مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 5 (امرداد 1400)

Concerns about the usage of fossil fuels have led to more attention to renewable energies such as wind energy. Utilization of wind turbines in the urban industry is one of the challenging topics in wind energy area. Particularly, due to the limitations of the wind conditions in the urban areas (i.e. low speed wind, high level of variation of wind direction and high turbulence level) and the space limitations within the cities, utilization of vertical axis wind turbines in micro scales have become of interest. The focus of the present study is on these types of turbines and assessing the parameters affecting their aerodynamic performances. Two types of vertical axis wind turbines (Savonius and a straight blade Darrieus) were designed and constructed in micro-scale. The effect of wind speed on the Savonius turbine is experimentally studied. Results show that this turbine performs better at lower wind speeds. The effect of the aspect ratio and the vertical position of the blade struts on the performance of the Darrieus turbine is also experimentally assessed. The results show that the best vertical position for struts is the tip of the blades. It was also experimentally observed that the turbine with aspect ratio one has the best aerodynamic performance.

A way to increase the generated power of an available wind turbine blade without changing its base shape is to add proper add-on to the blade tip. In this paper, seven tip add-ons are added to the blade tip of the NREL Phase VI wind turbine, and their effect on generated power is studied using computational fluid dynamics. Reynolds averaged Navier-Stokes equations are used with k-ω SST turbulence model to simulate the flow over the blade. Results show that the tapered tip add-on does not have a notable effect on generated power, while the shark-tip add-on increases the output power by about 4%, which is a minor increase comparing to the other add-ons. The suction surface and pressure surface winglets (without sweepback) increase the power generated by 5.23% and 9.6% respectively, which shows the superiority of pressure surface winglet over suction counterpart. Afterwards, sweepback is added to winglets, showing 11.87% and 13.25% power increase for suction surface and pressure surface winglets respectively, which shows the positive effect of sweepback angle in generated power increase. This is obtained by only a 28 cm add-on to the base blade with a radius of 553 cm.

The calcium looping process is considered a promising technology to CO2 capture emissions from combustion plants in recent decades. To model this process, the bed hydrodynamics as well as the sorbent characteristics will affect the calcium looping efficiency. In this study, CaO/Al2O3 sorbent is first synthesized by sol-gel method and then its performance is compared with pure CaO sorbent through 20 carbonation/calcination cycles. In addition, a general model based on bed hydrodynamics as well as sorbent properties for this process is presented and then the influence of parameters such as superficial gas velocity, carbonator height and sorbent inventory on process efficiency is investigated. Thermogravimetric experiments reveal that CaO/Al2O3 sorbent preserves 73% of its activity at the end of 20 cycles, whereas it is obtained as 21 for pure CaO sorbent. The results obtained from modeling show that the adsorption efficiency is decreased from 78.69 to 22.68% for pure CaO, whereas, it is decreased from 86.5 to 74.1% for modified CaO/Al2O3 sorbent. Finally, by studying the affective parameters it is obtained that the solid inventory has a significant impact on the process efficiency while the gas velocity and the height of the carbonator are far less effective.

Artificial vein graft is one of the most commonly used surgeries in the human body, in which the stenosis is replaced with an artificial prosthesis. The mechanical behavior of this prosthesis must be very close to the normal behavior of the vein in order to have an appropriate operation. The carotid artery is one of the major arteries in the blood supply to the human brain. In this paper, the effect of blood fluid on natural and prosthetic vessel walls in normal and occluded cases has been analyzed. Blood flow as a non-Newtonian fluid in the carotid artery has been simulated using ANSYS CFX software. According to the obtained results, the stenosis increases the velocity, shear stress, von Mises stress, deformation as well as local pressure reduction in the occlusion zone. Maximum value of deformation and von Mises stress occurs near bifurcation in the common carotid artery. Then Dacron and polyurethane polymers have been used as replacements for natural carotid artery and von Mises stress and deformation values have been calculated for these polymers in the normal and occluded cases. According to the obtained results, usage of Dacron polymer as a replacement for the natural carotid artery is more appropriate than polyurethane.

