Heat transfer and flow characteristics in a rectangular channel with combined delta winglet inserts

•Effect of combined winglet inserts on heat transfer and fluid flow of rectangular channel is studied.•The rotation angle and spacing among the combined delta winglet are varied.•Nusselt number, friction factor and synergy analysis are displayed.•Case 1 shows the highest heat transfer enhancement due to a larger swirl generated.•Good agreement is obtained between the current experiments and current CFD results. Flow and heat transfer characteristics in a rectangular channel with combined delta winglet vortex generators are studied using numerical simulations and experiments. This is undertaken for Reynolds numbers ranging between 500 and 1500 based on channel characteristics. The effects of four cases of different arrangements of delta winglets, their spacing between a delta winglet and the centre of the combined delta winglet, and rotation angles of the combined delta winglet for Case 1, on heat transfer and flow performance are numerically investigated. Experimental test of similar configurations for Case 1 highlights a good agreement with numerical results, indicating the simulations to be a good representation of the heat transfer and fluid flow. The numerical study shows that there is a general increase in the Nusselt number of the system with the inclusion of the combined winglet. The maximum Nusselt number of the channel is enhanced by 92%, however the friction factor is increased by 178% compared to that of a smooth rectangular channel. Both of the Nusselt number and friction factor first decrease and then increase with the rotation angle. The Nusselt number and friction factor are also observed to decrease with spacing between a winglet and the centre of the combined winglet as the number of the vortices is reduced leading to the disturbance of the combined winglet on fluid weakening. The best observed heat transfer performance is obtained when the spacing is equal to zero with a rotation angle of 60°. This work indicates that the longitudinal vortices play a significant role leading to the enhancement of heat transfer in consequence of the addition of winglet configurations, however optimization of the winglet configuration is needed to achieve maximum performance.