EXPERIMENT AND MODELING GRANULAR SEGREGATION IN VORTEX

• Main Author: M, Rahmawati
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/49058
• Institution: Institut Teknologi Bandung

The phenomena of granular segregations have attracted a lot of attentions. Many researchers have investigated such phenomena by various methods such as a kinetic sieving method to separate granules by employing the gravity and granular flow principle. This method was also used to explain the granular (stone) segregation in rivers and able to estimate the particle size distribution as well as densities. The simple and mostly used method for post-processing of harvest rices by human society is by winnowing the mixture to separate the filled and the empty grains. The process takes advantage of natural wind blowing or by self-generaiton of air blowing, particularly when performed in closed space. Winnowing using the wind blowing is performed by dropping the grain mixture from an elevation into the air stream. Different forces acting on grains of different densities (filled and empty) resulted in separation of then when they reach the ground. Winnowing by self-gerenarion air blowing (stream) is used by undulation of platelike tools such as tray repeatedly, above which the grain mixture is dropped. This method is frequently used for separating small grains from large grains of rice before used for cooking or other grain mixtures having small amount. Different acceleration received by different grains during falling back to the tray surface causing them to touch the surface at different positions. The position separation increases by repeating the undulation. From a modern perspective, segregation by winnowing can be easily understood, especially the process considering linear air flow. At low Reynolds number (low velocity or low dimension), the drag force acting on the grain by air flow satisfies f Av D ? , with A is the grain cross section and v is the velocity of the grain relative to the air. The acceleration experienced by the grains is then a ? Av /m, with m is the mass of the grain. Since the cross-sections of the filled and empty rice grains are nearly the same, the acceleration is largely controlled by the mass of the grains. The masses of the empty grains are smaller than those of the filled grains, so the empty grains experience higher accelerations and are displaced farther away. If r is the average radius of the grain, one has 2 A? r and 3 m??r so that ?1 ?1 a ?? r . Therefore, for grains of the same material, the acceleration increases when decreasing the average radius of the grain. The purpose of this work is to conduct an experiment for understanding the process of rice winnowing and develop physical foundation for describing the grain segregation in the process of rice winnowing by tray flapping. We demonstrated the grain segregation by manually flapping of the tray (by hand) to several samples of grain mixtures. To obtain quantitative data, we conducted an experiment for generating air vortex that is similar to the air flow generated by manually flapping the tray. Then modeling simulations were performed to show vortex circulation profile and grain trajectory when winnowing. Simulations have been done using Simflow software for computational fluid dynamic. This work will put a scientific explanation to the ancient (traditional) method that has been practiced in different continents for thousands of years. Indeed, the rice winnowing mechanism sounds scientific, although past societies have not documented this method. Although manual rice winnowing is currently rarely practiced due to the development of modern tools that function similarly, this practice can still be found in remote/rural settlements.