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The receiver is crucial in converting concentrated solar radiation to thermal energy in a solar parabolic dish collector. The most popular type of receiver is a cavity receiver, which reduces heat loss from the body. In this paper, an experimental investigation of the energy, exergy, and optical performance of two cavity-type receivers for a solar parabolic dish concentrator was carried out. The cylindrical and conical cavity tube receivers were designed and tested in the temperature range of 35 °C–190 °C, with water as the working fluid and a flow rate of 0.83 L per minute. The results showed that the conical cavity tube receiver is the optimum design, while the cylindrical shape is the second choice. The conical cavity tube design achieved 62% average thermal efficiency, 11% exergy efficiency, while the maximum optical efficiency was 82% under average solar irradiation of 865 W/m2. The overall heat loss coefficients of cylindrical and conical cavity receivers from the stagnation measure were 137 W/m2 and 125 W/m2, respectively. The maximum and average surface temperatures for both cylindrical and conical cavity tube receivers were reached at 140 °C, 115 °C and 155 °C, 125 °C, respectively. The simulation study was conducted using COMSOL Multiphysics software to investigate the temperature distribution of the receivers. The proposed lightweight conical cavity tube receiver was found to have a high capacity to be used with a solar parabolic dish-style concentrating device to operate heating water applications.

Phase field method has been widely utilized to study multiphase flow problems, but has seldom been applied to the study of imbibition. Previous methods used to simulate imbibition, such as moving mesh method, need to specify capillary pressure as a boundary condition a priori, whereas phase field method can calculate capillary pressure automatically for various geometries. Therefore, phase field method would be a versatile tool for the study of imbibition in various geometries. In this paper, phase field method is employed to solve dynamical imbibition problem in various geometries, including straight tube, conical tube and structures in which the topology changes. The variation of the imbibition height with respect to time from phase field simulation is verified with theoretical predictions from Lucas-Washburn law in a straight capillary tube with three gravitational scenarios. In addition, the capillary pressure and velocity field are found to be consistent with Laplace-Young equation and Hagen-Poiseuille equation in various geometries. The applicability and accuracy of the phase field method for the study of imbibition in structures with changing topology are also discussed. Cited as : Xiao, J., Luo, Y., Niu, M., Wang, Q., Wu, J., Liu, X., Xu, J. Study of imbibition in various geometries using phase field method. Capillarity, 2019, 2(4): 57-65, doi: 10.26804/capi.2019.04.01

Liquid gets imbibed into the capillary tube under the influence of capillary pressure. If capillary tube is shorter than Jurin height, oscillations of free surface are observed when contact line is confined by the edge of tube. Through numerical simulations and theoretic analyses, the velocity at the tube edge is determined, and the morphologies of meniscus at different stages of oscillation are identified. Finally, the oscillation period is found to be linearly correlated to r ρ L / γ through scale analysis, which is also verified by the simulation results. Discoveries in this paper show that new dynamics exists during the conversion between inertia and surface energy.