Changmin Son

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.

Comparative Study Between a Cut-Back and Conventional Trailing Edge Film Cooling System

The present experimental study investigates the aero performance differences between a conventional turbine blade trailing edge and a trailing edge with a sharp cut-back. Both geometries include trailing edge film cooling. A scaled model of a conventional turbine blade trailing edge and the trailing edge with a sharp cut-back including the scaled film cooling hole geometries were incorporated into flat plates. Experiments were conducted in a low speed wind tunnel to establish the performance change caused by introducing such a cut-back system on the trailing edge. Experiments were conducted at a Reynolds number of 1.9 × 106 at the trailing edge and for blowing ratios from 0.9 to 2.4. The experimental data presented include; static pressure variation on the plate surfaces; the change in discharge coefficient due to the cut-back; detailed total pressure measurements via a 2D traverse mechanism at a plane 40mm downstream of the flat plates. The total pressure measurements were used to establish the mixed out loss for both configurations.© 2006 ASME

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

Heat Transfer Characteristics of an Impingement Plate Used in a Turbine Vane Cooling System

Detailed heat transfer coefficient distributions have been measured on both surfaces of the impingement plate of an engine-representative impingement cooling system using the thermochromic liquid crystal (TLC) transient technique. The color images of the TLC on the impingement downstream surface provide evidence of a re-impingement flow. The reimpingement flow is found to contribute to local increases in the heat transfer on the impingement plate downstream surface. It was found that the average heat transfer coefficient on the impingement downstream surface is about 50% of the average target surface heat transfer coefficient. The results are compared with a previously reported correlation. Copyright

U-bend Shaped Turbine Blade Cooling Passage Optimization

The U-bend duct which turns flow through 180 is encountered in many engineering applications in mechanical and aerospace systems. One important example is in modern turbine blade cooling systems and, partly for this reason, the U-bend has been the subject of many fluid dynamic and heat transfer investigations. The bend flow combines many complex fluid dynamic phenomena and most of the investigations have focused on understanding of the flow physics or improving the accuracy of numerical simulations. However, reports of research into shape optimization of the U-bend are relatively sparse. The main purpose of the present investigation was to focus on finding a 3-D U-bend configuration with minimum pressure loss. The DOE (Design of Experiment) technique and surrogate design space model have been successfully applied by the authors, instead of direct optimization, to reduce computational time.

The Effect of Roughness Element Fillet Radii on the Heat Transfer Enhancement in an Impingement Cooling System

The combination of roughness elements with an impingement cooling systems offers an attractive means of achieving high heat transfer. Considerable care must be taken to choose the shape, size and the position of the roughness elements to maximise heat transfer and minimise pressure loss. In the last decade, many studies have been investigated the effect of changes in many of the geometric features, but little attention has been paid to the effect of the inevitable fillet. Blades and vanes are normally manufactured by casting so the fillet radius is unavoidable. The present paper investigates the effect of roughness element fillet radii on heat transfer enhancement in an impingement cooling system. Three configurations with streamwise ribs were studied. The streamwise ribs are all trapezoidal in cross section. In the three configurations the fillet radii are (1) 0mm (sharp-edged), (2) 3mm, and (3) 5mm. The extra heat transfer area of the sharp-edged, 3mm fillet and the 5mm fillet rib configurations are reported. Two-staggered arrays (a uniform & non-uniform hole diameter array) of impingement plates are used. The jets from odd numbered rows impinge between the ribs while the jets from even numbered rows impinge onto the ribs. 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. The transient liquid crystal technique was used to produce detailed Nusselt number distributions and row resolved average Nusselt number levels. The heat transfer enhancement and pressure loss due to the streamwise ribs are also compared to the smooth surface impingement cooling channel. The research showed that the streamwise ribs with fillet radii produced lower Nusselt number levels than both sharp-edged ribs and impingement onto a smooth surface.Copyright