In this paper, the air flow field around buildings is simulated to predict the dispersion of the fine solid pollutants. The large eddy simulation approach has been used to model the turbulence flow. In the first part of this research, a simple model including a building has been simulated and the obtained results are compared and validated with the experimental data obtained from a wind tunnel test. By setting the optimal parameters of the numerical model in the first part, in the second part, an area of Tehran city with high-rise buildings and irregular urban layout is considered and the velocity field and deposition of the contaminant particles in this model are also simulated. The result obtained in the first part of show good agreement with the experimental data and in both models the effect of some variables like the arrangement of buildings (in urban model) and the wind velocity are investigated. To analyze the value of the pollutant concentration in the urban area at each time, the integral of this variable on some important surfaces has been calculated over time and the effect of urban area layout on the integration is discussed.

This research represents the dual blanket structure and liquid metal fluid hydrodynamic characteristic under magnetic field. Numerical study of the flow inside the blanket which is separated by separator structure from the shell side is done. This structure is used for both thermal insulation and pressure drop reducing mechanism. As the fluid has electrical conductivity properties, magnetohydrodynamic analysis is also done. In the current study following analyses are done: magnetic field effect, wall electrical conductivity, baffle thickness and its distance from wall in on pressure drop, as well as explaining the behavior of velocity profile under magnetic field changes. According to the result increasing the magnetic field from 0.4 T to 1 T increasing the pressure drop by 4 times the initial value. Also, reducing the electrical conduction in the separating wall from 500 S/m to 5  S/m reduces the pressure drop by 35%. Studies on the different thicknesses of the separator structure in 16 different cases with constant distance between the separator and the wall does not have a significant effect on the pressure drop but by increasing the distance between the separator and the wall the pressure drop will decrease and consequently decreasing in pumping power.

In this paper, an efficient lattice Boltzmann method is applied for the simulation of two-phase flow problems at high density and viscosity ratios. The present lattice Boltzmann method employs the Allen-Cahn equation to model the interfacial dynamics between two phases and an appropriate collision operator is implemented to ensure the stability of the numerical solutions. The performance of the numerical algorithm is examined by studying droplet dynamics at different flow conditions. Herein, the equilibrium state of a droplet on the flat and curved walls is verified by considering the wetting properties, namely the hydrophilic and hydrophobic characteristics, for solid surfaces. The multiphase flow pattern and interfacial dynamics of an impinging droplet on a cylinder surface and a semicircular cavity are also investigated and the obtained results are compared with the available data. The present study demonstrates that the curved wall considering the wettability effects significantly affects the droplet dynamics, depending on the properties of the liquid phase and the flow conditions. This work also shows that the lattice Boltzmann method with the Allen-Cahn equation is more stable for simulation of liquid-gas systems at density ratio 1000 and viscosity ratio 100 which makes this method more suitable for predicting practical flow characteristics.

In this paper, nonlinear simulation of viscous fingering instability of miscible displacement involving nanofluid is investigated. Using vorticity and stream functions and the spectral method governing equations are obtained. Due to the fractality of fluid-fluid interface in instability phenomena, by using box counting method, its fractal dimension is calculated in different parameters such as deposition rate, mobility ratio and diffusion rates. The results show that increasing the deposition rate reduces the complexity of finger patterns and the diffusion rate of nanofluid has no effect on complexity of finger patterns while increasing the diffusion rate of displaced fluid has significant effect on patterns and makes it more complicated. The fractal analysis also shows that the effect of mobility ratio depends on the deposition rate.  By considering deposition rate, although the mobility ratio has no effect on fractal dimension and effective time is constant and equal to 275, start time of instability is delayed by 25 units. It can be concluded that fractal analysis of viscous fingering phenomena can be considered as one of the instability characteristics.

