Numerical and Experimental Study on the Effect of Guide Vane Insertion on the Flow Characteristics in a 90º Rectangular Elbow

Transcription

1 Numerical and Experimental Study on the Effect of Guide Vane Insertion on the Flow Characteristics in a 90º Rectangular Elbow Sutardi 1, Wawan A. W., Nadia, N. and Puspita, K. 1 Mechanical Engineering Department, FTI-ITS, Surabaya, Indonesia ABSTRACT: Pressure loss in an elbow is caused by friction as well as flow separation and secondary flow. One method to reduce pressure loss due to flow separation and secondary flow in a 90 o elbow is by insertion of guide vanes. On the contrary, the insertion of the guide vanes increases the friction loss due to the augmentation of the solid surface contact with the flow. Therefore, this study is intended to examine flow phenomenon in a 90 o elbow and to investigate the effect of insertion of guide vane on pressure loss. The study was performed numerically and experimentally. The numerical simulation was performed using commercial software Fluent , incorporated with Gambit Test section used in this study is a 90 o elbow with rectangular cross section and radius ratio (R m /D h ) equals to 1.875, with and without guide vanes. The air is drawn into the test section using a blower. Flow Reynolds numbers are set to be Re Dh 1.2 x 10 5 and 1.4 x 10 5 based on inlet freestream velocity and test section hydraulic radius (D h ). The stagnation pressure was measured using a Pitot tube, while the static pressure was measured using an inclined manometer. Mean velocity profiles show that the relaxation process has not been completed yet up to 4D h downstream of the guide vanes. Next, the insertion of guide vanes increases the flow pressure drop up to approximately three times compared to that of without guide vane. Finally, the insertion of guide vane in the elbow triggers the formation of cross passage flows and corner vortices. Key words: 90º Rectangular elbow, guide vane, pressure drop, secondary flow. 1. INTRODUCTION As the fluid flows inside a ducting system, it subjects to shear stress due to the no-slip condition at the duct wall. Pressure drop in such ducting system is mainly caused by skin friction effect, and in a straight duct, hundred percent of the pressure drop is from the wall shear stress. On a curved duct such as an elbow, on the other hand, the pressure drop is also contributed by the secondary flow in the elbow, beside from the wall shear stress. The secondary flow in the elbow is the consequence of the presence of pressure difference between the convec and the concave walls of the elbow. The other effect of the pressure difference is the local flow separation from the convec and the concave wall of the elbow as studied by Cheng [1], Marn and Primoz [2], and Miller [3]. In many engineering applications, it is found not only circular elbows, but also rectangular elbows. The secondary flow in the rectangular elbows has more severe effect on the pressure drop than that is in the circular elbow. While in the circular elbow there is no wall that is mutually perpendicular, in the rectangular elbow, their walls are connected each other

2 perpendicularly. Fluid flow in the near two mutually perpendicular walls experiences fluid boundary layer interaction that leads to the formation of other shear layer near the walls. This shear layer is a three dimensional vortex motion that contributes to block the main flow, and it has a penalty to the increase of flow pressure drop. Formation of the secondary flow can be prohibited or reduced by an insertion of one or more duct splitters of guide vanes, as it was studied by Liou and Lee [4]. On the contrary, the additional guide vanes in the duct increases the fluid-wall surface contact that results in the increase wall shear force in the fluid flow. The last condition leads to the increase in the flow pressure drop. Finally, the increase of the fluid-wall surface contact has more significant effect on the flow pressure drop as the fluid velocity increases. Therefore, a systematic study of the effect of the additional guide vane in the elbow on the flow pressure drop is essential. Based of the aforementioned background, the purpose of the present study is to investigate of the effect of guide vane insertion in a 90 o rectangular elbow as the flow Reynolds number is increased. The study is performed experimentally and numerically. 2. METHODS OF STUDY The experimental and numerical studies were peformed in this research. Experiment was conducted in Fluid Mechanics Laboratory, while the numerical study was conducted in the Computer Aided Design Laboratory, Mechanical Engineering Department, ITS, Surabaya. 2.1 Experimental Study Main component of this experimental set-up is a 90 o rectangular elbows without and with guide vanes. Air flow was driven by an induced fan/blower with flow capacity more than 2300 m 3 /min, and the flow was passed through honey comb and contraction area before entering the test section. Figure 1 shows the main test sections, and Fig. 2 shows the complete experimental set-up. The inner radius (R i ) of the elbow is 100 mm, while the outer radius (R o ) is 150 mm. The elbow cross section is rectangular with 50 mm width and 100 mm height. Details of elbow geometry are given in Fig. 3. Pressure measurements were conducted using an inclined manometer filled with red oil having the specific gravity (SG) of The manometer is connected to wall pressure taps located at the test section walls and also connected to a Pitot tube. A total pressure tube was used to measure total pressure at any location at a cross section. This total pressure together with static pressure at corresponding location are then used to obtain velocity profile at a particular test section. For the purpose of wall static pressure distribution, as many as 154 and 174 static pressure taps were located on the duct inner and outer wall, respectively. Also, the velocity profiles were obtained in 12 duct cross-sections, where the streamwise location can be seen in Fig. 3. The corresponding location of the 12 sections can be seen in Table I. a) Rectangular elbow 90 without guide vane b) Rectangular elbow 90 with three guide vanes Figure 1: 90 o rectangular elbow with and without guide vane 2

