Twist-bending of a 90° elbow or pipe bend

Out of plane bending by an end couple applied to a 90° elbow or pipe bend of circular cross-section is considered. There is conversion of bending to twist so that at 90° the elbow is held in equilibrium by a twist couple. Using the methods of toroidal elasticity, the six stresses of the first order stress field are determined from the zero-order field. The zero-order field constitutes a permissible initial deformation as verified by strain compatibility equations.The equations in the text are derived from the general theory of toroidal elasticity by applying the method of successive approximations. The equations of the general theory are expanded in a series in $\text{1}\text{R}$ (where R is the toroidal radius). This leads to the working equations given in the paper.     

An experimental investigation of coaxial turbulent jets

The flow field generated by two coaxial jets was investigated experimentally with hot-wire anemometers. The area ratio between the external and internal nozzle was varied as well as the velocity issuing from each of the nozzles. The distribution of the mean velocities, turbulence intensities, and shear stresses were determined for the various cases. The development of the flow field and its approach to a self-preserving state is discussed. The Reynolds numbers based on the nozzle diameters varied from 0 to 10s and the velocities were low enough that the flow can be considered incompressible.     

Numerical simulation of transient 3-D turbulent heated jet into crossflow in a thick-wall T-junction pipe

The present work is to investigate the transient three-dimensional heated turbulent jet into crossflow in a thickwall T-junction pipe using CFD package.Two cases with the jet-to-crossflow velocity ratio of 0.05 and 0.5 are computed,with a finite-volume method utilizing k-ε turbulent model.Comparison of the steady-state computations with measured data shows good qualitative agreement.Transient process of injection is simulated to examine the thermal shock on the T-junction component.Temporal temperature of the component is acquired by thermal coupling with the fluid.Via analysis of the flow and thermal characteristics,factors causing the thermal shock are studied.Optimal flow rates are discussed to reduce the thermal shock.     

A study on viscoelastic fluid flow in a square-section 90-degrees bend

It is well known that the drag-reducing effect is obtained in a surfactant solution flow in a straight pipe. We investigate about a viscoelastic fluid flow such as a surfactant solution flow in a square-section 90° bend. In the experimental study, drag-reducing effect and velocity field in a surfactant solution flow are investigated by measurements of wall pressure loss and LDV measurements. For the numerical method, LES with FENE-P model is used in the viscoelastic fluid flow in the bend. The flow characteristics of viscoelastic fluid are discussed compared with that of a Newtonian fluid.     

Mixing of two intersecting plane turbulent air jets

Temperature distributions within two intersecting air jets have been studied using a Mach Zehnder interferometer. The form of the resultant air stream was found to be qualitatively similar to that of a single free air jet, but its larger angle of spread renders it more useful for many air-conditioning applications.     

Developing turbulent heat transfer in a 360° bend of square cross section

This paper reports the computational results on the developing turbulent heat transfer in a 360° bend of square cross section. The centrifugal force acting on the fluid in a bend flow induces strong cross-stream motion. It was found from the computations that the counterrotating vortex pair caused by the centrifugal force are broken into a multicell pattern after the θ = 90° station of the bend and the continuous arising and ceasing of the vortices directly affects the variation of heat transfer through the walls of the bend. Particular attention was paid to the developing process of the vortices because they exert the most significant effects on the convective heat transfer through the bend walls. A low Reynolds number second moment turbulence closure was employed to simulate the near wall turbulence behaviors of the bend. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 77–88, 1999     

The structure and flame propagation regimes in turbulent hydrogen jets

Experiments on flame propagation regimes in a turbulent hydrogen jet with velocity and hydrogen concentration gradients have been performed. Horizontal stationary hydrogen jets released at normal and cryogenic temperatures of 290, 80 and 35 K with different nozzle diameters and mass flow rates have been investigated. Sampling probe method and laser PIV techniques have been used to evaluate the distribution of hydrogen concentration and flow velocity. High-speed photography combined with a Background Oriented Schlieren (BOS) system was used for the visual observation of the turbulent flame propagation. In order to investigate different flame propagation regimes the ignition position was changed along the jet axis. It was found that the flame propagates in both directions, up- and downstream of the jet flow if hydrogen concentration is >11%, whereas in case [H₂] < 11%, the flame propagates only downstream. This means that at normal temperature the flame is able to accelerate effectively only if the expansion ratio σ of the H₂-air mixture is higher than a critical value σ* = 3.75 defined for a closed geometry.     

