Heat transfer enhancement in turbulent boundary layers with a pulsed slot jet in crossflow

TL;DR

This study experimentally investigates how pulsed slot jets in crossflow can enhance heat transfer in turbulent boundary layers, analyzing effects of actuation frequency and duty cycle, and proposing a simplified model for optimization.

Contribution

It introduces a detailed experimental analysis of pulsed slot jets in crossflow for heat transfer enhancement and develops a simplified model decoupling pulsation effects.

Findings

Heat transfer increases with duty cycle and jet penetration.

Maximum Nusselt number occurs at a frequency independent of duty cycle.

Lowest duty cycle offers the most efficient heat transfer enhancement.

Abstract

The convective heat transfer enhancement in a turbulent boundary layer (TBL) employing a pulsed, slot jet in crossflow is investigated experimentally. A parametric study on actuation frequencies and duty cycles is performed. The actuator is a flush-mounted slot jet that injects fluid into a well-behaved zero-pressure-gradient TBL over a flat plate. A heated-thin-foil sensor measures the time-averaged convective heat transfer coefficient downstream of the actuator location and the flow field is characterised by means of Particle Image Velocimetry. The results show that both the jet penetration in the streamwise direction and the overall Nusselt number increase with increasing duty cycle. The frequency at which the Nusselt number is maximised is independent of the duty cycle. The flow topology is considerably altered by the jet pulsation. A wall-attached jet rises from the slot accompanied by a pair of counter-rotating vortices that promote flow entrainment and mixing. Eventually, a simplified model is proposed which decouples the effect of pulsation frequency and duty cycle in the overall heat transfer enhancement, with a good agreement with experimental data. The cost of actuation is also quantified in terms of the amount of injected fluid during the actuation, leading to conclude that the lowest duty cycle is the most efficient for heat transfer enhancement.