Solar thermal cracking of methane produces two valuable products, hydrogen gas and solid carbon, both of which can be used as a fuel and as a commodity. During the course of this two-phase phenomenon, carbon particles tend to deposit on the solar reactor window, wall, and exit. When they accumulate at the reactor exit, the agglomeration of these particles completely blocks the exit. This problem has been the major issue preventing solar cracking reactors from running continuously. To address this problem, a cyclone solar reactor was designed to enhance the residence time and allow carbon particles to rotate in the reactor instead of moving towards the exit inlarge particle groups together. A prototype reactor was manufactured to test the concept, to better understand and explain the flow dynamics inside the solar cyclone reactor and to analyze the flow via particle image velocimetry (PIV). Advanced measurement and computational techniques were applied to build the prototype reactor. Computational fluid dynamics (CFD) analysis employing discrete phase model (DPM) was used to predict the particle transport phenomenadel (DPM), whereas PIV was applied for the experimental part of the work. To understand the flow evolution along the vortex line, several images in the axial direction along the vortex line were captured. The results showed that when the main flow was increased by 25%, the axial velocity components became larger. It was also observed that the vertical vortices along the vortex line showed stronger interaction with outward fluid in the core region. This implied that the horizontal twisting motion dominated the region due to the main flow, which could trap the particles in the reactor for a longer time. Furthermore, when the main flow was increased by 50%, the flow displayed a cyclone-dominated structure. During the velocity evolution along the vortex line, more vortices emerged between the wall region and core region, implying that the energy was transferred from order to disorder. In summary, by appropriate selection of parameters, the concept of an aero-shielded solar cyclone reactor can be an attractive option to overcome the problem of carbon particle deposition at the reactor walls and exit.

A hydrogen-producing solar reactor was experimentally tested to study the cyclone flow dynamics of the gas–particle two-phase phenomenon. Two-dimensional particle image velocimetry (PIV) was used to observe the flow and to quantify the vortex formation inside the solar reactor. The vortex flow structure in the reactor was reconstructed by capturing images from orientations perpendicular and parallel to the geometrical axis of the reactor, respectively. The experimental results showed that the tangential components of the fluid velocity formed a Rankine-vortex profile. The free vortex portions of the Rankine profile were synchronized and independent of the axial position. The axial components showed a vortex funnel of higher speed fluid supplied by a reversing secondary flow. According to the inlet channel design, the geometry dominates the flow dynamics. A stable processing vortex line was observed. As the vortex flow evolves toward the exit, the vortex funnel expands radially with decreasing tangential velocity magnitude peak as a result of the vortex stretching. An optimal residence time of the flow was found by changing the cyclone flow inlet conditions. The swirl number versus the main flow rate change was obtained. Upon completion of the experimental studies, a thorough numerical analysis was conducted to model the flow dynamics inside the solar reactor and to verify the results by comparison to the experimental results. Three turbulence models including the standard k–ϵ, k–ϵ renormalization groups (RNG), and Reynolds stress transport models were used. Computational fluid dynamics (CFD) simulations were coupled with heat transfer analysis via discrete ordinate (DO) model. Particle tracing in Lagrange frame was applied to simulate the particle trajectory. A comparison between the turbulence modeling results for the room temperature and high temperature cases, as well as the experimental results for room temperature cases is presented.

A hydrogen producing solar reactor was experimentally tested to study the cyclone flow dynamics of the gas-particle two-phase phenomenon. Two dimensional PIV (particle image velocimetry) was used to observe the flow and to quantify the vortex formation inside the solar reactor. The vortex flow structure in the reactor was reconstructed by capturing images from orientations perpendicular and parallel to the geometrical axis of the reactor respectively. The experimental results showed that the tangential components of the fluid velocity formed a Rankine-vortex profile. The free vortex portions of the Rankine profile were synchronized and independent of the axial position. The axial components showed a vortex funnel of higher speed fluid supplied by a reversing secondary flow. According to the inlet channel design, the geometry dominates the flow dynamics. A stable precessing vortex line was observed. As the vortex flow evolves towards the exit, the vortex funnel expands radially with decreasing tangential velocity magnitude peak as a result of the vortex stretching. An optimal residence time of the flow was found by changing the cyclone flow inlet conditions. The swirl number versus the main flow rate change was obtained. Upon the completion of the experimental studies, a thorough numerical analysis was conducted to model the flow dynamics inside the solar reactor and to verify the results by comparison to the experimental results. Three turbulence models including the standard k-ε, k-ε RNG and Reynolds Stress Transport models were used. CFD simulations were coupled with heat transfer analysis via Discrete Ordinate model. Particle tracing in Lagrange frame was applied to simulate the particle trajectory. A comparison between the turbulence modeling results for the room temperature and high temperature cases, as well as the experimental results for room temperature cases is presented.

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