Effect of blade dip angle on internal characteristic of hydraulic retarder

Chen-guang LAI，Meng-hua DUAN，Yan ZHUANG，Yong-yan CHEN（College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China）（Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan）

Effect of blade dip angle on internal characteristic of hydraulic retarder

Chen-guang LAI1，2，Meng-hua DUAN1*，
Yan ZHUANG1，Yong-yan CHEN1
（1College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China）
（2Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan）

Based on CFD software platform，the joint surface between the rotor and stator of the hydraulic retarder was defined as interface to transfer the flow data of different subdomains by moving mesh theory.Numerical simulation of different dip angle of hydraulic retarder was performed with the RNG k-ε turbulence model and SIMPLEC algorithm simultaneously.The characteristics of velocity and pressure distribution were analyzed through the numerical simulation and post-processing.By the calculation braking torque and the simulation result，the brake increases with the blade dip angle decreasing，but the brake torque decreases when the dip angle is smaller than the angle of 43 degree.

Hydraulic retarder，Blade dip angle，CFD，Numerical simulation，Braking torque

1 Introduction

Most of car brakes are friction brakes use the friction between the working parts to produce braking force，and converting the car’s kinetic energy to thermal energy［1］.Braking torque is sharply reduced when the temperature of the brake shoe friction surface increase.So wheel brakes are not suitable for long time work.This problem can be solved by hydraulic retarder which is an important auxiliary braking device［2］.It has been used as a standard configuration for many domestic heavy-duty vehicles，buses and construction vehicles［3］.The calculation and analysis the hydraulic retarder in domestic are based on beam theory.This theory can express the relationship between the design parameters and the macroscopic properties of the product at a certain level.But it’s inadequate to fully reveal the internal characteristics of the flow field and the variation between the internal and external characteristics［4］.Blade Angle is one of the most important factors that affect the hydraulic retarder braking performance，so in this study，CFD was used to analyze the impact of different blade angle on the performance of hydraulic retarder.

2 Hydraulic retarder works

A schematic representation of the structure of hydraulic retarder is shown in Figure 1［5］.When the hydraulic retarder works，driven by high-pressure air，hydraulic oil is pressed into the tubes and pass through the tubes to the working chamber of hydraulic retarder.Then，retarder wheel which is connected to the vehicle drive system is rotated at high speed. Driven by the dynamic wheel，the oil that enters retarder inside movement in the axial direction and blade.Oil that move in the blade direction dumped the fixed wheel.Fixed wheel blade reacts to oil and hydraulic oil acting on the wheel.This prevents rotation of the drive wheels，and result in brake torque to decelerate the vehicle.In this process，oil temperatures continue to rise，and mechanical energy of vehicle can be converted into internal energy of the oil. Under the action of pressure，high-temperature oil return reservoir chamber through the tubing.In this process，the oil heat is taken away by the cooling system［7］.

3 The model of internal flow field in hydraulic retarder

3.1 Control equation

For the flow area，according to the laws of the Reynolds Average Navier-Stokes，the continuity equation and momentum equation as follow：

Where，ρ is the liquid density，ui，ujis the average velocity； is the arterial velocity components；p is the average pressure；ρuthe Reynolds stress；μis the dynamic viscosity，with different processing method for Reynolds stress cause different turbulence model.

This paper used the RNG k-ε turbulence model，which is appropriate for the situation of bending wall and repair turbulence intensity at the time of calculate.the RNG k-ε turbulence model is good at solving the movement problem of rotating，whirlpool and strong streamline bending.The k equation and ε equation as follow：

Where，Gkis the turbulence kinetic energy caused by average velocity gradient，Gbis the turbulence kinetic energy caused by buoyancy，Gb=0 for incompressible flow，C1ε，C2ε，C3εis the empirical formula，C1ε= 1.42，C2ε=1.92，C3ε=1.68，μeff=μ+μt，the situation of high Reynolds μtcan instead μeff［7］.

3.2 Calculation model

The internal flow，is shown in Fig.1，of hydraulic retarder is extracted by design model of Ansys software.The stator and rotor have the same dip angle，they have 36 and 31 pieces blades，respectively.

Fig.1 Extraction modeI of hydrauIic retarder

There is a simple explanation about the blade front rake of hydraulic retarder.The inlet and the outlet of inclined blade is radial，and the angle is shaped by the plane of each inclined blade laid and the axis plane that through the inlet and outlet of the blades. This angle is called blade dip angle.The tilt direction of the front tilt blade and the rotation direction of the rotor are the same.As shown in Fig.2，the α is the blade front rake.

Fig.2 The schematic of the bIade of hydrauIic retarder

The parameters of model are shown in Table 1.

TabIe1 Parameter of retarder internaI fIow fieId

ANSYS pre-processing，software ICEM is used to generate the grid of the model，the total element of grid is 703，800.

The model is simplified as follows before starting CFD numerical simulation.

