علیرضا آقایی

The sun is a renewable resource that helps reduce carbon dioxide emissions and reduces the impact of industries on nature pollution. In the present study, the effects of using the vortex generator in different geometrical states in the absorber tube of the parabolic solar collector were investigated using the numerical method and with the help of the finite volume method. Vortex generators were modeled with four geometric states, Case A, Case B, Case C, and Case D, and their effect on different parameters in the output of parabolic solar collector was investigated. According to the results, using the vortex generator increased the heat transfer coefficient and the thermal efficiency of the parabolic solar collector. The maximum increase in thermal performance increased by 65.03% compared to when the parabolic solar czollector was a simple.

In this study, lead zirconate titanate (PZT) and Nb-doped PZT (PZTN) were synthesized by two-step solid-state reaction method. To this end, ZrTiO4 (ZT) was first synthesized at 1450 °C for four hours. In the next step, PZT was synthesized by calcination of appropriate amounts of PbO and ZT. Calcination of (PbO+ZT) was performed at 850 °C. Next, the prepared PZT powder was doped by 0.02, 0.05, and 0.12 mol % of Nb2O5. The cylindrical pellets of the PZT and PZTN powders were sintered at 1300 °C. The electrical properties of PZT and PZTN samples were studied at 1 KHz. The experimental results indicated that upon increasing the amount of Nb up to 0.05 mol % in the PZTN samples, the piezoelectric charge coefficient (d33), relative dielectric constant (εr), and dissipation factor (tanδ) would also increase. However, the increase in the amount of Nb over 0.05 resulted in a negative effect on the electrical properties. The values of the mentioned electrical properties in the 0.05Nb-doped sample were 360 pC/N, 1400, and 2.4, respectively.

The present paper is a comprehensive empirical program of a group of faculty members in University of Sistan and Baluchestan on decreased fraud solutions for e-tests. The group views and experiences were reviewed and shared on a dedicated online platform. In early 2020, shortly after the epidemic of Covid-19 and almost simultaneously with the opening of universities, this platform was established to share the experiences related to applying e-learning methods for courses that were mainly in person so far. This is a qualitative interpretative phenomenology study. Materials, views, measures, and recommendations of a group of faculty members for decreased cheating in online tests were classified under some headings. In addition to the literature review, the most effective solutions to increase awareness and enhance students’ continence and honesty against the tendency to cheat and practical suggestions to reduce students’ fraud in online tests were provided. Research samples included faculty members of basic sciences, education and psychology, and engineering. Hence, however, some proposed strategies and approaches are widely used in a particular educational field and may be less practical in some other specialized areas.

In the present study, the performance of a parabolic trough solar collector using a hybrid nanofluid and its comparison with a mono nanofluid and a base fluid were theoretically investigated based on a thermal model. Therminol VP-1 was selected as the base fluid and Fe3O4 and Nanocomposite Multi-walled carbon nanotubes- Fe3O4 were selected as the nanoparticles. The fluid flow inside the absorber tube of the collector is assumed to be turbulent. The rsults showed that the energy and exergy efficiencies using hybrid and mono nanofluids were higher than the base fluid. However, their amount using of hybrid nanofluid was slightly less than that of mono nanofluid. The highest enhancement in energy efficiency for hybrid and mono nanofluids compared with the base fluid was equal to 2.17% and 2.22% respectively. Also, the highest enhancement in exergy efficiency was 1.49% and 1.58% respectively. On the other hand, the friction factor of mono nanofluid in average was about 7% higher than that of hybrid nanofluid. The results showed that the thermal-hydrodynamic efficiency (performance evaluation criterion) of the collector (performance evaluation criterion) using hybrid nanofluid was higher than that of mono nanofluid.

This investigation evaluates the first and second laws of thermodynamics for a flat plate sheet and tube based solar collector located in Abu-Musa Island (in Persian Gulf) in Hormozgan province, Iran where a suspension of molybdenum disulfide (MoS2) nanoparticles with different shapes in water is the working fluid. The main aim of this study is to analyze morphology effects of MoS2 nanoparticles on heat transfer and entropy generation and due to fulfill this demand, analysis four different nanoparticles shapes (blades, platelets, bricks, and cylinders) are chosen. The results of Nusselt number, outlet fluid temperature, pressure drop, friction factor, performance evaluation criterion (PEC) and entropy generation are calculated and reported for different shapes of nanoparticles and also nanoparticles volume fractions up to 4% in turbulent flow with two different mass flow rates of 0.50 and 0.75 kg/s. According to obtained results, the PEC of model with ϕ=3% and bricks nanoparticle shape in ṁ=0.5 kg/s is found to be the best among all models and its value is around 1.269. But in case with ṁ=0.75 kg/s this value for nanofluid with ϕ=4% and bricks nanoparticle shape is found to be the best among all models and is about 1.182. It was also found that for mass flow rate of 0.50 kg/s, adding the bricks and blade shaped nanoparticles to base fluid is not advantageous from the second law viewpoint, whereas for mass flow rate of 0.75 kg/s it was seen that using volume fractions of 1% and 4% of bricks and blade can reduce the entropy generation rate compared to base fluid till 8%

