Experimental and CFD investigation on heat transfer enhancement in evacuated tube solar collector with coiled wire inserts

ABSTRACT

This article presents a novel design of a modified evacuated tube collector fabricated for air heating applications by fixing helical coiled inserts in the vacuum tube. The investigations on modified evacuated tube collector (ETSC-HI) were conducted experimentally and using Computational Fluid Dynamics (CFD) and assessed with the unmodified ETC solar air heater under analogous conditions. The working of the modified evacuated tube collector (ETSC-HI) has been scrutinized under different modes, geometrical parameters, flow parameters and different operating conditions. Experimentally, the influence of these variables on outlet air and thermal efficiency of the modified collector have been studied in detail. The CFD findings have been visualized by plotting graphical contours, vectors and streamlines for various cases. Optimal geometrical parameters for maximum heat transfer coefficient were calculated from the CFD investigations and were considered for further experimental analysis by fixing the coiled wire on the vacuum pipe. It was seen that ETSC-HIperforms better contrasted to the unmodified ETC, registering Nusselt number enhancement of 3.23 times corresponding to P/D_h = 1.5 e/D_h = 0.074 at m = 0.015 kg/s. The highest thermal efficiency for ETSC-HI and unmodified collector are obtained as 70.99% and 64.86% respectively showing significant improvement in the performance. On average, the enhancement of 5.85% in daily average efficiency was seen for the modified ETSC-HI.

KEYWORDS:

• Evacuated tube collector
• Helical coil insert
• Energy
• roughness
• solar air heater

Nomenclature

ETC	=	Evacuated Tube Collector
ETSC-HI	=	Evacuated Tube Solar Collector with helical Inserts
ETSC	=	Evacuated Tube Solar Collector
FPC	=	Flat Plate Collector
CFD	=	Computational Fluid Dynamics
SAH	=	Solar Air Heater
D	=	diameter of pipe, mm
Ta	=	Ambient temperature, K
Ti	=	Inlet temperature, K
To	=	Outlet temperature, K
n	=	Number of Evacuated tubes
I	=	Solar radiation intensity, W/m2
ηth	=	Thermal efficiency
ηeff	=	Effective efficiency
ΔP	=	Pressure Drop, Pa
ΔPo	=	Pressure Drop across orifice, Pa
W	=	specific weight of water (9.81 KN/m2)
H	=	Difference in column heights of U-tube manometer, mm
m	=	Mass flow rate, kg/s
Cd	=	Coefficient of discharge
Β	=	Diameter ratio of orifice to pipe diameter
AO	=	Area of orifice, m2
v	=	velocity, m/s
Do	=	Absorber tube outer diameter, mm
Dcp	=	Central pipe diameter, mm
W	=	Width of duct, mm
H	=	Height of duct, mm
Re	=	Reynolds Number
Ap	=	Aperture area, m2
Qu	=	Useful thermal energy gain, W
CP	=	Specific heat of air at constant pressure, kJ/kgK
Pm	=	Pumping power, W

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Inderjeet Singh

Inderjeet Singh is serving as an Assistant Professor of Mechanical En-gineering at CT University, Ludhiana, Punjab. He received his Ph.D. in Me-chanical Engineering with research in the field of solar energy systems. His research interests are towards renewable energy, fluid mechanics, heat transfer, solar thermal systems and CFD. He has published number of arti-cles in international journals and conferences of repute.

Sachit Vardhan

Sachit Vardhan is working as an Associate Professor and Head of De-partment, Mechanical Engineering in CT University, Ludhiana. He received his Ph.D. in Mechanical Engineering from Sant Longowal Institute of Engi-neering & Technology (SLIET), Longowal, Punjab. He has published more than 50 articles in reputed international journals and conferences.