Search results

This article examines the influence of centrifugal buoyancy on the hydrodynamic and thermal behaviour in fully developed flow through an orthogonally rotating duct of aspect ratio 2:1. A series of computations have been performed at rotation numbers ranging from 0 to 0.2, for constant‐density flows (no buoyancy) and also for different levels of outward and inward buoyancy. The resulting comparisons reveal that for a Reynolds number of 32,500, rotational buoyancy effects become significant at Rayleigh number values greater than 107. In outward flows, buoyancy is found to strengthen the effects of the Coriolis force on the mean motion and, by raising turbulence levels, buoyancy also enhances wall heat transfer along both the pressure and the suction side of the rotating duct. In inward flows, it is found that strong buoyancy can reverse the direction of the Coriolis‐induced secondary motion, which causes a strong rise in wall heat transfer along the suction side and a similarly significant fall in heat transfer along the pressure side. The computed effects on heat transfer are in qualitative agreement with the findings of a number of experimental studies. For both inward and outward flows, at a constant Reynolds number, the modifications of centrifugal buoyancy on the side‐averaged levels of heat transfer correlate reasonably well with the rotational Rayleigh number.

To assess how effectively two‐layer and low‐Reynolds‐number models of turbulence, at effective viscosity and second‐moment closure level, can predict the flow and thermal development through orthogonally rotating U‐bends.

Heat and fluid flow computations through a square‐ended U‐bend that rotates about an axis normal to both the main flow direction and also the axis of curvature have been carried out. Two‐layer and low‐Reynolds‐number mathematical models of turbulence are used at effective viscosity (EVM) level and also at second‐moment‐closure (DSM) level. In the two‐layer models the dissipation rate of turbulence in the new‐wall regions is obtained from the wall distance, while in the low‐Re models the transport equation for the dissipation rate is extended right up to the walls. Moreover, two length‐scale correction terms to the dissipation rate of turbulence are used with the low‐Re models, and original Yap term and a differential form that does not require the wall distance (NYap). The resulting predictions are compared with available flow measurements at a Reynolds number of 100,000 and a rotation number (ΩD/U_bl) of 0.2 and also with heat transfer measurements at a Reynolds number of 36,000, rotation number of 0.2 and Prandtl number of 5.9 (water).

While the main flow features are well reproduced by all models, the development of the mean flow within the just after the bend in better reproduced by the low‐Re models. Turbulence levels within the rotation U‐bend are under‐predicted, but DSM models produce a more realistic distribution. Along the leading side all models over‐predict heat transfer levels just after the bend. Along the trailing side, the heat transfer predictions of the fully low‐Re DSM with the differential length‐scale correction term NYap are close to the measurements, with an average error of around 10 per cent, though at the bend exit it rises to 25 per cent. The introduction of a differential form of the length‐scale correction term to improve the heat transfer predictions of both low‐Re models.

The numerical models assumed that the flow remains steady and is not affected by large‐scale, low frequency fluctuations. Unsteady RANS computations or LES must also be tested in the future.

This work has expanded the range of complex turbulent flow over which the effectiveness of RANS models has been tested, to internal cooling flows simultaneously affected by orthogonal rotation and strong curvature.

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.

A numerical investigation has been undertaken to study fluid flow and heat transfer through artificially rib‐roughened channels. Such flows are of particular interest in internal cooling of advanced gas turbine blades. The main objective is to test the suitability of recently developed variants of the cubic non‐linear k‐ε model for the prediction of cooling flows through ribbed passages. The numerical approach used in this study is the finite‐volume method together with the SIMPLE algorithm. For the modelling of turbulence, the Launder and Sharma low‐Re k‐ε model and a new version of the non‐linear low‐Re two equation model that have been recently shown to produce reliable thermal predictions in impinging jet flows and also flows through pipe expansions, have been employed. Both models have been used with the form of the length‐scale correction term to the dissipation rate originally proposed by Yap and also more recently developed differential version, NYap. The numerical results over a range of flow parameters have been compared with the reported experimental data. The mean flow predictions show that both linear and non‐linear k‐ε models with NYap can successfully reproduce the distribution of the measured streamwise velocity component, including the length and width of the separation bubble, formed downstream of each rib. As far as heat transfer predictions are concerned, the recent variant of the non‐linear k‐ε leads to marked improvements in comparison to the original version of Craft et al. Further improvements in the thermal prediction result through the introduction of a differential form of the turbulent length scale correction term to the dissipation rate equation. The version of the non‐linear k‐ε that has been shown in earlier studies to improve thermal predictions in pipe expansions and impinging jets; it is thus found to also produce reasonable heat transfer predictions in ribbed passages.

