List of symbols
WmK−1	Thermal conductivity
m	Length of the heat exchanger, l
mV	Zeta potential
J/kg·K	Specific heat of water, S
m	Diameter of Cu-tube
m	Length
K	Δθ
K	ΔT
cP	Viscosity, μ
Re	Reynolds number
ζ	zeta potential
μ	Viscosity
T	Temperature
Q	the rate of forced convection heat transfer (W)
T s	the solid surface temperature (K)
T f	the fluid temperature (K)
A	the area of the surface that is in contact with the fluid (m2)
h	the convective HTC (W/m2·K)
m	the mass of hot water, kg,
S	the specific heat of water, J/kg·K
Δθ	the difference in water temperature before and after releasing temperature, K.
Abbreviations
ZnO	Zinc oxide
NR	Nanorod
BF	Base fluid
NF	Nanofluid
XRD	X-ray diffraction
TEM	Transmission electron microscopy
UV–vis	Ultraviolet-visible
HTC	Heat transfer coefficient
EG	Ethylene glycol
NM	Nanomaterial
NaOH	Sodium hydroxide
PVP	Polyvinylpyrrolidone
JCPDS	Joint committee for powder diffraction studies