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HVAC is a highly competitive industry and vital to the global economy. Due to the need for comfortable living and working space, the demand for HVAC services are at an all time high. HVAC services play an important role in improving the productivity and life safety of its consumers. We are at a time of increased global climate change due to anthropogenic causes and HVAC is one of the major direct/indirect contributors. There is an increased demand and not to mention a moral obligation to design energy efficient HVAC systems. Due to the competitive nature of this industry, energy efficient HVAC systems must also be cost effective in order to achieve commercial success and wide usage. Wide usage of highly efficient HVAC systems will greatly contribute in reducing the global emission rate. But designing highly efficient HVAC systems that are cost effective are a challenge and to achieve this on a large scale is very demanding. In order to achieve this demanding task we need to get back to the basics of HVAC. HVAC is a branch of mechanical engineering that deals with fluid & thermodynamics dynamics. Most of the uncertainty a HVAC engineer faces while designing a HVAC system is the complex and sometimes unpredictable nature of fluid which in this case is air. Better understanding the fluid behaviour will greatly help a HVAC engineer to designing effective HVAC systems. This is where CFD (Computational Fluid Dynamics) comes into picture. Since CFD is the study of fluid and its behaviour, it can better assist HVAC engineers in understanding HVAC systems. This article will discuss and provide insight into the application of CFD Analysis as a tool to help HVAC professionals in designing cost effective and efficient HVAC systems. We will also discuss the commercial benefits of using CFD from a HVAC point of view.

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

The post How to Design Highly Efficient HVAC Systems using CFD appeared first on Flowturb Solutions.

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The global pandemic has ravaged world and its economy. It had brought great uncertainty to the lives of millions of people. It had highlighted, how closely intertwined we are. A virus that originated in China has spread all over the world with great speed. More than a year has passed, but an end to this pandemic is still not in sight. It might take years to get back to normalcy. It would be prudent to think about long term mitigation strategies to prevent further spread of infection & also to improve the productivity. Since people spend most of their time indoors, preventing spread of infection and avoiding cross contamination in indoor space is of great importance. In an indoor environment, airborne transmission of infection is a major source of concern and number of studies have shown how contaminants from an infected person can infect other people in the same room even when they are seated far apart. It has also been proven that HVAC (Heating Ventilation and Air Conditioning) systems play a major role in the spread of airborne contaminants.

This discussion will focus on the effectiveness of different HVAC systems in containing airborne contaminants from a strictly fluid dynamics point of view. CFD Analysis is used to simulate multiple indoor scenarios with infected & non-infected people in the same indoor space and with different HVAC systems. Based on these CFD simulation results a quantitative conclusion will be draw and good HVAC implementation practices will be derived from the conclusion to effectively reduce the spread of infections in indoor environment.

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 use CFD simulations to study the movement of contaminates in an indoor space under multiple 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.

Physics of Airborne Transmission

To prevent airborne transmission of infection, first we need to study its physics to better understand how the transmission takes place. And whenever I mention transmission in this discussion, I strictly mean airborne transmission.

An infected person can transmit disease vectors through coughing, sneezing, talking & breathing [1]. Coughing & sneezing are low occurrence activities but emits high concentration of particles. Talking & breathing on the other hand are high occurrence activities but they emit low concentration of infected particles. It has been documented that even people who are asymptomatic and rarely cough or sneeze also transmit disease vectors through talking & breathing.

Avoiding Cross Contamination in an Indoor Environment using CFD

To give some context on the concentration of particles emitted, below table mentions the maximum concentration of particle emitted during each of the 4 activities [8].

Avoiding Cross Contamination in an Indoor Environment using CFD

Regarding the airborne particles, they are broadly classified into two categories based on their size, they are large droplets & aerosol particles.

Large Droplets: These particles have size larger than 5µm (micro meter). Because of their large size, they have high momentum and can accommodate large concentrations of harmful disease vectors. But they have low Atmospheric Residence Time (ART), which means these particles can stay air bourn for only short duration of time. Usually large droplets can travel short distances and settle at a distance less than 1m [2].

Aerosol Particles: These particles are of size less than 5µm (micro meter). Due to small size they have low momentum and can accommodate small large concentrations of harmful disease vectors. But they have height ART and can stay airborne for longer time. It has been documented that they can stay airborne for a period of 41 hours to 21 days.

Note : Atmospheric Residence Time (ART) is a measure of how long a particle can stay airborne.

The safe distance of 2m has been derived from studying the large droplets. But when we consider the transmission form aerosol particles especially in an indoor environment the safe distance of 2m is not a valid assumption. The safe distance in an indoor environment is largely determined by the indoor air movement of the building. Since the indoor air movement is largely controlled by HVAC systems, they are vital in controlling as well as spreading the contaminants.

