Donald Wroblewski

Development of a MEMS microvalve array for fluid flow control

A microelectromechanical system (MEMS) microvalve array for fluid flow control is described. The device consists of a parallel array of surface-micromachined binary microvalves working cooperatively to achieve precision how control on a macroscopic level. Flow rate across the microvalve array is proportional to the number of microvalves open, yielding a scalable high-precision fluidic control system. Device design and fabrication, using a one-level polycrystalline silicon surface-micromachining process combined with a single anisotropic bulk etching process are detailed. Performance measurements on fabricated devices confirm feasibility of the fluidic control concept and robustness of the electromechanical design. Air-flow rates of 150 ml/min for a pressure differential of 10 kPa were demonstrated. Linear flow control was achieved over a wide range of operating flow rates. A continuum fluidic model based on incompressible low Reynolds number flow theory was implemented using a finite-difference approximation. The model accurately predicted the effect of microvalve diaphragm compliance on flow rate. Excellent agreement between theoretical predictions and experimental data was obtained over the entire range of flow conditions tested experimentally.

Diagnostics and Control in the Thermal Spray Process

The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose are described along with the demonstration of a closed loop process controller based on these measurements.

Development of a rapid-response flow-control system using MEMS microvalve arrays

A method for providing high-resolution gas flow control using microelectromechanical systems (MEMS) has been developed and tested. The micromachined component consists of an array of 61 synchronized microvalves operating in parallel. A number of tests were conducted on microvalves of various designs to characterize their operation. The best performing of these was used with a prototype flow controller. Additionally, a mathematical model of the flow system and controller was derived to predict the response of the system to various changes in operating conditions. This work describes the design, modeling, and testing of a compact, stand-alone mass flow controller (MFC) to demonstrate high resolution, fast response flow control using MEMS microvalves. The device consists of a microvalve array packaged with a micro flow sensor and a microprocessor-based control system. The high bandwidth of microvalves allows an atypical flow control architecture. The controller regulates a pulsewidth-modulated (PWM) signal sent to the valve array and is capable of both open- and closed-loop control. A mathematical model was also developed to predict the dynamic performance of the system under various operating conditions. Additional advantages of the MEMS flow-control system include low-power consumption, low fabrication costs, and scalable precision.

Cliff-Ramp Patterns and Kelvin-Helmholtz Billows in Stably Stratified Shear Flow in the Upper Troposphere: Analysis of Aircraft Measurements

Abstract Cliff–ramp patterns (CR) are a common feature of scalar turbulence, characterized by a sharp temperature increase (cliff) followed by a more gradual temperature decrease (ramp). Aircraft measurements obtained from NOAA best aircraft turbulence probes (BAT) were used to characterize and compare CR patterns observed under stably stratified conditions in the upper troposphere, a region for which there are few such studies. Experimental data were analyzed for three locations, one over Wales and two over southern Australia, the latter in correspondence with the Southern Hemisphere winter subtropical jet stream. Comparison of observed CR patterns with published direct numerical simulations (DNS) revealed that they were likely signatures of Kelvin–Helmholtz (KH) billows, with the ramps associated with the well-mixed billows and the cliffs marking the highly stretched braids. Strong correlation between potential temperature and horizontal velocity supported the KH link, though expected correlations with ...

Solidification modeling of plasma sprayed TBC: Analysis of remelt and multiple length scales of rough substrates

A two-dimensional, finite-element model based on an enthalpy formulation, was developed to simulate a splat solidifying on a rough substrate (with an idealized, sinusoidal-shaped roughness) capturing the multiple-length scales seen in real coatings as well as different aspect ratios. The model was used to study the effects of substrate temperature, splat temperature, and roughness characteristics on the onset and extent of remelt. Remelt is studied since it is indicative of local heat transfer conditions and might explain observed coating properties. Multiple splats were simulated using the two-dimensional model for short-time cooling coupled to a one-dimensional model for long-time cooling to predict substrate temperature rise prior to subsequent splat impacts. The presence of roughness promoted substrate remelting at conditions under which no remelting was observed for a smooth surface, suggesting that substrate roughness is an important parameter to include in splat solidification studies. The effects of splat temperature and substrate temperature on remelt were consolidated into a single nondimensional parameter, which captured a number of critical phenomena including characterization of the onset of remelt with a nondimensional remelting point.

