You may look into the literature if any investigator has come across such a situation. The concept can be useful for comparison of your experimental / computational data.

Else,  a parameter similar to hydralic diameter which is used for non circular cross section can be utilized.  (It is stated as duct in the title and tube later).   It is assumed from your statement that the duct is twisted only with its cross section remaining constant.  The volume of the duct before the duct has been twisted must be known.  Based on this presumption, the surface area may be defined for a twist ratio as the ratio of volume to the hydraulic diameter of the duct.  Clarity in the question is needed, if required.  

Hi Saurabh,

You can make use of commercial software like FLUENT for complicated geometries.

Regards,

Rajesh.

If you know the wall thickness, the length of the central axis, and the material, why not simply weigh it?

Volume = m/rho

and volume = wall_thickness*area (to a good approximation)

How accurately do you need to know this?

You may look into the literature if any investigator has come across such a situation. The concept can be useful for comparison of your experimental / computational data.

Else,  a parameter similar to hydralic diameter which is used for non circular cross section can be utilized.  (It is stated as duct in the title and tube later).   It is assumed from your statement that the duct is twisted only with its cross section remaining constant.  The volume of the duct before the duct has been twisted must be known.  Based on this presumption, the surface area may be defined for a twist ratio as the ratio of volume to the hydraulic diameter of the duct.  Clarity in the question is needed, if required.