(409e) Mixing Characteristics of High Viscous Fluid by a Multi-Holed Static Mixer

In order to save energy and space in mixing process and to realize high mixing efficiency, static mixers have been often applied in chemical industrial processes. Static mixers commonly used realize division and twist processes in a pipe flow without additional power. Then, industrial process becomes very simple. Until now, some types of static mixers are suggested such as Kinetics, Sulzer and Lightnin. They have been applied in many situation of mixing and many reports have been published on their efficiencies and mixing characteristics. However, the efficiency of such a static mixer seriously depends on fluid properties, pipe size and streamwise space. Additionally, it is very difficult for such static mixers to respond various situations because they have very complex structures inside in order to realize effective mixing. In this paper, a multi-holed static mixer with a simple structure was treated. This new type of a static mixer consists of some simple elements only with several holes tapered at the inlet and outlet in a simple disk-shaped plate mounted normal to a flow. The effective mixing is realized by a combination of these elements. This has a large advantage on scale-up problems because of its simplicity of the structure. This paper reveals its efficiency for high viscous fluid mixing by flow visualization experiments and image processing methods. As working fluid, starch syrup aqueous solution was adopted. The Newtonian viscosity of the fluid was changed from 0.033Pa?s to 0.433 Pa?s by adjusting the concentration of starch syrup. A static mixer treated in this paper consists of a pair of elements with 4 and 5 holes. Each hole has 45 degree taper at the inlet and the outlet as the inlet and outlet diameters of each hole are kept at 6mm. Five pairs of elements, that were totally 10 elements, were applied in the present experiments. The hole-size of each element was set uniform in a respective combination. The hole-sizeÅ@of each combination was changed in 3 steps of 2, 3 and 5 mm in diameter. Such a combined static mixer was mounted in experimental flow apparatus, which consists of a reservoir tank, a pump, a tracer injection part, a test section and an observation window. The size of the test straight pipe was set at 18mm diameter. Flow visualization experiments were performed at a cross-section downstream of the static mixer. The test cross-section was located at 0.11m downstream of the end of the static mixer and a slit light with 5mm width was inserted there from the outside of the test pipe. The dyed tracer fluid whose viscosity was set at the same as the bulk flow viscosity was inserted from a nozzle installed at a center axis of the test pipe as the velocity of tracer fluid takes the same value as the bulk fluid velocity. The tracer injection nozzle with 4mm diameter was mounted at 0.07m upstream of the inlet of the static mixer in the present experiments. Digital photos of the test cross-section were taken from an observation window at the downsteam end of the test pipe. By using of an image processing method, the occupation area of tracer dye was calculated by a computer. The occupation ratio of dyed area in the test cross-section for each case was investigated as an index of mixing characteristics in this paper. The effects of fluid viscosity, hole diameter of the elements and velocity through the hole on the occupation ratio of dyed tracer in the test cross-section are investigated carefully. From the results, the occupation ratio of dyed tracer in the test cross-section for each element was found to increase as flow velocity increases or be viscosity decreases. The lateral movement of center fluid is dominated by the hole velocity and its resistance depends on fluid viscosity. This indicates hole Reynolds number defined by the hole-size, the velocity through the hole and fluid viscosity could be a primary parameter characterizing the present mixing. It was found that the present experimental data are qualitatively consistent with this prediction. However, the occupation ratio seems to be weakly affected directly by the fluid viscosity and the velocity through the hole. This is partially caused by experimental errors, but mainly caused by the complex mixing in a hole of elements. The main mixing mechanism of the present static mixer is deeply related to a twist action in a hole. The feature of the twist action could be complex function of fluid viscosity and velocity. Then, the simple definition of Reynolds number could not enough characterize the present phenomena in spite of its simplicity of the present elements. In order to reveal this weak effect, more careful experiments or analysis are required in the near future.