To enhance the aerodynamic performance of aero engine blades, simulations and experiments regarding microtextures to reduce the flow loss on the blade surfaces were carried out. First, based on the axisymmetric characteristics of the impeller, a new simulation method was proposed to determine the aerodynamic parameters of the blade model through the comparison of flow field characteristics and simulation results. Second, the placement position and geometrical parameters (height, width, and spacing) of microtextures with lower energy loss were determined by our simulation of microtextures on the blade surface, and the drag reduction mechanism was analyzed. Triangular ribs with a height of 0.2 mm, a width of 0.3 mm, and a spacing of 0.2 mm exhibited the best drag reduction, reducing the energy loss coefficient and drag by 1.45% and 1.31% for a single blade, respectively. Finally, the blades with the optimal microtexture parameters were tested in the wind tunnel. The experimental results showed that the microtexture decreased energy loss by 3.7% for a single blade under 57° angle of attack and 136.24 m/s, which was favorable regarding the drag reduction performance of the impeller with 45 blades.
IntroductionIn order to survive, organisms in nature have undergone billions of years of evolution; their body structures have been adapted to the current environment and exhibit special functions on biological surfaces . For the purpose of drag reduction, valuable inspiration can be derived from rapidly moving animals, such as the “denticles” found on the surface of shark skin, which enable high-speed swimming , as well as the texture of bird feathers . The phenomenon of drag reduction can also be observed on the surface of plants. For example, there is a superhydrophobic structure on the surface of lotus leaves . A thin gas film captured by the superhydrophobic structure creates a slip interface between gas and liquid, which effectively improves the drag reduction and antifouling performance of lotus leaves . However, the structures on biological surfaces are rather complex and not directly applicable in practice. Therefore, researchers have explored the drag reduction mechanisms through replication or imitation of the microtexture found on biological surfaces; it was found that the contribution of microtextures to drag reduction primarily occurs within the boundary layer . Lang et al. constructed rectangular and sinusoidal grooves with 2 mm in width, 3 mm in depth, and 1 mm in spacing, thus mimicking the transverse grooves on the surface of dolphin skin. They observed the effect of the grooves on flow separation and boundary layer using digital particle image velocimetry. Xiao et al. analyzed the drag reduction mechanism of bionic microtextures and constructed simplified V-shaped, trapezoidal, and wavy ribs by grinding. Experimental and simulation studies on aeroengine blades with such microtextures showed that the drag reduction performance of wavy ribs is better than that of the other two structures. Tian et al. pointed out that, because of the complexity of microstructures on the shark skin surface, it is difficult to use a uniform method to characterize the skin surface. Triangular grooves or rectangular grooves can be used to simplify the microstructures on the shark skin surface to study the effects on hydrodynamics and aerodynamics.
Within these extremely small structures, a low-speed, stable fluid flow exists, which can mitigate turbulences and enhance the stability of fluid motion within the boundary layer, resulting in a reduction of frictional drag . Based on the above principles and for large-scale manufacturing, researchers imitated and simplified the microtextures of biological surfaces to form the structures with different sectional shapes, such as triangles, trapezoids, and ellipses , as shown in Figure 1.
Figure 1: Microtextures with different sectional shapes: (a) triangles; (b) trapezoids, and (c) ellipses. The variables s, h, L and U represent the spacing, height, length, and the inflow velocity, respectively.
