Experimental study of the flow in helical circular pipes: Torsion effect on the flow velocity and turbulence

An objective of the present paper is to experimentally clarify the torsion effect on the flow in helical circular pipes. We have made six helical circular pipes having different pitches and common non-dimensional curvature δ of about 0.1. The torsion parameter β ₀, which is defined by β ₀ = τ/(2δ)_1/2 with non-dimensional torsion τ, are taken to be 0.02, 0.45, 0.69, 1.01, 1.38 and 1.89 covering from small to very large pitch. The velocity distributions and the turbulence of the flow are measured using an X-type hot-wire anemometer in the range of the Reynolds number from 200 to 20000. The results obtained are summarized as follows: The mean secondary flow pattern in a cross section of the pipe changes from an ordinary twin-vortex type as is seen in a curved pipe without torsion (toroidal pipe) to a single vortex type after one of the twin-vortex gradually disappears as β ₀ increases. The circulation direction of the single vortex is the same as the direction of torsion of the pipe. The mean velocity distribution of the axial flow is similar to that of the toroidal pipe at small β ₀, but changes its shape as β ₀ increases, and attains the shape similar to that in a straight circular pipe when β ₀ = 1.89. It is also found that the critical Reynolds number, at which the flow shows a marginal behavior to turbulence, decreases as β ₀ increases for small β ₀, and then increases after taking a minimum at β ₀ ≈ 1.4 as β ₀ increases. The minimum of the critical Reynolds number experimentally obtained is about 400 at β ₀ ≈ 1.4.