Transient and steady-state measurement of boiling heat transfer from impinging jets
Abstract
Free-surface liquid jets are frequently used in primary metals processing operations to control cooling rates, thereby influencing the metallurgical and mechanical properties of finished products. The hydrodynamics and heat transfer of jet impingement boiling from high temperature surfaces (exceeding the temperature corresponding to maximum heat flux) are still poorly understood and modeled. In an effort to address these deficiencies, this study examined boiling heat transfer from free-surface axisymmetric and planar water jets using both transient and steady-state experimental techniques. Transient, or quenching, experiments were executed to quantify heat transfer across a range of surface temperatures, radial locations, and jet velocities for a single, circular free surface jet of water. Single phase convection Nusselt numbers and nucleate boiling data were collected at the stagnation point, and maximum heat flux and rewetting data were measured for radial locations up to ten jet diameters. At the boiling front, the jet was observed to deflect from the surface due to the momentum of vapor generated at the boiling front, and rewetting was observed to be a strong function of this separation. Using the transient experimental apparatus, measurements were also made of a two-phase jet generated by injecting air bubbles into the water jet upstream of the nozzle exit. The influence of air injection on heat transfer was observed to depend strongly on the boiling regime with enhancements observed in single-phase convection and rewetting and reductions measured downstream of the boiling front. To circumvent difficulties associated with transient experimental techniques, a steady-state apparatus was developed that employed three single-input, single-output PID control to specify electrical input to cartridge heaters embedded within a copper block and maintain a desired surface temperature distribution beneath a planar water jet. Measurements were made across a range of nozzle widths, jet velocities, and liquid subcooling for streamwise locations up to 200 mm. Deflection of the jet at the boiling front was observed to depend strongly on the wall jet momentum, and deflection was categorized into four regimes. Heat transfer in each of these regimes was quantified and presented in the form of temperature and heat flux distributions. To quantify jet deflection and its dependence on flow quantities, an integral momentum analysis was developed that balanced the streamwise components of the vapor and wall jet momentum at the boiling front to determine the jet deflection angle for both axisymmetric and planar jets. Results agreed well with experimental observations of jet deflection distances from the planar jet study across a range of jet velocity, boiling front location, subcooling, and nozzle width.
Degree
Ph.D.
Advisors
Viskanta, Purdue University.
Subject Area
Mechanical engineering
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