transient flow

This study investigated the turbulent flow behavior in a horizontal channel containing elliptical cylinders with aspect ratio 0.45 and 0.5 and Reynolds number 5000. Using the finite element method to solve heat and fluid equations, the effect of parameters such as heat transfer rate, Nusselt number, temperature distribution and velocity distribution on fluid flow has been investigated. The results show that the presence of cylinders increases the vortices and changes the flow patterns from symmetric to asymmetric. Also, as the Reynolds number increases, the flow becomes increasingly turbulent and the rotating mechanics of the cylinders contribute to a different distribution of temperature and velocity. In the investigation of the flow around the elliptical cylinders, it was observed that the maximum temperature point occurs between the third and fourth cylinders in the case of constant properties and behind the fifth cylinder in the case of variable properties. Also, the fluid velocity has a greater effect on the heat transfer rate than the heat exchange surface. The reduction of the aspect ratio leads to the reduction of the heat transfer rate in the turbulent flow and the temperature contours are more uniform in this flow, which is due to the mixing and transverse vibrations of the turbulent flow. These findings can help to improve the design of heat transfer and flow control systems in engineering applications.

Nowadays, the use of hydromagnetic micro pumps has gained a lot of attraction due to the lack of moving parts. The design and construction of this type of pump has its own subtleties and complexities, which is still the subject of today's research, so the numerical analysis of this type of pump can be a great help in designing and manufacturing them. In this research, the flow of a thixotropic non-Newtonian fluid in a hydromagnetic micropump is simulated in two dimensions and the effect of various parameters such as magnetic field intensity, electric field, channel width and height, the length of the electrodes that create the electric field of the fluid on the flow rate and the pump head have been investigated. The results showed that the dimensionless Hartmann parameter has an optimal value where the pump discharge has its maximum value. The flow rate and head of each pump in Hartmann are specified and maximized differently, The flow rate reaches its maximum value at Hartmann number 40, while the pump head reaches its maximum value at Hartmann number 100. The results showed that in order to simultaneously increase the flow rate and head in the micropump, the length of the electrode can be increased or the potential difference between the two ends of the electrode can be increased. Also, with the increase of the viscosity ratio, the flow rate of the pump is reduced, but its head is increased, and with the increase of this ratio, the effective amount of flow rate and the pump head are greatly reduced from this ratio, so that the effect of this factor is very less in the viscosity ratio of 4 and above.

In this research, the effect of titanium dioxide nanoparticles was investigated, in improving the anti-wear, friction and physical properties of epoxied soybean oil. This research consists of four stages. In the first step, to prepare nano-lubricant mixtures, titanium dioxide nanoparticles with concentrations of 0.1, 0.2 and 0.5 wt% were combined with epoxidized soybean oil. In the second step, an ultrasonic bath was used to disperse the nanoparticles in the lubricant, and span80 surfactant was used to stabilize the fluid and prevent solid nanoparticles from settling. In the third stage, the experimental tests were performed, including the wear test using the pin-on-disk device and the physical properties of nano-lubricants, including pour point, flash point, and viscosity, by relevant devices and based on international standards. In the fourth stage, the results of the analysis and the performance of nano-lubricants mixtures were compared with the base fluid. Based on the results of the tests, it was determined that the best performance is related to the nano-lubricant mixture with a concentration of 0.5 wt% of titanium dioxide nanoparticles, which reduced the coefficient of friction and wear by 7.93% and 57.14%, respectively. Also, the viscosity decreased by 11.95% and the flash point decreased by 3% compared to the base oil, but with increasing concentration, no significant change was observed in the pour point of nano-lubricants.

In this paper, the water hammer resulting from the fast closure of the valve in a pipeline is simulated using numerical solution of continuity and Navier-Stokes equations. Simulation has been performed for a high-viscosity oil and for water. The initial flow regime for water is turbulent and for the oil is laminar. The obtained results are compared with the experimental results and for both fluids, a good agreement between the simulation and experimental results at different times is obtained. Velocity contours at different times show two regions with different behavior in transient flow. The wall region and the pipe core region. In the wall region, the effects of fluid viscosity are dominant, the velocity gradients are sharper, and flow changes more rapidly. While the pipe core region is affected by fluid inertial forces. As the fluid viscosity decreases and the Reynolds number increases, the core region becomes larger. In addition, a parametric study has been conducted and the effect of different parameters on water hammer has been studied. The results show that by reducing the thickness or length of the pipe, reducing the mass flow rate, or using a pipe with a lower elastic modulus, the water hammer effects can be significantly reduced. For example, by reducing the length of the pipe from 60 meters to 18 meters, the maximum pressure decreases by 11%.

In this study, numerical simulation of a pulse-jet filter cleaning system is conducted and its performance is investigated under pre-defined conditions. In the first step, 3D simulation of the system from high pressure tank to filter inlet is performed and the output is used as the input of second step. In second step, simulation is performed for filters with inlet mass flow rates calculated from the previous step. To validate the results, the results are compared to experimental data showing acceptable agreement. The results show that regardless of valve type, cleaning pulse generates after 0.5s and suddenly decreases afterward. No shock or choking is observed in the system. Another interesting result is induced flow generated after the nozzles, which increases filter inlet mass flow rate. In addition, axial distribution of filter outlet flow rate is not uniform, degrading from inlet to outlet. Finally, a complete parametric study is performed to investigate the effect of the tank pressure on the pressure difference in the filter which is an important index in cleaning performance analysis.

