where To is the environment temperature (K), Tu is the useful temperature
(K), Ts is the temperature of source (K), Qu is the useful heat
transfer rate, Qs is the heat transfer rate of source.
Fig. 13 shows the relationship between exergetic efficiency and
Reynolds number. It can be seen from the figure that the exergetic
efficiency of a rough surface is higher than that obtained from a
smooth surface. This is because the high surface roughness leads
to the high turbulence and heat transfer rate of cold water inside
the flow channel (Qu). The temperature difference between hot
stream (Ts) and cold stream (Tu) decreases with increasing surface
roughness. This indicates that the exergy destruction resulted from
the heat transfer irreversibility decreases. According to Eq. (16),
these effects result in the increase of exergetic efficiency.
However, under the same surface roughness, the higher Reynolds
number results in the decrease of exergetic efficiency. This is
because the high Reynolds number leads to the high friction and
frictional irreversibility which results in a decrease of exergetic
efficiency.