Kam Chana

An Investigation on Turbine Tip and Shroud Heat Transfer

Detailed experimental investigations have been performed to measure the heat transfer and static pressure distributions on the rotor tip and rotor casing of a gas turbine stage with a shroudless rotor blade. The turbine stage was a modern high pressure Rolls-Royce aero-engine design with stage pressure ratio of 3.2 and nozzle guide vane (ngv) Reynolds number of 2.54E6. Measurements have been taken with and without inlet temperature distortion to the stage. The measurements were taken in the QinetiQ Isentropic Light Piston Facility and aerodynamic and heat transfer measurements are presented from the rotor tip and casing region. A simple two-dimensional model is presented to estimate the heat transfer rate to the rotor tip and casing region as a function of Reynolds number along the gap.

Analysis on the Effect of a Nonuniform Inlet Profile on Heat Transfer and Fluid Flow in Turbine Stages

This paper presents an investigation of the aerothermal performance of a modern unshrouded high pressure (HP) aeroengine turbine subject to non-uniform inlet temperature profile. The turbine used for the study was the MT1 turbine installed in the QinetiQ Turbine Test Facility (TTF) based in Farnborough (UK). The MT1 turbine is a full scale transonic HP turbine, and is operated in the test facility at the correct non-dimensional conditions for aerodynamics and heat transfer. Datum experiments of aero-thermal performance were conducted with uniform inlet conditions. Experiments with non-uniform inlet temperature were conducted with a temperature profile that had a non-uniformity in the radial direction defined by (T(max) - T(min))/(T) over bar = 0.355, and a non-uniformity in the circumferential direction defined by (T(max) - T(min))/(T) over bar = 0.14. This corresponds to an extreme point in the engine cycle, in an engine where the non-uniformity is dominated by the radial distribution. Accurate experimental area surveys of the turbine inlet and exit flows were conducted, and detailed heat transfer measurements were obtained on the blade surfaces and end-walls. These results are analysed with the unsteady numerical data obtained using the in-house HybFlow code developed at the University of Firenze. Two particular aspects are highlighted in the discussion: prediction confidence for state of the art computational fluid dynamics (CFD) and impact of real conditions on stator-rotor thermal loading. The efficiency value obtained with the numerical analysis is compared with the experimental data and a 0.8% difference is found and discussed. A study of the flow field influence on the blade thermal load has also been detailed. It is shown that the hot streak migration mainly affects the rotor pressure side from 20% to 70% of the span, where the Nusselt number increases by a factor of 60% with respect to the uniform case. Furthermore, in this work it has been found that a nonuniform temperature distribution is beneficial for the rotor tip, contrary to the results found in the open literature. Although the hot streak is affected by the pressure gradient across the tip gap, the radial profile (which dominates the temperature profile being considered) is not fully mixed out in passing through the HP stage, and contributes significantly to cooling the turbine casing. A design approach not taking into account these effects will underestimate to rotor life near the tip and the thermal load at mid-span. The temperature profile that has been used in both the experiments and CFD is the first simulation of an extreme cycle point (more than twice the magnitude of distortion all previous experimental studies): it represents an engine-take-off condition combined with the full combustor cooling.

Assessment of Empirical Heat Transfer Models for a CFR Engine Operated in HCCI Mode

Homogeneous charge compression ignition (HCCI) engines are a promising alternative to traditional spark- and compression-ignition engines, due to their high thermal efficiency and near-zero emissions of NOx and soot. Simulation software is an essential tool in the development and optimization of these engines. The heat transfer submodel used in simulation software has a large influence on the accuracy of the simulation results, due to its significant effect on the combustion. In this work several empirical heat transfer models are assessed on their ability to accurately predict the heat flux in a CFR engine during HCCI operation. Models are investigated that are developed for traditional spark- and compression-ignition engines such as those from Annand [1], Woschni [2] and Hohenberg [3] and also models developed for HCCI engines such as those from Chang et al. [4] and Hensel et al. [5]. The heat flux is measured in a CFR engine operated in both motored and HCCI mode and compared to the predicted heat flux by the aforementioned models. It is shown that these models are unable to accurately predict the heat flux during HCCI operation if the model coefficients are not properly calibrated. The models from Annand, Hohenberg and Woschni overestimate the heat flux, whereas the models from Chang et al. and Hensel et al. underestimate it during the entire engine cycle if the original model coefficients are used. If the model coefficients are properly calibrated, the models from Annand, Hohenberg and Hensel et al. are able to predict the heat flux during HCCI operation for one engine operating point. However, if the same model coefficients are used for another operating point, the models are unable to satisfactorily predict the heat flux.

