مجله مکانیک سیالات و آیرودینامیک
سال یازدهم شماره 2 (پاییز و زمستان 1401)

Solving many important industrial problems requires knowing the values of the heat transfer coefficient of passing of one or more fluid streams in different equipment, systems or pipes. In the present study, a numerical model has been developed to simulate compressible fluid flow at the inlet of a hot pipe with different angles to the horizon. In this zone, the hydrodynamic and thermal boundary layers of the fluid flow are developing. Due to the turbulence of the fluid flow due to the interaction of heat and fluid flow inside this tube, a three-dimensional turbulent model was used for this simulation. For this purpose, continuity and compressible Navier-Stokes equations, Reynolds stress model, and turbulent and compressible energy equation have been solved simultaneously. Then, using a set of numerical runs by the concepts of experimental design and optimization methods, a predictive formula for the Nusselt number for these flows has been obtained. Finally, the ability of this formula has been investigated using a set of laboratory data.

In This research numerical investigation of two phase flow structure and heat transfer in microchannels with0.55 and 0.7 millimeter hydraulic diameter has been done. For this purpose, Ansys Fluent software has beenused and has wrote a UDF code for modeling of phase change process in this software. Inlet flow has beenassumed as super heat vapor of R134a and microchannel wall boundary condition is considered as constantheat flux. Effect of geometric cross section of microchannel in 3 cross section circular, square and trapezoidwith inlet mass flux range 100-600 𝑘𝑔/(𝑚2𝑠) on heat transfer coefficient and pressure drop has beenevaluated. Also two phase flow structure in two mentioned hydraulic diameter with same boundary conditionhas been studied. Under the same conditions of input mass flux and cross-section of the microchannel, it wasobserved that heat transfer coefficient in microchannel with 0.55 millimeter hydraulic diameter, is higherabout 15%. Also in a specific range of input mass flux, pressure drop in 0.55 millimeter hydraulic diameteris 3 times higher than 0.7 millimeter hydraulic diameter. Also in both two hydraulic diameter 0.55 and 0.7millimeter, square, circular and trapezoid microchannel have higher heat transfer coefficient in respect.

In this paper, the Smoothed Particle Hydrodynamics (SPH) method with dynamic boundary condition has been used to simulate 2D floating of objects. Severe fluctuations in the field of pressure and velocity is one of the major problems in this method. In this paper, the fluctuations have been corrected using Delta and Shift algorithms. The simulation was numerically performed with three viscosity models including real fluid viscosity (laminar and turbulence), ideal fluid (without viscosity) and artificial viscosity. Validation of this method indicated that in the case of artificial viscosity and also ideal fluid, the Delta algorithm should be used and in the case of real fluid viscosity, using Delta and Shift algorithms could establish good agreement with experimental data. Finally, by simulating the floating experiment with the obtained optimal numerical models, the results showed that the optimal method in the case of real fluid viscosity had a better performance in modeling the horizontal, vertical and rotational movements of the floating body than other optimal methods.

Utilizing EG/Water solution as working fluid in an automotive radiator cooling system could cause a significant decrement in heat transfer rate and increases fluid pressure drop. According to the results of this numerical research, a 70%-30% EG/Water solution has about 12%-20% less heat transfer rate and 5%-20% more pressure drop compared to pure water. Geometrical changes affecting the heat transfer parameters could be used to enhance the heat transfer performance of the radiator. This study aims to numerically investigate the effect of employing the mini-channel tubes and (-) or (+) shaped cross-section tubes rather than simple tubes on heat exchanger performance. Comparing the vertical and the horizontal configuration of pipes on heat transfer function is also studied. Results show that utilizing the mini-channel tubes can raise the heat transfer rate by about 15%-67%, while the friction factor is also increased by about 6%-52%. A considerable increment of nearly 10%-91% is also reported as an impact of employing horizontally arranged tubes compared to vertical tube configuration.

