For decades, Computational fluid dynamics (CFD) has allowed scientists and engineers to model and predict flow performance in numerous application areas. As computational power increases, the limits of applications continue to grow.
It enables HVAC systems to achieve their purpose to provide ventilation, thermal comfort or other special indoor conditions. As the traditional method, ground rules and hand calculations are used to size ventilation equipment capacities and parameters. However, this approach undergoes physical testing at the final stage to ensure compliance with requirements.
The typical result of this approach is the specification of oversized equipment and, consequently, an increase in upfront costs and energy consumption. As a matter of fact, fans constitute 13% of the total energy consumption in the commercial area and 6% in the residences in the USA. CFD comes to the rescue in this situation and provides an accurate prediction of the performance of a ventilation system by modeling physical phenomena, ambient conditions and the detailed geometry of the area.
In this article, we’ll take a look at some common ventilation system design examples and the benefits of handling analysis with CFD.
Passive Ventilation
Also known as natural ventilation, this strategy takes advantage of natural phenomena such as wind flow and buoyancy to create pressure differences that drive air changes inside the building. CFD is a very powerful tool for predicting the behavior of passive ventilation systems as it can precisely model special events such as buoyancy due to temperature or humidity differences. It can also take into account almost any geometric feature, so innovative solutions can be tested and validated prior to prototyping or construction.
In this example case (Figure 1), the natural ventilation of a three-storey shopping mall building is simulated using CFD. Results are drawn for comparison with building requirements and we can discover if they are adequately met.
Ventilation Equipment
A forced ventilation strategy is needed when the passive ventilation strategy is not sufficient to meet the performance requirements. This is achieved by applying fans, blowers or compressors to create pressure differences and direct air changes in the target area.
CFD provides great assistance in the design process of air motion equipment. It allows engineers to predict performance in terms of pressure distributions, flow paths and velocities. Design variations can then be quickly tested and compared, all in a virtual environment.
In this case study (Figure 2), the aerodynamic performance of a centrifugal fan was investigated using CFD simulation. Performance curves are drawn and improvement areas are determined.
Displacement Ventilation
In the traditional mixed ventilation strategy, conditioned air intakes are placed at the top of the space and at high flow rates. In contrast, the displacement ventilation strategy places air intakes near ground level and uses relatively low flow rates. Hot air produced by occupants or equipment rises to ceiling level via buoyancy (called thermal clouds), where it is consolidated and removed from the space without recirculation. A common application of this strategy is found in classrooms and office spaces as it provides improved air quality.
In the presented example (Figure 3), CFD simulation was used to compare mixing and displacement ventilation strategies in an office space. The results are presented in terms of temperature distributions and local thermal comfort visualizations, revealing that a displacement ventilation strategy can have the advantage of lower energy consumption and improved air quality.
Jet Fans
Jet fans are used to direct and strengthen flow in large areas. When used in conjunction with the strategic placement of inlets and outlets, optimum flow behaviors can be achieved in difficult conditions.
In the example studied (Figure 4), this ventilation strategy is used in a large garage area to reduce pollution levels and eliminate areas of high concentration. In the use of CFD, fan placement and performance are all evaluated quickly and efficiently. The results revealed that the proposed strategy would reduce contamination levels by 55.1%.
Compliance with Thermal Comfort Standards
One area where CFD simulation is particularly useful is the assessment of standards compliance. CFD allows engineers to measure variables such as temperature and flow rate at any point in the geometric space under consideration. This means that standards performance criteria can be easily deduced and interpreted and areas for improvement identified.
Using CFD simulation, the thermal comfort performance of a cinema space is evaluated according to two standard codes:
EPBD and EN 15251
The Energy Performance of Buildings Directive (EPBD) was adopted in 2002 to increase the energy efficiency of buildings. As part of this directive, the EN-15251 standard was created to specify requirements for indoor thermal comfort. In this working example (Figure 5), thermal comfort in the theater was evaluated according to EN 15251. The findings reveal that the ventilation system is insufficient and areas for improvement have been identified.
ASHRAE 55
The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has been publishing Standard 55 “Thermal Ambient Conditions for Human Occupation” since 1966. This standard has also been adopted by international organizations such as ISO and many other countries.
In the operating situation, ASHRAE 55 metrics were used to compare thermal comfort performance between two ventilation strategies (Figure 6) simulated with CFD in the theater.
This article presents case studies in the HVAC industry and explains the benefits of CFD simulation for evaluating performance.
