Investigation of the compressible flow through the tip-section turbine blade cascade with supersonic inlet

The contribution deals with the experimental and numerical investigation of compressible flow through the tip-section turbine blade cascade with the blade 54″ long. Experimental investigations by means of optical(interferometry and schlieren method) and pneumatic measurements provide more information about the behaviour and nature of basic phenomena occurring in the profile cascade flow field. The numerical simulation was carried out by means of the EARSM turbulence model according to Hellsten [5] completed by the bypass transition model with the algebraic equation for the intermittency coefficient proposed by Straka and P?íhoda [6] and implemented into the in-house numerical code. The investigation was focused particularly on the effect of shock waves on the shear layer development including the laminar/turbulent transition. Interactions of shock waves with shear layers on both sides of the blade result usually in the transition in attached and/ or separated flow and so to the considerable impact to the flow structure and energy losses in the blade cascade.     

Performance of two equation turbulence models for prediction of flow and heat transfer over a wall mounted cube

This paper deals with the CFD predictions of the three dimensional incompressible flow over a wall mounted cubic obstacle placed in fully developed turbulent flow along with the heat transfer calculations. Reynolds number considered in this study is 1870 based on cube height, h and bulk velocity U_b. Our main objective is to find out the appropriate two equation turbulence model for the complex flow structure which involves recirculation, separation and reattachment. We have used standard k–ε, low-Reynolds number k–ε, non-linear k–ε model, standard k–ω and improved k–ω models to solve the closure problem. The non-linear k–ε model and improved k–ω models along with standard models are validated with bench mark problem – flow through a backward facing step (BFS). Results showed that the improved k–ω model is giving overall better predictions of the flow field especially recirculation zone, mean streamwise velocity, and turbulent characteristics when compared to those by standard eddy viscosity models. The non-linear k–ε model is giving better prediction when compared to standard k–ε and low Reynolds number k–ε models. The complex vortex structure around the cube causes large variation in the local convective heat transfer coefficient. The maximum of the heat transfer coefficient occurred in the proximity of the reattachment points and the minimum is found at the recirculation zone.     

Numerical heat transfer predictions and mass/heat transfer measurements in a linear turbine cascade

The effect of secondary flows on mass transfer from a simulated gas turbine blade and hubwall is investigated. Measurements performed using naphthalene sublimation provide non-dimensional mass transfer coefficients, in the form of Sherwood numbers, that can be converted to heat transfer coefficients through the use of an analogy. Tests are conducted in a linear cascade composed of five blades having the profile of a first stage rotor blade of a high-pressure turbine aircraft engine. Detailed mass transfer maps on the airfoil and endwall surfaces allow the identification of significant flow features that are in good agreement with existing secondary flow models. These results are well suited for validation of numerical codes, as they are obtained with an accurate technique that does not suffer from conduction or radiation errors and allows the imposition of precise boundary conditions. The performance of a RANS (Reynolds-Averaged Navier–Stokes) numerical code that simulates the flow and heat/mass transfer in the cascade using the SST (Shear Stress Transport) k–ω model is evaluated through a comparison with the experimental results.     

An evaluation of eleven discretization schemes for predicting elliptic flow and heat transfer in supersonic jets

The paper describes studies to quantify the numerical errors caused by ‘false diffusion’, and to compare the performance of alternative numerical schemes for describing elliptic convective flow and heat transfer, within supersonic jets mixing into supersonic or subsonic streams. Results obtained are presented and discussed. Eleven schemes were considered in this study, but converged solutions were obtained with only five of them. Results obtained with the successful schemes are presented and discussed. It is concluded that for the high-shearing, high-velocity flows considered, the ‘upwind’ differencing scheme is probably the best choice, despite its dissipative nature and that the numerical errors associated with its use are no more significant than those introduced by uncertainties in the turbulence models.     

