To size a heat exchanger, I need to know (among other things) the Re number as an indication of the flow conditions. The Re number depends on the viscosity. In a [shear thinning fluid](http://en.wikipedia.org/wiki/Shear_thinning), I can't assume a constant viscosity, instead it will depend on the shear rate that is not constant throughout the pipe. [I've asked on Physics SE about the shear rate in turbulent shear thinning fluid flow in a pipe](http://physics.stackexchange.com/questions/86178/what-is-the-apparent-viscosity-in-shear-thinning-turbulent-flow-through-a-pipe), but received no helpful answer.

No w I'm not looking for an in-depth examination of the beauty of the Navier stokes equation, but for a practical approach to sizing a heat exchange for such a fluid. How to go about it?