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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|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|
|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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