An experimental study of air-solid two-phase flow in a 90° bend using LDV system

The measurements of the mean streamwise and radial velocities, the associated turbulence and the relative particle densities were made in an air-solid two-phase flow in a square sectioned (30mm×30mm) 90° vertical to horizontal bend using laser Doppler velocimetry. The radius ratio of the bend was 2.0. Glass beads of 100μm in diameter were employed to form the solid phase. The measurements of air and solid phases were performed separately at the same bulk velocity 19.34m/s, corresponding to a Reynolds number of 3.87×10⁴. The mass ratio of solid to air was 1.6%. The results indicate that the particle trajectories are very close to straight lines. The streamwise velocity profiles for the gas and the solids cross over near the outer wall with the solids having the higher speed. At θ=30° and 45°, particle-wall collisions happen mostly in the region from θ=30° to θ=75°, and cause a sudden change in solid velocity. The particles tend to move towards the outer wall in 90° bend. The particle concentration near the outer wall is much higher than that near the inner wall in the bend, and there are few particles in the inside of the bend. The bend leads to apparent phase separation: at θ=45°, the solids concentrate in the half of the duct near the outer wall. After θ=60° the second peak concentration appears, and goes gradually towards the inner wall.     

Numerical analysis of turbulent sonic jets from two-dimensional convergent nozzles

The numerical analysis of a turbulent sonic jet from a two-dimensional convergent nozzle has been carried out using the compressible k-ɛ turbulence model and TVD finite difference scheme. Numerical conditions have been varied over a range of operating pressure ratios from 1.893 to 6.0 which cover the jet flow conditions from correctly expanded to underexpanded. Numerical flow visualization of sonic jet structure using the computer schlieren, a relation between shock cell length in the jet with the operating pressure ratio and the pressure distribution along jet centerline are obtained. Also, a transition process of a two-dimensional sonic jet from correctly expanded to underexpanded conditions is shown in detail and a flow model of jet structure is proposed.     

Thermal behavior in a 180-deg turned channel with the perforation divider under rotational condition

This study provided a new configuration of the 180-deg turned channel with a perforation divider. The perforations cross the divider let the coolant at the first duct to bypass into the second duct early, preventing from the local high temperature at the downstream zone. Additionally, the perforation-induced disturbing flow should enhance the total heat transfer. This work experimentally investigated the heat transfer behaviors of such system under the rotational condition. The results indicated that the perforation would reduce the raise of the local heat transfer at the turned region. Besides, the rotation would obviously influence the local heat transfer on the leading and trailing surfaces. Furthermore, the effect of rotation on the total heat transfer was insignificant. Finally, the perforation system with the relative perforation diameter d/D_h = 2/4 had the similar total heat-transfer capacity with that without perforation; the total heat-transfer capacity of the perforation system with d/D_h = 1/4 was around 20% higher than the non-perforation one.     

Numerical study of the near-field of highly underexpanded turbulent gas jets

For safety issues related to the storage of gases (e.g. hydrogen) under high pressure, it is necessary to determine how the gas is released in the case of failure. In particular, there exist limited quantitative information on the near-field properties of gas jets, which are important for establishing proper decay laws in the far-field. Simulations of the near-field of highly underexpanded (high pressure) gas jets have been performed using Finite-Volume solver of the CAST3M code and validated using several sources available in the literature. The numerical model solves the 3D Compressible Multi-Component Navier–Stokes equations directly without relying on the compressibility-corrected turbulence models. It provides sufficiently fair mean predictions both in the case of one-component air–air and two-component helium-air releases. Possible initial conditions for the far-field simulations are suggested in terms of distance from the source, as well as the turbulence characteristics and gas-dynamic parameters at this location. In addition, these results are used to evaluate several notional nozzle concepts in order to determine the one physically consistent.     

