CFD is short for Computational Fluid Dynamics. CFD is widely used to optimize air flow movement in residential and commercial buildings:
CFD for Commercial buildings:
CFD has wider application for commercial building due to the scale of the HVAC systems and the possible different fire hazards, presence of certain pollutants and the generation of noise. CFD solutions are applied to address following design criterion:
- HVAC system design: CFD can assist to ensure certain levels of air circulation occur inside the complex commercial buildings. CFD allows to optimize the placement and the number of air terminals and diffusers and sensors.
- Exhaust system design: Carparks, plants, sensitive storage spaces and warehouses, commercial food production facilities can use CFD to cost effectively exhaust pollutants. Using CFD, engineers and architect can make sure of the effectiveness of their designs.
- Noise isolation: Through CFD, one assure that zones subject to noise pollution are sufficiently insulated/isolated from internal or external sources of noise.
- Impact of wind: CFD wind analysis assists building designers to find the best location for draw-through or blow-through fans.
- Other applications: CFD may also be used for evaluation of the effectiveness of natural ventilation, condensation and formation of condensed water, spread of fire, effectiveness of outdoor evaporative cooling, etc.
CFD for Residential buildings:
Governing councils in Australia often require new buildings to address several key sustainability parameters. The scheme in which these sustainability criteria has been addressed is called Environmentally Sustainable Development (ESD). There are two key sustainable parameters where CFD solutions may assist builders and architects to obtain better score and more sustainable build:
Indoor environment quality:
- Ventilation: a good ventilated house is the one that maintains a good circulation of air and thus demands less cooling energy. Our CFD specialists assist architects in the design stage for a better naturally ventilated building. This is achieved by placing certain openings, alteration of the partitions, changing the building orientation to capture more natural air movement.
- Air quality: with the use of CFD analysis, we assure that level of pollutants and nanoparticles are minimized through by natural ventilation or fan forced air movement.
- Noise: through the acoustic analysis in a CFD software, we can make assure that levels of noise inside the building are minimized.
Governing councils across Australia encourage builders and architects to innovate and explore passive design solutions to minimize the need for artificial ventilation, cooling and heating. CFD can contribute to the following passive house concepts:
- Building orientation to maximize passive ventilation.
- Thermal mass (e.g. Trombe wall) contribution to offset heating demand.
- Solar chimneys or solar-aided natural ventilation.
- Passive cooling system (e.g. earth tubes)
What is CFD?
CFD is a field in the study of heat and fluid in/on mostly complicated geometries. CFD code in a commercial software consists of set of validated empirical or analytical formulas predicting the flow of fluids and/or heat in a domain (e.g. inside a room).
CFD is a numerical and computation-based approach in solving complex problems. It is a scientific method of evaluating systems in pre-design stage that has been developed in the midst of 20th century. Evolution of CPUs in the late 20th century made it possible for this branch of science to make its way in the commercial section of human efforts. Today, CFD is widely used in the universities, R&D section of production facilities as well as professional energy companies.
How CFD modelling helps you design better and save money?
Heat and flow are complicated matters that humans need to control for a more comfortable, healthier and safer way of living. Complicated as it is, there’s always not a straight-forward way to predict:
- How well the air inside the building can be naturally ventilated to offset the need to cool.
- How well the exhaust system in an enclosed carpark works to exhaust the air to the limits required by the law.
- How well plants block air movement .
- How to innovate and win more ESD points installing state-of-art solar chimneys or earth tubes.
In order to find the best answers, this questions are almost always best to be addressed with simulation tools called CFD.
Why use CFD?
We always prefer to use simple empirical or analytical formulas using our calculators! However, in the real world, there are several cases where these simple formulas are useless for an engineer to predict the flow of fluids or heat. This is majorly due to:
- Complexity of the flow or heat domain (e.g. server room)
- Presence of various physical phenomena (e.g. radiation from lights and cool air flow in a tightly-spaced grow room)
- Presence of complex physical phenomena (e.g. smoke dispersion in a commercial car park, buoyancy-driven flows and turbulent air flows)
In these situations, there are only two ways to face above problems with:
- Build the system/domain/model or a small scale of it:
This approach works but there are downsides to it:
- It is expensive.
- It lacks the ability to compare two or more models or to optimise without adding to the cost.
- It depends on the accuracy of measuring tools and reliability of data gathering and analysis.
- Use computer-based CFD tools:
CFD approach is relatively cheaper to conduct and could be applied to any set of situations where flow and/or heat has a great impact on the key parameters of the design. Once CFD analysis is undertaken and its results validated, finding an optimised solution for the particular problem is rather easy. Several cases may be compared upon which, insights for a good design is formed.
How is CFD performed?
A typical CFD analysis consists of three major steps. For the ease of understanding, we try to explain these steps using an example:
Problem: we have a 760m2 enclosed carpark with a ducted air exhaust system. Carpark has a non-rectangular shape and there are several columns behaving as obstacles to free air flow. For this system to effectively exhaust smokes emitted from cars, at least 6 Air change per hour (6 ACH) must be achieved throughout the carpark to avoid secondary flows and smoke stagnation. Due to the relatively complex carpark geometry and the presence of columns, there is no straight forward solution to predict air velocity at any given location for the recognition of the minimum 6 ACH.
Solution Using CFD:
The best way to recognize the effectiveness of the exhaust system is by undertaking a CFD analysis, hence, there are three steps:
Pre-processing consists of what you’ll have to do before running the CFD model. Considering carpark exhaust system, this step requires CFD engineer to obtain architectural drawings of carpark and to model a geometry with all the major obstacles to the exhaust air. The model of the carpark is then exported to a software for discretizing the continuous domain of the modelled geometry into small volumes that is called (control volumes or cells). The discretized geometry is called “computational domain” (some prefer “mesh”) and is now suitable for computational (or rather numerical) processing.
- Processing (solver):
At this stage, the computational domain is exported to a CFD software. The major efforts of this stage is to accurately define the properties of air (e.g. density and viscosity with respect to temperature), boundary conditions (e.g. inlet air velocity), turbulence models, solver methods and the acceptable criteria for numerical convergence. Once the numerical convergence is achieved, the computation is done.
After the domain is solved, it’s time to gather data and analyze results and determine whether the exhaust system is effective. For exhaust systems, its best to define a parameter called LMAOA (Local Mean Age of Air) into the code. This parameter determines the exhaust duration for each particle in any given location. This way we can estimate the effectiveness of the current exhaust system design or to optimize it.