Experimental Determination of Hydrodynamic Parameters of Air-Water Two-Phase Slug Flow in Horizontal Pipes

Two phase slug flow is the most common flow pattern for horizontal and near-horizontal pipelines. This study is designed to determine experimental velocities of elongated bubbles, lengths of liquid slugs and elongated bubbles, and slug frequencies for twenty flow rates combinations of a two phase air-water system that belong to a slug type pattern in horizontal pipes with a non invasive electronic device made of Photo-diodes (emitter) and photo-transistors (receiver) in a non visible length wave of 940 nanometers. The non intrusive electronic device is validated by simultaneously taking pictures with a high speed camera, (Kodak model Ektapro 4540 mx Imager, at shooting speed of 4500 frames per second, the picture resolution is 256 × 256 pixels), through a visualization cell filled with glycerin. This work is done with acrylic pipes of 0,03175 m inner diameter, to ensure complete flow development, the pictures are taken from a visualization cell located at a x/D = 249, the electronic device is located at x/D = 250. Air superficial velocity ranged between 0.156 and 0.468 m/s while water superficial velocity ranged between 0.159 and 1.264m/s. It is found that the non intrusive electronic device formed by photo diodes and photo transistors is an accurate technique that can be used in the determination of elongated bubble velocities, lengths and slug frequencies.

Experimental Study of Aerodynamic Noise From a Double Shaft Fan

Noise pollution from wind turbines and blowers operating in the vicinity of residential buildings has in recent years been the focus of intensive research. This paper reports on the outcome of an experimental investigation to reduce the noise pollution through design, build and testing of a counter-rotating-double-row-fan with variable spacing. A single-row fan was selected as the benchmark fan. The mechanical noise and the background ambient noise were measured using the system operating with no-blades. The aerodynamic noise from the fan was then focused and air velocity, shaft-revolution, input-electric-power to fan, amplitude (dB) and Center Frequency (CF) in Hz of noise were measured using frequency-weighting of both “A” and “Linear”. Coefficients of performance (COP), dB, CF, Tip speed ratio (TSR) were plotted for a range of spacing between two-blade-rows. It was noticed that double-shaft-fan relative to the benchmark single shaft fan has operated: a) At a lower TSR due to division of the motor power between two shafts. b) At a higher COP of up to 18% due to a higher air velocity generated at the same motor power. c) Quieter at a lower dB (of up to 10 dB). d) At the minimum noise levels (80 dB) at the spacing of 15mm-to-25mm, using the “Lin”-weighting. e) At the minimum noise levels (20 dB) at the spacing of 12mm-to 50 mm when measured at the “A”-weighting. f) With no significant change in frequency of noise when operate at the same TSR.

Numerical Prediction of Particulate Flow Over a Backward-Facing Step Followed by a Filter Medium

Flow in air filter housings often is characterized by separation upstream of the filter. The effect of the separation on the motion of particles and their distribution at the filter is important to filter performance. The current research investigates these effects by applying CFD modeling to turbulent particulate flows over a backward-facing step followed by a porous medium representing a filter. The two-dimensional step flow was selected as it is an archetype for separated flow with many studies in the literature. The flow examined has a step expansion ratio of 1:2, with an entrance length of 30 step heights to the step followed by a length of 60 step heights. Computations were performed at step Reynolds numbers of 6550 and 10,000 for the step without a porous medium and with the medium placed 4.25 and 6.75 step heights downstream of the step. The mesh was developed in ICEM CFD and modeling was done using the Fluent commercial CFD package. The carrier phase turbulence was modeled using the RNG k-epsilon model. The particles were modeled using the discrete phase model with dispersion modeled using stochastic tracking. The boundary conditions are uniform velocity at the inlet, no-slip at the walls, porous jump at the porous medium, and outflow at the outlet. The particle boundary condition is “reflect” at the walls and “trap” at the filter. The numerical results for the no filter case matched experimental results for recirculation zone length and velocity profiles at 3.75 and 6.25 step heights well. The computed velocity profiles at 3.75 step heights do not match experimental profiles for the filter at 4.25 step heights so well, though the results show a profound effect on the recirculation zone length, matching the experiments. Differences are attributed to different velocity profiles at the step. With the medium 6.75 step heights downstream, the effect on the recirculation zone is negligible, again matching experimental results. The discrete phase model tracks injected particles and provides results which are qualitatively similar to the literature. It is observed that particles with lower Stokes number, and thus lower momentum, tend to follow the flow and enter the recirculation zone while particles with higher Stokes number tend to move directly to the porous medium. When the filter is moved downstream to 6.75 step heights, the increased length of the recirculation zone results in more particles entering the recirculation zone. Results for monodispersed and polydispersed particles agree.

