| Braking device is directly related to traffic safety, especially the heavy transport vehicles need large braking torque. Conventional friction braking deviceshave endanger driving safety problems such as low-efficient heat dissipation, long braking time, overheat braking, venerability to be damaged. It is an important way to solve the problems mentioned above by installation of auxiliary braking system, namely hydraulic retarder. Therefore, we carried out the research and development of hydraulic retarder for nearly ten years and developed several types of products. Since so many factors affecting the rapid braking process of hydraulic retarder, it is difficult to explain many technical foundation problems, like theoretical expression of the nonlinear variation characteristics of work to heat transfer and dissipation in unsteady process of oil filling, complex internal disturbance in system by properties of filling medium and their variation, flow pattern, position,dynamic change of power and heat transfer intensity, that changes the nature of work to heat and dissipation in turn,mechanism of fluid solid interaction in transient process. Therefore, the study take the comprehensive characterization of braking performance parameters of hydraulic retarder as entry point, deep analyses the interaction mechanism and inner-relationship among the working medium of hydraulicretarder, qualitative characteristic parameter of device geometric structure, characteristic parameter of movement, to make clear of the relationship and characteristicsbetween themedium temperature and viscosity, between the filling ratio and rheological properties of the medium, between the medium viscosity and braking torque, and between the filling speed and braking torque with different filling media. Taking THB40 hydraulic retarder jointly developed by Shenzhen Terca Technology Co., Ltd. and us as study object, make dimensional analysis and similarity argument asthe main task, reveal the law of power heat transfer and dissipation in unsteady process, clarify the braking mechanism of hydraulic retarder by the combination of theoretical analysis CFD calculation and bench testresearch.The research work and innovation of this paper are mainly reflected in the following aspects:1) Based on the analysis of domestic and foreign research of hydraulic retarder and combined with the THB40 retarder jointly developed by the research group and Shenzhen Terca Transportation Technology Co., Ltd., the author proves that during the process of braking, there is a qualitative change point for the medium to change from the dilatant state to plastic state, and to pseudoplastic fluid state on the basis of analyzing the principle, structure and existing problems of the hydraulic retarder.2) This paper selects the two-equation turbulence model k SST-ω based on the calculation of governing equation of typical fluid mechanics and feature of flow field of hydraulic retarder, and provides a solution to key parameters in the governing equation. Then, it introduces the solution process and the calculation of the braking torque of hydraulic retarder.3) On the basis of gas-liquid two-phase homogeneous model, this paper makes a CFD simulation calculation of the hydraulic retarder with different filling ratio, temperature, rotation speed and viscosity. The calculation results show that the braking torque increases with the increase of the filling ration when the rotation speed and media temperature remain constant; the braking torque will increase with the increase of the temperature when the filling ratio and rotation speed remain constant, but its increased rate will decrease with the increase of the temperature and it will level off when the temperature is above 90 ℃; when the filling ratio and temperature remain constant, the braking torque will change with the change of the filling ratio, and when the filing ratio is 95%, the braking torque will increase with the increase of the rotation speed.4) Test the viscosity of Helix ultra SM 5W-40 Full Synthetic lubricating engine oil used for the research at different temperatures with automatic wide-range viscometerat HVM472 and verifies the Walther equation, which is used to test the viscosity-temperature characteristics of oil medium. Makes a braking performance test of the hydraulic retarder on the bench with different temperature-viscosity, control pressure, rotation speed, and then compares the results of braking torque to CFD experimental results obtained under the same conditions, both of which show basically same changing tendency. When the filling ratio is 95% and the working medium temperature is 95℃, the braking torque increases as the velocity increases, the maximum error of the braking torque with different rotation speeds will be 3.6%; when the filling ratio is 95% and the rotation speed is 600r/min, the braking torque increases as the temperature increases, decreases as the superficial dynamic viscosity increases, the maximum error of the braking torque with different viscosity and temperatures will be 5.83%; and when the rotation speed is 600r/min and the medium temperature is 70 ℃, the braking torque increases as the filling ratio increases, the maximum error of the braking torque under the corresponding pressure filling ratio will be 4.3%. The results show that the CFD calculation model is correct, and the result is credible.5) This paper study the dimensionless of braking performance parameters of hydraulic retarder by π theorem of dimensional analysis, obtain the relation between braking torque and the parameters of hydraulic retarder systemafter deduces the dimensionless numbers of μπ 、,λπ 、,pπ, eTπ by taking density, velocity, specific heat and length scale as the basic physical parameters. Then the relationship between the braking mechanism of hydraulic retarder and Reynolds numbereR, Prandtl numberrP, Euler number uE and Berkeley numbereP is studied respectively, and the results show that with different filling medium, the four dimensionless numbers maintain different relations with the braking torque:eR and eP will decrease when the rotation speedincrease under the filling ratio is 38%, but rP and uE will increase under the same conditions; when the filling ratio is 95% will be in turn. The braking torque increases as eR andeP increase and decreases as uE and rP increases,it veryfy the correctness of relational formula of breaking torque based on dimensional analysis.6) The relationship between the medium parameters and the braking mechanism of hydraulic retarder is studied based on the bench test and CFD calculation results, and the findings are as follows.When the filling ratio is over 72%, working medium of the hydrodynamic retarder is a pseudoplastic non-Newtonian fluid, the viscosity of which decreases as the shear rate increases, while the braking torque increases as the rotation speed increases; when the filling ratio is 68% or less, the working medium is a dilatant non-Newtonian fluid, the viscosity of which increases as the shear rate increases, while the braking torque decreases as the rotation speed increases; there is an approximately linear correspondence between filling ratio and braking torque; the control pressure and the filling rate have a corresponding relationship, the maximum control pressure is 2.8bar equivalent to a filling ratio of 95%, 2.0bar corresponds to a filling ratio of 82%, 1.8bar corresponds to a filling ratio of 72%, 1.5bar corresponds to a filling ratio of 68%, 1.0bar corresponds to a filling ratio of 55%, and 0.66 bar corresponds to a filling ratio of 38%; the medium viscosity and temperature maintain a relationship of power function with a negative 1.8 index and K coefficient, while the coefficient K maintains a positive linear relation with the filling ratio, which are verified to be correct. And the braking torque of the hydraulic retarder will reduce as the dynamic viscosity of the working medium increases, but there are differences in its reducing way when the working medium is in a form of pseudoplastic non-Newtonian fluid or dilatant non-Newtonian fluid. The braking torque of the hydraulic retarder maintains a linear relationship with the density of the medium. |
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