Droplets need HELP!!!

Maria

Hi all! I am new in CFX and I could really use some help.....! The simulation that I am trying to set is a transient case with fuel injection into stationary air. I am using the Enhanced Blob Primary break-up model and I am trying to set somehow the mass flow rate. I dont want to use a fixed flow rate but a variable with time. But in the particle Injection region tab..there are some things that I dont understand... 1)What is the number of positions...I am setting it to be proportional to mass flow rate but it requests a number with units (1/kg). Is this the total injected mass..? 2)For the particle mass flow rate I tried to use an expression as a function of time, but it crashes because it says that this variable (mass flow rate) should be dimensionless.....

Can anyone please help me on how to set a flowrate which will be variable in time for the injection conditions....?

Thank you guys...! maria