Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Research Paper Heat transfer enhancement and flow characteristics of vortex generating jet on flat plate with turbulent boundary layer Ni-oh Puzu, Suteera Prasertsan, Chayut Nuntadusit ⁎ Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla 90110, Thailand HIGHLIGHTS • Vortex generating jet is investigated for heat transfer enhancement on flat plate. • Effect of jet-mainstream flux ratio is studied experimentally and numerically. • The heat transfer enhancement is explained by behavior of generated counter-rotating vortex pair. ABSTRACT This paper presents the results of flow and heat transfer characteristics of vortex generating jet on a flat plate. A circular jet from the pipe nozzle is injected perpendicularly to the mainstream flow on flat plate Longitudinal vortex flow is induced for enhancing heat transfer on a constant heat flux surface. The effect of jet- mainstream flux ratio (J) on heat transfer was investigated by fix the mainstream flow at 10/s and varied the jet velocity corresponding to three jet-mainstream momentum flux ratios, J , of 0.06, 2.25, and 12.25. Temperature distribution on heat transfer surface with constant heat flux is captured by an infrared camera. The flow field is also investigated with numerical simulation. The results reveal that all heat transfer rates on the surface are higher than under the No-jet conditions due to generated counter-rotating vortex pair. Simulation results indicate that the low momentum flux vortex of J = 0.06 moves close to the surface and directly affects heat transfer enhancement, with a maximum increase of 40%. With a greater jet velocity the vortex pair moves away from the surface, the mainstream flows around the more stronger jet, and counter-flow secondary vortex is formed downstream of jet exit. Thus for J = 2.25, the disturbed mainstream exerts more influence on the surface heat transfer, up to a maximum of 50%. For J = 12.25, a maximum heat transfer, at 68% better than the No-jet scenario, is clearly achieved through the secondary vortex over a longer and larger area. 1. Introduction Boundary layer perturbation is an effective solution to increase the heat transfer rate on heat transfer surface. Solid turbulators are com- monly used to generate longitudinal vortices to spiral flow along downstream direction over the heat transfer surface, and are usually found attached to almost-maintenance-free cooling fins and other heat exchangers. However, solid turbulators impede flow and hence create an increase in friction loss to the flow. Fiebig [1] used wing-type vortex generators on a plane surface and showed a greater heat transfer enhancement; the generated vortex in- creased the rate of kinetic energy flux and greatly changed the transi- tion Reynolds number, the temperature profile and gradients at the wall. Aris et al. [2] studied 3-D delta wing tabs attached to a fin surface and reported that the tabs could improve heat transfer rate up to 37% compared to that without attachment. Smulsky et al. [3] investigated the effect of rib attachment orientation angle on a heat transfer surface and found that at 50-degree angle the heat transfer rate was best at approximate 40% higher than when the rib was perpendicular to the mainstream. Jacobi and Shah [4] reported that solid vortex generators (solid turbulators) could considerably increase heat transfer, again with an increase in friction loss. Most solid turbulators can improve heat transfer on a surface due to generated longitudinal vortices or spiral flow near the surface, but, on top of the problem that the heat transfer rate cannot be controlled due to stationary fixture, an additional power is needed to deal with pres- sure drop from flow blockage of turbulators. To address this dis- advantage, boundary layer perturbation using Vortex Generating Jet (VGJ) in Jet-In-Crossflow (JICF) investigations is of interest. In JICF, jet fluid injects from an exit at an angle into mainstream. The shear layer along the upstream edge of the jet develops unstable flow which roll up into large bundle of jet fluids and traverse around the front edge of the jet. Helmholtz instability near the jet penetration region leads to formation of shear layer vortices that initiate a Counter https://doi.org/10.1016/j.applthermaleng.2018.11.035 Received 10 July 2018; Received in revised form 30 October 2018; Accepted 10 November 2018 ⁎ Corresponding author. E-mail address: chayut.n@psu.ac.th (C. Nuntadusit). Applied Thermal Engineering 148 (2019) 196–207 Available online 13 November 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved. T