A	area (m2)	Greek symbols
cp	specific heat transfer (J/kg.K)	ϑ	kinematic viscosity (m2/s)
f	friction coefficient	μ	dynamic viscosity (kg/ms)
f0	friction coefficient of plain channel	ρ	density (kg/m3)
F	body force (N)	ϕ	nanofluid concentration
g	gravitational acceleration (m/s2)	θ	dimensionless temperature
h	heat transfer coefficient (W/m2K)	Subscripts
k	thermal conductivity (W/mK)	ave	average
n	number of phases	b	bulk
p	pressure (Pa)	m	mixture
q″	heat flux (w/m2)	f	fluid
Re	Reynolds number	i	inner tube
T	temperature (K)	o	outer tube
V	velocity vector (m/s)	nf	nanofluid
Vdr	drift velocity	Abbreviations
Vf	fluid velocity	Co-STT	co-swirling twisted tapes
Vp	particle velocity	Counter-STT	counter-swirling twisted tapes
		PHE	plain heat exchanger
		IT	inner tube
		OT	outer tube