In this article, the waterjet propulsion system is numerically designed and simulated for use in an amphibious vehicle with a design speed of 12 km/h. CFturbo and PumpLinx software are used for 3D design and fluid flow simulation of the system, respectively. The proper value of propeller design parameters and other components are obtained by using the CFturbo software, and applied according to the desired input parameters and based on valid references and experimental data. To evaluate the performance of the designed system, fluid flow simulation of the system is also performed by PumpLinx software, specifically software designed for turbomachinery simulations. The results of fluid simulation confirm the design parameters obtained by CFturbo software to a great extent, which shows the high efficiency and accuracy of this software in the design of blades and waterjet systems. Finally, with the help of these two powerful software and considering the solid and fluid mechanics design, an optimal waterjet system with high efficiency is designed, which meets the desired need. The results show that the required propulsion force can be achieved by using a waterjet pump with 3 vanes impeller and 7 vanes stator. The amphibious vehicle will obtain the velocity of 12 km/h at 1700 rpm revolution speed. The flow field distribution after passing the stator section becomes axial and uniform which shows the proper design of the stator.

In the presented research, the analysis and finding optimal values of a heat exchanger used in the exhaust system of a diesel engine has been discussed. The main purpose of using this heat exchanger is to reduce the temperature of combustion products exhausting from the engine. This can be done by mounting a heat exchanger at the exhaust gas path. In order to analyze the issue, different designs of heat exchangers have been regarded. Three- and five-tube heat exchangers reduce the temperature of the smoke to an acceptable level by increasing the contact surface between the smoke and the cooling water. However, the pressure drop created on the side of the smoke is high in these designs. The use of annular fins and longitudinal fins on the smoke side also increase the heat transfer by increasing the contact surface between the smoke and water flow and the turbulence in the smoke path. However, in this design the heat exchanger outlet temperature is higher than allowed value (350 ). Furthermore, in this design, the pressure drop also increases. After the investigations carried out to reduce the temperature of the smoke, shell and tube heat exchangers have been chosen as the final designs. The shell and tube heat exchangers is capable to reduce the exhaust temperature to 309.42 .

Controlling the flow on the cylinder and postponing the flow separation area from the surface of the cylinder, Shrinking the wake area and reduces the drop of vortices and increases the life of the structure. The phenomenon of fatigue, which is caused by loading and fluctuating stresses on the structures, causes depreciation and reduces the life of the structures. Flow controllers play an inhibitory role in creating the phenomenon of fatigue. In this research, the effect of two control cylinders for different distances from the main cylinder at Reynolds 140 was evaluated. The main goal of this research is to find a certain distance to place the control cylinders from the main cylinder in which the vortex shedding distance is reduced to an optimal level and neutralized. For this purpose, transient numerical analysis was performed by Fluent software. The results of this research indicate that there is a certain distance to neutralize the fall of vortices. This distance, for Reynolds 140, is equal to 0.8 of the diameter of the main cylinder, and it can be referred to as the golden distance, which is considered an important component in the design.

In this study, the effects of a cavity on the pitching characteristics of NACA0012 airfoil under dynamic stall conditions were examined numerically and the transient incompressible turbulent flow was simulated in two dimensions. Two different sets of circular cavities with R=0.05c were set at two different positions of x=0.13c and x=0.6c from the leading edge (LE) to investigate the effect of cavity position on the aerodynamic parameters of the airfoil such as lift, drag, and pitching moment coefficients as well as aerodynamic efficiency (lift to drag ratio), using Re=106 and reduced frequency of kf=0.15. Results indicated that the cavity at distant location from the airfoil LE showed a better performance in improving the lift coefficient along with aerodynamic efficiency of the pitching airfoil and the averaged values of the lift coefficient for cavities at x=0.13c and x=0.6c locations increased by 3.57% and 0.18%, respectively compared to the baseline. The averaged value of the drag coefficient for the cavity located at x=0.6c decreased by 3.25% and for the cavity at x=0.13c went up by 3.97% in comparison to the clean airfoil.

