مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 9 (آذر 1400)

In this study, the effect of fluid-solid interaction on forced convection flow in a channel with the two-dimensional incompressible fluid flow is investigated. One surface can exchange heat and the other is elastic and insulated. As the fluid flows through the hot and oscillating elastic surfaces, the rate of heat transfer to the fluid varies. In this case, the heat exchange rate behaves as a function of the conditions of the oscillating elastic surface, one of the factors affecting the heat exchange is the vibration amplitude of the elastic surface. Therefore, the aim of the simulation is to investigate the application of the replacement of the elastic boundary with the rigid boundary in a part of the channel and the effect of the maximum size of the amplitude of vibration of the vibrating elastic surface on the heat transfer rate. It was found that the average Nusselt number and the average temperature of the air leaving the channel increase with the replacement of the elastic surface with a part of the rigid channel boundary. Also, with increasing the maximum amplitude of oscillation wall vibration, the Naselt number, the average temperature of the output fluid, and the rate of heat transfer from the constant temperature level to the operating fluid increases.

In this paper, a new model is developed for rotating stall in low speed axial compressors and fans. The theory is developed from Moore’s theory. The modified model makes it possible to predict the transient behavior of the stall cells, which is not possible with Moore’s theory. The general assumptions such as the layout of the compression system, the lags in the entrance and exit ducts, and the small disturbances are assumed to be similar to those of Moore’s theory. However, a second order hysteresis is used in the current work for the pressure rise of the rotor and stator rows. Comparing the experimental results with the theory shows that the modified model can predict the transient behavior of the stall cells fairly accurately. Furthermore, the current model makes it possible to study the effects of different parameters such as the stagger angle, number of stages, and number of stall cells. It has been suggested in the current study that the number of stall cells should reduce to one in a fully developed rotating stall pattern.

The common carotid artery is a large vessel which supplies oxygenated blood to the large front of the brain. The artery geometry is extracted from computed tomography angiography images of a healthy 20-year-old volunteer. ANSYS-Fluent commercial software is utilized to simulate the blood transient laminar flow in common, external and internal carotid arteries. In addition to the Newtonian viscosity model, two non-newtonian generalized power law and the modified Casson models have been selected for comparison. The quantitative and qualitative results include the distribution of the low density lipoprotein concentration, the wall shear stress and its fluctuations, and the volume and shape of the recirculation zone. Computations show that the low density lipoprotein Concentration estimated by non-Newtonian models is higher than by the Newtonian model. On the other hand, the carotid bulb and the beginning part of the external carotid artery, contain a large volume of the recirculation flow. Also, the low density lipoprotein particles concentration Comparison between the two modified Casson and Newtonian models in the common carotid artery zone shows the difference of about 12.5 percent. The results of this study show that the vortex region volume and shape are changed during the cardiac period cycle. The findings also reveal that Newtonian and non-Newtonian models present different results in predicting the flow parameters and secondary flow estimation.

In the present study, the two-dimensional lock-exchange turbidity current under the influence of wind flow is modeled using open-source software. To solve this, the large eddy simulation method has been used in order to observe turbulent phenomena more accurately. By developing the two-phase solver of the software so that the equations of the volume of fluid method are coupled with the scalar equation of concentration, the three-phase problem is simulated as a phase of a mixture of dense fluid and pure water next to the air phase. The results show that an increase in wind speed reduces the buoyancy force driving the turbidity current and increases the entrainment, which means faster pollution of water areas. This increase in wind speed also increases the wall shear stress, with the difference that the amount of wall shear stress at low wind speeds is not significant. So this prevents a significant change in the deposition behavior of the current. Studying the current's sedimentation behavior, showed that at high wind speeds, the co-current wind flow corresponding to the turbidity current has more harmful effects than the reverse wind flow and its sediment accumulation is getting higher.

