Maybe you can check these two latest manuscripts and an online calculator. I hope it may help you.

1. https://www.ajdesigner.com/phppeclet/peclet_number_equation.php

2. Influence of Composition and Spray-Drying Process Parameters on Carrier-Free DPI Properties and Behaviors in the Lung: A review

3. Dispersibility and Storage Stability Optimization of High Dose Isoniazid Dry Powder Inhalation Formulations with L-Leucine or Trileucine

Pe number is a dimensional less number. Just like Re and other numbers in engineering. It puts conventive and diffusive transport phenonmenon in correlation. It can be defined in two ways, heat transfer perspective and mass transfer perspective.

Peclet number= Mass transfer due to convection/Mass transfer due to diffusion

In that paper which you have mentioned, there the author has adressed peclet number according to mass transfer perspective. Because the droplet is exposed to the hot air due to which evaporation is taking place from surface of droplet. You can call it conventive mass transfer because it is happening due to bulk flow of air. If you go inside the droplet, it has dissolved components. These components because of the diffusion travel to the surface of droplet.

Now If the rate (i.e. speed) of droplet evaporation is lower than the rate of diffusion of components inside the droplet (note: components are trying to move towards surface). The components will go to surface of droplet and form a shell. Then you end up with spherical particles.

In opposite case when evaporation is fast and components diffusion towards surface is slow. You will not see any shell formation. Therefore, it will lead to despahed partilces. In this scenario you have peclet number value more than 1 as you could see from above mentioned definition.

In the paper you have mentioned with a link, the researchers are not interested in exact values of peclet number. They are interested if it is more than 1 or less than 1. This you can figure out by having ratio of two terms i.e. rate of evaporation and mass transfer diffusion coefficient.

There is no well established method to measure diffusion coefficient. Most of the people use already developed empirical correlations. These correlations you need to look for your system (components and solvent). May be this link helps you in finding some ways.

https://perminc.com/resources/fundamentals-of-fluid-flow-in-porous-media/chapter-3-molecular-diffusion/diffusion-coefficient/measurement-techniques/

Check this paper, they also talked about buckling of particles

https://hal.sorbonne-universite.fr/hal-01368558/file/Lintingre_2016_Control_of_particle.pdf

Thank you very much!