Journal of Transport Phenomena in Nano and Micro Scales
Volume:8 Issue: 2, Summer-Autumn 2020

Crack is one of the common types of defect in sensors that may cause system failure. In this paper, crack fault detection is considered in piezoelectric sensors. Piezoelectric sensors are assumed in micro-scale and cantilever-based MEMS sensors. Therefore, the usual methods used for macro-scale systems and FEMs to find natural frequencies cannot be used. To find the natural frequencies of the piezoelectric sensors, Modified Couple Stress Theory (MCST) and the Hamilton principle are used. Crack is modeled with a torsional spring whose stiffness depends on the depth and location of the cracks and the material length scale parameter. The PSO optimization algorithm is used to find the depth and location of the crack in the sensor. The results of optimization indicate the proper performance of the Particle Swarm Optimization (PSO) algorithm for detecting the crack in piezoelectric sensors. The results of the PSO algorithm are accurate for cracks near the fixed end of the sensor and are acceptable for cracks near the free end.

Many studies have shown critical heat flux (CHF) enhancement by adding metals and nonmetals nanoparticles to water in flow boiling. In this study, we investigated critical heat flux enhancement mechanism by adding multi-walled carbon nanotubes (MWCNTs) to pure water in flow boiling in a 2 m horizontal tube under atmospheric pressure. Also, the feasibility of MWCNTs-GA/water nanofluid as an advanced and economic coolant was assessed for cooling the high power thermal systems. For preparing this nanofluid, gum Arabic as a surfactant and MWCNTs as nanoparticles were used in the ratio 1:1 with concentrations of 0.001, 0.005, 0.01 wt%. The measuring zeta potential showed nanofluid stability. Results indicated the relative stability of suspension in all concentrations. Also the results of the experiments showed that the critical heat flux of the nanofluid increases with increasing in concentration and mass flux at the inlet temperatures of 60 and 70 °C. The CHF enhancement was observed for nanofluids and it was greater than that for pure water. It is due to the deposition of MWCNTs nanoparticles and improvement of wettability in the heat transfer surface. The maximum CHF enhancement was observed at 0.01 wt% concentration, a mass flux of 620 kg/m2s and the inlet temperature of 60 oC. The local exit equality of the nanofluid in a fixed mass flux at the inlet temperatures was less than that for pure water and the lowest local exit equality of nanofluid was at inlet temperature of 60 °C and concentration of 0.001wt%.

Today, green synthesis method for manufacturing of the nanoparticles (NPs) is the common rout for biomedical application. In this article, ZnFe2O4 NPs are fabricated by solgel method in the presence of surfactants. The samples are studied by x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), field effect scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM), x-ray fluorescence (XRF) and fourier transform infrared spectroscopy (FTIR). The XRD results show hexagonal wurtzite structure of ZnO and spinel phase of rhombohedral α-Fe2O3. The crystallite size of annealed samples are calculated around 57 nm. The SEM images show that the NPs change from rod-shape to nanoparticle clusters of ZnFe2O4 by increasing temperature after annealing. The TEM studies show the formation of Fe2O3 shell around NPs with a diameter of 10 nm for as-prepared samples. The sharp peaks in FTIR spectrum indicate the stretching vibrations of Fe and Zn groups in the frequencies of 675, 602 cm-1 and 476 cm-1. The results of magnetic measurements show coercive field and saturation magnetism around 2298 G and 34 memu/g respectively, for as-prepared samples. The XRF analysis demonstrate the decreasing of the Fe weight percent from 41.28 %Wt. to 39.64 %Wt., by increasing temperature.

