Thermal Stress in Externally Irradiated Tubes of Solar Central Receivers with a Gas-Particle Fluidized Dense Suspension

One of the objectives of the new generation of Concentrating Solar Power (CSP) plants is to increase the actual temperature limit (565 $^{\circ}$C) up to or near 1000 $^{\circ}$C. An alternative Heat Transfer Fluid (HTF) that could help to reach this objective is the use of a fluidized dense gas-particle suspension ascending through a vertical tube. The main objective of this work is to numerically study the thermomechanical behaviour of a non-uniform externally irradiated tube where particles are fluidized and moves upwards. The heat transfer problem in the two media present (i.e. dense suspension of particles and tube wall) was solved separately: a Computational Particles Fluid Dynamic (CPFD) model solves the heat transfer in the particles, whereas a 2-D Finite Volume model simulates the heat conduction through the tube wall to obtain the temperature profile, which serves as input to calculate the thermal stress of the tube with an analytical method.Higher thermal stresses were obtained for an absorbed power of 250 kW/m$^2$ ($\sigma_{VM,max}=219\,$MPa) compared to that for an absorbed power of 500 kW/m$^2$ ($\sigma_{VM,max}=72\,$MPa). Although the maximum temperatures reached are higher for 500 kW/m$^2$ ($T_{w,max}=1402\,^{\circ}$C compared to $1017\,^{\circ}$C for $250\,\text{kW}/\text{m}^{2}$), the maximum temperatures difference between the front and the rear of the tube are lower ($\Delta T_{w,max}=134$ K compared to 174 K for $250\,\text{kW}/\text{m}^{2}$). The position where the maximum stress is located depends also on the incident power, being located on the outer surface of the front of the tube for an incident irradiation lower than $250\,\text{kW}/\text{m}^{2}$ and on the inner surface of the rear part of the tube for $500\,\text{kW}/\text{m}^{2}$. The thermomechanical behaviour of the receiver with particles were compared to that for a molten salt receiver: for an incident solar flux of 500 kW/m$^2$, a reduction of 73 \% in the maximum thermal stress was obtained for the particle receiver compared to the molten salt receiver, due to the lower temperature differences in the wall of the tube.