How to send a second welcome email to those who haven't opened my previous one?

Hi there @Rara,

Thanks for sharing.

You can do this by setting up a triggered split in your Flow similar to this : 

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The only thing that you would have to add is a time delay for the Split. So if you wanted to wait 24 hours - you would apply a one day time delay after the first email. 
After that day wait, they would come into the Split where it checks if the user opened the email. If Yes, they will go down the YES path, if no - you can add the email in the NO path again. 

Do you know the profile you need to resend to? If so, it'll be easier to just follow the instructions for "Resending from a profile" (document here). 
If you want to send from the flow analytics, you'd click "View all analytics". Then go to the Recipient tab and find the person you're looking for.

Hope this helped,

Alex