Payam Rahimi

The effect of gas-injector location on bubble formation in liquid cross flow

Liquid flows incorporating small-size bubbles play a vital role in many industrial applications. In this work, an experimental investigation is conducted on bubble formation during gas injection from a microtube into the channel of a downward liquid cross flow. The tip of the air injector has been located at the wall (wall orifice) and also at several locations from the wall to channel centerline (nozzle injection). The size, shape, and velocity of the bubbles along with liquid velocity field are measured using a shadow-particle image velocimetry/particle tracking velocimetry system. The process of bubble formation for the wall orifice and the nozzle injection configurations is physically explained. The effect of variation in water and air flow rates on the observed phenomena is also investigated by considering water average velocities of 0.46, 0.65, and 0.83 m/s and also air average velocities of 1.32, 1.97, 2.63, and 3.29 m/s. It was observed that shifting the air injector tip toward the center of the c...

Evaluation of StereoPIV measurement of droplet velocity in an effervescent spray

Particle image velocimetry (PIV) is a well known technique for measuring the instantaneous velocity field of flows. However, error may be introduced when measuring the velocity field of sprays using this technique when the spray droplets are used as the seed particles. In this study, the effect of droplet number density, droplet velocity profile, and droplet size distribution of a spray produced by an effervescent atomizer on velocity measurement using a StereoPIV has been investigated. A shadowgraph-particle tracking velocimetry (S-PTV) system provided measurement of droplet size and velocity for comparison. This investigation demonstrated that the StereoPIV under-estimates velocity at near-field dense spray region where measurement accuracy is limited by multi-scattering of the laser sheet. In the dilute far-field region of the spray, StereoPIV measurement is mostly in agreement with velocity of the droplet size-class which is close to the mean diameter based on droplet number frequency times droplet cross sectional area.

Effect of Gas-Liquid Ratio on Droplets Centricity and Velocity of an Effervescent Atomizer

Effervescent atomization is a twin fluid atomization mechanism which is based on the injection of gas into a liquid stream at an upstream location of the exit nozzle. This atomization mechanism is capable of producing droplets with Sauter Mean Diameter (SMD) comparable to other types of atomizers but at lower injection pressures. For the conditions investigated in this paper, liquid at the nozzle exit is an annular film surrounding the gas phase. A shadowgraph system is used to visualize particles shape and study droplets evolution in the spray field. Measurement of droplet SMD and centricity has been conducted at several axial and radial locations for different Gas-Liquid Ratios (GLRs). The shadowgraphy images reveal some non-spherical droplets which consist of both elliptical and coalescing droplets. Results also demonstrate that higher numbers of non-spherical droplets are observed at the near nozzle region and at higher liquid flow rate. In this work, spatial structure of the liquid phase velocity field has also been studied using a StereoPIV technique. The velocity field from StereoPIV measurements has been compared with the shadowgraphy velocity results averaged over different droplet size classes. This comparison has been conducted for the atomizer operating at different GLRs. Comparison of the results demonstrates that at the far-field, StereoPIV velocity field measurement is biased toward the velocity of droplets size classes which have relatively higher probability.Copyright

Contact angle hysteresis on smooth and homogenous surfaces in gravitational fields

In an experiment conducted on a space shuttle flight, contact angle hysteresis was observed for the water-glass system. Surface heterogeneities have usually been taken as the cause of the observed hysteresis. In order to examine if other factors such as adsorption could be the cause of the observed hysteresis, contact angles on homogenous, smooth surfaces of glass were examined in ground-based experiments. Ten glass cylinders with different inside diameters were constructed with a capillary tube in the centre of each cylinder. The cylinders were filled with prepared water and the meniscus heights inside the capillary tube and in the glass cylinder were measured. Using the values of these parameters, the pressure in the vapour and the contact angle at the three-phase line of each interface were calculated from the equilibrium conditions. In each of the cylinders, a difference in the contact angle at the three-phase line of the capillary and the three-phase line of the cylinder was observed. This contact angle hysteresis cannot be explained by surface heterogeneity or line tension effect. The observed contact angle hysteresis is predicted to be the result of the pressure profile in the system. This suggests adsorption and its extraordinary sensitivity to vapour-phase pressure as the source of the contact angle hysteresis.