(87e) Experimental Investigation of PVA – Borax Gel Reaction Flow and Mixing in a Non–Element Mixer

Flow characteristics and pressure variation with PVA–borax gel reaction in a Non-element mixer were experimentally investigated. The Non-element mixer consisted of a main flow pipe (inner diameter 10 mm) and three branch flow pipes (inner diameter 4 mm) which are respectively named B1, B2, and B3 from upstream side. The working fluids are 10 mass% polyvinyl alcohol (PVA) solution and 3 mass% borax solution. When these two fluids are mixed, a gel is formed under crosslinking reaction. PVA solution flows in main flow pipe with a steady state condition and borax solution which is colored by blue dye to visualize flow pattern is injected from B2. At first, the experiment is conducted to clarify the relation of flow pattern and pressure variation with gel reaction in the Non-element mixer. Reynolds number of main flow (Re) is varied from 0.5 to 30.0 and the injection velocity from B2 (v_inB2) is varied from 7.5 to 60 mm/s. Injection time is varied from 4 to 32 s to supply same volume of borax solution to the main flow pipe. Gage pressure variation at 100 mm upstream from B1 is measured by a pressure sensor. The conclusions are as follows.

Three distinct flow patterns with gel reaction are described as, 1. Capsule flow, 2. Capsule + stretched flow and 3. Capsule + stretched + fingering flow. In the capsule flow, the injected borax forms a block namely “capsule” and it flows to downstream direction. The pressure increases when the capsule is formed. However it recovers to the initial pressure when the capsule is discharged. The capsule is produced by gel sheet which is generated the interface between the injected borax and PVA solution. As a result, the injected borax is almost completely separated from PVA region. In the capsule + stretched flow, the capsule is formed in the tip of injected borax and stretched by shear stress of main flow. Therefore gel is generated and adhered on upper side of flow path. In the capsule + stretched + fingering flow, the injected borax forms a capsule and is stretched. Additionally, fine finger shape product is observed ahead of the capsule. Interface between borax and PVA is unstably spread due to viscosity difference of two fluids. This phenomena is similar to viscous fingering. After that, gel is adhered on the main flow pipe. The pressure cannot recover to the initial pressure because formed gel is adhered to the wall of main flow pipe. The flow patterns can be determined by the Input borax fraction and the Mixture velocity. The Input borax fraction is the flow ratio of borax solution and the total flow rate of PVA and borax solutions. The Mixture velocity is a total velocity of main and branch flows. When the Input borax fraction > 0.4 and the Mixture velocity > 40 mm/s, the capsule flow is stably formed.

In order to increase contact surface area between borax and PVA solution to enhance mixing and the gel reaction under the capsule flow condition, we have conducted the experiment using B3. The borax injection velocity from B2 (v_inB2) is 30 mm/s, Re is less than unity which satisfies capsule flow condition. The injection time of B2 is 8 s. After the injection from B2, PVA solution is continuously supplied to the main flow pipe from B3. The injection velocity from B3 (v_inB3) is varied from 7.5 to 240 mm/s. When v_inB3 is increased such as 240 mm/s, the discharged product forms a sphere which the blue borax solution is covered with thin gel sheet and the pressure decreases to negative pressure when the spherical product is discharged from the main flow pipe. The shape of discharged product and pressure variation can be qualified by first Damköhler number which is the ratio of characteristics of flow time and chemical reaction time.