Laminar Diffusers – Do They Really Work?

Laminar diffusers are often employed in cleanrooms, hospital operating rooms, laboratories, and similar other spaces where high level of cleanliness is desired. These air supply outlets (diffusers) are designed to provide unidirectional flow–often referred as “laminar” flow in the HVAC industry. Whereas non-unidirectional, mixing air flows are referred as “turbulent flow”. Laminar flow is required to deliver clean air to a “critical zone” where high level of cleanliness is required. Ideally the unidirectional flow should sustain downward motion of the supply air without significant recirculation, mixing, and entrainment from the surrounding air. There are no easy means to quantify the directionality of laminar flows and entrainment from the surrounding air. In such situation Computational Fluid Dynamics (CFD) can help predict and optimize the performance of laminar diffusers. We performed a CFD study to systematically analyze the impact of supply airflow rate (discharge velocity) on the dynamics of unidirectional airflow under isothermal and non-isothermal conditions. This study provided valuable insights into the dynamics of laminar flows which can be helpful to architects and HVAC engineers.

Under isothermal conditions when the cooling capacity of the supply air is much higher than the sensible cooling load in the space, such as in cleanrooms with high air change rates, the supply air flow rate (discharge velocity) has a little impact on the directionality of the laminar flow. In these situations irrespective of the supply airflow rate,the downward air jet can maintain the unidirectional flow up to 55 percent of the distance, and then it spreads sideward as it realizes downstream obstructions such as equipment, workbench, people, and other similar objects located on the floor.

When the flow rate or the cooling capacity of the supply air is much lower than the sensible cooling load, the discharge air cannot sustain the downward unidirectional motion due to upward resistance of the buoyant plume from the heat sources located near the floor. However, such airflow patterns can form a localized protective layer of hot air surrounding the heat sources and create nearly isothermal conditions in other locations. When the cooling capacity of the supply air exceeds about three times the cooling load, the air jet can sustain the downward unidirectional motion. However, non-isothermal conditions surrounding the laminar diffuser, created by heat released from the lighting or thermal stratification-saturation of hot air near the ceiling, can cause entrainment of the surrounding air into the supply of clean air. Reducing such thermal gradients can help reduce entrainment.This study further indicates that local thermal gradients can significantly affect the directionality of laminar flow and entrainment from the surrounding air. The layout of the room including the locations of the supply and exhaust grilles as well as location and strength of various heat sources can affect the air flow patterns and temperature distribution in the space, which in turn affects the performance of unidirectional (laminar) flows. In such cases an in-depth CFD analysis can help in visualizing airflow patterns, temperature distribution, and in predicting potential entrainment from the surrounding. Insights gained through such analyses help in optimizing the HVAC design to meet the desired level of cleanliness in a space. Laminar diffusers–do they really work? The answer depends on the situation and thermal environment in which they are installed.
Numerical computations for this study were performed using ANSYS/Fluent CFD software.