Computational Modeling and On-Sun Model Validation for a Multiple Tube Solar Reactor with Reflective Cavity Walls
A three-dimensional steady state computational fluid dynamics model is developed for a solar receiver consisting of a reflective cavity with a windowed aperture enclosing an array of five tubes. The computational model couples radiative transfer with fluid flow, heat transfer, mass transfer, and chemical reaction kinetics. The accuracy of the finite volume (FV) method for the approximate solution of the radiative transfer equation is examined for a 2D cross-section of the receiver for the solar and emitted energy components on each of four unstructured grids with three sets of control angles and constant pixelization. FV model accuracy was judged by comparison with Monte Carlo solutions. FV results for the solar component either failed to approach the MC solution or required highly refined spatial and angular grids to achieve accurate, non-oscillatory solutions whereas solutions for the emitted component showed only a weak dependence on the angular and spatial grids.
Based on these results, radiation heat transfer is included in the full 3D model via a combination of ray tracing, Monte Carlo, and finite volume methods. Ray trace modeling of the concentrating system provides the magnitude and direction of the solar energy incident on the receiver window surface. Transport of solar radiation in the cavity is decoupled from all other transport processes occurring in the receiver and profiles of the absorbed solar radiation are determined via a Monte Carlo model using only the receiver geometry, magnitude/direction of radiation incident on the window, and spectral/directional optical properties. A finite volume model is implemented in conjunction with the overall CFD model to account for thermal radiation emitted by heated surfaces in the receiver. Absorption coefficients, scattering coefficients, and the scattering phase function for the cloud of particles in each tube are determined via Mie theory. Transport of the aerosol particles is modeled using a single fluid mixture model with a population balance including transport by convection, Brownian motion, and thermophoretic diffusion. Gasification of 40nm carbon particles is used as a test reaction with Arrhenius kinetic parameters from the literature. Temperature distributions and, correspondingly, reaction conversions among the five tubes are found to be considerably non-uniform, with large temperature gradients developing between the front and back surfaces of each tube. Validation of the base heat transfer and reaction models is accomplished using experimental data taken on-sun at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) with both inert and reacting materials reaching temperatures up to 1400°C.
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