In this study, the effect of wind speed on the energy and exergy efficiencies of the LS-2 parabolic trough solar collector have been studied. The wind speed on the reflector and receiver has been considered between zero and 27 m/s. Three working fluids of Therminol VP-1, Syltherm 800 and Dowtherm A have been used. The results showed that at the inlet fluid temperature of 650 K and volumetric flow rate of 50 L/min, if the wind speed increases to 27 m/s, the energy efficiency decreases using Therminol VP-1, Syltherm 800 and Dowtherm A are 0.87%, 1.16% and 0.85%, respectively and the exergy efficiency decreases are 0.88%, 1.18% and 0.86%, respectively. Also the results showed that the variations of energy and exergy efficiencies for wind speeds greater than 10 m/s are negligible. The results indicated that although wind speed increase has little effect on the energy and exergy efficiencies of the collector, but the rate of these decreases are comparable with the rate of efficiency increases observed in previous studies using different efficiency enhancement strategies such as turbulators and nanofluids, so wind speed is important. The performance evaluation study revealed that using Therminol VP-1 and Dowtherm A are more suitable than Syltherm 800 to be used as working fluids in parabolic trough solar collectors.

A parabolic dish solar collector system is one of the main types of concentrated solar power systems that are based on point focusing and is more beneficial than the other kinds of concentrated solar power systems. Most of the literature concerning concentrated solar power systems focused on thermal losses and their relationship to the receivers with different geometries. A few former researches have investigated the impacts of the real solar flux distribution on the receiverschr('39') absorber surface in parabolic dish solar collector systems. The inaccuracy level appertaining to the isothermal assumption is more than that of a receiver’s walls with constant heat flux, simply due to heat transfer fluid running through the receiver. On the other hand, the constant heat flux approach cannot be so accurate due to the non-uniform distribution of solar heat flux at receiverchr('39')s internal walls. The current paper investigates the usage of a two-phase nanofluid in a baffled parabolic dish solar collector under a non-uniform distribution of solar heat flux. The geometrical parameters of the collector are analyzed in this work; for this purpose, the SIMPLEC algorithm and Finite Volume Method are employed. The heat transfer fluid is based on water/Al2O3 two-phase nanofluid. The most expected average Nusselt number is achieved at Re = 15,000 in June. In the next section, the effects of different Reynolds numbers and months on the predicted average Nusselt numbers will be investigated in detail. Finally, the PDSC with Z = 70 mm and F = 600 mm filled with nanofluid at  = 4% and dnp = 20 nm is introduced as the most efficient model in the present investigation.

In the present paper, a two-phase simulation of atmospheric aerosol (air, pollution, fine particles of dust, and vehicles pollutants) is carried out to investigate the pollutants dispersion and heat transfer (conduction-convection-radiation) in urban street canyons under haze-fog conditions. Numerical calculations were considered for four time periods during the day, including: 10 am, 1 pm, 4 pm, and 8 pm on the first days of April, July, October, and January. For this purpose, Computational Fluid Dynamics (CFD) method and the simulation are used and dynamic characteristics of airflow and heat transfer have been obtained. The results show that the trend of the ground temperature variation in different months is the same and the maximum value of it has always occurred at 1 pm. In addition, in all the hours and months that examined in the present study, the natural convection for the fluid flow than the forced convection is insignificant, and the main of the fluid flow and pollutants is due to the wind velocity at the inlet boundary condition and the pollutants velocity. In cases with higher windward wall and small aspect ratio between buildings and street, the lowest pollutant concentration in passengerchr('39')s level is achieved.

The flow affected by a magnetic field is applied in cooling electronic devices and voltage transformers and in physical phenomenon such as geology. In this study, the effects of magnetic field on the flow field and heat transfer of Cu-water nanofluid natural convection and by considering the Brownian motion of nanoparticles have been studied in a trapezoidal enclosure. The study has been done for Rayleigh numbers 103 to 1010, Hartmann numbers 0 to 100 and the nanoparticles volume fraction of 0 to 0.04. The governing equations have been solved numerically by use of finite volume method and SIMPLER algorithm. The results showed that by applying the magnetic field and increasing it, the nanofluid convection and the strength of flow decrease and the flow tends toward natural convection and finally toward pure conduction. For all of the Reynolds numbers and volume fractions which are considered, by increasing the Hartmann number, the average Nusselt number decreases.

