Turbulence Shear Stress Transport Research Articles

Numerical study of transverse hydrogen injection in high-speed reacting crossflow

A high-speed compressible solver capable of solving detailed chemical reaction mechanisms is developed by coupling the open-source computational fluid dynamic toolbox OpenFOAM® and Cantera 2.5.0. A sonic hydrogen jet discharging from a circular injector into a high enthalpy supersonic crossflow over a flat plate is selected as a test case for the developed solver. The incoming boundary layer is laminar, and an adverse pressure gradient-induced transition is expected due to transverse injection. The test case is selected to serve two purposes. First, to validate the developed solver. Second, to inspect the capability of Reynolds-Averaged Navier–Stokes (RANS) in predicting the flame characteristics in high-speed flows involving laminar to turbulent transition. The present study features three-dimensional RANS simulations with Shear Stress Transport (SST) k–ω and Langtry–Menter SST k–ω turbulence models, with three values of inlet turbulent intensity: I = 0.5, 1, and 2. Analysis showed that laminar to turbulent transition plays a significant role in the resulting flame structure. A fully turbulent SST k–ω model showed several discrepancies from the experiment, especially near the boundary layer. On the other hand, the Langtry–Menter SST k–ω model predicts transition onset and hence the flame structures accurately. Furthermore, the transition onset and the flame structure strongly depend on I. The low-velocity recirculation regions near the injector aid in flame stabilization upstream of the injector. At the same time, the horseshoe vortex dictates the flame spread in a spanwise direction. The reflected shock–boundary layer interaction helps in flame stabilization downstream of the injector.

Maximizing the thermal performance index applying evolutionary multi-objective optimization approaches for double pipe heat exchanger

The thermal performance index (TPI) known as the efficiency parameter evaluates the highlight of the heat transfer for the same pumping power requirement. The optimum design of a heat exchanger with minimum pressure drops and efficient heat transfer is a big challenging task from an energy-saving point of view. One way to optimize this efficiency parameter is by changing the geometric parameters of the heat exchanger. For this purpose, the multi-objective reinforcement learner non-dominated sorting genetic algorithm (NSGA-RL) is employed maximizing TPI and Nusselt number and minimizing Fanning friction factor to the optimal design of a double pipe heat exchanger, with perforated baffles in the annulus side. Its performance is compared with a non-dominated sorting genetic algorithm - version II (NSGA-II), and a chaotic non-dominated sorting genetic algorithm (CNSGA). The calculations were carried out using the turbulent shear stress transport k-ω model. The multi-objective optimization techniques presented promising results in terms of the number of nondominated solutions, Euclidean distances to the origin point (reference point), and hypervolume indicators. However, in general, the results based on mean values indicate that the NSGA-RL outperforms the NSGA-II and CNSGA concerning the three performance indicators for the two double pipe heat exchanger evaluated cases. TPI increases 78% for case 1, and 108% for case 2 compared without baffles in a double pipe heat exchanger (DPHE). Numerical results revealed that DPHE employing internal perforated baffles improved heat transfer significantly with Nusselt number increased around 7.93–8.25 times and friction factor increased by 6.5–9.75 times compared with plain DPHE.

Study on the multi-field coupling model of electrolyte temperature distribution in electrochemical machining

The temperature distribution of electrolyte in ECM affects the distribution of machining gap and the precision of forming. Considering the influence of hydrogen bubbles and electrolyte with low velocity near the wall on heat transfer, the Euler–Euler two-fluid model was used to calculate the gas volume fraction distribution and the turbulent shear stress transport (SST) model was used to calculate the electrolyte velocity. The multi-field coupling model of electric field, flow field, and temperature field was established to solve the temperature of electrolyte. The temperature detection device is set up to detect the inlet, outlet temperature of the electrolyte, and the temperature of the sampling points on the workpiece. The experimental results show that the results of the coupled model considering the hydrogen bubbles and the low flow velocity electrolyte are more accurate than those of the turbulent SST and k − e models ignoring the influence of the bubbles.

Mathematical simulation of fluid dynamics during steel draining operations from a ladle

Fluid flow dynamics during ladle drainage operations of steel under isothermal and nonisothermal conditions has been studied using the turbulence shear stress transport k-e model (SST k-ω) and the multiphase volume of fluid (VOF) model. At high bath levels, the angular velocity of the melt, close to the ladle nozzle, is small rotating anticlockwise and intense vertical-recirculating flows are developed in most of the liquid volume due to descending steel streams along the ladle vertical wall. These streams ascend further downstream driven by buoyancy forces. At low bath levels, the melt, which is close to the nozzle, rotates clockwise with higher velocities whose magnitudes are higher for shorter ladle standstill times. These velocities are responsible for the formation and development of a vortex on the bath free surface, which entrains slag into the nozzle by shear-stress mechanisms at the metal-slag interface. The critical bath level or bath height for this phenomenon is 0.35 m (in this particular ladle design) for a ladle standstill time of 15 minutes and decreases with longer ladle standstill times. At these steps, the vertical-recirculating flows are substituted by complex horizontal-rotating flows in most of the liquid volume. Under isothermal conditions, the critical bath level for vortex formation on the melt free surface is 0.20 m, which agrees very well with that determined with a 1/3 scale water model of 0.073 m. It is concluded that buoyancy forces, originated by thermal gradients, as the ladle cools, are responsible for increasing the critical bath level for vortex formation. Understanding vortex mechanisms will be useful to design simple and efficient devices to break down the vortex flow during steel draining even at very low metal residues in the ladle.