This paper aims to compare the electrical and thermal performance of different designs of hybrid photovoltaic-thermal collectors. The main advantage of photovoltaic-thermal collectors in comparison to common photovoltaic modules is decreased cell temperature and an associated increase in their electrical efficiency. In addition, the combination of photovoltaic module and solar thermal collector makes it possible to produce both heat and electricity in a single device and reduces the area required for collector and module installation. In this research, the electrical and thermal efficiency of different designs of photovoltaic-thermal collectors is investigated. The heat transfer fluid considered for heat dissipation is water. A theoretical analysis of eight types of different photovoltaic-thermal collectors, including sheet and tube with spiral (circular cross-section) and parallel tube (circular, square and rectangular cross-sections) designs were implemented based on thermal modeling. These include collectors with different flow paths and different cross-section geometries. According to the results, sheet and tube design with circular cross-section has minimum and sheet and tube design with rectangular cross-section has maximum thermal and total efficiency. Also, glass cover reduces the electrical efficiency and increases the thermal efficiency and total thermal energy.

The advantage of micro thermophotovoltaic systems is the direct conversion of heat energy into electrical energy without any moving parts. For an adequate performance of thermophotovoltaic systems, uniform and high temperature along the micro-chamber wall is required. In the present study, a laminar premixed combustion of hydrogen-air in a micro porous chamber is studied. Non-equilibrium thermal condition between gas and solid phases and radiative transport equation in solid phase is considered . Using numerical simulation, the effect of several parameters on the radiation efficiency of thermophotovoltaic system including equivalence ratio, porosity, porous thermal conductivity and inlet mixture velocity have been studied. The results show that increasing the equivalence ratio up to 1 increases the wall temperature and increasing the thermal conductivity of the porous medium, results in a more uniform temperature distribution. Also decreasing the inlet velocity, porosity and thermal conductivity of the porous medium increases the system's radiation efficiency. The convection heat transfer between the gas and solid phases inside the porous and the radiation and conduction heat transfer in the porous for the porosity of 0.4 and 0.8 were compared and it was shown that the role of radiation heat transfer inside the porous is negligible.

Following concerns about air pollution and global warming in recent years, the use of heavy duty gas engines has become favorable in major industries such as the marine industry, power plants, etc. Heavy duty diesel engines designed for similar applications were used and made by modifying their structure or adding new parts or a combination of the two approaches, because of the fact that heavy duty gas engines are more similar to diesel engines with similar emissions. They have less emission and also less power. Various technologies can be used to increase the power of the gas engines. One of these new technologies is the pre-combustion chamber, which results in an increase in power output. The pre-chambers are categorized into two types of refueling in terms of how the refueling is shared with the main chamber and refueling independent of the main chamber. The pre-combustion chamber should be used to increase the efficiency of the pre-chamber and overall engine efficiency, which is suggested after considering the use of the pre-combustion chamber with a stand-alone fuel system.

The present study is conducted to find a compatible combustion model, in case of single room compartment fire. The large eddy simulation was used with one-equation sub-grid scale turbulence model by steady laminar flamelet and eddy dissipation models were acquired as the combustion model. OpenFOAM solver based on C++ programming language was developed to use the flamelet model. The benefit of the flamelet model employment than the eddy dissipation model was regarding the lower computational cost which was about 14 percent lower in this case. Moreover, steady laminar flamelet model considered the detailed chemical kinetic of GRI 3.0, however, eddy dissipation model treated the chemical kinetics of the model with an irreversible single-step Arrhenius global reaction which is only able to estimate the main products of combustion. Deviations of velocity and temperature at the doorway showed that the steady laminar flamelet model predictions were accurate with an uncertainty error of 3.3 % for temperature and 8 % for velocity, respectively. Prediction of the temperature inside the room with a steady laminar flamelet model was estimated to have 3.2 % accuracy.