Velocity and Temperature Structure Functions in the Upper Troposphere and Lower Stratosphere from High-Resolution Aircraft Measurements

Abstract High-resolution measurements obtained from NOAA “best” atmospheric turbulence (BAT) probes mounted on an EGRETT high-altitude research aircraft were used to characterize turbulence in the upper troposphere and lower stratosphere at scales from 2 m to 20 km, focusing on three-dimensional behavior in the sub-kilometer-scale range. Data were analyzed for 129 separate level flight segments representing 41 h of flight time and 12 600 km of wind-relative flight distances. The majority of flights occurred near the tropopause layer of the winter subtropical jet stream in the Southern Hemisphere. Second-order structure functions for velocity and temperature were analyzed for the separate level-flight segments, individually and in various ensembles. A 3D scaling range was observed at scales less than about 100 m, with power-law exponents for the structure functions of the velocity component in the flight direction varying mostly between 0.4 and 0.75 for the separate flight segments, but close to ⅔ for the ...

Effect of periodic unsteadiness on heat transfer in a turbulent boundary layer downstream of a cylinder-wall junction

Abstract The heat transport and wall heat transfer in a turbulent boundary layer downstream of a cylinder-wall junction has been investigated experimentally. The heat transfer effect of local periodic, large-scale unsteadiness, as a result of vortex shedding from the cylinder, was examined using a conditional data-sampling and analysis technique that separated the large-scale periodic motion from the background turbulence. Detailed measurements of temperature and velocity were obtained using a cold wire and a custom-designed heat-flux probe. Thin-film, heat-flux surface sensors were used to obtain the time-resolved wall heat flux. Experiments were conducted approximately 5 diameters downstream of the leading edge of the cylinder, at Re D = 36,000 and Re L = 970,000. The boundary-layer transport and wall heat transfer were affected by the presence of large-scale periodic unsteadiness arising from vortex shedding in a region spanning approximately 2 diameters on either side of the cylinder. Large-scale fluctuations of the time-resolved wall-Stanton number were caused by the thinning and thickening of the near-wall thermal layer by periodic fluctuations in the vertical component of velocity.

Effect of large-scale periodic unsteadiness on the heat transport in the downstream region of a junction boundary layer

Abstract The heat transport in the boundary layer downstream of a junction between a streamlined cylinder and a wall is examined. This flow is characterized by large-scale periodic motions, attributable to vortex shedding, which affect the turbulent heat transport and may contribute to enhanced wall heat transfer. Experimental data obtained using a triple-wire turbulent heat-flux probe were analyzed using triple decomposition and a conditional sampling technique that separated the large-scale periodic motions from the background turbulence. The periodic motions lead to large-scale movements of the instantaneous thermal boundary-layer interface and contribute significantly to the overall Reynolds heat fluxes. The intermittency of the temperature signal revealed that the hot/cold interface penetrated much closer to the wall than in an undisturbed two-dimensional (2-D) boundary layer. Two different mechanisms that might be responsible for the observed transport phenomena are discussed: (1) a portion of the shed vortex filament that is skewed by the mean strain field of the wake and boundary layer; and (2) large counter-rotating stream-wise vortices identified from mean-flow measurements that oscillate in the unsteady wake of the obstacle.

Modeling and Parametric Analysis of Plasma Spray Particle State Distribution for Deposition Rate Control

Plasma spray for depositing thermal barrier coatings features large distributions of particle states that result in significant variations in coating quality. These variations arise from distributions of particle sizes, large spatial gradients of plasma thermal-fluid fields, and temporal variations of the arc and jet. This paper describes a simplified approach for studying how particle state distributions are influenced by torch conditions and powder distributions, and the implications for deposition rate monitoring and control. The approach combines a simplified jet model with a more detailed particle model. The important fluid-thermal spatial gradients in the plasma jet are captured using a three zone model: a core region, modeled by growth of a turbulent shear layer around a laminar core, a transition region and a similarity region. Plasma-particle momentum and thermal interactions, particle phase transitions, internal particle temperature gradients, and collapse of in-flight hollow particles have been modeled using a multi-lumped particle model. Effects of distributions of particle size, particle morphology, injection velocity, and carrier gas flow were studied for YSZ spray in an Ar-He plasma. The results provide guidance on sensor design and operation and on approaches for plume location control.Copyright

Sensor and Control Design Issues for Developing Real-Time Deposition Rate Control for Plasma Spray

Real-time control offers the potential to reduce plasma spray variations that affect yield and coating quality. Important factors for designing such controllers are discussed including sensor issues, dominant nonlinearities, and cross-coupling interactions. The performance of several alternative strategies to achieve better coating thickness control is evaluated.Copyright