The present study investigates the thermal performance of an ultra-high temperature (> 1000 °C) latent heat thermal energy storage system that utilizes silicon as a phase-change (PCM) material. ...Application of this system in the residential sector is studied, when integrated with a solar PV as an energy source and a hybrid thermionic-photovoltaic (TIPV) converter to produce both electricity and heat and subsequently cover corresponding domestic demand. A one-dimensional (1D) dynamic model has been developed in Dymola modelling tool for predicting the temperature profile and total charging and discharging time of the PCM heat storage system, as well as the produced thermal and electric power from the hybrid TIPV converter. A sensitivity analysis both for the melting and the solidification stage of the PCM has been performed based on key parameters, such as material thermal properties –including specific heat capacity, thermal conductivity and latent heat of fusion- and other operating and design parameters. The results have showed an increasing linear dependence of charging and discharging time on specific heat, latent heat of fusion and density and a decreasing dependence on thermal conductivity. Finally, the integration of the heat storage system on a building level has showed the potential for high coverage of either heat demand (over 100%) or electricity demand (over 69%) for a typical Southern European household, depending on the generation priority strategy followed.
The central binary collision of two unequal sized droplets is numerically investigated using the volume of fluid (V.O.F.) methodology. The numerical method based on the solution of the continuity and ...momentum equations in axi-symmetric formulation is coupled with a recently developed adaptive local grid refinement technique, thus allowing an accurate representation of the interface between the liquid and gas phase. Mass transfer mechanisms are reproduced by solving a transport equation for a colour function representing the mass of one of the colliding droplets before and after collision and mixing. The investigation is performed assuming either constant relative velocity of the colliding droplets or constant total energy of the system, thus creating a combination of the standard non-dimensional parameters affecting the collision process, i.e. Weber (We) and Ohnesorge (Oh) numbers as also droplet diameter ratio (Δ). The reliability of the procedure is first established by comparing predictions with available experimental data. The effect of the above mentioned parameters on ligament’s formation, maximum deformation of the two droplets, the penetration of one droplet into the other and satellite droplet formation is quantified.
The paper presents an extensive experimental and numerical study on a cross-ventilated building providing important features of the induced flow patterns at the two openings as a function of the free ...stream wind velocity's magnitude and its incidence angle. The experimental data are measured via anemometers across the openings, whilst the numerical methodology is based on the time-dependant solution of the governing Navier–Stokes equations. The experimental data are compared to the corresponding numerical results, revealing the unsteady character of the flow field especially at large incidence angles. Furthermore, additional information regarding the flow field near the opening edges, not easily extracted by experimental methods, provide an in depth sight in the main characteristics of the flow field both at the openings but also inside the building. Finally, a new methodology for the approximation of the volume flow rate aerating the building based on experimental measurements of the velocity field at the openings is presented.
Variation of fuel properties occurring during extreme fuel pressurisation in Diesel fuel injectors relative to those under atmospheric pressure and room temperature conditions may affect ...significantly fuel delivery, fuel injection temperature, injector durability and thus engine performance. Indicative results of flow simulations during the full injection event of a Diesel injector are presented. In addition to the Navier-Stokes equations, the enthalpy conservation equation is considered for predicting the fuel temperature. Cavitation is simulated using an Eulerian-Lagrangian cavitation model fully coupled with the flow equations. Compressible bubble dynamics based on the R-P equation also consider thermal effects. Variable fuel properties function of the local pressure and temperature are taken from literature and correspond to a reference so-called summer Diesel fuel. Fuel pressurisation up to 3000bar pressure is considered while various wall temperature boundary conditions are tested in order to compare their effect relative to those of the fuel heating caused during the depressurisation of the fuel as it passes through the injection orifices. The results indicate formation of strong temperature gradients inside the fuel injector while heating resulting from the extreme friction may result to local temperatures above the fuel's boiling point. Predictions indicate bulk fuel temperature increase of more than 100°C during the opening phase of the needle valve. Overall, it is concluded that such effects are significant for the injector performance and should be considered in relevant simulation tools.
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