Theory

The air velocity varies from point to point over a cross section of a duct. The velocity profile of air flow in a duct depends upon Reynolds Number, surface roughness of the duct. Typically for laminar flow, velocity profile resembles to parabolic shape and for turbulent flow, it takes ‘D’ shaped profile. Hence to estimate average flow velocity, traversing is usually carried out at a cross section of the duct. There are two types of traversing conducted to measure average velocity: continuous traversing and precise traversing. However, traversing is more time consuming. In order to obtain very rapid estimate of average air velocity, the point measurement of velocity may be obtained, which then be multiplied by a method factor to obtain average velocity. Hence, to do this the method factor of the duct is to be known in priori. The value of method factor decreases with decrease in Reynolds Number. For high Reynolds Number (in the order of 50,000 and above) the method factor can be taken to be constant.

To determine method factor of a duct, both central velocity and average velocity may be taken, and then it can be calculated by the following expression.

Method Factor = Average Velocity (V_arg)/Central Velocity (V_c)

The central velocity can be measured with the help of a pitot tube by placing it at the centre of the duct, and the average velocity can be measured with anemometer traversing. The average velocity can be estimated within ±5% of accuracy by this method.

Apparatus Used:

• Pitot tube
• Inclined tube manometer
• Anemometer

Demo