Nomenclature
a	Annulus diameter ratio, dimensionless
ci	Correlation fitting constant, dimensionless
cp	Specific heat capacity, J·kg−1·K−1
d1	Inner diameter of the inner tube, m
d2	Outer diameter of the inner tube, m
d3	Inner diameter of the outer tube, m
dh	Hydraulic diameter, m
f	Darcy friction factor, dimensionless
h	Convection coefficient, W·m−2·K−1
k	Thermal conductivity, W·m−1·K−1
LMTD	Logarithmic mean temperature difference, °C
Lh	Effective heating length, m
Lp	Effective pressure drop length, m
Nu	Nusselt number, dimensionless
Pr	Prandtl number, dimensionless
$\dot{Q}$	Heat flow rate, W
R	Thermal resistance, K·W−1
Re	Reynolds number, dimensionless
T	Temperature, K, °C
v	Average velocity of the fluid, m·s−1
$\dot{V}$	Volume flow rate, m3·s−1
vol %	Percentual graphene nanoplatelets volume concentration, dimensionless
wt %	Percentual graphene nanoplatelets mass concentration, dimensionless
Δp	Pressure drop, Pa
η	Thermal performance factor, dimensionless
ρ	Density, kg·m−3
φm	Graphene nanoplatelets mass concentration, dimensionless
φv	Graphene nanoplatelets volume concentration, dimensionless
ψ | ω	Correction factors for the Nusselt number in the annulus, dimensionless
Subscripts
bf	Base fluid
hw	Heating water
in	Inlet of the test section
nf	Nanofluid
out	Outlet of the test section
ov	Overall
s	Steel
tf	Tested fluid
w	Water
wall	Tube wall