Posted by Prahlad Parmar on November 27th, 2014
Building HVAC is a widely studied topic among the engineering community in order to develop systems that operate with high efficiency, maintaining required temperature conditions inside. While it involves the study of complex fluid flow phenomenon, experimental model tests are often not adequate enough to design efficient HVAC systems.
Moreover, increasing energy saving concerns and government regulations force design engineers to reduce energy consumption as much as possible and focus on green building goals. The use of CFD does provide benefit in understanding flow characteristics and temperature conditions inside the building, but the complexity lies in developing the computational domain that exactly represents real conditions. As such the application of CFD requires enough understanding about the real physical conditions to ensure productive simulation results.
Considering the example of an Airport, which in itself is a large domain, consists of several heat loads which affect the air conditioning system considerably. Since, the building will be public intensive; the heat loads of individuals will increase the temperature inside the building.
Moreover, there will be shops and offices across the floor to cater the passengers in the waiting area, which again will have impact on the room temperature. Additionally, furniture employed in the building also serves as heat load, which affect the inside environment and human comfort. The addition of heat inside the building is also significant by frequent opening and closing of doors and heat transfer from window glasses.
The capacity of the HVAC system is thus calculated considering these heat loads and accordingly, fresh air outlets are provided inside the building. However, for an optimum thermal environment inside the building, it is also required that the air velocity remains under 0.2 m/s, as larger velocity air flows is considered to provide discomfort to human body and also causes cold injuries.
To have a clear understanding of air flow inside the building physical experiments on test models aren’t enough. The situation becomes even worse when the frequency of occupants visiting inside the building increases during peak hours. Thankfully, CFD simulations for HVAC system evaluation can allow designers to visualize the flow characteristics, determine velocity profiles and temperature conditions to optimize the system accordingly.
While the use of CFD in such cases requires building large domain, which can consume considerably amount of computational power, the benefits that can be reaped are substantial.
The CFD model of the building can be developed using CAD tools, considering the location of shops, offices and furniture, which can be later treated as heat loads. The frequency of the people during normal and peak hours can also be added in the model as heat loads. A fluid flow analysis can depict critical regions where temperature is not being maintained along with the velocity profiles. The simulation can also provide details about the air flow velocity at different heights in the building, which can be utilized to tune the air flow outlets.
The capacity required to maintain the inside temperature comfortable can then be calculated, as the simulation results will closely resemble real world conditions. Since the visualization allows identifying critical regions where temperature and velocity parameters are not satisfied, the position and design of the outlets can be adjusted. Overall, a comprehensive study of the HVAC system can be performed through the use of CFD. However, the accuracy relies in the hands of an analyst and his engineering judgements.
Image Source: http://www.symscape.com/blog/internal-flow-with-cfd
About Author: Prahlad Parmar is an Engineering Specialist working at Mechanical 3D Modelling for the past 4 years. He caters critical engineering challenges with ease and performs exhaustive procedures to develop robust, well-engineered and high performance designs. He can always be found in the lab discussing, brainstorming and tweaking designs.