The pressure drops were measured for water, microbubble/water mixtures, and complex fluids (spherical micelle surfactant solution and polymer solution) in flows through micro-apertures (micro-orifices, circular pores, and hexagonal pores). For water, agreement between the resultant pressure drops and the predictions of the Navier-Stokes equation was obtained. For microbubble/water mixtures, drag reduction effect was suggested over a Reynolds number of about 1.0 × 101 in micro-orifice flows. Surfactant solutions exhibited the same results as microbubble/water mixtures. For polymer solutions, significant drag reduction was shown. Moreover, a drag reduction effect, which was independent of the used test fluids, was observed for Reynolds numbers over 1.0 × 101 in the flow through circular pores and hexagonal pores. To explain this phenomenon, the size effect, visco-elastic property, electric interaction, and interfacial tension are considered. The results suggest that electric interaction at the wall (interfacial tension) is a contributing factor. In addition, drag reduction rates were estimated.
In this study, we investigated the influence of wall surface roughness on the pressure drop in micro-channel flow of dilute polymer solutions with two different channel heights, 100 and 50 μm. Tested channel surface roughness are Ra = 0.2, 1.0, 1.6, 2.5, 4.7, and 10.0 μm. Moreover, we changed surface wettability on these surfaces, namely hydrophilic or hydrophobic. The experiment showed the pressure drop increased with an increase of surface roughness for all surface conditions. In case of Ra > 1.0 μm, the pressure drop on hydrophobic surfaces were lower than that on hydrophilic surfaces. The pressure drop of dilute polymer solutions decreased in the case of hydrophilic surface roughness comparing that of distilled water. On the other hand, for hydrophobic surface roughness, the pressure drop of dilute polymer solutions increased.
In previous study, we measured the dynamic surface tension using droplet on the vibrating plate. As the apparent gravity changes, the droplet deformation is occurred locally, which results in the increase and decrease of the local surface area. However, the dynamic surface tension gradient along the droplet surface under vibration has not been fully understood. Therefore, we studied the droplet internal flow by numerical and experimental analysis. When the surface tension gradient is imposed along the droplet surface, the internal flow of droplet was generated due to the Marangoni effect along the droplet surface. From experimental point of view, we observed movements of the tracers per period at the highest and the lowest position, respectively. The displacements of the tracers were very small. We concluded that the dynamic surface tension on the droplet surface in the vibrating condition may be almost uniform.
An experimental study has been performed in order to investigate the optimum rheological characteristics for a technique of heat transfer augmentation in a cavity between ribs mounted in parallel plates. The rheological characteristics of visco-elastic fluid were changed by controling the molar ratio of counter-ion to a cationic surfactant. Flow visualization experiments and pressure loss measurements were conducted in the range of solvent Reynolds number from 100 to 2,500 and in the range of molar ratio from 0.50 to 10. Under this condition, the zoro-shear Reynolds number and the Weissenberg number ranged from 2.01 to 120 and from 1.94 × 10-2 to 9.15, respectively. From the results, flow penetration into the cavity occurred in the cases from 1.5 to 5.0 of molar ratio of counter-ions. However, the pressure loss became large in the cases when the remarkable Barus effect can be observed. On the other hand, the pressure loss became almost the same as that of water in the case when the molar ratio of counter-ions was set at 5.0. From this, it was concluded that the optimum fluid for the heat transfer augmentation in a cavity is required to have suitable elasticity and low viscosity as the molar ratio of counter-ions is set around 5.0.
A flow experiment of a dilute suspension and aggregation/dispersion analysis of fine particles in a non-uniform shear field were performed. We investigated the dispersion characteristics in a microchannel with an abrupt contraction section. The dispersed particles and dispersion media were polystyrene and aqueous glycerine, respectively. The particle diameter and solid volume fraction were 3.5 μm and 0.001, respectively. Citric acid (0.5 and 1.0 wt%) was added as a non-ionic salt in order to investigate the effect of the aggregational force. The suspension was introduced into the microchannel with a syringe pump at a volume rate of 0.3 μl・min-1. The aggregation/dispersion characteristics of the fine particles were observed with a microscope and analyzed by image analysis. We observed the breakup and re-aggregational behavior of the fine particles in a dilute suspension. The cumulative frequencies of fine particles fluctuated through the channel. From the results, we posit the possibility that the non-uniformity of ions affect the aggregational characteristics of fine particles in a shear flow.
Three-dimensional numerical simulations of the flow dynamics of milk using a volume-of-fluid (VOF) method were carried out in order to optimize the process in which milk is filled into a gable top carton with a volume of 1,000 mL. In the numerical simulations, the shear-thinning property of milk was modeled with the Carreau-Yasuda model. The sensitivity of the milk flow dynamics due to the prescribed lifting motion of the gable top carton and due to the filling speed (the number of milk cartons filled per hour) was investigated. When there was no prescribed lifting motion, it was computationally shown that milk which impinged on the bottom of the carton largely jumped and flowed outside the carton. In other words, it was found that a prescribed lifting motion can significantly reduce the amount of milk that escapes the carton. Also, it was found that the efficiency for filling a milk carton was sensitive to the filling speed. The optimum filling speed determined from fully three-dimensional numerical simulations was found to be consistent with previous investigations that used two-dimensional numerical simulations.
The shear-banding and the shear-induced structure (SIS) of CTAB/NaSal wormlike micellar solution in large amplitude oscillatory shear in concentric cylinder flow cell were investigated by the simultaneous macroscopic and microscopic observation. The crossed-Nicols polarizer method was used to detect the distributions of birefringence and orientation angle along the radial direction. The shear-banding was visualized by the distribution of birefringence and generation of SIS was estimated by the change of the stress-optical coefficient. The critical conditions of the shear-banding and SIS in the large amplitude oscillatory shear were plotted on the shear strain - shear rate chart. The SIS condition coincided those the step shear reported previously.
A noble measurement method for planar elongation viscosity was proposed. A bullet shaped bob was pushed into a sample liquid held in a cylindrical cup with a constant velocity and the resistance force acting on the bullet bob was measured. The bullet bob was designed to generate steady planar elongation flow through the gap between the bob and the cup. The trial experiments were carried out for standard viscous fluids and aqueous polymer solutions. The resistant forces were measured as a function of the planar elongation rate. The forces were compared to flow analysis of Newtonian, non-Newtonian and viscoelastic fluids. These forces were generally consistent with experimental data.
In multi-particle collision dynamics (MPCD), a solvent is modeled as a large number of point-like particles with a given mass, and particle collisions are expressed in terms of a redistribution of momentum among them. The present study proposes a computational model for dilute spheroidal colloid particle suspensions using an MPCD method called stochastic rotation dynamics (SRD). The present model describes the effect of the colloidal particle volume fraction on the shear viscosity of suspensions by modeling the colloidal particles as SRD particles in a similar manner to solvent particles.
The drag reduction (DR) of nonionic surfactant (oleyldimethylamine oxide, ODMAO) in ethylene glycol (EG) aqueous solutions was investigated at the solution temperature of 20 ºC. The pressure drop and flow rate were measured in the pipe flow system with the inner diameter of 5 mm. The concentration of ODMAO solutions was 1000, 2000, and 3000 ppm by weight. Regardless of the ODMAO concentration, the DR could not be obtained in EG aqueous solutions, in which the concentration of EG was fixed at 30 % by weight. It was revealed, however, that the addition of a small amount of salicylic acid noticeably improved the drag-reducing ability at the ODMAO concentration of 2000 and 3000 ppm even in the 30 % EG aqueous solution, as well as the pure aqueous solution, in which the higher DR was achieved at the lower concentration.