Experimental Studies on Heat Transfer in the Tip Gap of a Sectorial Turbine Cascade

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Endwall heat transfer and film cooling measurements in a turbine cascade with injection upstream of leading edge

Detailed heat transfer measurements were conducted on the endwall surface of a large‐scale low‐speed turbine cascade with single and double row injection on the endwall upstream of leading edge. Local film cooling effectiveness and the heat transfer coefficient with coolant injection were determined at blowing ratios 1.0, 2.0, and 3.0. In conjunction with the previously measured flow field data, the behaviors of endwall film cooling and heat transfer were studied. The results show that endwall film cooling is influenced to a great extent by the secondary flow and the coverage of coolant on the endwall is mainly determined by the blowing ratio. An uncovered triangle‐shaped area with low effectiveness close to pressure side could be observed at a low blowing ratio injection. The averaged effectiveness increases significantly when injecting at medium and high blowing ratios, and uniform coverage of coolant on the endwall could be achieved. The averaged effectiveness could be doubled in the case of double row injection. It was also observed that coolant injection made the overall averaged heat transfer coefficient increase remarkably with blowing ratio. It was proven that film cooling could reduce endwall heat flux markedly. The results illustrate the need to take such facts into account in the design process as the three‐dimensional flow patterns in the vicinity of the endwall, the interactions between the secondary flow and coolant, and the augmentation of heat transfer rate in the case of endwall injection. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 141–152, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20007     

Fluid flow characteristics and convective heat transfer improvement on a gas turbine blade

The flow field features and heat transfer enhancement are investigated on a gas turbine blade by applying the jet impingement cooling method. The distribution of the flow field and the Nusselt number (Nu) was determined on the targeted surface in the cooling channel. The injection holes of different shapes, such as circular, square, and rectangular were considered. The Reynolds numbers (Re) of the airflow in the range of 2000–5000 and aspect ratios of 0.5–2 were particularly focused. The flow vortices and recirculation in the cooling channel and their influence on the heat transfer enhancement were analyzed in detail under different airflow and geometric conditions. Decreasing the ratio of the distance between jet-to-target plate to the diameter of the jet orifice (H/d) increased the heat transfer rate and produced high-intensity vortices and recirculation zones. It was noticed that the formation and generation of vortices and recirculation have important effects on the convective heat transfer rate at the impingement surface. Local Nusselt number, formation of complex vortices, and airflow recirculation in the cooling channel decreased with the increase in the distance between the jet hole and the targeted surface. It was found that with the increase in the Reynolds number of the jet, heat transfer between cold airflow and the targeted surface increased. Moreover, it was observed that the cooling performance of the round and square jet holes was better than the rectangular holes.     

Effect of Nonequilibrium Homogenous Condensation on Flow Fields in a Supersonic Nozzle

INTRODUCTIONManystudies[1-131onthecondensationshockwaveoccurringinthecaseoftheraPidexPansionofmoistairorsteaminasupersonicnozzlehavebeenper-formed,andthecharacteristicsofcondensationshock'wavehavenearlybeenclarilied.Acondensationshockwavealsooccursinthebladepassageinasteamturbinel14,15]andsuchacondensationshockwavinteractswiththeboundarylayeronthesurfaceoftheblade.Thus,thefiowinthebladepassageofthesteamturbinewiththecondensationshockwavehasnotyetbeenclariliedl16'17].InthepreseDtstudythee…     

Transient thermal structure, turbulence, and heat transfer in a reattaching slot jet flow

The role of turbulent fluctuations on mean heat transfer coefficient in a reattaching slot jet flow is studied experimentally. Convective heat transfer rate and near-wall fluid flow are examined in the recirculation, reattachment, and post-reattachment regions for two nozzle-to-surface spacings of 0.25 and 0.75 times the width of the nozzle bottom plate. In the reattachment region, results indicate a strong correspondence between variances of near-wall velocity fluctuation and peak heat transfer rate for both spacings. Thermal structures that vary in the spanwise direction are identified in the recirculation region from low-frequency transient infrared thermographs of the heated surface. While these thermal structures are confined to regions in the vicinity of nozzle bottom plate for the low nozzle spacing, they span the entire recirculation region at larger spacings. Thermal streaks are observed past reattachment for the larger nozzle spacing, suggesting a periodic breakup and re-formation of the jet curtain. The scaling of heat transfer distribution is affected by the flow structure in the geometrically non-similar area of the recirculating flow beneath the nozzle. A correlation for peak Nusselt number is presented.     

