Pool and Convective Boiling Heat Transfer Control/Design Laboratory Department of Mechanical Engineering Yonsei University

Introduction h (W/m 2 K) 5~25 Natural convection Forced convection 10~15,000 Pool boiling(liquid) 2,500~35,000 h (W/m 2 K) 5,000~100,000 Impinging jet boiling Heat transfer enhancement Impinging jet Convective boiling (liquid)

Introduction – Applications Slab/Billet CastingHot rolling of Steel …X-ray medical equipment, laser weapons and textile dryers Medical Instruments Gas Turbines

Hydrodynamics of jet impingement Circular, submerged jet Wall jet region x or r V d Nozzle Stagnation region H v=V Potential core length y Impingement surface (target surface) Boundary layer Free jet region Liquid

Heat transfer regimes Wall Superheat, log ΔT sat Wall Heat Flux, log q Single- phase convection Nucleate boiling Transition boiling Film boiling Critical heat flux Minimum heat flux Fully developed nucleate boiling Partial boiling Boiling incipience (G) (C) (B) (A’)(A) (E) (D) (F)

Pool boiling(1) Microporous Coating – pool boiling enhancement barecoated bare coated Nucleate boilingNear CHF S. M. You (U. of Texas, Arlington)

Pool boiling(2) Microporous Coating – pool boiling enhancement S. M. You (U. of Texas, Arlington) SEM Image of Surface Micro-Structure of DOM Coating (side view )

Experimental Setup Main Reservoir Line drain & Vacuum port Secondary Reservoir Test Section Relief valve Filter or Filter/dryer Data Acquisition system Flow meters Constant temp. bath Power supply & controller Cooling water Immersion Heater Flow control valve Heat exchanger Drain Vacuum port Magnetic pump Pressure gage Thermo- couples Pressure transducers Temp. control relays Filling line TC1 TC3 TC2 Filling line

Test Section (Thin-Plate Heater Module) Heated Surface - Inconel alloy mm thick Heated Surface - Inconel alloy mm thick

Hydraulic Characteristics Unconfined single circular jets for H/d=9 V=1.7 m/s V=5.0 m/s V=10.0 m/s V=15.6 m/s Unconfined array circular jets for H/d=9 V=1.8 m/s V=2.5 m/s V=3.3 m/s

Confined Free-Surface Planar jet Convection coefficient distributions at H/w=4, V=1.7 m/s and z/w=0.0. Convection coefficient distributions at H/w=4, V=1.7 m/s and z/w=0.0. Velocity effects on normalized single-phase Convection coefficient distributions at H/w=4, and z/w=0.0. Velocity effects on normalized single-phase Convection coefficient distributions at H/w=4, and z/w=0.0.

Free-Surface Planar jets: Confined vs. Unconfined Confinement effects on temperature distributions at H/w=4 and V=1.7 m/s Confinement effects on temperature distributions at H/w=4 and V=1.7 m/s Confinement effects on convection coefficient distributions at H/w=4 and V=1.7 m/s. Confinement effects on convection coefficient distributions at H/w=4 and V=1.7 m/s.

Confined Free-Surface Planar jets: Boiling Curves h vs.  T f h vs.q Convection coefficient increase at H/w=4, V=1.7 m/s and z/w=0.0. q vs.  T sat (=T w -T sat ) q vs.  T f (=T w -T f ) Boiling curves at H/w=4, V=1.7 m/s and z/w=0.0.

Confined Free-Surface Planar jets: Subcooling effects Effects of subcooling on boiling curves at H/w=4, V=1.7 m/s and z/w=0.0. Effects of subcooling on boiling curves at H/w=4, V=1.7 m/s and z/w=0.0. Effects of subcooling on boiling curves at H/w=4, V=1.7 m/s and z/w=0.0. Effects of subcooling on boiling curves at H/w=4, V=1.7 m/s and z/w=0.0. q vs.  T f (=T w -T f ) q vs.  T sat (=T w -T sat )

Nozzle Geometry Effects on Confined Jets Identical Flowrate (Q=3.0 l/m) Non-normalized Normalized (single-phase convection) Heat Transfer Control/Design Lab. Yonsei University Nozzle geometry effects on convection coefficient distributions at H/d or H/w=1 and z=0.0. V=16.2, 5.3, and 3.3 m/s for single-circ., array-circ., and planar jet, respectively. Nozzle geometry effects on convection coefficient distributions at H/d or H/w=1 and z=0.0. V=16.2, 5.3, and 3.3 m/s for single-circ., array-circ., and planar jet, respectively.

Summary of Jet impingement Boiling Under a thin supercritical wall jet condition, the boundary layer transition to turbulence precipitated by the bubble-induced disturbance caused considerable decrease in wall temperature of further downstream only in the highest velocity case in this study. The confinement letting the free surface exist had only slightest effect. The highly-confined jet which allowed no free-surface produced a sooner and salient transition to turbulence, increasing overall heat transfer. With the developed boiling, the heat transfer characteristics became similar for all the tested cases. Circular jets provided remarkable heat transfer enhancement with a high confinement in either single or array form. Heat Transfer Control/Design Lab. Yonsei University