Reza Kashani

Electrohydrodynamically Augmented Micro Heat Pipes

An experimental investigation was conducted to evaluate the potential benefits of electrohydrodynamic (EHD) forces on the operation of micro heat pipes. In these experiments, electric fields were used to orient and guide the flow of the dielectric liquid within the micro heat pipe from the condenser to the evaporator. The experiments indicate the heat transport capability of the EHD micro heat pipes is increased by up to six times of that of conventional ones. In parallel, an analytical model was developed to predict the maximum heat transport capability for various electric field intensities and micro heat pipe geometries. The analytical model agrees well with the experimental results for the geometry studied experimentally. The model shows that large pore sizes are optimal from a heat transport capacity perspective. Finally, a critical assessment of the experimental results suggest an alternative design capable of achieving as much as a 180 times improvement in the heat transport capacity in comparison to traditional micro heat pipes.

Temperature Control of Electrohydrodynamic Micro Heat Pipes

Abstract Active thermal control was achieved by using an electrohydrodynamically (EHD) assisted micro heat pipe array. A simulation model of temperature control of EHD micro heat pipes was established in a Matlab Sinulink environment. An experimental model was designed and fabricated to verify the model and identify the factors most influential to the thermal control via EHD micro heat pipe array. Good correspondence between simulations and experiments was achieved. Electric field intensity, set-point temperature and the gap between the upper and lower set-point temperatures were shown to have a dramatic influence on the temperature control.

Active Isolation Using a Kalman Estimator—Based Controller

A novel feed-forward active vibration/shock isolation scheme that is based on the use of a Kalman estimator (KE) has been developed. It does not require real-time adaptation and yet maintains its effectiveness when the attributes (such as the frequencies) of the disturbance change. The proposed controller is always second order, regardless of the complexity of the structure under control. In addition to feeding forward the measured base acceleration, the acceleration at the location of interest on the structure is measured and fed back through the KE gain. Using numerical and experimental data, the KE-based controller is demonstrated to be highly effective in reducing the transmission of the base excitation to the structure.

Floor Vibration Control Using Three-Degree-of-Freedom Tuned Mass Dampers

The use of high-strength material in buildings has resulted in the use of less building materials and, consequently, a high level of flexibility in buildings, making them vibration prone. For example, high-strength concrete has lowered the thickness of concrete slabs used in the floors of steel/concrete buildings, such as office buildings and shopping centers, resulting in excessive floor vibration stemming from heavy traffic and normal human activity. Although not dangerous, such vibration is highly annoying to the occupants of the building. The authors have been working on the use of three-degree-of-freedom (3-DOF) tuned mass dampers (TMD) to abate floor vibration. Such TMDs can provide improved effectiveness over a traditional one-degree-of-freedom TMD and yet possess all of the attractive features of a traditional TMD; namely, simplicity and low cost. As in a 1-DOF TMD, this device will be installed on a concrete floor slab, at an optimally designed/chosen location.Copyright

Micro-Scale Electrohydrodynamic Pumped High Performance Actuation

A new type of actuation device has been conceptualized that meets the needs of both large displacement force and bandwidth within a package more compact than the currently available magnetostrictive and stack-type piezoelectric actuators of similar rating. This concept relies on micro-scale electrohydrodynamic (EHD) pumping of a dielectric liquid within small channels. Configured as an actuator, the EHD pump(s) would be used to move fluid between two reservoirs—each having a compliant membrane that interfaces to the world to provide the means to achieve vibration cancellation or micro actuation. Ordinarily limited to generating flow in macroscale applications, the EHD pump, when operating in a thermal induction mode, is shown to exhibit an exciting scaling law as its size is reduced. As the pump volume to surface area decreases, the energy going toward increasing pressure in the pump has an increasingly larger effect. Since the volume/surface area is proportional to 1/a, where a is the characteristic width or diameter of the channels comprising the pump, the pressure head generated scales similarly. Analytical and numerical studies have shown the EHD-pumped actuator to be capable of delivering equal force and bandwidth to magnetostrictive and stack-type piezo actuators, but with considerably greater displacement and a smaller size. Further, this type of actuator offers the possibility for deployment in active vibration control or micro actuation applications at significantly greater temperatures than for piezoelectric and magnetostrictive devices.

The Impact of Perforation Geometry on Acoustic Damping Attributes of a Perforated Liner With Bias Flow

