Measurement of the Air Pressure for a Rotary Vane Compressor

This study introduces an experimental method that can measure air pressures in the vane segments when a sliding-vane rotary compressor performs suction and compression phases in stable or unstable rotational speeds. When the air pressures of these two phases can be measured, the intake effect of the compressor’s inlet and the seal effect of the vane segments can be evaluated, respectively. Because a frequency converter provides unstable rotational speeds when it controls rotational speeds of a motor with a compressor, an encoder mounted on the output shaft of the motor was applied to record the angular location of the compressor rotor. Two strain gauge type pressure transducers were inserted into the cover plate of the compressor to measure air pressures in the vane segments. Comparing the signals of the encoder with pressure transducers, the air pressures in completions of suction and compression phases could be determined in stable or unstable rotational speeds. The air pressures when the compressor performed suction and compression phases were 99.5 kPa and 153 kPa, respectively, in 1400 rpm. The air pressure when the compressor performed suction phase decreased with the rotational speed faster than 800 rpm. The size or shape of the inlet port of the compressor should be enlarged or modified to provide the suction air pressure without dropping too much. The designed air pressure when the compressor performed compression phase was 244 kPa in 140 rpm, the manufacture precision of the compressor should be increased to decrease leakage.

Error Assessment and Error Reduction in Production-Level RANS-Based Drag Predictions

Assessments of the effects of several numerical parameters on RANS-based drag prediction accuracy are presented. The parameters include grid cell size adjacent to solid walls, grid stretch ratio, grid stretch transition, artificial dissipation scheme, and artificial dissipation coefficient. Results from extensive parametric studies on a two-dimensional flat plate are reported. Based on the results of these studies, guidelines for high-accuracy drag predictions using both second- and fourth-order accurate, finite-difference-based solvers are proposed. In addition, error assessments obtained with a single grid using second- and fourth-order accurate solutions are compared to multiple-grid Richardson’s extrapolation approaches. The single-grid approach is shown to provide a significant improvement in both accuracy and error assessment relative to the multiple-grid approach.

Near-Surface Velocimetry Using Evanescent Wave Illumination

Total internal reflection fluorescent microscopy (TIRFM) is used to measure particle motion in the near wall region of a microfluidic system. TIRFM images have minimum background noise and contain only particles that are very close to channel surface, where slip velocities may be present. Submicron sized fluorescent particles suspended in water are used as seed particles and images are analyzed with a PTV algorithm to extract information about apparent slip velocity. At relatively low shear rates (less than 2500 sec−1), an apparent slip velocity, proportional to the shear rate was observed. However, numerical simulations show that this observation is a direct consequence of the small, but finite thickness of the illuminated region, and most likely not due to physical slip at the surface. The statistical difference in apparent slip velocities measured over hydrophilic and hydrophobic surfaces is found to be minimal. Issues associated with the experimental technique and the interpretation of the experimental results are also discussed.

High-Pressure Microhydraulic Actuator

Actuation forces of 2.1 and 5.3 pounds (9.3 and 24 N) at velocities of 1 and 0.5 mm/s have been demonstrated with compact electrokinetic pumps producing 200 μL/min at 400 psi (2.8 MPa) and 100 μL/min at 1000 psi (6.9 MPa). This output compares favorably with electromechanical actuators (solenoid, piezoelectric, stepper motor) of similar size and is achieved silently and with no moving parts. Electrokinetic pump monoliths based on phase-separated porous methacrylate polymer monoliths and slurry-packed, sintered silica monoliths have been developed that can generate electrokinetic pressures of 3 psi/V (21 kPa/V) and 8 psi/V (6.9 kPa/V), respectively. Corresponding maximum power conversion efficiencies of 1% and 3% have been demonstrated in 10 mM TRIS-HCI at pH 8.5. Gas-bubble-free electrodes have been demonstrated to deliver 2 mA and seal to 1200 psi (8.3 MPa) for microhydraulic actuation.

Characterization of the Time-Dependent Rheological Behavior of Fluids for Electronics Packaging

To effectively control the dispensing process by which fluids are delivered onto substrates in electronics packaging, one of the key issues is to understand and characterize the flow behavior of the fluids being dispensed. However, this task has proven to be a demanding one as the fluids used for electronics packaging usually exhibit the time-dependent rheological behavior, which has not been well documented in the literature. In this paper, the characterization of time-dependent rheological behavior of fluids is studied. In particular, a model using the structural theory is proposed and applied to the characterization of the decay and recovery of fluid behavior, which are typically encountered in a dispensing process. Experiments are conducted to validate the proposed model.

