We present an experimental investigation of the thermal performance of a 1-kW directly irradiated suspension-flow solar particle receiver with an open (windowless) aperture under simulated solar conditions. Silicon carbide and air were used as the particulate and fluid media, respectively. A 5 kW xenon-arc solar simulator was used as the energy source. An outlet-suction strategy was employed to induce a net inflow through the aperture as means to mitigate particle and hot fluid egress from the device. The influence of various operating parameters, namely particle loading, air inlet mass flow rate and net air ingress through the aperture on the global thermal performance (thermal efficiency, exergy disruption, specific heat losses and wall temperature distribution) of the device was investigated systematically. It was found that the ratio of the solar input to total heat capacity of the two-phase flow and the level of suction have controlling influence on the global performance of the device. For the operating conditions investigated here, the heat absorbed by the air through the receiver was found to be 3-15 times higher than that of the solid phase, depending primarily on the mass loading and total inlet fluid flow rate. The presence of the solid phase was found to enhance the total heat absorbed by the two-phase flow albeit at the expense of lowering the value of the maximum outlet air temperature.

The air entraiment induced by vorticity-free-surface interaction is here numerically investigated using a two-fluid model which describes the flow in air and water as that of a single incompressible fluid whose density and viscosity vary smoothly across the interface. The numerical approach is used for the simulation of a viscous vortex pair vertically rising toward the free surface. Several flow conditions are studied aimed at understanding the role played by vortex intensity, surface tension and gravity forces on the amount of entrained air and the mechanisms forits entrainment.

The free surface dynamics induced by a rising vortex pair is here numerically investigated through a two-fluids Navier-Stokes solver coupled with a Level-Set technique for the interface capturing. Surface tension effects are modelled in the form of a continuum force acting on a small neighbourhood of the interface. The numerical model is applied to simulate the motion of a viscous vortex pair rising toward the free surface leading to a strong vorticity?free-surface interaction. A careful validation of the present two-fluids numerical approach is carried out for a gentle rising vortex, recovering single-fluid results available in literature obtained by using moving grid technique. When the same flow is simulated for higher Froude numbers a much larger free surface distortion takes place, eventually giving rise to the entrainment of an air bubble which significantly change the dynamics of the primary vortex. The Weber number is found to play an important role on that respect as well.

Surface tension effects onto the two-dimensional wave breaking flow produced by a hydrofoil moving beneath the free surface are investigated. The study is carried out numerically by a finite difference approach which solves the Navier-Stokes equations. The air-water interface is embedded in the computational domain and it is captured via a Level-Set technique. A heterogeneous unsteady domain decomposition approach is used, allowing to focus the computational effort onto the free surface vicinities, while the flow about the body is approximated by a potential flow model. Surface tension effects are investigated by progressively reducing the length scale, while keeping Froude and Reynolds number constant. Different flow regimes are recovered, ranging from intense plunging jet, eventually resulting in large amount of entrapped air, up to a micro-breaker, in which case air entrapment is suppressed and the jet is replaced by a bulge growing on the wave crest. At this scale, the surface tension is responsible for the large curvature at the toe of the bulge, and when the bulge slides upon the forward face of the wave, an intense shear layer develops from the toe. Instabilities of this shear layer are observed, and, when increasing the Reynolds number, the shear layer breaks up into coherent vortex structures that interact with the free surface, eventually leading to the formation of large surface fluctuations which propagate downstream.

The role played by surface tension onto the wave breaking flow generated by a submerged hydrofoil is numerically investigated by a two-fluids Navier-Stokes approach. Both the initial breaking establishment and the successive evolution are addressed. Increasing surface tension is found to progressively suppress the initial jet formation and the entrainment of air.

In the present paper the two-dimensional wavy flow generated by an hydrofoil moving beneath the free surface is studied by means of a finite difference Navier-Stokes solver coupled with a level-set technique that captures the interface location in a computational domain that encloses both air and water. The presence of the solid body is mimicked by introducing suitable body forces on the grid points inside the body contour. Simulations performed at a moderate Reynolds number show that an unsteady separation from the suction side occurs. This leads to a weaker wave production with respect to experimental data, obtained at a larger Reynolds number, and, downstream, to an intense interaction between vorticity and free surface.