Modeling of nonreacting and reacting turbulent spray jets using a fully stochastic separated flow approach

Numerical modeling of several turbulent nonreacting and reacting spray jets is carried out using a fully stochastic separated flow (FSSF) approach. As is widely used, the carrier-phase is considered in an Eulerian framework, while the dispersed phase is tracked in a Lagrangian framework following the stochastic separated flow (SSF) model. Various interactions between the two phases are taken into account by means of two-way coupling. Spray evaporation is described using a thermal model with an infinite conductivity in the liquid phase. The gas-phase turbulence terms are closed using the k–ε model. A novel mixture fraction based approach is used to stochastically model the fluctuating temperature and composition in the gas phase and these are then used to refine the estimates of the heat and mass transfer rates between the droplets and the surrounding gas-phase. In classical SSF (CSSF) methods, stochastic fluctuations of only the gas-phase velocity are modeled.Successful implementation of the FSSF approach to turbulent nonreacting and reacting spray jets is demonstrated. Results are compared against experimental measurements as well as with predictions using the CSSF approach for both nonreacting and reacting spray jets. The FSSF approach shows little difference from the CSSF predictions for nonreacting spray jets but differences are significant for reacting spray jets. In general, the FSSF approach gives good predictions of the flame length and structure but further improvements in modeling may be needed to improve the accuracy of some details of the predictions.     

Direct numerical simulation of ignition in turbulent n-heptane liquid-fuel spray jets

Direct numerical simulation was used for fundamental studies of the ignition of turbulent n-heptane liquid-fuel spray jets. A chemistry mechanism with 33 species and 64 reactions was adopted to describe the chemical reactions. The Eulerian method is employed to solve the carrier-gas flow field and the Lagrangian method is used to track the liquid-fuel droplets. Two-way coupling interaction is considered through the exchange of mass, momentum, and energy between the carrier-gas fluid and the liquid-fuel spray. The initial carrier-gas temperature was 1500 K. Six cases were simulated with different droplet radii (from 10 to 30 μm) and two initial velocities (100 and 150 m/s). From the simulations, it was found that evaporative cooling and turbulence mixing play important roles in the ignition of liquid-fuel spray jets. Ignition first occurs at the edges of the jets where the fuel mixture is lean, and the scalar dissipation rate and the vorticity magnitude are very low. For smaller droplets, ignition occurs later than for larger droplets due to increased evaporative cooling. Higher initial droplet velocity enhances turbulence mixing and evaporative cooling. For smaller droplets, higher initial droplet velocity causes the ignition to occur earlier, whereas for larger droplets, higher initial droplet velocity delays the ignition time.     

Numerical study of bubble generation and transport in a serpentine channel with a T-junction

The two-phase flow problem is one of the most critical challenges for direct methanol fuel cells (DMFC). With carbon dioxide generated in the anode catalyst layer, a bubble that could not be removed from the flow channel would hinder the oxidation reaction. In this paper, bubble generation and flow in a micro-serpentine channel is modeled using a multi-phase three-dimensional Navier–Stokes plus volume of fluid (VOF) method. The bubble generation process, which results from the gas–liquid flow and the effects of surface tension and viscosity, are discussed and validated by a comparison with a serious of experimental results. The results prove that the VOF method is an effective method to simulate flow fields where the bubble flow phenomena exist, such as the two-phase flow fields in DMFCs.     

Stress fields produced by a vertical seismic force acting on a 90° elbow or pipe bend

A vertical component of seismic force acts upon a 90° elbow or curved pipe bend lying in an X-Y plane. The stress field, due to the force of magnitude Z, is established and consists of an initial stress field S(0) and a corrective stress field. The solution satisfies the equilibrium and compatibility equations of toroidal elasticity (Ref. 1).     

Stress fields for a normal force acting on the end of a 90° elbow or pipe bend

In a previous paper, eight stress fields were determined for a 90° elbow or pipe bend acted upon by an end shear force directed inwardly. The present paper determines the eight stress fields when an end normal force acts on a 90° elbow or pipe bend. The two problems are dual problems in the sense that they differ only by adjustment of a stress field of pure bending.