David R. H. Gillespie

Turbulent heat transfer measurements using liquid crystals

Abstract The transient method of measuring heat transfer coefficients that uses liquid crystals, since its beginnings in the early 1980s, has become one of the best ways of determining full surface distributions of heat transfer coefficient. The turbomachinery research group at Oxford has concentrated on the application of the method to numerous mechanical engineering thermal problems specific to the jet engine. The paper summarises some of the recent developments in the technique including the implementation of an elegant way of producing a change in the fluid temperature. Recent, high-density heat transfer coefficient measurements are discussed together with the advantages such resolution offers in terms of flow field interpretation. A means of integrating the measurements into finite element software for subsequent data analysis is presented. The paper should be of interest to engineers interested in using the most modern heat transfer measurement techniques in their research and development programmes.

Full surface local heat transfer coefficient measurements in a model of an integrally cast impingement cooling geometry

Cast impingement cooling geometries offer the gas turbine designer higher structural integrity and improved convective cooling when compared to traditional impingement cooling systems, which rely on plate inserts. In this paper, it is shown that the surface that forms the jets contributes significantly to the total cooling. Local heat transfer coefficient distributions have been measured in a model of an engine wall cooling geometry using the transient heat transfer technique. The method employs temperature-sensitive liquid crystals to measure the surface temperature of large-scale perspex models during transient experiments. Full distributions of local Nusselt number on both surfaces of the impingement plate, and on the impingement target plate, are presented at engine representative Reynolds numbers. The relative effects of the impingement plate thermal boundary condition and the coolant supply temperature on the target plate heat transfer have been determined by maintaining an isothermal boundary condition at the impingement plate during the transient tests. The results are discussed in terms of the interpreted flow field.

Heat Transfer and Flow Characteristics of an Engine Representative Impingement Cooling System

A study of a large-scale model of an engine representative impingement cooling system has been performed. A series of tests were carried out to characterize the behavior of the system fully. These included cold flow diagnostic tests to determine the pressure loss and the static pressure distribution, and flow visualization to assess surface shear. The surface shear stress pattern provided by multiple stripes of colored paint applied to the target surface yielded important information on the near-wall flow features far from the jet axis. The row solved flow and pressure distributions are compared to industry standard predictions. Heat transfer tests using the transient liquid crystal technique were also conducted using coatings comprised of a mixture of three thermochromic liquid crystals. Analysis of the thermochromic liquid crystal data was enhanced by recent developments in image processing. In addition, an energy balance analysis of signals from fast-response thermocouples for air temperature measurement was applied to verify the levels of heat transfer coefficients on surfaces not coated with the temperature-sensitive liquid crystal.

A Novel Transient Liquid Crystal Technique to Determine Heat Transfer Coefficient Distributions and Adiabatic Wall Temperature in a Three-Temperature Problem

Transient liquid crystal techniques are widely used for experimental heal transfer measurements. In many instances it is necessary to model the heat transfer resulting from the temperature difference between a mixture of two gas streams and a solid surface. To nondimensionally characterize the heat transfer from scale models it is necessary to know both the heal transfer coefficient and adiabatic wall temperature of the model. Traditional techniques rely on deducing both parameters from a single test. This is a poorly conditioned problem. A novel strategy is proposed in which both parameters are deduced from a well-conditioned three-test strategy. The heat transfer coefficient is first calculated in a single test; the contribution from each driving gas stream is then deduced using additional tests. Analytical techniques are developed to deal with variations in the temperature profile and transient start time of each flow. The technique is applied to the analysis of the heat transfer within a low aspect ratio impingement channel with initial cross flow.

The Effect of Initial Cross Flow on the Cooling Performance of a Narrow Impingement Channel

Impingement channels are often used in turbine blade cooling configurations. We examine the heat transfer performance of a typical integrally cast impingement channel. Detailed heat transfer coefficient distributions on all heat transfer surfaces were obtained in a series of low temperature experiments carried out in a large-scale model of a turbine cooling system using liquid crystal techniques. All experiments were performed on a model of a 19-hole, low aspect ratio impingement channel. The effect of flow introduced at the inlet to the channel on the impingement heat transfer within the channel was investigated. A novel test technique has been applied to determine the effect of the initial cross flow on jet penetration. The experiments were performed at an engine representative Reynolds number of 20,000 and examined the effect of additional initial cross flow up to 10 percent of the total mass flow

A Novel Transient Liquid Crystal Technique to Determine Heat Transfer Coefficient Distributions and Adiabatic Wall Temperature in a Three Temperature Problem

