In the past decades, several experimental investigations performed to the hydrodynamics and heat transfer in microscale laminar flow have showed divergences to the Darcy friction factor and the Poiseuille and Nusselt numbers, when the results obtained for the same ones were compared to those provided by classical theory. There are reports of deviations to the Darcy friction factor and the Poiseuille and Nusselt numbers attributed to geometric imperfections at the cross-section of the microchannels. The aim of this numerical study is to analyze how the hydrodynamic and heat transfer characteristics in single-phase laminar flow, of a fluid with constant thermophysical properties, can be affected by imperfections at the cross-section of the microchannels. The results obtained for single-phase laminar flow of water in microchannels with imperfections at the cross-section are compared to those obtained for a geometrically perfect microchannel through Poiseuille and Nusselt numbers. Deviations at Poiseuille and Nusselt numbers due to imperfections at the cross-section of the microchannels were verified. The results showed that Nusselt number is more sensitive to shape of the cross-section of the microchannels than Poiseuille number. This study showed that knowledge about the geometrical shape of the cross-section of the microchannels is important to determine properly the hydrodynamic parameters of the flow, such as Poiseuille number. This study was carried out through computational fluid dynamics (CFD) and the numerical model made up by mass conservation, Navier-Stokes and energy equations.
Reversed Tainter gates are often used to control flow in the filling and emptying locks navigation systems of high fall. High speeds and pressure fluctuations may occur in the flow downstream of these gates, the flow cavitation may occur, damaging the structure. One way proposed to mitigate this problem is through geometric changes in the channel downstream of the gate. In this study was analyzed using CFD the effect of an expansion in the roof and the base of the conduit, followed by a straight section and a subsequent contraction until returning to the original geometry. It was observed that the pressure at the base of the conduit increases with the modification, however peaks of positive and negative pressures occur due to the shape change of the geometry of the corners not be smooth. The size of recirculation downstream of the gate increases with the geometrical alteration, increasing the distance required for pressure recovery.
The thermal heat storage it’s an effective way to suit the energy availability with the demand schedule. It can be stored in the means of sensible or latent heat, the latter applying a material denominated Phase Change Material (PCM), which is provided as organic compounds, hydrated salts, paraffins, among others. The latent heat storage systems offer several advantages, like the practically isothermal process of loading and unloading and the high energy density. However, the low thermal conductivity makes the cycle prolonged on these systems, restricting its applicability. Applying computational fluid dynamics, the behavior of the PCM melting process was studied in cylindrical cavities with horizontal and vertical fins, aiming the optimization of the fin geometry. In this way the fin area was kept constant, varying its aspect ratio. The numerical model was validated with results from the literature and it’s composed of the continuity, momentum and energy equations increased by the phase change model. Qualitative and quantitative results are presented, referring to mesh independence, contours of velocity, net fraction and temperature at different moments of the process. The results of the study indicate that the position of the fin in the heat exchanger influences the melting process, although the vertical fins have a faster total melting process, horizontal fins can reach larger partial liquid fractions in less time in the heat exchanger. Such as the position of the fin, the increase of its length propitiates the reduction of the melting time, evidencing the optimal aspect ratio.
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