Semester

Fall

Date of Graduation

2008

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

James E. Smith.

Abstract

Due to the rising power costs and lack of nonrenewable energy sources, the cooling of houses is becoming more expensive. Looking for alternative methods applicable to this process is becoming not only an option, but also a necessity. Changes in the roof structure of buildings can be applied in order to achieve a more favorable thermal transmission behavior. The utilization of a tilted roof, composed of two separated surfaces, generates natural convection currents in the channel between them. These currents, after driving off part of the transferred heat, decrease the temperature of the lower surfaces and consequently, the heat flux through the ceiling into the living areas.;The natural convection phenomenon is treated by numerical means, and the influence of the dimensions of the proposed design on the ventilation rates is analyzed in order to determine the most efficient geometry. The comparison of thermal performances between the proposed roof and a typical unventilated design is also established in order to realize the quantitative advantage of the proposed model.;Results show that the separation between surfaces strongly influences the process within certain values; i.e. a reduction in the heat flux through the ceiling achieved by the system of 32.9% can be raised to 45.4% by increasing the width of the channel from 0.05m to 0.15m, and keeping the other dimensions constant. Moreover, higher tilt angles also improve natural ventilation rates. For example, a 32.8% reduction obtained by the system at a 30 degree tilt angle grows up to 41.6% by raising the tilt angle to 65 degrees. A vertical extension or exhaust channel on the top increases the reduction of heat flux too, but with less intensity. In this sense, the heat flux reduction achieved by the system, when the vertical exhaust length is 12.5% of the length of the roof, increases from 32.9% to 45.5% when a considerably bigger vertical extension is used, 60% of the roof length.

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