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

Abstract

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.

Numerical predictions of jet impingement heat transfer on square pin-fin roughened plates

“…Taslim et al [8] and Son et al [9] confirmed that the roughness elements enhanced the total heat transfer performance of the impingement system with very low pressure penalty when the roughness elements was appropriately located at the low heat transfer coefficient region. Pin-fins have been arranged onto the test surface in an impingement system for its excellent capability of convective heat transfer enhancement.…”

“…Quan et al [10] measured the cooling performance of an impingement system combined with pin fins by using infrared technology, and the overall cooling effectiveness was above 0.8. Funazaki et al [11,12], Son, et al [9] and Yamawaki et al [13] investigated the impingement cooling combined with circular pin fins by using transient liquid crystal technology. Their results showed that the wetted-area averaged Nusselt number of pin-fin roughened target plate was lower than flat plate; however the total heat transfer quantity of the pin-fin roughened plate increased by up to 50%, and the maximum pressure loss only increased by 10%.…”

Experimental and numerical investigation of impingement heat transfer on a flat and micro-rib roughened plate with different crossflow schemes

“…They found that the co-flow arrangement, for which the crossflow is parallel to the ribs, is better. Son et al [16,17] investigated the application of roughness elements to enhance heat transfer in an impingement cooling system by conducting flow field and transient heat transfer experiments. The results showed that the roughness elements enhance the total performance of the impingement cooling system with very low pressure penalty.…”

An Experimental and Numerical Study of Heat Transfer From Arrays of Impinging Jets With Surface Ribs

“…It was concluded that the choice of rib configuration allows altering the local heat transfer distribution significantly, e.g., by shifting the point of maximum heat transfer, and thus allows for a tailoring of local cooling needs. Detailed heat transfer measurements in the form of local distributions were also presented by Son et al [20,21] for different element types. The results showed that the elements enhance total heat transfer performance of the impingement cooling system at low pressure penalty.…”