Gary D. Lock

A Converging Slot-Hole Film-Cooling Geometry—Part 1: Low-Speed Flat-Plate Heat Transfer and Loss

This paper presents experimental measurements of the performance of a new film cooling hole geometry - the Converging Slot-Hole or Console. This novel, patented geometry has been designed to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems. The physical principles embodied in the new hole design are described, and a typical example of the console geometry is presented. The cooling performance of a single row of consoles was compared experimentally with that of typical 35° cylindrical and fan-shaped holes and a slot, on a large-scale, flat-plate model at engine representative Reynolds numbers in a low speed tunnel with ambient temperature main flow. The hole throat area per unit width is matched for all four hole geometries. By independently varying the temperature of the heated coolant and the heat flux from an electrically heated, thermally insulated, constant heat flux surface, both the heat transfer coefficient and the adiabatic cooling effectiveness were deduced from digital photographs of the colour play of narrowband thermochromic liquid crystals on the model surface. A comparative measurement of the aerodynamic losses associated with each of the four film-cooling geometries was made by traversing the boundary layer at the downstream end of the flat plate. The promising heat transfer and aerodynamic performance of the console geometry have justified further experiments on an engine representative nozzle guide vane in a transonic annular cascade presented in Part 2 of this paper [1].

Aerothermal Investigations of Tip Leakage Flow in Axial Flow Turbines—Part I: Effect of Tip Geometry and Tip Clearance Gap

A numerical study has been performed to investigate the effect of tip geometry oil the tip leakage flow and heat transfer characteristics in unshrouded axial flow turbines. Base line flat tip geometry and squealer type geometries, namely, double squealer or cavity and suction-side squealer, were considered. The performances of the squealer geometries, in terms of the leakage mass flow and heat transfer to the tip, were compared with the flat tip at two different tip clearance gaps. The computations were performed using a single blade with periodic boundary conditions imposed along the boundaries in the pitchwise direction. Turbulence was modeled using three different models, namely, standard k-epsilon, low Re k-omega, and shear stress transport (SST) k-omega, in order to assess the capability, of the models in correctly predicting the blade heat transfer The heat transfer and static pressure distributions obtained using the SST k-omega model were-found to be in close agreement with the experimental data. It was observed that compared to the other two geometries considered, the cavity tip is advantageous both from the aerodynamic and from the heat transfer perspectives by providing a decrease in the amount of leakage, and hence losses, and average heat transfer to the tip. In general, for a given geometry, the leakage mass flow and the heat transfer to the tip increased with increase in tip clearance gap. Part II of this paper examines the effect of relative casing motion on the flow and heat transfer characteristics of tip leakage flow. In, Part III of this paper the effect of coolant injection on the flow and heat transfer characteristics of tip leakage flow is presented.

Heat transfer and aerodynamics of turbine blade tips in a linear cascade

Local measurements of the heat transfer coefficient and pressure coefficient were conducted on the tip and near tip region of a generic turbine blade in a five-blade linear cascade. Two tip clearance gaps were used: 1.6% and 2.8% chord. Data was obtained at a Reynolds number of 2.3 X 10 5 based on exit velocity and chord. Three different tip geometries were investigated: A flat (plain) tip, a suction-side squealer, and a cavity squealer. The experiments reveal that the flow through the plain gap is dominated by flow separation at the pressure-side edge and that the highest levels of heat transfer are located where the flow reattaches on the tip surface. High heat transfer is also measured at locations where the tip-leakage vortex has impinged onto the suction surface of the aerofoil. The experiments are supported by flow visualization computed using the CFX CFD code which has provided insight into the fluid dynamics within the gap. The suction.vide and cavity squealers are shown to reduce the heat transfer in the gap but high levels of heat transfer are associated with locations of impingement, identified using the flow visualization and aerodynamic data. Film cooling is introduced on the plain tip at locations near the pressure-side edge within the separated region and a net heat flux reduction analysis is used to quantify the performance of the successful cooling design.

