Search results

This study evaluates low Reynolds number models of turbulence for numerical computations on the heat transfer and fluid flow behavior in a rectangular channel with streamwise‐periodic ribs mounted on one of the principal walls. The models include k − ε models of Launder and Sharma (1974), Chien (1982), k − ε model of Lin and Hwang (1998), Wilcox’s k−ω model (Wilcox, 1994) and Durbin’s model k − ε −v² (Durbin, 1995). The numerical results show that all these models can predict the flowfield reasonably well, and the inclusion of the Yap term (Yap, 1987) in the ε – equation (or ε – equation) can further improve the prediction in these k − ε models, k − ε model and k − ε − v² model. However, these models behave differently in heat transfer computations. The k − ω model leads to too low a level of heat transfer and turbulence. Among these k − ε models and the k − ε model, Lin’s model with the Yap term predicts the heat transfer level best. Durbin’s model with extra v², f equations and the Yap term exhibits further improvement.

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.

This paper presents finite volume computations of turbulent flow through a square cross‐sectioned U‐bend of curvature strong enough (Rc/D =0.65) to cause separation. A zonal turbulence modelling approach is adopted, in which the high‐Re k‐ε model is used over most of the flow domain with the low‐Re, I‐equation model of k‐transport employed within the near‐wall regions. Computations with grids of different sizes and also with different discretization schemes, demonstrate that for this flow the solution of the k and ε equations is more sensitive to the scheme employed in their convective discretization than the solution of the mean flow equations. To avoid the use of extremely fine 3‐Dimensional grids, bounded high order schemes need to be used in the discretization of the turbulence transport equations. The predictions, while encouraging, displayed some deficiencies in the downstream region due to deficiencies in the turbulence model. Evidently, further refinements in the turbulence model are necessary. Initial computations of flow and heat transfer through a rotating U‐bend, indicate that at rotational numbers (Ro = ΩD/W_b) relevant to blade cooling passages, the Coriolis force can substantially modify the hydrodynamic and thermal behaviour.

This paper examines various turbulence models for numerical simulation of a steady, two-dimensional (2-D) plane wall jet without co-flow using the commercial CFD software (ANSYS FLUENT 14.5). The purpose of this paper is to decide the most suitable and most economical method for steady, 2-D plane wall jet simulation.

Seven Reynolds-averaged Navier–Stokes (RANS) turbulence models were evaluated with respect to typical jet scaling parameters such as the jet half-height and the decay of maximum jet velocity, as well as coefficients from the law of the wall and for skin friction. Then, a plane wall jet generating from a rectangular slot of 1:6 aspect ratio located adjacent to the wall was investigated in a three-dimensional (3-D) model using large eddy simulation (LES) and the Stress-omega Reynolds stress model (SWRSM), with the results compared to experimental measurements.

The comparisons of these simulated flow characteristics indicated that the SWRSM was the best of the seven RANS models for simulating the turbulent wall jet. When scaled with outer variables, LES and SWRSM gave generally indistinguishable mean velocity profiles. However, SWRSM performed better for near-wall mean velocity profiles when scaled with inner variables. In general, the results show that LES performed reasonably well when predicting the Reynolds stresses.

The main contribution of this article is in determining the capabilities of different RANS turbulence closures and LES for the prediction of the 2-D steady wall jet flow to identify the best modelling approach.

A numerical study is performed to investigate turbulent flow characteristics in a pipe rotating around the axis. Emphasis is placed on the effect of pipe rotation on the friction coefficient and velocity distribution in the hydrodynamically, fully‐developed flow region. The k—ε turbulence model is modified by taking the swirling effect into account, in which the model function including the Richardson number is introduced to the ε equation. The governing boundary‐layer equations are discretized by means of a control volume finite‐difference technique for numerical computation. Results obtained from the modified model agree well with experiment data in the existing literature. It is found from the study that (i) an axial rotation of the pipe induces an attenuation in the turbulent kinetic energy, resulting in a reduction in the friction coefficient, the turbulent and (ii) an increase in the velocity ratio causes substantial decreases in the friction coefficient, the turbulent kinetic energy and the streamwise velocity gradient near the wall.

The use of natural ventilation by large openings to maintain thermal comfort conditions in the premises is a concept that is perfectly integrated into the traditional architecture of countries in the Mediterranean region or in tropical climates. In a temperate climate where the architecture is not usually designed to respond to the use of natural ventilation is seasonal and is done at the initiative of the occupants by making changes in the design of their doors. The European interest in natural ventilation, as a passive building air-conditioning technology, is increasing and has been the subject of a research program commissioned by the European Community. In this work, the authors consider a part of a housing compound as a refreshing floor. This floor is maintained at a constant cold temperature, the one vertical wall at hot temperature and other surfaces are adiabatic. Various scenarios are considered for this work. Mixed convection for different boundary conditions and different configurations is carried out. In addition, an airflow is injected through a window and extracted on the opposite window. Classical conclusion and transitional value on Richardson number have been completed by the new thermal configuration with nonsymmetric thermal conditions. The complex 3D flow structure is more obvious when one of the two flows (ventilation or natural convection) dominates. However, the induced heat transfer is less sensitive to the added ventilation. In this study, the authors consider a part of a housing compound as a refreshing floor. This floor is maintained at a constant cold temperature, the one vertical wall at hot temperature and other surfaces are adiabatic.

