A diffuser is commonly applied in fluid-flow engineering applications with its simplest form of expanding area in the flow direction. A curve diffuser is one of its kinds often associated with secondary flow separation, thus improvement via installing passive flow control devices such as a vortex generator is to explore. The present work aims to numerically investigate the potential of four (4) types of vortex generators, i.e. triangle, rectangle, tapered, wishbone to improve the 90Â° curve diffuser performance. The results suggest that using vortex generators on a curve diffuser can improve performance. Triangle vortex generator provides the most optimum pressure recovery and flow uniformity of respectively 0.250 and 2.14.&nbsp; This promises improvement of approximately 31.3% and 25.4% relative to the benchmark case, without vortex generator

Anuar, Muhammad Musleh

Hariri, Azian

Hau, Chin Yong

Mohamed Rasidi, Shamsuri

Nordin, Normayati

Shariff, Muhammad Zahid Firdaus

Teo, Wen Yong

English

Journals of Universiti Tun Hussein Onn Malaysia (UTHM)

 JOURNAL OF ADVANCED MECHANICAL ENGINEERING APPLICATIONS VOL. 2 NO. 2 (2021) 53-60      © Universiti Tun Hussein Onn Malaysia Publisher’s Office  JAMEA  Journal homepage: http://penerbit.uthm.edu.my/ojs/index.php/jamea  Journal of Advanced Mechanical Engineering Applications  e-ISSN : 2716-6201   *Corresponding author: mayati@uthm.edu.my 2021 UTHM Publisher. All rights reserved. penerbit.uthm.edu.my/ojs/index.php/jamea 53 Effect of Employing Vortex Generator on Curve Diffuser Performance  Muhammad Musleh Anuar1, Normayati Nordin1*, Muhammad Zahid Firdaus Shariff1, Azian Hariri1, Shamsuri Mohamed Rasidi1, Hau Chin Yong1, Teo Wen Yong1  1Center for Energy and Industrial Environment Studies, Faculty of Mechanical and Manufacturing Engineering,   Universiti Tun Hussein Onn Malaysia, 86400 Batu Pahat, Johor, MALAYSIA  *Corresponding Author  DOI: https://doi.org/10.30880/jamea.2021.02.02.006 Received 08 September 2021; Accepted 20 September 2021; Available online 12 December 2021  1. Introduction A diffuser is a common engineering device which has the simplest design of an expanding area in the flow direction. The main and basic function of a diffuser is to reduce the velocity of the flow. A wind tunnel is one of the many installations that incorporates a diffuser [1]. A typical closed-circuit wind tunnel would normally incorporate a wide-angle diffuser before the nozzle in order to transform the geometry from a smaller to a larger area [2]. An additional diffuser is also installed behind the working section to recover static pressure from the kinetic energy to increase the efficiency of the power source. Similarly, higher propulsion efficiency can be achieved as a result of pressure recovery obtained by installing a diffuser at the inlet of an aircraft engine. The use of diffusers to increase the handling stability of racing cars is also well established. Unfortunately, the desired performance of a diffuser is often compromised by detrimental flow phenomena induced by the very nature of its geometry.  Bernoulli’s principle implies that an increasing area along the path of an incompressible flow, as in a diffuser, causes the velocity to decrease �𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕< 0�  and the pressure to increase (𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕> 0). Under a strong adverse pressure gradient, the boundary layer on the diffuser wall is likely to separate because the flow cannot sustain the momentum required to maintain a stable boundary layer [3-4]. Ultimately, flow detachment occurs because the fluid particles at the near-wall region experience a greater retarding shearing force than the pressure force pushing it. Flow separation is undesirable in many fluid systems as it would increase the pressure drag, decrease the core flow area, reduce the handling stability, and enhance the structural vibration.  