A Converging Slot-Hole Film-Cooling Geometry: Part 2 — Transonic Nozzle Guide Vane Heat Transfer and Loss

This paper presents the first experimental measurements on an engine representative nozzle guide vane, of a new film-cooling hole geometry, a convergingslot-hole or console. The patented console geometry is designed to improve the heat transfer and aerodynamic performance of turbine vane and rotor blade cooling systems. These experiments follow the successful validation of the console design in low-speed flat-plate tests described in Part 1 of this paper. Stereolithography was used to manufacture a resin model of a transonic, engine representative nozzle guide vane in which seven rows of previously tested fan-shaped film-cooling holes were replaced by four rows of consoles. This vane was mounted in the annular vane ring of the Oxford cold heat transfer tunnel for testing at engine Reynolds numbers, Mach numbers and coolant to mainstream momentum flux ratios using a heavy gas to simulate the correct coolant to mainstream density ratio. Heat transfer data were measured using wide-band thermochromic liquid crystals and a modified analysis technique. Both surface heat transfer coefficient and the adiabatic cooling effectiveness were derived from computer-video records of hue changes during the transient tunnel run. The cooling performance, quantified by the heat flux at engine temperature levels, of the console vane compares favourably with that of the previously tested vane with fan-shaped holes. The new console film-cooling hole geometry offers advantages to the engine designer due to a superior aerodynamic efficiency over the fan-shaped hole geometry. These efficiency measurements are demonstrated by results from midspan traverses of a four-hole pyramid probe downstream of the nozzle guide vane.

Work placement experience: should I stay or should I go?

The opportunity to experience work placements that complement taught and practical courses in higher education has become a central strand of many undergraduate degree programmes. While there is tacit agreement that such placements are a good thing, in recent years the numbers of students opting for work placements has been declining. This paper uses a mixed method research design to probe the learning outcomes, attitudes and perceptions of undergraduate students who choose not to go on a work placement. Findings highlight some areas of concern that could be considered by institutions of higher education working to enhance good practice in students’ work placement experiences.

The application of thin-film technology to measure turbine-vane heat transfer and effectiveness in a film-cooled, engine-simulated environment

Abstract Thin-film technology has been used to measure the heat transfer coefficient and cooling effectiveness over heavily film cooled nozzle guide vanes (NGVs). The measurements were performed in a transonic annular cascade which has a wide operating range and simulates the flow in the gas turbine jet engine. Engine-representative Mach and Reynolds numbers were employed and the upstream free-stream turbulence intensity was 13%. The aerodynamic and thermodynamic characteristics of the coolant flow (momentum flux and density ratio between the coolant and mainstream) have been modelled to represent engine conditions by using a foreign gas mixture of SF 6 and Argon. Engine-level values of heat transfer coefficient and cooling effectiveness have been obtained by correcting for the different molecular (thermal) properties of the gases used in the engine-simulated experiments to those which exist in the true engine environment. This paper presents the best combined heat transfer coefficient and effectiveness data currently available for a fully cooled, three-dimensional NGVs at engine conditions.

Transient heat transfer measurements using thermochromic liquid crystal. Part 1: An improved technique

Abstract It is common practice to employ thermochromic liquid crystal (TLC) to determine heat transfer coefficients, h , in transient experiments. The method relies on the solution of Fourier’s conduction equation, usually with the boundary condition of a step-change in air temperature. In practice a step-change can be difficult to achieve, and a more general solution to the one-dimensional conduction equation is presented here for a “slow transient,” where the rise in air temperature is represented by an exponential series. An experimental method, based on this technique, requires only a single measurement of surface temperature history, and this has the advantage that narrow-band TLC can be used. As an example, measurements of h are presented from an experiment modelling the internal flow of cooling air inside a gas turbine engine. The measurements are analysed using both the conventional step-change method and the exponential-series technique, and the results show that using the step-change method can give rise to significant errors in the calculated values of h . The new technique should be applicable to many other slow transient heat transfer measurements.

