Read the article on Flow Science Web Site Figure 1. Figure 2. … [Read more...]
Low Pressure Die Casting, early solidification issues
In Low Pressure Die Casting processes, the liquid metal is moved from a furnace through a rising tube, typically ceramic, into a metal mould. To do that, the furnace is incrementally pressurized to control the height of the metallic fluid and to impose a final high compression once the part is completely filled. In this process risers are not necessary, reducing the trimming costs, and it is possible to obtain parts with a very good surface finishing and with very low porosity. Anyway, the flow rate of the metal and the die temperature should be accurately calculated to find the best compromise between the velocity of the process and quality of the cast part. In fact, a too fast flow rate could lead to a highly turbulent flow and to an excessive entrainment of air; on the other hand, a too slow filling (in combination with a low temperature of the die) could provoke an early solidification, preventing the complete filling of the part. Using FLOW-3D® CAST it has been possible to reproduce through an accurate simulation, the real process, in which a complete filling is not obtained. The flow in the rising pipe is not included in the setup, imposing the flow rate directly at the entrance in the metal mould, with its real temperature. Figure 1 shows the fluid at the half of the filling phase, coloured with the solid fraction. From this picture the early solidification can already be noticed, underlining the excessive cooling rate. Figure 1 - Solidification front Figure 2, instead, shows the final shape of the aluminium, compared with a picture of the real part. The solidification of the metal front creates a blockage that forces the still liquid metal to flow around it to the upper part of the casting, where it also solidifies generating a big hole in the final shape. Figure 2 - Early solidification defects The video, finally, shows the complete dynamic of the flow, underlining the phase of the early solidification and showing in details how the metal slows down and stops due to the increasing solid fraction. https://youtu.be/x-VaIm05q6s Video of the dynamic of the filling and solidification … [Read more...]
Pelton turbine simulation – starting transient up to regime
Pelton turbines (or Pelton wheel) are the most worlwide used type turbines for electricity generation in hydraylics powerplant, due to their high efficiency. Its design belongs to 1870 but, with some modifications, they are still the first choiche in modern powerplants. In a Pelton turbine the energy is extracted from the kinetic energy of the water, in contrast with other types of turbines where the hydrostatic pressure is used: the water, coming from an upper basin, is accelerated and ejected against the Pelton paddles. Paddle geometry is designed properly to absorb as much as possible the kinetic energy of the fluid, starting rotating. The rotational speed of the turbine is then converted to electric power through a conductive coil. The simulation analyse the initial transient of the turbine, where water at over 100 m/s impact on the Pelton’s paddle providing torque and angular acceleration. https://www.youtube.com/watch?v=lb2xEbHmWKw All geometries and data used in the simulation are realistic and coherent with the real phenomena: wheel geometry has real shape and mass property, fluid is water with a reasonable speed, and the nozzle contains a doble valve, used in real turbines to adjust water flow rate. Interesting is the plot of the angular speed of the wheel. For Pelton turbines, it is known that the top efficiency is reached when the peripheral speed of the wheel is about half the speed of the water at the nozzle. For this purpose, a probe has been located at the centre of the nozzle in order to monitor the fluid speed, while another probe has been attached to a wheel paddle, in order to catch the peripheral speed. The two quantities can be directly showed as output from the simulation. The videos make a large use of Flowsight features: transparency based on the value of the variables, moving camera, fine tuning of light and reflections, multi-plots and multi-viewport visualization. https://youtu.be/TddbeL1lK9I … [Read more...]
