Flow Behavior and Friction Factor. in Internally Grooved Pipe Wall

Transcription

1 Adv. Studies Theor. Phys., Vol. 8, 2014, no. 14, HIKARI Ltd, Flow Behavior and Friction Factor in Internally Grooved Pipe Wall Putu Wijaya Sunu Mechanical Engineering Department, Bali State Polytechnic, Bali, Indonesia I N.G. Wardana A.A. Sonief Nurkholis Hamidi Copyright 2014 Putu Wijaya Sunu, I N.G. Wardana, A.A. Sonief and Nurkholis Hamidi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Friction factor cause by internally grooved pipe has been investigated experimentally. The result indicates that grooves reduce friction factor. However, in a certain groove numbers, it increases friction due to a series of fluid cycling near the pipe wall. Keywords: Friction factor, flow behavior, grooves, pipe flow

2 644 Putu Wijaya Sunu et al. Introduction Most energy loss in piped fluid flow systems is caused by friction. Passive technique developed for reducing this is through cutting grooves into internal pipe walls. This is an effective method, as it needs no extra energy. Much effort has gone into ways to reduce friction energy loss and to control fluid flow close to pipe wall. And in the fields of heat transfer, where space can be limited, size together with efficient heat transfer capabilities are extremely important, grooves meet both of these criteria, as studied by [1, 2, 3]. In the application of fluid flow which actually turbulence, here grooves can have a significant effect, as well as on viscous areas near pipe walls reported by [4, 5, 6]. In pipes, flow patterns are unique due to their being symmetrical and wall curvature effect, and to date very little research reconstructed the effects of grooves on friction and flow behavior in pipe. Thus the aim of this study was to look into the effects of grooves on friction and flow structure in radial/lateral plane of pipes. Experiment Set-up Figure 1 shows the sketch of the experimental equipment. Fig.1 Schematic of experimental setup The flow was driven by a centrifugal pump which stabilizes using electric stabilizer at a water temperature of 27 (± 1 ). Flow control valves were fully open so resulting in a Reynolds Number of The pipe test sections were made of a PVC pipe with internally incised grooves, 1 meter in length and an internal diameter of 2.6 cm. The elbows before and after the test sections were made of transparent acrylic pipes for ease of lighting and flow visualization. Grooves were incised using a conventional etching technique, and were 1 x 1 mm

3 Flow behavior and friction factor 645 in size (rectangular groove). There were differing 6 grooved pipe test sections, namely, 2, 4, 8, 16 and 32 grooves and a smooth pipe for comparison. The pipe entrance length of the experimental apparatus was 70 cm and the flow in it was considered to have been fully developed. Two pressure taps were installed in the test pipe and where connected to pressure transducer and a data logger, so nominal differing levels could be recorded. Pressure drop data from each of the grooved pipes as well as the smooth pipe were converted to friction factors. Flow visualization was performed by using threads at 240 fps. And, dye visualization was recorded at 480 fps to increase both photo clarity and accuracy. Result and discussion The data from measuring pressure differences was converted into those friction and the results are laid in Figure 2. Friction Factor 0,035 0,030 0,025 0, Change of Friction (%) a. b. Groove Number 15 Groove Number Fig.2 Friction factor as a function of groove number. a) Friction factor; b) Change of friction factor The dotted lines in figure 2a denote the friction factors from the smooth pipe, and figure 2a shows that for pipes with 2, 8, and 32 grooves, the friction factors were lower than that for the smooth pipe. However, for 4 and 16 grooves pipe, the friction factors were higher than that of the smooth pipe. Figure 2b gives the change of friction factors, the negative percentages indicate that there was a fall in friction and likewise a rise where these were positive. For ease of the visualization of the causes of the rises and falls in the individual friction factors, plastic threads were attached to internal pipe and a dye was introduced to the flows and recorded for nearly 400ms Fig. 3 Dye pattern at the outlet sections of the pipes (a) no grooves; (b) 2 grooves; (c) 4 grooves; (d) 8 grooves; (e) 16 grooves; (f) 32 grooves.

