Wednesday 6.03.2024 at 14:00

Wolfson Building of Mechanical Engineering, Room 206

Modeling, Fabrication and Characterization of

Curved Tunable Micro-plates

Asaf Asher

Ph.D. Student, under the supervision of Prof. Slava Krylov and Prof. Rivka Gilat

School of Mechanical Engineering, Tel Aviv University, Tel Aviv, Israel

Bistable curved microplates prone to snap-through (ST) buckling manifest extremely high sensitivity in the vicinity of the critical configurations when a minor change in the external stimuli results in an essentially different response. This feature lies in the foundations of a broad range of bifurcation-based sensors such as threshold accelerometers, optical switches, pressure, gas, and mass sensors.

Here we study both theoretically and experimentally the stability and spectral response of curved circular microplates actuated by a distributed electrostatic force. The aluminum-made plates with thickness of a few hundred nanometers and radii varying between 300 µm and up to 700 µm were micro fabricated by an innovative process combining deep reactive ion etching (DRIE) of silicon wafer and mold-less forming. The plate’s dynamics were investigated theoretically using the Von-Karman theory combined with the reduced order (RO) Galerkin approximation and fully coupled electromechanical finite element (FE) analysis. In the experiments, the electrostatic force containing both steady (dc) and time-harmonic (ac) components was applied to the plate. The responses in air and vacuum were measured using scanning laser Doppler vibrometry (LDV) and the spectral content of the plates, their resonant frequencies, and modal shapes, were obtained. Fabrication-related residual stress in the free-standing Al layer was measured based on the resonant responses of the flat plates and was shown to play an instrumental role in the forming process.   Consistently with RO and FE model predictions, large frequency tunability up to 50% and increased sensitivity to loading of the curved plates were observed in the configurations close to the stability boundaries. In the case of an initially flat plate of similar geometry, frequency tunability of only 2% was achieved at the same voltage. Bistability of the plates undergoing linearly increasing electrostatic loading was also demonstrated in the experiments.

Fig.1 (a) Frequency shift due to the DC voltage increase. Experiment (solid blue line) and FE model (dashed-dot red line) results. (b) Image of fabricated, radius ≈300 μm, Center elevation ≈25 μm and thickness ≈0.4 μm

SCHOOL OF MECHANICAL ENGINEERING SEMINAR

Monday 4.03.2024 at 14:00

Wolfson Building of Mechanical Engineering, Room 206

Evaporation of liquid flowing downward in three parallel pipes

Itamar Bartal

M.Sc. Student, under the supervision of Prof. Dvora Barnea and Prof. Yehuda Taitel

School of Mechanical Engineering, Tel Aviv University, Tel Aviv, Israel

Evaporating flow in parallel heated pipes may occur in power plants such as steam, solar, geothermic or nuclear power plants or in cooling systems such as electronic devices cooling and air conditioning systems.

Parallel heated pipes are used to enhance heat transfer due to their large surface area. However, systems of parallel heated pipes may be subjected to instability problems, flow rate maldistribution and oscillations that may impair their performance.

In the present work evaporation of liquid flowing downward in three parallel pipes with common inlet and outlet headers is studied theoretically and experimentally. Subcooled liquid enters an upper manifold, and the pipes exit is placed somewhat below a water surface open to the atmosphere. Evaporation of liquid in parallel pipes was mainly investigated in co-current horizontal and upward flows. Recently Hayat et al. (2022) studied the behavior of a system of two parallel heated pipes with an upper inlet header. They found that the static instability (Ledinegg) is not sufficient to predict some of the phenomena occurring in such systems since upward flow and oscillations of pressure and flow rates may take place in certain regions of inlet flow rates. As can be expected the degree of complexity of the behavior of three parallel heated pipes increases significantly compared to that of two parallel pipes. This is reflected both in the steady state solutions, where several steady state flow rate distributions may be obtained for the same inlet flow rate and in the various transient trajectories towards the steady state solutions. Cases of upward flow in one pipe and downward flow in the two other pipes may take place as well as upward flow in two pipes and downward flow in one pipe may occur for the same inlet flow rate, resulting in different oscillatory behavior.

Acknowledgments

Support for this project was provided by the ISF (Grant No. 1098/18) and Ministry of Energy (Grant No. 053-11-220).