1、CFD Analysis of Axial Flow Fans for Radiator Cooling in Automobile EnginesAbstract：Radiators are installed in automobiles to removeheat from the coolant. The use of higher output engineswith highly compacted under hood packaging, the addition of new emission components, and aerodynamic front end sty
2、ling with narrow openings are creating a hostile thermal environment in the engine compartment.This results in a smaller volume of under hood cooling air.So, to handle higher volume flow rates of air axial flow fans are used to cool the radiators. These fans consume considerable amount of power from
3、 the engine and hence the performance of the axial flow fan is an important parameter of efficiency of the engine cooling system. CFD is used as a major design tool to investigate the major issues related to the performance of fan like volume flow rate and static pressure rise etc. The present work
4、investigates the characteristics of the flow over axial flow fans, which are used for radiator cooling using CFD code FLUENT 6.0. Comparison between CFD simulation and results from Blade Element Theory of two typical commercial axial flow fans with different blade profiles are presented. It is notic
5、ed that the fan efficiency can 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 into the radiator.Key words: Axial Flow Fan, CFD simulation, Turbulent kinetic ene
6、rgy, blade element01 IntroductionWhile running, an automobile engine produces large amount of heat which has to be dissipated, otherwise the engine gets overheated and finally resulting into failure. The basic purpose of a fan is to move a mass of gas or vapor at the desired velocity. For affecting
7、the air flow, fan develops a total pressure difference over the inlet and outlet air streams. The total pressure rise comprises of static pressure which depends on the blade profile, number of blades, pitch, hub space and aerodynamic characteristics of the fan impeller and dynamic pressure which dev
8、elops due to velocity or kinetic energy imparted to the air stream.2 Analysis of Axial Flow FansGlauert Blade Element Theory: A relatively simple method of predicting the performance of a fan is the use of Blade Element (BE) Theory. In this method the fan is divided into a number of independent sect
9、ions along the length. At each section a force balance is applied involving 2D section lift and drag with the thrust and torque produced by the section. At the same time a balance of axial and angular momentum is applied. This produces a set of non-linear equation that can solved by iteration for ea
10、ch blade section.The resulting valuse of section thrust and torque can be assumed to predict the overall performance of the fan.2.1 Blade Element SubdivisionThe lift and drag components normal to and parallel to the propeller disk are calculated and there by the contribution to thrust and torque of
11、the complete fan from this single element are also calculated. rh to rt are calculated making use of the following equations2.2 Inflow Factors:A major complexity in applying this theory arises when trying to determine the magnitude of the two flow components V0 and V2. V0 is roughly equal to the veh
12、icles forward velocity (Vi) but is increased by the fans own induced axial flow into a slipstream. V2 is roughly equal to the blade sections angular speed ( r) but is reduced slightly due to the swirling nature of the flow induced by the propeller. To calculate V0 and V2 accurately both axial and an
13、gular momentum balances must be applied to predict the induced flow effects on a given blade element. Figure 2 shows the induced flow components which are factors for increasing or decreasing the major flow components. The local flow velocity and the angle of attack for the blade section isV1 = (Vo2
14、 + V22) and = ( tan-1(Vo/V2)2.3 Axial and Angular Flow Conservation of Momentum:The governing principal of Conservation Momentum is applied for both axial and circumferential directions. For the axial direction, the change in flow momentum along a stream-tube starting upstream, passing through the p
15、ropeller at section AA and then moving off into the slipstream must equal the thrust produced by this element of the blade. To remove the unsteady effects due to the fans rotation, the stream-tube used is one covering the complete area of the fan disk swept out by the blade element and all variables
16、 are assumed to be time averaged values.F = (2r)dr Vo (V slipstream - Vi) Applying Bernoullis equation and conservation of momentum, for the three separate components of the tube, from free stream to face of disk, from rear of disk to slipstream far downstream and balancing pressure and area versus
17、thrust, the axial velocity at the disk will be the average of the free stream and slipstream velocities. V0 = (Vi + Vslipstream)/2, Hence Vslipstream = Vi (1 + 2a).Thus by solving the integral equations, F and T are calculated as followsF = 4Vi a (1+a) (rt -rh )/2 Since these final forms of the mome
18、ntum equation balance still contain the variables for element thrust and torque, they cannot be used directly to solve for inflow factors. With these approximate values of thrust and torque equations gives the improved estimates of the inflow factors a and b. This process is repeated until values fo
19、r a and b have converged to within a specified tolerance.3 Calculations:The theoretical analysis of axial flow fan is generally done by Glauret Blade Element theory. This theory is applied to calculate performance characteristics like volume flow rate of air, static pressure etc. For the evaluation
20、of performance characteristics of fans, one is having no swept blade (fan1) and the other is having forward swept blade (fan2) are considered for the study.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 f
21、actor 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, and the properties
22、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 co
23、efficient, 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 airS.No.InletVelocityV (m/s)AxialvelocityV (
24、m/s)TangentialVelocityV (m/s)Static pressurerise( pascal )10019.27223.1017.34303.33344.5816.81264.49466.1615.27208.07588.3612.62135.2061010.2111.2670.3871212.0211.0932.19Table 2:Performance Characteristics of Fan 2 at various inlet velocities of airS.No.InletVelocityV (m/s)AxialvelocityV (m/s)Tangen
25、tialVelocityV (m/s)Static pressurerise( pascal )10023.12385.93223.2619.82359.21344.9217.67336.48466.5716.73294.71588.6815.75242.3061010.8112.28155.0771212.2511.9685.334 CFD SimulationsThe control volume of fluid flow across fan is developed using GAMBIT1. Fan is located behind the radiator, in order
26、 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 d
27、ownstream of fan.The flow velocity of air after passing through radiator increases due to the suction created by the fans 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 o
28、f 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
29、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 Fan1The domain of fan1 contains 154,676 tetrahedral cells as shown in Figu
30、re 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- model.Fig. 4 Tetrahedral Meshing of control volume of air flow across Fan1The boundary conditions for the model are taken as follows: a)
31、 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 gri
32、d 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 () of air as 1.
