional(2-D) computational fluid dynamics (CFD) analysis of the turbulent natural convection flow in the roof cavity. The transferred heat flux predicted by the developed model with the proposed correlations is compared with that predicted by a full CFD model, and a good agreement is obtained between the two predictions. The present third achievement is the theoretical and experimental analysis of the impact of the cavity width on the natural convective heat transfer in the inclined roof cavities with low width-to-spacing ratios. A three-dimensional (3-D) CFD model is built up firstly to analyze the airflow movement and natural convective heat transfer in the inclined roof cavity. The 3-D model is validated by the experiments performed on the inclined and vertical cavities with low width-to-spacing ratios. A close agreement is found between the theoretically simulated and experimentally measured cavity surface temperatures, airflow temperatures and velocities. After validation, the 3-D CFD model is employed to analyze the impacts of cavity width on the induced airflow velocity and convective heat transfer coefficient in the cavities. The last achievement is the theoretical and experimental study of the thermal performance of forced-ventilated roofs. A CFD model is built up to analyze the airflow movement and mixed convective heat transfer in the forced-ventilated roof. The CFD model is validated further by the experiments performed on vertical and inclined cavities. The validated CFD model is then employed to analyze the impacts of the induced airflow velocity on the mixed convective heat transfer in the forced- ventilated roof cavities.