Solar Loads in Northern Climates
By Craig Makens and Amit Shah, ThermoAnalytics, Calumet, Michigan, USA and Matthew Monte, Monte Consulting Company, Houghton, Michigan, USA
A cutting plane through the fluid domain shows the wind velocity magnitude; the building geometries are colored by surface temperature with exterior walls ghosted to show internal rooms |
Advanced Simulation often requires the use of multiple analysis codes to account for all desired phenomena. As an example, an approximate representation of the Raymond L. Smith Mechanical Engineering Building on the campus of Michigan Technological University in Houghton, Michigan was recently used for an environmental simulation in the presence of transient solar radiation. The model demonstrated the combined use of FLUENT and RadTherm® software for architectural analysis.
Traditional HVAC and energy design makes use of empirical methods based on one and two dimensional approaches. These methods, while very powerful and established, often neglect wind effects and many other relevant situational factors, such as the surrounding terrain and neighboring structures. By combining FLUENT's accurate prediction of wind flow around a building with RadTherm's comprehensive environmental analysis, transient diurnal thermal results can be generated with minimal computation time. Wind flow patterns and eddy current effects are thus captured in FLUENT, while accurate solar shadowing, solar loading through glass, and infrared band heat transfer are rapidly computed in RadTherm. Interior heat loads, infiltration and ventilation effects are also accounted for in the thermal calculation. This combined approach allows architects and building designers to more accurately improve the thermal performance of each zone of a planned structure, to test the efficacy of energy saving devices like low-e windows, or to develop passive thermal heating and cooling designs.
The R. L. Smith Building geometry was created in Rhinoceros and meshed with ANSA for thermal analysis in RadTherm. The thermal model geometry consisted of 32,000 surface quads, including interior walls and floors. A preliminary thermal analysis was performed using RadTherm's built-in wind model. The building exterior wall temperatures were then exported to FLUENT to be used as a boundary condition profile in the next phase of the solution.
Diurnal surface temperatures of buildings and terrain, viewed from the southeast; varying environmental parameters include diffuse and direct solar loads, direct solar angle, cloud cover, and apparent sky temperature |
For the FLUENT calculation, a high resolution CFD mesh of 1.5 million tetrahedral cells was generated in GAMBIT. A steady-state analysis was performed using a bulk air temperature of 6.8°C and wind speed of 5 m/s from the north, representing a cold north wind - a common occurrence for autumn in this location. The flow analysis in FLUENT captured advective effects as the wind moved around the structure.
After the flow analysis was completed, the standard FLUENT menu export command was used and "RadTherm" chosen as the file type. This command exported a Patran Neutral file containing surface mesh geometry and convection data (convection coefficients and fluid temperatures on an element level basis). These data were then imported into RadTherm and mapped onto the lower resolution geometry as a boundary condition for the external surfaces of the buildings. Interior building surfaces retained the 1D fluid nodes used for natural and forced convection computation in RadTherm. A complete multimode thermal analysis was then carried out, including transient solar conditions with loading through the windows. The solar model considers global position, time of day, cloud conditions for predicting direct and diffuse solar loads, surface characteristics and glass characteristics. Post processing was carried out in EnSight, courtesy of CEI.
This model illustrates the comprehensive in-situ analysis that FLUENT and RadTherm can provide to buildings and downtown areas where complex flow and radiation effects render traditional empirical methods inadequate. The approach used in this model provides architects and building designers with more accurate heating and cooling requirements, which leads to proper and efficient HVAC sizing for each zone. The increased efficiency yields fast returns on the engineering investment, especially during high cost energy markets.
Particle trace of wind pathways colored by velocity magnitude. Note the high speed wind canyon between the R.L. Smith Mechanical Engineering Building and the Chemical Sciences Building to the east (right). In November 1994, several windows in the northwest area of the Chemical Sciences building were cracked by high speed winds. The windows had no mechanism to be opened, so it was not the result of them slamming shut, but rather the intense flexing within their rigid frames. The Smith building was constructed in the early 70s, and no wind analysis was done at the time. Wind canyons between these buildings can be quite severe, as many students and professors can testify |
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