1）The blades and shell of retarder is regarded as rigid body.

2）Ignore the temperature change of working liquid at runtime.

3）Ignore the leakage of working liquid between wheels.

4）In order to avoid collision between the stator and the rotor，there is an axial interval about 3 mm between two wheels，that is about 1%of the entire circle diameter.Because of that the impact of the interval is tiny on the whole flow field，it is regarded as the extension of runner on both sides.

5）The working cavity is filled with oil，which is need to meet the requirements of SAE10W30 standards.The density of the oil is 860 kg/m3，and dynamic viscosity is 0.006 1 kg/（N·m）.

In the working processes the rotor blade makes oil ratate with high-speed and the oil circulatory flow in the flow passage which is composed of stator and rotor.Oil either move with working wheel or move relative to the direction of blade in flow channel.In order to simulate the phenomenon that working liquid flow in and out at the same time better，the sliding mesh method is adopted.

3.3 Numerical calculation method

In numerical simulation，the pressure-based.SIMPLE algorithm method is still the most widely applied，the SIMPLEC algorithm is used in order to improve the computation speed and be convenient for solutions. Correction SIMPLEC algorithm can solve the speed and pressure revised inconsistent problem better，the faster calculation speed can reduce the overall computation time.The turbulence model used is RNG k-ε model.

4 The results of numerical calculation analysis

Different blades dip Angle of hydraulic retarder are simulated under the premise of full of liquid and rotor speed of 1 200 r/min.The braking torque is gained in blade dip angle that change from 36 degree to 51 degree is gained，as shown in Fig.3.

Fig.3 The brake torque at different bIade dip angIe

The static pressure of different blade dip angle at the string plane of radius R=160 mm is shown in Fig. 4.From the Fig.4 we can know that the static pressure distribution trend about the same when the dip angle is different.Due to the impact of a large amount of fluid in the angle at the outer and pressure surface of the blade，fluid change the direction and produce a high pressure area in this place（B，D，F）.The locally low pressure area is formed by high speed working fluid at the suction surface and blade twist（A，C，E）. The largest local high pressure occur at the blade angle of 43°，and the pressure at the blade angle of 41° is the minimum，in the flow field，the maximum pressure gradient appear at blade angle of 43°，that of 45° is second，and that of 41°is the minimum.

Fig.4 The static pressure contour of string surface

Fig.5 is the velocity vector of string surface.The string plane is located close to the outer ring，so the fluid flow direction is from the rotor to the stator blades.The high speed oil obtain energy from rotor flow toward the pressure surface of the stator.The flow angle change drastically，that result in flow separation phenomenon in the liquid flow and the suction surface of the blade，and flow separation often occurs in the entire blade surface，that lead to great loss.The greater relative velocity of circular circle center，the greater impact angle of the flow，it makes the shock loss bigger.The maximum speed is 26.2 m/s and the relative speed difference is the largest when the blade angle is 43°as shown in the figure（b），thus compared（a）and（c）the energy loss is larger，easier to produce braking torque.

Fig.5 The reIative veIocity vector distribution of string surface

5 Conclusions

In this paper，based on the numerical simulation on retarder of different blade angle，the velocity field and pressure field in the interal flow field is analyzed. The result of brake torque on different blade angle are compared.The results show that，in the range from 35°to 51°，as the blade angle increases，the speed of the working fluid in the loop center increases first then decrease，and the same to braking torque.

Acknowledgements

This paper is support by National Natural Science Foundation of China（51305477）；Project Supported by Program for Innovation Team Building at Institutions of Higher Education in Chongqing（KJTD201319）；Prat of the work was carried out under the Collaborative Research Project of the Institute of Fluid Science，Tohoku University，Japan.

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葉片傾角對液力緩速器內流場的影響分析

賴晨光1，2，段孟華1*，莊 嚴1，陳永燕1
1.重慶理工大學車輛工程學院，重慶400054
2.Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan

基于CFD軟件平臺，利用滑移網格的方法，將液力緩速器定子和轉子之間的接合面命名為網格分界面（interface），用它來傳遞不同子域間的工作液的流動信息。采用了RNG k-ε模型和SIMPLEC算法對不同葉片傾角的液力緩速器進行三維數值模擬和分析，得到了緩速器內部流場的壓力及速度分布云圖，進一步對制動力矩進行比較。結果表明：葉片傾角在36°到51°的范圍里，隨著葉片傾角的逐漸增大，制動力矩逐漸增加大；當葉片傾角增大到43°后，制動力矩開始逐漸減小。

液力緩速器；葉片傾角；CFD；數值模擬；制動力矩

10.3969/j.issn.1001-3881.2015.12.006Document code：A

U463.53+3

15 December 2014；revised 7 February 2015；accepted 1 March 2015

Chen-guang LAI，Professor.E-mail：chenguanglai@cqut. edu.cn

*Corresponding author：Meng-hua DUAN，

E-mail：305034059@qq.com