Possible Solutions

Taking into account the reasons that cause the thermal gradient studied in the previous section various solutions could be considered:

  1. Getting as much horizontal symmetry as possible in the component layout within the instrument D4 by means of
    • the removal of the support and keeping the sample “levitating” within the instrument.
    • the coupling of a stem in the upper region of the sample with similar geometrical and physical characteristics to the support ones.
  2. Reducing the heat absorbed by the top of the sample, by limiting or deflecting the heat delivered by the cylinder in this area. For instance, by removing the upper shields.
  3. Minimizing the heat transferred between the sample and the support by means of the reduction of the thermal conductance of the support.

 

Getting horizontal symmetry within the instrument D4

repilcar o quitar soporteAlthough the option of removing the support and keeping the sample under levitation is the one that would have less heat losses and, therefore, it would be the most efficient one from an energetic point of view, it is, also, the less feasible option and, thus, it will dismissed.

The option of coupling an “inverted copy” of the support on the top of the sample, nevertheless, could be a feasible option to take into account, since more heat losses would be added to the top of the sample that would compensate the heat losses between the bottom of the sample and the support. Thus, a more symmetrical thermal distribution would be obtained, reducing, this way, the temperature gradient in the sample.

Although this option could give good results with regard to the enhancement of the temperature gradient in the sample, it has been decided not to study it since with the addition of heat losses to the sample the system would become less efficient, requiring a higher energy contribution in order to get the same mean temperature in the sample.

 


Reducing the heat absorbed by the top of the sample

sin shields superioresThe radiative heat transfer between the cylinder and the container can be increased by extracting, or reducing, the number of upper shields, reducing, thus, the temperature of the upper region of the cylinder. By doing this, a more symmetrical temperature distribution in the cylinder could be obtained and the cells from the top of the sample would be seeing points from the cylinder at lower temperatures, similar to the ones from the bottom of the sample.

Both the option of adding an “inverted copy” of the support in the top of the sample and the option of extracting the upper shields have a negative impact on the system energy efficiency.

Therefore, the horizontal thermal asymmetry of the sample would be reduced and, thus, the temperature gradient would also decrease in the sample. Nonetheless, this solution, like the previous one, is adding even more thermal losses in the sample, getting a less energy-efficient system. This solution, therefore, will be avoided in favour of the system energy efficiency.

 


Reducing the support thermal conductance

In order to reduce the magnitude of the heat transferred between the sample and the support and to smooth, in this way, the temperature profile of the sample, it is necessary to modify one of the support parameters that define the heat transfer through it. These parameters, already mentioned, are the thermal conductivity of the support components (changing the materials used in the support components) and its own geometry.

With the purpose of reducing the thermal losses between the sample and the support, the decision of designing a new alumina tube with a lower thickness has been taken. The thickness reduction causes a decrease of the effective heat transfer area  by conduction, making difficult, in this way, the heat flux across the support.

It has been considered a thickness reduction of one millimeter, going from an original thickness of 2 mm to a goal thickness of 1 mm. This thickness reduction of the alumina tube causes a decrease in the heat transfer area by conduction of 43.75%.

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