Cooling Fan on the Aerodynamic Performance

We studied the effect of the structure parameters of engine annular cooling fan with 11 outer ring on the aerodynamic performance by means of experiments and model simulation in 12 fluent®. Firstly, based on the experiment, a computational model is developed to calculate and 13 analyze the aerodynamic performance of the tested annular fan. The model is validated by 14 comparing the test results with the calculated data. Besides, the aerodynamic performance 15 differences between two types of fans (common fan without outer ring and annular fan with outer 16 ring) are discussed. Based on the computational model, the relation between aerodynamic 17 performance and the outer ring structure parameters are investigated. The results show that the 18 relative parameter on the axial direction has great influence on the aerodynamic performance; 19 while the effect of radial relative parameter is minor. In addition, the outer ring with arc chamfer 20 structure in the downstream side can improve its static pressure efficiency effectively. 21


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The performance of engine cooling fan includes aerodynamic performance and aerodynamic 25 noise [1]. And the former is the primary consideration for designers, it includes the flow rate, the 26 static pressure, the fan power and the static pressure efficiency. In the premise of a constant flow 27 rate, the static pressure efficiency is expressed as: where η is the static pressure efficiency, q is the flow rate, psp is the static pressure, a is the 29 coefficient, and P is the power of the fan. Equation (1) shows that the static pressure efficiency takes

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For annular cooling fan, an outer ring is added around the blade tip as shown in Figure. 1b, 34 which is different from the common cooling fan shown in Figure.

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The main motivation of this paper is to present some proposals of outer ring design for annular

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Based on Ref. [1], an aerodynamic performance test bench for the cooling fan is set up, and the 115 schematic diagram is shown in Figure. 3. The test bench mainly consists of three parts: the air-flow 116 pipe, the mechanical transmission and the test control board.
where αε is the air flow coefficient number, and it approximately equals 0.96 in this paper [1]; d 131 is the diameter of the duct section where U-type pressure gauge (pitot tube) locates at the inlet side; 132 psi is the static pressure at the same section, which is measured by the gauge 1 as shown in Figure. 3;

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and ρ is the air density.

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The total pressure (ptp) of the air at a certain section of the bench is composed of static pressure (psp) 135 and dynamic pressure (pdp). The total pressure ptp can be measured by the pitot tube, so do the psp, 136 which is generated from pressure perpendicular to the duct wall. Based on the above description, pdp 137 can be calculated which is the difference between ptp and psp. The total pressure of the cooling fan is 138 defined by the total pressure difference between the section of the outlet and inlet, which can be 139 expressed as: The static pressure of cooling fan is defined by the difference between the total pressure and the 141 dynamic pressure at the outlet section, which can be expressed as: Because of the complex flow-field of bench ducts, the pressure loss (Δp) is composed of frictional 143 resistance loss (Δpf) and local pressure loss (Δpl), based on the test experience [1], the Δp can be 144 expressed as: Based on (3)-(5), the static pressure of the fan can be expressed as: In all above equations, the subscripts 1 and 2 represent the inlet and the outlet section.
effective fan power (Pesp). The fan power of the fan is equal to the energy transmitted from the 150 driving motor, which can be expressed as: Here, T is the output torque of the driving motor, n is the rotating speed of the cooling fan.

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These values are all measured by the speed-torque sensor as shown in Figure. 3.

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The effective fan power is equal to the energy absorbed by air when it flows through the 154 rotating fan during per unit time, which can be expressed as:

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As shown in Figure.

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Under a steady state numerical calculation, the grid independence is necessary to be verified.

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Four grid plans are presented in Table 1.

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In this part, the static pressure is used to be the evaluation index to determine which plan is 176 finally to be adopted. The calculation is implemented with the flow rate is 2.81m 3 /s. The static 177 pressure value differences of the four plans are less than 2%. In order to reduce the computational 178 burden, the Plan 2 with the fewest grids is considered in this paper.

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The multi-reference frame method (MRF) is a robust algorithm, which is computationally efficient 181 with acceptable accuracy. Therefore, the MRF method is adopted to calculate the aerodynamic

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The external physical characteristics of annular fan with outer ring are shown in Figure. 7.

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Three dimension structure parameters about the outer ring will be described in this section: the axial 214 projection width (W), the diameter (D) and its shape. First of all, all axial lengths in this paper are projected along the airflow direction. i is a custom 217 axial structure parameter being discussed in this paper that is termed as "aperture opening ratio",

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and it can be expressed as: In Figure. 7, Wf is the total axial length of the annular cooling fan, W is the width of the outer 220 ring, and Wb is the axial length of the fan blade.
between the front-edge-wall of the outer ring and the blade trailing edge. Therefore, i is always lager 223 than zero.

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As aforementioned, the winglet that is added on fan blades can change the inherent frequency 226 of the fan by changing its radial installation position. The outer ring is evolved from it to improve the 227 aerodynamic performance in this paper. Therefore, Lr is another custom radial structure parameter 228 termed as "overhang length" in this paper, which is expressed as: Based on Figure.

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The flow state can be changed or even improved by setting flared structure for the duct flow 233 field. Based on the flow direction, the flared structure is divided into forward and reverse directions.

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As shown in Figure.         As shown in Figure. 16, the static pressure contour profiles on blades of the 2 fans are similar, 296 but negative pressure region is observed on the blade tip marked in Figure. 16(b). The region 297 mentioned above is produced for the complex blade shape and high rotating speed. However, the 298 pressure value of this region is lower than the inside of the outer ring. Therefore, the answer is that a 299 quantifiable pressure difference is produced here that leads to another back flow around the outer 300 ring as shown in Figure.      All three fans are tested on the test pipeline bench and the test data are obtained to be 338 compared. From Figure. 20, it can be found that both the static pressure and the fan power are 339 significantly increased after adding the outer ring. At the flow rate of 3.8m 3 /s, the static pressure 340 increases with a degrees of 15.4% and 9.6% for the final annular fan and the interim fan, respectively.

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However, the static pressure efficiency for the interim cooling fan is lower than the common fan at 342 all flow rates, and the largest decline of it reaches 6.1% at the flow rate of 2.9 m 3 /s. The main reason of 343 it is that the outer ring has a wider axial projection width and a larger back-flow area would be 344 produced based on the pattern shown in Figure. 13.

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In conclusion, based on the evaluation index of static pressure, the aerodynamic performance of 346 annular cooling fan with outer ring is better than common fan without outer ring. However, the 347 aerodynamic performance of the cooling fan cannot be always improved by adding an outer ring.

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In this paper, the aerodynamic performance indexes of engine axial flow cooling fan are described.

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The test and simulation methods of the cooling fan are introduced. Besides, the accuracy of the 351 simulation model is verified.
Based on the "small winglet" structure, annular cooling fan with outer ring is designed. Three 353 important structural parameters of the outer ring are defined and discussed.

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The aperture opening ratio and shape of the outer ring have great influence on the aerodynamic 355 performance of the annular cooling fan, while the length of blade extension has minor influence on 356 the performance. Therefore, when designing the outer ring, the aperture opening ration and shape of 357 the outer ring are the primary parameters to be considered.

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If the flow rate remains constant, the aerodynamic performance increase first and then decrease 359 with the increase of aperture opening ratio. Therefore, it is necessary ti select the appropriate 360 parameter value of the structure to optimize the aerodynamic performance.

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The downstream side of the outer ring can smoothly guide the air flow. Because the diameter of 362 the outer ring increases gradually along the flow direction, it can accelerate air velocity, reduce the 363 possibility of backflow and improve the flow condition.