Why CFD Analysis?
To answer this question effectively, I need to make some unorthodox explanation along with an analogy. I would like to see HVAC as a field that manipulates air and its properties. When I say manipulation of air, it means increasing or decreasing any of the properties of air (like velocity, temperature, RH & contaminants). So it is the job of a HVAC engineer to manipulate air in a given volume of interest. A CFD expert on the other hand, spends his/her time understanding complex fluid behaviour. Since fluid behaviour is inherently complex and counter intuitive, manipulation of air becomes difficult to achieve in many cases. Hence it is admissible to say that a CFD or a fluid dynamics expert will be of great help to a HVAC professional. Now that we have completed the unorthodox explanation, we will get to the analogy part.
Analogy: CFD a tool for HVAC
“ A medical doctor uses multiple tools(like thermometer, BP machine, scans..etc) to understand the physics that is taking place inside the body in order to make a better diagnosis of the illness in a patient and to prescribe necessary medication to counter it. Of course the doctor can use only his skills without using his tools to make a diagnosis. However it is much more efficient & easy to do the same job with the necessary tools and moreover it will give the doctor more confidence/peace of mind in his own diagnosis.
Similarly CFD is like one of the tools that would help a HVAC engineer better understand the system and the work in hand in order to efficiently complete his job with greater confidence/peace of mind.“
What is CFD
CFD stands for Computational Fluid Dynamics. It is a branch of fluid dynamics which solves fluid flow problems using computer (simulations). The logic behind the operation of CFD software is complex and explaining it would require a complete discussion on its own. Since we do not wish to deviate from the topic in hand and for the sake of simplicity, we can consider the CFD Analysis software as a tool to predict fluid flow behaviour. CFD software can be used to simulate any fluid flow scenario and can be used to study fluid behaviour under any given circumstances. In the case of HVAC, CFD analysis is used as a validation tool in studying the effectiveness of the HVAC systems. CFD has been used widely in the HVAC sector to study the flow, temperature, contaminant distribution, humidity, & pressure pattern in an indoor space. In this discussion we are going to see how CFD analysis can be used to study, improve & optimize the performance of different HVAC systems. The CFD software used in this study is ANSYS Fluent and the 3D modelling of the indoor environment is done using ANSYS Spaceclaim.
Transport Phenomenon of Fluids
Since this discussion is focussed on the effectiveness of HVAC systems from a fluid dynamics point of view, I need to give the reader some context on the behaviour of the fluid with respect to the contaminant particles. Fluid has the ability to transport heat, moisture & contaminates wherever it travels. This property is called transport phenomenon of fluids. If we are interested in controlling temperature, humidity & concentration of contaminants in an area of interest then it is absolutely imperative that we control the fluid flow pattern of that area of interest.
How to Do a CFD Analysis to Optimize HVAC Systems: A Simple Example
For the sake of understanding, I am going to choose a simple HVAC system for optimization. So we can use a ductless car parking basement ventilation system with one fresh air and one exhaust. Figure-01 shows a typical car parking basement with one exhaust on the right hand side and the fresh air through ramp.
The 2D CAD layout of the basement will be converted into a 3D geometry using 3D modelling software. The location of the fresh air & exhaust will also be marked in the 3D model. The exhaust will be labelled as velocity outlet boundary condition (meaning this label will have a constant velocity and move air out of the basement). The ramp will be labelled as pressure inlet (meaning this label will behave as having ambient pressure and will move air inside the basement depending on the internal negative pressure).
Flow Physics of the Basement: Now let’s imagine what will happen when we turn on the exhaust fan in this car parking basement. Flow from the ramp will develop & move towards the exhaust and after some time the flow becomes fully developed. When we examine the fully developed flow, we can identify multiple flow phenomenon. The first and most prominent flow phenomenon is the main flow current (MFC), this MFC will originate from the fresh air and move towards the exhaust and will have high velocity. The second phenomenon of interest is the re-circulation region, these re-circulation regions will often be found in the periphery of the MFC and are one of the unfavourable flow phenomenon. The third phenomenon of interest is the stagnation regions; these are regions that have low or no air movement. Now that we have classified the flow profile based on their characteristics, we can see what happens when cars enter the basement.