A Review of the Oxford Turbine Research Facility

Gas turbine engine efficiency and reliability is generally improved through better understanding and improvements to the design of individual components. The life limiting component of the modern gas turbine is the high pressure (HP) turbine stage due to the arduous environment. Over the last 50 years significant research effort has been focused on advancing blade cooling designs and materials.Due to practical limitations little fundamental research on the turbine system is performed in the operating gas turbine engine. Consequently different types of experimental approaches have been developed over the last 4 decades to study the flow and in particular the heat transfer and cooling in turbines.In general the facilities can be divided into continuous running or short duration and cascade or rotating. Over the last 30 years short duration facilities have dominated the research in the study of turbine heat transfer and cooling.The Oxford Turbine Research Facility (formerly known as the QinetiQ Turbine Test Facility, The Isentropic Light Piston Facility and The Isentropic Light Piston Cascade) is a short duration facility developed and built in the late 1970s and early 1980s for turbine heat transfer and cooling studies.This paper presents the developments and measurements taken on the facility over the last 35 years, including the type of research that has been conducted and, the current capability of the facility.Copyright

Applying Design of Experiments to Determine the Effect of Gas Properties on In-Cylinder Heat Flux in a Motored SI Engine

Models for the convective heat transfer from the combustion gases to the walls inside a spark ignition engine are an important keystone in the simulation tools which are being developed to aid engine optimization. The existing models have, however, been cited to be inaccurate for hydrogen, one of the alternative fuels currently investigated. One possible explanation for this inaccuracy is that the models do not adequately capture the effect of the gas properties. These have never been varied in a wide range because air and ‘classical’ fossil fuels have similar values, but they are significantly different in the case of hydrogen. As a first step towards a fuel independent heat transfer model, we have investigated the effect of the gas properties on the heat flux in a spark ignition engine. The effect of the gas properties was decoupled from that of combustion, by injecting different inert gases (helium, argon, carbon dioxide) into the intake air flow of the engine under motored operation. This paper presents the results of the experiment, which was designed with DoE techniques. The paper shows that the three investigated effects (throttle position, compression ratio and gas) and the interaction between the throttle and the compression ratio are significant with a significance level of 1%. Both the individual and combined effects of the gas properties are investigated. The most remarkable effect observed in the data was that the dynamic viscosity influences the heat flux in two contrasting ways. At the one hand, it increases the heat flux by increasing the gas temperature, at the other hand, it reduces the heat flux through the convection coefficient. A preliminary test shows that modeling under motored operation could be based on classical concepts. However, some scatter occurs in the data which needs further investigation.

Unsteady Flow Phenomena in Turbine Rim Seals

While turbine rim sealing flows are an important aspect of turbomachinery design, affecting turbine aerodynamic performance and turbine disc temperatures, the present understanding and predictive capability for such flows is limited. The aim of the present study is to clarify the flow physics involved in rim sealing flows and to provide high quality experimental data for use in evaluation of CFD models. The seal considered is similar to a chute seal previously investigated by other workers, and the study focuses on the inherent unsteadiness of rim seal flows, rather than unsteadiness imposed by the rotating blades. Unsteady pressure measurements from radially and circumferentially distributed transducers are presented for flow in a rotor-stator disc cavity and the rim seal without imposed external flow. The test matrix covered ranges in rotational Reynolds number, Re∅, and non-dimensional flow rate, , of 2.2 –3.0x106 and 0 – 3.5x103 respectively. Distinct frequencies are identified in the cavity flow and detailed analysis of the pressure data associates these with large scale flow structures rotating about the axis. This confirms the occurrence of such structures as predicted in previously published CFD studies and provides new data for detailed assessment of CFD models.

