Recent Advances in Heat Transfer Applications Using Sweeping Jet Fluidic Oscillators

Journal of Thermal Science and Engineering Applications

2023

An ongoing research effort has been conducted for years to investigate the feasibility of mixing the air with mist (micron-level water droplets) to enhance heat transfer. It becomes very interesting to investigate how these tiny droplets behave in conjunction with the periodic sweeping jets. How would the interaction between the droplets and the sweeping jets affect the film cooling effectiveness? To answer this question, Part II of this paper specifically focuses on investigating the droplet dynamics and thermo-flow behavior of droplet evaporation on film cooling effectiveness in a sweeping jet. The multiphase CFD model employs an Eulerian-Lagrangian approach. The Eulerian method is used for the continuous phase including air and water vapor and the Lagrangian method in terms of the discrete phase model (DPM) is used to simulate the dispersed phase in a continuous phase. A mist ratio of 10% with a droplet size of 10 microns were used in this study. The results show that, for a blowing ratio of 1, using mist provides better film cooling performance with an average enhancement of 50-90% in comparison to that without mist. Using a mist ratio of 10% could save approximately 50% of the cooling air. It is observed that the liquid droplets mostly follow the main sweeping flow and its vortical structure and horseshoe vortices, but with a phase lag between the droplets and the main sweeping jet. This phase lag further enhances both temporal and spatial film cooling surface protection during the sweeping motion.

Computational Analysis of Air/Mist Film Cooling Using A Sweeping Jet Fluidic Oscillator–Part II: Sweeping Mist Jets

Journal of Thermal Science and Engineering Applications

2023

Computational Analysis of Air/Mist Film Cooling Using a Sweeping Jet Fluidic Oscillator–Part I: Sweeping Air-Only Jet With Detailed Analysis of Vortex Dynamics

Journal of Thermal Science and Engineering Applications

2023

This two-part paper presents a numerical study to investigate the application of using air/mist sweeping jets in film cooling for protecting turbine airfoils. Part I focuses on validating the computational mode by comparing the thermal-flow and heat transfer behavior between steady and sweeping air-only jets to ensure they are consistent with published information. Part II focuses on the mist behavior and its effect on heat transfer enhancement in the sweeping jet film cooling by adding micro-liquid droplets. An unsteady Reynolds Averaged Navier-Stokes (URANS) simulation accompanied by the k-ω SST turbulence model was used in this study. A comparison is made between steady and sweeping jets at two blowing ratios (BR=1 and 2). The results show that the steady jet provided better film cooling performance along the centerline compared to that of the sweeping jet for both blowing ratios. However, the sweeping jet provided better and more uniform film cooling performance in the spanwise direction. The sweeping jet caused an increase of 9.5% in total pressure losses compared to the steady jet. A detailed analysis is presented to understand the instantaneous vortex dynamics of the sweeping jet that leads to the inward rotating CRVP. The result shows that the pair of counter-rotating vortices is just a time-averaged image of a single vortex sweeping back and forth in the domain; it does not actually exist in real time.

Investigation of Thermal-Flow Behavior and Droplet Dynamics of Mist Sweeping Impinging Jet Cooling

ASME Journal of Heat and Mass Transfer

2023

This paper presents a 2-D numerical study to investigate the fluid flow behavior and cooling characteristics caused by injecting tiny droplets into the sweeping air jet through a fluidic oscillator. An unsteady Reynolds-averaged Navier-Stokes simulation accompanied with the k-ω SST turbulence model is used in this study. The movement and evaporation of the mist are simulated by using the discrete phase model. The study has been conducted for a target wall with a constant heat flux of 3,000 W/m2, jet-to-wall distance of 4, ReD=2,500, and a mist /air mass ratio of 5% with a droplet size of 5 microns. A comparison between the cooling performance of steady and sweeping jets is presented for two impingement schemes. The approach of using a slip upper wall boundary condition as an alternative of the unconfined impingement scheme is investigated as well. The results show that adding mist provided a temperature reduction of 5-10% on the target wall in all cases when compared to the air cases. The liquid droplet coalescence phenomenon prevails in the sweeping jet case. For the confined impingement, both mist jets provided the similar average temperature reduction. However, the steady mist jet introduced a 58% more target wall shear compared to the sweeping mist jet. For the unconfined impingement, the steady mist jet achieved a better average cooling performance compared to that of the sweeping mist jet. However, the steady mist jet introduced a 72% more target wall shear.