As air distribution solutions become increasingly bespoke to achieve good quality indoor air while meeting aesthetic design requirements, Joe Jones, engineering and development manager at Waterloo, looks at how manufacturers are turning to computational fluid dynamics for optimum results.
At some point in our lives, most of us will have experienced that unpleasant waft of stale air, perhaps walking into an office space or stuffy hotel room. Reach for the air conditioning switch, and the loud noise from the blast of fresh air assails our ears. Then, to top it off, we find we’re caught in a nasty cold draught.
Fortunately, there is growing momentum on the importance of ensuring good indoor air quality (IAQ) to protect both the well-being and health of building occupants and create better buildings. Modern buildings are designed to be more airtight, but while this makes them more energy efficient, an effective ventilation system is essential to prevent a potentially harmful build-up of pollutants. Providing a healthy indoor environment is therefore a key focus in new building design.
Ensuring efficient and effective ventilation also ultimately benefits employers, as revealed in a recent study commissioned by Harvard Business Review. In a survey of over 1,600 North Americans, better air quality was the top environmental factor cited to improve the happiness, well-being and, ultimately, the productivity of employees. This was followed by access to natural light and the ability to personalise their workplace.
An efficient and effective air flow will also use less energy, so it’s a cost-effective means for businesses and organisations to simultaneously address a building’s energy efficiency and occupant wellbeing.
So how to go about optimising air distribution? Forward-thinking manufacturers are turning to technology – specifically to Computational Fluid Dynamics (CFD) – as a rapid, accurate tool to deliver the best possible air flow in every environment from their products.
Optimising air distribution
Creating a comfortable indoor climate involves multiple tasks, such as providing the proper volume of fresh air, keeping a stable temperature, and minimising the draught exposure.
With a conventional diffusion arrangement, primary air is supplied over the occupied zone where it mixes with room or secondary air. This process causes the initial temperature and velocity difference between the supply and room air to decrease, so that when the supply jet reaches the occupied zone, the velocity and temperature are close to room conditions.
This is achieved through air terminal devices such as grilles and diffusers, with the location, type and size determining the manner in which the supply jet and resultant room air motion behave.
The image below shows the occupied zone which is defined as the area up to 1.8m from the floor and as close as 150mm from any room surface.
|Vr||Room Air Velocity||0.1…0.25||ms-1|
|Vt||Air Velocity from the Air Terminal Device (ATD)||0.15…0.5||ms-1|
|Tr||Room Air Temperature||20…24||°C|
|Ta||Supply Air Temperature||10…34||°C|
Fast and accurate solution
How, then, can CFD benefit the design and product selection process? CFD is applied to a wide range of industries, including aerodynamics, aerospace, weather simulation and environmental engineering. When applied to air distribution, CFD modelling can help accurately identify the most appropriate solutions by using numerical analysis and data structure to analyse and optimise air flow.
The technology enables a 3D case study model of the area to be produced using simplified polygons. Influencing parameters or conditions can then be added, such as air temperature, pressure, humidity, velocity, flow and volume.
The CFD simulation produces cross sections of the room, with arrows indicating the pressure, temperature and velocity of the air flow, similar to a meteorological report. CFD enables us to analyse ventilation patterns such as different inlet directions and positions. Effectively, it enables the air to be visualised more easily than using probes in a laboratory.
Visualising air flow
Let’s consider an office space. Using CFD, we can visualise the direction and speed of air travel within the room. In the image below, the arrows show the air entering the room from the ceiling viaduct through a four-way diffuser. The air blasts into the room, then slows down as it loses energy and mixes with the room air. While the occupant would be comfortable at their desk, a draught would be felt walking past the grille.
Spreading the air out over more diffusers is one means of keeping the draught and air supply away from the occupant. Alternatively, the air could be introduced through the floor or wall to diffuse the air over a larger area, as shown in the image below, where the air is introduced through a floor grille. Air stream that is discharged close to and parallel to a surface, tends to cling to the surface. This is known as the coandra or ceiling effect.
There can be no one-size-fits all solution to product selection as a number of factors, including thermal and acoustic criteria, will influence specification. The air distribution requirements in a hotel bedroom, for example, will differ from those in of a restaurant or bar area. Architectural and aesthetic requirements also increasingly need to be considered alongside functionality when determining the type and location of a grille or diffuser.
Adopting a bespoke approach to design will undoubtedly result in the best solution, but it can be a complex and time-consuming challenge. CFD allows engineers to design the air flow more rapidly and accurately to meet the customer specification for the environment. And for manufacturers like Waterloo, it provides an efficient means of ensuring that the most effective products are designed to ensure optimum high-performance results.