In this paper, the superconducting carbon dioxide cycle is re-examined and compared from the perspective of advanced and thermoconomic exergy analysis to identify real potentials and prioritize the improvement of cycle components. In advanced exergy analysis, in addition to calculating the total exogenous exergy destruction for each component, the contribution and effect of each of the other components and their combination in causing this inefficiency have also been identified. In thermoeconomic analysis of the system, the unit cost of the product, the cost of investment and the cost of destroying the exergy for the components of the system are calculated. Improvements based on advanced exergy analysis are assigned to high temperature recuperator, turbine, compressor 1, preheater, low temperature recuperator, compressor 2 and reactor, respectively. Also, based on thermoeconomic analysis, improving the turbine and reactor is not economically justified. However, the results show that even by abandoning the improvement of these two components, due to their high economic cost and by improving other components of the cycle based on the prioritization of advanced exergy analysis, it is possible to increase the efficiency of the exergy cycle from 4/29/47. There is 63% to 47.4% and cycle energy efficiency from 34.15% to 45.84%.

In this study, two new multi-generation (hydrogen, power, heating) systems are thermodynamically analyzed and optimized. For the proposed cycles, the two systems are distinguished by the power generation cycle, so that the organic Rankine cycle and the Kalina cycle are used to produce power. Both systems also use domestic water heater for heating and proton exchange membrane electrolyzer for hydrogen production. After the thermodynamic simulation, a comprehensive study was performed for evaluating the parameters affecting hydrogen production, net output power, heating, thermal efficiency and exergy efficiency of two cogeneration systems and finally, an optimization was performed from an exergy efficiency point of view. According to the results of this study, for the organic Rankine cycle-based tri-generation system, when evaporator temperature increases exergy efficiency and hydrogen production show optimum values while for Kalina cycle-based tri-generation system, hydrogen production and exergy efficiency increase. Also, according to the study of various operating fluids for the organic Rankine cycle, the R152a as an organic Rankine cycle fluid produces more hydrogen. Furthermore, based on the optimized results for 120 °C heat source temperature, the Kalina cycle-based tri-generation system has more exergy efficiency and more hydrogen production than the organic Rankine cycle-based tri-generation system.

In the present research, a novel bi-evaporator combined cooling and power cycle based on the integration of a double-stage organic Rankine cycle and an ejector refrigeration cycle is devised to recycle heat from the exhaust gas of a marine diesel engine. Instead of using conventional pure organic fluids, various appropriate zeotropic mixtures are screened for the proposed system and the results are discussed in terms of the first and second laws of thermodynamics. The results indicated that by recycling 434kW energy from the exhaust gases and using R142/Pentane with 51/49 percent a maximum thermal efficiency of 43.28% and an overall cooling load of 166.36 kW can be achieved. In this case, the net produced electricity and exergy efficiency are obtained as 21.83 kW and 20.22% which can be increased by selecting other appropriate mixtures. Additionally, using R142/Pentane with 51/49 percent as the working mixture, it is figured out that the auxiliary vapor generator contributes to the highest exergy destruction by 62.3 kW out of overall exergy destruction of 122.15 kW. Also, the energy and exergy efficiencies of the system can be increased simultaneously by increasing the evaporator bubble point temperature.

Many researchers are interested in powering sensors and electrical circuits in wireless networks through energy harvesting from environmental waste energies. In this study, a flexible nanogenerator is designed and fabricated based on the reverse electrowetting concept. The performance of the nanogenerator has been investigated in different conditions including various bias voltage, different excitation frequency, and several external loads. The nanogenerator comprises of water droplets, as a strong dipole fluid, and two dielectric layers; polymethylsiloxane polymer. The latter has good hydrophobicity and flexibility. These two dielectric layers are formed on the surface of copper electrodes by using a spin coater. It is shown that increasing the excitation frequency augments the generated power to some extent that the capacitor is not fully discharged. The nanogenerator power output increases with the external load up to equality between the external load and the nanogenerator's internal resistance. The results show that the fabricated nanogenerator can generate a power density equal to 1.08 W/m2 using 1 ml water droplet, 7V bias voltage, and an excitation frequency of 1 Hz.