The drag reduction effect of biomimetic microtextures can reduce friction and turbulence pulsation on blade surfaces, thus, improving the aerodynamic performance of blades . The research on drag reduction of microtextures on blade surfaces can be traced back to the 1980s. In 1982, Walsh et al. from NASA Langley Laboratory conducted a pioneering microtexture study on surfaces. Their experiments focused on longitudinal grooves with various shapes, revealing that symmetrical V-shaped grooves exhibited a remarkable drag reduction effect at low flow rates. The highest drag reduction rate (DRR) attained was 8%. Chamorro studied fans with a grooved surface and found that, under certain operating conditions, the drag reduction effect of local coverage on the textured blade surface surpassed that of a complete covering. Additionally, they designed microgrooves of various sizes on the suction surface to achieve the optimal drag reduction effect. Zhang et al. proposed a method to determine the placement position of microtextures by using finite element analysis. The suggested microtextures were arranged on blade surfaces and exhibited drag reduction compared to smooth blades. Mischo et al. improved the cooling capability of turbine blades by adding grooves to the blade tips of axial turbines. Experimental and numerical simulation results showed that the addition of grooves increased turbine efficiency by 0.2% and 0.38%, respectively. In order to reduce aerodynamic losses in turbines caused by tip leakage, Parkash et al. added grooves at the blade tips and verified their effectiveness through computational fluid dynamics (CFD) simulations. After the incorporation of grooves, the turbine efficiency improved by 0.1% to 0.2%.
It is evident that arranging microstructures on blade surfaces can optimize the aerodynamic performance of the blades, thereby achieving energy savings. However, the rational arrangement of microstructures on blade surfaces also requires investigation, as the shape, size, and placement position of microstructures can all affect drag reduction performance . Wu et al. investigated the effect of different sizes of triangular grooves on the drag of NACA 0012 airfoils, finding an optimal DRR of 9.65% when the microstructure dimensions were s = h = d = 0.1 mm. Liang et al. arranged various sizes of triangular microstructures on rotating disks and found through comparative analysis that microtextures with dimensions of s = 0.5 mm and h = 0.2 mm yielded a maximum DRR of 8.46%, with the DRR varying with changes in Reynolds number. Yang and Baeder designed wavy structures on the trailing edge (TE) of a wind turbine blade to reduce the recirculation flow and coherent vortex shedding; the influence of wave depth, TE thickness, and chord length on the drag force were investigated by numerical simulations. The results indicated that the maximum drag was observed at a ratio of wave depth/TE thickness = 0.25. Hossain et al. constructed inward and outward dimple-like structures on the upper surface of a NACA 4415 airfoil. The results from wind tunnel experiments showed that the dimples on the airfoil surface delayed flow separation; the inward dimples increased lift by 16.43% and reduced drag by 46.66%. However, while these researches have yielded encouraging achievements in the application of microtextures on blade surfaces, achieving good drag reduction performance relies heavily on the appropriate microtextures regarding size, type, and position. The determination of such microtextures is typically impractical for the following two reasons: (1) There is a lack of analysis on the flow field over smooth blades. The phenomenon of flow separation on blades occurs because of the complex curved surface in the air flow. The placement position of microstructures needs to be determined based on the locations of flow separation on blade surfaces. (2) The analysis of drag reduction performance is mostly based on numerical simulation results, with a lack of technology research regarding the processing of microtextures and of analysis of experimental results. On the one hand, the small size of microtextures may lead to significant errors in conventional machining methods, thereby increasing the difficulty of verifying microtexture drag reduction. On the other hand, the cost associated with experiments required for microtexture testing, such as wind tunnel tests, is high.
The present study employs numerical simulations, high precision milling, and wind tunnel experiments for solving the two problems discussed above. The main contributions of this article are as follows: (1) Based on the highly symmetrical characteristics of rotating machinery such as compressors, a new numerical simulation method for blade analysis was proposed to determine the flow field on the smooth blade surface and provide references for the placement of microtextures. (2) The influence of microstructure size on drag reduction of blade surfaces was analyzed to determine the optimal parameters for microstructures. The drag reduction mechanism of the microstructures was also analyzed based on simulation results. (3) A high-precision five-axis computerized numerical control (CNC) milling machine was used to process the microtextured blades. In order to obtain high-quality surfaces on the microtextured blades, three types of end milling tools were utilized to rough- and fine-mill blade and microtextures. The microtextured blades were tested in a wind tunnel to obtain drag reduction results.