When cars enter the basement, they emit contaminants through their exhaust. When the contaminants are emitted inside the MFC they are transported out of the basement as the MFC moves all the contaminants towards the exhaust. When the contaminants are emitted in a re-circulation region, it will be sucked to the centre of the re-circulation region. This is because of the centrifugal force present in the re-circulation region. Larger the re-circulation size and velocity magnitude the stronger the centrifugal force and higher the concentration of the contaminants at the centre of this region. Any contaminates that are emitted in the stagnation region, they simply stays there and gets accumulated when more contaminants are emitted here.
So it is clear that the contaminants are accommodated by the stagnation & re-circulation regions. If we are able to remove these Undesirable Flow Phenomenon’s (UFP), then we have removed locations of high contaminant concentration inside the basement.
Now that we have a clear understanding of the flow physics, we can get back to the CFD Analysis. Once the labels are provided to the fresh air and exhaust, CFD CO Analysis will be carried out without considering the jetfans. This analysis will simulate a working car parking basement where constant pollutants are emitted. In the CFD CO Analysis, CO (Carbon Monoxide) is the contaminant.
Figure-02 shows the flow profile of the basement and the locations of the UFP are identified. Figure-03 shows the CO concentration inside the basement and it is clear that the locations of high concentrations are the location with UFP’s. Now that we have identified the fluid phenomenon that causes undesirable conditions inside the basement, we can strategically place the jetfan to remove these UFP’s and achieve optimum flow profile to achieve effective ventilation.
Secondary CFD CO Analysis with jetfans strategically placed will be carried out to optimize the ventilation. Since fluid flow is very complex, we will be carrying out multiple simulations to achieve complete optimization.
Procedure to Optimize a HVAC System using CFD
Since CFD is one of the tools in designing the HVAC system (from a HVAC engineer’s point of view) it is highly productive to formulate operational procedures to achieve a streamline workflow and to prevent time wastage.
- Run CFD Simulation of the Base Design (BD) that requires optimization.
- Identify UFP’s that causes ineffective HVAC System design.
- Make design changes to the HVAC system that will mitigate the losses and improve performance.
It should be noted that any unfavourable conditions ( like hot/cold pockets, high contamination level,. etc. ) are just the effects of the problem. The source of the problem is always an Undesirable Flow Phenomenon (UFP). So to solve the problem it is prudent to attack the source of the problem rather than its effects.
Commercial Benefits of using CFD:
When CFD is not being used, HVAC engineers must use assumptions to calculate the ventilation load and choose HVAC systems accordingly. To prevent inadequate ventilation, safety factors are also used to increase the capacity of the HVAC system as a failsafe. Hence the capacity of HVAC system used is always considerably more than the required system. It is always good to have redundancy in the system to address unexpected operational failure. But the level of redundancy used must be justifiable. With increased capacity of the system comes increased initial and running cost(CAPEX, OPEX). Hence a HVAC engineer must always choose a system considering redundancy & being competitive.
When CFD is used, HVAC engineers need not rely on assumptions & approximate calculations to choose the HVAC system. They can use the calculated HVAC system as the Base Design(BD) and use CFD simulations to study and optimize the HVAC system to achieve optimum performance without losing competitiveness.
Conclusion
It could be summarised from the above discussion, that CFD simulations can provide valuable foresight into the operation of a designed HVAC system. Using CFD as a HVAC tool brings number of advantages and removes approximations/uncertainties in the design. Wide use of CFD to design HVAC systems will reduce global energy consumption and help developed highly sustainable and efficient ventilation design. With increased climate change and global warming, sustainable and energy efficient HVAC designs are needed now more than ever to combat this global crisis.
Author
Antony
Director – CFD Operations
