An optimum HVAC system provides occupant thermal comfort and good indoor air quality throughout the year, irrespective of the outside weather conditions. Besides achieving these performance criteria, HVAC systems should also be energy efficient. If these performance criteria aren't met, the indoor environment can impede productivity, cognition, and the general health and well-being of occupants.

CFD comes to the rescue in this situation, allowing precise forecasting of the performance of HVAC systems by modeling the physical phenomena, ambient conditions, and the detailed geometry of the space.

This helps designers make decisions about design optimization without needing to physically build and install the system.

In the HVAC industry, CFD has the following principal applications -

1. For better Indoor Air Quality

2. For optimum Thermal Comfort

CFD empowers an engineer to visualize different parameters virtually at every point in space. After post-processing, engineers can visualize data differently using Surface Plots, Contour Plots, Iso-Volume Plots, Iso-Surface Plots, and Flowlines to diagnose the space.

We will examine examples of how CFD can be used as a diagnostic tool.

Indoor Air Quality (IAQ) refers to the air quality within and around buildings and structures, especially concerning the health and comfort of building occupants. If too little outdoor air enters indoors, pollutants can accumulate to levels that can pose health and comfort problems making HVAC systems critical in ensuring indoor air quality isn't unhealthy.

Indoor carbon dioxide concentrations are driven by a combination of outdoor CO2, indoor breathing, and the ventilation rate of the building. To make ventilation systems more energy efficient, conditioned air is recirculated rather than being exhausted to the environment. As per ASHRAE 62.2, CO2 concentrations should be below 1000 ppm in a space. Concentrations over 1000 ppm can lead to occupants feeling drowsy and losing attention, and increased heart rate.

The mean age of air refers to the average time spent by air in a building zone—the Greater the time, the more significant the chances of contaminants accumulating in a space. The mean age of air can help gauge the ventilation effectiveness of a system. The lesser the average time spent by air, the higher the indoor air quality.

In the below-shown contour plot, the orange-colored sections show regions in the space where circulated air is getting accumulated for more time. This will decrease the IAQ in this region for the occupants. A remedy for this issue can be adding a supply diffuser to lower the mean age of air, improving IAQ.

Designing the HVAC system to get optimized thermal comfort is an iterative process.‘Comfort’ is a state of mind or a personal feeling - not a quantifiable metric. Thanks to Prof. Fanger for providing us with the parameters to evaluate occupant comfort. The various parameters that can determine thermal comfort are Predicted Mean Vote (PMV), Predicted Percentage Dissatisfaction (PPD), Draft Rating Index (DR), and Effective Draft Temperature (EDT). With AHC 2023, one can design for Thermal Comfort by considering all these parameters.

Predicted mean vote (PMV) is an index that predicts the mean value of a large group of persons' thermal sensation votes (self-reported perceptions) on a sensation scale expressed from –3 to +3. PMV values for space should lie between +0.5 to -0.5 to ensure occupant thermal comfort, according to ASHRAE 55.

Predicted percentage of dissatisfied (PPD) is an index that establishes a quantitative prediction of the percentage of thermally dissatisfied people determined by PMV. Comfort criteria are achieved when PPD values are below 10 %. PPD levels below 10% don't ensure that all the occupants in the space are comfortable. 10% of the occupants will still feel uncomfortable.

Draft Rating and Effective Draft Temperature assess local effects from high-velocity drafts of extremely cold or hot air.

A draft is an unwanted local cooling of the body caused by air movement. Draft rating is an index that establishes a quantitative prediction of the percentage of occupants dissatisfied due to the draft. Comfort Criteria are achieved when the percentage of dissatisfied people due to draft is below 20%, as mentioned in the ASHRAE 55.

Effective draft temperature is a calculated temperature difference that combines the effect of air temperature and air motion on the human body. Comfort Criteria are achieved when the effective draft temperature (EDT) is between -3 °F to +2 °F, and the air velocity is less than or equal to 70 fpm.

Below are the horizontal slices at occupant neck level (seated) for effective draft temperature. Simulations are done for 2 configurations of sidewall supply diffuser locations. In configuration 1, the diffusers are located at a height, while in configuration 2, diffusers are located near the floor. The results of flowlines for supply air help visualize the airflow and air diffusion parameters like throw, spread, and drop. One can also visualize if the path of supply air is obstructed due to furniture, as in images 1 and 3.

In configuration 1, the draft ratings of the space were higher than 20% making it uncomfortable for occupants. This has been resolved by changing the location of diffusers which can be viewed in configuration 2. The results of draft ratings are below 20%, ensuring minimum occupant discomfort due to air drafts which can also be confirmed using EDT contour plots.

In places with too dry or too humid air, humidity control is critical in ensuring occupant thermal comfort. In AHC, one can design an HVAC system for humidity control using Relative Humidity contour plots as described in the images below. It’s recommended to have humidity levels between 40% and 60% to achieve thermal comfort. The contour plot below shows the relative humidity levels in the space and lies between a range of 40% to 60% ensuring maximum comfort.

Temperature plots from CFD simulation can be viewed to visualize temperature distribution across space. One can use them to view hot and cold spots in space.

The mean radiant temperature (MRT) expresses the influence of surface temperatures on occupant comfort. Mean radiant is a dominant element in the thermal comfort equation which is an integral part of indoor environmental quality and building performance. Heat gain in the form of radiation from all the surfaces can be calculated using MRT contour plots. MRT plots can ensure that space occupants will be comfortable in different sections of the space. The below image shows MRT results near exposed surfaces like walls and windows to understand the effect of solar radiation.

Now that we have established the importance of CFD in designing optimum HVAC systems, let's understand the workflow for performing a CFD simulation in any commercially available software.

1. Modeling the building geometry
2. Model flow domain and cad clean-up
3. Generate mesh
4. Establish boundary conditions and initial conditions
5. Choose turbulence models and solver schemes
6. Perform simulation
7. Iterative convergence
8. Post-processing to visualize results
9. Iteration for performance parameters
10.Final design

Lack of CFD knowledge, extensive training requirements, expensive software licenses, and the need for in-house, high-performance computing equipment have kept CFD from being widely implemented.

What if I tell you that there is an application that will automate this entire process for you and help you achieve the performance parameters with ease of mind? Presenting to you the Autonomous HVAC CFD (AHC) application, which does all the heavy lifting of the CFD simulation process (fluid volume extraction, meshing, selecting the suitable numerical models and solver schemes, monitoring convergence, and postprocessing) autonomously. The only inputs the app needs are the Building Information Models (BIMs) of indoor spaces and information about the HVAC system, weather conditions, and occupant density.

AHC app is designed to help HVAC solution providers and HVAC manufacturers design suitable airside systems by accurately evaluating thermal comfort and indoor air quality in conditioned spaces.

It provides vital information like Predicted Mean Vote(PMV), Percentage of People Dissatisfied (PPD), Relative Humidity, Air Velocity, Temperature, Mean Radiant Temperature of Air, Mean Age of Air, and CO2 levels for any conditioned space.

Nothing demonstrates the power of a product more than its real-world examples. View case studies of how AHC is used for estimating thermal comfort.