3 2.2 Numerical Study In this numerical study, Computational Fluid Dynamics (CFD) softwares were utilized. The softwares are Fluent and Gambit Common steps are used in this numerical study: pre-processing, solving, and post-processing. Pre-processing step includes grid generation, turbulence model determination, material, operating and boundary condition choices. In solving step, iterations up to minimal error 10-6 were performed. Finally, in postprocessing step, velocity and pressure distribution and pathlines are obtained. Table II shows the steps performed in the numerical method. Figure 2: Complete experimental set-up Figure 3: Details of the test section 3. RESULTS AND DISCUSSION 3.1 Mean Velocity Profiles Mean velocity profiles at a downstream location of the elbow are shown in Figs. 4a and 4b. The downstream location is 4D h from the end of the guide vanes. In Fig. 4a it is shown the velocity profiles for elbow without guide vane at two different Re Dh, obtained experimentally 3

4 as well as numerically. For the two elbow configurations, without and with three guide vanes, the velocity profiles are affected by the value of Re Dh, for both experimental results and numerical results, except the profiles from the numerical study for the elbow with three guide vanes. The experimental results show that the flow is still developing or relaxing after passing through the elbow, as at this location the profiles have not been fully developed. One important feature is noticed from these profiles. Unlike the profiles from the experiments, the profiles obtained from the numerical study with three guide vanes show no significant different for the two different Reynolds numbers. Table I: Static pressure tap locations on the duct walls at twelve streamwise directions Inner Wall Outer Wall Section Distance from the Distance from the x/d elbow inlet (mm) h Section elbow inlet (mm) x/d h Tabel II: Steps in the numerical study Step Sub-step Detail Preprocessing Grid Using Gambit; model grid: map Model double precision; segregate; k-ε realizable Material density = kg/m 3 ; viscosity = 1.57 x 10-5 kg/m.s; Operating Condition Pascal Boundary Condition - Velocity inlet: U ref 28.0 and 33.0 m/s - outlet B.C.: outflow Solving Convergence criteria: 10-6 Postprocessing - velocity and pressure distribution - pathlines 3.2 Wall Pressure Coefficient (Cp) Wall pressure coefficient (Cp) is plotted as a function of Reynolds number (Re Dh ) and the number of guide vanes, for both experimental and numerical results. Figure 5 shows the distribution of Cp along the wall of the test section as a function of Re Dh for the elbow without guide vane, at Re Dh = 1.2 x 10 5, while Fig. 6 shows the distribution of Cp, similar to that of Fig. 5, but it is for Re Dh = 1.4 x In those figures, it is shown that there is a significant different in static wall pressures between that is at the inner and the outer radius of elbow 4

5 walls. This pressure difference allows the fluid particles move from the outer to the inner wall (radius) of the elbow, as reported by Miller [3]. The distribution of the pressure in the radial direction at the elbow is shown in Fig. 7. The location is corresponds to x/d h = 2.83, based on the inner wall of the elbow (section no. 3, Table I). In Fig. 7, it is shown that the pressure increases monotonically as the radius increases, for the two Reynolds numbers, either obtained from the experiment or from numerical study. Although the numerical values of Cp obtained from the experiment are slightly different from that obtained form the numerical study, their pressure differences between the inner and the outer walls of the elbow are almost similar. Table III shows the pressure difference between outer and inner walls of the elbow, for the two Re Dh. (a) (b) Figure 4: Velocity profiles at section 8 (4D h ) for elbow without (a) and with (b) three guide vane as a function of Re Dh. Figure 5: Comparison of experimental and numerical results of Cp distribution at the inner and outer radius of the elbow walls without guide vanes at Re Dh = 1.2 x

