Session PP: Multiphase Flows VII

Multi-Discipline Collaboration for Sustainability in Heating Buildings

It was a dark and stormy night. The storyteller said, ``Let us each tell a story.'' The physicist expounded, ``Capture heat from rain on roofs to melt stored ice. Re-freeze melted ice with heat pumps. My new through-wall, multi-phase, mass-flow meter controls collecting, storing, transferring and pumping heat.'' At dawn, the engineer explained, ``Design a system to collect roof-heat from rain, solar and wind inputs. Heat is stored in freeze-thaw tanks and in soil under buildings and driveways.'' The architect adapted the new designs to beguile builders with plans for zonal heating that offers rapid zonal recovery, on demand. The businessman spun a tale of a new industry to mass produce affordable systems. The storyteller next instructed the team, ``Make it so.'' It was a dark and snowy night five years later. The homeowner said, ``My heat pump uses electricity from wind power to pump two thirds of my heat using stored energy from rain, sun, wind and soil.'' Sustainable heating of buildings will not be mythical if physicists develop new models for fluid motion and collaborate on educating other team members.   

Particle Velocity Fluctuations in a Liquid Fluidized Bed

The random motion of solid particles in a liquid fluidized bed has been investigated using high speed video, in the range of high particle Reynolds and moderate Stokes number. Matching the refractive index of both phases allows the recording of a single colored particle 3-D trajectory within the bed. The particle velocity \textit{pdf} and variance following the particle motion have been derived for each velocity component, in a wide range of solid fraction. Instantaneous velocity signal is composed of large amplitude low frequency fluctuations and small amplitude fluctuations at higher frequency. The large scale motion is much more pronounced in the longitudinal direction than in the transverse one, resulting in a significant anisotropy of the fluctuating motion. For each component, the velocity \textit{pdf} is centered on zero and can be well fitted by a Gaussian distribution at low to moderate solid fraction. The velocity variance of each component is found to be a decreasing function of the sold fraction. These results have been interpreted in the frame of an averaged statistical model derived from kinetic theory of granular media.   

Effect of Ambient Turbulence on the Drag Force of Particle at High Stokes Number

Velocity of solid particle (diameter is 2 mm) free-falling in a nearly isotropic turbulent airflow has been investigated using an ingenious experimental setup to achieve high Stokes number. Turbulent intensity around the particle is large enough to have eddies of comparable size to the thickness of boundary layer (approximately 0.2 mm) that is estimated in a laminar flow. As a result of measurement, an ensemble averaged particle velocity is larger than the velocity predicted with Schiller and Naumann's drag coefficient (Muto \textit{et al}., 2007). To investigate this reduction of drag force, flow aspects near the particle are observed using a numerical simulation of rotating spherical particle (periodically rotates in opposite direction) in a uniform flow. As a result, a modulation of drag force is found and it depends on period and amplitude of the rotation. A reason of the change of drag force in both experiment and numerical simulation is deduced that eddies included in an approach flow to particle, or periodic rotation of particle affect its boundary layer, and the wake of particle is suppressed.   

A study on the interactions between femtosecond-pulse laser and water from a viewpoint of multiphase flows

Femtosecond-pulse lasers (fs pulses) cause very interesting phenomena due to their extremely high energy density. The effects on substances are not thermal, but multi-photon absorption. When this multi-photon absorption of fs pulses operates on water, extraordinary phenomena different from laser-induced cavitation by usual laser such as nano- or pico-pulse laser are induced. Fs pulses of 60 femtoseconds in duration, 1kHz in repetition rate and 0.2--0.9$\mu $J in pulse energy are focused at pure water in a glass cell through several types of lens. The fs pulses split from original beams through a beam splitter are used as probe light. The Femtosecond-order Time-resolved Optical Measurement is realized by adjusting a light path length of the probe light (fs pulses). We elucidate the changes of refraction index of the water, the bubble generation process and the bubble properties. On the basis of these results, we discuss a relationship between the bubble motion and the field irradiated by fs pulses.   