Laminar Flow Around a Sinusoidal Interface Between a Porous Medium and a Clear Fluid

A number of natural and engineering systems can be characterized by some sort of porous structure through which a working fluid permeates. Atmospheric boundary layers over tropical forests and vegetation can be modeled as flow over a porous layer of irregular surface. In addition, in engineering systems one can have components that make use of a working fluid flowing over irregular layers of porous material. This paper presents numerical solutions for such hybrid medium, considering here a channel partially filled with a sinusoidal porous layer saturated by a fluid flowing in laminar regime. One unique set of transport equations is applied to both regions. Effects of Reynolds number, porosity and permeability on mean and turbulence fields are investigated. For a fixed inlet mass flow rate, increase of either porosity or permeability reduced the strength of the recirculating motion over the porous layer.

LES of Particle-Laden Flow With Inter-Particle Collisions

By analogy with kinetic approach, the gas-solid turbulent flow was considered as an ensemble of interacting both stochastic liquid and solid particles. In this way, the motion equation for the solid particle along a smoothed trajectory has been derived. To close this equation, the statistical temperature of particles has been introduced and expressed by statistical properties of turbulence. The smoothed particles dynamics was then computed along with large-eddy simulation (LES) of turbulent channel gas flow with “two-way” coupling of momentum. The calculated results are compared with the experiment of Kulick et. al. (1994) and with computation of Yamomoto et. al. (2001), where the inter-particle interaction has been simulated by hard-sphere collisions with prescribed efficiency. It has been shown that our computation with smoothed motion of particle is relatively in agreement with experiment and computations of Yamomoto et. al. (2001). At the same time, the model presented in the paper has a following advantage: it, practically, does not require an additional CPU time to account for inter-particle interactions. The turbulence attenuation by particles and the preferential concentration of particles in the low-turbulence region have been shown.

New Evolutionary Techniques for Optimization of Energy Systems Utilizing Computational Fluid Dynamics

Optimization techniques that search a solution space without designer intervention are becoming important tools in the engineering design of many thermal fluid systems. Evolutionary algorithms are among the most robust of these optimization methods because the ability to optimize many designs simultaneously makes evolutionary algorithms less susceptible to premature convergence. However application of evolutionary algorithms to thermal and fluid systems described by high fidelity models (e.g. computational fluid dynamics) has been limited due to the high computational cost of the fitness evaluation. This paper presents a novel technique that combines two technologies used in the optimization of thermal fluids systems. The first is graph based evolutionary algorithms that are implemented to help increase the diversity of the evolving population of designs. The second is an algorithm utilizing a feed forward neural network that develops a stopping criterion for computational fluid dynamics solutions. This reduces the time required for each future evaluation in the evolutionary process and allows for more complex thermal fluids systems to be optimized. In the system examined here the overall reduction in computational time is approximately 8 times.

CFD Comparison of Clearance Flow in a Rotor-Casing Assembly Using Asymmetric vs. Symmetric Lobe Rotors

The performance of a Rotor-Casing Assembly is influenced more by the internal air leakages than by any other thermo-fluid aspect of its behaviour. The pressure difference driving the air along a leakage path varies periodically and does so in a manner that may not be the same for every leakage path. So the distribution of leakage through the various leakage paths within the machine is important for the improvement of its performance. The total volume of air leakage and the distribution of the leakage among the different paths depend on the rotor-rotor and rotor-casing clearances as well as the geometry of the rotors’ lobes. Computational Fluid Dynamics (CFD) analysis was carried out using the FLUENT. Geometry definition, mesh generation, boundary and flow conditions, and solver parameters have all been investigated as the part of the numerical analysis. This analysis was conducted for static rotors at different positions. The results indicate that the size of the clearances as well as the geometry of the rotors’ lobes can have a significant effect on the total volume of the air leakage as well as the distribution of the leakage among the three main leakage paths. The results can be used to ascertain the proper levels of clearances to be used and the best rotor lobes geometry to be used for the practical reduction of air leakage.

Treatment of Thermophoretic Deposition in Tube Flow for Materials Processing Applications

Material processing techniques based on vapor or particle transport involve a number of interrelated hydrodynamic and thermal effects. For cases with large temperature gradients or small (nano) particles, thermal gradients can induce motion referred to as thermophoresis. Thermophoresis directly impacts material quality and production rates for chemical vapor deposition (CVD) and related processes. A simple yet useful geometry for the study of thermophoretic deposition is axial tube flow. This geometry is particularly useful as it closely represents one process used to produce fiber optic preforms. While considerable research has been conducted on thermophoretic deposition, a global description and understanding of the phenomena is still difficult. This paper will first review the analytical study of thermophoretic deposition. Starting with the simple Graetz problem the development of more complex solutions to the governing Navier-Stokes equations will be detailed from the literature. Next, the creation of a coupled, two code Eulerian approach to solve this problem will be presented. Finally, the results of numerical case studies performed by the author will be discussed. These results will be used to compare and contrast the influence of different factors.