In the present study, the efficiency of the finlet as a means of passive trailing-edge noise control has been experimentally investigated. Surface pressure spectra, spanwise length scale, and eddy convection velocity in the trailing-edge region are important parameters in determining far-field trailing-edge noise. In the present study to measure the above parameters, a flat-plate model equipped with unsteady surface pressure transducers has been designed and built. Results have shown that the flow behavior downstream of the finlets is strongly affected by the spacing between the finlets. The use of finlets with coarse spacing leads to a reduction in the surface pressure spectrum at mid to high frequencies and an increase in the spanwise length scale at low to mid frequencies. On the other hand, for the finlets with fine spacing, while the surface pressure spectrum has been further reduced at high frequencies, there has been an undesirable increase at low to mid frequencies. Moreover, fine finlets can significantly reduce the coherence and eddy convection velocity at mid to high frequencies. Finally, the Amiet-Roger model has been used to evaluate the changes in far-field trailing-edge noise and the results have shown the effectiveness of finlets in the mid and especially high frequency range.

Continuous blowing and synthetic jet actuators were implemented to investigate their effects on a fully stalled airfoil. An opening tangential to the boundary layer configuration was installed over the suction surface of the Selig-Donovan airfoil at the angle of attack of 16° and Reynolds number of 60,000. An optimization analysis was carried out to look for the optimum operational design point. Genetic algorithm, artificial neural network, and computational fluid dynamic simulations were combined to perform the optimization. Inserting location, opening diameter, velocity amplitude, and synthetic jet frequency were considered as design variables. Results indicated a significant improvement in aerodynamic characteristics, performance, and lift and drag coefficients. Using unsteady actuation caused a better improvement in aerodynamic characteristics compared to the steady case and also led to a remarkable reduction in the applied momentum coefficient. Contours of different flow field parameters were depicted for both cases and their similarities and dissimilarities were identified. Moreover, the synthetic jet actuator displayed a lower increase in the friction coefficient than the continuous blowing actuator. Therefore, it showed a higher performance improvement in comparison with the continuous blowing jet.

In the present study, numerical simulation of the steady blood flow through the carotid artery with a non-planar geometry model and considering mild (20%), moderate (50%), and severe (80%) occlusion was performed. In this research, the shear-thinning behavior of the blood fluid is incorporated by the Carreau–Yasuda model, and the viscoplasticity of blood was ignored. Furthermore, concentric and eccentric geometries were considered for stenosis. By comparing the non-Newtonian and Newtonian viscosity results, significant differences were found in the secondary flow lines. Shear-thinning behavior affects the secondary flow lines so that the vortices are either not formed or are smaller in size in the middle of the stenosis and subsequent sections. Moreover, axial velocity profiles in the non-planar branch decreased by increasing stenosis percentage, and in estimating the maximum wall shear stress, the Newtonian model had a significant error compared to the non-Newtonian one, and the estimated values by the Newtonian model were less than the non-Newtonian in most cases (up to 37% for an 80% stenosis). In addition, variation of velocity and shear rate caused by stenosis reveals the importance of the non-Newtonian model in calculating streamlines and velocity magnitudes. Plus, as the percentage of stenosis increased, the vessel's curvature effect, which causes the velocity field to deviate to the inner wall, decreased.

In this study, the effect of protuberance on the thrust vector of a supersonic jet was investigated as a new method in thrust vector control. For this purpose, a convergent-divergent nozzle was designed and fabricated. This nozzle is such that the nominal Mach number in the nozzle exit in full expansion condition is 2. The wall of the nozzle is equipped with pressure holes to measure pressure variations. Also, there is a hole on the nozzle wall to apply a protuberance inside the nozzle. Pressure sensors for pressure measurement and also the Schlieren system are used to check the outlet flow field. The total pressure in all experiments is constant and equal to NPR=6.6. Three-dimensional and multi-block numerical code is used for flow modeling. Also, the turbulence model k-ε, RNG is used to model the nozzle flow. An unstructured mesh has been used for modeling the flow field within the nozzle and the outside domain. The results of this study show that the depth of penetration of the protuberance in the flow field has a significant effect on the amount of deviation and even the direction of the jet stream exited from the convergent-divergent nozzle. The maximum deviation of the jet outlet from the nozzle is 9.8 degrees, which is equal to a penetration ratio of 0.4. In addition, these results indicate that with the increase in protuberance penetration within the nozzle, the nozzle axial thrust has slightly decreased.

