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ABSTRACT
Annular impinging jets create a more uniform flow on the impact surface compared to circular impinging jets, allowing the surface to cool better. Additionally, periodic flow oscillations significantly increase heat transfer by reducing the thermal resistance on the surface. Therefore, this study experimentally investigated the heat transfer characteristics of a synthetic annular jet impinging on a flat surface with constant heat flux. In the experiments, the jet-target surface distance (H/D), jet Reynolds number (Rej), oscillation amplitude (Ao), and Womersley number (Wo) were changed. In contrast, the Prandtl number (Pr) and other geometric parameters were kept constant. The effects of these parameters on heat transfer were analyzed and the results were compared with continuous circular and annular impinging jets. Local temperature values on the target surface were obtained for different parameters and heat transfer from the surface was calculated. Experimental results showed that heat transfer increased with decreasing H/D ratio for all jet types. The highest heat transfer on the surface was achieved in synthetic jet flow. Heat transfer increased as the oscillation amplitude decreased. It was observed that there is a specific value for the Womersley number (Wo = 94) and that the heat transfer decreases after this value. For Re = 50000 and H/D = 2, the annular jet provided 27% higher heat transfer than the circular jet. In synthetic jet flow, heat transfer at H/D = 2 was improved by 21% compared to H/D = 8 for Re = 6000, Ao = 1.87 and Wo = 163.
Nomenclature
| CAJ | = | Contunious anular jet |
| CCJ | = | Contunious circular jet |
| SAJ | = | Syntetic annular jet |
| Ao | = | Dimensionless amplitude |
| B | = | Dimensionless jet-target surface distance |
| cp | = | Specific heat (kJ/kgoC) |
| D | = | Nozzle diameter (m) |
| h | = | Heat transfer coefficient (W/m2K) |
| H | = | Distance between jet-target surface (m) |
| k | = | Conduction coefficient (W/mK) |
| L | = | Radius of target plate (m) |
| N | = | Total data number |
| Nu | = | Nusselt number |
| Pr | = | Prandtl number |
| q” | = | Heat flux (W/m2) |
| Re | = | Reynolds number |
| T | = | Temperature (oC) |
| U | = | Average velocity (m/s) |
| Wo | = | Womersley umber |
| xm | = | Piston stroke (m) |
| Greek symbols | = | |
| α | = | Thermal dissipation coefficient (m2/s) |
| μ | = | Dynamic viscosity (Pa/s) |
| ν | = | Kinematic viscosity (m2/s) |
| ρ | = | Density (kg/m3) |
| τ | = | Cycle time |
| ω | = | Angular velocity (rad/s) |
| subscripts | = | |
| h | = | hydraulic |
| m | = | mean |
| in | = | inner |
| out | = | outer |
| ∞ | = | ambient condition |
| w | = | wall |
| j | = | jet |
Acknowledgments
The authors gratefully thank ASU-BAP (Scientific Research Project Unit of Aksaray University, ASU-BAP project no. 2023-05) for the financial support provided.
Disclosure statement
No potential conflict of interest was reported by the author(s).