In this research, the performance, efficiency, and properties of anti-fouling and flux of poly vinylidene fluoride (PVDF) nano-composite membranes with concentration of 15wt.% and 18 wt.%, mixed with different functional  carbon nanotubes (-OH, -COOH, -NH2), were made and studied using phase inversion and normal methylpyrrolidone (NMP) solvent; moreover, the  nano-composite membranes tested for flux, fouling, contact angle, porosity and  protein rejection rate. Also, by using empirical test results, (1) flux and (2) fouling parameters were modeled based on the input variables including nanoparticle percentage, polymer percentage, porosity, contact angle and protein rejection rate. In this model, four intelligent systems including multiple layer percepton, radial basis function, least squares support vector machine and adaptive neuro-fuzzy inference system and  three optimization algorithms including generic algorithm, simulated annealing and particle swarm optimization have been used. The results showed that for both flux and fouling parameters, the best models are GA-RBF and Conjugate-ANFIS with high correlation coefficient. In the next section, modeling was used to obtain optimal values of the best models made for both outputs (minimum fouling and maximum flux) and then the combined algorithm of the genetic algorithm and particle swarm optimization values were obtained. Afterward, by using optimization results for each type of polymer (15wt% and 18wt%), the membranes were made in the laboratory, and then flux, fouling, contact angle and porosity tests were performed, and the results were compared with the results of  the model. Finally, the results showed that 0.07 wt.% single-walled carbon nanotube-PVDF nanocomposite membrane functionalized with hydroxyl group and 0.17 wt.% single-walled carbon nanotube-PVDF nanocomposite membrane functionalized with  hydroxyl group had  the best performance with the polymers of 15 wt.% and 18 wt.% of PVDF respectively.

In this paper, flux and antifouling performance of PVDF (15wt.%) nanocomposite ultrafiltration mambrane with different functionalized carbon nanotubes (-COOH, -OH, -NH2) are considered. Membranes are prepared with phase inversion method using NMP as solvent. This is the first time that hydrophilicity of different nanocomposite membranes are changing gradually as performance of nanocomposite membrane are considered. Gradually, the change of hydrophilicity explaines the behavior and performance of nanocomposite membrane. Analysing surface pore size by SEM showed no significant diffferent in mean pore size (a few tenths of a micron) of nanocomposite memebranes in comparison with pristine memebrans. The cross section morphology showed that all prepared membranes are asymmetric structure with a compact toplayer and a bottom layer with large pore channels. All modified nanocomposite membranes show better hydrophilicity surface in comparison with pristine PVDF membrane (87°). The results of pure water flux are approved by the results of contact angle and porosity which have been measured. Pure water flux of nanocomposite membranes with optimized carbon nanotube amount is increased more than twofold to pristine membrane (146 L/m2.h). AFM also revealed that nanocomposite membranes have smaller roughness in comparison with pristine membrane (Sa=270 nm). This subject is shown that functional carbon nanotubes have not any detrimental effects on mechanical resistance in modified membranes.

In this work, the fluid flow and heat transfer and entropy generation of mixed convection in "Γ" -shaped cavity filled with Al2O3-Water-EG have been studied. For this propose, we used from a FORTRAN program based on finite volume method (FVM) and SIMPER algorithm. This investigation have done for volume fraction from 0 to 0.04, Richardson number between 0.01 and 100, and Grashof number equal to 104 for two situation, moving top wall with velocity of U from right to left and moving left vertical wall equal to V from up to bottom. In all cases average Nusselt number increase with adding of nanoparticle or decreasing of Richardson number. This phenomenon is sensitive with reduction of Richardson number. In addition for all Richardson number and for two cases total entropy generation and entropy generation due to conduction increase with nanoparticle volume fraction.

In the present research, stabilization of premixed flames with porous media is studied experimentally. First, for comparison, the effects of flame speed, equivalence ratio, and thermal power on the stability limit of a premixed free flame are investigated. Furthermore, the stabilization of the premixed flame with Al2O3 porous media is studied. Porous aluminum oxide ceramics with pore densities of 10, 20 and 30 ppm are used in the burner. Experiments are performed for 5 and 8 cm distances between the porous medium and the mixture outlet. The results show that the flame stability limit is independent of pore density and the flame is formed in the porous medium at all conditions at an equivalence ratio of about 0.55. Moreover the increase of porous medium distance from the burner causes the stability boundary in higher equivalence ratios. The amount of pollutants depend on distance and type of the porous medium. It is observed that the use of porous medium reduces NOX and increases CO in comparison with the free flame burner.

