A simulation study of liquid drops on inclined surfaces is performed in order to understand the evolution of drop shapes, contact angles, and retention forces with the tilt angle. The simulations are made by means of a method recently developed for dealing with contact angle hysteresis in the public-domain Surface Evolver software. The results of our simulations are highly dependent on the initial contact angle of the drop. For a drop with an initial contact angle equal to the advancing angle, we obtain results similar to those of experiments in which a drop is placed on a horizontal surface that is slowly tilted. For drops with an initial contact angle equal to the mean between the advancing and the receding contact angles, we recover previous results of finite element studies of drops on inclined surfaces. Comparison with experimental results for molten Sn-Ag-Cu on a tilted Cu substrate shows excellent agreement.

A thermodynamic model is developed for predicting the performance records of a solar hybrid gas turbine power plant with variable irradiance and ambient temperature conditions. The model considers a serial solar hybridization in those periods

This paper presents a general thermodynamic model for hybrid Brayton central tower thermosolar plants. These plants have been proved to be technically feasible but R+D efforts need to be done in order to improve its commercial interest. From the thermodynamic viewpoint it is necessary to increase its performance to get larger power production with reduced fuel consumption, and so reduced emissions. We develop a model for multi-step compression and expansion stages with that aim. The model is flexible and allows to simulate recuperative or non-recuperative plants, with an arbitrary number of stages and working with different subcritical fluids. The results for multi-step configurations are compared with those obtained for a plant with one turbine and one compressor. Different working fluids are analyzed, including air, nitrogen, carbon dioxide, and helium. Several plant layouts and the corresponding optimal pressure ratios are analyzed. It is concluded that configurations with two-stages compression with intercooling combined with one or two expansion stages can significantly improve overall plant efficiency and lower fuel consumption. Power block efficiencies can reach 0.50 and overall plant efficiency can attain values about 0.40 working with air or CO 2. For instance, comparing with a single stage plant running with air, a plant with subcritical CO 2 , two compression stages with intercooling and single step expansion can reach an overall efficiency about 19% larger and a fuel conversion rate around 23% larger. For such configuration, the specific fuel consumption is predicted to be about 108 kg/(MW h) at design point conditions.

A working fluid performs a Brayton cycle that is fed by a heat input from a solar power tower and from a combustion chamber, which burns natural gas. This hybrid system is described by a complete model that includes all the main losses and irreversibility sources (optical and thermodynamic). Numerical implementation and validation is performed based on a Spanish commercial plant. On-design computations are carried out varying the pressure ratio for four working fluids (dry air, nitrogen, carbon dioxide, and helium), for different number of stages and for recuperative and non-recuperative configurations. When adjusting the pressure ratio, an improvement of about 7 % in overall thermal efficiency is predicted for a dry air single-stage recuperative configuration with respect to a standard commercial gas turbine. A study about the main energy losses in each plant subsystem for some particular plant layouts is accomplished. A two-compression and expansion stages recuperative Brayton cycle working with air is expected to give overall thermal efficiencies about 0.29 at design conditions, which is about a 47% increase with respect to the simplest single-stage configuration. It is stressing that fuel consumption from the reheaters maybe higher than that of the main combustion chamber for multistage layouts. Off-design hourly curves of output records for the four seasons throughout a day are analyzed. Greenhouse emissions are also analyzed. Specific carbon dioxide emissions are smaller for helium than for dry air, when they both work in a single-stage non-recuperative configuration.

A simple model is proposed to simulate contact angle hysteresis in drops on a planar surface. The model is based on assuming a friction force acting on the triple contact line in such a way that the contact line keeps fixed for contact angles comprised between the advancing angle and the receding one and is allowed to move in order to avoid angles outside this interval. The model is straightforwardly applied to axisymmetric drops for which a simple solution of the Young-Laplace equation can be obtained. A variation of the method has also been implemented for nonaxisymmetric drops by resorting to the public-domain "Surface Evolver" software. Comparison with experiments shows the excellent performance of the model.