Experimental Research on Turbulent Mixing Layer Flow with Polymer Additives

We present the results of an experimental study of fluids with polymer additives mixed by a specially designed splitter plate in a vertical rectangular channel. This arrangement is perhaps the simplest in which mixing effects can drive instability in the fluid. The velocity ratio between high and low speed is 4:1 and the Reynolds number based on the velocity difference of two steams and hydraulic diameter of the channel ranges are from 22800 to 87120. The flow field and turbulent parameters of different concentration polymer additives are measured and compared with water flow, which shows that the dynamic development of mixing layer is great influenced by polymer addictives. Our investigations reveal that similar with pure water case, the Reynolds stress and voticity still concentrate in a coniform area of central mixing flow field part. But compared with pure water case, the coniform width of polymer additives case is larger which means the polymer additives will lead to the diffusion of coherent structure. The peak value of vorticity in different cross section will decrease with the development of mixing layer. Compared with pure water case, the vorticity is larger at the beginning of the mixing layer but decreases faster.   

Measurements of the average properties of a bidisperse suspension of bubbles rising in a vertical channel

This investigation presents an experimental study of a system for which the bubble size is not monodisperse. In this work an experimental equipment was designed to study the behaviour of a bidisperse suspension of bubbles rising in a vertical channel, in which the dual limit of small Weber and large Reynolds number is satisfied. Bubbles were produced using capillaries of two distinct inner diameters. Using water and water-glycerin mixtures, the range of Reynolds numbers was extended from 50 to 500, approximately. To avoid coalescence, a small amount of salt was added to the interstitial fluid, which did not affect the fluid properties significantly. Measurements of the size, bubble velocity, aspect ratio as well the equivalent diameter of the bubbles were obtained as a function of gas volume fraction. We found that the bidisperse nature of the flow changes the dynamics in a significant manner. We observed a modification of the flow agitation, characterized by the liquid velocity variance. Although the decrease of the mean velocity with gas volume fraction is similar to that observed for monodisperse flows (Mart\'inez et. al. 2007), a general increase of the magnitude of fluctuations is observed for certain combinations of bubble size and gas fraction ratios.   

Image Treatment of Hybrid Drops in Low Reynolds Number Flow

Hybrid drops, also known as compound drops, consisting of two dissimilar configurations are encountered in processes such as melting of ice particles in the atmosphere, liquid membrane technology, evaporation of drops in a superheated liquid and in other various industrial operations. The flow fields around such hybrid drops form a basis for a better understanding of these industrial applications. Here we provide exact analytic solutions for a class of $3D$/$2D$ hybrid drops immersed in an infinite viscous domain in the limit of low Reynolds number. For mathematical convenience, the geometry of the multiphase droplet is composed of two overlapping spheres (infinitely long cylinders for $2D$ case) $S_a$ and $S_b$ of radii $a$ and $b$, respectively, intersecting at a vertex angle $\frac{\pi}{2}$. The composite inclusion has the shape resembling a {\it figure-eight lens} type of object with a vapor $S_a$ partly protruded into a fluid sphere $S_b$ with a viscosity different from that of the host fluid. The mathematical problem with this twin-sphere assembly in the Stokes flow environment is formulated in terms of Stokes stream function with mixed boundary conditions and solved using the classical method of images. Singularity solutions are obtained for the hybrid droplet embedded in several unbounded flow fields and the force acting on the drop is computed in each case. Streamline topologies show interesting flow patterns and surprising, but interesting flow features are noticed in the case of two-dimensional flows.   

Heat transfer in ordered and random arrays of spheres at low Reynolds number

Direct simulation of passive scalar transport in steady flow past arrays of spheres is performed using the immersed boundary method. We investigate the dependence of the Nusselt number on different sphere arrangements (simple cubic, face--centered cubic and random) as a function of solid volume fraction and Reynolds number ($0.01 < Re < 20$) for Prandtl number $\mbox{Pr} =0.7$. Our results compare well with the established correlations for low solid volume fractions ($<0.1$). At higher solid volume fractions, existing correlations are found to underpredict the heat transfer with significant departures in the Nusselt number at the highest volume fraction of $0.4$. The simulations motivate an improved heat transfer correlation for gas-solids flow at low Reynolds numbers.