The heat pipes are usually simulated by using a two phase model and a model describing the phase-change process. The computational costs of the two-phase approaches are relatively high and the model generally needs small-size time steps, which leads to a long simulation run times in the order of several days. In the present study, a variable conductance heat pipe is simulated by using a set of single-phase fluid flow models. It is shown that the proposed approach needs to a simulation time in the order of minutes that considerably facilitates the parametric study process of the variable conductance heat pipe. The effect of heat rate, sink temperature, mass of non-condensable gas, vapor radius, and wick porosity on the performance of variable conductance heat pipe are investigated. For the considered variable conductance heat pipe, the obtained numerical results indicate that sink temperature has the greatest effect on distributions of average wall temperature, overall heat transfer coefficient, the active length of condenser, and its average temperature. By increasing the sink temperature of 10, the active length of condenser is increased about 48 and average wall temperature is increased about 6.4.

This study aimed to investigate the heat transfer of water/Fe3O4 nanofluid in a square cross-sectional channel with dimensions of 80 cm ⨯1 cm ⨯1 cm under the influence of a uniform heat flux perpendicular to the laminar flow of ferrofluid in the presence of a magnetic field. Firstly, the production of ferrofluid with concentrations of 0.5 vol.% and 1vol.%, their quality, and the quality of the production method was investigated. The results of zeta potential and vibrating-sample magnetometer tests show the good quality and stability of the produced ferrofluid. The thermophysical properties of the made ferrofluid are compared and evaluated with existing experimental correlations. The heat transfer of the produced ferrofluids under the influence of heat fluxes of 134-546 Watts is investigated in the absence of an external magnetic field. Then, the effect of the external magnetic field on the heat transfer at 0.5 vol.%, under the influence of a total heat flux of 1258.2 Watts is investigated. The magnitude of increase of heat transfer coefficient compared to pure water, without external field, for ferrofluid with 1 vol.%, under total heat fluxes of 134, 545, and 321.3 Watts, are 30%, 48%, and 38% respectively. At a heat flux of 1258.2 Watts, the heat transfer coefficient in the presence of an external magnetic field increases by 3.16% at 0.5 vol.% compared to the absence of a magnetic field.

The main purpose of this study is to investigate the effect of the pulsating of the inlet jet on the heat transfer rate short distances of the nozzle from the concave surface. For this purpose, three-dimensional simulation of flow and heat transfer of sinusoidal pulsed jets on the concave surface has been performed at distances of 0.5 times of nozzle diameter to 4 and for Reynolds numbers of 7000 and 14000. The results of the numerical simulation are in good agreement with the experimental results of the steady jet. The result shows that the effect of pulsating the flow with the sine function decreases at short distances between the jet and the concave surface. So that at a distance of 4 times of nozzle diameter, pulsating jet led to a 10% increase in the average Nu, while this value is equal to 5% for a distance of 0.5 times of nozzle diameter. It can be found that pulsating the flow decreases Nu at low frequencies, and then with increasing the frequency of the pulsed jet, the Nu number increases. Furthermore, with increasing the distance between the surface and the inlet jet, the Nu number decreases significantly. This rate of reduction is lower in comparison to the steady jet.

