[AsadEl]

Dear Members,
Hope that everyone doing well.
First of all, I am not a fluid mechanic engineer, however, I have to design an experimental work for my Ph.D. During, the design I have faced several challenges, I am sure these challenges for you will be a piece of cake.

I will try to explain them here, however, more details can be found in the attached file.
Please refer to the setup indicated in the attached file.
The first question is for the entrance length (EL)
I have plan to install velocity sensors in pipe #1, therefore I want to know the best formula that I can use to estimate the EL between the two pipes ( 1 and 2 ) if the flow rate is designed to be 150 gpm ( US gallon per minute )
The second question about the selection of Pipe#3.
As you can see in the attached file, the fluid must exit pipe 2 and go along pipe #3 , my question, How I can find out the best ID for pipe #3 , so that all the fluid will move to pipe #3 ( the flow rate must be 150 gpm )
Because if I select a small pipe, most of the fluid will raise up ( this the case which I do not want to have, I want to make sure that all the fluid will exist through pipe#3 when the flow rate is 150 gpm ( which is the upper limit flow rate.
Can you please help me to solve these 2 design issues

Thanks a lot in advance , hoping to get your answer

Best regards

Asad