发布时间 : 星期日 文章汽车发动机轴流式冷却风扇的CFD分析外文文献翻译、中英文翻译、外文翻译更新完毕开始阅读d6b5cd30ec630b1c59eef8c75fbfc77da269972d
The inlet velocity of air is 2 m/s and the fan is rotating at 1800 rpm clockwise, in the direction of flow. In the first step the axial inflow factor ‘a’ is assumed and the axial component of velocity of air, Vo is calculated. Similarly the swirl flow factor ‘b’ is calculated and also the tangential component of velocity V2. The magnitude and direction of resultant velocity of air, V1 is calculated. Knowing the values of V1, Φ and the properties of blade section of the fan (i.e. CD and CL ), the thrust developed by the fan on air and the torque required to rotate the fan are calculated. And finally ‘a’ and ‘b’ are calculated. The above procedure is repeated until the values of ‘a’ and ‘b’ are close to the values from previous iteration. The flow coefficient, static pressure rise across the fan, velocity components, flow coefficient, static pressure rise across the fan are obtained as follows for fan1 and fan 2 (tables 1 & 2).
Table 1:Performance characteristics of Fan 1 at variousinlet velocities of air
Inlet Axial Tangential Static S.No. Velocity velocity Velocity pressure V (m/s) V (m/s) V (m/s) rise ( pascal ) 1 2 3 4 5 6 7
0 2 4 6 8 10 12 0 3.10 4.58 6.16 8.36 10.21 12.02
Table 2:Performance Characteristics of Fan 2 at various inlet velocities of air Inlet Axial Tangential Static S.No. Velocity velocity Velocity pressure V (m/s) rise V (m/s) V (m/s) ( pascal ) 1 2 3 4 5 6 0 2 4 6 8 10 0 3.26 4.92 6.57 8.68 10.81 5
19.27 17.34 16.81 15.27 12.62 11.26 11.09 303.33 264.49 208.07 135.20 70.38 32.19 23.12 19.82 17.67 16.73 15.75 12.28 385.93 359.21 336.48 294.71 242.30 155.07 7
12 12.25 11.96 85.33 4 CFD Simulations
The control volume of fluid flow across fan is developed using GAMBIT1. Fan is located behind the radiator, in order to induce the air flow across the radiator (i.e., an induced draught fan). Otherwise the fan itself offers resistance to incoming air if it is located in front of the radiator. There is 30cm gap between radiator and the fan. The inlet to the fan is taken at 30cm upstream and the outlet is at 70cm downstream of fan.
The flow velocity of air after passing through radiator increases due to the suction created by the fan’s rotational movement which is the convergent portion of control volume at 30cm ahead of fan and the flow becomes straight in the duct that is surrounded by the fan. The function of the duct is to direct the flow axially, there by increasing the axial component of velocity and hence the volume flow rate across the fan. The clearance between blade tip and duct is 2.5cm. The fan is rotating at 1800 rpm clockwise direction about positive X-axis. The air flow being forced by the fan becomes divergent while passing from duct to surroundings. The 1D control volume shown in fig. 3 can be simulated to a 3D model using GAMBIT1.3, is shown in fig.4.
Fig.3 Simulated Model of Control Volume of air flow across Fan1
The domain of fan1 contains 154,676 tetrahedral cells as shown in Figure 4. The simulate model in GAMBIT is exported to FLUENT6.0 where the fluid flow analysis is carried out. The effects of turbulences were modeled using standard k-ε model.
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Fig. 4 Tetrahedral Meshing of control volume of air flow across Fan1
The boundary conditions for the model are taken as follows: a) Inlet- velocity of air 2m/s along X-axis b) Turbulent intensity-5% c) Turbulent viscosity ratio - 0.05
d) Outlet- uniform pressure at atmospheric conditions
e) Fan1- moving reference frame rotating at 1800 rpm clockwise about f) Duct wall- no slip in absolute frame.
Figure 5 is the computational grid developed for fan2 having forward swept blades. The fan domain is divided into 195,115 tetrahedral cells. The boundary conditions are also similar to that of fan1. The properties of air are assumed to be constant and the density of air is taken as 1.2kg/m3 and the dynamic viscosity (μ) of air as 1.789x10-05.
Fig.5 Simulated Model of Control Volume of air flow across Fan 2
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Fig.6 Tetrahedral Meshing of control volume of air flow across Fan2
5 Conclusion
The static pressure rise of the fluid over fan2 having forward swept blades is more than that fan1 having unswept blades. The maximum value of static pressure is higher in case of fan1 but the average pressure is lower than that of fan2 and though it creates more vacuum in upstream side it is not able to pressurize into downstream. This may lead to ‘stall’ the flow at fan outlet. The static pressure decreases with increase in air inlet velocity for both fans but fan2 handles air at higher pressures than that of fan1. So, fan2 is more efficient than fan1 at any volume flow rates of air. If the volume flow rate of air is the main criteria then the number of blades can be reduced in order to increase the free flow area for air. The fan efficiency can further be improved by a fixed ring at the fan tip which avoids the back flow from casing. The heat from the engine coolant can be converted into work using an Axial-flow turbine before entering in radiator.
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