Turbulent development in a free-surface jet and impingement boiling heat transfer

David Herman Wolf, Purdue University

Abstract

Convective cooling through liquid jet impingement is the preferred method for achieving high rates of heat transfer in many practical applications. This study focuses on some of the fundamental issues that most influence impingement heat transfer to a free-surface jet, both in the absence and presense of phase change. For single-phase convection, primary emphasis is on the relationship between jet turbulence and local impingement heat transfer for a free-surface, planar jet of water. Employing a thermal anemometer system, measurements of the mean velocity and turbulence intensity are reported at different streamwise and spanwise locations throughout the jet. The flow conditions at the nozzle discharge were controlled by using different nozzle designs (parallel-plate and converging) and flow manipulators (wire and and screens). Flows involving turbulence manipulators (wires and screens), although exhibiting very high initial turbulence levels (as high as 12%), experience large rates of turbulent decay such that, within about 5 to 10 nozzle widths, the turbulence intensity declined to approximately 2%. Despite streamwise turbulence intensities within the jet that differed by as much as 345%, the influence on the stagnation line convection coefficient is limited to enhancements of less than 44%. Correlation of the results yielded approximately a one-quarter power dependence of the stagnation line convection coefficient on turbulence intensity. For two-phase convection, local boiling curves are presented at several streamwise distances from the stagnation line and are complemented by distributions of the surface temperature and convection coefficient along the surface at several heat fluxes. The position downstream of the stagnation line influenced heat transfer predominantly in the single-phase convection regime and had no appreciable effect on fully-developed nucleate boiling. Surface location was shown to influence the extent of the partial boiling regime. The effect of jet velocity on heat transfer was most pronounced in the single-phase and partial boiling regimes, where convective transport is dominated by the hydrodynamics of the bulk flow and not bubble motion. Within the fully-developed boiling regime, heat transfer was insensitive to jet velocity because the convective transport was dominated by intense mixing induced by bubbles leaving the surface.

Degree

Ph.D.

Advisors

Viskanta, Purdue University.

Subject Area

Mechanical engineering

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