Dimples in turbulent pipe flows: experimental aero-thermal investigation

•Experimental investigation on the turbulent flow structures' effects on the local heat transfer in dimpled pipes.•Comparison of the aero-thermal performances among dimpled and helically ribbed tubes.•Reynolds number effects on the heat transfer in dimpled pipes.•Uncertainty quantification of t...

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Bibliographic Details
Published in:International journal of heat and mass transfer 2020-08, Vol.157, p.119925, Article 119925
Main Authors: Virgilio, M., Dedeyne, J.N., Van Geem, K.M., Marin, G.B., Arts, T.
Format: Article
Language:English
Subjects:
Aerodynamics
Coefficient of friction
Computational fluid dynamics
Diameters
Dimples
Dimpling
Enhanced heat transfer
Fluid flow
Heat exchangers
Heat transfer
Holes
Kinetic energy
Liquid crystals
Liquid crystals thermography
Low pressure
Particle image velocimetry
Pipe flow
Pipes (defects)
Pressure drop
Pressure loss
Secondary flow
Skin friction
Stereo-PIV
Thermodynamic efficiency
Thermography
Tubes
Turbulence
Turbulent flow
Vorticity
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Summary:•Experimental investigation on the turbulent flow structures' effects on the local heat transfer in dimpled pipes.•Comparison of the aero-thermal performances among dimpled and helically ribbed tubes.•Reynolds number effects on the heat transfer in dimpled pipes.•Uncertainty quantification of the Liquid Crystals Thermography over highly deformed surfaces. An experimental investigation is performed to study the effects of turbulent flow structures on the wall heat transfer, specifically inside the spherical cavities of partially dimpled tubes. The geometry under investigation consists of an array of spherical cavities distributed over the inner surface of half a tube. Each row has five indentations separated by an angular offset of 36° over the half circumference. The dimples are characterized by a depth-to-print diameter (d/Dp) of 0.18, a print diameter-to-pipe diameter (Dp/D) of 0.25, and a streamwise distance (xd/D) between consecutive cavities of 0.56. At Re=20000, flow separation, reattachment, and secondary flows are captured using both planar Particle Image Velocimetry (PIV) and stereo-PIV. The time-averaged flow field, the vorticity levels, and Turbulent Kinetic Energy (TKE) fields inside the dimples can be used for numerical validation. Moreover, the local heat transfer field inside the cavity is measured for Re=20000−80000, by using Liquid Crystals Thermography (LCTG). At the same Re and Pr, an overall heat transfer enhancement of 1.8 (1.8 times the Nusselt number value (Nu) in bare pipes) has been measured. The use of dimples in heat exchangers thus results in a rise in thermal efficiency at the cost of an increased pressure drop. Comparing the skin friction coefficient (Cf) of the dimpled pipe with the one measured in ribbed tubes for Re=20000−80000, it is shown that higher levels of the aerothermal performance (high heat transfer and low-pressure losses) are achieved by employing the dimple technique.
ISSN:0017-9310
1879-2189
DOI:10.1016/j.ijheatmasstransfer.2020.119925