Understanding Boiling Water Heat Transfer in Metallurgical Operations

Understanding Boiling Water Heat Transfer in Metallurgical Operations Dr. Mary A. Wells Associate Professor Department of Mechanical and Mechatronics Engineering University of Waterloo

Microstructural Engineering Mathematical Models Metallurgical Process Characterize using principles of: Heat transfer Fluid flow Deformation Material Response & Properties Characterize using principles of: Physical metallurgy

Quantification of Heat Transfer BC s Role of water quenching in metallurgical processes: Continuous casting of steel Direct-chill casting of nonferrous metals Runout table spray cooling of steel products Quantification of the surface heat flux: Inverse Heat Conduction (IHC) analysis based on the measured thermal history Non-linear relationship with surface temperature

The Boiling Curve NB TB FB T crit T L (T s )

Direct Chill (DC) Casting Water jet impingement point (IP) Water film free falling zone (FFZ)

DC Casting Moulds Hot Top mould refractory plate Low Head Composite (LHC) mould water chamber & exit holes graphite casting surface

Boiling curves Good agreement for the nucleate boiling regime (below 200 C) Significant scatter in the transition and film boiling regimes (above 200 C)

Importance of Water in DC Casting? Variation in intensity of the water spray heat transfer coefficient causes changes in process behaviour leading to scrap or poor quality butt cracks in rolling block start-up cracks in billet staining mould deposits and blockages

Hot tear in a magnesium billet Hot Tear

Factors Which Affect the Boiling Curve Water properties: Quality Flow rate/jet velocity Temperature Material properties: Surface morphology Thermophysical properties Nozzle conditions: Stand-off distance Nozzle geometry/configuration (angle) Nozzle size

Water Flow Rate Heat flux (W/m 2 ) Heat flux (W/m 2 ) 4.0E+06 3.5E+06 3.0E+06 2.5E+06 2.0E+06 1.5E+06 1.0E+06 5.0E+05 4.0 g/min 5.0 g/min 6.0 g/min 6.7 g/min 7.3 g/min 0.0E+00 0 50 100 150 200 250 300 350 400 450 500 Surface temperature ( C) 4.0E+06 3.5E+06 3.0E+06 2.5E+06 2.0E+06 1.5E+06 1.0E+06 5.0E+05 Impingement point Free-falling zone 4.0 g/min 5.0 g/min 6.0 g/min 6.7 g/min 7.3 g/min 0.0E+00 0 50 100 150 200 250 300 350 400 450 500 Surface temperature ( C) (143-261 l/min-m or 0.003-0.004 m 3 /s-m) As water flow rate increases the boiling curve increases up to a maximum. (Olden 1996, Matsuda 1996, Li 1999). Lower water flow rate promotes film boiling (Grandfield 1997, Li 1999).

Preferred operating conditions Boiling water heat flux ( MW/m 2 ) Preferred operating conditions steady-state 6 5 4 3 2 1 0 Preferred operating conditions start-up Preferred operating conditions Steel Aluminum 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Temperature ( C)

Hot Rolling Mill Schematic 1. 2. 3. 4. 5. 6. 7. 8. Reheat Furnace Descaler Roughing Stands Crop Shear Descaler Finishing Stands Runout Table Down Coiler Source: http://www.ussteel.com/corp/sheet/hr/pmhsmill.htm

Results Effect of speed (early pass) Temperature Heat flux 0.3 m/s Plate Movement Plate Movement 1.0 m/s Plate Movement Plate Movement Video image

Results Effect of speed (later pass) Temperature Heat flux 0.3 m/s Plate Movement Plate Movement 1.0 m/s Plate Movement Plate Movement Video image

Effect of speed

Determination of Heat Transfer BC s Thermocouple Data Thermocouple Position Thermophysical Properties FEM Conduction Model Inverse Algorithm Boiling Water Curves Experimental Model

Thermocouple Theory Two wires of different materials Electrical potential difference (voltage) generated at the welded junction Insulating material (e.g. mullite) with a relatively low thermal conductivity