33、789x10-05.Fig.5 Simulated Model of Control Volume of air flow across Fan 2Fig.6 Tetrahedral Meshing of control volume of air flow across Fan25 ConclusionThe static pressure rise of the fluid over fan2 having forward swept blades is more than that fan1 having unswept blades. The maximum value of stat
34、ic 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
35、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 effici
36、ency 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.汽车发动机轴流式冷却风扇的CFD分析摘要：散热器安装在汽车中，从冷却剂中去除热量。在发动机罩的包装下，增加了新的排放元件，以及空气动力学前端的设计，在发动机室中形成了一
37、个充满敌意的热环境。这将导致发动机罩冷却空气的体积变小。因此，为了控制空气轴流风机的流量，风机被用来冷却散热器。这些风扇从发动机中消耗相当大的功率，因此轴流风机的性能是发动机冷却系统效率的重要参数。CFD作为主要的设计工具来调查的主要问题相关的性能风扇体积流率和静态压力上升等。目前工作调查的特点,流在轴流风扇,用于散热器冷却使用CFD代码流利的6.0。摘要对两种典型商业轴流风机叶片元件理论的计算结果进行了比较。人们注意到，风扇顶端的一个固定环可以改善风扇的效率，避免从套管中回流。在进入散热器之前，发动机冷却剂的热量可以转换为使用轴流涡轮机。关键词:轴流风机、CFD仿真、紊流动能、叶片元件1介绍
38、在运行时，汽车发动机产生大量的热量，必须消耗掉，否则引擎会过热，最终导致失败。风扇的基本目的是以理想的速度移动大量的气体或蒸汽。为了影响气流，风机在进气和出水气流中产生总压差。总压强上升由静压力组成，这取决于叶片轮廓、叶片的数目、螺距、毂空间和风机叶轮的空气动力特性，以及由于速度或动能向气流传递而产生的动力压力。2 轴流风机的分析格劳厄特叶片元素理论:一种比较简单的预测扇子性能的方法是使用叶片元素(BE)理论。在这个方法中，扇形被分成若干独立的部分。在每一部分中，一个力平衡被应用于二维截面升力和阻力和由截面产生的扭矩。同时，还应用了轴向和角动量的平衡。这就产生了一组非线性方程，可以通过对每个叶
39、片节的迭代来解决。由此产生的截面推力和扭矩的结果可以用来预测风扇的整体性能。2.1叶片元素细分图2.1图2.2通过计算，算出了桨盘的升降分量，并对其进行了计算，并计算出了该单的全扇的推力和扭矩。到的计算方法是利用下列方程2.2流入的因素:当试图确定两个流分量V0和V2的大小时，应用该理论的一个主要复杂性就出现了。V0大致等于车辆的前进速度(Vi)，但增加的是风扇自己的感应轴流到一个滑流。V2大致等于叶片截面的角速度(r)，但由于螺旋桨的流动特性而减小了。为了计算V0和V2的精度，必须将轴向和角动量平衡应用于预测给定叶片单元的引流效应。图2显示了产生的流组件，这些组件是增加或减少主要流组件的因素
40、叶片截面的局部流速和攻角。V1 = (Vo2 + V22) and = ( tan-1(Vo/V2)2.3轴和角动量守恒动量守恒保护动量的管理原则适用于轴向和周向的方向。在轴向的方向上，流动量的变化沿著流管从上游开始，通过在a节的螺旋桨，然后移到滑流中，必须等于叶片的这个元素所产生的推力。为了消除风扇旋转造成的不稳定影响，使用的流管是一个覆盖风机盘的完整区域，所有的变量被假定为时间平均值。F = (2r)dr Vo (V slipstream - Vi) 应用伯努利方程和动量守恒,管的三个独立的组件,从自由流到磁盘,从后方的磁盘冲流到下游和平衡压力和面积与推力,轴向速度的磁盘将自由流和气流速
