New Correlation for Two-Phase Flow in 90 Degree Horizontal Elbows

The comparison of experimental data and results obtained from four global models — homogeneous, Dukler, Martinelli and Chisholm, used to evaluate the two-phase flow pressure drop in circular 90° horizontal elbows — is presented in this paper. An experimental investigation was carried out using three galvanized steel 90° horizontal elbows (E1, E2, E3) with internal diameters of 26.5, 41.2 and 52.5 mm, and curvature radii of 194.0, 264.0 and 326.6 mm, respectively. According to the experimental results, the model proposed by Chisholm best fitted them, presenting for each elbow an average error of E1 = 18.27%, E2 = 28.40% and E3 = 42.10%. Based on experimental results two correlations were developed. The first one is the classical Chisholm model modified to obtain better results in a wider range of conditions; it was adjusted by a dimensionless relationship which is a function of the homogeneous volumetric fraction and the Dean number. As a result, the predictions using modified Chisholm model were improved presenting an average error of 8.66%. The second developed correlation is based on the entire two-phase mass flow taken as liquid and adjusted by the homogeneous volumetric fraction ratio. The results show that this last correlation is easier and accurate than the adjusted Chisholm model, presenting an average error of 7.75%. Therefore, this correlation is recommended for two-phase pressure drop evaluation in horizontal elbows.

Thermoacoustic-Stirling Heat Pump for Domestic Applications

Domestic heating contributes to a significant amount of energy usage in the Netherlands. Due to scare energy resources, attention to develop new and efficient technologies is increasing. At ECN, a burner driven heat pump employing thermoacoustic technology is being developed for possible applications in households and offices. The desired temperature lift is from 10 °C to 80 °C. As a first step the heat pump is driven by a linear motor. Measurements and performance analysis of the heat pump are presented in this paper. The heat pump has a coefficient of performance which is the ratio of heat produced to the work input of 1.38 when operating between 10 °C to 80 °C. The performance relative to maximum possible Carnot value is 26.5%.

Design of a Mechanical Resonator to Be Coupled to a Thermoacoustic Stirling-Engine

This paper describes the design of a mechanical resonator for a thermoacoustic Stirling-engine. The engine was previously run with a quarter-wavelength acoustic resonator. The advantage of the mechanical resonator is that it is compact and would dissipate less acoustic power. The mechanical resonator consists of a twin piston-spring assembly moving in opposite phase to cancel vibrations. The system uses flexure springs to suspend the piston in a cylinder leaving a narrow gap between them. The narrow gap acts as a dynamic seal between the fronts and back sides of the piston. Simulation calculations show that the mechanical resonator dissipates 40% less acoustic power than the acoustic one. This will lead to more useful acoustic power output from the thermoacoustic Stirling-engine. In addition, the size of the system is reduced considerably.

Creating Small Gas Bubbles in Flowing Mercury Using Turbulence at an Orifice

Pressure waves created in liquid mercury pulsed spallation targets have been shown to create cavitation damage to the target container. One way to mitigate such damage would be to absorb the pressure pulse energy into a dispersed population of small bubbles, however, creating such a population in mercury is difficult due to the high surface tension and particularly the non-wetting behavior of mercury on gas-injection hardware. If the larger injected gas bubbles can be broken down into small bubbles after they are introduced to the flow, then the material interface problem is avoided. Research at the Oak Ridge National Labarotory is underway to develop a technique that has shown potential to provide an adequate population of small-enough bubbles to a flowing spallation target. This technique involves gas injection at an orifice of a geometry that is optimized to the turbulence intensity and pressure distribution of the flow, while avoiding coalescence of gas at injection sites. The most successful geometry thus far can be described as a square-toothed orifice having a 2.5 bar pressure drop in the mercury flow of 8 L/s for one of the target inlet legs. High-speed video and high-resolution photography have been used to quantify the bubble population on the surface of the mercury downstream of the gas injection site. Also, computational fluid dynamics has been used to optimize the dimensions of the toothed orifice based on a RANS computed mean flow including turbulent energies such that the turbulent dissipation and pressure field are best suited for turbulent break-up of the gas bubbles.

An Experiment Study of Wall Slot Jets Pertinent to Trailing Edge Cooling of Turbine Blades

An experimental study was conducted to quantify the flow characteristics of wall jets pertinent to trailing edge cooling of turbine blades. A high-resolution stereoscopic PIV system was used to conduct detailed flow field measurements to quantitatively visualize the evolution of the unsteady vortex and turbulent flow structures in cooling wall jet streams and to quantify the dynamic mixing process between the cooling wall jet streams and the main stream flows. The detailed flow field measurements are correlated with the adiabatic cooling effectiveness maps measured by using pressure sensitive paint (PSP) technique to elucidate underlying physics in order to improve cooling effectiveness to protect the critical portions of turbine blades from the harsh ambient conditions.

