ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
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ASHRAE IJHVAC 16-6-2010
Scope
INTRODUCTION Airflow in enclosed spaces can be complicated due to complex flow features such as flow transition and lack of stability. Many indoor airflows are transitional when the Reynolds number based on the supply air grille is in the region of 2000 < Re < 3500. Turbulence is generated at the grille where the fluctuating component of velocity is a fraction of the mean velocity. As the air travels further into the room@ the fluctuating component may decay gradually due to the decrease in the mean flow gradient and damping effect of the solid surfaces. Therefore@ relaminization may occur within the occupied space@ and the flow is transitional. In addition to flow transition@ in many indoor environments@ airflow with relatively high air change rates (between 5 to 20 ach) can be unstable under the transitional Reynolds number. One reason for this is that the transitional phenomenon makes the flow unstable. In the transitional region@ the inertial force is approximately balanced by the viscous force. A random small impact from the main stream can break down this balance and lead to transitional flow. This mechanism results in instability of the main flow. Another reason for unstable flow is the interaction between different flow features. Many enclosed environments are mechanically ventilated. However@ the air can also be driven by buoyancy in the occupied zone@ where the thermal plume can be as strong as the flow from the mechanical ventilation system. The two flows interact@ and result in instability.