Sven Olaf Neumann

The Effect of Turning Vanes on Pressure Loss and Heat Transfer of a Ribbed Rectangular Two-Pass Internal Cooling Channel

Gas turbine blades are usually cooled by using ribbed serpentine internal cooling passages, which are fed by extracted compressor air. The individual straight ducts are connected by sharp 180 deg bends. The integration of turning vanes in the bend region lets one expect a significant reduction in pressure loss while keeping the heat transfer levels high. Therefore, the objective of the present study was to investigate the influence of different turning vane configurations on pressure loss and local heat transfer distribution. The investigations were conducted in a rectangular two-pass channel connected by a 180 deg sharp turn with a channel height-to-width ratio of HIW=2. The channel was equipped with 45 deg skewed ribs in a parallel arrangement with e/d h = 0.1 and P/e = 10. The tip-to-web distance was kept constant at W el /W = 1. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Furthermore static pressure measurements were conducted in order to determine the influence of turning vane configurations on pressure loss. Additionally, the configurations were investigated numerically by solving the Reynolds-averaged Navier― Stokes equations using the finite-volume solver FLUENT. The numerical grids were generated by the hybrid grid generator CENTAUR. Three different turbulence models were considered: the realizable k-e model with two-layer wall treatment, the k-ω-SST model, and the v 2 -f turbulence model. The results showed a significant influence of the turning vane configuration on pressure loss and heat transfer in the bend region and the outlet pass. While using an appropriate turning vane configuration, pressure loss was reduced by about 25%, keeping the heat transfer at nearly the same level in the bend region. An inappropriate configuration led to an increase in pressure loss while the heat transfer was reduced in the bend region and outlet pass.

Large-Eddy Simulations and Heat-Flux Modeling in a Turbulent Impinging Jet

This article documents the results of an investigation into aspects of the simulation and modeling of turbulent jets that impinge orthogonally on a target surface. The focus is on the case of a jet which issues from a circular pipe into stagnant surrounding at the relatively high value of Reynolds number of 23,000 (based on nozzle diameter and bulk velocity) for which experimental data are available. Large-eddy simulations were performed to obtain details of the mean flows and the turbulence fields including distributions of all components of the turbulent heat fluxes. The outcome of these simulations were used to assess three alternative models for the turbulent heat fluxes which differ from the conventional Fouriers Law by not being based on the assumption of proportionality between the eddy and thermal diffusivities via a constant Prandtl number. It was found that only one of the models considered succeeds in representing the effects on the heat fluxes of the complex strain field associated with the stagnation region and the subsequent development into the wall-jet region. The reasons for this outcome are discussed.

An Experimental and Numerical Investigation of the Effect of Cooling Channel Crossflow on Film Cooling Performance

This paper presents an experimental and numerical investigation into film cooling performance over a flat plate. As previous studies have shown, the flow situation at the entry-side of the cooling hole shows a notable effect on film cooling performance. The present investigation takes this into account feeding the cooling holes from an internal cooling channel and not from a stagnant plenum. High resolution heat transfer coefficient and adiabatic film cooling effectiveness distributions received from transient liquid crystal experiments are presented. The Reynolds numbers of the hot gas channel and the coolant crossflow feeding the holes are varied. Furthermore, the effects of 45° angled ribs, introduced into the cooling channel, are investigated. The experiments are performed at constant blowing, momentum and pressure ratios. Numerical calculations of the adiabatic film cooling effectiveness for selected configurations using FLUENT are presented. Comparison reveals the influence of coolant channel Reynolds number and the introduced ribs on the cooling hole flow pattern leading to a changed film cooling performance.Copyright

The Effect of Side Wall Mass Extraction on Pressure Loss and Heat Transfer of a Ribbed Rectangular Two-Pass Internal Cooling Channel

