Search results

The purpose of this paper is to study flap application in the airfoil comprising a cross flow fan by experiment and numerical simulation.

An airfoil was made and tested in a blowing wind tunnel. Because of complicated shape of the airfoil, distributed quantities in the flow field cannot be measured. They were computed by establishing a CFD code validated by the experimental data. The k‐ε model was used for the Reynolds stress modeling. Flow was considered incompressible, two dimensional and steady‐state. The pressure‐velocity coupling was performed by the SIMPLEC algorithm and convection terms were discretized by using the second‐order upwind discretization scheme.

Computed aerodynamic coefficients were in good agreement with the experimental results. Flap augmented lift and pitching moment coefficients of the airfoil considerably. It was perceived that the airfoil aerodynamic coefficients decrease with the Reynolds number, its lift and pitching moment coefficients increase and its drag coefficient decreases with the fan speed. Static pressure difference between the airfoil surfaces increased with the flap angle and consequently at higher flap angles it must have larger aerodynamic coefficients as proved by the experiments. This pressure difference increases with the Reynolds number that is equivalent to higher aerodynamic forces. It was shown by the numerical solution that surface pressure on the airfoil upper wall decreases with the fan speed while it is not sensitive to the fan speed on the airfoil bottom wall.

This is the first instance in which flap application in the airfoil with forced airflow provided by an integrated cross flow fan is studied.

Performance of various k‐ε models on turbulent forced convection in a channel with periodic ribs is assessed.

The influence of the Yap correction and the non‐linear stress‐strain relation on the predictions of mean‐flow, turbulence quantities and local heat transfer rate is examined. The effect of thermal boundary conditions on the heat transfer predictions is investigated by employing both the prescribed heat flux approach and the conjugate heat transfer approach.

It was found that the inclusion of the Yap correction in the ε‐equation significantly improves the predictions of mean velocity and wall heat transfer for both high‐Reynolds number and low‐Reynolds number k‐ε models in the present ribbed channel flow with massive flow separation. The employment of the non‐linear stress‐strain relation only marginally improves the predictions of turbulence quantities: the turbulence anisotropy is reproduced although the level of turbulence intensity is still too low. In general, the conjugate heat transfer approach predicts better average Nusselt number than the prescribed heat flux approach. However, both approaches under‐predict the experimental value by about 28‐33 percent when the low‐Reynolds number k‐ε model of Lien and Leschziner (1999) with the Yap term is adopted.

Thorough numerical treatments of the thermal boundary conditions at the solid‐liquid interface, and detailed periodic condition in the periodic regime, were given in the paper to benefit researchers interested in solving similar problems.

This paper aims to describe the numerical simulation of a three‐dimensional turbulent free jet issuing from a sharp‐edged equilateral triangular orifice into still air.

The numerical simulation was carried out by solving the governing three‐dimensional Reynolds‐averaged Navier‐Stokes equations. Several two‐equation eddy‐viscosity models (i.e. the standard k‐ε, renormalization group (RNG) k‐ε, realizable k‐ε, shear‐stress transport (SST) k‐ω), as well as the Reynolds stress models (i.e. the standard RSM and the SSG) were tested to simulate the flowfield. The numerical predictions were compared with experimental data in order to assess the capability and limitations of the various turbulent models examined in this work. Findings –The vena contracta effect was predicted by all the tested models. Among the eddy‐viscosity models only the realizable k‐ε model showed good agreement of the near‐field jet decay. None of the eddy‐viscosity models was capable of predicting the profiles of the jet turbulence intensities. The RSMs, especially the standard RSM, were able to produce much better predictions of the features of the jet in comparison with the eddy‐viscosity models. The standard RSM predictions were found to agree reasonably well with the experimental data.

The conclusion, that among the tested RANS turbulence closure models, the RSM appeared the only one capable of reproducing reasonably well the experimental data concerns only the jet flow case examined here. Also, the average computational time for a single run was quite long, i.e. 340 h, but it is believed that parallel computing will reduce it considerably.

The numerical results reported in this paper provide a comparison between several RANS turbulence closure models for simulating a turbulent free jet issuing from an equilateral triangular nozzle.