6 Figure 6: Comparison of experimental and numerical results of Cp distribution at the inner and outer radius of the elbow walls without guide vanes at Re Dh = 1.4 x Figure 7: Distribution of wall pressure coefficient (Cp) normal to the streamline of the elbow without guide vane Re Dh = 1.2 and 1.4 x Table III: Pressure difference between elbow outer and inner walls, without guide vane. Re Dh ΔCp Δp (Pa) (outer-inner) (outer-inner) Num. Exp. Num. Exp The insertion of three guide vanes (Figs. 8 and 9) reduces significantly of the pressure difference between that is at the inner and the outer walls (radius) of the elbow. This phenomenon indicates that the cross flow of the fluid particles from the outer to the inner radius of the elbow should be suppressed. Although the insertion of guide vanes can suppress the formation of secondary flow, hence, the blockage effect on the main stream, the presence of the vanes contributes to the increase in skin friction drag. The increase in skin friction drag is due to the increase of the solid-fluid surface area contact, and this effect is more pronounced at higher Reynolds numbers (see Table IV). The gross effect of the guide vane insertion is the 6

7 increase in pressure drop at Re Dh 1.2 x 10 5 and 1.4 x The results obtained from the experiments (Fig. 8) are in good agreement with that obtain from numerical simulation (Fig. 9), at the two Re Dh. Figure 8: Comparison of experimental results of Cp distribution at the inner and outer radius of the elbow walls with guide vanes at Re Dh = 1.2 and 1.4 x Figure 9: Comparison of numerical results of Cp distribution at the inner and outer radius of the elbow walls with guide vanes at Re Dh = 1.2 and 1.4 x Table IV: Pressure drop comparison in the elbow with three guide vanes at two Re Dh. Re Dh vanes ΔCp Δp (Pa) (inlet-outlet) (inlet-outlet) %Δp w/o w/o w w

8 3.3 Secondary Flow Figure 10 shows the flow topology at the vicinity of the leading edges in the elbow with three guide vanes at Re Dh = 1.2 x The formation of forward saddle point and 3-D flow separation are also shown in the figure. In addition, there is also cross passage flow initiates from the leading edge of the guide vanes. This cross passage flow crosses the main flow toward the region with lower energy, i.e. at the region with smaller radius of curvature. Figure 11 shows the corner vortices inside the elbow without guide vane at Re Dh = 1.4 x These vortices are the results of boundary layer interaction developing on each mutually perpendicular walls. The figure is obtained at section 4, based on Table I. The formation of such corner vortices actually are not only happened at this particular section, but also happened at each section of the rectangular elbow. The formation of these vortices, together with the formation of the cross passage flows and the increase of fluid-solid surface contact on the guide vane surface is responsible to the total increase in the flow pressure drop. flow separation forward saddle point Figure 10: Flow Topology in the vicinity of the guide vane leading edge Re Dh = 1.2 x Figure 11: Corner votex in the elbow without guide vane at section 4. Re Dh = 1.4 x

9 4. CONCLUSION From present experimental and numerical studies on the flow inside 90 o rectangular elbows with and without guide vanes can be drawn following conclusion: 1. Velocity profiles show that at a location of 4D h downstream of the elbow with guide vanes have not been relaxed back to the normal profiles. 2. Insertion of three guide vanes contributes for the increase in pressure drop up to approximately 300 and 200 percent, respectively, compared to that without guide vane counterpart. 3. The insertion of guide vanes in the elbow triggers the formation of other secondary flows, such as cross passage flows and corner vortices. REFERENCES [1] Cheng, D.Y., 1994, Laminar Flow Elbow System and Method, U.S. Patent Documents, No. 5: p. 323,661. [2] Marn, J. & Primoz, T., 2006, Laminar Flow of Shear-Thichkening Fluid in 90º Pipe Bend, Fluids Dynamics Research : pp [3] Miller, D.S., 1990, Internal Flow Systems, 2 nd edition, BHRA (Information Service). [4] Liou, T.M. & Lee, H.L., 2001, Effect of Guide-Vane Number in a Three-Dimensional 60 Deg Curved Side-Dump Combuster Inlet, J. of Fluids Engineering, 123, pp