Numerical study of effects of vortex generators on heat transfer deterioration of supercritical water upward flow

•The effects of VGs on HTD of supercritical water upward flow is numerically studied.•The size, position and number of VGs have suppression and delay effects on HTD.•HTD suppression is caused by the weakening of the buoyancy effect.•HTD delay is caused by the boundary layer recovery effect. The occu...

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Bibliographic Details
Published in:	International journal of heat and mass transfer 2019-07, Vol.137, p.489-505
Main Authors:	Eze, Chika, Wong, Kin Wing, Gschnaidtne, Tobias, Cai, J., Zhao, Jiyun
Format:	Article
Language:	eng
Subjects:	Boilers Boundary layers Catastrophic failure analysis Channels Computational fluid dynamics Delay Deterioration Downstream effects Fluid flow Heat flux Heat transfer Heat transfer coefficient Heat transfer coefficients Heat transfer deterioration Kinetic energy Operational problems Redevelopment Shear stress Supercritical pressures Supercritical water Suppression Temperature distribution Turbulence models Vortex generator Vortex generators Wall temperature
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Summary:	•The effects of VGs on HTD of supercritical water upward flow is numerically studied.•The size, position and number of VGs have suppression and delay effects on HTD.•HTD suppression is caused by the weakening of the buoyancy effect.•HTD delay is caused by the boundary layer recovery effect. The occurrence of heat transfer deterioration (HTD) in supercritical pressure boilers, where the wall temperature rises abruptly is an undesirable phenomenon that is limiting the design of new promising engineering applications. This may cause operational problems such as tube burnouts which could result in a catastrophic system failure. Understanding ways of eliminating HTD is of great importance for the design of safe and reliable supercritical pressure boilers. In this paper, vortex generators (VGs) are installed in circular and annular channels to investigate HTD phenomenon of supercritical water flowing upward at high heat flux and low mass flux using shear stress transport (SST) k-ω turbulence model in ANSYS-FLUENT. Results on wall temperature distribution for the smooth and enhanced channels are compared with available experimental data, and good agreements are obtained. The effects of VG’s size, position, and number on HTD are investigated. The results indicate that VG’s size slightly suppresses and delays HTD downstream of the VG, and there exists an optimum size beyond which HTD starts to aggravate. A strong effect of VGs on heat transfer coefficient (HTC) profiles is observed at locations where VGs are installed and at the downstream locations, with the normalized HTC showing that the wall temperature oscillates a couple of times before becoming stable once the flow returns to fully developed state. The VG’s position has the most significant effect on HTD for any single VG. Increasing the number of VGs installed in-line at the start of the major peak baselines significantly suppresses and delays the peak. Mechanism analysis based on radial distributions of velocity and turbulent kinetic energy (TKE) at different axial positions of both channels shows that HTD suppression is caused by the weakening of the buoyancy effect due to increased TKE near the wall downstream of the VG, whereas the delay is caused by the boundary layer recovery effect due to flow redevelopment after being disrupted by the VG.
ISSN:	0017-9310 1879-2189