Different groove angles are used to study performance characteristics of two-axial groove journal bearing. In this study two grooves are located at ±90º to the load line. The various angles of grooves have been taken as 10° to 40° in the interval of 5°. Different equations such as Reynolds equation, three-dimensional energy equation and heat conduction equation have been solved using finite element method and finite difference method. Pressure distribution in fluid is found by using Reynolds equation. The three-dimensional energy equation is used for temperature distribution in the fluid film and bush. One-dimensional heat conduction equation is used for finding temperature in axial direction for journal. There is a very small effect of groove angle on film thickness, eccentricity ratio and pressure. There is a drastic change in attitude angle and side flow. Result shows that there is maximum power loss at large groove angle. So the smaller groove angle is recommended for two-axial groove journal bearing.

The finite element method is used for solving Reynolds equation for pressure distribution in fluid. The finite difference method is adopted for finding temperature distribution in bush, fluid and journal.

Pressure distribution in fluid is found out. Temperature distribution in bush, fluid and journal is found out. There is a very small effect of groove angle on film thickness, eccentricity ratio and pressure.

The groove angle used is from 10 to 40 degree. The power loss is more when angle of groove increases, so smaller groove angle is recommended for this study.

The location of groove angle predicts the distribution of pressure and temperature in journal bearing. It will show the performance characteristics. ±90° angle we will prefer that will get before manufacturing of bearing.

Due to this study, we will get predict how the pressure and temperature distribute in the journal. It will give the running condition of bearing as to at what speed and load we will get the maximum temperature and pressure in the bearing.

The finite element method is used for solving the Reynolds equation. Three-dimensional energy equation is solved using the finite difference method. Heat conduction equation is also solved for journal. The C language is used. The code is developed in C language. There are different equations which depend on each other. The temperature is dependent on pressure viscosity of fluid, etc. so C code is preferred.

In this work, the performance characteristics of thrust bearings are studied by taking into account bearing distortions.

The pressure distribution in the bearing is found using Reynolds' equation. Reynolds' equation is solved under isothermal conditions by assuming oil viscosity constant. The amount of deformation in the bearing is determined using constitute equations applied to linear elastic materials. It is preferred to work with dimensionless equations for the result to be meaningful. The results of the finite differences method gives superior performance for analyzing the behavior of a bearing undergoing on elastic deformation. The amount of deformation which corresponds to a certain pressure and load carrying capability of the bearing after and before deformation are found and compared.

The comparison is carried out for steel‐steel and steel‐bronze material pairs. It is found that there is a decrease of 2.13 percent in the load carrying capacity of the bearing for the material pair of steel‐bronze. This decrease may be important for some applications.

The present investigation includes distortion of pads only. Distortion of runner is ignored since it is assumed to be rigid. This assumption may cause serious deviations in computing performance characteristics.

This paper aims to investigate the fluid flow regime and the elastic effect in a plain cylindrical journal bearing subjected to highly severe operating velocity to better predict the behavior of the bearing for the turbulent flow regime.

A numerical analysis of the behavior of an elastohydrodynamic for cylindrical journal bearing finite dimension coated with antifriction material in turbulent regime, is implemented using the code-ANSYS CFX. This analysis is performed by solving the Navier–Stocks equations of continuity by the method of finite volume for rotational speeds ranging from 6,000 to 15,000 rpm, that is to say for different Reynolds number.

This study aims to better predict the elastic behavior in a journal bearing subjected to severe operating conditions. The speed of rotation varies from 6,000 to 15,000 rpm.

The results clearly show that significant pressures are applied in the extreme case of speed, that is to say to the turbulent regime. There is an emergence of new rupture zone pressure, we do not usually see the regime established; the level of the supply groove. Displacement of shaft relative to the bearing is remarkable by introducing the elastic effect and the turbulent regime.

An Unsteady Reynolds‐Averaged Navier‐Stokes (URANS) equation method has been applied to compute the flow over two‐dimensional smooth topography and compared with conventional RANS and large‐eddy simulation (LES) results. The URANS calculation with sufficient grid resolution near solid surface and an appropriate near‐wall model has been shown to simulate much of the large‐scale unsteadiness and some of the turbulent motion for flows with and without separation. Although the results with unadjusted model constants do not show an overwhelming improvement over a standard two‐equation model, it is demonstrated that it may be improved and, more importantly, can be generalized to a new simulation technique by refining the model, considering such factors as grid‐dependent length scales and by making a three‐dimensional calculation.

