Pool and Convective BoilingPool and Convective Boiling

Heat Transfer Control/Design Laboratory

Department of Mechanical Engineering

Yonsei University

Introduction

h (W/m2K) 5~25

Naturalconvection

Forcedconvection

10~15,000

Pool boiling(liquid)

2,500~35,000

h (W/m2K) 5,000~100,000

Impinging jetboiling

Heat transferenhancement

Impinging jet

Convective boiling (liquid)Convective boiling (liquid)

Introduction – Applications

Slab/Billet Casting Hot 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

dNozzle

Stagnation region

Hv=VPotential

core length

y

Impingement surface (target surface)

Boundary layer Free jet region

Liquid

Liquid

Heat transfer regimes

Wall Superheat, log ΔTsat

Wal

l Hea

t F

lux,

log

q

Single-phase

convection

Nucleate boiling

Transition boiling

Film boiling

Critical heat flux

Minimum heat flux

Fully developed nucleate boiling

Partial boiling

Boilingincipience

(G)

(C)(B

) (A’)(A)

(E)

(D)

(F)

Pool boiling(1)

Microporous Coating – pool boiling enhancement

bare coated bare coated

Nucleate boiling Near 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

Cooling water

Cooling water

Immersion Heater

Immersion Heater

Flow control valve

Heat exchanger

Drain

Vacuum port

Magnetic pump

Pressure gage

Pressure gageThermo-

couples

Pressure transducers

Temp. control relays

Filling line

TC1

TC3 TC2

Filling line

Test Section (Thin-Plate Heater Module)

Heated Surface

- Inconel alloy 600

- 0.467 mm thick

Heated Surface

- Inconel alloy 600

- 0.467 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. Tf h vs.q

Convection coefficient increase at H/w=4, V=1.7 m/s and z/w=0.0.

q vs. Tsat (=Tw-Tsat) q vs. Tf (=Tw-Tf)

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. Tf (=Tw-Tf)q vs. Tsat (=Tw-Tsat)

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