Question

What are the main differences between natural and forced convection flows? In the lab experiments, explain how we did use one experimental test rig to demonstrate the characteristics of both flows. ?

Answer 1

In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, i.e. the rise of warmer fluid and fall of the cooler fluid, without any aid of external agent. Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan or by means of a blower.In forced convection, buoyancy has little effect on the direction of flow.Low temperature distribution in natural convention, whereas it is increased in case of forced convention.

When Ar >> 1, natural convection dominates and when Ar << 1, forced convection dominates.The Archimedes number is the ratio of Grashof number and the square of Reynolds number, which represents the ratio of buoyancy force and inertia force, and which stands in for the contribution of natural convection

====> While performing experiment on natural and forced convention, we use same test rig to demonstrate characteristics of both flows.

The main difference in setting up the apparatus for different flows is in switching on or keeping blower(fan) off throughout the experiment.

In natural convention , the heat input is given by applying voltage across the setup. After reaching a steady state temperatures are noted down along with ambient temperature.Caution should be taken that , throughout the experiment, the blower is OFF.

In forced convention , heat input is given. Later blower is switched ON.Manometer head is set as required.Test pipe is connected to the delivery side of the blower along with the orifice to measure flow of air through the pipe.Airflow is measured with the help of orifice meter and the manometer fitted on the board.