Concentration Distribution During Pulse Jet Mixing

Obtaining real-time, in situ slurry concentration measurements during unsteady mixing can provide increased understanding into mixer performance. During recent tests an ultrasonic attenuation sensor was inserted into a mixing vessel to measure the slurry concentration during unsteady mixing in real time during pulse jet mixer operation. These pulse jet mixing tests to suspend noncohesive solids in Newtonian liquid were conducted at three geometric scales. To understand the solids suspension process and resulting solids distribution, the concentration of solids in the cloud was measured at various elevations and radial positions during the pulse jet mixer cycle. In the largest scale vessel, concentration profiles were measured at three radial locations: r = 0, 0.5 and 0.9 R where R is the vessel radius. These radial concentration data are being analyzed to provide a model for predicting concentration as a function of elevation. This paper describes pulse jet mixer operation, provides a description of the concentration probe, and presents transient concentration data obtained at three radial positions: in the vessel center (O R), midway between the center and the wall (0.5 R) and near the vessel wall (0.9 R) through out the pulse to provide insight into pulse jet mixer performance.

Turbulent Two-Phase Flows and Particle Deposition in a Duct at High Concentrations

Two-phase flows including particle-particle collisions and two-way coupling in a turbulent duct flow were simulated using a direct simulation approach. The direct numerical simulation (DNS) of the Navier-Stokes equation was performed via a pseudospectral method was extended to cover two-way coupling effects. The effect of particles on the flow was included in the analysis via a feedback force that acted on the fluid on the computational grid points. The point particle equation of motion included the Stokes drag, the Saffman lift, and the gravitational forces. Several simulations for different particle relaxation times and particle mass loading were performed, and the effects of the inter-particle collisions and two-way coupling on the particle deposition velocity, fluid and particle fluctuating velocities, particle normal mean velocity, and particle concentration were determined. It was found that when particle-particle collisions were included in the computation, the particle deposition velocity increased. When the particle collision was neglected but the particle-fluid two-way coupling was accounted for, the particle deposition velocity decreased slightly. When both inter-particle collisions and two-way coupling effects were taken into account in the simulations, the particle deposition velocity increased. Comparisons of the present simulation results with the available experimental data and earlier numerical results are also presented.

Four-Way Coupling of Dense Particle Beds of Black Powder in Turbulent Pipe Flows

The modeling of particle deposition and transport in pipes is one of the most challenging problems in multiphase flow, because the underlying physics is multi-faceted and complex, including turbulence of the carrier phase, particle-turbulence interaction, particle-wall interactions, particle-particle interactions, two-way and four-way couplings, particle agglomeration, deposition and re-suspension. We will discuss these issues and present new routes for the modeling of particle collision stress. Practical examples like black powder deposition and transport in gas pipelines will be presented and discussed. The model employed is based on dense-particle formulation accounting for particle-turbulence interaction, particle-wall interactions, particle-particle interactions via a collision stress. The model solves the governing equations of the fluid phase using a continuum model and those of the particle phase using a Lagrangian model. Inter-particle interactions for dense particle flows with high volume fractions (from 1% to close packing ∼60%) have been accounted for by mapping particle properties to an Eulerian grid and then mapping back computed stress tensors to particle positions. Turbulence within the continuum gas field was simulated using the V-LES (Very Large-Eddy Simulation) and full LES, which provides sufficient flow unsteadiness needed to disperse the particles and move the deposited bed.

Scouring Below Pipelines: The Role of Vorticity and Turbulence

A numerical study on the turbulence and vorticity of local scour underneath an offshore pipeline placed on a non-cohesive sandy seabed and forced by a steady flow current is presented. The numerical model solves the Navier-Stokes equations using an innovative Level Set technique. The model predicts the behavior of the movable sediments through both drift and lift force components. Mean and turbulent flow quantities were extracted by temporal averaging. Results on the distribution and evolution of turbulent kinetic energy and vorticity will be illustrated at the conference.

Flow Visualization Techniques for the Evaluation of Non-Contact Trace Contraband Detectors

Efforts are underway in the Surface and Microanalysis Science Division at the National Institute of Standards and Technology to study trace aerodynamic sampling of contraband materials (explosives or narcotics) in non-contact trace detection systems. Trace detection systems are designed to screen people, personal items, and cargo for particles that have contaminated surfaces. In a typical implementation of people screening, a human subject walks into a confined space where they are interrogated by a series of pulsed air jets and are screened for contraband materials by a chemical analyzer. The screening process requires particle and vapor removal, transport, collection, desorption, and detection. Aerodynamic sampling is the critical front-end process for effective detection. In this paper, a number of visualization techniques are employed to study non-contact aerodynamic sampling in detail. Particle lift-off and removal is visualized using high-speed videography, transport of air and particles by laser light scattering, and desorption surface heating and cooling patterns by infrared thermography. These tools are used to identify sampling inefficiencies and may be used to study next-generation screening approaches for aerodynamic sampling of particles and vapors.