Flow over an aerofoil without and with a leading-edge slat at a transitional Reynolds number

Abstract In this study, a multi-element aerofoil including NACA2415 aerofoil with NACA22 leading-edge slat is experimentally and computationally investigated at a transitional Reynolds number of 2×105. In the experiment, the single-element aerofoil experiences a laminar separation bubble, and a maximum lift coefficient of 1.3 at a stall angle of attack of 12° is obtained. This flow has been numerically simulated by FLUENT, employing the recently developed, k—k L—ω and k—ω shear—stress transport (SST) transition models. Both transition models are shown to accurately predict the location of the experimentally determined separation bubble. Experimental measurements also illustrate that the leading-edge slat significantly delays the stall up to an angle of attack of 20°, with a maximum lift coefficient of 1.9. The fluid dynamics governing this improvement is the elimination of the separation bubble by the injection of high momentum fluid through the slat over the main aerofoil — an efficient means of flow control. Numerical simulations using k—k L—ω are shown to accurately predict the lift curve, including stall, but not the complete elimination of the separation bubble. Conversely, the lift curve prediction using the k—ω SST transition model is less successful, but the separation bubble is shown to fully vanish in agreement with the experiment.

Aerothermal Investigations of Tip Leakage Flow in Axial Flow Turbines—Part II: Effect of Relative Casing Motion

A numerical study has been performed to investigate the effect of casing motion on the tip leakage flow and heat transfer characteristics in unshrouded axial flow turbines. The relative motion between the blade tip and the casing was simulated by moving the casing in a direction from the suction side to the pressure side of the stationary blade. Base line flat tip geometry and squealer type geometries, namely, double squealer or cavity and suction side squealer, were considered at a clearance gap of 1.6% C. The computations were performed using a single blade with periodic boundary conditions imposed along the boundaries in the pitchwise direction. Turbulence was modeled using the shear stress transport k-w model. The flow conditions correspond to an exit Reynolds number of 2.3 × 10 5 . The results were compared to those obtained without the relative casing motion reported in Part I of this paper. In general, the effect of relative casing motion was to decrease the tip leakage mass flow and the average heat transfer to the tip due to the decrease in leakage flow velocity caused by a drop in driving pressure difference. Compared to the computations with stationary casing, in the case of all the three geometries considered, the average heat transfer to the suction surface of the blade was found to be larger in the case of the computations with relative casing motion. At a larger clearance gap of 2.8%C, in case of a flat tip, while the tip leakage mass flow decreased due to relative casing motion, only a smaller change in the average heat transfer to the tip and the suction surface of the blade was noticed.

Experimental Measurements of Ingestion Through Turbine Rim Seals—Part I: Externally Induced Ingress

Part I of this two-part paper presented experimental results for externally-induced (EI) ingress, where the ingestion of hot gas through the rim seal into the wheel-space of a gas turbine is controlled by the circumferential variation of pressure in the external annulus. In Part II, experimental results are presented for rotationally-induced (RI) ingress, where the ingestion is controlled by the pressure generated by the rotating fluid in the wheel-space. Although EI ingress is the common form of ingestion through turbine rim seals, RI ingress or combined ingress (where EI and RI ingress are both significant) is particularly important for double seals, where the pressure asymmetries are attenuated in the annular space between the inner and outer seals. In this paper, the sealing effectiveness was determined from concentration measurements, and the variation of effectiveness with sealing flow rate was compared with theoretical curves for RI ingress obtained from an orifice model. Using a nondimensional sealing parameter phio the data could be collapsed onto a single curve, and the theoretical variation of effectiveness with phio was in very good agreement with the data for a wide range of flow rates and rotational speeds. It was shown that the sealing flow required to prevent RI ingress was much less than that needed for EI ingress, and it was also shown that the effectiveness of a radial-clearance seal is significantly better than that for an axial-clearance seal for both EI and RI ingress.