Challenge
On their most recent project, they modeled a new building and discovered that, without appropriate supply air distribution and window constructions, temperatures would be too high in the atrium during the summer and too low near windows in the winter. Air jet behavior and areas of high velocity are important to discover to determine appropriate thermal comfort. With this information in hand, they made modifications to the air distribution and heating systems and re-ran the CFD analysis to validate their ability to solve the problems. “Computer simulation significantly improved the indoor air quality of this building while avoiding the expense that would have otherwise been required to modify the HVAC (heating ventilation air conditioning) system after it had already been installed,” said Tuomas Laine, Manager of Research and Development for Olof Granlund Oy.
Olof Granlund Oy provides design, consulting and technical computing services over the whole life cycle of a building. The company is the largest building services design and consulting company in Finland and employs about 270 people. During the entire construction process, the company provides systematic supervision in order to achieve design targets and ensure the continuity of information flow to facilities management. Granlund has also developed a comprehensive family of integrated design and facilities management software, including commercial applications for international markets as well as proprietary tools.
In this project, the company was asked to evaluate and if necessary improve the comfort level of a new office building in Helsinki, Finland. The designers were concerned about thermal comfort because of the large surface area of windows and skylights and the large atrium. While many people think of Finland as a very cool climate, the Helsinki area actually has greater solar thermal loading during the summer than Paris. “Keeping the atrium warm enough is difficult in the winter because of the large window and skylight area.” said Sami Lestinen, CFD Specialist for Granlund. “On the other hand, keeping the atrium cool in the summer months is challenging because skylight and windows caused significant solar heat source to atrium .”
Previous method only provides average temperatures
In the past, Granlund would have used only an energy analysis program that accepts as input a description of the building layout, systems, construction, usage, and utility rates, along with weather data. This program performs an hourly simulation of the building and estimates utility bills. The limitation of this type of program is that it is primarily intended to evaluate energy usage and cost and thus can only predict average temperatures for a particular space.
Of course, knowing that the average temperature in the atrium is 72 degrees Fahrenheit (22 degrees Celsius) would be small comfort to someone standing in an area of the room where the temperature is 85 degrees Fahrenheit (29 degrees Celsius).
“In the past, about all that consultants could do was to make sure the average temperatures were right and hope that there wasn't too much variation,” Laine said. “In this building, because of the large window and skylight areas, we were almost certain that this method wouldn't be good enough. The real risk faced by the building owner was that after the building was finished the occupants would report major heating and cooling problems. It would then be necessary to go through a lengthy and expensive trial and error process in order to remedy the problem. The owner asked us to use advanced tools such as CFD in order to avoid this kind of headache.”
One of the reasons for coming to Granlund was the company's experience in the use of CFD to simulate indoor airflow and thermal conditions. CFD involves the solution of the governing equations for fluid flow, heat transfer, and chemical reactions at thousands of discrete points on a computational grid representing the flow domain. A CFD analysis yields values for gas or liquid velocity, temperature and pressure throughout the solution domain, providing more data points than conventional testing.
Based on the CFD analysis, a designer or an engineer can optimize fluid flow patterns by adjusting either the geometry of the system or the boundary conditions such as inlet velocity or temperature. “We use CFX-5 CFD software from ANSYS Inc. because we have found that it can quickly and easily model the complex geometries found in most large building projects,” Laine said. “We also like the wide range of physical models and powerful visualization capabilities found in CFX-5.”
Modeling the new building
Lestinen created the model by importing the computer aided design geometry of the building in IFC format into CFX-5 and generating an unstructured tetrahedral mesh. One of the challenges in modeling indoor thermal conditions is the need to account for small-scale supply air diffusers, while at the same time accommodating large open areas of the atrium. He added mesh controls to the areas with large gradients near supply air devices and heat sources and then set the mesh density to an appropriate value. This approach made it possible to provide high levels of accuracy in critical area while keeping the model to a reasonable size of 600,000 nodes and 3,000,000 elements.
Lestinen took advantage of the ability of an energy analysis program to determine dynamic thermal behavior of the atrium and office spaces in order to reduce the time required to perform the CFD analysis. He used a program developed by his company, called RIUSKA, which uses the DOE2 thermal dynamics program as a calculation engine, to determine surface temperatures and heat gains in the building. Air terminal boundary conditions were provided by a software program called CFD Blockmaster from Halton Group specifically designed to simulate diffuser performance. Inputting these values into the CFD analysis as boundary conditions greatly reduced the time that would have otherwise been required to calculate them from scratch.
“In analyzing a heating and cooling system, schedules and costs have to meet the requirements of real projects,” Laine said. “By efficient use of CFD simulation as part of our analysis tool environment we were able to provide necessary modifications to the building design by reasonable cost.”
“When we looked at the simulation results of different air distribution solutions, we found clearly the system for air distribution throughout the building,” Lestinen said.
“One general problem was that air velocities were too high for comfort in a number of occupancy zones. Temperatures were too high in certain areas of the atrium during the summer due to the large amount of solar thermal loading through the skylights. We also noticed that optimization of supply air distribution in offices was important for thermal comfort. We addressed the airflow problem and the heat distribution in the atrium by evaluating a number of different diffuser configurations. We finally found a diffuser design that reduced airflow to comfortable levels while improving thermal distribution to the point that the temperature remained within an acceptable range in all occupied areas of the atrium under summer conditions. We also modeled a variety of different window heating systems until we found one that solved the problems.”
The value of CFD
“CFD supplements energy analysis software by providing far more detailed results that makes it possible to predict, for example, temperatures and air velocities at any point in a building,” Laine concluded. “The challenge in CFD analysis is providing results at a level of accuracy and in a timeframe that makes it possible to impact the building design. CFX-5 makes it possible to meet the challenge of providing results at a high level of accuracy in time to impact the building design. We found solutions to initial design in cases where thermal comfort is important. The building owner knows to have design with advanced simulation methods,” Lestinen said “and the fact is that correctly implemented CFD is the only believable way to simulate indoor air flows.”
Photorealistic visualisation of the glazed lobby.
ANSYS CFX analysis of the lobby for temperature.
ANSYS CFX analysis of the lobby for velocity.
ANSYS CFX model of the air temperature in an office.
ANSYS CFX model of the air velocity in an office.
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