Hi Mr. Dastmalchi,

from the Reynolds number alone one could think that the flow is laminar.

This is, however, misleading whenever buoyancy (natural convection) is involved in addition to the forced convection. Thus, a mixed convection prevails.

The ratio between natural and forced convection is often correlated to the so-called Richardson numer Ri=Gr/Re^2. For very small numbers of Ri

thank you for your answer

Can the flat plate in the way of flow change flow regime to turbulent?

Also the chimney is the tube of natural gas water heater.

hello

for natural convection, you need to calculate the Rayleigh number and found the critical value in all books about heat transfer.

The Richardson is just to know if you are in natural, fixed or forced convection regime.

I would need a more accurate description of the geometry. Is the Reynolds number based on the diameter of the tube.? Is the plate in the tube?

I found Bastian Schmandt's very interesting, but how to find experimental data.

I have been working over 30 years with Ljungström Air Preheaters and experimental data were made with a heat transfer test rig. We usually always got a break point in the Colburn factor at approx 2000.  Reynolds number was based on the hydraulic diameter for all types of elements.

Below Re=2000 the flow was laminar and over Re = 2000 the flow was turbulent.

thank you for answers

suction in chimney pipe is Natural. So I think criterion of turbulent flow is Ra not Re. Since plate is in the tube, how is Ra defined?

The Ra number indicates transition to turbulence for an object in free space. In a confined space as a chimney the Reynolds number based on the flow velocity, the pipe diameter and the fluid viscosity is the relevant parameter. However the geometry is quite important. One can distinguish fully developed flows from flows at local discontinuities. In a fully developed flow for smooth walls turbulence will not be maintained for Re