Heat Transfer Performance Of A Linear Fresnel Solar Collector Absorber Tubes With Circumferential Non-Uniform Heat Flux Distributions
Izuchukwu F. Okafor
National Centre for Energy Research and Development, Africa Centre of Excellence-Sustainable Power and Energy Development University of Nigeria, Nsukka
Keywords: absorber tube, solar heat flux, numerical simulation, heat transfer coefficients
Abstract
This study investigated the heat transfer performance of a linear Fresnel solar collector absorber tubes with circumferential non-uniform heat flux distributions. A 3D steady-state numerical simulation was implemented on ANSYS Fluent code version 14. The non-uniform solar heat flux distribution was modelled as a sinusoidal function of the concentrated solar heat flux incident on the circumference of the absorber tube. The k-ε model was used to simulate the turbulent flow of the heat transfer fluid through the absorber tube. The tube-wall heat conduction and the convective and irradiative heat losses to the surroundings were also considered in the model. The average internal and axial local transfer coefficients were determined for the sinusoidal circumferential non-uniform heat flux distribution span of 160°, 180°, 200° and 240°, and a 360° span of circumferential uniform heat flux for a 10 m long absorber tubes of a 62 mm inner diameter and a 52 mm wall thickness with thermal conductivity of 16.27 W/mK between the Reynolds number range of 2 600 and 100 000 based on the fluid inlet temperature. The results showed that the average internal heat transfer coefficients for the 360° span of circumferential uniform heat flux with different concentration ratios were approximately the same. The average internal heat transfer coefficient for the absorber tube with uniform heat flux was approximately the same as that of the absorber tubes with the sinusoidal circumferential non-uniform heat flux distributions. The average axial local internal heat transfer coefficient for the uniform heat flux distribution was slightly higher than that of non-uniform flux distributions at the Reynolds number of 4 000. The averaged internal heat transfer coefficient increased with the decrease in the inner diameter of the absorber tube and the wall thickness. However, a decrease in the inner diameter of the tube resulted in an increase in pressure drop and the consequent increase in the pumping power required to overcoming the pressure drop and the turbulent dissipation of the heat transfer fluid. The numerical results showed good agreement with the Nusselt number experimental correlations for fully developed turbulent flow available in the literature.