Numerical model of solar external receiver tubes: Influence of mechanical boundary conditions and temperature variation in thermoelastic stresses

Abstract

Failure in solar external receivers is mainly originated from the thermal stress, caused by the high non-uniform transient solar flux. The heat-up and cooldown of tube receivers in daily cycles produce low-cycle fatigue that limits the lifetime of tubes. The corrosion of tube materials produced by incompatibility between the decomposed heat transfer fluid and tube material may increase this issue.The temperature spatial distribution in these tubes has strong variations in radial, circumferential, and axial directions. The stress field, produced by the temperature gradients, has been commonly analyzed using bidimensional models in isolated tube cross sections, without taking into account the axial temperature variation, the mechanical boundary conditions, and the temperature-dependent thermomechanical properties.In this work, a three-dimensional finite element model has been developed in order to calculate the stress field distribution, without performing any geometrical simplification. In addition, appropriate mechanical boundary conditions have been imposed in order to adequately simulate the tube behavior. Besides, radial, circumferential and axial temperature variations have been studied separately to analyze how each of them influences the maximum stress distribution. This 3D model has been compared with analytical solutions for the two-dimensional thermal stress problem in circular hollow cylinders.The results show that the boundary conditions have a significant effect on the tube stresses, increasing the axial stress component and therefore the equivalent stress.The analysis of each of the temperature variations showed that the circumferential variation temperature is the one that produces most of the stress, since it tries to strongly bend the tube, which is impeded by the boundary conditions.The results also present that 2D models are not capable of obtaining the correct stress distribution along the tube, since they are not taking into account the longitudinal supports. By contrast, the maximum stress can be obtained with confidence using the analytical stress solution of the angular and radial temperature variation around a hollow circular cylinder.