Low‐Re turbulence models are used in the computation of convective heat transfer in two‐dimensional ribbed passages. The cases computed include ribbed annular channels, pipes and plane channels. The models investigated cover both zonal models, that obtain the near‐wall dissipation rate from the wall distance, and full low‐Re models. Effective viscosity modes and simple (basic) second‐moment closures are used. Zonal models display predictive weaknesses in the rib‐induced separation region, but return reasonable heat transfer levels. For the low‐Re models an alternative length‐scale‐correction term to the one proposed by Yap is developed, which is independent of the wall distance. This wall‐independent correction term is found to improve heat transfer predictions, especially for the low‐Re k‐ε model. The low‐Re models produce a more realistic heat transfer variation in the separation region and reasonable Nusselt number levels. The differential second‐moment closure (DSM) models improve heat transfer predictions after re‐attachment and over the rib surface. The effect of Reynolds number on the Nusselt number is not, however, fully reproduced by the models tested.

This paper aims to predict turbulent flow and heat transfer through different channels with periodic dimple/protrusion walls. More specifically, the performance of various low-Re k-ε turbulence models in prediction of local heat transfer coefficient is evaluated.

Three low-Re number k-ε turbulence models (the zonal k-ε, the linear k-ε and the nonlinear k-ε) are used. Computations are performed for three geometries, namely, a channel with a single dimpled wall, a channel with double dimpled walls and a channel with a single dimple/protrusion wall. The predictions are obtained using an in house finite volume code.

The numerical predictions indicate that the nonlinear k-ε model predicts a larger recirculation bubble inside the dimple with stronger impingement and upwash flow than the zonal and linear k-ε models. The heat transfer results show that the zonal k-ε model returns weak thermal predictions in all test cases in comparison to other turbulence models. Use of the linear k-ε model leads to improvement in heat transfer predictions inside the dimples and their back rim. However, the most accurate thermal predictions are obtained via the nonlinear k-ε model. As expected, the replacement of the algebraic length-scale correction term with the differential version improves the heat transfer predictions of both linear and nonlinear k-ε models.

The most reliable turbulence model of the current study (i.e. nonlinear k-ε model) may be used for design and optimization of various thermal systems using dimples for heat transfer enhancement (e.g. heat exchangers and internal cooling system of gas turbine blades).

Re<95,000 based on hydraulic diameter, heat transfer and turbulent flow through a rectangular‐sectioned 90° bend was investigated numerically and experimentally. To develop turbulence level, square prism and cylindrical obstacles was placed in the center of the bend.

For heat transfer, uniform heat flux of 5,000 W/m² from bend surfaces is assumed. Numerical analysis was realized for both the turbulent flow and heat transfer. For numerical study, FLUENT 6.1.22 code, RSM turbulence model, hybrid hexahedral‐tetrahedral cell structures and uniform inlet velocity assumption were selected. For the pressure distribution in the bend and velocity profile at the outlet of the bend, the experiments was carried out by means of manometers with ethyl alcohol, Mano‐air 500 Equipment and pitot‐static tube.

There was a high level of validation obtained between the numerical and the experimental results. Thereby, the mentioned numerical calculation method can be used most engineering applications. For Re>20,000, the square prism obstacles provide higher turbulence level and more favorable heat transfer than cylindrical obstacles. For Re<20,000, the obstacle use would not require for enhanced heat transfer aim. The obstacle in the bend cause considerably pressure drop in the bend.

The turbulent flow in the bend without obstacle has been numerically investigated by various turbulence models with the non‐refined mesh structure and various wall functions. For numerical solution of the turbulence flows and the heat transfer in the rectangular bend with obstacles, the FLUENT code and RSM turbulence model with enhanced wall functions are selected. In order to adapt the cell size and number to the turbulent flow the mesh structure was refined over curvature of turbulence dissipation rate in the bend.

