Computational Modeling and On-Sun Model Validation for a Multiple Tube Solar Reactor with Specularly Reflective Cavity Walls. Part 1: Heat Transfer Model

A three-dimensional, steady state computational model coupling radiative transfer with convective and conductive heat transfer is developed to describe a solar receiver consisting of an array of five tubes enclosed within a specularly reflective cylindrical cavity with a windowed aperture. Ray trace modeling of the concentrating system provides the magnitude and direction of solar energy incident on the aperture. Transport of solar radiation in the cavity space is decoupled from all other transport processes occurring in the receiver and profiles of the absorbed solar energy are determined via a Monte Carlo technique requiring only the receiver geometry, solar profile at the aperture, and spectral directional optical properties. A finite volume method is utilized to account for thermal radiation emitted by heated surfaces and implemented in conjunction with a computational fluid dynamics model. Maximum temperatures of 1820. K, 1355. K and 1536. K are predicted for the center, front, and back tubes, respectively for a solar power input of 6. kW though temperature gradients as high as 340. K develop between the front and back sides of the center tube. More than 79% of the solar energy is absorbed by tube surfaces. Emission losses account for 11-25% of the solar input whereas conductive heat losses account for 55-69% of the solar input and arise predominantly from conduction along the tube length to cooled cavity walls. Average discrepancies between theoretically predicted and experimentally measured temperatures are 44. K (4%) for silicon carbide tubes and 21. K (2%) for Inconel tubes over temperature ranges of, respectively, 600-1700. K and 700-1400. K.