In order to enhance convective heat transfer, turbulence promoters or vortex generators (VGs) are often used to manipulate the flow field and to benefit from their effect on thermal performance. The current investigation is directed towards a detailed understanding of the generated vortex flows and their impact on heat transfer for wedge shaped full-body VGs in internal flows. The main focus is on longitudinal and parallel arrangements of two or three VGs, where interaction of the induced flow field plays an important role. A single VG introduces a main vortex pair moving longitudinally downstream which is symmetric to the mid-plane of the turbulator itself. By using arrangements of several VGs it is possible to take advantage of the vortex interaction and define or deflect zones of enhanced heat transfer. In certain cases (e.g. due to manufacturing reasons) sharp edges on the elements cannot be realized. The effect of this discrepancy in the designated geometry is also investigated. Data for heat transfer behind a sharp-edged VG is compared with data for VGs manufactured with two different edge radii. In the present experimental setup the VGs are mounted on the bottom wall of a rectangular channel. For Reynolds numbers of 150,000 up to 550,000 the heat transfer coefficient is measured with the transient thermochromic liquid crystal (TLC) thermometry which is based on the measurement of the wall temperature response to a given step change in the fluid temperature. Numerical simulations using a Reynolds-Stress Model describe the flow field around the arrangements and are used for further interpretation of the experimental heat transfer distributions. Effects of vortex interactions on the heat transfer distribution are described for parallel and longitudinal arrangements. In the experimental data for elements with rounded edges a significant reduction in heat transfer can be observed.
The effect of single full-body Vortex Generators (VGs) on heat transfer was investigated experimentally. The delta shaped devices with different geometries were examined in a rectangular channel for a Reynolds number range of 80,000 up to 600,000. The research included heat transfer as well as flow measurements. Detailed heat transfer results were obtained by a steady state thermochromic liquid crystal (TLC) method using heater foils. This full surface measurement shows heat transfer enhancement evoked by the longitudinal vortices produced by the VGs. Data for secondary flow structures were determined by Particle Image Velocimetry (PIV) measurements. The comparison of vortex position and heat transfer distribution shows that the local heat transfer maximum due to downflow regions of the secondary flow does occur at positions shifted slightly towards the centerline of the channel compared to the existing vortex cores. Experimental data for the flow field were also compared to numerical calculations using FLUENT.
Sequential impingement channels can reduce the adverse effect of crossflow in narrow impingement channels, as well as increase the cooling efficiency. In this work, sequential impingement channels are experimentally investigated using the transient liquid crystal technique to assess their thermal performances. A low heat transfer region is identified in the downstream part of the first channel where the flow is discharged into the second plenum. Various means of increasing the heat transfer at this location are investigated. Ribs on the target plate allow for an increase of the average heat transfer coefficient with small losses in pressure. Reducing the channel cross-section increases the mean flow velocity and, combined with the ribs, allows for a further increase of the heat transfer. Additionally, the geometrical changes of the channel caused by the addition of a ramp with a rounded corner, allow to decrease the pressure losses associated with the discharge into the second plenum, which is not optimal in the baseline configuration due to the sharp corner of the purge hole. Further reducing the cross-section to increase the heat transfer, however, increases the pressure losses due to the small open area in the transition zone.
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