Date of Award
Master of Science
Attic energy modeling is a topic of interest as current whole building energy models have a difficult time predicting the thermal performance of attics. Recent developments in attic energy modeling has allowed, for the first time, many different attic shapes to be analyzed. This allows analysis of roofs beyond standard gable, shed, flat, and saltbox type roofs. With this generalization of attic geometries, the calculation of view factors becomes a potential issue for these diffusely emitting nonconvex polygon shaped attics. The presence of air ducts further complicate calculation of view factors and radiation heat transfer in attics. Currently there are two methods to produce these view factors, 1) solving the double integral that defines the view factor or 2) simple calculations based on engineering assumptions to allow for the view factors to be easily calculated. The first approach can be computationally intensive as air ducts and complex geometry can drastically increase the number of surfaces to be calculated in the integral equation, and can be tedious to set up. On the other hand, the second (very fast) approach is built upon assumptions in ASHRAE RP-717. Very limited analysis has been documented on the validity of these assumptions. Furthermore, with current advances in the ability to model different attic geometries these assumptions may no longer be valid. This work focuses on updating the view factor assumptions to provide a fast and accurate method for calculating view factors in attics containing circular air ducts under more generalized conditions. The assumptions are updated by first systematically evaluating the old assumptions to determine whether a modification is needed, followed by designing and testing an alternative approach when needed. In all cases, extensive evaluation and comparison of proposed alternative approach with the older assumptions and double integral calculations are performed. Finally, a comparison between approach in RP-717 and the new rules developed in this thesis is reported. We envision that these updated assumptions can easily be included in whole building or attic specific energy modeling software to better capture the thermal effects of radiation heat transfer between air ducts and attic surfaces.
Tran, Hiep, "View factors for circular air ducts in attics: fast and accurate approximations" (2016). Graduate Theses and Dissertations. 15443.