Transient liquid crystal techniques are widely used for experimental heat transfer measurements. In many instances it is necessary to model the heat transfer resulting from the temperature difference between a mixture of two gas streams and a solid surface. To non-dimensionally characterise the heat transfer from scale models it is necessary to know both the heat transfer coefficient and adiabatic wall temperature of the model. Traditional techniques rely on deducing both parameters from a single test. This is a poorly conditioned problem. A novel strategy is proposed in which both parameters are deduced from a well conditioned three test strategy. The heat transfer coefficient is first calculated in a single test; the contribution from each driving gas stream is then deduced using additional tests. Analytical techniques are developed to deal with variations in the temperature profile and transient start time of each flow. The technique is applied to the analysis of the heat transfer within a low aspect ratio impingement channel with initial cross flow.Copyright

An Investigation of the Application of Roughness Elements to Enhance Heat Transfer in an Impingement Cooling System

The inclusion of roughness elements on the target surface of a turbine aerofoil impingement cooling system is an attractive means of heat transfer enhancement. In such a system, it is important to minimise additional pressure loss caused by the roughness elements and thus their shape, size and position need to be optimised. The research showed how heat transfer enhancement is normally achieved at the expense of extra pressure loss. A hexagonal roughness element designed by the authors showed up to 10% heat transfer enhancement with minimal extra pressure loss. The present work includes shear pattern visualisation on the target surface, pressure loss measurements and heat transfer coefficient measurements for an impingement cooling system with simply shaped roughness elements-specifically cylindrical & diamond pimples. Flow visualisation results and pressure loss measurements for the above configurations provided criteria for selecting the shape, size and position of the roughness elements. The detailed heat transfer measurements on the target surface and over the roughness elements were used to explain the heat transfer enhancement mechanisms. It was found that the largest contribution to heat transfer is the impingement stagnation point and the developing wall jet regions. However, the research showed that the low heat transfer coefficient region could be made to contribute more by using strategically located roughness elements. A hexagonal rim was designed to cover the complete low heat transfer coefficient region midway between neighbouring jets. The effect of the height, cross sectional shape and wall angle of the hexagonal rim were studied using a series of heat transfer and pressure loss experiments. The transient heat transfer tests were conducted using a triple thermochromic liquid crystal technique and the thermal transient was produced by a fine wire mesh heater. The heat transfer coefficient over the pimples was measured using a hybrid transient method that analysed the thermal transient of the copper pimple. The detailed heat transfer coefficient distributions over the complete area of the target surface provided comprehensive understanding of the performance of the hexagonal rim. Tests were conducted at three different mass flow rates for each configuration. The average and local jet Reynolds numbers varied between 21500 and 31500, and 17000 and 41000 respectively.Copyright

Detailed Flow and Heat Transfer Coefficient Measurements in a Model of an Internal Cooling Geometry Employing Orthogonal Intersecting Channels

Cast interconnecting passage Lattice cooling geometries offer the gas turbine designer higher structural integrity and improved convective efficiency when compared to traditional aerofoil rear cooling strategies. In this paper, local heat transfer coefficient distributions were measured in a model of an idealised engine lattice cooling geometry, with flow ejection through film-cooling holes. The measurements were made using the transient liquid crystal technique in a large-scale perspex model at low temperature. The technique allows very high data resolution. Heat transfer patterns on all surfaces of the device including the internal web are presented at engine representative Reynolds numbers. The results are discussed in terms of the interpreted flow field. Furthermore, a subsequent analysis which accounted for the changing driving gas temperature and mass flow rate through the model has allowed the heat transfer coefficients to be derived based on the mixed bulk temperature, and local passage Reynolds number.© 2000 ASME

Detailed Measurements of Local Heat Transfer Coefficient in the Entrance to Normal and Inclined Film Cooling Holes

The local heat transfer inside the entrance to large-scale models of film cooling holes has been measured using the transient heat transfer technique. The method employs temperature-sensitive liquid crystals to measure the surface temperature of large-scale perspex models. Full distributions of local Nusselt number were calculated based on the cooling passage centerline gas temperature ahead of the cooling hole. The circumferentially averaged Nusselt number was also calculated based on the local mixed bulk driving gas temperature to aid interpretation of the results, and to broaden the potential application of the data. Data are presented for a single film cooling hole inclined at 90 and 150 deg to the coolant duct wall. Both holes exhibited entry length heat transfer levels that were significantly lower than those predicted by entry length data in the presence of crossflow. The reasons for the comparative reduction are discussed in terms of the interpreted flow field.

A Comparison of Full Surface Local Heat Transfer Coefficient and Flow Field Studies Beneath Sharp-Edged and Radiused Entry Impinging Jets

Full heat transfer coefficient and static pressure distributions have been measured on the target surface under impinging jets formed by sharp-edged and large entry radius holes. These geometries are representative of impingement holes in a gas turbine blade manufactured by laser cutting and by casting, respectively. Target surface heat transfer has been measured in a large scale perspex rig using both the transient liquid crystal technique and hot thin film gauges. A range of jet Reynolds numbers, representative of engine conditions, has been investigated. The velocity variation has been calculated from static pressure measurements on the impingement target surface. The heat transfer to the target surface is discussed in terms of the interpreted flow field.Copyright