Influence of circumferential solar heat flux distribution on the heat transfer coefficients of linear Fresnel collector absorber tubes

摘要

The absorber tubes of solar thermal collectors have enormous influence on the performance of the solar collector systems. In thisnumerical study, the influence of circumferential uniform and non-uniform solar heat flux distributions on the internal and overall heattransfer coefficients of the absorber tubes of a linear Fresnel solar collector was investigated. A 3D steady-state numerical simulation wasimplemented based on ANSYS Fluent code version 14. The non-uniform solar heat flux distribution was modelled as a sinusoidal functionof the concentrated solar heat flux incident on the circumference of the absorber tube. The k–e model was employed to simulate theturbulent flow of the heat transfer fluid through the absorber tube. The tube-wall heat conduction and the convective and irradiative heatlosses to the surroundings were also considered in the model. The average internal and overall heat transfer coefficients were determinedfor the sinusoidal circumferential non-uniform heat flux distribution span of 160 , 180 , 200 and 240 , and the 360 span of circumferentialuniform heat flux for 10 m long absorber tubes of different inner diameters and wall thicknesses with thermal conductivity of16.27 W/mK between the Reynolds number range of 4000 and 210,000 based on the inlet temperature. The results showed that the averageinternal heat transfer coefficients for the 360 span of circumferential uniform heat flux with different concentration ratios on absorbertubes of the same inner diameters, wall thicknesses and thermal conductivity were approximately the same, but the average overallheat transfer coefficient increased with the increase in the concentration ratios of the uniform heat flux incident on the tubes. Also, theaverage internal heat transfer coefficient for the absorber tube with a 360 span of uniform heat flux was approximately the same as thatof the absorber tubes with the sinusoidal circumferential non-uniform heat flux span of 160 , 180 , 200 and 240 for the heat flux of thesame concentration ratio, but the average overall heat transfer coefficient for the uniform heat flux case was higher than that of the nonuniformflux distributions. The average axial local internal heat transfer coefficient for the 360 span of uniform heat flux distribution ona 10 m long absorber tube was slightly higher than that of the 160 , 200 and 240 span of non-uniform flux distributions at the Reynoldsnumber of 4 000. The average internal and overall heat transfer coefficients for four absorber tubes of different inner diameters and wallthicknesses and thermal conductivity of 16.27 W/mK with 200 span of circumferential non-uniform flux were found to increase with thedecrease in the inner-wall diameter of the absorber tubes. The numerical results showed good agreement with the Nusselt number experimentalcorrelations for fully developed turbulent flow available in the literature.