Suitability of three different two-equation turbulence models in predicting film cooling performance over a rotating blade

The suitability of three different two-equation turbulence models in predicting film cooling effectiveness on a rotating blade was investigated and they are the commonly used standard k-ε model, the k-ω model and the shear stress transport k-ω model. To fulfill this target, both numerical simulation and the experimental investigation were carried out for a rotating blade having a flat test surface with a 4 mm diameter straight circular cooling hole in 30° inclined injection. The blade rotated at five different speeds of 0, 300, 500, 800 and 1000 rpm. The momentum ratio was set to be 0.285 and the Reynolds (Re_D) number based on the mainstream velocity and hydraulic diameter of the mainstream channel is 1.45 × 10⁵. The averaged density ratio was chosen to be 1.026 with air as both the coolant and the mainstream. Comparison between the numerical work and the experimental results indicated that (1) the rotating speed is the most critical parameter influencing the film cooling effectiveness distributions and the pressure surface could be remarkably different from the suction surface, (2) as for the algebraic averaged film cooling effectiveness, numerical predictions of the three turbulence models all overshoot compared with the experimental results, (3) among the three turbulence models, the standard k-ε model gave the poorest prediction.     

Analysis of 2D flow and heat transfer modeling in fracture of porous media

Heat and mass transfer between porous media and fluid is a complex coupling process,which is widely used in various fields of engineering applications,especially for natural and artificial fractures in oil and gas extraction.In this study,a new method is proposed to deal with the flow and heat transfer problem of steady flow in a fracture.The fluid flow in a fracture was described using the same method as Mohais,who considered a fracture as a channel with porous wall,and the perturbation method was used to solve the mathematical model.Unlike previous studies,the shear jump boundary condition proposed by Ochoa-Tapia and Whitaker was used at the interface between the fluid and porous media.The main methods were perturbation analysis and the application of shear jump boundary conditions.The influence of permeability,channel width,shear jump degree and effective dynamic viscosity on the flow and heat transfer in the channel was studied by analysing the analytical solution.The distribution of axial velocity in the channel with the change of the typical parameters and the sensitivity of the heat transfer was obtained.     

Stereoscopic PIV measurements of the flow field in a turbine cascade

This paper presents experimental measurements of the flow field in a Low-speed Turbine Cascade using a stereoscopic particle-image velocimetry(SPIV). During the measurements, a pair of frame-straddling-based CCD cameras were configured at different sides of the laser light sheet, and appropriate tracing particles(DEHS) were employed. The measurements were conducted at the incidence angle of 0 degree and exit Reynolds number of 1.7 × 105 with the tip clearance 1.18% of blade chord. The tip flow features, such as the evolution and breakdown of tip leakage vortex, the horseshoe vortex, turbulence characteristics of tip leakage flow, were studied for the flow field analysis. The results showed that the tip leakage flow/vortex mainly dominate flow fields in the tip region. The tip leakage vortex performs as a concentrated vortex before its breaking down and splitting into small vortices. The highest turbulence intensity mainly occurs in the tip region along with the trajectory of tip leakage vortex, and when the vortex breaks down, the turbulence intensity reduces rapidly. Additionally, the SPIV with this configuration also shows an advantage in investigating the flow structures and mechanism inside the turbine cascade.     

Performance of flow and heat transfer in vertical helical baffle condensers

The feed water heaters in power plants are actually the condensers using turbine extracting steam to heat feed water. The vertical feed water heater occupies less area than the horizontal one and convenient to lift tube bundles out in maintenance. However, the lower heat transfer coefficient due to thick condensate film limits its application. A novel trisection helical baffled vertical condenser (feed water heater) is proposed with liquid dams and gaps for facilitating condensate drainage. The flow and condensation heat transfer characters of two vertical condensers with variable angled trisection helical baffles of both single-thread and dual-threads and a variable spanned segmental baffled one were numerically studied with Mixture model of Fluent software. The distributions of velocity, pressure, volume fraction of condensate, and local heat transfer coefficient in these heat exchangers were demonstrated. The simulation results show that the inclined baffles with liquid dam and drainage gaps could drain condensate effectively from tube bundle surfaces and prevent liquid film from entraining into vapor, and that the variable angled trisection helical baffled vertical condenser with dual-threads could greatly improve the condensation heat transfer coefficient up to 35.7% higher than that of the variable spanned segmental baffled one.     

