To investigate the rheology of multi-phase fluids and especially foams under realistic field conditions a downhole flow loop has been constructed and tested. The interactions between gravitational, frictional and other (e.g. gas solubility) factors have been investigated and isolated in a series of experiments. Here the system design of the multi-phase fluid flow loop and the special considerations of the design and their justification are described. To evaluate the data a new two-phase flow equation and a methodology of interpretation are outlined. Applications of the methodology in the treatment of sample data are also presented. The choice of a vertical instead of a horizontal apparatus is justified from the fundamental treatment in this work. It shows that the presence of the gravitational effects magnifies the frictional effects, allowing their more accurate estimation.