In the present paper, the effects of the interaction of a high-density liquid oxygen jet with high-velocity hydrogen in the presence of a pseudo-boiling phenomenon are investigated. The pseudo-boiling phenomenon causes a sudden expansion in the flame, which leads to the formation of a recirculation zone. Different turbulence models have been investigated and it has been shown that the selection of a suitable turbulence model for the trans-critical reacting flow is much more important than subcritical and supercritical flames. Also, contrary to expectations, the dense core of liquid oxygen disappears faster in the non-reacting case than the reacting flow, which is due to the displacement of the mixing layer in the reacting flow due to the intense expansion (because of the pseudo-boiling phenomenon). The effects of mass flux ratio were also investigated and it was observed that by increasing the mass flux ratio from 5 to 24, a strong recirculation is formed at the flame front and the flame becomes like a bubble, similar to LOX-GCH4 flame. Increasing the mass flux ratio leads to an increase in the strength of the shear layer that causes the pseudo-boiling phenomenon to occur at a higher rate. Finally, increasing conversion of the liquid-like oxygen to gas-like conditions leads to the formation of a strong vortex in the flame front.

One way to improve air-fuel mixing in a gas diffusion flame is to produce targeted vortices and circulate the flow using a bluff body. In this study, the influences of radius, thickness, and location of a disk-shaped bluff body on the performance of a methane gas diffusion flame are numerically studied. This investigation is carried out under both cold mixing and hot mixing (with combustion reaction) conditions. The present simulation is verified against experimental data. The results show the substantial influence of the mentioned parameters on the size and intensity of downstream vortices, and a direct dependence is observed between the sizes of inner and outer recirculation zones and air-fuel mixing. It is also observed that the flow pattern and level of air-fuel mixing are more dependent on the bluff body’s radius than its thickness. Based on the hot mixing simulation results and regarding the dependence between the rates of the chemical reaction and turbulence mixing, the higher rate of air-fuel mixing is associated with the decreased flame length. Among the cases investigated, the bluff body with a radius of 6mm, the thickness of 5mm, and axial location of 5mm away from the air channel exit results in the best air-fuel mixing.

In this study, the hybrid of organic Rankine cycle with heat recovery system of low temperature gases in Electric Arc furnace has been investigated. Moreover, the effect of the steam accumulator on stabilizing the mass and heat of exhaust gases of the heat recovery boiler is shown. Hence, constant thermal power has been achieved for a longer period of time for the organic Ranking cycle. The steam accumulator thermodynamic model is simulated based on the non - equilibrium thermal model for the liquid and vapor phases. Furthermore, the steam accumulator pressure variations with different mass outflow rates have been investigated. Constant and continuous thermal power has been reached with an output mass flow rate of 2.84 kg/s during four processes of the electric arc furnace. The transient state of the aforementioned hybrid system has been studied from the energy and exergy points of view. The energy and exergy efficiencies of the whole system are calculated with three working fluids Hexamethyldisiloxane, Toluene, and R245fa of the organic Ranking cycle. Toluene with thermal and exergy efficiencies of 16.4% and 27.1%, respectively, is suitable for use in the organic Ranking cycle compared with the other two fluids.

Driving cycles represent the vehicle speed as a function of time and are used in vehicle design, fuel management, and the improvement of standard indicators. In this study, four combined driving cycles were extracted using real data. The data was collected from a passenger car with a gasoline engine under real driving conditions while driven from Tehran to Amol based on the car chasing method. A code was generated in MATLAB software to create the desired cycle using support vector machine and K-means algorithms considering mid-range and mean values as group centers. The characteristic parameters of the cycles such as the average speed and the percentage of the car travel time at idle, cruise, accelerating, and decelerating conditions were also calculated. These cycles were compared based on the mean relative error, the root-mean-square error, and the Chi-square test. The results showed that the cycles extracted by the support vector machine were closer to the allowable time interval (less than 1800 seconds); however, the cycle extracted by the K-means algorithm with the mean value as the centers of the generated categories, recorded the least errors. This cycle, in addition to spending most of its time in accelerated motion, represented a greater amplitude of acceleration and velocity fluctuations than other cycles.