A Swirling Pipe Flow Course Project

Students designed a rotating pipe flow apparatus for the fluids laboratory. The project was funded by the ASHRAE Senior Undergraduate Project Grant Program. This paper describes a project where a group of undergraduate engineering students in the manufacturing processes, finite element methods and fluid mechanics courses designed, built, and tested a swirling pipe flow apparatus for measurements of friction factors. The overall objective was to engage the students in a design project. The paper will also provide details of assessment and outcomes for the project. The students had to choose materials, minimize production cost, and determine fabrication techniques for the apparatus. Students designed the apparatus using SolidWorks, and SolidWorks Flow Simulation software was used to simulate the swirling pipe flow. Students designed a pipe flow apparatus with static mixers, diffuser, settling chamber, honeycomb, screens and a contraction in order to minimize the disturbance level of the flow entering the pipe section. The pipe had a section that could be rotated in order to study the effects of swirling motion on the stability of pipe flow. Students were involved in the building of the apparatus and they performed pressure drop measurements and determined friction factors. Introduction This project involved the design, building and testing of a swirling pipe flow laboratory setup for undergraduate engineering students. The laboratory will enable students to conduct hands on measurements of laminar, transitional and turbulent pipe flows with swirl. Swirling motion and related losses are created in piping systems from common components such as bends, elbows, fittings, flow meters, tees, and valves. Piping systems are frequently included in different engineering designs. The water that we use in our homes is distributed through piping networks. Both air and water that flows through pipes is commonly used in heating, ventilation, air conditioning, and refrigeration applications. Piping has numerous HVAC related applications including sizing of pipes and pump head calculations, see Taylor and McGuire 1 . Pipe flow and piping designs are commonly studied in elementary fluid mechanics and HVAC 2 courses in mechanical engineering. Pipe flow was studied by Reynolds 3 in his transitional flow studies 125 years ago but is still a puzzle 4 . The flow in a pipe is generally considered to be turbulent for Reynolds numbers Re = UmD/ > 2,000 based on mean velocity Um, inner pipe diameter D, density and the dynamic viscosity of the fluid. However, transition in pipe flows is highly dependent on the disturbance level of the flow and pipe flow has been shown in carefully controlled experiments 5 to be laminar up to Re = 100,000. In this project the students designed an apparatus with diffuser, settling chamber, honeycomb, screens and a contraction in order to minimize the disturbance level of the flow entering the pipe section. A short stationary section of the pipe will be followed by a longer section that can be axially rotated in order to study the effects of swirling motion on the stability of pipe flow. Swirl has been shown to be destabilizing for laminar pipeflow 6 but stabilizing for turbulent pipe flow 7 . Furthermore, in turbulent pipe flow the pressure drop along the pipe is decreasing with increasing rotation rate 8 . The pipe was made of clear polycarbonate tubing for durability and transparency. The inner diameter of the pipe was D = 44.45 mm and the total length L of the pipe section is limited to 7.14 m due to lab space restrictions. This means that the total length of the pipe equals 161 pipe diameters. The hydrodynamic entry length Lh is the distance between the pipe entrance and the location where the flow is fully developed. The entry length is approximately given by Lh =0.05DRe for laminar pipe flow and much shorter in turbulent flow Lh =10D. Fully developed laminar pipe flow can be attained in the proposed laboratory project for Reynolds number up to 3,000. The pressure drop in pipe flow is described by P = fLUm 2 /2D where f is the DarcyWeisbach friction factor defined as f = 8wUm 2 and w is the wall shear stress. The Fanning friction factor Cf is commonly used and is equal to one-fourth the DarcyWeisbach friction factor. For laminar flow in a circular pipe it can be shown that f = 64/Re. The Fanning friction factor for turbulent flow in a smooth pipe is given by f = 0.316/Re 1⁄4 . For rough pipe flow the friction factor can be determined from the wellknown Moody chart. The velocity profile for fully developed laminar pipe flow is parabolic in shape but there are four different layers of the turbulent pipe flow velocity profile including viscous sublayer, buffer layer, overlap layer and the turbulent layer. Experimental turbulent pipe velocity profiles are often compared with the power law velocity profile. The influence of rotation on pipe flow is determined by the swirl number S = Uw /Um where Uw =D/2 is the velocity of the pipe wall and is the angular velocity of the rotating pipe. Experimental Set-Up The contraction design for the pipe flow apparatus is important since it reduces turbulence and velocity fluctuations in the flow 9 . The shape of the contraction is often chosen as a fifth-order polynomial in order to avoid boundary layer separation on the convex part of the contraction and growth of Gortler vortices triggered by the centrifugal force on the concave part of the same contraction. It is well known that such vortices can develop on concave surfaces such as airfoils but also in contractions for pipe flows 10 . The shape of the contraction, see figure 1, was chosen to be y(x) Ri (Re Ri) 6 x L 5 15 x L 4 10 x L 3 (1) where L = 263.4 mm is the length of the contraction and