Introduction to Food Engineering

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Free (Natural) Convection

Learning Outcomes

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

  1. describe the natural flow of fluid over a heated surface
  2. learn how to calculate Grashof and Rayleigh Numbers
  3. calculate rate of heat transfer in fluids under conditions of natural convection

In this module, we will learn how to determine the rate of heat transfer under conditions of free convection (also called natural convection). Free convection involves movement of a fluid over a solid surface only under the influence of gravity; there is no forced movement of fluid (no fans, pumps, blowers, etc.). In the following video, you will learn how to calculate two dimensionless numbers important in situations involving free convection, namely, Grashof Number and Rayleigh Number. One of the critical parameters in these numbers is the characteristic dimension. It is the dimension of the object that influences the movement of the fluid over it. To solve problems, you will require information on appropriate dimensionless correlations describing fluid flow over prevailing orientations such as inside or outside horizontal pipes or flow over vertical walls. This information is available in textbooks.


In this module, you learned how to calculate the rate of heat transfer for free convection. We considered relevant dimensionless numbers, Grashof Number, and Rayleigh Number.