Maria A. Kuczmarski

Numerical Studies of a Fluidic Diverter for Flow Control

The internal flow structure in a specific fluidic diverter is studied over a range from low subsonic to sonic inlet conditions by a time-dependent numerical analysis. The understanding will aid in the development of fluidic diverters with minimum pressure losses and advanced designs of flow control actuators. The velocity, temperature and pressure fields are calculated for subsonic conditions and the self-induced oscillatory behavior of the flow is successfully predicted. The results of our numerical studies have excellent agreement with our experimental measurements of oscillation frequencies. The acoustic speed in the gaseous medium is determined to be a key factor for up to sonic conditions in governing the mechanism of initiating the oscillations as well as determining its frequency. The feasibility of employing plasma actuation with a minimal perturbation level is demonstrated in steadystate calculations to also produce oscillation frequencies of our own choosing instead of being dependent on the fixed-geometry fluidic device.

Numerical Studies of a Supersonic Fluidic Diverter Actuator for Flow Control

The analysis of the internal flow structure and performance of a specific fluidic diverter actuator, previously studied by time-dependent numerical computations for subsonic flow, is extended to include operation with supersonic actuator exit velocities. The understanding will aid in the development of fluidic diverters with minimum pressure losses and advanced designs of flow control actuators. The self-induced oscillatory behavior of the flow is successfully predicted and the calculated oscillation frequencies with respect to flow rate have excellent agreement with our experimental measurements. The oscillation frequency increases with Mach number, but its dependence on flow rate changes from subsonic to transonic to supersonic regimes. The delay time for the initiation of oscillations depends on the flow rate and the acoustic speed in the gaseous medium for subsonic flow, but is unaffected by the flow rate for supersonic conditions.

Numerical Studies of an Array of Fluidic Diverter Actuators for Flow Control

In this paper, we study the effect of boundary conditions on the behavior of an array of uniformly-spaced fluidic diverters with an ultimate goal to passively control their output phase. This understanding will aid in the development of advanced designs of actuators for flow control applications in turbomachinery. Computations show that a potential design is capable of generating synchronous outputs for various inlet boundary conditions if the flow inside the array is initiated from quiescence. However, when the array operation is originally asynchronous, several approaches investigated numerically demonstrate that resynchronization of the actuators in the array is not practical since it is very sensitive to asymmetric perturbations and imperfections. Experimental verification of the insights obtained from the present study is currently being pursued.

CFD-guided development of test rigs for studying erosion and large-particle damage of thermal barrier coatings

Burner rigs are routinely used to qualifymaterials for gas turbine applications. The most useful rig tests are those that can replicate, often in an accelerated manner, the degradation that materials experience in the engine. Computational fluid dynamics (CFD) can be used to accelerate the successful development and continuous improvement of combustion burner rigs for meaningful materials testing. Rig development is typically an iterative process of making incremental modifications to improve the rig performance for testing requirements. Application of CFD allows many of these iterations to be done computationally before hardware is built or modified, reducing overall testing costs and time, and it can provide an improved understanding of how these rigs operate. This paper describes the use of CFD to develop burner test rigs for studying erosion and large-particle damage of thermal barrier coatings (TBCs) used to protect turbine blades from high heat fluxes in combustion engines. The steps used in this study-- determining the questions that need to be answered regarding the test rig performance, developing and validating the model, and using it to predict rig performance--can be applied to the efficient development of other test rigs.

Buoyancy suppression in gases at high temperatures

Abstract The computational fluid dynamics code FLUENT was used to study Rayleigh instability at large temperature differences in a sealed gas-filled enclosure with a cold top wall and a heated bottom wall (Benard problem). Both steady state and transient calculations were performed. Instability boundaries depending on the geometry, temperature, and pressure were defined that showed the system tended to become more unstable when the hot-wall temperature increased beyond a certain level, a result of the dampening effect of gas viscosity at higher temperatures. Results also showed that the eventual system stability depended on the final pressure reached at steady state, regardless of how fast the bottom-wall temperature was ramped up to minimize time spent in the unstable region of fluid motion. It was shown that the final system state can differ depending on whether results are obtained via a steady-state or transient calculation, demonstrating that the history of the flow structure development and corresponding temperature fields in this type of system has a profound effect on the final state. Finally, changes in the slope of the pressure-versus-time curve were found to be good indicators of flow pattern changes, and can be a convenient experimental tool for diagnosing the expected changes in flow behavior in such systems.

Pressure Response in Enclosures During and After Large-Scale Flame Spread: Testing and Modeling

As part of the risk mitigation effort supporting the development of a zero-gravity largescale fire demonstration experiment, normal-gravity flame-spread tests were conducted on thin fuels in a sealed chamber capable of accommodating large-scale samples. The primary objective of these tests was to measure pressure rise in a large sealed chamber during and after flame spread and to characterize that data as a function of sample material, burning direction (upward/downward), heat release rate, total heat release, and heat loss mechanisms. Ignition at the bottom of a sample resulted in a rapid acceleratory upward turbulent flame spread. As the amount of fuel burned increases, the pressure rise in the chamber increases since the volume of hot gas generated is larger. The upward flame spread slows down as initial pressure is reduced, causing a drop in peak pressure and a shift to the right in time for peak pressure. Ignition at the top of a sample resulted in a slow, steady flame spread with a very small flat flame across the top of each burning cheesecloth sample. For downward flame spread, the pressure rises to a maximum steady-state pressure during the burn. This indicates that the heat dissipation to the surroundings has matched the heat generated by the flame for this long, slow burn. Heat sinks were used for overpressure mitigation above the burning samples so that the flame plume would pass through the heat sink and deposit the heat there instead of heating the atmosphere. For upward and downward flame spread, the heat sink was very effective in reducing the pressure by a factor of at least four. The heat sink was determined to be an effective mitigation strategy for the flight experiment.

Method for Measuring Thermal Conductivity of Small Highly Insulating Specimens

A hot plate method for the measurement of thermal conductivity is described, which is unique in that it combines all the following capabilities: (1) Measurements of very small specimens, (2) measurements of specimens with thermal conductivity on the same order of that as air, and (3) the ability to use air as a reference material. As with other approaches, care is taken to ensure that the heat flow through the test specimen is essentially one-dimensional. However, unlike other approaches, no attempt is made to use heated guards to minimize the flow of heat from the hot plate to the surroundings. Results indicate that since large correction factors must be applied to account for guard imperfections when specimen dimensions are small, simply measuring and correcting for heat from the heater disk that does not flow into the specimen may be preferable. Extensive computational heat transfer modeling and experimental measurements taken in a prototype apparatus show that this approach is feasible. Suggestions are made for further improvements based on analyses of the generated data.