07 convestav heat transfer during air-mist spray...

CCC ANNUAL REPORTUIUC, August 18, 2011

Heat Transfer During Air Mist Spray Cooling

*Alberto HernandezHumberto Castillejos

Air-Mist Spray Cooling

Humberto CastillejosAndres AcostaXiaoxu Zhou

* Brian G ThomasBrian G. Thomas

* University of Illinois at Urbana ChampaignUniversity of Illinois at Urbana-ChampaignCINVESTAV, Unidad Saltillo, Mexico

Project Objectives

Q tif h t t f t d i i i t• Quantify heat transfer rate during air-mist spray cooling:

• measure: size, velocity, flow rate, and impact densitymeasure: size, velocity, flow rate, and impact density distributions of water droplets from commercial nozzles spraying air-water mists

• interpret steady-state heat-transfer measurements with• interpret steady-state heat-transfer measurements with computational models

• develop empirical heat flux relation based on f d t l t d l t tfundamental water droplet parameters

_____________________________________________________________________________________________________________________________________________University of Illinois at Urbana-Champaign • Metals Processing Simulation Lab • BG Thomas 2

Steady Heat Transfer Measurement

• Induction heating is used to heat up platinum sample

• Power controller is used to maintain• Power controller is used to maintain sample temperature at a specific value


Experiment Details

Side Vie TC Wire

Cylinder Plastic CoverSpray Nozzle

Side View

Ceramic BodyX

Top ViewY

Z Platinum Sample

Top ViewBox and SampleY

C

Copper Coil

Quartz Glass

TC Wires

• Plastic cover and quartz glass are used to keep spray water from ceramic body, only exposing front surface of platinum


sample to the water

Typical Ts and Itot Measurements Water Flow Rate=4.6lpm, Air Flow Rate=104lpm,Nozzle Centered (June 26, 09)

Total current measurement approaches steady state at the end of the 8 min for each stage

25 s20 s

Sample temperature Ts=300 C


Heat Source Distribution and Temperature Distributionand Temperature Distribution

Temperature distributionHeat Source distribution

670680690

, oC D

620630640650660670

Tem

pera

ture

C

Heat Generated, W Heat Out, W

Sample 264.49Spray 229.14

Coil 206.22

Window 14 96


6200 0.5 1 1.5 2 2.5 3 3.5 4

T

Platinum Sample Radius, mm

Coil 186.19Window 14.96

Natural convection 0.44

Total 450.68 Total 450.76

Nozzle CenterSpray Heat Transfer Coefficientsp y

Y=0mm(Nozzle Centered)

24

26

K

WaterFlowRate=4.6lpm, Heating

WaterFlowRate=4.6lpm, Cooling

16

18

20

22

24

cien

t, kW

/m^2

K WaterFlowRate 4.6lpm, CoolingWaterFlowRate=3.5lpm, Heating


WaterFlowRate=2.5lpm, Heating


10

12

14

16

Tran

sfer

Coe

ffi

WaterFlowRate 2.5lpm, Cooling

2

4

6

8

Spra

y H

eat T

•Increasing water flow rate increases heat transfer coefficient.During heating HTC peaks around 150 200 C then decreases as sample surface increases

00 100 200 300 400 500 600 700 800 900 1000 1100 1200

Sample Surface Temperature, C


•During heating, HTC peaks around 150~200 C, then decreases as sample surface increases•During cooling, HTC keeps increasing gradually.•Hysteresis is shown in heat transfer coefficient curves. HTC is independent of temp at temp >850 C

Nozzle Centered - Spray Heat Flux

( )

4.5

5.0Nucleate boiling

Transition boiling

Stable Film boiling

3.5

4.0

W/m

^2

g

2.0

2.5

3.0

y H

eat F

lux,

MW

WaterFlowRate=4.6lpm,HeatingWaterFlowRate=4.6lpm,CoolingWaterFlowRate=3.5lpm,HeatingWaterFlowRate=3.5lpm,CoolingWaterFlowRate=2.5lpm,HeatingWaterFlowRate=2.5lpm,CoolingTransient 2 02lpm

0 5

1.0

1.5Spra

y Transient_2.02lpmTransient_1.761lpmTransient_3.321lpmTransient_3.34lpm

•Increasing water flow rate increases spray heat flux

0.0

0.5

0 100 200 300 400 500 600 700 800 900 1000 1100 1200Sample SurfaceTemperature, C


•Increasing water flow rate increases spray heat flux.•Heat transfer almost independent of temperature above ~850 oC•Hysteresis is observed below Leidenfrost temperature ~850 oC•Steady measurement gives higher heat flux than transient measurement

--Transient results by Sami Vapalahti, etc, Spray Heat Transfer Research at CINVESTAC,P26, CCC report, 2007

Mechanism of Hysteresis

sample

Steam LayersampleWater Layer

Spray DropletWater Layer

•Heating process: Fig. 1 Fig. 2

•spray droplet impinges on water layer, touches surface, boils, and takes heat away. (Fig. 1)•High heat removal keeps surface cold.

