The key difference between natural and forced convection is that in natural convection, the motion of the fluid is influenced by natural means whereas, in forced convection, the motion of fluids is influenced by external means. The difference between natural and forced convection related to heat transfer is that there are no external factors affecting the heat transfer in natural convection while external factors can cause heat transfer in forced convection.
Convection is a method of heat transfer via bulk movement of the molecules in a fluid (such as a gas or a liquid). It is in two types as natural and forced convection according to the way of initiation of the movement of fluid.
CONTENTS
1. Overview and Key Difference
2. What is Natural Convection
3. What is Forced Convection
4. Side by Side Comparison – Natural vs Forced Convection in Tabular Form
5. Summary
What is Natural Convection?
Natural convection is a method of heat transfer in which natural means influence the motion of the fluid. There is no influence from external facts. This movement of molecules in the fluid is due to the differences between densities of different regions of the same fluid. The density of a fluid decreases when it heats and vice versa. That is because of the thermal expansion of the fluid (the speed of molecules increase with the temperature increase, which results in the increase of the volume of the fluid. Although the volume increases, the mass remains constant. Therefore the density decreases).
When we heat a fluid in a container from its bottom, the density of the bottom layer of the fluid decreases. Then the lower density region tends to move to the top of the container. Then the cooler fluid at the top of the container replaces the bottom region. This continues, as a result, convection occurs.
Figure 01: Mechanism of Natural Convection
Examples of natural convection include cooling down a boiled egg when kept in the normal air, loss of cool of a cool drink can, etc. When considering the mechanism of natural convection, first, the temperature of the outside of a hot object (kept in cold air) drops down. At the same time, the temperature of the air adjacent to the object will rise due to heat transfer. Then the density of this adjacent layer of air decreases. As a result, the air rises upward. Cold air will replace this region. Then the convection continues. In the end, the object will cool down.
What is Forced Convection?
Forced convection is a method of heat transfer in which external means influence the motion of the fluid. There, external sources such as pumping, fans, suction devices, etc. are useful in generating the fluid motion. This method is very valuable because it can efficiently transfer heat from a heated object. Some common examples of this mechanism include air conditioning, steam turbines, etc.
When considering the mechanism of forced convection, it is has a complicated mechanism than the natural way. That is because, in this method, we have to regulate two factors; fluid motion and heat conduction. These two factors have a strong connection since the fluid motion can enhance the heat transfer. Ex: higher the rate of motion of the fluid, higher the heat transfers.
What is the Difference Between Natural and Forced Convection?
Natural convection is a method of heat transfer in which the motion of the fluid is influenced by natural means. Forced convection is a method of heat transfer in which the motion of the fluid is influenced by external means. When considering the factors affecting the heat transfer, there are no external factors affecting the heat transfer in natural convection while external factors can cause heat transfer in forced convection.
The fluid motion in natural convection generates as a result of the change of the density of fluid when heated. However, the fluid motion in forced convection generates as a result of an external source such as pumping, fan, suction devices. Cooling down a boiled egg when kept in the normal air, loss of cool of a cool drink can, etc. can be included as examples of natural convection, and air conditioning, steam turbines, etc. are examples of forced convection.
Summary – Natural vs Forced Convection
Convection is in two forms as natural convection and forced convection. The difference between natural and forced convection is that, in natural convection, the natural means influence the motion of the fluid whereas, in forced convection, external means influence the motion of the fluid.
Reference:
1. “Natural Convection.” Wikipedia, Wikimedia Foundation, 17 Apr. 2018. Available here
2. msg15 Follow. “Forced Convection.” LinkedIn SlideShare, 21 Apr. 2015. Available here
Image Courtesy:
1.’Natural Convection Circulation by Asperitas’By Rolf Brink – Own work, (CC BY-SA 4.0) via Commons Wikimedia