基于空化流动计算的混流式水轮机尾水管的压力脉动

为了保证混流式水轮机运行安全,分析了尾水管内部的空化流场。假设气液混相均质,并考虑不可凝结气相,采用基于组份输运方程,求解了尾水管内气液混相均质流的雷诺平均N-S方程以及气相组分输运方程。结果表明:不同装置空化系数下尾水管压力脉动主频率变化不大,但在极低的装置空化系数下,削波现象导致更低频率成分的出现。在较大的装置空化系数下,空化涡带直径较小,压力脉动幅值随装置空化系数的变化不明显;当装置空化系数小于某一临界值后,随装置空化系数的降低,压力脉动幅值增加并达到一个最大值。计算结果解释了模型试验现象。

A Review on Effects of Personalized Ventilation Systems on Air Quality and Thermal Comfort in Aircraft Cabin Mini-Environments

This study conducts an evaluation of air quality,dispersion of airborne expiratory pollutants and thermal comfort in aircraft cabin mini-environments using a critical examination of significant studies conducted over the last20 years.The research methods employed in these studies are also explained in detail.Based on the current literature,standard procedures for airplane personal ventilation and air quality investigations are defined for each study approach.Present study gaps are examined,and prospective study subjects for various research approaches are suggested.

Prediction and optimization of aerodynamic noise in an automotive air conditioning centrifugal fan

The high aerodynamic noise induced by automotive air conditioning systems has important effects on the ride comfort, and the centrifugal fan is the largest noise source in these systems. It is very important to reduce the aerodynamic noise generated by the centrifugal fan. The flow field and the sound field on the whole centrifugal fan configuration have been carried out using the computational fluid dynamics. Simulation results show that the sound pressure level near the outlet of the centrifugal fan is too high. Based on the relationship between flow characteristics and the aerodynamic noise, four parameters of the centrifugal fan, i.e., impeller blade＇s outlet angle 0, volute tongue＇s gap t, collector inclination angle fl, and rotating speed n, were selected as design variables and optimized using response surface methodology. While keeping the function of flow rate unchanged, the peak noise level is reduced by 8 dB or 10.8%. The noise level is satisfactorily reduced.

A modified PANS model for computations of unsteady turbulence cavitating flows

A modification to the PANS(partially averaged Navier-Stokes) model is proposed to simulate unsteady cavitating flows. In the model, the parameter fk is modified to vary as a function of the ratios between the water density and the mixture density in the local flows. The objective of this study is to validate the modified model and further understand the interaction between turbulence and cavitation around a Clark-Y hydrofoil. The comparisons between the numerical and experiment results show that the modified model can be improved to predict the cavity evolution, vortex shedding frequency and the lift force fluctuating in time fairly well, as it can effectively modulate the eddy viscosity in the cavitating region and various levels of physical turbulent fluctuations are resolved. In addition, from the computational results, it is proved that cavitation phenomenon physically influences the turbulent level, especially by the vortex shedding behaviors. Also, the mean u-velocity profiles demonstrate that the attached cavity thickness can alter the local turbulent shear layer.

Computational modeling of cavitating flows in liquid nitrogen by an extended transport-based cavitation model

Developing a robust computational strategy to address the rich physical characteristic involved in the thermcdynamic effects on the cryogenic cavitation remains a challenge in research. The objective of the present study is to focus on developing mod- elling strategy to simulate cavitating flows in liquid nitrogen. For this purpose, numerical simulation over a 2D quarter caliber hydrofoil is investigated by calibrating cavitation model parameters and implementing the thermodynamic effects to the Zwart cavitation model. Experimental measurements of pressure and temperature are utilized to validate the extensional Zwart cavi- tation model. The results show that the cavitation dynamics characteristic under the cryogenic environment ale different from that under the isothermal conditions： the cryogenic case yields a substantially shorter cavity around the hydrofoil, and the pre- dicted pressure and temperature inside the cavity are steeper under the cryogenic conditions. Compared with the experimental data, the computational predictions with the modified evaporation and condensation parameters display better results than the default parameters from the room temperature liquids. Based on a wide range of computations and comparisons, the extension- al Zwart cavitation model may predict more accurately the quasi-steady cavitation over a hydrofoil in liquid nitrogen by pri- marily altering the evaporation rate near the leading edge and the condensation rate in the cavity closure region.