MethodsThis section introduces the modeling of microtextures on blade surfaces, as well as the equipment and processes in the actual processing of microtextures. Finally, the wind tunnel experiment platform used to measure the drag reduction of the microtextured blades is described.
ImpellerThe research object of this paper is the impeller of an axial flow compressor, which consists of a hub and blades . In order to generate high pressure, axial flow compressors typically comprise multiple stages of impellers, as shown in Figure 2a. When the compressor is working, the air flow is driven by the rotation of the impeller from the inlet to the outlet, as shown in Figure 2b. The working conditions of the impeller mainly include the rotational speed and the environmental conditions (temperature and pressure) at inlet and outlet. The impeller considered in this paper has 45 blades and is designed to operate at a rotational speed of 2880 rpm. The operating environment for the impeller is at standard temperature (25 °C) and pressure (101325 Pa).
Modeling of microtextures on blade surfaceTo determine the geometry and position of the microtextures on the blade surface, a new simulation method is proposed based on the axisymmetric characteristics of rotating machinery. The complete flow domain model of the compressor was simplified into a single flow channel model, so that the flow field on the smooth blade surface could be obtained quickly and accurately. A flow diagram consisting of four steps is shown in Figure 3.
Figure 3: Flow chart of the simplified numerical simulation method. Images used courtesy of ANSYS, Inc.
Step 1The compressor model has rotational symmetry, and each blade is uniformly installed on the compressor. Therefore, the compressor model was evenly divided according to the number of blades to obtain a calculation domain model including a single blade and a single flow channel. In the simulation setup, the walls on both sides of the channels were set as periodic boundaries, which can simulate the flow domain with symmetry and make up for the calculation error caused by the simplified model. Through the above simplification, the calculation cost can be greatly reduced while ensuring calculation accuracy.
Step 2The microtexture placement position is determined according to the flow field of the smooth blade. Flow separation occurs during high-speed air flow over the curved blade surface. Here, a reasonable arrangement of microtexture can effectively improve the drag reduction. Therefore, CFD was employed to simulate a single channel model of the blade and the flow separation region was obtained. In order to verify the accuracy of the simulation calculation, the theoretical calculated value of the angle of attack was compared with the simulation results.
Step 3First, the drag reduction performance of four microtextures was compared by numerical simulations to determine the geometric type with the optimal drag reduction. Then, different widths (w), spacings (s), and heights (h) of the microtextures were compared to determine the scale range with drag reduction. In the simulation setup, the initial conditions and the flow domain are consistent with the single flow domain of the blade. The coefficient of friction and the DRR from the simulation results were compared to determine the geometric types and size ranges of the microtexture with drag reduction performance.
Step 4According to the flow field on the smooth blade surface, groove and rib microtextures were arranged before and after the flow separation point on the suction surface. The difference between grooves and ribs will described in the passage referring to step 4 in the Results and Discussion section. Also, based on the microtexture types and size range determined in step 3, the drag reduction results of grooves and ribs with different size parameters were compared to determine the combination of microtexture parameters with the best drag reduction performance. This parameter combination was employed for machining microtextures on the blade surface, and the microtextured blade was placed in the wind tunnel for experiments.
Details of step 1 and step 3Here, the details about the simulation setup method and the determination method of microtextures in step 1 and step 3 of the proposed method are described. To enhance readability, the relevant results from steps 2, 3, and 4 will be analyzed in the Results and Discussion section.
Step 1The whole impeller with 45 blades was divided evenly, that is, each single flow domain occupies an 8° fan-shaped flow channel, as shown in Figure 4. The length of the flow channel is 300 mm, and the bottom radius and top radii are 300 and 410 mm, respectively.
Figure 4: The size of flow domain and the single blade.
