Heat Transfer and Fluid Flow Investigations in Ribbed Ducts and Impinging Jets Using Liquid Crystal Thermography and PIV

University dissertation from Division of Heat Transfer, Lund Institute of Technology, Box. 118, SE-221 00 Lund, Sweden

Abstract: This thesis presents experimental investigations of local heat transfer and detailed flow measurements in ribbed rectangular ducts and impinging jets. The advanced measurement methods, Liquid Crystal Thermography (LCT) and Particle Image Velocimetry (PIV) are adopted to provide full field characteristics. The work consists of two parts, regarding rib-roughened rectangular ducts and impinging slot jets, respectively. Previous results have shown that rib-roughened ducts have good performance providing heat transfer enhancement by setting up large-scale secondary flows. Experiments in rectangular ducts of aspect ratio 1 to 8, roughened by ribs are carried out, to understand the fundamental mechanisms and enable further improvements. The rib arrangements of parallel and V-shaped ribs are considered in the heat transfer measurements. The flow separation zones behind the ribs are responsible for the streamwise sawtooth distribution of the heat transfer coefficient. Significant spanwise variations of the heat transfer coefficients are observed. Along the parallel rib-roughened wall, the heat transfer coefficients exhibit the highest values at the upstream ends of the ribs, decrease continuously along the spanwise direction, and reach the lowest values at the downstream ends of the ribs. For the V-shaped ribs, the distribution fashion is the same as parallel ribs for each half rib. The PIV investigation is then carried out to study how different secondary flows are generated, and how they interact with the original base flow and temperature field. More rib arrangements are included in the flow field experiments to study different effects. It is found that the main flow field is strongly altered by the inclined ribs. The parallel ribs cause the streamwise velocity component to decrease continuously along the spanwise direction from the upstream ends of the ribs to the downstream ends, coinciding with the spanwise heat transfer distribution. The expected secondary flows over the entire cross section are deduced from the measurements in different planes. The high momentum fluid carried by the downwash flow leads to high heat transfer coefficients at the upstream ends of the ribs, while the low momentum carried away from the ribbed walls by the upwash flow causes low heat transfer coefficients at the downstream ends of the ribs. In the second part of this work, LCT is applied on a flat surface on which confined impinging slot jets impinge to show the local heat transfer characteristics. The effects of the Reynolds number, the cross flow, the nozzle-to-plate spacing and the slot width are investigated. The heat transfer is proved to be enhanced by exhausting the spent air through symmetric exhaust ports, compared to those with cross flow effects. The multiple jets interact with each other at certain circumstances when their independent flow cell structure cannot be sustained. The slot width has a definite effect on the heat transfer coefficients and the narrow slot is found to give a larger average heat transfer coefficient at the same jet exit velocity.

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