by 86rocco » Mon Nov 27, 2006 10:57 am
At it's most basic, a flow bench is just a box with some blower motors, your head or what ever else might be testing sit on the box and the blowers make air flow through the head. Inside the box usually between the head and the blower is some means of measuring airflow and some way of controlling the amount of flow.
To measure the air flow, we most commonly use either an orifice of a pitot tube. An orifice provides a restriction to the airflow as a result, there's a pressure difference across the orifice by measuring this difference, we can calculate the airflow. Most commercially available flowbenchs use this method.
Pitot tubes are another measurement method popular with the members of these forums. A Pitot tube works by measuring the impact pressure of the air. Think of sticking your hand out of a moving car, the faster the car goes, the harder the air pushes against your hand. Pitot tubes provide a way of measuring the force of the moving air and from that the air velocity and volume can be calculated.
With either the orifice type of bench or the Pitot tube type, it's important to accurately measure the pressures generated and for this we use manometer, they are just fluid filled tubes and the amount of pressure applied moves and fluid and the amount of movement measures the pressure. Also, increasing common are electronic manometers.
What I found useful when I was trying to get my head around the principles of the flow bench was to study and thoroughly understand the math involved in the calculations. I would draw small diagrams of a flowbench, figure out where the pressure measurements needed to be made and by running through the math, I could understand how things needed to be sized etc.
Anyways, you've come to the right place to learn about these things, many of the people here are very experienced and willing to share their knowledge. Good Luck.