Purpose: The purpose of this paper is to study the transient cavitation process in torque converters with a particular focus on cavitation suppression with a passive flow control technique. Design/methodology/approach: The transient fluid field in a torque converter was simulated by RANS-based computational fluid dynamics (CFD) in a full three-dimensional (3D) model. A homogeneous Rayleigh–Plesset cavitation model was used to simulate the transient cavitation process and the results were validated with test data. Various secondary flow passages (SFP) were applied to the stator blade. The cavitation behavior and hydrodynamic performance were simulated and compared to investigate the effect of SFP geometries on cavitation suppression. Findings: Presented results show that cavitation in the torque converter is highly unstable at stall operating condition because of the combination of a high incidence angle and high flow velocity. The addition of an SFP to the stator blade produces a disruption of the re-entrant jet and reduces the overall degree of cavitation, consequently inhibiting the unstable cavitation and reducing performance degradation. Originality/value: This paper provides unique insights into the complicated transient cavitation flow patterns found in torque converters and introduces effective passive flow control techniques useful to researchers and engineers in the areas of fluid dynamics and turbomachinery.

Hydraulic torque converter is a kind of high speed rotating machine using viscosity hydraulic oil as working medium, and its internal flow field is very complex. Thereby cavitation can occur easily in the working process, resulting in severe degradation of torque converter performance, noise, vibration and even failure. In order to reveal the effect of rotating speeds on the cavitation characteristics, a full flow passage geometry and a computational fluid dynamics (CFD) model with cavitation were developed to analyze the flow behavior in the torque converter. The results show that cavitation occurs when the speed difference between pump and turbine exceeds 1400 rpm for the basic model torque converter, which could be used as a useful indicator for the occurrence and degree of severity of flow cavitation. The increase of pump rotating speed or the decrease of speed ratio will intensify cavitation, which reduces the hydraulic transmission capacity and efficiency by over 20%, and seriously alters the shape, size, vapor volume fraction and region of cavitation bubbles. In extreme cases, more than 80% of the area on the suction side of the stator blade could be covered by cavitation bubbles. Moreover, the increase of pump rotating speed also changes the critical cavitation number and extends the cavitation range towards high speed ratio conditions not previously affected. These findings can provide guidance on how to choose the operating conditions of the hydraulic torque converter and how to improve its hydrodynamic performance and stability.

Traditional one-dimensional (1D) design theory no longer satisfies modern torque converter design requirements due to the lack of accuracy and long trial-and-error process. A computational fluid dynamics (CFD) based optimization approach was brought forward. This study is devoted to two key processes in bringing CFD into torque converter design – blade parameterization/evaluation and optimization. A six-parameter blade camber line design method was employed to represent the blade shape. The blade profile was evaluated based on manufacturability considerations. A low-fidelity CFD model was developed for parameter study and optimization, and a high-fidelity CFD model considering the transient and cavitation effects was employed to evaluate the optimization outcome. The parameter sensitivity study was performed using design of experiment (DOE) technique and the variables were categorized into primary design variable, effective design variable and insignificant design variable groups. Optimization on the primary variables was carried out using the genetic algorithm. Two optimum solutions were evaluated with the transient full 3D CFD model and then tested. The optimum torque converters’ test results indicated significant performance improvement. The proposed design procedure can improve the design efficiency & accuracy and shorten the design cycle by the application of different CFD models in conjunction with an optimization algorithm.

Hydraulic torque converters are prone to cavitation due to their high impeller rotational speeds and their complex three-dimensional flow characteristics. Since the blades are the core components of torque converters, the shapes of the blades are important to the hydraulic performance and cavitation characteristics. Different cavitation computational fluid dynamics (CFD) models for a torque converter were developed to simulate the internal cavitation flow for different pump and turbine blade exit angles, and the influence of the blade angles on the cavitation characteristics and cavitation flow field in the torque converter was investigated. Experimental prototypes were produced and tested for verification. The results indicate that the pump and turbine blade exit angles had significant effects on the cavitation number of the torque converter. Increasing the pump and turbine blade exit angles promotes the generation and intensification of cavitation, resulting in severe changes in the shapes and locations of the cavitation bubbles due to changes in the fluid impact angles. Additionally, cavitation is quickly suppressed and the performance is improved when the blade exit angles are reduced within an appropriate range, in particular, that of the turbine blade. These research results can provide guidance for the design of a high-performance hydraulic torque converter cascade system and the suppression of cavitation for practical engineering applications.

Cavitation inside a torque converter induces noise, vibration and even failure, and these effects have been disregarded in previous torque converter design processes. However, modern torque converter applications require attention to this issue because of its high-speed and high-capacity requirements. Therefore, this study investigated the cavitation effect on a torque converter using both numerical and experimental methods with an emphasis on the influence of the charging oil feed location and charge pressure. Computational fluid dynamics (CFD) models were established to simulate the transient cavitation behaviour in the torque converter using different charging oil pressures and inlet arrangements and testing against a base case to validate the results. The CFD results suggested that cavitating bubbles mainly takes place in the stator of the torque converter. The transient cavitation CFD model yielded good agreement with the experimental data, with an error of 7.6% in the capacity constant and 7.4% in the torque ratio. Both the experimental and numerical studies showed that cavitation induced severe capacity degradation, and that the charge pressure and charging oil configuration significantly affects both the overall hydrodynamic performance and the fluid behaviour inside the torque converter because of cavitation. Increasing the charge pressure and charging the oil from the turbine-stator clearance were found to suppress cavitation development and reduce performance degradation, especially in terms of the capacity constant. This study revealed the fluid field mechanism behind the influence of charging oil conditions on torque converter cavitation behaviour, providing practical guidelines for suppressing cavitation in torque converter.

Cavitation is a transient phase transition between liquid and vapor, and it often occurs in fluid machinery, especially in a hydraulic torque converter that uses oil as the working medium to transmit speed and torque. The complex and strongly coupled fluid flow in the torque converter is prone to cavitation due to high rotating speed and high-temperature working conditions. Cavitation seriously affects the working performance, transmission smoothness, and service life of the torque converter. The flow pressure in the stator of a torque converter under various charging conditions and high rotating speeds was measured. The pressure data on the stator blade were analyzed in the time domain and frequency domain to identify and evaluate the cavitation characteristic. The transient cavitation flow inside the torque converter was also simulated with the computational fluid dynamics model. The results show that the shedding of cavitation seriously reduced the hydraulic performance, hindered the fluid flow, and destroyed the stability of the flow field. Moreover, cavitation aggravates the complexity and nonlinearity of the pressure frequency and hydraulic performance oscillation of the torque converter, and seriously affected the shaft/blade interaction frequency between the pump and stator. Meanwhile, the occurrence and degree of cavitation in the torque converter can be evaluated by A_PS.shaft/A_PS.blade (the amplitude ratio of the shaft interaction frequency and blade interaction frequency between pump and stator) with spectrum analysis of the dynamic pressure, and the critical value was 1.6 for the test torque converter. The research revealed the influence of cavitation on the internal flow field of the torque converter and provided a novel practical cavitation evaluation technique.