•Cooling process:•At high sample surface temperature (>~860oC), a stable steam layer forms on the sample surface. (Fig. 2)•This steam layer acts as a barrier to heat transfer and decreases heat removal


•This steam layer acts as a barrier to heat transfer and decreases heat removal.•Low rate of heat removal sustains the air gap to low temperatures before droplets finally can penetrate through

•Result: difference in heat transfer at intermediate temperatures according to history (heating or cooling)

Nozzle Water Flow Rate=4.6lpm--Spray Heat Transfer Coefficientsp y

2426

W/m

2 K

WaterFlowRate=4.6lpm

Y=0mm,HeatingY=0mm Cooling

25

30000

35000100 C300 C500 C700 C

1416182022

Coe

ffici

ent,

kW

Y=0mm,CoolingY=9mm,HeatingY=9mm,CoolingY=18mm,HeatingY=18mm,Cooling

15

20

20000

25000

30000, l/m2 s

700 C900 C1100 CWater Flux Rate

468

101214

Hea

t Tra

nsfe

r C

10

10000

15000

20000

r Flux Rate

HTC,W/m2 K

024

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Spr

ay

Sample Surface Temperature, C0

5

0

5000

10000

Wate

•Hysteresis exists for different location from spray centerline.•Moving further away from spray centerline decreases HTC.

00

0 3 6 9 12 15 18Y Direction,mm


•Difficult to correlate water flow rate footprint measurements with HTC. •More details of spray dynamics needed (droplet distribution, size, velocity, etc, --collaboration work at CINVESTAV, Mexico)

Heat Transfer in Film-Boiling Regime

Stable

No hysteresis

Almost independent of surface temperature


Flow-rate and Pressure of air and water are related in a nozzle

Operating Diagram(Delavan W19822 nozzle)

Water Flow Rate (L/s)


(Delavan W19822 nozzle)

Droplet Parameter Measurements

For a given nozzle, water flow rate and air pressureflow rate, and air pressure,Measure distributions of droplet:droplet:• Size• VelocityVelocity• Weber number


Measure water flux and impact density mapsp y p

For a given: - nozzle, - water flow rate, and- air pressure


Title


New Correlations

Heat Flux (W/m2)0.319 0.317 0.144 0.036

300.307 z wq w u T d−− =

0 318 0 330 0 895 0 024

Heat Transfer Coefficient (W/m2K)0.318 0.330 0.895 0.024

30379.93 z wh w u T d− −=

2w = local water flux density (L/m2s)uz = volume-weighted mean droplet velocity (m/s)Tw = surface temperature (K)


w p ( )d30 = volume-mean droplet diameter (mm)

Agreement of fit: 96% of h measurements fall within +/-25% of predictionp


Conclusions

• Spray heat transfer coefficient and heat flux show hysteresis likely related to formation of vapor layer on sample surface.

• Heat transfer in stable film boiling regime above ~650 oC experiences no hysteresis, (above Leidenfrost temperature)

• New correlation to predict air-water spray mist heat transfer has been developed based on fundamental droplet parameters: flux density, size, velocity, & surface temperatureparameters: flux density, size, velocity, & surface temperature

• New correlation fits measurements for 3 different nozzle types and many different water-flow and air-pressure

diticonditions

• New correlation shows heat transfer depends mainly on water flux density with velocity also important but is


water flux density, with velocity also important, but is almost independent of surface temperature & droplet size.

References (for further details)

• C.A. Hernández, J.I. Minchaca, A.H. Castillejos, F.A. Acosta, X. Zhou, and B.G. Thomas, “Measurement of Heat Flux in Dense Air-Mist Cooling: Part gI. A Novel Steady-State Technique”, CCC Report 201101, Aug. 18, 2011.

• C.A. Hernández, J.I. Minchaca, A.H. Castillejos, F.A. Acosta, X. Zhou, and B.G. Thomas, “Measurement of Heat Flux in Dense Air-Mist Cooling: Part II. The Influence of Mist Characteristics on Heat Transfer”, CCC Report 201102, Aug. 18, 2011.

• X. Zhou, Heat Transfer During Spray Water Cooling Using Steady Experiments, MS Thesis, University of Illinois, 2009.


Acknowledgementsg• Continuous Casting Consortium Members

(ABB A l Mitt l B t l T t St l(ABB, Arcelor-Mittal, Baosteel, Tata Steel, Magnesita Refractories, Nucor Steel, Nippon Steel, Postech Posco SSAB ANSYS-Fluent)Postech, Posco, SSAB, ANSYS Fluent)

• National Science FoundationNational Science Foundation– GOALI DMI 05-00453


07 convestav heat transfer during air-mist spray...

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