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 momentum, heat, moisture & contaminates where ever it travels. This property is called transport phenomenon of fluids. In our case we are interested in the ability of air in transporting the harmful contaminates inside the area of interest. So air is responsible for the movement of contaminants, it simply transports them where ever they travel. To control the concentration & distribution of contaminants in an indoor pace, one need to study & control the flow air flow pattern in that space.

CFD Study Overview

For the CFD study, we have chosen a typical air conditioned office space as the area of interest. In this office space we are going to place 3 infected individuals at different locations. Person-01 will be placed at maximum distance from the return air, person-02 at minimum distance from the return air & person-03 at an intermediate distance from the return air. 3D model of the office space will be modelled accurately considering the furniture in the room. A series of CFD analysis will be carried out on the office space with different air conditioning systems. In all the simulations, the geometry of the office space, location of the infected persons, the quantity of infections from the infected persons & the flow rate of the supply/return air remains the same. The only variable in the simulations is the use of different AC systems.

Case:01 HVAC System without False Ceiling

In this case supply air travels from the AHU through the supply air duct and enters the office space through the 4 way supply air diffusers placed throughout the office space. The conditioned air then collects the heat and contaminants and moves toward the return air through the occupancy zone. In this case there is only one return air close to the AHU. Image below provides a simple illustration of this HVAC system.

3D model of an office space with this HVAC system will be modelled along with the 3 infected persons placed at different locations.

Avoiding Cross Contamination in an Indoor Environment using CFD

Flow Illustration of Case Study-01

Avoiding Cross Contamination in an Indoor Environment using CFD

Isometric View of the Office Space in Case Study-01[3D Model]

Avoiding Cross Contamination in an Indoor Environment using CFD

Top View & Side view of the Office Space in Case Study-01

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1.5m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.5m height

Following observations could be drawn from the CFD results of Case-01,

The infections from Person-01 could reach and plausibly infect half of the office space. Since the supply air moves towards the return air, the infected particles emitted by person-01 moves towards the return air. But due to turbulence and fluid mixing in the fluid domain, the contamination particles diffuse as it moves towards the return air. By the time it has reached the middle of the office space, the concentration of the particles have completely diffused. So the infection from person-01 has large spread distance. The concentration of contaminated particles are high close to the person-01 and reduced as we move away from him.

For Person-02 the spread and the concentration of the contaminated particles are low as he is sitting close to the return air. Since there is only one return air, the velocity close to the return air is high and hence it facilitates the effective removal of the contaminated particles.

For Person-03 the spread area of the infection is low due to his proximity to the return air. The concentration of contaminated particles is high close to person-03. It could also be seen that the CP move towards the return air through the occupancy zone, which means any one seated between person-03 and the return air had high risk of infection.

Case-02: HVAC System with False Ceiling

In this case supply air travels from the AHU through the supply air duct and enters the office space through the 4 way supply air diffusers placed throughout the office space. The conditioned air then collects the heat and contaminants and moves toward the return air 4 way diffuser paced throughout the office space, then the return air moves towards the AHU through the plenum (space above the false ceiling). Image below provides a simple illustration of this HVAC system.

3D model of an office space with this HVAC system will be modelled along with the 3 infected persons placed at different locations.

Avoiding Cross Contamination in an Indoor Environment using CFD

Flow Illustration of Case Study-02

Avoiding Cross Contamination in an Indoor Environment using CFD

Isometric View of the Office Space in Case Study-02 above false Ceiling [3D Model]

Avoiding Cross Contamination in an Indoor Environment using CFD

Isometric View of the Office Space in Case Study-02 below false Ceiling [3D Model]

Avoiding Cross Contamination in an Indoor Environment using CFD

Top View & Side view of the Office Space in Case Study-02

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1.5m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.0m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.5m height

Following observations could be drawn from the CFD results of Case Study-02,

When compared to case-01, the infection from person-01 has reduced drastically in terms of both spread area and concentration of infection. This is due to the presence of localised return air diffusers. The air from one supply diffuser will try to move towards the nearest return air diffuser, hence the flow is localised. The spread from preson-01 is in the range of 10-15m in all directions.

For person-02 & 03, the concentration of CP has reduced but the spread has comparatively increased due the absence of a single large return air grill with high velocity. The increase of the spread area is due to the movement of CP towards multiple return air diffusers. This results in diffusion of the CP concentration due to fluid mixing & turbulence.

Case-03: HVAC System with False Ceiling & Raised Floor Supply Air Grills

[ Return air Grills: 22no , Supply Air Grills: 78no]

In this case supply air travels from the AHU through the supply air plenum(space below the false floor) and enters the office space through the raised floor grills placed throughout the office space. The conditioned air then collects the heat and contaminants and moves toward the return air 4 way diffuser paced throughout the office space, then the return air moves towards the AHU through the plenum (space above the false ceiling). Image below provides a simple illustration of this HVAC system.

