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

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.

• author: Gao, Xiufang
• supervisor: [unknown] [unknown]
• opponent: Dr Murray, Darina, University of Dublin, Ireland
• organization: Heat Transfer
• publishing date: 2002
• type: Thesis
• publication status: published
• subject: Energy Engineering
• keywords: Thermal engineering, applied thermodynamics, Termisk teknik, termodynamik, Particle Image Velocimetry (PIV)., Liquid Crystal Thermography (LCT), secondary flow, impinging jets, Enhanced heat transfer, ribbed ducts
• pages: 180 pages
• defense location: Sal M:B, M-huset, Ole Römers väg 1, LTH
• defense date: 2002-12-17 10:15:00
• external identifiers: other:ISRN:LUTMDN/TMHP - - 02/1008 - - SE
• ISBN: 91-628-5467-4
• language: English
• LU publication?: yes
• additional info: Article: Paper 1. Gao X. and Sundén B., 2000, “Detailed Measurements of Heat Transfer Coefficients in a Rectangular Duct Using Hue-Based Calibrated Liquid Crystal”, Int. Comm. Heat Mass Transfer, Vol. 27, No. 1, pp. 13-22. Article: Paper 2. Gao X. and Sundén B., 2000, “Heat Transfer Measurements in a Rectangular Duct with Inclined Ribs Using Liquid Crystal Thermography”, Paper No. NHTC2000-12091, 34th National Heat Transfer Conference, Pittsburgh, Pennsylvania, USA, Aug. 20-22. Article: Paper 3. Gao X. and Sundén B., 2001, “Heat Transfer and Pressure Drop Measurements in Rib-Roughened Rectangular Ducts”, Exp. Thermal Fluid Science, Vol. 24, pp. 25-34. Article: Paper 4. Gao X. and Sundén B., 2002, “PIV Measurement of the Flow Field in Rectangular Ducts with 60 Degree Parallel, Crossed and V-shaped Ribs”, submitted for publication. Article: Paper 5. Gao X. and Sundén B., 2002, “Thermal and Flow Field Characteristics in V-shaped Rib-Roughened Ducts”, submitted for publication. Article: Paper 6. Gao X. and Sundén B., 2002, “Effects of Inclination Angle of Ribs on the Flow Behavior in Rectangular Ducts”, submitted for publication. Article: Paper 7. Gao X. and Sundén B., 2001, “Heat Transfer Measurement of Impinging Two-Dimensional Slot Jets”, Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, G. P. Celata, P. Di Marco, A. Goulas and A. Mariani (Eds.), Edizioni ETS, Pisa, Vol. 2, pp. 1081-1086. Article: Paper 8. Gao X. and Sundén B., 2003, “Experimental Investigation of the Heat Transfer Characteristics of Confined Impinging Slot Jets”, to appear in Experimental Heat Transfer, Vol. 16.
• id: da1e1063-64a1-4314-a218-b277a7ee0b30 (old id 465306)
• date added to LUP: 2016-04-01 16:45:38
• date last changed: 2018-11-21 20:44:00