Definition and Factors Affecting Airflow
– Airflow is the movement of air caused by air pressure differentials.
– Airflow can be described as a volumetric flow rate or a mass flow rate.
– Airflow is influenced by factors such as altitude, temperature, and composition of the air.
– Mechanical means, like fans, can induce airflow, or it can occur passively due to pressure differentials.
– Airflow can exhibit laminar or turbulent flow patterns, depending on the smoothness of the flow and presence of irregularities.
Velocity Profiles and Reynolds Number
– Velocity profiles describe the distribution of instantaneous velocity vectors across a cross section.
– Factors like geometric configuration, fluid properties, and engineered components affect velocity profiles.
– Streamlines connect velocities and can be curved and independent of the container shape.
– Reynolds number predicts the transition from laminar to turbulent flow based on viscous and inertial forces.
– Different equations are used to predict the behavior of laminar, turbulent, and transitional flows.
Units of Airflow Measurement
By volume:
– Cubic meters per minute (/min)
– Cubic meters per hour (/h)
– Cubic feet per hour (/h)
– Cubic feet per minute (CFM)
– Liters per second (l/s)
By mass:
– Kilograms per second (kg/s)
Air changes per hour (ACH) can also indicate full replacement of air in a space.
Measurement of Airflow
– Airflow is measured using airflow meters and anemometers.
– Anemometers can use ultrasound or resistive wire to measure air velocity.
– Hot-wire anemometers detect airflow by analyzing temperature changes in a wire.
– Anemometers can also calculate air flow, wet bulb temperature, dew point, and turbulence.
– These measurements are crucial in various fields, including building science and environmental analysis.
Simulation and Control of Airflow
– Computational Fluid Dynamics (CFD) modeling and wind tunnels can simulate and observe airflow patterns.
– Airflow simulations help predict airflow around vehicles, aircraft, buildings, and pollution concentrations.
– Equipment like dampers and air handlers regulate airflow in ducts.
– Fans generate airflow by creating pressure differentials.
– HVAC systems use airflow control to modulate the airflow rate for temperature and ventilation regulation.