In the simulation setup, the k–epsilon (k-ε) model with enhanced wall treatment, which has better prediction results for rotation, boundary layer separation with large back pressure gradient, and backflow phenomena, was used for turbulence modeling in Ansys® CFX, Release 2020 R2 . The boundary conditions were set according to the working conditions of the impeller, that is, the inlet of the impeller was set as the velocity inlet with a value of 75 m/s; the outlet was set as the pressure outlet with the value of 101325 Pa. Periodic boundary conditions were applied on both sides of the flow domain, and the upper and lower walls were no-slip walls, as shown in Figure 4. According to the above impeller parameters, we used the method of speed triangle to calculate the theoretical value of the 0.5 blade height (span = 0.5). The relative airflow velocity was determined to be 130.67 m/s, with an angle of attack of 54.97°.
The height of the first layer mesh should be calculated according to the requirement of y+ (dimensionless distance from the wall), which is determined according to each turbulence model requirements. In order to obtain good mesh quality and meet the k-ε model requirements used in this paper, y+ needs to be between 1 and 5 . In this paper, the height of the first layer mesh is 0.005 mm, and y+ = 1.5, as shown in Figure 5.
Figure 5: Meshed model of (a) a smooth blade surface and (b) a microtextured surface with 0.005 mm height of the first layer mesh according to y+ = 1.5. Images used courtesy of ANSYS, Inc.
Step 3The method of dimensionless size was used to determine the size range of microtextures with drag reduction performance . In this section, the microtexture sizes (i.e., h, w, and s) were determined according to the boundary layer theory as shown in Figure 6.
Figure 6: Characteristic parameters of (a) grooves and (b) ribs.
The dimensionless size calculation formula of microtextures with drag reduction performance are as follows :
where μ is the dynamic viscosity, v is the kinematic viscosity, u is the average flow velocity, uτ is the wall stress shear rate, τw is the wall shear stress, and ρ is the density.
where δ is the thickness of the boundary layer. The flow condition around the flat plate wall can be determined by the dimensionless local Reynolds number.
where x is the distance from the inlet along the fluid flow direction. For Rex < 3 × 105, the flow in the boundary layer is laminar, and the following equation yields δL:
The flow is turbulent if Rex > 3 × 106, and the thickness of δT is calculated as:
The flow in the boundary layer is transitional when Rex is between 3 × 105 and 3 × 106. The turbulent area is selected for the arrangement of microtextures. Therefore, Equation 9 is entered into Equation 6:
Entering Equation 10 into Equation 4 gives:
Entering Equation 11 into Equation 1, Equation 2, and Equation 3, respectively, yields:
Based on Equations 12–14, the dimensionless sizes corresponding to microtextures under different Rex can be obtained; Rex needs to be determined according to the flow velocity and the characteristic dimensions (x) of the calculation domain. For our blades, u is 75 m/s, and the maximum value of x is 300 mm, as shown in Figure 4. Therefore, according to Equation 13 and the range of h+ given in Table 1, the values of h of the microtextures with drag reduction performance were first determined. The range determination for w and s in Table 1 will be described in the Results and Discussion section.
Table 1: Microtexture parameters.
Parameter Values direction spanwise, longitudinal type grooves, ribs height (h) 5 < h+ < 25 width (w) <3h spacing (s) <3h position front, back Experimental methodThe overall process of experiments involves machining the microtextures on the blade surface and conducting experiments with the microtextured blades in a wind tunnel. The Results and Discussion section will give details about the determined microtexture types and sizes.
Experimental equipmentA list of equipment used in the experiment is shown in Table 2.
Table 2: Overview of the equipment used in the experiment.
