Mehrdad Raisee Dehkordi; Arman Rokhzadi
Abstract
Roughness elements or turbulence promoters have been widely used to enhance heat transfer in cooling passages of modern gas turbine blades. Although such ribs substantially enhance heat transfer, the heat transfer coefficient is reduced immediately at corner downstream of each rib, creating hot spots. ...
Read More
Roughness elements or turbulence promoters have been widely used to enhance heat transfer in cooling passages of modern gas turbine blades. Although such ribs substantially enhance heat transfer, the heat transfer coefficient is reduced immediately at corner downstream of each rib, creating hot spots. To remove such hot spots some of the ribs can be detached from the channel walls. In this paper, this idea is investigated using numerical methods. In this study, turbulent flow and heat transfer through three types of channels, namely: 1) a channel with detached ribs from a wall 2) a channel with detached ribs from both walls and 3) a channel with alternative attached-detached ribs on both walls have been investigated. Computations presented in this research have been obtained with the linear and non-linear k-? models. The numerical method in this work is finite volume methodology and simple algorithm. The governing equations are discretized in a semi-staggered grid system. In all equations the convective terms are approximated using Hybrid scheme. The numerical results for the channel with detached ribs close to one principal wall showed that both the linear and non-linear k-? models are able to predict the length and width of the wake downstream of the detached ribs, although both models produce weaker wakes compared with experimental data. Both models "specially the non-linear k-? model" predict lower stream-wise velocity and turbulent intensities closed to the ribbed wall. As a result, both turbulence models "and specially the non-linear k-? model" substantially under-predict the wall heat transfer. For the channel with ribs detached close to both walls, both turbulence models produce better heat transfer predictions though still predict lower heat transfer levels. Finally, for the channel with alternative attached-detached ribs, both turbulence models fail to predict reliable heat transfer levels in first half of the channel but return acceptable Nusselt levels in the second half of the channel.
Hooman Naimi; Mehrdad Raisee
Abstract
The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square cross-section. Such flows are of direct relevance to the internal cooling system of modern gas turbine blades. The main objective is to assess how a recently developed variant ...
Read More
The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square cross-section. Such flows are of direct relevance to the internal cooling system of modern gas turbine blades. The main objective is to assess how a recently developed variant of a cubic non-linear model (proposed by Craft et al. (1999)), that has been shown to produce reliable thermal predictions through axi-symmetric and plane two-dimensional ribbed passages (Raisee et al. (2004)), can predict flow and heat transfer characteristics through more complex three-dimensional ribbed ducts. To fulfil this objective, the present paper discusses turbulent air flow and heat transfer through two different configurations, namely: (I) a square duct with “in-line” ribs normal to the flow direction at and (II) a square duct with normal ribs in a “staggered” arrangement at . In this paper the flow and thermal predictions of the linear model (EVM) are also included, as a set of baseline predictions. Both turbulence models have been used with the form of length-scale correction term to the dissipation rate originally proposed by ‘Yap’ and also a differential version of this term, ‘NYP’. The mean flow predictions show that both linear and non-linear models can successfully reproduce most of the measured data for stream-wise and cross-stream velocity components. Moreover, the non-linear model, which is sensitive to turbulence anisotropy, is able to produce better results for the turbulent stresses. As far as heat transfer predictions are concerned, it was found that both EVM and NLEVM2, the more recent variant of the non-linear , with the algebraic length-scale correction term, overestimate the measured Nusselt numbers for both geometries examined. While the EVM with the differential length-scale correction term underestimates heat transfer levels, the Nusselt number predictions with the NLEVM2 and the ‘NYP’ term are in close agreements with the measured data. Comparisons with our earlier work, Iacovides and Raisee (1999), show that the NLEVM2 thermal predictions are of similar quality to those of a second-moment closure. This modified version of the non-linear model, that in earlier studies was shown to improve thermal predictions in axi-symmetric and plane ribbed passages, is thus now found to also produce reasonable heat transfer predictions in three-dimensional ribbed ducts.