Characterization of airflow through an air handling unit using computational fluid dynamics

Abstract

HVAC equipment manufacturers spend a considerable amount of time and effort updating existing product lines in order to meet the ever-increasing demand for energy efficient systems. As a major part of HVAC systems, an air handling unit (AHU) controls the airflow through the system and regulates the indoor air quality. Plenum fans used in AHUs inherently produce a rotational airflow, which can create highly unstructured airflow as it enters a heat exchanger located downstream. This in turn leads to lower heat transfer rates and premature heat exchanger failure. As such, airflow uniformity is presently regarded as an important consideration in designing these systems. Through advancements in computer technologies within the last decade, computational fluid dynamics (CFD) has become an economical solution allowing HVAC equipment designers to numerically model prototypes and reduce the time required to optimize a given design and identify potential failure points. While CFD analysis also offers the ability to visualize and characterize the airflow through an AHU system, it has often been used to model individual components such as fans or heat exchangers without analyzing them as a single unit. This work presents the CFD models used to characterize the airflow within an AHU in order to aid in understanding the effects that flow uniformity has on heat exchanger performance. The airflow uniformity was analyzed over a range of volumetric flow rates, and experiments were used to validate the baseline simulations. Different baffle designs were then added into the validated simulations to observe their influence on both airflow uniformity and heat transfer performance. Results indicate that airflow uniformity is, by itself, an insufficient metric to predict heat transfer performance. Additionally, steady-state CFD analyses performed on simplified geometries are shown to provide a sufficient model to be used for further optimization, when the inlet conditions are well specified.