4 646 Putu Wijaya Sunu et al. Figure 3 shows that the diameter of vortices of 4.7, 9.5, 4.5, 8.1, 4.1 and 12.0 pixels respectively formed. As a reference, the width of the grooves was 4.7 pixels. In the 2, 8 and 32 groove pipes, vortices sizes were greater than that of the grooves width causing them to remain stationary above the grooves so that the shear stress failed to approach pipe wall, resulting in low friction factors. Whereas, for the 4, and 16 grooves pipes, the vortices were of lesser size than groove width, which enabled them to fall into groove valleys and thus increase shears stress, thus their friction factors were higher. Fig. 4 Dye pattern at the inlet sections of the pipes. (a) no grooves; (b) 2 grooves; (c) 4 grooves; (d) 8 grooves; (e) 16 grooves; (f) 32 grooves. Fig. 5 Thread pattern of the pipes. (a) no grooves; (b) 2 grooves; (c) 4 grooves; (d) 8 grooves; (e) 16 grooves; (f) 32 grooves. Figure 4 indicates the results from the size of vortices close to pipe walls, it can be seen that in the sizes of the vortices in the 4-groove, and 16-groove pipes are smaller than those of the grooves width, resulting in whirling flows. The fluid movement is circular inducing a high velocity gradient between center of pipe and near wall causing flows to be attracted to pipe walls. This phenomenon is confirmed by figure 5, which shows thread patterns, which are extended and tend to be close to pipe walls and thus raises flow friction factors. Whereas, the 2, 8, and 32 groove pipes had vortices of greater than their groove widths, and thus their flows were not circular. Visualization indicates that the fluid collects with little diffusion, meaning the velocity gradients were low. This was also confirmed by figure 5, which shows the thread movement was limited and tended to collect together, which demonstrates low shear stress leading to lower friction factors than that of the no-groove pipe. Conclusion It can be concluded that in the 2, 8 and 32-groove pipes, fluid flow experienced reduced friction factors as the size of vortices formed were greater than groove widths so that flows tended to be straight with small velocity gradient. Whereas in the 4 and 16-groove pipes, since the vortices formed were of a smaller

5 Flow behavior and friction factor 647 size than groove width, friction factors increased causing vortex movement tended to become circular with high velocity gradient. Thus, choosing the appropriate number of grooves will reduce the energy loses. References [1] Aroonrat, K., Jumpholkul, C., Leelaprachakul, R., Dalkilic, A.S., Mahian, O.,Wongwises, S., Heat Transfer and Single-Phase Flow in Internally Grooved Tube. International Communication in Heat and Mass Transfer, 42 (2013), [2] Choi, K.S. and Orchard, D.M., Turbulence Management Using Riblets for Heat and Momentum Transfer. Experimental Thermal and Fluid Science, 15 (1997), [3] Katoh, K., Choi, K.S.,Azuma, T., Heat-transfer Enhancement and Pressure Loss by Surface Roughness in Turbulent Channel Flows. International Journal of Heat and Mass Transfer, 43 (2000), [4] Lee, S.J. and Jang, Y.G., Control Of Flow Around A Naca 0012 Airfoil With A Micro-Riblet Film. Journal of Fluid and Structure, 20 (2005), [5] Litvinenko, Y.A., Chernoray, V.G., Kozlov, V.V., Loefdahl, L., Grek, G.R., Chun, H.H., The Influence Of Riblets On The Development Of A Structure And Its Transformation Into A Turbulent Spot, Doklady Physics, 51(3) (2006), doi: /s [6] Lokhov D.S, A. V. Boiko, and D. S. Sboev, Controlling the development of stationary Longitudinal structures in the boundary layer On a flat plate using riblets, Journal of Applied Mechanics and Technical Physics, Vol. 46, No. 4 (2005), Received: May 5, 2014