A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface
Impinging jets are used in many industrial applications because they produce high transfer coefficients with relatively low pressure drops. Applications include drying of papers and films; tempering of glass; and cooling of turbine blades, metal, and electronic components. A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re = 250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W = 5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the subcritical steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. The flow at a supercritical Reynolds number of 750 was found to be unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency of about 13 Hz that was found to correspond to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation (or buckling) of the jet, leading to a lateral flapping jet instability. Augmentation in the target wall heat transfer was readily attributable to the enhanced transport in the unsteady coherent secondary flow.
- Research Organization:
- Univ. of Arizona, Tucson, AZ (US)
- OSTI ID:
- 20002520
- Report Number(s):
- CONF-990805--
- Country of Publication:
- United States
- Language:
- English
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