The monolithic oxynitride glasses in SiO2-Al2O3-B2O3 system were synthesized using sol-gel method. The porous, dried gels were heated in flowing ammonia for different times and varying temperatures. The nitrogen content of the oxynitride glasses was determined using an oxygen-nitrogen analyzer. The chemical bonding state and properties of glasses were investigated by means of Fourier transform infrared spectroscopy, dilatometry, and Vickers hardness methods. The crystallization behavior of glasses was studied with X-ray diffraction. Spectroscopic evidence indicated that nitrogen was chemically dissolved in the glass network. The results showed that the chemical composition of the glass has an effect on the nitridation processes. The nitrogen content of the glasses was increased to 3.8% wt. by increasing the soaking time of nitridation from 2 to 15h. The dilatometric softening point temperature of the glasses was increased approximately 25°C, the micro-hardness value reached 10.89 GPa and thermal expansion coefficient was decreased from 3.88×10-6 to 3.39×10-6 after the nitridation process at optimal composition. The X-ray diffraction pattern of the glasses confirmed that nitrogen does not enter the crystalline phase.

In this paper, forced turbulent convection flow and heat transfer of air inside a desert helicopter cabin was studied. The main goal was to provide human thermal comfort, by using cooling system, for a desert helicopter pilot in summer. A body subdomain was considered around the pilot that includes the pilot's using area in the cabin. The governing equations were numerically solved by the control volume approach based on the SIMPLE technique and standard k-ε turbulent model. The effects of air supply parameters velocity and temperature on pilot's thermal comfort are presented and the optimization was carried out to find the best case with the minimum predicted percentage dissatisfied (PPD). It was found that temperature and velocity of supply air have a remarkable influence on thermal comfort characteristics. The air temperature and air velocity in pilot's body subdomain decreased and increased respectively, by increasing supply air velocity. Finally, the predicted mean vote (PMV) and PPD indexes were calculated for different supply air performances and the minimum PPD was obtained for the case with supply air temperature and velocity of Tin=10 °C and Vin=0.4 m/s, respectively.

In this study, the effects of using corrugated absorber plate on heat transfer and turbulent flow in solar air-heater collectors were numerically investigated. The two-dimensional governing equations were solved by utilizing finite volume method and SIMPLE algorithm. The absorber plate with triangle, rectangle and sinuous corrugations in turbulent flow regime and Reynolds number was considered within the range of 2500-4000. Proper geometry was selected based on the best performance evaluation criteria (PEC) and increasing the air temperature from collector inlet to outlet (ITIO). Simulations were performed for two different tilt angles of collector, which are the optimum six month tilts for flat plate solar collectors in Kashan. The results revealed that using corrugated absorber plate has a considerable influence on the flow field and heat transfer. For the whole year the highest PEC was obtained for the sinusoidal corrugated model, however, the highest ITIO was observed for rectangular corrugated model. To achieve best ITIO and the highest PEC, the optimum Reynolds number of 2500 was also noticed.

Common fluids such as water, ethylene glycol and oil have low thermal conductivity compared to metals and metal oxides. Hence, in recent decades, use of nanofluids has received significant interest. These fluids are colloidal of a base fluid (such as water or oil) with metal, ceramic or polymer nanoparticles. In this study, multi walled carbon nanotubes/water nanofluid in a forced convection heat transfer was investigated numerically in turbulent flow through a curved tube (180°) with constant wall temperature. Effects of Reynolds number and nanoparticles volume fraction on convection heat transfer were considered. Results showed that with increase in Reynolds number from 4000 to 6000, convection heat transfer coefficient was enhanced by 24%. Also, with increasing the volume friction of nanoparticles up to 0.01, convection heat transfer coefficient increased by 21% compared to the base flow.

In this experimental study dynamic viscosity of hybrid engine oil (5w-50)-Cuo-MWCNT nanofluid for volume fractions of 0.05, 0.1, 0.25, 0.5, 0.75 and 1 percent of nanoparticles for temperatures of 5, 15, 25, 35, 45, 55 °C has been measured. This hybrid nanofluid has been prepared utilizing the two steps method. For viscosity measurement, the Brookfield viscometer has been used. The experimental measurments indicate that by increasing volume fraction of nanoparticles the viscosity increases; also by increasing the temperature the viscosity decreases. Based on the experimental results the maximum and minimum viscosity increases with volume fraction increase from 0.05 to 1 at a constant tempearture are 35.52 and 12.92 percent, respectively, relating to 55 and 15 °C. Measurement of the nanofluid viscosity with different volume fractions, shear rates and tempeartures indicate its Newtonian behavior. A new temperature and volume fraction dependent viscosity correlation, developed in this study to be used in numerical simulations, shows very good agreement with experimental results.