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Applications of Computational Fluid Dynamics (CFD) for Design and Optimization of Building HVAC

Sep 28, 2021 | Blogs | 0 comments

HVAC for Built Environment

The main purpose of heating, ventilation, and air conditioning (HVAC) for buildings is to maintain a healthy and comfortable indoor environment for occupants. Air is the primary carrier of heat, moisture, and airborne contaminants in indoor spaces. The distribution of clean supply air and resulting airflow patterns, therefore, play a crucial role in determining the thermal comfort of occupants and the quality of indoor air. The airflow patterns, the flow path of airborne contaminants, and thermal comfort of occupants can depend on several factors including the number, location, and type of supply diffusers; supply airflow rates (air change rates) and associated diffuser throws; supply air temperature; number, size, and locations of return/exhaust grilles; the location and strengths of various heat sources in a room; an arrangement of furniture and other obstructions to airflow; location, type, and capacity of in-room air cleaners; and importantly, the relative positions of contaminant and heat sources in space.

Effectiveness of HVAC Performance

The breathing zone which is typically located between 4 to 6 feet height from the finished floor is the most critical zone for the health and comfort of occupants in indoor spaces. Ideally, the clean supply air should sweep the contaminants from the breathing zone of occupants without significant recirculation and stagnation that generally create pockets of high concentration and zone of high and low temperature. At the same time, the clean air should not escape or short-circuit the space without collection and removal of contaminants and heat from the space. Since the air takes the path of least resistance, which is often not intuitive, the effectiveness of ventilation depends on several factors related to the design and operation of HVAC in indoor spaces.

Why CFD Analysis?

Physical testing and real-time measurements of all the parameters that affect the ventilation performance of enclosed spaces are often time and labor-intensive, if not impossible. Moreover, such measurements are not possible during the design phase before the construction of a facility. In such situations, CFD analyses provide a feasible tool to gain valuable insights into ventilation performance. CFD analyses, if performed properly with adequate expertise, can provide valuable insights into the airflow patterns, the flow path of airborne contaminants, and thermal comfort of occupants. Our proprietary methodology of performing CFD analyses for built environment help optimize HVAC performance for creating comfortable and healthy indoor environments for the people.

Limitations of Building Codes and Standards

Most building codes and standards provide prescriptive minimum design requirements and are generally developed based on the consensuses of experts and experienced professionals. These guidelines cannot ensure the best possible airflow performance for all spaces when each space is unique in many aspects. Such codes and standards enforce minimum requirements and provide valuable guidance to design engineers. However, a human-centric HVAC design that should ensure optimum thermal comfort and a healthy indoor environment for all occupants requires a holistic approach beyond just high-level prescriptive guidelines. CFD helps in such comprehensive analysis of the indoor environment even during the early stages of HVAC design.

What is Computational Fluid Dynamics (CFD)?

Computational Fluid Dynamics (CFD) is a science that deals with the simulation and analysis of fluid flow, heat transfer, mass transfer, and other similar transport processes. CFD involves laws of Physics such as conservation of mass, momentum, and energy. CFD employs numerical methods to solve the underlying transport equations. It thus predicts temporal and spatial variations of governing entities such as velocity, pressure, temperature, chemical concentrations, etc. CFD simulations yield a wealth of information related to time-varying three-dimensional distributions of these entities that is difficult to obtain through prototyping or physical testing.

How CFD Works?

The transport of mass, momentum, energy, and chemical species are governed by a general principle of conservation and can be presented in a single form of a differential equation. During the CFD analysis, first, the calculation domain (extent of space) is divided into several non-overlapping control volumes, such that there is one control volume surrounding each grid point. Then, each governing differential equation is iteratively balanced over each control volume to conserve the mass, momentum, energy, and other similar physical entities. During the iterative process, the residual error for each governing equation is monitored and reduced. This process continues until the overall balance in conserving all governing entities is obtained to a certain acceptable level. Finally, such converged numerical solutions reveal a detailed distribution of pressure, velocities, turbulence parameters, temperature, and the concentration of chemical species, etc. in the calculation domain.

Applications of CFD for Building HVAC

Our proprietary methodology of CFD analysis for built environment predicts:

  • Intuitive airflow patterns and insightful airflow animations
  • Flow path and resulting distribution of airborne contaminants
  • Temperature distribution identifying hot and cold zones in space
  • Thermal comfort of occupants as per ASHRAE Standard 55
  • Spread Index – a measure of ventilation effectiveness applicable to a variety of situations

With the help of these predictions, we employ cause-and-effect analysis to identify the root cause of underlying problems and suggest appropriate mitigation strategies to optimize the HVAC performance.

Read more about CFD solutions for a specific HVAC application.

About the author

Dr. Kishor Khankari

ASHRAE Fellow, ASHRAE Distinguished Lecturer

Dr Kishor Khankari is the founder of AnSight LLC. As a specialist in Computational Fluid Dynamics (CFD), his passion for solving engineering problems and providing sound scientific solutions has led to innovations and optimized designs in the industry.

A noted expert in his field, he has a Ph.D. from the University of Minnesota and has published in several technical journals and trade magazines. As a well sought-after speaker Dr. Khankari makes regular presentations in various technical conferences and professional meetings worldwide.

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