Different Areas to Use

From Chemepedia


Interfacial Area

This refers to the total area of contact at the interface between fluid A and fluid B.

Surface Area

The surface area of an object measures the total area at the outermost border of the object; in other words, at the surface.

Cross-Sectional Area

This is the area that a flowing fluid would see when passing through an object. For example, a fluid flowing horizontally through a cylinder would see a circular path as it moves down, thus the cross-sectional area here would be the area of a circle.



Surface Area:


(Note: In most cases, the second term is almost always neglected as the fluid continuously flows, so the ends of the cylinder are not considered in calculations involving total surface area.)

Cross-sectional Area:



Surface Area:


Cross-sectional Area:


(Note: Think about why this area is equal to the corresponding area in cylinders.)

When to Use What

In some cases, texts will use different variables to denote different areas when discussing heat and mass transfer. For the most part, though, an area will most likely be symbolized by , making it important to be aware of which area an equation is referring to.

Heat Transfer


Here, the on the right-hand-side of the equation corresponds to the convective heat transfer, and uses the surface area of the object the fluid is flowing through.

Mass Transfer


Again, the area term on the right-hand-side corresponds to the surface area of the object through which the fluid is flowing.

Flow of an Object

When calculating the velocity of a moving object, the area used in the appropriate equations beforehand correspond to the cross-sectional area of the object itself, as opposed to the area through which it is moving. As a baseline, calculations involving Reynold's number usually follow these guidelines.


To calculate the above velocity , one might use the volumetric flow rate and area as follows,



  • is the cross-sectional area of the object (usually a particle) moving through a fluid.