Equipment name Model Purpose Manufacturer five-axis CNC machine tool JDGR400-A13S processing of blades and microtextures Beijing Jingdiao Technology Group Co., LTD, China. flat end mill tool ⌀8 × 37 × ⌀8 × 81 × 3Fa processing of blades Shanghai Mituo CNC Equipment Co., Ltd, China. ball end mill ⌀0.3 × 0.6 × ⌀4 × 50 × 2Fa roughening of microtextures MISUMI (China) Precision Machinery Trading Co., Ltd. ball end mill ⌀0.2 × 0.3 × ⌀4 × 50 × 2Fa finishing of microtextures MISUMI (China) Precision Machinery Trading Co., Ltd. trinocular stereo microscope JSZ6S observing the processed blades Nanjing Jinsong Optical Instrument Co., Ltd, China. three-dimensional video microscope KH-7700 high-precision 3D imaging QUESTAR Corporation, Japan intermittent wind tunnel customized equipment aerodynamic performance testing Nanjing Power Tiger Electromechanical Technology Co., Ltd, China.a⌀(tool diameter in mm) × (cutting edge length in mm) × ⌀(shank diameter in mm) × (overall length in mm) × (number of flutes)F.
Microtexture processing of blade surfacesA JDGR400-A13S five-axis CNC machine tool was used to process the blade and the microtextures; the processing steps are shown in Figure 7. First, the blank blade was installed in the machine tool, the size of the blank is h × l × w = 140 × 100 × 25 mm3. Second, the end milling tool was used to mill the blank roughly to improve the processing efficiency. Third, the high-quality blade models were obtained by finishing the rough blade models. Finally, the microtextures were machined on the blade surfaces; this step was also divided into roughening and finishing because the end milling tool would break if the tool with a smaller radius was used to process the microtextures directly. The tool parameters used in different processing stages are given in Table 3.
Figure 7: The manufacturing procedure of the microtextured blade.
Table 3: Tool parameters in different machining stages.
Process stage Tools Tool radius (mm) Number of flutes roughening of blades flat end mill 4 3 finishing of blades flat end mill 4 3 roughening of microtextures ball end mill 0.15 2 finishing for microtextures ball end mill 0.1 2 Wind tunnel platformThe experiment was conducted at the intermittent wind tunnel platform at the College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics. Pictures and a schematic diagram of the wind tunnel test platform are shown in Figure 8. Figure 8b shows that the wake measurement device consisting of a three-hole probe, a motor, and a guide rail. Ten probes were utilized to measure the wake, while the motor facilitates control and adjustment of their position. The total pressure (TP), static pressure (P) and velocity of the air flow (V) in the experiment were obtained by the three-hole probe measuring device. These results can be calculated according to Equations S5–S8 in Supporting Information File 1 to obtain Mach number (Ma) and energy loss coefficient (ξ). The angle of attack can be set by controlling the motor and, thus, turning the disc (Figure 8c). In Figure 8c, the inlet of the test platform is connected with the high-pressure gas source, which is a 100 m3 high-pressure gas tank with a maximum of 25 atm; the air extraction source is a 200 m3 vacuum tank with a minimum of 0.1 atm.
Figure 8: Experimental platform and schematic diagram. (a) Wind tunnel test platform. (b) Three-hole probe measuring device. (c) Schematic diagram of the wind tunnel test platform.
Experimental stepsTo ensure the accuracy of experiments, the velocity and angle of attack for blade heights of 0.25, 0.5, and 0.75 were selected to carry out multiple tests to verify the drag reduction effect of the microtextured blade. The specific steps of the experiment are as follows: (1) preparation of two blades in contrast, that is, one smooth blade and another blade with a microtexture on the surface; (2) test the smooth blade first; adjust the wind tunnel flow velocity and the angle of attack to 123.98 m/s and 52.8°, respectively; (3) measurement of the TP and P at the inlet and outlet, respectively, and calculation of ξ and observation of the wake loss distribution; (4) change of velocity and angle of attack to 130.67 m/s and 54.8°, respectively, and continuation according to step 3; (5) change of velocity and angle of attack to 137.54 m/s and 57.0°, respectively, and continuation according to step 3; (6) installation of the blades with microtextures and repetition of steps 3–5. The results obtained from the above steps will discussed in the “Results of the experiments” section.