Avoiding Cross Contamination in an Indoor Environment using CFD

Flow Illustration of Case Study-03

Avoiding Cross Contamination in an Indoor Environment using CFD

Isometric View of the Office Space in Case Study-03 above false Ceiling [3D Model]

Avoiding Cross Contamination in an Indoor Environment using CFD

Top View & Side view of the Office Space in Case Study-03

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1.5m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.0m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.5m height

Following observations could be drawn from the CFD results of Case Study-03,

When compared to case-01 & 02, the infection from person-01,02 & 03 has reduced drastically in terms of both spread area and concentration of infection. This is due to the presence of localised return air diffusers & supply air grills. The maximum spread from all the 3 infected persons is in the range of 5m in all directions.

Case-04: HVAC System with False Ceiling & Raised Floor Supply Air Grills

[ Return air Grills: 44no , Supply Air Grills: 78no]

This case is similar to case study-03, with the only difference in the number of return air diffusers. In case-03 the number of return air diffusers are 22no and in this case the return air diffusers are increased to 44no.

Avoiding Cross Contamination in an Indoor Environment using CFD

Top View & Side view of the Office Space in Case Study-04

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1.5m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.5m height

Following observations could be drawn from the CFD results of Case Study-04,

The increase of return air diffusers have further reduced the spread area & concentration of CP. This is due to the increased localization of the flow.

Case-05: HVAC System with False Ceiling & Raised Floor Supply Air Grills

[ Return air Grills: 77no , Supply Air Grills: 78no]

This case is similar to case study-03 & 04, with the only difference in the number of return air diffusers. In case-03 & 04 the number of return air diffusers are 22no and 44no respectively. In this case the return air diffusers are increased to 77no.

Avoiding Cross Contamination in an Indoor Environment using CFD

Top View & Side view of the Office Space in Case Study-05

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 1.5m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2m height

Avoiding Cross Contamination in an Indoor Environment using CFD

CFD Results- Particle Concentration Contour at 2.5m height

Following observations could be drawn from the CFD results of Case Study-04,

The increase of return air diffusers have further reduced the spread area & concentration of CP. This is due to the high localization of the flow.

Results Comparison

The results from all the 5 cases are extracted and plotted in the form of graphs side by side for ease of understanding and to make quantitative comparison of all the cases. Graph-01 plots the maximum concentration of CP in all the 5 cases & Graph-02 plots the area of contamination spread. It could be seen that case-01 has substantially high concentration of CP & spread area when compared to other cases. This is due to the fact, since there is only one return air, most of the CP remain inside the office space and hence increasing the overall concentration. In case-02, by introducing a false ceiling and adding multiple return air diffusers the overall CP concentration has been reduced by half and the spread area has been reduced substantially. In case-03 the concentration & the spread area has further reduced by increasing the number of return air diffusers, this is due to the fact increasing the supply and return air points will make the flow highly localised and hence reduces the overall travel distance of the CP. In the case-04, the concentration of CP has substantially reduced by the use of floor mounted supply grills and ceiling mounted return air grills. It should also be noted that the spread area has reduced but not substantially in case-04 compared to case-03. In case-05 the concentration is at a minuscule level and the spread area is at the lowest of all the cases.

Avoiding Cross Contamination in an Indoor Environment using CFD

Area Weighted Average of Contaminant Particle Concentration

Avoiding Cross Contamination in an Indoor Environment using CFD

Area of Contaminated Particle Spread

Design Recommendations Based on this Fluid Dynamics Study:
It is no less important to reiterate the fact that in all the 5 cases, the flow rate of supply/return air, geometry of the building along with its furniture, concentration of CP emitted by the infected persons, location of the infected persons all remains the same. The only variable in these case studies are the type of air distribution systems. But still we can see vast differences in the probability of infection in all these cases.

Return air extraction by using return air ducts or plenum reduces the probability of infection substantially. Or can we say having a single return air point in a large office space, will substantially increase the probability of infection.

Increasing the number of supply air & return air points will further help in mitigating the infection rate.

Placement of floor or furniture mounted supply air points will be effective if placed correctly, taking into account the geometry of the area of interest and the location/size of return air diffusers.

Conclusion

There are number of studies conducted to understand the physics of contaminating transmission and ways to prevent them. There are number of recommendations made by different health and safety regulatory agencies to prevent the spread of infections. It is to be noted that the above recommended design practices are not to replace the recommendations and guidelines prescribed by health and safety regulatory agencies but rather to compliment them. We sincerely hope that this humble study will make a small contribution in reducing the spread of COVID and help humanity overcome this great suffering.

Author

Antony
Director – CFD Operations

The post Avoiding Cross Contamination in an Indoor Environment using CFD appeared first on Flowturb Solutions.

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