Minsuk Choi

Effects of cavitation on performance of automotive torque converter

Cavitation is a phenomenon whereby vapor bubbles of a flowing liquid are formed in a local region where the pressure of the liquid is below its vapor pressure. It is well known that cavitation in torque converters occurs frequently when a car with an automatic transmission makes an abrupt start. Cavitation is closely related to a performance drop and noise generation at a specific operating condition in a car and a torque converter itself. This study addressed the relation between cavitation and performance in an automotive torque converter in a quantitative and qualitative manner using numerical simulations. The cavitation was calculated at various operating conditions using a commercial flow solver with the homogeneous cavitation model, and the torque converter performance was compared with the experimental data. Numerical results well match to the data and indicate that the cavitation causes significant performance drop, as the pump speed increases or both speed ratio and reference pressure decrease.

Effects of recessed blade tips on the performance and flow field in a centrifugal compressor

This article presents a numerical investigation of the influence of a recessed blade tip on the performance and flow field in a centrifugal compressor with a vaned diffuser. Steady computations were performed for three different blade tip geometries over their whole operating ranges. A flat tip blade was used as a baseline case to assess two different recessed blade geometries. It was found that recessed blade tip designs could improve the total-to-total pressure ratio and efficiency over whole operating ranges. The recessed tips were also effective in reducing the tip leakage flow. However, the recess cavity had an adverse effect on the efficiency due to the generation of a vortex in the cavity. As a result, the overall stage loss was reduced in the recessed cases because the positive effect of the reduced tip leakage flow ended up being superior to the negative effect of a strong vortex in the cavity. In addition, the diffuser performance of the centrifugal compressor could be enhanced with the recess cavity, because the pressure recovery coefficient was higher in the recessed cases than in the flat tip case. A parametric study was conducted to determine the geometry of a recess cavity with improved performance. This study revealed that as the cavity volume increases, the tip leakage flow continuously decreases but the strength of the cavity vortex increases. Therefore, it may be possible to optimize the recess cavity geometry further.

Unsteady flow simulations of Pelton turbine at different rotational speeds

This article presents numerical simulations of a small Pelton turbine suitable for desalination system. A commercial flow solver was adopted to resolve difficulties in the numerical simulation for Pelton turbine such as the relative motion of the turbine runner to the injector and two-phase flow of water and air. To decrease the numerical diffusion of the water jet, a new topology with only hexagonal mesh was suggested for the computational mesh around the complex geometry of a bucket. The predicted flow coefficient, net head coefficient, and overall efficiency showed a good agreement with the experimental data. Based on the validation of the numerical results, the pattern of wet area on the bucket inner surface has been analyzed at different rotational speeds, and an attempt to find the connection between rotational speeds, torque, and efficiency has been made.

Development of Wheel-Terrain Interaction Device for Mobility Prediction of Off-road Vehicle

Received 29 September 2014; received in revised 3 November 2014; accepted 4 November 2014ABSTRACT This paper presents on the development of wheel-terrain interaction device using low-pricedsensors, which will be used to predict the drawbar pull and optimal slip of off-road vehicle inreal time. The essential variables obtained in the device to predict the mobility of vehicles aredetermined based on semi-empirical model describing the wheel-terrain interaction. Using thedeveloped device, the experiments about the wheel-terrain interaction were performed on thesoil of the Jumunjin standard sand, which yielded dynamic weight, motor driving torque, draw-bar pull, and sinkage with respect to wheel slip ratio. Finally, the repeatability of the measureddata are verified through repeating the experiments three times on the same condition.Key Words: Drawbar pull, Off-road vehicle, Wheel-terrain interaction device

On the Recessed Blade Tip With and Without a Porous Material in an Axial Compressor

This paper addresses the effect of the recessed blade tip with and without a porous material on the performance of a transonic axial compressor. A commercial flow solver was employed to analyze the performance and the internal flow of the axial compressor with three different tip configurations: reference tip, recessed tip and recessed tip filled with a porous material. It was confirmed that the recessed blade tip is an effective method to increase the stall margin in an axial compressor. It was also found in the present study that the strong vortex formed in the recess cavity on the tip pushed the tip leakage flow backward and weakened the tip leakage flow itself, consequently increasing the stall margin without any penalty of the efficiency in comparison to the reference tip. The recessed blade tip filled with a porous material was suggested with hope to obtain the larger stall margin and the higher efficiency. However, it was found that a porous material in the recess cavity is unfavorable to the performance in both the stall margin and the efficiency. An attempt has been made to explain the effect of the recess cavity with and without a porous material on the flow in an axial compressor.Copyright