Fig. 4 is plotted to show the effects of mass flow rate on the curve
characteristics of solar collector for different nanofluid concentrations.
At the first glance, it is found that the mass flowrate has no considerable
effect on the thermal efficiency when the heat loss parameter limits to
zero. However, with increasing of the heat loss parameter, a greater
mass flow rate gives a higher efficiency at each concentration. Based
on the definition of efficiency in Eq. (1), a higher mass flow rate
indicates a higher efficiency. On the other hand, with increasing of
mass flow rate the magnitude of radiation absorption by working fluid
reduces, hence, the temperature difference between the inlet and outlet
of solar collector decreases. Therefore, the balance between the mass
flow rate and temperature differences determines the final efficiency.
Fig. 5 shows the variation of absorbed energy parameter with the
volume fraction for different mass flow rates. It is observed that the
absorbed energy parameter increases with an increase in nanofluid
concentration. Themaximumgrowth in the absorbed energy parameter
is experienced by the mass flow rate of 0.18 kg/s where the concentration
increases from 0 to 1%.