41、度的平均值。V0 = (Vi + Vslipstream)/2, Hence Vslipstream = Vi (1 + 2a).通过解积分方程，计算出F和T的积分F = 4Vi a (1+a) (rt -rh )/2 由于动量方程平衡的最终形式仍然包含元素推力和扭矩的变量，它们不能直接用于求解流入因子。用这些近似的推力和力矩方程给出了对流入因子a和b的改进估计。这个过程会重复，直到“a”和“b”的值收敛到一个指定的容忍度。3计算对轴流风机的理论分析一般采用青光眼叶片元件理论。该理论应用于计算空气、静态压力等性能指标，对风机性能特性进行评价，无风吹叶片(fan1)，另一种是前向叶片(fan2)
42、空气的进口速度为2米/秒，风扇以顺时针方向旋转1800转，方向为流动方向。在第一步中，对轴向流入因子a进行了假设，并计算了空气流速的轴向分量。同样的，漩涡流系数b也被计算，并且是速度V2的切线分量。计算了空气的总速度的大小和方向。知道的值V1,和风扇的叶片部分的属性(例如CD和CL),风机的推力由空气和所需的扭矩计算旋转风扇。最后是a和b。上面的过程会重复，直到“a”和“b”的值接近以前迭代的值。风机的流量系数、静压升高、速度分量、流量系数、风机的静压上升均为扇形和扇形2(表1和2)。表1:空气中的风机1的性能特性S.No.入口速度v（m/s）轴向速度v（m/s）切向速度v（m/s）静态压力
43、上升10019.27223.1017.34303.33344.5816.81264.49466.1615.27208.07588.3612.62135.2061010.2111.2670.3871212.0211.0932.19表2:风机2的性能特点在空气的各种入口速度S.No.入口速度v（m/s）轴向速度v（m/s）切向速度v（m/s）静态压力上升10023.12385.93223.2619.82359.21344.9217.67336.48466.5716.73294.71588.6815.75242.3061010.8112.28155.0771212.2511.9685.334 CFD
44、模拟通过GAMBIT1开发了风机的流体流量控制量。风扇位于散热器的后面，以引起散热器的气流。，一种诱导的通风风扇。否则，风扇本身就会对进入的空气产生阻力，如果它位于散热器的前面。散热器和风扇之间有30厘米的缺口。风扇的入口在上游30厘米处，排气口在风扇下游70厘米处。由于风机旋转运动所产生的吸力，空气流过散热器后的气流速度增加，在风机的前30厘米处的控制体积的收敛部分，在风扇周围的管道中，水流直冲直入。管道的作用是通过增加速度的轴向分量，从而使气流在风扇上流动。叶尖与导管之间的间隙为2.5厘米。风扇以顺时针方向转动，大约是正x轴。当气流通过管道向周围流动时，气流被强迫产生发散。图4所示，图3所
45、示的1D控制体积可通过GAMBIT1.3对3D模型进行模拟。图3模拟了扇形1的气流控制体积fan1的域包含154,676个四面体单元，如图4所示。对GAMBIT的模拟模型导出到FLUENT6.0，进行流体流分析。动荡的影响是通过标准k-模型进行建模。图4 .四面体网格的控制体积，在扇形1该模型的边界条件如下:a)进气-在x轴上的空气200米/ s速度b)湍流强度- 5%c)紊流粘度比- 0.05d)出口大气条件下的均匀压力e)Fan1 -移动参考帧以顺时针方向旋转1800 rpmf)管道壁-绝对框架内没有滑动。图5是为fan2所开发的用于前进叶片的计算网格。扇区被分为195,115四面体细胞。
46、边界条件也类似于fan1。空气的特性是假定为常数和空气的密度为1.2千克/立方米和动态粘度x10-05 1.789()的空气。图5模拟了风扇2的气流控制量图6四面体控制扇2内气流的控制体积5结论在扇形叶片上的流体的静压力上升超过了吹过叶片的扇形叶片。在fan1的情况下，静态压力的最大值是较高的，但是平均压力低于fan2，尽管它在上游产生了更多的真空，但是它无法向下游施加压力。这可能会导致风扇出口的“熄火”。静态压力随风扇的气流速度的增加而减小，但是fan2处理空气的压力比fan1大。所以，fan2比fan1更有效。如果空气流量是主要的标准，那么可以减少叶片的数量，以增加空气的自由流动面积。风扇顶端的固定环可以进一步改善风扇的效率，避免从套管中回流。在进入散热器之前，发动机冷却剂的热量可以转换为使用轴流涡轮机。14
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