It depends on the twist ratio, type of flow. It is not clear whether you performing steady or unsteady analysis. Select the proper twist ratio from literature and choose proper Reynolds number.

if reynolds number Re=(rho*v*l)/mu.

where l= (4A)/P;

for any cross section, I found less Reynolds number after using insert. insert is used for making more turbulence, means more Reynolds number. is it realistic that less Reynolds number after using insert?

These article may help you.

See, due to swirl the localization of flow is there. May be for that the outlet velocity is lower. Please keep the inlet Reynolds number constant and check for turbulence in outlet

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.820.593&rep=rep1&type=pdf

http://www.journal.multiphysics.org/index.php/IJM/article/download/184/176

http://ansannual.org/wp-content/data/polopoly_fs/1.3583582.1494346571!/fileserver/file/768009/filename/447.pdf

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3671&context=rtd

Due have adverse pressure gradient in the flow?

@naseem ali, some time back flow occur at the outlet. but after some iteration, back flow was eliminate.

To be honest, the question is not clear..what do you mean for adding more turbulence to turbulence? Could you show the details of your flow problem?

Do not forget that the results of you simulation strongly depends on the formulation. If you want to get quantitative results in a (dynamic/thermal) BL you need a well resolved grid layers. Furthermore, you did not write if RANS/LES/DNS is used.

the enhancement ratio depend on twisted ratio and as colleagues said above you have to choose according the literature, geometry that be used for your channel (circular, square ... etc.), Reynolds number for your problem should be same for baseline and modification cases. regards

I agree your view point Dr. Mohammed Saad Kamel.

If you refer to the following document: https://www.researchgate.net/publication/239407401

you will probably find the path to your answer as it is a review of passive heat transfer augmentation techniques. To quote the authors, "Twisted tape in turbulent flow is effective up to a certain Reynolds number range but not over a wide Reynolds number range."

Physically, the locations where the twisted tape intersects (or nearly intersects) the internal wall of the pipe will have restricted flow and probably localized laminar flow, which counteracts any additional turbulence added to the bulk flow.

I hope this helps.

@filippo, without insert the Reynolds number of my pipe flow is in turbulence region. now if I put a insert in the pipe, this creates more turbulence in the pipe!

now convective heat transfer will increased?

thanks @kamel, and @jim for your suggestions.

Some papers studies the mechanism, for example Article Heat transfer enhancement by tapered twisted tape inserts

It is not absolute fact that the twisted inserts will increase turbulence. The twisted inserts are working to stimulate the turbulence. It is not increasing the average Reynolds no. since its value depends on the characteristic length, average velocity and properties of fluid. The local Reynolds number will be change only. If Reynolds number is high enough to be stimulated locally by adding some inertial forces at some regions in the flow, you have to ensure that distance between the stimulation inserts and the boundary layer of pipe wall is not far so this stimulation will be diminished. So the ratio of insert diameter to pipe diameter must be taken in your consideration. If Reynolds number less than the transient Reynolds number, the viscosity effect will be more dominant and turbulence stimulation will fail.

I thought I would add my thoughts to the ones provided by others. A long time ago, we were asked by the customer to add the effects of free stream turbulence on skin friction and convective heat transfer rate in a boundary layer code. We used the following references:

1. McDonald, and Kreskovsky, J. P., "Effects of Free stream turbulence on the Turbulent Boundary Layer," Int. J. Heat of Heat and Mass Transfer, Vol. 17, pp. 705-716, 1974.

2. Rued, K and Witting, S., "Free stream Turbulence and Pressure Gradient Effects on Heat Transfer and Boundary Layer Development on Highly Cooled Surfaces," Journal of Engineering for Gas Turbines and Power Transactions of the ASME, Vol. 107, pp.54-59, Jan 1985.

There are definitely effects of free stream turbulence on boundary layer quantities, the magnitudes of which have to be determined empirically or experimentally, or thru current DNS methods. The way to generate the free stream turbulence is another matter.

So, happy researching !

I agreed with Filippo Maria Denaro