Gas turbine blades are often cooled by using combined internal and external cooling methods where for internal cooling purposes, usually, serpentine passages are applied. In order to optimize the design of these serpentine passages it is inevitable to know the influence of mass extraction due to film cooling holes, dust holes, or due to side walls for feeding successive cooling channels as for the trailing edge on the internal cooling performance. Therefore, the objective of the present study was to analyze the influence of side wall mass extraction on pressure loss and heat transfer distribution in a two-pass internal cooling channel representing a cooling scheme with flow towards the trailing edge. The investigated rectangular two-pass channel consisted of an inlet and outlet duct with a height-to-width ratio of H/W = 2 connected by a 180 deg sharp bend. The tip-to-web distance was kept constant at W el /W = 1. The mass extraction was realized using several circular holes in the outlet pass side wall. Two geometric configurations were investigated: A configuration with mass extraction solely in the outlet pass and a configuration with mass extraction in the bend region and outlet pass. The extracted mass flow rate was 0%, 10%, and 20% of the inlet channel mass flow. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Pressure losses were determined in separate experiments by local static pressure measurements. Furthermore, a computational study was performed solving the Reynolds-averaged Navier―Stokes equations using the commercial finite-volume solver FLUENT. The numerical grids were generated using the hybrid grid generator CENTAUR. Three different turbulence models were considered: the realizable k-e model with two-layer wall treatment, the k-ω-SST model, and the v 2 -f model. The experimental data of the investigation of side wall ejection showed that the heat transfer in the bend region slightly increased when the ejection were in operation, while the heat transfer in the section of the outlet channel with side wall ejection was nearly not affected. After this section, a decrease in heat transfer was observed, which can be attributed to the decreased mainstream mass flow rate.

Heat Transfer Enhancement From Single Vortex Generators

The effect of single full-body Vortex Generators (VGs) on heat transfer was investigated experimentally. The delta shaped devices with different geometries were examined in a rectangular channel for a Reynolds number range of 80,000 up to 600,000. The research included heat transfer as well as flow measurements. Detailed heat transfer results were obtained by a steady state thermochromic liquid crystal (TLC) method using heater foils. This full surface measurement shows heat transfer enhancement evoked by the longitudinal vortices produced by the VGs. Data for secondary flow structures were determined by Particle Image Velocimetry (PIV) measurements. The comparison of vortex position and heat transfer distribution shows that the local heat transfer maximum due to downflow regions of the secondary flow does occur at positions shifted slightly towards the centerline of the channel compared to the existing vortex cores. Experimental data for the flow field were also compared to numerical calculations using FLUENT.Copyright

Validation and Analysis of Numerical Results for a Varying Aspect Ratio Two-Pass Internal Cooling Channel

Numerical results for an internal ribbed cooling channel including a 180° bend with a 2:1 inlet and 1:1 aspect ratio outlet channel were validated against experimental results in terms of spatially resolved heat transfer distributions, pressure losses, and velocity distributions. The numerical domain consisted of one rib segment in the inlet channel and three ribs segments in the outlet channel to reduce the overall numerical effort and allow for an extensive parametric study. The results showed good agreement for both heat transfer magnitudes and spatial distributions and the numerical results captured the predominate flow physics resulting from the 180° bend. The production of Dean vortices and acceleration of the flow in the bend produced strongly increased heat transfer on both the ribbed and unribbed walls in the outlet channel in addition to increases due to the ribs. Numerical simulations were performed for a wide range of divider wall-to-tip wall distances, which influenced the position of the highest heat transfer levels on the outlet walls and changed the shape of the heat transfer distribution on the tip wall. Analysis of section averages of heat transfer in the bend and outlet channel showed a strong influence of the tip wall distance while no effect was seen upstream of the bend. A similarly large effect on pressure losses in the bend was observed with varying tip wall position. Trends in averaged heat transfer varied linearly with tip wall distance while pressure losses followed a non-linear trend, resulting in an optimum tip wall distance with respect to heat transfer efficiency.Copyright

Understanding the Dynamics and Control of a Turbulent Impinging Jet via Pulsation and Swirl Using Large Eddy Simulation