Calibration of a TFG Sensor for Heat Flux Measurements in a S.I. Engine

In the development of internal combustion engines, measurements of the heat transfer to the cylinder walls play an important role. These measurements are necessary to provide data for building a model of the heat transfer, which can be used to further develop simulation tools for engine optimization. This research will focus on the Thin Film Gauge (TFG) heat flux sensor. This sensor consists of a platinum RTD (Resistance Temperature Detector) on an insulating Macor® (ceramic) substrate. The sensor has a high frequency response (up to 100 kHz) and is small and robust. These properties make the TFG sensor adequate for measurements in the combustion chamber of an internal combustion engine. To use this sensor, its thermal properties - namely the temperature sensitivity coefficient and the thermal product - must be correctly calibrated. First, different calibration setups with a different temperature range are used to calibrate the temperature sensitivity coefficient of the TFG sensor. These results will be analyzed and discussed. Second, the DED (Double Electrical Discharge) calibration setup for the thermal product is extensively discussed.

New technique for the fabrication of miniature thin film heat flux gauges

This paper details the improvements made to the design and fabrication of thin-film heat flux gauges at Oxford. These improvements have been driven by the desire to improve measurement accuracy and resolution in short duration wind-tunnel experiments.A thin-film heat flux gauge (TFHFG) measures heat flux by recording the temperature history of thin film resistive temperature sensors sputtered onto an insulating substrate. The heat flux can then be calculated using Fourier’s law of heat conduction.A new fabrication process utilising technology from the manufacture of flexible printed circuit boards is outlined, which enables the production of significantly smaller and more robust gauges than those previously used.

Demonstrating the Use of Thin Film Gauges for Heat Flux Measurements in ICEs: Measurements on an Inlet Valve in Motored Operation

To optimize internal combustion engines (ICEs), a good understanding of engine operation is essential. The heat transfer from the working gases to the combustion chamber walls plays an important role, not only for the performance, but also for the emissions of the engine. Besides, thermal management of ICEs is becoming more and more important as an additional tool for optimizing efficiency and emission aftertreatment. In contrast little is known about the convective heat transfer inside the combustion chamber due to the complexity of the working processes. Heat transfer measurements inside the combustion chamber pose a challenge in instrumentation due to the harsh environment. Additionally, the heat loss in a spark ignition (SI) engine shows a high temporal and spatial variation. This poses certain requirements on the heat flux sensor. In this paper we examine the heat transfer in a production SI ICE through the use of Thin Film Gauge (TFG) heat flux sensors. An inlet valve has been equipped with 7 TFG sensors in a row. In literature only measurements on the piston, cylinder liner or cylinder head could be found. First, the construction of the heat flux sensor will be discussed. Second, the heat flux measurement technique and the implementation of the TFG sensors are discussed. The choice for Thin Film sensors is highlighted. Only compression operation (motored) measurements are currently considered and compared to literature. The effect of a variation in manifold air pressure on the heat flux is analysed.

Design of a Nonreacting Combustor Simulator With Swirl and Temperature Distortion With Experimental Validation

Nonuniform combustor outlet flows have been demonstrated to have significant impact on the first and second stage turbine aerothermal performance. Rich-burn combustors, which generally have pronounced temperature profiles and weak swirl profiles, primarily affect the heat load in the vane but both the heat load and aerodynamics of the rotor. Lean burn combustors, in contrast, generally have a strong swirl profile which has an additional significant impact on the vane aerodynamics which should be accounted for in the design process. There has been a move towards lean burn combustor designs to reduce NOx emissions. There is also increasing interest in fully integrated design processes which consider the impact of the combustor flow on the design of the HP vane and rotor aerodynamics and cooling. There are a number of current large research projects in scaled (low temperature and pressure) turbine facilities which aim to provided validation data and physical understanding to support this design philosophy. There is a small body of literature devoted to rich burn combustor simulator design but no open literature on the topic of lean burn simulator design. The particular problem is that in non-reacting, highly swirling and diffusing flows, vortex instability in the form of a precessing vortex core or vortex breakdown is unlikely to be well matched to the reacting case. In reacting combustors the flow is stabilised by heat release, but in low temperature simulators other methods for stabilising the flow must be employed. Unsteady Reynolds-averaged Navier-Stokes and Large eddy simulation have shown promise in modelling swirling flows with unstable features. These design issues form the subject of this paper.Copyright