This paper aims to compute flow and heat transfer through a straight, orthogonally rotating duct, with ribs along the leading and trailing walls, in a staggered arrangement and at an angle of 45° to the main flow direction.

Flow computations have been produced using a 3D non‐orthogonal flow solver, with two two‐layer models of turbulence (an effective‐viscosity model and a second‐moment closure), in which across the near‐wall regions the dissipation rate of turbulence is obtained from the wall distance. Flow comparisons have been carried out for a Reynolds number of 100,000 and for rotation numbers of 0 (stationary) and 0.1. Temperature comparisons have been obtained for a Reynolds number of 36,000, a Prandtl number of 5.9 (water) and rotation numbers of 0 and 0.2 and also at a Prandtl number of 0.7 (air) and a rotation number of 0.

It was found that both two‐layer models returned similar flow and thermal predictions which are also in close agreement with the flow and thermal measurements. The flow and thermal developments are found to be dominated by the rib‐induced secondary motion, which leads to strong span‐wise variations in the mean flow and the local Nusselt number and to a uniform distribution of turbulence intensities across the duct. Rotation causes the development of stronger secondary motion along the pressure side of the duct and also the transfer of the faster fluid to this side. The thermal predictions, especially those of the second‐moment closure, reproduce the levels and most of the local features of the measured Nusselt number, but over the second half of the rib interval over‐predict the local Nusselt number.

The work contributes to the understanding of the flow and thermal development in cooling passages of gas turbine blades, and to the validation of turbulence models that can be used for their prediction, at both effective viscosity and second‐moment closure levels.

This study is concerned with the computation of turbulent flow and heat transfer in U‐bends of strong curvature. Following the earlier studies within the authors' group on flows through round‐ended U‐bends, here attention is turned to flows through square‐ended U‐bends. Flows at two Reynolds numbers have been computed, one at 100,000 and the other at 36,000. In the heat transfer analysis, the Prandtl number was either 0.72 (air) or, in a further departure from our earlier studies, 5.9 (water). The turbulence modelling approaches examined, include a two‐layer and a low‐Re k‐ε model, a two‐layer and a low‐Re version of the basic differential stress model (DSM) and a more recently developed, realisable version of the differential stress model that is free of wall‐parameters. For the low‐Re effective viscosity model (EVM) and DSMs, an alternative, recently proposed length‐scale correction term, independent of wall distance has also been tested. Even the simplest model employed – two‐layer EVM – reproduces the mean flow development with reasonable accuracy, suggesting that the mean flow development is mainly influenced by mean pressure rather than the turbulence field. The heat transfer parameters, on the other hand, show that only the low‐Re DSMs produce reliable Nusselt number predictions for both Prandtl numbers examined.

The purpose of this paper is to investigate turbulent fluid flow and heat transfer through passages with an array of either detached or alternative attached‐detached ribs of square cross‐section.

The finite‐volume method in a partially staggered grid system has been applied. For the modeling of turbulence, the zonal as well as the linear and non‐linear low‐Reynolds number k  −  ε models have been employed.

The numerical results show that the presence of the ribs produces a very complex flow in the channel. The mean flow predictions for the channel with detached ribs show that the low‐Re k  −  ε models are able to reproduce most of the experimentally observed flow features away from the ribbed wall, but return lower stream‐wise velocities close to the wall. Additionally, all low‐Re k  −  ε models underpredict the stream‐wise turbulence intensity whilst producing correct cross‐stream turbulence intensity levels close to the measured data. All three turbulence models fail to completely reproduce the distribution of Nusselt number. Among three turbulence models examined in this work, the zonal k  −  ε model produces the best heat transfer predictions.

The work contributes in understanding of the flow and thermal development in passages with detached ribs. The present set of 2D and steady heat and fluid flow comparisons establishes a base‐level for more realistic three‐dimensional and unsteady computations. The results of this study may be of interest to engineers attempting to re‐design the internal cooling system of gas turbine blades and to researchers interested in the turbulent flow‐modification aspects of heat transfer enhancement of forced convection in ribbed passages.