Impinging jets are used in a variety of engineering applications like in chemical reactors, mixing devices, drying and cooling applications. Good quality simulations of this highly complex flow field is a challenging task. In this work, the flow field and heat transfer of turbulent pulsating and swirling jet impingements are investigated by Large Eddy Simulation (LES). The benchmark case of turbulent impinging jet with out swirl and excitation, recommended by ERCOFTAC, is simulated first. In all investigations the jet’s Reynolds number (Re) is 23000 and jet outlet-to-target wall distance (H/D) is 2. The agreement between experimental data and simulation gives encouragement for further investigation of complex flow of jets impingement with excited inlet velocity profiles and swirl. The pulsating jets are investigated at four different excitations modes. Where as the swirl is introduced at two different swirl numbers (S). The correlation between the heat transfer mechanism, flow kinematics and turbulence quantities is investigated.

Thermal & Flow Field Analysis of Turbulent Swirling Jet Impingement Using Large Eddy Simulation

Swirling jets are used in a variety of engineering applications like in chemical reactors, cyclone separators, mixing devices, drying and cooling applications. Good quality simulations of this highly complex flow field is a challenging task. In this work, the flow field and heat transfer of turbulent swirling and non-swirling impinging jets are computed using Large Eddy Simulation (LES). For the investigation of non-swirling jets, the ERCOFTAC recommended test case of an impinging jet at a Reynolds number of 23000 is simulated first. The agreement between experimental data and simulation gives encouragement for further investigation of complex flow of swirling jets impingement. Therefore, the swirling jets with Reynolds numbers of 21000 & 23000 and four different swirl numbers are investigated via LES. The results are compared with experimental data. The effect of inflow conditions and inlet temperature is investigated. The correlation between the heat transfer mechanism, flow kinematics and turbulence quantities is investigated. The numerical data computed within this investigation serve additionally for benchmarking results based on the solution of Reynolds Averaged Navier-Stokes Equations in complex flow configurations at even higher Reynolds numbers. This comparison is not shown here, but could be found at [ITLR].

Large Eddy Simulation of the Heat Transfer Due to Swirling and Non-Swirling Jet Impingement

Turbulent impinging jet is a complex flow phenomenon involving free jet, impingement and subsequent wall jet development zones; this makes it a difficult test case for the evaluation of new turbulence models. The complexity of the jet impingement can be further amplified by the addition of the swirl. In this paper, results of Large Eddy Simulations (LES) of swirling and non-swirling impinging jet are presented. The Reynolds number of the jet based on bulk axial velocity is 23000 and target-to-wall distance (H/D) is two. The Swirl numbers (S) of the jet are 0,0.2, 0.47. In swirling jets, the heat transfer at the geometric stagnation zone deteriorates due to the formation of conical recirculation zone. It is found numerically that the addition of swirl does not give any improvement for the over all heat transfer at the target wall. The LES predictions are validated by available experimental data.Copyright

Novel Turbine Endwall Contours for the Reduction of Heat Transfer Generated Using the Ice Formation Method

Three-dimensional contouring of vane endwalls has proven to be an efficient method for reducing aerodynamic losses or, respectively, endwall heat transfer by active manipulation of the complex vortical flow structures in the vane passage. The present study shows the application of the Ice Formation Method for endwall contouring of a guide vane row with the goal of reducing endwall heat transfer. Endwall contours for the guide vane row of a low pressure turbine are experimentally generated in form of ice contours and evaluated with respect to their heat transfer behavior. A comparison with the flat plate showed that average heat transfer is considerably reduced for the ice-contoured endwalls with reductions up to 42%. The generated endwall contours were also digitized and used in numerical simulations. The latter allowed for a comparison of endwall heat transfer for the novel contours with the heat transfer for a flat, uncontoured endwall. This showed that the new endwall contours also feature decreased average heat transfer compared to the flat endwall with the maximum obtained reduction being 12%.Copyright