In this research, a flat plate solar collector with a triangular geometry, which is designed and built by the author and based on the ASHRAE93-2010 standard, has been experimentally studied. The triangular solar collector has no riser and has spiral tubes containing fluid from the inlet to the outlet of the collector. To study water and Al2O3-water nanofluid in concentrations of 0.1, 0.2 and 0.3% and different flow rates from 0.0063 to 0.0378 lit/s have been used, which has been studied experimentally based on environmental criteria such as temperature and radiation and effective thermal parameters such as concentration and flow. Studies show that the triangular solar collector, due to its stable geometry and due to the reduction of the tube length in relation to the area, has a suitable performance for water heaters and can also be used. Time constant for triangular solar collector was between 2.5-4.5 minutes in flow rate ranges. The average efficiency increase when using nanofluids instead of water was about 12.4% and the maximum efficiency obtained at a concentration of 0.3% and a flow rate of 0.0378 was more than 68.3%. Also, in measuring the pressure drop in the collector, it was found that the pressure drop was very small and less than 0.1 bar.

In this paper, by employing the Computational Fluid Dynamics (CFD) and applying the NSGA II algorithm, the multi-objective optimization of frost formation in the interrupted Micro Channels Heat sinks (MCHS) is investigated considering microfluidic effects in slip flow regime. For numerical modeling, basic equations of humid air and frost including continuum, momentum, energy and phase change mechanism are numerically solved and results are compared with reported data and good agreements are observed. Knudsen number (Kn) is changed so that slip flow regime requirement is accomplished. The design variables are geometrical parameters of MCHSc. In the results section, the Pareto front, which simultaneously displays the changes of the heat transfer rate and the frost formation, will be presented, and it will be demonstrated that the Pareto front conveys very important results for the two phase thermal designing of MCHSc. Finally, the multi-objective optimization results computed in this paper are compared with the CFD data and very useful and valuable information is obtained.

This study investigates the effect of adding SiO2 nanoparticles to the amine solution on the mass transfer coefficients with the aim to obtain effective solvent in the CO2 capturing process. An aqueous mixture of the methyl di-ethanol amine (MDEA), activated by blending with Piperazine (PZ) (a-MDEA), was considered as base absorption solvent. The addition of SiO2 nanoparticles with six different concentrations to the base absorption solvent was then studied. The absorption process took place in an agitated batch reactor at 40 ºC. Considering the gas pressure drop in the reactor the CO2 absorption and mass transfer rates were obtained for all types of the solvents and were compared with each other. Results revealed that two regimes of fast and slow were involved in the CO2 absorption process, where the overall mass transfer of the fast regime was more than 100 times that of the slow one. The results indicated that the liquid phase controlled the mass transfer in the CO2 absorption process. Furthermore, the maximum increase in the absorption rate was happenned when the concentration of SiO2 nanoparticles was set at 25 ppm in the solvent. In this regard, the enhancement was at least 85% increase in the overall mass transfer coefficient. The findings of this study can be used to design new nanoparticle-based solvents and reduce the cost of the CO2 capturing process.

In this paper, the fully developed non-ideal gaseous slip flow in circular,parallel plates and rectangular microchannels is analyzed analytically by using Navier-Stokes equations to obtain the analytical exact solution.Van der Waals equation is used as the state equation of non-ideal gas. It is assumed that the flow is isothermal, incompressible, steady state, two-dimensional and fully developed,slip flow regime with consideration the first and second orders boundary conditions. It is developed the models for predicting the local and mean velocity, normalized Poiseuille number,and the ratio of density for the first and second orders boundary conditions.The results show that the rarefication process and Poiseuille number are a function of the Knudsen number and the cross-section geometry and Poiseuille number is independent of fluid material properties, velocity, temperature.Also, for circular microchannel,the rarefication process occurs faster than the others. Keywords: Analytical solution, Non-ideal gaseous slip, Microchannel In this paper, the fully developed non-ideal gaseous slip flow in circular,parallel plates and rectangular microchannels is analyzed analytically by using Navier-Stokes equations.The Van der Waals equation is used as the state equation of non-ideal gas. Also, it is assumed that the flow is isothermal, incompressible, steady state, two-dimensional and fully developed,slip flow regime with consideration the first and second orders boundary conditions.It is found that, density to inlet density ratio in non-ideal gas is different than ideal gas in length of microchannel under the same flow conditions and the type of microchannel geometry affect on gas behaviors. Keywords: Analytical solution, Non-ideal gaseous slip, Microchannel