A	Tube cross section area (m2)
$c_p$	Specific heat (J/kg·K)
Din	Tube inner diameter (µm)
$D_o$	Tube outer diameter (µm)
f	Darcy friction factor
h	Heat transfer coefficient (W/m2·K)
k	Thermal conductivity (W/m·K)
l	Tube length (cm)
Nu	Nusselt number
Δp	Pressure drop (N/m2)
Pr	Prandtl number
Q	Volume flow rate (m3/s)
q	Heat flux (W/m2)
Re	Reynolds number
$\dot{S}$	Entropy generation rate (W/m·K)
$T_{wi}$	Inner wall temperature (K)
$T_{wo}$	Outer wall temperature (K)
u	Bulk fluid velocity (m/s)
Greek Symbols
ρ	Density (kg m−3)
µ	Dynamic viscosity (N s m−2)
φ	Nanoparticles fraction (%)
Subscripts
av	Average
bf	Base fluid
eff	Effective
fr	Friction
hy	Hybrid nanofluids
nf	Nanofluid
np	Nanoparticles
th	Thermal