Cp	Specific heat capacity, (J/kg.K)	ν	Kinematic viscosity, (m2/s)
D	Absorber tube inner diameter, (m)	ρ	Density, (kg/m3)
d	Diameter of obstacle, (m)	τw	Wall shear stress, (Pa)
f	Friction factor	ϕ	Nanoparticle volume fraction, %
h	Heat transfer Coefficient, (W/m2.K)	φ	Circumferential angle, (°)
L	Tube length, (m)	Subscripts
ṁ	Mass flow reat, (kg/sec)	avg	Average
Nu	Nusselt number	b	Bottom
P	Distance between the obstacles, (m)	cal	Calculated
P	Pressure, (Pa)	D	Hydraulic diameter
∆P	Pressure drop, (Pa)	exp	Experiment
Pr	Prandtl number	f	Fluid
q	Heat flux, (W/m2)	in	Inlet
Re	Reynolds number	max	Maximum
T	Temperature, (K)	min	Minimum
v	Velocity, (m/s)	nf	Nanofluid
Greek symbols		np	Nanoparticle
α	Inclination angle of the obstacles, (°)	t	Top
δ	Deviation, (%)	x, y, z	Cartesian coordinates
ε	Rate of dissipation of turbulent kinetic energy	Abbreviation
k	Turbulent kinetic energy, (J/kg)	DISS	Direct Solar Steam
λ	Conductivity, (W/m.K)	DNI	Direct normal irradiance
HTF	Heat transfer fluid	PEC	Performance evaluation criteria
LCR	Local concentration ratio	PTC	Parabolic trough collector
MCRT	Monte Carol ray-tracing	PTR	Parabolic trough receiver
OD	Obstacles discs	RNG	Renormalization-group
OHD	Obstacles half discs