Study on the Instability of Two-Phase Flow in the Heat-Absorbing Tube of Trough Solar Collector

The Marangoni effect and Rayleigh-Benard effect in the two-phase region of solar trough heat-absorbing tube are simulated by FTM (front tracking method). Considering the Marangoni effect alone, although surface tension gradient and surface tension affect the interface wave, the two effects have different characteristics. The surface tension gradient caused by the temperature gradient is one of the factors that swing the interface. The amplitude attenuation of the interface wave decreases with the increase of the Marangoni number (Ma). In general, the surface tension gradient enhances the convection opposite to the temperature gradient. Under the gravity field, the Rayleigh-Benard effect influences the development of the vortex structure in the flow field, which in turn affects the velocity gradient near the interface to influence the evolution of the interface fluctuation. In a small Rayleigh number (Ra), the buoyancy convection reduces the velocity gradient, thus suppressing the evolution of the interfacial wave. In the range of Ra < 4.0E4, the larger the Ra, the stronger the inhibitory effect. However, when the Ra number is large (Ra > 4.0E4), the situation is just the opposite. The larger the Ra is, the stronger the promoting effect is

On diffuse interface modeling and simulation of surfactants in two-phase fluid flow

An existing phase-field model of two immiscible fluids with a single soluble surfactant present is discussed in detail. We analyze the well-posedness of the model and provide strong evidence that it is mathematically ill-posed for a large set of physically relevant parameters. As a consequence, critical modifications to the model are suggested that substantially increase the domain of validity. Carefully designed numerical simulations offer informative demonstrations as to the sharpness of our theoretical results and the qualities of the physical model. A fully coupled hydrodynamic test-case demonstrates the potential to capture also non-trivial effects on the overall flow

COMPUTATIONAL STUDY OF DROPLET AND CAPSULE FLOW IN CHANNELS WITH INERTIAL EFFECTS

The flow of droplets and capsules in channels is important for a variety of industrial and biological applications. Droplet flow is common in microfluidic devices and emulsion processing as well as oil recovery from porous materials. Capsules are used to encapsulate sensitive materials and can be used to study the mechanical properties of biological cells. A computational method was developed to study the two-phase flow of drops with and without surfactants, and capsules surrounded by a thin elastic membrane. This new computational method allowed for the inclusion of inertial effects on droplet and capsule flow which has not received much attention in the past. Results are presented for both the steady flow in straight cylindrical channels, and the transient flow in response to sudden expansions or contractions in the channel diameter. Increasing the Reynolds number was seen to cause non-monotonic trends in the capsule deformation and velocity. Parameters such as the drop viscosity and presence of surfactants were seen to have smaller effects when the Reynolds number became large. Capsules flowing in channels were seen to have limiting elastic capillary numbers above which no stable shape could be found. The transient deformation of drops and capsules moving through expansions depended strongly on the shape of the drop upstream of the expansion. The transient deformation increased with the capillary number up to a limiting value. The flow of droplets through channels was seen to produce large deformations that could break the drop apart at low viscosity ratios. The inclusion of inertial effects caused increases in the transient deformation as well as oscillations as the drops relaxed back into their steady shape

A computational fluid dynamics study of two-phase flows in the presence of surfactants

Drop formation in co-flowing fluids and drops rising in a tube are important in applications such as microencapsulation and enhanced oil recovery. A hybrid volume-of-fluid method with a front-tracking scheme is developed to study two-phase flows in the presence of surfactants at finite Reynolds numbers. Both fluids can be Newtonian or shear-thinning, and surfactants are soluble in the adsorption-desorption limit. A drop in the co-flowing geometry typically breaks up at the primary neck. The drop breaks faster with smaller volumes as the outer flow rate increases or the drop viscosity decreases. When surfactants are present, they accumulate in the neck region resulting in Marangoni stresses that slow down the neck thinning rate. This results in longer breakup times with larger drop volumes. At high surfactant coverages, the primary neck formation slows down enough and breakup occurs at the secondary neck. Increasing outer co-flowing flow weakens the retarding effect of the high surfactant coverage leading to breakup again at the primary neck. The adsorption-desorption kinetics also affects the neck breakup position, and the primary drop volume and breakup time depend non-linearly on the Biot number. The presence of a confining wall may lower the value of the critical equilibrium fractional coverage required for the drop to enter the no-necking regime. As the drop becomes more shear-thinning, the drop breaks up faster with a shorter remnant drop length. Multiple satellite drops are observed at breakup with strongly shear-thinning drop fluid at high coverage of soluble surfacants. The buoyancy-driven motion of drops in a tube is investigated by determining the steady shapes and velocities of the drops as a function of the drop size. Higher buoyancy force leads to larger deformation of drops and increased terminal velocities. Higher inertia increases the terminal velocity of drops and results in the development of negative curvatures at the rear of the drop. The non-uniform distribution of surfactants at the interface gives rise to Marangoni stresses that retard the drop motion though the drop shapes remain unaffected