In the present study, a single-cylinder research engine was utilized to capture experimental data at 9 compression ratio and 1800 rpm engine speed for dual fuel mixtures of 100%, 90%, 75% and 60% gasoline and the rest natural gas in skip-fire mode. Then, a gasoline- natural gas multi-zone thermodynamic entrainment simulation-code equipped with blow-by sub-model was developed. Two 200-cycle sets of free residual motoring and firing cycles were separated from the experimental data to check the response of the code. In motoring-case, the ensemble-average P-θ of the motoring set was compared with that of the code and the blow-by sub-model was verified. Next, in the firing-case, the results obtained from the code were compared with the ensemble-average P-θ of the firing set in each fuel combination and the code was validated. In the firing-case, the leakage to crevices was estimated to be considerably more than that of the motoring-case. In the firing mode of the code, the deviation of the obtained results of the code without blow-by option from the experimental results was more serious as compared to those of the code with blow-by, emphasizing the importance of the blow-by sub-model in the code.

One of the concerns about the operation of underfloor air distribution systems is the occurrence of local thermal discomfort such as draught and vertical air temperature difference. In the present study, to investigate these discomfort parameters in a room with the mentioned system, the temperature, mean velocity and turbulence intensity for vertical and horizontal supply air diffuser with 16 and 20°C inlet temperatures were evaluated. These parameters were measured at 0, 30 and 60cm distances from the center of diffuser of the underfloor air distribution system at 8 different heights from the floor. Also, by using the Fanger model, thermal sensation, percentage of thermal discomfort and the percentage of thermal discomfort caused by the draught were determined. The results indicated that the amount of thermal discomfort in a room with an underfloor air distribution system is significantly dependent on supply air direction (horizontal/vertical) and, distance from the inlet diffusers. Also, the results showed that by using proper diffusers, while subjects’ thermal comfort maintains constant, the supply air temperature can be increased from 16 to 20 °C, which is a major step towards reducing energy consumption.

The present paper, heat transfer and flow of shear-thinning non-Newtonian fluids in a circular tube under constant heat flux with a modified twisted tape, have been numerically studied in a laminar, steady-state and three-dimensional regime. The finite volume method was used to numerically solve the governing equations, modified power-law model be used to describe the dependence between the stresses and shear rates. The physical model is a circular tube with a standard twisted tape with decreasing its width, also a hollow tape in circular tube with an increase in the central cavity of the tape. The heat transfer and the overall performance are unfavorable by cutting off the tape edge. Instead, a decrease in tape width ratio, hollow tape with different removal ratios was used to improve thermal efficiency. the numerical results show that the removal ratio (hollow width of the tape divided by the initial width) equal to 0.3 in the fluids with behavioral indexes 0.86, 0.55 and 0.41 can cause 17.95%, 18.49% and 19.69% increase in thermal performance compared to the best thermal performance mode, respectively. Therefore, the hollow twisted tape is a promising technique for laminar convective heat transfer enhancement.

Boiling heat transfer is one of the most applicable heat transfer processes in the industry. In recent years, many studies have been investigated in nanofluid pool boiling field and reported some contradictory results. This research is a qualitative and quantitative investigation to understand the behavior of nanofluid during pool boiling heat transfer. For this purpose, a low concentration (up to 1000mg/l) of CuO-water nanofluid and a copper plate surface heater with a diameter of 10 mm and surface roughness of 7.5 nm were used. CuO-water nanofluids have been created by 40nm nanoparticles and 1 to 1000 mg/l of concentrations are used in this research. The measurement of critical heat flux at different concentrations of nanofluid showed that critical heat flux has improved 92% in optimized concentration of 100 mg/l compared to distilled water. Atomic force microscopy, scanning electron microscopy and contact angle measurements have been done for analyzing properties of surface and nanocoated which are formed after nanofluid boiling. Results demonstrate that there is a positive effect in increasing roughness and a negative impact of thickness enhancement on critical heat flux.