Ventilation by Thermal Buoyancy in the Air Cavity of Pitched Roofs - An Experimental and Numerical Study of Air Cavity Design and Natural Convection in Parallel Roof Constructions

Examensarbete för masterexamen

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Examensarbete ACEX30-19-79 (Martina Svantesson och Toivo Säwén).pdf32.12 MBAdobe PDFView/Open
Type: Examensarbete för masterexamen
Title: Ventilation by Thermal Buoyancy in the Air Cavity of Pitched Roofs - An Experimental and Numerical Study of Air Cavity Design and Natural Convection in Parallel Roof Constructions
Authors: Svantesson, Martina
Säwén, Toivo
Abstract: A parallel roof is a common roof type in Nordic countries, ventilated through an air cavity for the removal of heat and moisture. The air flow is driven by wind pressure and thermal buoyancy. A large amount of research has been performed on wind driven cavity ventilation for the purpose of heat removal. However, few studies have considered the thermal buoyancy as a driving force, or the perspective of moisture removal. Also, there is a lack of quantitative guidelines for the design of air cavities in roof constructions in Sweden, making it difficult to evaluate a proposed roof design. This study investigates how the air cavity design affects the thermal buoyancy by experiments and by numerical simulations. The experimental tests were performed on a full-scale roof model, with a cavity length of 3:5m, heated to simulate a solar heated roof. Cavity heights between 23 and 70 mm as well as roof inclinations between 5 and 45° were tested for different heat intensities applied to the system. Surface and air temperatures were measured and the air velocity in the cavity was determined by smoke tests. Numerical CFD modelling of the same heated air cavity was also performed in COMSOL Multiphysics, aiming to replicate the experimental results. The experimental and numerical results were used to characterise the driving forces and the resistances for air flow by using the dimensionless Grashof number. To also include the thermal conditions of the cavity, the dimensionless Rayleigh number was used and a relationship between Rayleigh number and air flow rate was derived. An analytical model of the thermal and mechanical behaviour in the air cavity was created, as a basis for further studies of the moisture conditions in an air cavity. The study shows that an increased heat intensity increases air and surface temperatures, which in turn causes larger air flow rates. An increased cavity height and a higher inclination cause larger air flow rates, while the air velocity has a maximum value. Higher flow rates cause decreased air and surface temperatures for a constant heat intensity. The results of the study imply that thermal buoyancy is of relevance when evaluating the performance of cavity ventilated roof constructions from a moisture perspective in Swedish climates. However, further research is required to ascertain the impact of these findings regarding moisture safety.
Keywords: parallel roof;cavity ventilation;natural convection;thermal buoyancy;CFD
Issue Date: 2019
Publisher: Chalmers tekniska högskola / Institutionen för arkitektur och samhällsbyggnadsteknik (ACE)
Collection:Examensarbeten för masterexamen // Master Theses

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