### Convective heat transfer

#### Learning Outcomes

At the end of this module, you will be able to:

- understand the role of boundary layers in fluid flow and heat transfer
- discuss the reason for using empirical approaches to determine rate of convective heat transfer
- describe the role of three dimensionless numbers - Reynolds, Nusselt and Prandtl numbers - in convective heat transfer

In this module, we will first consider the role of boundary layers in fluid flow and heat transfer. The complexity and chaotic behavior of fluids in motion make it difficult to obtain simple expressions to calculate the rate of heat transfer as we learned for conductive heat transfer. Still, we should get some understanding of how the boundary layers are formed in a fluid as it flows on a solid surface. In this first video, we will consider both the hydrodynamic boundary layer and the thermal boundary layer.

In the previous video, we observed some of the complexities of fluid flow. The physics and mathematics involved in describing the flow of fluids (air, gases, water) are highly complicated. It is still not possible to predict with high accuracy the movement of an air mass in the atmosphere that may impact our local weather despite the use of supercomputers. Since the analytical treatment of this subject is beyond the scope of this course, we will instead use empirical approaches that give us useful tools to determine the rate of convective heat transfer in food processing. In the next video, we will consider three dimensionless numbers important in forced convection, namely Reynolds number, Nusselt number, and Prandtl number. The empirical correlations that we will use in calculating convective heat transfer use these three dimensionless numbers; therefore, we must get well acquainted with them.

Recap

In this module, you developed an understanding of the complexities of heat transfer in fluids as they flow over a solid surface. The boundary layers and their thickness depend on the flow conditions and fluid properties. Instead of the complex analytical analysis, we will use empirical approaches involving results from experiments conducted for well-defined conditions. You learned about three dimensionless numbers used in describing empirical relationships that you will learn to use in the next module.