To discover the nonlinear characteristics of Pipe flow, we simulated the flow as a sum of many vortex rings. As a first step, we investigated the nonlinear interaction among a maximum of three vortex rings. The Pipe wall was replaced by many bound vortices. A free vortex ring moves right or left according to the radius, and that of a particular radius keeps the initial position. The energy of a free vortex ring, except when it is close to a wall, coincides with that without boundaries. Two vortex rings of equal radii always show a repeated overtaking process. In the case of three vortex rings, they show a wide variety of behavior. For certain combinations of radii and the axial spaces among them, the motion, which seems to be very complex, is limited on a curved surface in three-dimensional space whose axes correspond to the three radii. It was found that this simplicity comes from the momentum conservation law, and also that its direction depends on the energy contribution calculated from free vortex rings. Our results elucidate the nonlinear behavior among many vortices.      

The Pipe diffuser, an efficient kind of radial bladed diffuser, is widely used in centrifugal compressors for gas turbine engines. This paper investigates flow characteristics of a Pipe diffuser for centrifugal compressors by solving three-dimensional Reynolds-averaged Navier-Stokes equations. The results show that the Pipe diffuser is adaptable to high Mach number incoming flows, and its unique leading edge could uniform the flow distortion. Numerical analysis indicates that the choke in Pipe diffuser occurs suddenly, which leads to the dramatically steep performance curves near choke condition. Besides, it is found that the first half flow passage is particularly important to the Pipe diffuser performance as it influences the choking behavior, the static pressure distribution, and the matching, so more attention should be paid to this region when designing or optimizing a Pipe diffuser. Two counter-rotating vortices generated in the diffuser inlet region are captured by numerical simulation, and they can exist in the downstream of the diffuser passage. More detailed analysis show that these two vortices dominate the flow structure in the whole diffuser passage by shifting flow to certain positions and forming high-momentum flow cells and wake flow cells. The leading edge formed by the intersection of adjacent diffuser passages significantly affects this pair of vortices. In addition, these two vortices also affect the flow separation in Pipe diffuser flow passages, they suppress separation near the front wall and back wall while facilitate separation at center locations. Therefore, it is recommended to design the leading edge of the Pipe diffuser carefully to control the vortices and obtain a better flow field.

Double-Pipe heat exchangers are commonly used in various industries, including power plants, solar cells, refineries, and automotive. The double-Pipe heat exchanger is one of the most used simple exchangers in the industry. In this study, we employ computational fluid dynamics to investigate the flow characteristics of nanofluids within a double-Pipe heat exchanger featuring a square inner tube and a circular outer tube (SC). The simulations are conducted under constant heat flux conditions, exploring laminar and turbulent flow regimes. Numerical results for water flow under forced convection are compared with reference results for validation. The results indicate that as the Reynolds number increases, particularly in turbulent flow regimes, the Nusselt number in nanofluid flow increases more than in water flow. For instance, in the case of aluminum oxide nanofluid flow at a Reynolds number of 500, the Nusselt number demonstrates a nearly 5% enhancement over water flow. In contrast, at a Reynolds number of 20000, this enhancement escalates to approximately 20%. Three types of nanoparticles are considered to investigate the effect of nanoparticle type on heat transfer and pressure drop. The results show that the use of nanoparticles has a slight effect on the friction factor while significantly enhancing heat transfer.

By means of He's homotopy perturbation method (HPM) an approximate solution of velocity field is derived for the flow in straight Pipes of non-Newtonian fluid obeying the Sisko model. The nonlinear equations governing the flow in Pipe are for-mulated and analyzed, using homotopy perturbation method due to He. Furthermore, the obtained solutions for velocity field is graphically sketched and compared with Newtonian fluid to show the accuracy of this work. Volume flux, average velocity and pres-sure gradient are also calculated. Results reveal that the proposed method is very effective and simple for solving nonlinear equations like non-Newtonian fluids.

Abstract: In this investigation a two-dimensional airflow and heat transfer with sinusoidal boundary condition analysis was conducted. Both laminar and turbulent sinusoidal Pipe flows were investigated numerically for Remax=2000, 10000, 20000 and 60000. The results are compared with experimental results of previous researchers [2], [5]. The RNG k – E turbulence model was used for turbulence modeling. After investigation of this turbulence model, since the flow inside the exhaustport of internal combustion engine is sinusoidal, this model can be used for it [9, 10]. An available code, which is basis on finite volume and can solve Nervier-stokes equations on the structured "grids, is used After discretization equations on the control volume, physical properties flux on the control volume by means of first-order upwind difference scheme is calculated. The SIMPLE method was used for solving algebraic equations. Computational results are presented and discussed for velocity, pressure drop, wall shear, temperature, heat flux, and convection coefficient, for Pipe flow and flow around exhaust valve.

The spreading of a tracer in a bubbly two-phase grid-generated turbulent flow system is studied. In this work both particle image velocimetry (PIV) and planer laser-induced fluorescence (PLIF) are used to study the effect of the dispersed phase flow rate on the mixing characteristics of the tracer. The turbulent intensity of the continuous phase in the bubbly two-phase grid-generated turbulent flow is close to isotropic, and increasing the gas void fraction reduces the degree of non-isotropicity. The self-similarity of mean and RMS values of the cross-stream concentration distribution is observed. A new mathematical model is suggested to describe the self-similarity of the cross-stream profiles of the mean concentration based on two separate Gaussian curves into the central and outer region of the flow. The turbulent diffusivity is calculated using the Taylor hypothesis, which is based on the growth of the variance of the cross-stream profiles of the mean concentration, with a position along the direction of the flow. An increase in the void fraction does not affect the diffusivity of the superimposed distribution of the plume in the central region, however it did increase in the outer region.