Some aspects of free-convective heat transfer in eddy flow through a horizontal tube

Experimental investigation of unsteady-state free convection in a horizontal cylindrical channel is carried out for the case of non-uniform distribution of heat flux along a channel at a constant temperature on the wall. The averaged temperature field in a gas was investigated on a Mach-Zender interferometer. Hydrodynamic structures were investigated by the smoke visualization technique. Heat fluxes were calculated on the basis of thermocouple measurements. In the case of non-uniform heat flux on the channel wall, the existence of longitudinal large scale hydrodynamic structures has been noted. Longitudinal and lateral Rayleigh numbers varied from 0 to 4 × 10⁹ and from 0.8 × 10⁴to 1.2 × 10⁵, respectively. Investigations were carried out with air, carbon dioxide and helium flows.     

The rate of heat flow through a flat vertical wall due to conjugate heat transfer

Free convection along both sides of a vertical flat plate is studied within the framework of the laminar boundary-layer theory and for the case where only the temperature of the fluid far away from the wall is prescribed. Corrections to the Pohlhausen solution for the temperature at the plate surface are calculated. It is found that for good thermal conductors, the corrections are small (within a few percent), while for poor thermal conductors the corrections may be substantial (～30% for a wall with conductivity similar to brick). In addition, expressions for the heat transfer coefficient h as well as for the Nusselt number are derived and the corresponding convective heat transfer rate is determined.     

Effects of free-stream turbulence on high pressure turbine blade heat transfer predicted by structured and unstructured LES

Recent developments and demonstrations for the prediction of turbulent flows around blades point to Large Eddy Simulations (LES) as a very promising tool. Indeed and despite the fact that this numerical method still requires modeling and intense computing effort compared to Reynolds Average Navier–Stokes (RANS), this fully unsteady simulation technique provides valuable information on the turbulent flow otherwise inaccessible. Theoretical limits and scales of wall bounded flows are now well mastered in simple cases but complex industrial applications usually introduce unknowns and mechanisms that are difficult to apprehend beforehand especially with LES which is usually computationally intensive and bounded to code scalability, mesh quality, modeling performances and computer power. In this specific context, few studies directly address the use of fully structured versus unstructured, implicit versus explicit flow solvers and their respective impact for LES modeling of complex wall bounded flows. To partly address these important issues, two dedicated structured and unstructured computational solvers are applied and assessed by comparing the predictions of the heat transfer around the experimental high pressure turbine blade profile cascade of Arts et al. [6]. First, both LES predictions are compared to RANS modeling with a particular interest for the accuracy/cost ratio and improvement of the physical phenomena around the blade. LES’s are then detailed and further investigated to assess their ability to reproduce the inlet turbulence effect on heat transfer and the development of the transitioning boundary layer around the blade. Quantitative comparisons against experimental findings show excellent agreement especially on the pressure side of the profile. Detailed analysis of the flow predictions provided by both the structured and unstructured solvers underline the importance of long stream-wise streaky structures responsible for the augmentation of the heat transfer and leading to the transition of the suction-side boundary layer.     

Visualization of heat transfer distributions in a gas turbine scroll

To visualize heat transfer distributions in systems with complex internal geometries, an experimental technique using a combination of the transient method and the hysteresis effect of thermopaint was developed. A mercury compound based thermopaint was used as a temperature indicator. Features of the paint are its reusability and its hysteresis nature. Isothermal lines visualized by the thermopaint are preserved after the experiment by utilizing the hysteresis nature. Also heat transfer on those parts that are hidden behind other parts can be visualized. Effects of initial and air temperature on the measurement uncertainty were evaluated. With this method, local heat transfer coefficients were obtained on the model scroll of a single can type combustor. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 229–242, 1998