Understanding Boiling Water Heat Transfer in Metallurgical Operations

Transcription

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

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

3 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

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

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

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

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

8 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

9 Hot tear in a magnesium billet Hot Tear

10 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

11 Water Flow Rate Heat flux (W/m 2 ) Heat flux (W/m 2 ) 4.0E E E E E E E E g/min 5.0 g/min 6.0 g/min 6.7 g/min 7.3 g/min 0.0E Surface temperature ( C) 4.0E E E E E E E E+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 Surface temperature ( C) ( l/min-m or 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).

12 Preferred operating conditions Boiling water heat flux ( MW/m 2 ) Preferred operating conditions steady-state Preferred operating conditions start-up Preferred operating conditions Steel Aluminum Temperature ( C)

13 Hot Rolling Mill Schematic Reheat Furnace Descaler Roughing Stands Crop Shear Descaler Finishing Stands Runout Table Down Coiler Source:

14 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

15 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

16 Effect of speed

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

18 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

19 Heat flow around TC

20 Thermocouple Theory

21 Perturbation in sample temp. due to T/C Quench surface mm T/C insulation C 5.0 mm m m Cooling time (s) casting direction Distance from the center of thermocouple (mm) Temperature (ºC) to to to to to to to to to to to mm

22 FE Model of TC Holes at 45 and 0

23 Effect of TC hole orientation Temperature ( C) Cooling time (s) Thermal history NoT/C hole Heat flux (W/m 2 ) 1.2E E E E E+06 casting direction 2.0E+06 tensile specimen Predicted boiling curve No T/C hole E Temperature ( C)

24 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 E E E E E+00 Bi = hl/k Steel casting direction Bi=0.1, L=1mm tensile specimen Bi=0.1, L=2mm Aluminum Thermal conductivity (W/m C)

25 Experimental setup

26 Experimental Results - Steel Temperature ( C) E+06 4.E+06 Predicted boiling curve tensile specimen (T/C hole included in IHC analysis) Cooling time (s) Thermal history Heat flux (W/m 2 ) 3.E+06 2.E+06 1.E+06 0.E Surface temperature ( C)

27 Experimental results - aluminum 500 Temperature ( C) E E E+06 Predicted boiling curve tensile specimen (T/C hole included in IHC analysis) Cooling time (s) Thermal history Heat flux (W/m 2 ) 1.2E E E E+05 casting direction 4.0E E E Surface temperature ( C)

28 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 E E E E E+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 Radius of thermocouple hole (mm) tensile specimen

29 Equivalent Depth Technique TD ED = TD TR 1.1 Equivalent depth (mm) Null point Thermocouple radius (mm)

30 Use of Equivalent Depth Technique Surface heat flux (W/m 2 ) 4.0.E E E E E E E with TD 0 with TD 90 with ED 5.0.E E Surface temperature ( C)

31 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