LATERAL CONDUCTION AND AEROTHERMAL CHARACTERISATION OVER A RIBBED WALL OF INTERNAL COOLING PASSAGES

Abstract

Economic utilisation of energy is the eternal desire of modern time. This can be achieved by developing efficient thermal systems, and therefore, different techniques of heat transfer enhancement (HTE) become a vital area for research. Rib turbulators are commonly used passive techniques for HTE in many engineering applications. Gas turbine blade cooling channels, gascooled reactor fuel elements, a cooling device for electronic components, nuclear reactor and heat exchangers are key examples of industrial application of rib turbulators. The enhancement in heat transfer due to the introduction of ribs can be associated with an increase in surface area and the flow destabilization in the wall-bounded region. Typically, surface mounted ribs act as the turbulence promoters and consequentially host improvement of the momentum flux in a crosswise direction to the flow, leading to heat transfer enhancement with a pressure penalty; therefore, demands for an effective trade-off towards selecting an effective rib configuration. Herein, the potential of two sophisticated non-intrusive optical techniques naming liquid crystal thermography (LCT) and particle image velocimetry (PIV) is utilised to extract the detailed surface heat transfer and flow field information, respectively. In addition, pressure drop measurement is done using a digital micro-manometer. The detailed heat transfer and fluid flow investigation for a novel rib geometry proposed, i.e., round edge slit ribs have been done by carefully designed experiments. The heat transfer results have been evaluated by considering the lateral conduction effect and further analysed in terms of local and spanwise-averaged augmentation Nusselt number. The effect of lateral conductions have been analysed by using an analytical empirical correction method based upon the 1-D heat semi-infine conduction solution. The transient surface temperature distribution based upon liquid crystal thermography has been used during analysis. The results reveal the significant change in magnitude and spatial resolution of the HTC distribution while considering the lateral conduction in the spanwise direction. The influencing parameters such as slit angle (α) is varied from 0⁰ to 10⁰ in a step of 5⁰, whereas rib pitch to height ratio (p/e) has been varied from 5 to 15 in an increment of 5. The Reynolds numbers (Re) are varied as 6200, 8200, 10200, 12200 depending on the various mass flow rate condition. The effectiveness of the proposed rib configuration on mixing/heat transfer enhancement and the effect of the influencing parameters has been evaluated by examining the surface and spanwiseaveraged augmented Nusselt number distribution (i.e., 0 Nu / Nu and x / x,0 Nu Nu , respectively). Also, the performance parameters such as overall averaged augmentation Nusselt number ( 0 Nu / Nu ), friction factor ratio ( 0 f / f ) and Thermal Performance Factor (TPF) have been x calculated to evaluate the aerothermal behaviour of the ribbed duct. In addition, Response Surface Methodology (RSM) has also been implemented to analyse the thermal behaviour of the system based on the performance parameters. Correlations have also been developed to predict the behaviour of the ribbed duct, which can be used by design engineers. The results obtained from the correlations have been furthered analysed and found in accord with the experimental values within ±10% deviation. The individual and combined effect of the influencing parameters on the heat transfer augmentation and flow parameters are also studied. The averaged flow features such as mean velocities, critical flow structures, turbulent stresses, coherent structure distribution, the turbulent kinetic energy budget in the inter-rib region have been studied that help in understanding the driving mechanism responsible for better mixing and heat transfer augmentation. Flow investigation reveals a band of high-velocity magnitude, strong positive wall-normal velocity as well as velocity vectors behind the slitted ribs indicate the flow through the slit. The interaction between flow through the slit and the separated shear layer cause the different size of recirculation bubble observed downstream of the first rib, thus changing the reattachment length. The above observations properly justify the heat transfer behaviour of these ribs, i.e., matching with the existence of low and high heat transfer zone location and extent. However, additional flow structures (like two vortical structures with saddle point, swirl structure with different nodes and bifurcation lines) are witnessed at the top downstream corner of the slit rib (around y/e=0.8), which cannot be directly correlated with the heat transfer results but can be responsible for the pressure drop as it affects the formation of the separated shear layer. The overall effect of the Reynolds number can be seen as the heat transfer augmentation increases in magnitude with an increase in Reynolds number without observing any major change in the pattern. The effect of angle is found to be quite significant, especially in the vicinity of the rib. Better performance of 10⁰ slit ribs has been observed in terms of high heat transfer augmentation and reduced pressure losses. The flow, coming from the slit strikes directly like a jet on the bottom wall for the higher slit angle ribs, irrespective of the arrangements and Reynolds number. The effect of rib arrangements (p/e) has also been examined, which reveals a better heat transfer augmentation for p/e=10 value for all the rib configurations and flow conditions studied. This enhanced heat transfer can be linked with the flow field information observed, which indicates the secondary boundary layer developed near the wall after the reattachment point, having different thicknesses for different arrangements. Further, flow field results combined with the heat transfer distribution are studied by superimposing each flow quantity separately over the local Nusselt number (Nu) distribution and found to be in accord with the heat transfer results.