Flow Over Finned Tube Heat Exchanger

Finned tube heat exchangers consist of an array of tubes each having a dense fin stack. The fluid passes through the tubes and conducts the heat to the fins, which dissipate heat to air that is blown over the fin via forced convection. These types of heat exchangers are generally employed where the heat transfer coefficient is much higher on one side of the heat exchanger than the other. Applications typically involve heating or cooling air or other gases. Typical examples of applications where finned tubes are used include economizers, waster heat recovery units, fired heaters, oil heaters, air coolers, etc.

Geometry and Meshing

Geometry and Meshing courtesy @Ansys

Setup and Boundary Conditions

Flow Type - Laminar
Fluid- Air
Solid- Aluminium
inlet -Pressure inlet
outlet - Mass flow outlet : 9.1 e5 kg/sec
Tube Wall- Symmetry at 350K temperature

Results

The airflow comes to a stop in the stagnation regions at the upstream fin tips and tube walls before turning and flowing around the tube.
As the flow progresses downstream through the gaps between the fins, its velocity is reduced by the viscous friction with the fin and tube walls.
Due to the staggered arrangement of the tubes, large portions of downstream tubes remain in the flow path allowing higher heat transfer compared to the aligned configuration.
The pressure drop across the tube array can be seen in the pressure contour plot below.

Velocity Contour

Velocity gradient can be seen across the tubes. Change in velocity can deeply affect the temperature distribution across the body.

Pressure Contour

Pressure Distribution can be seen as the flow passes the tubes following Bernoulli's Principle.

Tempreture Contour

The cross flow carries heat from the tube walls and the extended surfaces via convection.
This can be seen from the temperature contour plots shown on the right.
As aluminum has high thermal conductivity, it provides low thermal resistance to heat conduction and as a result, the fins are at a higher temperature than the steel fins.
Thus, the aluminum fins allow a higher heat to be extracted from the tubes.
Another important thing to note is that the tubes fins upfront cool down more compared to the ones at the back, as the front row is exposed to the cold air stream while the back rows experience air already heated by the front rows.

Result : Heat Transfer Analysis

We can see that the average temperature of the fins is higher in the case of aluminum fins compared to the steel fins due to the higher conductivity of aluminum.

The total heat transfer rate is therefore significantly higher for the aluminum fins compared to the steel fins.

There is a significant reduction in heat transfer for rows 3 and 4 as compared to rows 1 and 2, as the first two rows are exposed to the stream of ambient cold air, while rows 3 and 4 facing hot wakes of the first two rows.

The total heat transfer rate from the fins is higher compared to the tube walls because of the larger surface area available for convective heat transfer.

Another interesting thing to note here is that the heat flux from the tube walls of row 2 is higher than the heat flux from the tube walls in row 1.

The flow accelerates as it bends and turns while flowing around the row 1 tubes, the incident flow for row 2 tube walls is at a higher velocity as shown by the red colored pathlines .