This is a qualitative preliminary study of a 2D–3D flow. The authors examine the competition between the natural convective flow and the added airflow on the flow structure and indoor air quality. The numerical model shows a good agreement with that obtained by researchers analytically and experimentally. To deal with turbulence, the RNG k-ε model has been adopted in this study.

The transfer is more sensitive between the 2D and 3D cases for the present analyzed case.

The study of ventilation efficiency has shown the competition between the big and small structures and the induced discomfort.

Impinging jets have been widely studied, and the addition of swirl has been found to be beneficial to heat transfer. As there is no literature on Reynolds-averaged Navier Stokes equations (RANS) nor experimental data of swirling jet flows generated by a rotating pipe, the purpose of this study is to fill such gap by providing results on the performance of this type of design.

As the flow has a different behaviour at different parts of the design, the same turbulent model cannot be used for the full domain. To overcome this complexity, the simulation is split into two coupled stages. This is an alternative to use the costly Reynold stress model (RSM) for the rotating pipe simulation and the SST k-ω model for the impingement.

The addition of swirl by means of a rotating pipe with a swirl intensity ranging from 0 up to 0.5 affects the velocity profiles, but has no remarkable effect on the spreading angle. The heat transfer is increased with respect to a non-swirling flow only at short nozzle-to-plate distances H/D < 6, where H is the distance and D is the diameter of the pipe. For the impinging zone, the highest average heat transfer is achieved at H/D = 5 with swirl intensity S = 0.5. This is the highest swirl studied in this work.

High-fidelity simulations or experimental analysis may provide reliable data for higher swirl intensities, which are not covered in this work.

This two-step approach and the data provided is of interest to other related investigations (e.g. using arrays of jets or other surfaces than flat plates).

This paper is the first of its kind RANS simulation of the heat transfer from a flat plate to a swirling impinging jet flow issuing from a rotating pipe. An extensive study of these computational fluid dynamics (CFD) simulations has been carried out with the emphasis of splitting the large domain into two parts to facilitate the use of different turbulent models and periodic boundary conditions for the flow confined in the pipe.

In this paper a numerical simulation, based on finite difference techniques, has been developed in order to analyse turbulent natural and mixed convection of air in internal flows. The study has been restricted to two‐dimensional cavities with the possibility of inlet and outlet ports, and with internal heat sources. Turbulence is modelled by means of two‐equation k‐ε turbulence models, both in the simplest form using wall functions and in the more general form of low‐Reynolds‐number k‐ε models. The couple time average governing equations (continuity, momentum, energy, and turbulence quantities) are solved in a segregated manner using the SIMPLEX method. An implicit control volume formulation of the differential equations has been employed. Some illustrative numerical results are presented to study the influence of geometry and boundary conditions in cavities. A comparison of different k‐ε turbulence models has also been presented.

Convective heat transfer features of a turbulent slot jet impingement are comprehensively studied using two different computational approaches, namely, URANS (unsteady Reynolds-averaged Navier–Stokes equations) and SAS (scale-adaptive simulation). Turbulent slot jet impingement heat transfer is used where a considerable heat transfer enhancement is required, and computationally, it is a quite challenging flow configuration.

Customized OpenFOAM 4.1, an open-access computational fluid dynamics (CFD) code, is used for SAS (SST-SAS k-ω) and URANS (standard k-ε and SST k-ω) computations. A low-Re version of the standard k-ε model is used, and other models are formulated for good wall-refined calculations. Three turbulence models are formulated in OpenFOAM 4.1 with second-order accurate discretization schemes.

It is observed that the profiles of the streamwise turbulence are under-predicted at all the streamwise locations by SST k-ω and SST SAS k-ω models, but follow similar trends as in the reported results. The standard k-ε model shows improvements in the predictions of the streamwise turbulence and mean streamwise velocity profiles in the zone of outer wall jet. Computed profiles of Nusselt number by SST k-ω and SST-SAS k-ω models are nearly identical and match well with the reported experimental results. However, the standard k-ε model does not provide a reasonable profile or quantification of the local Nusselt number.

Hybrid turbulence model is suitable for efficient CFD computations for the complex flow problems. This paper deals with a detailed comparison of the SAS model with URANS and LES for the first time in the literature. A thorough assessment of the computations is performed against the results reported using experimental and large eddy simulations techniques followed by a detailed discussion on flow physics. The present results are beneficial for scientists working with hybrid turbulence models and in industries working with high-efficiency cooling/heating system computations.

The swirling flow of sudden‐expansion dump combustor with central V‐gutter flameholder and six side‐inlets is studied by employing the SIMPLE‐C algorithm and Jones‐Launder k‐ε two‐equation turbulent model. Both combustion models of one‐step with infinite chemical reaction rate and two‐step with finite chemical reaction rate of eddy‐breakup (EBU) model are used to solve the present problem. The results agreed well with available prediction data in terms of axial‐velocity and total pressure coefficient along combustor centerline. The flowfield structure of combustor considered is strongly affected by swirling, flameholder and side‐inlet flow. For the fixed strength of swirling, the length of central recirculation zone is decreased when the angle of V‐gutter is increased. The outlet velocity of combustor in reacting flow is higher than that in cold flow because the released heat of combustion causes the decrease of density throughout the combustor flowfield. The distribution of mass fraction of various species in reacting process depends on the mixing effect, chemical kinetic and the geometric configuration of combustor.