A curve diffuser is regularly companion with secondary flow separation that calls for improvement [5-7]. Despite enormous literature on diffusers are available, much less attention has been given to improve performance of curve Abstract: A diffuser is commonly applied in fluid-flow engineering applications with its simplest form of expanding area in the flow direction. A curve diffuser is one of its kinds often associated with secondary flow separation, thus improvement via installing passive flow control devices such as a vortex generator is to explore. The present work aims to numerically investigate the potential of four (4) types of vortex generators, i.e. triangle, rectangle, tapered, wishbone to improve the 90° curve diffuser performance. The results suggest that using vortex generators on a curve diffuser can improve performance. Triangle vortex generator provides the most optimum pressure recovery and flow uniformity of respectively 0.250 and 2.14.  This promises improvement of approximately 31.3% and 25.4% relative to the benchmark case, without vortex generator.   Keywords: Curve diffuser, vortex generators, flow separation, pressure recovery, flow uniformity index Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  54 diffuser by means of applying passive flow control devices. Vortex generator is one of its kinds that has already been proven to provide significant improvement of fluid flow in many engineering applications such as aircraft wings and car bumpers [4-9]. A specific configuration and arrangement of the vortex generator is used to produce delayed flow separation, which would reduce the drag force [10, 11]. In this study, the potential of 4 types of vortex generator (rectangle, triangle, tapered, wishbone) to improve performance of a sharp 90o curve diffuser is numerically investigated. The most optimum configuration of vortex generator to resolve flow separation and improve pressure recovery is proposed.   Nomenclature: 𝜌𝜌 Air density (kg/m3) 𝜎𝜎𝑜𝑜𝜕𝜕𝑜𝑜 Flow uniformity index 𝐶𝐶𝐶𝐶 Pressure recovery coefficient 𝑁𝑁 Number of measurement points 𝑃𝑃𝑖𝑖𝑖𝑖  Average static pressure at inlet (Pa) 𝑃𝑃𝑜𝑜𝜕𝜕𝑜𝑜  Average static pressure at outlet (Pa) 𝑉𝑉𝑖𝑖 Local air velocity at outlet (m/s) 𝑉𝑉𝑖𝑖𝑖𝑖  Mean air velocity at inlet (m/s) 𝑉𝑉𝑜𝑜𝜕𝜕𝑜𝑜  Mean air velocity at outlet (m/s) 𝑉𝑉𝑉𝑉 Vortex generator  2. Methodology This study involves the designing of 90° curve diffuser and 4 types of vortex generator models using Solidworks 2018 software. The model will be used in Ansys FLUENT 19.2 to simulate the data and obtain the result when employing vortex generator on curve diffuser performance.   2.1 Modeling and Meshing The 90° curve diffuser design was based on the previous study by Nordin et al. [12]. To analyze the design in Ansys, only path flow of the fluid was designed in Solidwork as shown in Fig. 1. Basic dimensions applied are shown in Table 1. In this study, 4 types of vortex generators which are rectangle, triangle, tapered and wishbone were considered. The model and configuration of each vortex generator is shown in Fig. 2. Table 2 presents the dimension of each VG design.   Fig. 1 - 90° curve diffuser model       Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  55 Table 1 - Basic dimension of curve diffuser Angle (°) W1 (mm) X1 (mm) W2 (mm) X2 (mm) rin (mm) Lin (mm) rm (mm) Lm (mm) 90 50 130 108 130 120 199.5 175 245.1    (a)      (b)     (c)     (d)   Fig. 2 - Types and configurations of vortex generator (a) rectangle; (b) triangle; (c) tapered; (d) wishbone  Table 2 - Main parameters of vortex generator Vortex Generator Type Height (mm) Length (mm) Width (mm) Lateral spacing (mm) Angle (°) Rectangular 8.23 16.46 1 20.58 18 Triangle 8.23 16.46 1 20.58 18 Tapered 8.23 34.29 1.2 23.78 - Wishbone 8.23 41.15 2.39 11.23 - Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  56 As shown in Fig. 3(a), hybrid mesh to consist of hexahedral and tetrahedral elements was generated to provide acceptable quality of skewness less than 0.3 [10]. Enhanced wall treatment of y+≈1.0 was applied to allow an optimum number of nodes obtained particularly close to the inner wall region to capture presences of flow separation. Failure to observe this essential flow phenomenon may disrupt the results. Grid independence test (GIT) was conducted, as shown in Fig. 3(b) to verify the most optimum mesh to represent the case.      (a) (b) Fig. 3 - (a) hybrid mesh with enhanced wall treatment of y+≈1.0; (b) grid independence test result  2.2 Solver Setting As depicted in Table 3, three types of boundary operating conditions were imposed. The inlet velocity, 𝑉𝑉𝑖𝑖𝑖𝑖was varied in the range 12.92 m/s corresponding to the 𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 5.786 x 104 until 39.66 m/s corresponding to the 𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 1.775 x 105. The inlet turbulent intensity, 𝐼𝐼𝑖𝑖𝑖𝑖= 3.5% - 4.1 % was estimated for fully developed turbulent flow. At the outlet boundary, the pressure was set at atmospheric pressure (0 gage pressure). At the solid wall, the velocity was zero due to the no-slip condition. Table 4 lists the details of the solver setting applied. The governing equations were independently solved using a double-precision pressure-based solver with a robust pressure-velocity coupling algorithm, SIMPLE been applied. Second order scheme was employed for the discretization of the pressure and momentum equations, while first order scheme for the turbulent kinetic energy and dissipation rate equations. Standard k- ɛ (SKE) turbulence model equipped with enhanced near-wall treatments was applied for the simulation as it has been proven to be successful in simulating similar cases based on previous works [3, 5, 8, 12-17].   Table 3 - Boundary conditions Inlet: Type of boundary Velocity-inlet Velocity magnitude, 𝑉𝑉𝑖𝑖𝑖𝑖  (m/s) 12.92 (𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 5.786 x 104) 14.25 (𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 6.382 x 104) 22.94 (𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 1.027 x 105) 31.21 (𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 1.397 x 105) 39.66 (𝑅𝑅𝑅𝑅𝑖𝑖𝑖𝑖 = 1.775 x 105) Turbulent intensity, 𝐼𝐼𝑖𝑖𝑖𝑖 (%) 4.1 4.0 3.8 3.6 3.5 Hydraulic diameter, 𝐷𝐷ℎ (mm) 72 Outlet: Type of boundary Pressure-outlet Pressure (Pa) 0 gauge pressure Wall:  Type of boundary Smooth wall Shear condition No-slip Working fluid properties: Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  57 Working fluid Air Temperature (oC) 30 Density, ρ (kg/m3) 1.164 Dynamic viscosity, µ (kg/m.s) 1.872 x 10-5  Table 4- Solver details Solver Scheme SIMPLE Gradient Least Square Cell Based Pressure Second Order Momentum Second Order Upwind Turbulent Kinetic Energy First Order Upwind Turbulent Dissipation Rate First Order Upwind Turbulence models Standard k-ɛ (SKE) model Near wall treatment Enhanced wall treatment (EWT)  Pressure recovery coefficient (Cp) and flow uniformity index (σout) are the parameters used to assess the curve diffuser performance [2, 3, 12]: 𝐶𝐶𝐶𝐶 =  2(𝑃𝑃𝑜𝑜𝜕𝜕𝑜𝑜 − 𝑃𝑃𝑖𝑖𝑖𝑖)𝜌𝜌𝑉𝑉𝑖𝑖𝑖𝑖2 (1)   𝜎𝜎𝑜𝑜𝜕𝜕𝑜𝑜  =  �1𝑁𝑁− 1� (𝑉𝑉𝑖𝑖 − 𝑉𝑉𝑜𝑜𝜕𝜕𝑜𝑜)2𝑁𝑁𝑖𝑖=1 (2)  The Cp indicates how much kinetic energy is successfully converted to pressure energy. The main problem in achieving high pressure recovery is flow separation, which results in dissipation of energy and non-uniform flow distribution [14-17]. The σout is used to measure the dispersion of local velocity from the mean velocity. It strongly depends on the distribution of the core flow and the presence of secondary flow. The flow is considered uniform with the presence of secondary flow of less than 10% [17-19]. 3. Results and Discussion Effects of employing different types of vortex generator on curve diffuser performance are assessed. Ultimately, the most optimum configuration is proposed.  3.1 Numerical Validation Previous experimental work by Shariff et al. [14] was referred to validate the present simulation. As shown in Fig. 4, the present simulation model resembles well the experimental case with an average deviation percentage of approximately 0.72%.    Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  58  Fig. 4 - Numerical validation 3.2 Effect of Employing Vortex Generators on Flow Characteristics Effects of employing VGs (rectangle, triangle, tapered and wishbone) relative to a benchmark case and without VG on flow characteristics were observed as in Fig. 5. There is substantial back-flow to form separation occurred in the diffuser without VG. The installation of VGs is seen to reduce back-flow and secondary flow separation. These VGs function to disturb the flow of air flowing on the inner-wall (convex region), producing a vortex of air between the high and low energy flows [8]. This attracts a flow of high energy air from the free flow down into the boundary layer, increasing the energy of the boundary layer. High energy air attaches to the inner-wall much more effectively and thus improves the flow attached. The triangle VG is seen to assist the flow well to produce minimal back-flow separation, ultimately leads to the best flow uniformity of 2.14, as depicted in Table 5. This promises 25.4% of improvement relative to the benchmark case, without VG.         (a) (b) (c)    0.0000.0500.1000.1500.2000.2500.3000.3500.4000.4500.5005.786E+04 6.382E+04 1.027E+05 1.397E+05 1.775E+05Pressure recovery coeeficient, CpInflow Reynold Number (Rein)PreviousExperimentMuhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  59   (d) (e)  Fig. 5 - Flow characteristics of curve diffuser (a) without VG; (b) with rectangle VG; (c) with triangle VG; (d) with tapered VG; (e) with wishbone VG  Table 5 - Flow uniformity (𝝈𝝈𝒐𝒐𝒐𝒐𝒐𝒐) of curve diffuser with different vortex generators Vortex Generator Type Flow Uniformity  Index, 𝝈𝝈𝒐𝒐𝒐𝒐𝒐𝒐 Percentage of  Improvement (%) Without VG 2.87 - Rectangle 2.26 21.3 Triangle 2.14 25.4 Tapered 2.61 9.1 Wishbone 2.39 16.7  3.3 Effect of Employing Vortex Generators on Pressure Recovery Effects of employing VGs (rectangle, triangle, tapered and wishbone) relative to a benchmark case and without VG on pressure recovery are presented in Table 6. It is shown that the triangle VG produces the highest recovery of 0.25 corresponding to improvement of 31.3% relative to the benchmark case without VG. The triangle VG can minimize considerable losses due to form drag that often associates with flow separation. It was also observed that the vortex generators should be positioned exactly in the transition region of the boundary layer. However, the situation is complicated because of fact that the transition region depends on the flow conditions and the angle of attack. Therefore, future work shall be conducted to investigate the optimal mounting position of VGs.   Table 6 - Pressure recovery (𝑪𝑪𝒑𝒑) of curve diffuser with different vortex generators  Vortex Generator Type Recovery  Pressure, 𝑪𝑪𝒑𝒑 Percentage of  Improvement (%) Without VG 0.190 - Rectangle  0.243 27.6 Triangle 0.250 31.3 Tapered 0.205 7.6 Wishbone 0.230 20.7  4. Conclusion In conclusion, the effects of employing vortex generators on curve diffuser performance have successfully been investigated. Results show that there is a potential performance of applying vortex generators on curve diffuser. Triangle vortex generator provides the most optimum pressure recovery and flow uniformity of respectively 0.250 and 2.14. This promises an improvement of approximately 31.3% and 25.4% relative to the benchmark case, without vortex generator. Future work should be conducted to investigate further the optimal mounting position of vortex generators as it really affects the overall performance of diffuser.   Muhammad Musleh Anuar et al., Journal of Advanced Mechanical Engineering Applications, Vol. 2 No. 2 (2021), p. 53-60  60 Acknowledgement This research was supported by Ministry of Higher Education through Fundamental Research Grant Scheme (FRGS/1/2018/TK03/UTHM/02/7). 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Effect of Employing Vortex Generator on Curve Diffuser Performance

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Effect of Employing Vortex Generator on Curve Diffuser Performance

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