Adhesion of Wax Droplets to Porous Substrates

The adhesion of solid wax ink droplets to porous polyethylene and Teflon substrates was studied experimentally. Wax droplets with a diameter of 3 mm and an initial temperature of 110°C were dropped onto test surfaces from heights varying from 20–50 mm. The Teflon surfaces had holes drilled in them to create idealized porous surfaces while the porous polyethylene sheets had mean pore sizes of either 35 or 70 μm. The force required to remove the wax splats from the substrates was measured by a pull test. The detachment force increased with droplet impact velocity. A simple analytical model is proposed to predict the force attaching the wax splat to the surface: it has an adhesive component, calculated by multiplying the contact area between the splat and substrate by the strength of adhesion; and a cohesive component, calculated by multiplying the area of the pores into which wax penetrates by the ultimate tensile strength of wax. Predictions from the model agreed reasonably well with measurements.

Unsteady Modeling of Powdered Activated Carbon Deposition on Collection Electrode of a Lab-Scale Electrostatic Precipitator (ESP)

Particulate sampling in the flue gas at the Electrostatic Precipitator (ESP) outlet during injection of powdered activated carbons (PACs) has provided strong anecdotal evidence indicating that injected PACs can penetrate the ESP in significant concentrations. The low resistivity of PAC is consistent with poor collection efficiency in an ESP and lab-scale testing has revealed significantly different collection behavior of PAC in an ESP as compared to fly ash. The present study illustrates the use of a commercial CFD package — FLUENT — to investigate precipitation of powdered activated carbon (PAC) in the presence and absence of electric field. The computational domain is designed to represent a 2-D wire-plate ESP channel. The governing equations include those covering continuous phase transport, electric potential, air ionization, and particle charging. The particles are tracked using a Lagrangian Discrete Phase Model (DPM). In addition, a custom user-defined function (UDF) uses a deforming boundary condition and a prescribed critical particle velocity to account for particle deposition and dust-cake growth on the electrodes. The effect of Electrohydrodynamics (EHD) induced flow on the ESP collection efficiency under various flow and particle characteristics as well as different ESP configurations are illustrated.

Control of the Flow Direction Induced by Plasma Actuator

It is known that a dielectric barrier discharge plasma actuator (DBD-PA) induces tangential flow in the vicinity of the wall. In the conventional studies the induced flow was generated in the direction from the upper electrode to the lower electrode. The direction of the induced flow was one-way using DBD-PA composed of asymmetric electrodes. However, in this experiment, it was found that the flow induced by DBD-PA can be generated in the opposite direction in contrast with conventional results. In this study, effects of DBD induced flow on applied voltage characteristics were investigated by means of PIV analysis. When square wave voltages biased negative are applied to the electrodes of DBD-PA, the flow is generated in the direction from the lower electrode to the upper electrode. It should be noted that it is not reversed by sine waves even with same amplitudes and frequencies when the reverse flow was generated by DBD-PA, a vortex is induced above the area of plasma. The vortex height was about 10 mm, and it tended to change slightly its shape based on the electrode width. It seems that the vortex on the plasma area play an important role in the mechanism of the reverse flow. The plasma actuator has been focused as an actuator for aerodynamic flow control. If it becomes possible to control the flow direction as well as the velocity by changing the applied voltage, the applications of the plasma actuator can be spread across a wide area of industry.