Analysis of natural gas transient flow in transmission pipelines is one of the most important issues in the gas industry. Despite the previous studies, the accuracy and the computational time have yet considered as two important challenges in this field. In this paper, a parallel algorithm for numerical simulation of isothermal and non-isothermal gas flows is presented. Numerical analysis of the flow is performed using the implicit Steger-Warming flux vector splitting method. For parallelization, the computer program has been parallelized using Message Passing Interface library. In order to demonstrate the capabilities of the developed computer program, the flow inside two pipelines with different conditions is solved, and the results are validated. Then, some factors such as the computational time, reduction of the time, and the speed up criteria are obtained to demonstrate the computational efficiency of the proposed method. The results show that parallel processing method can significantly reduce computational time of natural gas flow in long transmission pipelines. Moreover, it is shown that application of this approach on the fine computational grids is more efficient than on the coarse grids.

The main purpose of this study is to experimentally investigate transient convective heat transfer from a fluid stored inside a closed reservoir. Different cooling methods using helical and straight tubes are considered for heat transfer from the fluid reservoir. This paper attempts to determine the thermal advantages of helically coiled versus a straight tube. Fluid stored in the reservoir is cooled by water flow in the tube section at 27oC inlet temperature. The pressure drop and heat transfer parameters are measured over a wide range of Reynolds numbers (covers 500 to 5500). In these experiments, the effects of a couple of different parameters such as Reynolds number, time and geometrical parameters have been studied. Both heat transfer and pressure drop fluid flow in two test sections have been also reported and discussed. The experimental data indicate that using of helical coil instead of straight tube leads to increase heat transfer sharply. The coiled tube results show 42% reduction in of reservoir temperature compared to straight tube.

In this research the transient flow analysis in viscoelastic pipes considering Fluid Structure Interaction have been performed utilizing a newly developed formulation of Transfer Matrix Method in frequency domain. To obtain this extended form of TMM, mathematical processes was accomplished. Time domain governing equations have been transformed to frequency domain and then a suitable matrix form of them is used to study transient flow due to sudden valve closure. Obtaining a set of algebraic equations instead of integral equations and the ability to analyze this phenomenon without need to solve complex convolution integral, are some of the benefits of the frequency domain tools, which have been applied in this research. To verify the model, initially two cases of rigid and elastic pipe wall have been analyzed. Results showed good conformity comparing to experimental data and analytical solution available. Then having a set of reliable experimental data of transient flow in VE pipe, MatLab code was adopted to the model and fortunately here also results were in good compatibility with the experimental results. Also it has been showed that this model will be a suitable tool for parametric analysis and for determining the critical situations of the system. The results obtained from this research prove that using frequency domain tools will lead to an effective and precise model for simulating the transient flow characteristics in VE and also normal transmitting pipelines.

In the large scale pipe systems the use of pump group connected in series is evitable to meet the desired pressure head. If one or more of the pumps shuts down، water hammer event occurs which manifests in pressure and rare faction wave، along the pipeline these waves result in huge stresses on the system. In this regard، this paper aims at investigating the pump stations performance during steady and transient flows، in which the focus is placed on various possibility of pump failure in a system of pumps in series. The mathematical model to carry out this research includes continuity and momentum equations (transformed to two characteristic equations) as well as equations to govern mechanical behavior of pump stations. These governing relations are numerically solved and programmed in a MATLAB code. The results reveal that the use of the same pumps leads to larger efficiency compound to using different pumps. Furthermore، if the pumps shut down one after other، the water hammer pressures remarkably alleviate. The result for this sequential pump station failure are quantitatively presented and discussed.

This note presents a theoretical analysis and numerical simulation of hydraulic transients in pressurized pipeline system made of a local polyethylene pipe-wall located at a steel pipeline system. The continuity and momentum equations are solved by the method of characteristic (MOC) taking into account the viscoelastic effect of the pipe-wall for polyethylene pipe. The polyethylene pipe length and location at the pipeline and the discharge flow rate are changed and their influence on transient flow is investigated. By comparing this pipeline system with one that is made of polyethylene pipe totally، the possibility of using local polyethylene pipe due to its effect on the pressure wave is investigated.

Numerical simulation of non-isothermal transient gas flow is performed using implicit Steger-Warming finite difference method. Because of nonlinearity of the governing equations، they are linearized at each time step. The linearization either reduces computational effort or analyzes the flowfield more conveniently. In order to validate and evaluate the accuracy of current numerical method، Fanno and shock tube flows are investigated first. Then، transient flow in a gas pipeline that its inlet pressure changes with time is simulated. The results of present study show that Steger-Warming finite difference scheme can well captured the sudden changes in the flowfield. Moreover، the present method is able to analyze transient gas flows as nearly accurate as the nonlinear one with less computational effort.

In case of studying and modeling gas flow in the pipeline network, there are two types of flow: steady flow and transient flow. The natural gas pipeline network consists of pipelines, pressure compressor stations, pressure reduction stations, line break valves and storage fields. The pipeline is the key element in determining the overall network dynamic behavior. Dynamic behavior of a gas pipeline due to changes of gas flow rate for isothermal transient status has been modeled and investigated in this study. The modeling of isothermal transient flow is resulted in differential equations using mass conservation equations, momentum and an equation of state. By solving these equations according to time and place, the behavior of gas pipeline will be obtained. Three cases of the operating conditions for gas transmission pipelines in transient status have been studied. The modeling results of each case have been shown and interpreted in curves of flow rate and pressure variations along pipeline in different time spots.