The purpose of this paper is to obtain a better understanding of the rotor–stator interaction in the vaneless region of a centrifugal pump.

A third-order sub-grid scale (SGS) model containing the rotation rate tensor named the dynamic cubic non-linear model (DCNM) is used for simulating the flow field in a centrifugal pump with a vaned diffuser. The pressure coefficient and velocity distributions are compared with the experimental data. Focusing on the vaneless region, the pressure pulsation, Reynolds stress pulsation and Reynolds stress transport equation are analyzed.

The comparison of the calculation results with the experimental data indicates that the DCNM can accurately capture the distributions of pressure and velocity in the vaneless region. Based on the instantaneous pressure signals, the pressure pulsation is analyzed to show that in the vaneless region, the dominant frequency near the impeller is twice the blade passing frequency, whereas it is equal to the blade passing frequency near the diffuser. Further exploration of the Reynolds stress pulsation shows the correlation between the two variables. Additionally, the extreme low frequency of Reynolds stress near the diffuser is found to be related to the rotation instability. To explore the turbulence characteristics in the vaneless region, the Reynolds stress transportation equation is studied. In the vaneless region, the rotation term of the Reynolds stress transport equation is negligible compared to the production term, although the rotation instability is obvious near the diffuser. The production of the Reynolds stress plays the role of redistributing the energy from the uu component to the vv component, except for the region near the impeller outlet.

The third-order SGS model DCNM has proved to be promising in simulating the rotor–stator interaction. The analysis of the rotation instability and the Reynolds stress transport equation shed light on the further understanding of the rotor–stator interaction.

The purpose of this paper is to examine the thermal and dynamic performance of the plate-fin heat sink fitted with a shield in the bypass.

The governing equations were solved using the finite volume method based on the SIMPLE algorithm. The k-ω Shear Stress Transport was used to model turbulence. The thermal and dynamic results were presented in term of average Nusselt number and friction factor, respectively. The effect of the height (Hs=6, 10 and 13) and the position (X=0, 1/3, 1/2, 2/3 and 3/4) of the shield was studied for a Reynolds number ranging from 2×103 to 12×103 and compared with a heat sink without shield. To evaluate the performance of different heat sink geometries, the efficiency was presented and discussed.

By adding a shield in the bypass, a greater amount of air is injected between the heat sink fins, which improves the heat transfer (advantage) of the one part, and increases the friction on the other hand (disadvantage). The efficiency of the heat sink varies inversely proportional with the Reynolds number.

The originality of this work is the method for enhancement of heat transfer.

The purpose of this paper is to develop new semi-empirical heat transfer correlations for turbulent flow of liquid metals in the tubes, and then to compare these correlations with the experimental data. The Prandtl and Reynolds numbers can vary in the ranges: 0.0001 ≤ Pr ≤ 0.1 and 3000 ≤ Re ≤ 10⁶.

The energy conservation equation averaged by Reynolds was integrated using the universal velocity profile determined experimentally by Reichardt for the turbulent tube flow and four different models for the turbulent Prandtl number. Turbulent heat transfer in the circular tube was analyzed for a constant heat flux at the inner surface. Some constants in different models for the turbulent Prandtl number were adjusted to obtain good agreement between calculated and experimentally obtained Nusselt numbers. Subsequently, new correlations for the Nusselt number as a function of a Peclet number was proposed for different models of the turbulent Prandtl number.

The inclusion of turbulent Prandtl number greater than one and the experimentally determined velocity profile of the fluid in the tube while solving the energy conservation equation improved the compatibility of calculated Nusselt numbers, with Nusselt numbers determined experimentally. The correlations proposed in the paper have a sound theoretical basis and give Nusselt number values that are in good agreement with the experimental data.

Heat transfer correlations proposed in this paper were derived assuming a constant heat flux at the inner surface of the tube. However, they can also be used for a constant wall temperature, as for the turbulent flow (Re > 3,000), the relative difference between the Nusselt number for uniform wall heat flux and uniform wall temperature is very low.

Unified, systematic approach to derive correlations for the Nusselt number for liquid metals was proposed in the paper. The Nusselt number was obtained from the solution of the energy conservation equation using the universal velocity profile and eddy diffusivity determined experimentally, and various models for the turbulent Prandtl number. Four different relationships for the Nusselt number proposed in the paper were compared with the experimental data.