Directional solidification has a wide interest due to its importance to the iron and steel
industry. Examples of further application can be found in the aerospace industry
regarding the manufacture of turbine blades and the semiconductor industry regarding
single-crystal growth applications. Solute convection in the solidification process results
in channel formation, which has a freckle-like appearance in cross-section and has a
critical effect on the mechanical strength of a casting. For a solidification process that
occurs via planar solidification from a solid boundary, one may consider the presence of
three distinct regions often identified as horizontal layers, i.e. a fluid binary mixture (the
melt), the solid layer and a two-phase (fluid-solid) mushy layer, separating the other two.
The mushy layer is practically a porous medium consisting of an interconnected solid
phase having its voids filled with the melt binary fluid. Channelling in the mushy layer
and the creating of freckles are being considered the main reasons for non-homogeneous
solidification and production of defects in the resulting solid product. The production of
defects adversely affects the mechanical properties of the solid product leading to
undesirable constraints on its industrial use.
The purpose of this study is to evaluate the effect the vibrations have on the heat transfer
during the solidification process as well as on the average density of the solid product and
void formation. Experimental as well as theoretical investigations related to the
solidification process were undertaken. Two effects that have been observed in previous experimental studies when metals and
metal alloys are vibrated during solidification are a decrease in dendritic spacing, which
directly affects density, and faster cooling rates and associated solidification times.
Because these two effects happen simultaneously during solidification it is challenging to
determine the one effect independently from the other. Most previous studies were on
metals and metal alloys. In these studies, the one effect, i.e. the decrease in dendritic
spacing, might influence the other, i.e. the faster cooling rates, and vice versa. The direct
link between vibration and heat transfer has not yet been studied independently. The
purpose of this study was to experimentally investigate the effect of vibration only on
heat transfer and thus solidification rate. Experiments were conducted on paraffin wax,
because it had a clearly defined macroscopic crystal structure consisting of mostly large
straight-chain hydrocarbons. The advantage of the large straight-chain hydrocarbons was
that the dendritic spacing was not affected by the cooling rate. Experiments were done
with paraffin wax inside hollow plastic spheres of 40 mm diameter with 1 mm wall
thickness. The paraffin wax was initially in a liquid state at a uniform temperature of
60°C and then submerged into a thermal bath at a uniform constant temperature of 15°C,
which was approximately 20°C below the mean solidification temperature of the wax.
Experiments were conducted in approximately 300 samples, with and without vibration at
frequencies varying from 10 – 300 Hz. The first set of experiments were conducted to
determine the solidification times. In the second set of experiments, the mass of wax
solidified was determined at discrete time steps, with and without vibration. The results
showed that paraffin wax had vibration independent of solid density contrary to other
materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker
solidification times and possible control over the heat transfer rate. The increase in heat
transfer leading to faster solidifcation times was observed to first occur, as frequency
increased and then to decrease.
Experimental results showed that paraffin wax had vibration independent of solid density
contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer
resulted in quicker solidification times and possible control over the heat transfer rate.
The increase in heat transfer leading to faster solidifcation times was observed to first
occur, as frequency increased and then to decrease. Theoretical results of heat convection in a porous layer heated from below and subject to vibrations are presented by using a
truncated spectral method in space. The partial differential equations governing the mass,
momentum, heat, and solute transport were tranformed into a set of ordinary differential
equations via a truncated modal expansion. Then the resutling equations were solved to
identify the variety of regimes, and transitionbetween them, i.e. from steady convection,
via periodic and quasi-periodic convection, towards chaotic or weak turbulent
convection. The theoretcial results show that the heat convection subject to vibration is
generally reduced when compared with the corresponding convection without vibrations.
The exception for a certain frequency range shows about a 10% enhancement in the weak
turbulent regime of convection, however, a 10% enhancement is still lower than the heat
transfer prior to the transition to weak turbulence. Therefore, the heat transfer mechanism
can be excluded as the main reason behind the improvement in solidification when
vibrations are applied. Both experimental and theoretical results show an enhancement in
heat transfer which correlate qualitativally.