Better by design

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Published: 18 October 2018


Rachael Hopwood Jarvis of Envenio explains the benefits of CFD simulation in air conditioning.

The Carbon Trust expects around 40% of all commercial space to be air conditioned by 2020, compared with only 10% at the end of 1994. This continued growth in the use of air conditioning brings with it an obvious increase in electricity consumption and power station carbon dioxide emissions – a reality in contrast to the UK government’s energy efficiency targets. The costs involved with comfort cooling are also considerable, and the need to address and eradicate poor performing air conditioning systems is vital.
For this reason, designers and engineers have turned to cost-effective technology in order to gain a better understanding of how a potential air conditioning system will perform in a given environment, or to identify areas where an existing system could be improved. 

One of these emerging technologies is computational fluid dynamics or CFD simulation – a tool that is proving to be both effective and efficient when it comes to designing and optimising air conditioning systems.

CFD – what is it?
CFD is a computational technology that uses numerical methods to simulate, solve and analyse problems that involve fluid flows and thermal issues. Put simply, CFD allows you to simulate an environment to analyse or predict the performance of an air conditioning system or unit. Identifying where air is stagnating for example, or fluctuations in temperature across a room.
With++advances in modern CFD software meaning tools are more accessible and easier to use, users do not need to be engineers to benefit from this technology. 

New system design
CFD simulation enables designers to predict how an air conditioning unit or system will actually perform in a particular environment thus allowing for testing and analysis before installation, and highlighting potential problems early in the design process. As a result, air conditioning system designers can save time, costs and prevent unnecessary physical design changes down the line.
CFD simulation tools can prove beneficial across all three types of generic air conditioning systems:
  • Centralised air systems
  • Partially centralised air systems
  • Local systems
Simulation modeling can also prove useful for both constant volume (CV), variable air volume (VAV) centralised, and displacement ventilation air systems.

Optimisation of existing systems
The components of an air conditioning system, including the way they have been set up, have a substantial impact on energy use as well as the level of occupant comfort. Faulty or poor performing air conditioning systems therefore have the potential to be wasteful of energy, time and money and should be corrected where possible.
With CFD simulation, a detailed virtual model can be built, even with the possibility of importing complex geometries from existing CAD designs, before airflow and heat transfer can be truly analysed in explicit detail.
CFD simulation models also provide a greater understanding of intakes and returns, age-of-air, and highlight issues with stagnation or fluctuations. Essentially showing how an air conditioning system is impacting the environment in which it is working in.

Application example:
In this case, Envenio simulated three different rooms including an open laboratory, a robotics facility and a chemical preparations area, to assess the impact of established air conditioning cooling systems.
For each room, three HVAC conditions were simulated; low, operational and high.
The rooms were cooled by chilled beams or chilled beams with a supplemental dump diffuser. A volumetric energy source was applied to the entire volume of each room to simulate the room heat loading. Model parameters included energy, buoyance and turbulence. The images are known as flood plots, depicting velocity magnitude and temperature within the plane of the image. Results such as these are used to provide air conditioning technicians and designers with a better understanding of mixing, circulation patterns, and occupant comfort.
The spaghetti-like strands in the image below are streamlines seeded along a line at the chilled beam outlets. The streamline tubes are coloured by temperature. The stream traces are a way of visualizing where the jets of air go from the exit of the chilled beams, how quickly the temperature changes as the jets mix and entrain surrounding air, and how the jets behave as walls or adjacent jets impinge the flow.

CFD simulation tools can be used to solve problems such as:
  • Energy consumption increasing without explanation, suggesting that air conditioning systems may not be working effectively.
  • Maintenance costs are increasing due to air conditioning equipment failing regularly through wear.
  • Regular complaints from occupants – feeling too hot or cold, for example.
  • Following changes in a building structure or layout.

Locations where CFD simulation for air conditioning can prove beneficial include:
  • Atrium designs, concert halls, auditoriums
  • Operating rooms, clean rooms
  • Office buildings
  • Manufacturing facilities
  • Apartment blocks
  • Farm buildings
  • Restaurants, hotels, retail outlets
  • Aircraft, boats, cars

Simulation for Smart system validation
Air conditioning systems can account for as much as 25% of some businesses’ total energy consumption so it is perhaps unsurprising that advancing IoT (internet of things) research is looking to identify ways to adopt and implement smart building technologies across the sector. Smarter air conditioning systems such as those controlled remotely from mobile phones or wearable devices are just one example of this, and are proving incredibly effective at assessing conditions and initiating an immediate response. The air conditioning system itself however, must still perform effectively to achieve the desired result as well as satisfying energy efficiency targets. Step forward, CFD simulation. 

Cloud CFD tools and usability
Cloud computing means more professionals across the air conditioning and refrigeration sector can benefit from high performance CFD simulation software for a fraction of the traditional price. Cloud tools like Envenio’s EXN/Aero can be accessed when needed, with a pay-as-you-go monthly model replacing the restrictive licenses that have dominated this space for many years.
As mentioned earlier in this article, users no longer need extensive engineering knowledge to benefit from CFD. Of course, some experience is required to understand and effectively translate the simulation results, but vendors are working to match user training with intuitive, easy to use software platforms.
In summary, the benefits of using CFD simulation for air conditioning design include:
  • Uptime: Designers can better understand failure scenarios and reduce areas of system redundancy.
  • Operational costs: By simulating various scenarios, optimal operational costs can be achieved for an AC system. 
  • Capital costs: Designers can make large investment decisions with full confidence and information by having a better understanding of the environment in which they are working.
  • Maximise available space: Designers can optimise the use of their available equipment and AC systems. 
  • Satisfy legislation: Through simulation, designers can satisfy legal standards and legislation around thermal comfort for occupants.
With so many potential advantages and an increase in accessibility, it’s not hard to see why many within the HVACR space are ‘dipping their toe’, incorporating CFD simulation to help them design better and more effective air conditioning systems.  

An essential guide to CFD simulations and a free demonstration are available on the Envenio website.