Acoustic damping properties of perforated liners are highly dependent on a number of variables which can be categorized as “flow variables” such as the extent and Mach number of grazing flow as well as bias flow and “geometric variable” such as the shape of the hole which can be rectangular, cylindrical, conical with diverging or converging nozzle, thickness to radius ratio, radius to hole spacing ratio and hole orientation which can be normal to or inclined with respect to the perforated plate. Many of these variables were not incorporated in previous studies.Theoretical and empirical approaches have provided the foundation for understanding the damping properties of liners but they are based on certain simplifying assumptions making them inadequate in addressing the more realistic conditions encountered in industrial applications. These limitations have highlighted the importance of numerical methods for studying damping behavior of liners. Acoustic attributes of perforated plates (mainly in terms of impedance which is a frequency-dependent complex quantity) as a function of non-dimensional variables like Reynolds, Strouhal, Mach, and Helmholtz numbers have been studied by various researchers, including the authors, using a variety of numerical tools starting from the simple 1D network scheme based on linear acoustics and the wall compliance concept introduced by Howe all the way to the computationally intensive Large-Eddy Simulations (LES) and Scaled Adaptive Simulation (SAS) reconstructing the full unsteady turbulent structures. Although the impacts of some geometry variations such as hole inclination angle and diameter, in conjunction with various fluid dynamic parameters, have been investigated using 1D network tools, the focus of LES has been mainly on analysis of a single circular hole with periodic boundary conditions as the representation of multi-perforation (assuming the perforations are spaced far enough from each other so that there is no interaction between neighboring holes). There is certainly a need for thorough investigation of the acoustics impact of these geometric parameters as well as shape of the holes using LES.In an on-going research we are extending the numerical modeling work on characterizing the acoustic damping attributes of a perforation, beyond the current state of the art, by including the geometric variables including hole size, shape, orientation, and radius to thickness ratio, amongst others, in the study. In this paper, following a short review of the research conducted in the recent past for comprehension of the acoustic-vortex interaction mechanism in perforated liners resulting in acoustic absorption, we present the findings on the impact of thickness/radius ratio on the acoustic damping attribute of a perforation. The verification of the CFD results are done by comparing the data with analytical solutions.Copyright

Distributed Parameter Acoustic Modeling of a Perforation With Bias Flow

Perforated acoustic liners (screech liners) with bias flow are commonly used for mitigation of thermoacoustic instabilities in augmentors. In addition to cooling the liner, the flow of air thru the liner perforation (dubbed ‘bias flow’) improves the damping effectiveness of the liner thru enhancing its energy dissipation. These liners are currently being designed using empirical design rules followed by build-test-improve steps, basically trial and error. The development of physics-based tools to assist in the design of such liners is of great interest to practitioners. In this paper, the existing work in developing analytical, semi-empirical, and numerical techniques such as Large-Eddy Simulations (LES) in exploring the damping effectiveness of an acoustic liner with bias flow are reviewed. The paper continues with presenting the research in progress that has been conducted by the authors in this area with the goal of expanding the numerical modeling work beyond the current state of the art by including the variables that were not incorporated in previous studies including, but not limited to, hole orientation, combined effect of tangential grazing flow and bias flow interaction with acoustics, and different flow characteristics (Mach and Reynolds number). In addition, the spatial distribution of pressure and velocity over the aperture area (instead of the current practice of averaging these variables) are being looked at.Copyright

Low-Frequency Thermoacoustic Instability Mitigation Using Adaptive-Passive Acoustic Radiators

A commonly used technique for mitigating thermoacoustic instability in an enclosed combustion environment is removing more acoustic energy from the combustor, at the frequency corresponding to the acoustic mode(s) of the combustor which are sympathetic to such instability. This approach is based on adding tuned acoustic damping to the combustion environment. By incorporating in-situ adjustability into acoustic damping devices, they can change their mechanical attributes, e.g., mass and/or stiffness, and adapt themselves in a semi-active manner to the varying instability frequency. Adaptive-passive thermoacoustic mitigation solutions have less weight penalty than the alternative active solutions mainly because the adaptation is done in a semi-active way, at slow pace, with a small and less power-hungry actuation mechanisms. Moreover, the flexibility they offer make them highly desirable for land and marine instability mitigation applications. In this work, semi-active adjustment of a novel tuned acoustic damper, namely an acoustic radiator, is explored. The paper describes the inner working of a semi-active (adaptive-passive) acoustic radiator and the relevant control schemes to adapt them to the instability frequency on hand. The damping effectiveness of the proposed damper, is demonstrated experimentally. It should be mentioned that the semi-active control strategies developed for acoustic radiators can also be used, with minor modifications, for semi-active control of other acoustic damping mechanisms such as Helmholtz resonators and quarter-wave tubes.Copyright

Thermoacoustic Control Modeling

Thermoacoustics can be viewed as the combination of two feedback coupling mechanisms between heat release fluctuation and the acoustics of the combustor: one resulting from the perturbations in velocity causing fluctuation in flame surface area and the other from perturbation in equivalence ratio, both causing perturbation in heat release. As in most feedback system, instability could occur when heat release perturbation is in phase with the acoustic pressure perturbation of the combustion enclosure. The emphasis of this paper is the state space formulation of the acoustics model to be used as part of the thermoacoustics model containing the first two interaction mechanisms.Copyright

Tuned Damping of Floor Vibration

Control of floor vibration, via damping, using a three-degree-of-freedom (DOF) tuned mass damper (TMD) is explored. Three-DOF TMDs can provide as much effectiveness as traditional one degree-of-freedom TMDs while possessing less internal damping than the traditional TMDs. In this work, the use of such a system as a damping treatment for floor vibration is explored. To evaluate the effectiveness of the proposed damping-enhancement system, a scaled floor with the first resonant frequency of around 9 Hz (typical first resonant frequency of most commercial building floors) is fabricated. A 3-DOF TMD, appended to the center of the floor, is tuned to the first mode of the floor to provide damping at that mode. The comparison of the floor vibration without and with this damping treatment exhibited an average reduction of close to 10 dBs in the magnitude of the targeted mode, signifying the high damping effectiveness of the treatment.Copyright