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In this paper, a new method for microbubble generation independent of porous media has been introduced. The approach used in the microbubble generator is based on high turbulent intensity rotational flow. At first, a simple cylindrical model is investigated experimentally. In the model, water enters the device via six diagonal nozzles and mixes with air coming from six holes at the bottom of the device. Passing a rotational trace around central motionless rod, air particles are broken up to tiny bubbles. Size of bubbles is measured and bubbles smaller than 1mm in diameter were found. Finally, improved microbubble generator with the similar mechanism has been introduced. Internal flow passes longer trajectory around the central rotating hub because of its design characteristic that let it rotate by inlet water excitation. Bubbles with diameters less than 300 microns are generated by this method. Size of bubbles is measured experimentally in different void fractions for different outlet gap size. Outlet flow gap changes from 20mm to 8mm and 3mm in three experiments. The best result is observed in the third experiment at which microbubbles smaller than 100 micron are observed. The desirable bubbles’ size is achieved at void fraction of 15% that could be increased up to 60%.

Heat transfer in a thermally-positioned doubly-clamped bridge, at the micro- and nano-scale, is simulated to investigate the effect of convective cooling on the mechanical response of the system. The mechanical response of the system is defined as the displacement at the center of the bridge. The heat conduction equation is solved numerically using a finite difference method to obtain the temperature distribution in the bridge. Then, thermal stress due to the temperature difference with respect to the wall temperature is calculated. The thermo-structural equation is solved numerically to get the displacement along the beam. Two systems are compared: one doubly clamped beam with a length of 100 microns, a width of 10 microns, and a thickness of 3 microns, and a second beam with a length of 10 microns, a width of 1 micron, and a thickness of 300 nanometers, in air at a pressure from 0.01 Pa to 2 MPa. Conduction within the beam as well as convection between the beam and the gas are considered. A constant heat load with respect to the time is applied to the top of the beam varying from 10 to 600 μW/μm2. The simulations use both free molecular and continuum models to define the convective coefficient, h. The simulations are performed for three different materials: silicon, silicon carbide, and diamond. The numerical results show that the displacement and the response of thermally-positioned nano-scale devices are strongly influenced by ambient cooling. The displacement depends on the material properties, the geometry of the beam, and the Biot number. In the free molecular model, the displacement varies significantly with the pressure at high Biot numbers, while it does not depend on cooling gas pressure in the continuum case. The significant variation of displacement starts at Biot number of 0.1 which occurs at gas pressure of 27 KPa in nano-scale. As the Biot number increases, the dimensionless displacement, δ* = δk/q″αl2 decreases. The displacement of the system increases significantly as the bridge length increases, while these variations are negligible when the bridge width and thickness change. Thermal noise analysis shows silicon carbide has the most physically meaningful displacements in comparison with silicon and cvd diamond.

Heat transfer in a thermally positioned doubly clamped bridge is simulated to obtain a universal scaling for the behavior of microscale and nanoscale bridge structures over a range of dimensions, materials, ambient heat transfer conditions, and heat loads. The simulations use both free molecular and continuum models to define the heat transfer coefficient, h. Two systems are compared: one doubly clamped beam with a length of 100 μm, a width of 10 μm, and a thickness of 3 μm, and a second beam with a length of 10 μm, a width of 1 μm, and a thickness of 300 nm, in the air at a pressure from 0.01 Pa to 2 MPa. The simulations are performed for three materials: crystalline silicon, silicon carbide, and chemical vapor deposition (CVD) diamond. The numerical results show that the displacement and the response of thermally positioned nanoscale devices are strongly influenced by ambient cooling. The displacement depends on the material properties, the geometry of the beam, and the heat transfer coefficient. These results can be collapsed into a single dimensionless center displacement, δ* = δk/q″αl2, which depends on the Biot number and the system geometry. The center displacement of the system increases significantly as the bridge length increases, while these variations are negligible when the bridge width and thickness change. In the free molecular model, the center displacement varies significantly with the pressure at high Biot numbers, while it does not depend on cooling gas pressure in the continuum case. The significant variation of center displacement starts at Biot number of 0.1, which occurs at gas pressure of 27 kPa in nanoscale. As the Biot number increases, the dimensionless displacement decreases. The continuum-level effects are scaled with the statistical mechanics effects. Comparison of the dimensionless displacement with the thermal vibration in the system shows that CVD diamond systems may have displacements that are at the level of the thermal noise, while silicon carbide systems will have a higher displacement ratios.

The Time Transformation Method in ANSYS CFX is investigated as an efficient substitution to Transient Rotor Stator (TRS) analysis for rotating stall prediction in a centrifugal compressor stage. The computational study was performed by varying the number of blade sectors to determine how the circumferential extent of the computational domain affects the accuracy of the stall prediction. The results obtained using a minimum number of blades, approximately one-quarter the full blade count, and approximately one-half the full blade count were compared to both TRS and steady simulations on the same mesh to characterize the predictive capability of each approach. It is shown that both steady and unsteady methods are able to predict the formation of stall cells, but significant qualitative and quantitative differences exist in the flowfield results. The largest mass flow rate at which rotating stall was captured and the number of stall cells were in good agreement with the experimental data.