Spark-ignition internal combustion engines are sensitive to ignition time. The optimum ignition timing depends on speed, engine load and even fuel type. If this time is manipulated for any reason, it affccts on engine performance and combustion products and consequently the amount of pollutants. Due to the fuel crisis and the need to use engines with higher performance and lower fuel consumption, various strategies for reducing fuel consumption and increasing efficiency of internal combustion engines are presented.‌ Since the XU7/JPL3, made by Peugeot, is the second high production engine in Iran, it has been selected in this investigation. The test results in full load condition show the ignition advance increment to a certain extent, increases power and NOx emissions relative to gasoline mode. The half load results indicate that the increment of the ignition advance angle reduces the brake specific fuel consumption, and NOx and HC emissions, whereas does not have any effect on CO emissions. Optimum amounts of the ignition advance in half and full load conditions have been presented for the gas engine.

In Zn-Ag2O flow batteries, the electrolyte as one of active substances in electrochemical reactions flows circulatory in the narrow distance between electrodes, and in order to avoid of short circuit inside the battery cells, the spacers are used between their electrodes. In these batteries, the hydrogen gas bubble due to electrochemical reactions is produced on the cathode and then released within the electrolyte flow and a two-phase current is formed. Especially at high-rate discharge processes, this event can reduce the electrochemical active surface of the electrodes, increase the ohmic resistance, and thereby, reduce in the battery capacity. In this paper in order to solve this problem, the behavior of gas bubbles on the electrodes and within the electrolyte flow and their effect on the electrochemical active surface by considering two types of cross-sectional surfaces for spacers, is studied numerically. The results shown that the gas bubbles within the two-phase electrolyte flow spent more time behind the spacers with square cross-section compared to the case of spacers with a circular cross-section at the narrow distance between the electrodes. Therefore, this delay has led to other gas bubbles reaching each other in the electrolyte flow and as a result, they are combined. This event is investigated with two positive and negative approaches in this article.

The emission and distribution of dust particles are of the most important issues in the field of environmental studies. The transfer of dust particles is influenced by natural and environmental factors and always has destructive effects on the environment and especially on people's health. In present research, the numerical simulation of the emission of dust particles around low-lying hills has been done. The emission of dust particles is influenced by factors such as airflow velocity, environmental conditions, surface and also the type and structural characteristics of the particles. In order to accurately predict the flow field and investigate the way of dust particles emission, a numerical method based on the large eddy simulation has been used. Validation of the model was performed based on the comparison of simulation results with the laboratory data. It shows an appropriate accuracy for modeling based on the large eddy simulation. In the current research, the numerical simulation of fluid flow has been done with the Fluent module from the Ansys software package. The results of this research, including changes in the concentration of dust particles under the influence of factors such as airflow velocity and the diameter of the particles, have been presented and discussed. The results show an increase in the concentration of sand particles by reducing their diameter from 200 microns to 20 microns in this study. Also, the higher velocity of the inlet air flow into the computing domain, the lower the concentration of particles on the surface of the hill and its surrounding environment.

In this study, mixed convection inside a square cavity with a movable cap and baffle was simulated numerically using the finite volume method. The under-study cavity was two dimensional and affected by gravity and rotated perpendicular to the plane. Right and left walls of the cavity were adiabatic and the upper wall was warm source at a constant temperature. Lower surface was a movable cap that moved from the center to both sides and was assumed to be a cold source at constant temperature. The baffle was assumed to be at the same temperature as cold wall and had a height equal to two thirds of the side of the cavity. Experimental data was used for the thermal conductivity coefficient of the nanofluid. Simulations were performed at a constant Reynolds number to investigate the effects of three parameters of Richardson number, volume fraction of solid particles and cavity slope angle on isothermal lines, streamlines and mean Nusselt value, which created 36 different states. It was found that increasing of slop angle of cavity with respect to reference surface (0 to 90 deg), increasing Richardson number (0.01 to 100) and increasing the volume fraction (0 to 0.05), increase the mean Nusselt value, where the maximum value of which is equivalent to state , , . Increasing the volume fraction of the nanofluid causes an increment in average Nusselt number. It was also observed that at low Richardson values, cavity slope angle has no effect on the results.