<p>Secondary cooling during continuous casting is a delicate
process because the cooling rate of water spray directly affects the slab
surface and internal quality. Undercooling may lead to slab surface bulging or
even breakout, whereas overcooling can cause deformation and crack of slabs due
to excessive thermal residual stresses and strains. Any slab which does not
meet the required quality will be downgraded or scrapped and remelted. In order to remain competitive and continuously
produce high-quality and high-strength steel at the maximum production rate,
the secondary cooling process must be carefully designed and controlled. Efficient
and uniform heat removal without deforming or crack the slab is a significant
challenge during secondary cooling. In the meantime, the on-site thermal
measurement techniques are limited due to the harsh environment. In contrast, experimental measurements
are only valid for the tested conditions, and the measurement process is not
only labor-intensive, but the result might be inapplicable when changes in the
process occur. On the other hand, the high-performance computing (HPC)-powered
computational fluid dynamics (CFD) approach has become a powerful tool to gain
insights into complex fluid flow and heat transfer problems. Yet, few
successful numerical models for heat transfer phenomena during secondary
cooling have been reported, primarily due to complex phenomena. </p>
<p> </p>
<p>Therefore, the current study has proposed two
three-dimensional continuum numerical models and a three-step coupling
procedure for the transport of mass, momentum, and energy during the secondary
cooling process. The first numerical model features the simulation of water
spray impingement heat and mass transfer on the surface of a moving slab considering
atomization, droplet dispersion, droplet-air interaction, droplet-droplet
interaction, droplet-wall impingement, the effect of vapor film, and droplet
boiling. The model has been validated against five benchmark experiments in
terms of droplet size prior to impingement, droplet impingement pressure, and
heat transfer coefficient (HTC) on the slab surface. The validated model has
been applied to a series of numerical simulations to investigate the effects of
spray nozzle type, spray flow rate, standoff distance, spray direction, casting
speed, nozzle-to-nozzle distance, row-to-row distance, arrangement of nozzles,
roll and roll pitch, spray angle, spray water temperature, slab surface
temperature, and spray cooling on the narrow face. Furthermore, the simulation
results have been used to generate a mathematically simple HTC correlation,
expressed as a function of nine essential operating parameters. A graphic user
interface (GUI) has been developed to facilitate the application of
correlations. The calculated two-dimensional HTC distribution is stored in the universal
comma-separated values (csv) format, and it can be directly applied as a boundary
condition to on-site off-line/on-line solidification calculation at steel mills.
The proposed numerical model and the generic methodology for HTC correlations should
benefit the steel industry by expediting the development process of HTC
correlations, achieving real-time dynamic spray cooling control, supporting
nozzle selection, troubleshooting malfunctioning nozzles, and can further
improve the accuracy of the existing casting control systems.</p>
<p> </p>
<p>In the second numerical model, the volume-averaged
Enthalpy-Porosity method has been extended to include the slurry effect at low
solid fractions through a switching function. With the HTC distribution on the
slab surface as the thermal boundary condition, the model has been used to
investigate the fluid flow, heat transfer, and solidification inside a slab
during the secondary cooling process. The model has been validated against the
analytical solution for a stationary thin solidifying body and the simulation
for a moving thin solidifying body. The effects of secondary dendrite arm
spacing, critical solid fraction, crystal constant, switching function
constant, cooling rate, rolls, nozzle-to-nozzle distance, and arrangement of
nozzles have been evaluated using the validated model. In addition, <a>the solidification model has been coupled with the
predictions from the HTC correlations, and the results have demonstrated the availability
of the correlations other than on-site continuous casting control. </a>Moreover,
the model, along with
the three-step coupling procedure, has been applied to simulate the initial
solidification process in continuous casting, where a sufficient cooling rate
is required to maintain a proper solidification rate. Otherwise, bulging or
breakout might occur. The prediction is in good agreement with the
measured shell thickness, which was obtained from a breakout incident. With the help of
HPC, such comprehensive simulations will continue to serve as a powerful tool
for troubleshooting and optimization.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15040452 |
| Date | 23 July 2021 |
| Creators | Haibo Ma (11173431) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/ON_HEAT_TRANSFER_MECHANISMS_IN_SECONDARY_COOLING_OF_CONTINUOUS_CASTING_OF_STEEL_SLAB/15040452 |
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