Date of Award
Doctor of Philosophy
School of Engineering
Associate Professor Yasir Al-Abdeli
Associate Professor Mehdi Khiadani
Numerous industrial applications rely on impinging jets to impart convective heat and mass transfer in processes ranging from the cooling of electronic devices and gas turbine blades to drying of paper and food products. Conventionally, non-swirling impinging jets have been employed, but some studies have shown that inducing swirl allows better control of uniformity and improved convective fluxes. A better understanding of the underlying physical mechanisms that lead to such behaviour warrants deeper insights into the flow and heat transfer characteristics of impinging jets, both swirling and non-swirling. Whilst important to achieve, the flow field of an impinging jet is already quite complex even before the addition of swirl which, in free (not impinging) jets, induces vortex breakdown and other instability modes. The addition of swirl to impinging jets thus has the potential to affect the transient and steady-state convective behaviour, both of which are crucial in industrial applications.
This study features experimental and numerical investigations of incompressible turbulent impinging air jets that utilize aerodynamically generated swirl. The research focuses on the velocity field, upstream near the nozzle exit plane as well as further downstream, and the way in which it affects heat transfer at the impingement plane, both under transient and steady-state conditions. Boundary conditions at the nozzle exit were measured using Constant Temperature Anemometry. The surface temperature distribution of a thin foil heater, which forms the impingement surface cooled by the ambient temperature jet, was measured using infrared thermography for a range of Reynolds numbers (Re=11,600-35,000), swirl numbers (S=01.05), and impingement distances (H/D=2-6). The effects of different inflow conditions for non-swirling and weakly swirling impinging jets were also simulated (numerically) using ANSYS Fluent (version 16.2). Particle Image Velocity was utilized to resolve the flow field, over low (S=0.30) and higher (S=0.74) swirl over a range of Reynolds numbers (Re=11,60035,000) and nozzle-to-plate distance (H/D=2 and 4).
Whilst the use of non-intrusive infrared thermography has been widely reported in studies of the steady-state heat transfer behaviour of impinging jets, an image processing methodology to resolve the time-dependant (transient) convective heat transfer behaviour was lacking. In this context, a MATLAB based method was developed to quantify the role of various impinging jet parameters on the time to reach steady-state. The effect of spatial discretization, image resolution, and the threshold value of time-dependent Nusselt number, on the time to reach steady-state, was also analysed.
The role of various operating (Re, S) and geometric conditions (H/D) on the temporal evolution of turbulent impinging jets was also resolved. By applying the innovative image processing methodology developed, results show that for non-swirling jets, transient heat transfer characteristics at some conditions (H/D=4) are distinct if compared to others (H/D=2 and 6) and that the heat transfer distribution over the impingement plate changes significantly over a small interval of time. For swirling jets, the peak Nusselt number shifts to the wall jet region as the intensity of the swirl increases. Two correlations (no-to-low swirl, moderate-to-high swirl) are proposed to predict the time needed to reach a steady-state for Re=35,000.
Computational Fluid Dynamics was then used to resolve the role of various (upstream) nozzle exist conditions (velocity profiles) on the emerging heat transfer characteristics at the impingement plane. Results showed that under some conditions (S=0.31, uniform velocity profile) a small recirculation zone, stabilised on the impingement plane, affects the heat transfer compared to other tested velocity profiles. This study also gave valuable insights on the impact of using (simple) geometric inserts to generate for swirl into impinging jets, a method widely used for its simplicity. Results showed that this can fundamentally perturb the results unlike the use of aerodynamic swirl which relies on tangential air ports.
For the experimentally measured flow field, vortex breakdown is observed for two of conditions (Re=11,600 and 24,600 at S=0.74) out of the six tested. Impingement affects the position, shape, and strength of the vortex breakdown. For Re=24,600, impingement significantly affects (shape and position) the recirculation bubble when compared to impingement at Re=11,600. Heat transfer characteristics at high swirl are compared with low swirling impinging jets. The vortex breakdown (at high swirl) affects the impingment heat transfer and showed comparatively uniform heat transfer distribution in contrast to low swirling impinging jets. Vortex breakdown significantly deteriorates stagnation zone heat transfer and the Nusselt number peak occurs in the wall jet region.
Benefits derived from this study include identifying impingement conditions that allow quicker stabilisation of heat transfer (shorter transients) as well as an improved understanding for the role of impingement on the upstream and downstream velocity field and heat transfer characteristics.
Ikhlaq, M. (2021). Flow and heat transfer characteristics of turbulent swirling impinging jets [thesis]. https://ro.ecu.edu.au/theses/2389