T. V. Jones

Liquid crystal measurements of heat transfer and surface shear stress

Liquid crystals have become an accurate and convenient means of measuring surface temperature and heat transfer for the gas turbine and heat transfer research communities. The measurement of surface shear stress using liquid crystals is finding increasing favour with aerodynamicists and developments in these techniques ensure that liquid crystals will continue to provide key thermal and shear stress data in the future. The increasing use of three-dimensional finite element computational models has allowed industry to capitalize on the advantages of the full surface data generated. The paper reviews the use of these complex materials in research with a special emphasis on recent developments in the field. The aim is to provide the reader with an up to date background in this measurement technology and allow the researcher to decide whether liquid crystals would be suitable in specific applications.

Detailed Measurements of Local Heat Transfer Coefficient and Adiabatic Wall Temperature Beneath an Array of Impinging Jets

A transient method of measuring the local heat transfer under an array of impinging jets has been developed. The use of a temperature-sensitive coating consisting of three encapsulated thermochromic liquid crystal materials has allowed the calculation of both the local adiabatic wall temperature and the local heat transfer coefficient over the complete surface of the target plate. The influence of the temperature of the plate through which the impingement gas flows on the target plate heat transfer has been quantified. Results are presented for a single in-line array configuration over a range of jet Reynolds numbers

The development of a new direct-heat-flux gauge for heat-transfer facilities

A new type of direct-heat-flux gauge (DHFG) comprising an insulating layer mounted on a metal substrate has been developed. The gauge measures the heat flux across the insulating layer by measuring the top surface temperature employing a sputtered thin-film gauge (TFG) and the metal temperature using a thermocouple. The TFGs are platinum temperature sensors with physical thickness less than 0.1 µm. They are instrumented on the insulating layer. The thermal properties and the ratio of the thickness over the thermal conductivity of the insulating layer have been calibrated. A detailed method of analysis for calculating the surface heat flux from DHFG temperature traces is presented. The advantages of the DHFG include its high accuracy, its wide range of frequency response (from dc to 100 kHz) and, most significantly, that there is no requirement for knowledge of the structure of the metal substrate. Since the metal substrate is of high conductivity, few thermocouples are required to monitor the small spatial variation of the metal temperature, whereas multiple thin-film gauges may be employed. The DHFGs have been applied to a gas turbine nozzle guide vane and tested in the Oxford Cold Heat Transfer Tunnel successfully.

Embedded micromachined fiber-optic Fabry-Perot pressure sensors in aerodynamics applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical-fiber pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m squared, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m-thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a single mode fiber sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by three wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of 1.6 radbar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 8 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.

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.

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

A color 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 color (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 T{sub gas} is reported. The uncertainty in all measurements has been quantified and examples of applications of both techniques given.

Heat Transfer Enhancement Within a Turbine Blade Cooling Passage Using Ribs and Combinations of Ribs With Film Cooling Holes

A transient heat transfer method using liquid crystals has been applied to a scale model of a turbine rotor blade passage. Detailed contours of local heat transfer coefficient are presented for the passage in which the heat transfer to one wall was enhanced first by ribs and then with ribs combined with holes. The hole geometry and experimental dimensionless flow rates were representative of those occurring at the entrance to engine film cooling holes. The results for the ribbed passage are compared to established correlations for developed flow. Qualitative surface shear stress distributions were determined with liquid crystals. The complex distributions of heat transfer coefficient are discussed in light of the interpreted flow field.

Embedded micromachined fibre optic Fabry-Perot pressure sensors in aerodynamics applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical fibre pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m square, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a singlemode fibre sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by 3 wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of /spl sim/1.8 rad bar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 10 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.

An Advanced Method of Processing Liquid Crystal Video Signals From Transient Heat Transfer Experiments

A new method of processing the liquid crystal color change data obtained from transient heat transfer experiments is presented. The approach uses the full-intensity history recorded during an experiment to obtain an accurate measurement of the surface heat transfer coefficient at selected pixels. Results are presented for a model of a turbine blade cooling passage with combined ribs and film cooling holes. The implementation of the technique and the advantages to be gained from its application are discussed

The effect of hot-streaks on HP vane surface and endwall heat transfer : An experimental and numerical study

Pronounced nonuniformities in combustor exit flow temperature (hot-streaks), which arise because of discrete injection of fuel and dilution air jets within the combustor and because of endwall cooling flows, affect both component life and aerodynamics. Because it is very difficult to quantitatively predict the effects of these temperature nonuniformities on the heat transfer rates, designers are forced to budget for hot-streaks in the cooling system design process. Consequently, components are designed for higher working temperatures than the mass-mean gas temperature, and this imposes a significant overall performance penalty. An inadequate cooling budget can lead to reduced component life. An improved understanding of hot-streak migration physics, or robust correlations based on reliable experimental data, would help designers minimize the overhead on cooling flow that is currently a necessity. A number of recent research projects sponsored by a range of industrial gas turbine and aero-engine manufacturers attest to the growing interest in hot-streak physics. This paper presents measurements of surface and endwall heat transfer rate for a high-pressure (HP) nozzle guide vane (NGV) operating as part of a full HP turbine stage in an annular transonic rotating turbine facility. Measurements were conducted with both uniform stage inlet temperature and with two nonuniform temperature profiles. The temperature profiles were nondimensionally similar to profiles measured in an engine. A difference of one-half of an NGV pitch in the circumferential (clocking) position of the hot-streak with respect to the NGV was used to investigate the affect of clocking on the vane surface and endwall heat transfer rate. The vane surface pressure distributions, and the results of a flow-visualization study, which are also given, are used to aid interpretation of the results. The results are compared to two-dimensional predictions conducted using two different boundary layer methods. Experiments were conducted in the Isentropic Light Piston Facility (ILPF) at QinetiQ Farnborough, a short-duration engine-sized turbine facility. Mach number, Reynolds number, and gas-to-wall temperature ratios were correctly modeled. It is believed that the heat transfer measurements presented in this paper are the first of their kind.