Computational fluid dynamics (CFD) is a vital tool for providing insights into the design, optimization, and evaluation of solar receivers/reactors. CFD facilitates detailed predictions and deeper understanding of system heat and mass transfer.  An ongoing challenge for CFD modeling in concentrating solar applications is coupling directional intensities from solar concentrating infrastructure with other relevant physics (*e.g.*, fluid mechanics, heat transfer, and chemical reaction behavior). Current modeling techniques for solar inputs require either significant simplifications to radiative boundary conditions or the inclusion of the concentrating solar infrastructure within the computational domain, reducing model accuracy or increasing computational and development time/cost.

Monte Carlo ray tracing (MCRT) and finite volume approximations to the radiative transport equation (FV–RTE) are two methods for modeling radiative transport from concentrating solar infrastructures and within solar receivers/reactors. Challenges in implementing MCRT occur when modeling optically thick media, wavelength or temperature-dependent radiative properties, and complex geometries. These conditions are often more tractable using FV–RTE methods such as the discrete ordinates (DO) method, which is typically supported in commercial CFD software. However, FV–RTE methods are computationally expensive for domains requiring high mesh resolutions, like combined models of solar receivers/reactors and collecting/generating infrastructure.

The use of MCRT to model the concentrated solar input to the receiver/reactor and FV-RTE to capture internal radiative transport within the reactor reduces the model development time and computational expense. The combined models, when correctly paired, are powerful tools for capturing radiative heat transfer from concentrating solar infrastructure and within solar receivers/reactors. However, the pairing of the models is a potential source of significant modeling error in concentrated solar applications, as the radiative input is both highly directionally- and spatially-varying.

A novel, computational method for mapping surface irradiation and volumetric absorption from MCRT to CFD models is presented in this work. The method is well-suited for the directional and spatial radiative inputs of concentrating solar infrastructures. It employs a simple algorithm for directly mapping each absorbed ray from the MCRT model domain to boundary or volumetric source terms in the CFD domain. The algorithm is compatible with unstructured, two and three-dimensional CFD meshes with varying face and cell element shapes.

The method was developed and demonstrated for a custom MCRT of a high-flux solar simulator written in C++. The MCRT was mapped to an ANSYS Fluent model of a directly irradiated, windowed solar thermochemical reactor model via user-defined functions in Fluent. The method is presented generally to allow implementation in numerous other commercial and in-house MCRT/CFD software. To evaluate the method, four case studies were performed, and the results were compared to other current mapping methods. The method was demonstrated to be energy conservative and to preserve spatial variation when mapping rays from the MCRT to the CFD model, even in the cases of complex geometries and high irradiative gradients.


Pairing Directional Solar Inputs From Ray Tracing to Solar Receiver/reactor Heat Transfer Models on Unstructured Meshes: Development and Case Studies

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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