higher for lower gas flow rates. This trend was observed for
both experimental and numerical results. When the gas
flow rate is low, the gas flow heat capacity is small respect
to tube wall heat capacity. Therefore, temperature drop
can be higher for lower gas flow rates. In other words, heat
transfer takes place within the tube in two directions, convection
heat transfer from gas into tube wall and axial conduction
heat transfer within the tube wall. In the case of
low gas flow rates, convection heat transfer from gas into
tube wall is low. So, the conduction heat transfer within
the tube wall is dominated and wider surface area (longer
length of tube wall) is affected by longitudinal heat conduction.
In this situation, gas flow within the tube is
exposed to wider cold surface area and its temperature
drops down.