An Efficient Immersed Boundary Method Based on Penalized Direct Forcing for Simulating Flows Through Real Porous Media

A computationally efficient Immersed Boundary Method (IBM) based on penalized direct forcing was employed to determine the permeability of a real porous medium. The porous medium was composed of about 9000 glass beads with an average particle diameter of 1.93 mm and a porosity of 0.367. The forcing of the IBM depends on the local solid volume fraction within a computational grid cell. The latter could be obtained from a high-resolution X-ray Computed Tomography (CT) scan of the packing. An experimental facility was built to determine the permeability of the packing experimentally. Numerical simulations were performed for the same packing based on the data from the CT scan. For a scan resolution of 0.1 mm the numerical value for the permeability was nearly 70% larger than the experimental value. An error analysis indicated that the scan resolution of 0.1 mm was too coarse for this packing.

CFD Simulation of Particle Transport and Dispersion in Indoor Environment by Human Walking

Particle resuspension is an important source of particulate matter in indoor environments that significantly affects the indoor air quality and could potentially have adverse effect on human health. Earlier efforts to investigate indoor particle resuspension hypothesized that high speed airflow generated at the floor level during the gate cycle is the main cause of particle resuspension. The resuspended particles are then assumed to be dispersed by the airflow in the room, which is impacted by both the ventilation and the occupant movement, leading to increased PM concentration. In this study, a three dimensional model of a room was developed using FLUENT™ CFD package. A RANS approach with the RNG k-ε turbulence model was used for simulating the airflow field in the room for different ventilation conditions. The trajectories of resuspended particulate matter were computed with a Lagrangian method by solving the equations of particle motion. The effect of turbulent dispersion was included with the use of the eddy lifetime model. The resuspension of particles due to gait cycle was estimated and included in the computational model. The dispersion and transport of particles resuspended from flooring as well as particle re-deposition on flooring and walls were simulated. Particle concentrations in the room generated by the resuspension process were evaluated and the results were compared with experimental chamber study data as well as simplified model predictions, and good agreement was found.

Using Graphics Processing Units to Accelerate Numerical Simulations of Interfacial Incompressible Flows

We present a GPU accelerated numerical solver for incompressible, immiscible, two-phase fluid flows. This leads to a significant simulation speed-up and thus, the capability to have finer grid sizes and/or more accurate convergence criteria. We solve the Navier-Stokes equations, which include the surface tension force, by using a two-step projection method requiring the iterative solution to a pressure Poisson problem at each time step. However, running a serial linear algebra solver on a CPU to solve the pressure Poisson problem can take 50–99.9% of the total simulation time. To remove this bottleneck, we employ the large parallelization capabilities of GPUs by developing a double-precision parallel linear algebra solver, SCGPU, using NVIDIA’s CUDA v.4.0 libraries. The performance of SCGPU in serial simulations is presented, in addition to an evaluation of two pre-packaged GPU linear algebra solvers CUSP and CULA-sparse. We also present preliminary results of a GPU-accelerated MPI CPU flow solver.

Interactions Between High-Viscosity Droplets Deposited on a Surface: Experiments and Simulations

A combined numerical and experimental investigation of coalescence of droplets of highly viscous liquids dropped on a surface has been carried out. Droplets of 87 wt% glycerin-in-water solutions with viscosity 110 centistokes were deposited sequentially in straight lines onto a flat, solid steel plate and droplet impact photographed. Impacting droplets spread on the surface until liquid surface tension and viscosity overcame inertial forces and the droplets recoiled, eventually reaching equilibrium. Droplet center-to-center distance was varied and droplet line length was measured from photographs. As droplet spacing was increased there was less interaction between the droplets. A three dimensional parallel code has been developed to simulate fluid flow and free surface interaction by solving the continuity, momentum and volume-of-fluid (VOF) equations. The two-step projection method was employed to solve the governing equations for the whole domain including both liquid and air phases. The continuum-surface-force (CSF) scheme was applied to model surface tension and the piecewise-linear-interface-construction (PLIC) technique used to reconstruct the free surface. Computer generated images of impacting droplets modeled droplet shape evolution correctly and compared well with photographs taken during experiments. Accurate predictions were obtained for droplet line length during spreading and at equilibrium.