Heat flow around TC

Thermocouple Theory

Perturbation in sample temp. due to T/C Quench surface -5 1.5 mm -4-3 -2-1 T/C insulation C 5.0 mm 1 2 3 4 5 12.0m m Cooling time (s) casting direction Distance from the center of thermocouple (mm) -5-4 -3-2 -1 1 2 3 4 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Temperature (ºC) 680+ 644 to 680 607 to 644 571 to 607 535 to 571 498 to 535 462 to 498 425 to 462 389 to 425 353 to 389 316 to 353 280 to 316 0.8 0.9 1 0.8mm

FE Model of TC Holes at 45 and 0

Effect of TC hole orientation Temperature ( C) 700 600 500 400 300 200 100 0 0 0.2 0.4 0.6 0.8 1 Cooling time (s) Thermal history NoT/C hole 0 45 90 Heat flux (W/m 2 ) 1.2E+07 1.0E+07 8.0E+06 6.0E+06 4.0E+06 casting direction 2.0E+06 tensile specimen Predicted boiling curve No T/C hole 0 45 90 0.0E+00 0 200 400 600 800 Temperature ( C)

Criterion to determine when to include T/C hole in the IHC analysis 3.0E+04 Heat transfer coefficient (W/m 2 C) 2.5E+04 2.0E+04 1.5E+04 1.0E+04 5.0E+03 0.0E+00 Bi = hl/k Steel casting direction Bi=0.1, L=1mm tensile specimen Bi=0.1, L=2mm Aluminum 0 20 40 60 80 100 120 140 160 180 200 220 240 Thermal conductivity (W/m C)

Experimental setup

Experimental Results - Steel Temperature ( C) 800 700 600 500 400 300 200 90 45 0 5.E+06 4.E+06 Predicted boiling curve tensile specimen 90 90 (T/C hole included in IHC analysis) 45 0 100 0 0 5 10 15 20 25 Cooling time (s) Thermal history Heat flux (W/m 2 ) 3.E+06 2.E+06 1.E+06 0.E+00 0 100 200 300 400 500 600 700 800 Surface temperature ( C)

Experimental results - aluminum 500 Temperature ( C) 400 300 200 100 90 45 1.8E+06 1.6E+06 1.4E+06 Predicted boiling curve tensile specimen 90 45 90 (T/C hole included in IHC analysis) 0 0 5 10 15 Cooling time (s) Thermal history Heat flux (W/m 2 ) 1.2E+06 1.0E+06 8.0E+05 6.0E+05 casting direction 4.0E+05 2.0E+05 0.0E+00 0 100 200 300 400 500 600 Surface temperature ( C)

Comparison to criterion Heat transfer coefficient (W/m 2 C) Maximum HTC in experiments (steel) = 10,000 W/m 2 C Maximum HTC in experiments (aluminum) = 7,000 W/m 2 C 2.5E+04 2.0E+04 1.5E+04 1.0E+04 5.0E+03 0.0E+00 Aluminum at L=0.8 mm, h=20,000 W/m 2 C Steel at L=0.8 mm, h=2,500 W/m 2 C Steel casting direction Aluminum 0 1 2 3 4 5 Radius of thermocouple hole (mm) tensile specimen

Equivalent Depth Technique 2.5 2.0 TD ED = TD TR 1.1 Equivalent depth (mm) 1.5 1.0 Null point 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Thermocouple radius (mm)

Use of Equivalent Depth Technique Surface heat flux (W/m 2 ) 4.0.E+06 3.5.E+06 3.0.E+06 2.5.E+06 2.0.E+06 1.5.E+06 1.0.E+06 90 with TD 0 with TD 90 with ED 5.0.E+05 0.0.E+00 0 100 200 300 400 500 600 700 800 Surface temperature ( C)

Summary Process modeling important activity in manufacturing Important to measure real process and compare to model predictions Often problems much more complicated than imagined - leads to new ideas for fundamental research Boiling water heat transfer extremely complicated especially in the transition and film boiling regimes