Thermal Characteristics Of Parabolic Trough Solar Absorber With Non-uniform Heat Flux

The solar collector, the crucial element of solar thermal energy system, is a devicewhich converts solar energy into heat energy. The heat conversion efficiency of the solarcollector will directly affect the efficiency and economy of the whole solar energysystem. With the characteristics of the time intermittent and spatial non-uniformity ofthe solar irradiation, the incident heat flux on the surface of the heat absorber is alwaysvariational which leads to temperature change repeatedly on the surface of the heatabsorber in the running process of the solar thermal power generation system, resultingin temperature gradient and mechanical stress in absorber wall. For the solar collector inthe middle and low temperature, because of the relatively low temperature on thesurface of the absorber, the temperature difference in the heat exchanger is small, andthe thermal stress is not high enough to influence the working performance of theabsorber. However, for the solar thermal power generation system with high opticalconcentration ratio, such as the parabolic trough solar absorber pipe, only part of thetube wall is heated, and the other part is not heated. Furthermore, the heat fluxdistribution on the heating side also has the characteristic of nonlinearity. In this processof non uniform heating, where the wall temperature is up to400℃, the thermal stresswhich is caused by the temperature gradient of the large temperature difference can notbe ignored.In this thesis, the heat transfer, thermal stress and deformation of the parabolictrough solar absorber are numerically studied using FLUENT and ANSYS commercialsoftware. The potential ways to reduce thermal stress and deformation of the solarabsorber are analyzed. The main points are as follows:A heat transfer model for the parabolic trough solar absorber is established. Fluentis used to obtain the temperature distribution of the single-phase region (the subcooledand superheated zone of the heat transfer fluid) of the trough solar collector in thenon-uniform heat flux distribution boundary condition. The influence of differentconditions such as the heat flux density, mass flow rate and the distribution of heat flux,on the thermal behavior are analyzed. The temperature field in the heat pipe wallcalculated in FLUENT is used as the additional load to analyze the stress distributionand the bending deformation of the tube wall by ANSYS WORKBENCH software.With reasonable simplifications, the heat transfer of the solar absorber in two-phase flow regime is studied. According to the different steam quality, the absorbertube is segmented into different regions (vapor and liquid phases) which are endowedwith the different heat transfer coefficients. On this basis, the thermal stress and thebending deformation caused by thermal stress of absorber under different heat fluxdensity, different steam quality and different heat flux distribution are analyzed intwo-phase flow region.The calculation results of the single-phase region and two-phase region of the solarabsorber under non-uniform heat flux are compared. Under the same conditions, theaxial, circumferential and radial distribution of thermal stress and bending deformationof the absorber in different flow regime are also analyzed;In order to study the potential ways to reduce the thermal stress and deformation ofthe absorber, the heat transfer models of the steel-copper compound wall absorber andhelically coiled absorber are also established. The numerical simulation of the thermalstress and deformation of these two kinds of absorber with different wall thickness,shapes and heat flux are carried out.The research work in this thesis provides the numerical foundation and thereference for the optimization design of the parabolic trough solar absorber for directsteam generation.