Results and Discussion Determined microtexturesThe details about the results obtained in steps 2–4 of the simulation method described in the Methods section are given here. In step 2, the simulation results of a single impeller blade were analyzed to determine the flow field characteristics around the blade, such as angle of attack, velocity, and air flow state. As shown in Figure 9a, we sliced the calculation domain to analyze the simulation results and selected the green plane at 0.25 of the total length of the flow domain in streamwise direction. In the radial direction, three curved surfaces with spans of 0.25, 0.50, and 0.75 of the blade height, progressing from the bottom to the top, were chosen for analyzing the velocity distribution at the intersection between these curved surfaces and the green plane. Figure 9b indicates an increase in the peripheral speed of the blade as the radius increases. The average velocity at span = 0.5 is 131.5 m/s, with an error of only 0.6% from the theoretical value of 130.67 m/s. This result serves as evidence supporting the reliability of the simplified simulation method; hence, the velocity in the local area simulation was set at 130.67 m/s.
Figure 9: Single blade and simulation results. (a) Slices of the calculation domain. (b) Simulation results of flow velocity at span = 0.25, 0.5, and 0.75. Figure 9a used courtesy of ANSYS, Inc.
In Figure 10, two-dimensional flow streamlines of the curved surface at span = 0.25, 0.5, and 0.75 are analyzed. The relative velocity angle of airflow and blade changes as the blades rotate. A comparison of simulation results is shown in Table 4, the error between the simulation value and the theoretical value of the angle of attack is only 0.31%. Hence, the flow field of the smooth blade surface at span = 0.5 is further analyzed.
Figure 10: Simulation values of attack angle at (a) span = 0.25, (b) span = 0.50, and (c) span = 0.75. x is the axial direction, y is the radial direction, and z is the rotation direction of the blade. Images used courtesy of ANSYS, Inc.
Table 4: The comparison of simulated and theoretical values of the angle of attack.
Surface Simulation values (°) Theoretical values (°) Error (%) span = 0.25 52.30 52.80 0.95 span = 0.50 54.80 54.97 0.31 span = 0.75 56.50 57.00 0.88Table 5 presents the resistance results of the smooth blade, where the total drag (Td) in the direction of airflow was divided into pressure drag (Pd) and friction drag (Fd). The primary impact of the microtextures is to modify the flow state of the boundary layer near the wall, reducing of Fd. For our blade, the contribution of Fd is small, accounting for only 2.39% of Td. Thus, this paper primarily focuses on assessing the influence of microtextures on system energy loss.
Table 5: Aerodynamic parameters of the smooth blade.
Type Pd (N) Fd (N) Td (N) Fd/Td (%) smooth 1.3288 0.0325 1.3613 2.39From the leading edge to the trailing edge of the blade, the pressure surface exhibits a favorable pressure gradient, whereas the suction surface presents an adverse pressure gradient. This observation is complemented by Figure 11a, which illustrates that the turbulent kinetic energy (k) on the blade surface is small. The position of X = 0 mm in Figure 11a corresponds to the highest point of the blade surface (point B in step 4 of Figure 3). Observing the distribution of k on the periodic boundary shows that k is zero in the front region of the blade (X ≤ 0 mm). However, k begins to rise sharply from X = 40 mm, indicating that boundary layer separation at this position generates turbulence. The peak of k indicates that this position corresponds to the center of the turbulent vortex. According to Figure 11b, the turbulent vortices manifest on the adverse pressure surface within the system. Therefore, the microtextures were arranged on the adverse pressure surface.
Figure 11: The distribution of (a) k and (b) the flow field on the blade surface. Figure 11b used courtesy of ANSYS, Inc.
Based on Figure 11, the air flow separation is initiated at X = 30 mm. As a result, the suction surface can be divided into two regions (front and back) at X = 30 mm, which serves as the critical point to discuss the drag reduction performance of the microtexture.
In step 3, the geometric types and size ranges of the microtextures were determined. Because of the high flow velocity on the blade surface, transverse microtextures would significantly increase the projected area in the flow direction, leading to a drastic increase in Pd. Therefore, longitudinal microtextures were considered here. Because the flow projection area in longitudinal microtextures is small, F
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