Peter T. Ireland

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

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.

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.

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 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.

Compact Double-Side Liquid-Impingement-Cooled Integrated Power Electronic Module

This paper presents a compact integrated power electronic module (IPEM) which seeks to overcome the volumetric power density limitations of conventional packaging technologies. A key innovation has been the development of a substrate sandwich structure which permits double side cooling of the embedded dies whilst controlling the mechanical stresses both within the module and at the heat exchanger interface. A 3-phase inverter module has been developed, integrating the sandwich structures with high efficiency impingement coolers, delink capacitance and gate drive units. Full details of the IPEM construction and electrical evaluation are given in the paper.

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

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