Peter Ireland

Heat Transfer Measurements to a Gas Turbine Cooling Passage With Inclined Ribs

The local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage have been measured in great detail. A new method of determining the heat transfer coefficient to the rib surface has been developed and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficient was found to be considerable. The vortex-dominated flow field was interpreted from the detailed form of the measured local heat transfer contours. Computational Fluid Dynamics calculations support this model of the flow and yield friction factors that agree with measured values. Advances in the heat transfer measuring technique and data analysis procedure that confirm the accuracy of the transient method are described in full.

An Investigation on the Onset of Wake-Induced Transition and Turbulent Spot Production Rate Using Thermochromic Liquid Crystals

Wakes shed by upstream blade rows are known to cause boundary layer transition in both the compressor and turbine stages of axial flow gas turbines. This transition process is believed to take place via discrete zones of turbulence known as turbulent spots which occur in an otherwise laminar boundary layer. However, the process of transition over the blade surface cannot, at present, be reliably predicted. This is due to a lack of information on where and when these turbulent spots form and how they grow and merge as they convect downstream to form the turbulent boundary layer.This paper presents detailed experimental information on the process of boundary layer transition induced by a bar generated wake travelling over a zero pressure gradient laminar boundary layer on a flat plate. The Reynolds number was 3×105. The peak turbulence intensity within the wakes varied from 3 to 6 % by using different bar of different diameters. An encapsulated cholesteric liquid crystals coating has been employed on a heated flat plate to reveal detailed information over the full surface. The information includes the thermal characteristics, the spot onset and formation rate. Data were obtained at high resolution on a grid of 30,000 points. The results were compared to intermittency plots and time-distance diagrams obtained by using surface-mounted thin film gauges and found to be similar. The data were also consistent with well established correlations and other published data from the literature for existing wake-induced transition models.Copyright

Techniques for detailed heat transfer measurements in cold supersonic blowdown tunnels using thermochromic liquid crystals

The paper presents methods for measurement of convective heat transfer distributions in a cold flow, supersonic blowdown wind tunnel. The techniques involve use of the difference between model surface temperature and adiabatic wall temperature as the driving temperature difference for heat transfer and no active heating or cooling of the test gas or model is required. Thermochromic liquid crystals are used for surface temperature indication and results presented from experiments in a Mach 3 flow indicate that measurements of the surface heat transfer distribution under swept shock wave boundary layer interactions can be made

Visualization of turbulent spots and unsteady wake-induced boundary-layer transition with thermochromic liquid crystals

Abstract A method of using thermochromic liquid crystals has been developed to visualize the thermal footprints of turbulent spots convecting downstream in an otherwise laminar boundary layer over a heated surface. This technique has been employed to visualize the development of turbulent spots under the influence of adverse pressure gradients. It has also been used to visualize the transitional events that occur during unsteady wake-induced boundary layer transition typically of those occurring in multi-stage turbomachines. The results show that liquid crystal is not only capable of providing quantitative information about the growth and development of individual spots but also allows a detailed study of formation of turbulent spots occurring naturally during a complicated transition process.

Influence of Film Cooling Hole Angles and Geometries on Aerodynamic Loss and Net Heat Flux Reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (α), compound angle (β), hole inlet geometry and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR = 0.16, 0.64 and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different α and β but with the same angle between the mainstream and coolant flow directions (angle κ). This relationship is assessed through experiments by testing two sets of cylindrical holes with different α and β: one set with κ = 35°, another set with κ = 60°. The data confirm the stated relationship between α, β, κ and the aerodynamic mixing loss. The results show that the designer should minimise κ to obtain the lowest loss, but maximise β to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α = 35.0°, β = 0°) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is ≈ 50% of the loss of the fan-shaped hole at IR = 0.64 and 1.44.Copyright

A Robust Radial Traverse Temperature Probe for Application to a Gas Turbine HP/IP Stage

The requirements to reduce engine fuel burn costs and gaseous emissions combine to ensure that gas turbine engine manufacturers continually seek to increase the peak cycle temperatures of new engine designs. Consequently, high-pressure turbine components must be developed that can withstand increasing gas temperatures, resulting in the continuous introduction of new technologies that allow appropriate service life. Accurate gas path measurements are vital for early understanding of the performance of a new design, although the accurate measurement of fluid temperature in a turbine stage is becoming increasingly difficult. The use of active probe-assembly cooling is important to ensure a sufficiently robust measurement system. Cooling issues may also affect the measurement performance because the component to which the temperature instrumentation is attached is cooled (for example, a guide vane). The use of a radial traverse total temperature device in the turbine section of a large civil aero-engine has previously been reported and the results analysed and compared to expectation. One outcome of ongoing work has been a proposal for a new design of turbine traverse probe with improved total temperature measurement accuracy. The new design directly addresses those uncertainties caused by conduction of heat from the thermocouple junction and into the cooled probe support. Extensive conjugate CFD modelling followed by validation tests in a high temperature free-jet rig confirmed the success of the design in reducing the magnitude of the thermal conduction error. The probe is likely to be used in future tests to improve engine performance validation.Copyright

Characteristics of an Underexpanded Jet and Its Surface Impingement for Combustion Burnthrough

The flow from an underexpanded free jet and a jet impinging on a perpendicular planar surface has been characterised for a pressure ratio (N) of 40 with a nozzle to plate spacing (L/D) of 3 nozzle diameters. Surface heatflux (Q), temperature (T), pressure (P) and flow visualisation techniques have been used extensively to examine the flow within the jet and on the plate surface. From these it is shown that the flow is non-axisymmetric in nature. The experimental programme has been conducted across several facilities within the UK which has allowed the influence of Reynolds number to be determined. The results of this work show marked similarities in flow characteristics irrespective of Reynolds number variation. Heat transfer on the impingement plate is largely determined by Taylor-Gortler vortex flow and total temperature separation resulting from shearing flows. Evidence for Taylor-Gortler vortex flow within the jet has been reinforced by comparison of the free jet element of this work with other research. Infra-Red camera measurements have confirmed the presence of strong total temperature separation on the impingement surface with local surface temperature step variation of greater than 200K (360°F) in the presence of a high temperature jet running at approximately 1900K (2960°F). This feature is dictated by this N and L/D and is shown to occur over a wide range of jet total temperature.Copyright

A Colour Image Processing System for Transient Liquid Crystal Heat Transfer Experiments

A colour image processing system for liquid crystal heat transfer experiment has been developed. The system is capable of digitizing and processing the complete liquid crystal surface colour (hue) change history in a transient test and, together with a calibration, can give the complete history of surface temperature over a full surface. Two methods for automatically processing the hue history to give heat transfer coefficient distributions are presented. Both methods raise the accuracy of the transient technique above other approaches by using the redundancy inherent in the multiple surface temperature measurements. The first regression approach applied to the determination of both h and Tgas is reported. The uncertainty in all measurements has been quantified and examples of applications of both techniques given.Copyright