考虑不可凝结气体的空化流模型及数值模拟

以均相流假设为基础建立了一种基于输运方程的空化流模型,该模型在质量传输方程中不仅考虑了蒸发和凝结的机理,而且考虑了不可凝结气体的影响.采用RNG k-ε湍流模型,并引入与混合密度相关的修正函数对湍流涡黏性系数进行修正.应用文中的空化流模型,对NA-CA66翼型进行了定常空化流动数值模拟,翼型吸力面的压力系数分布曲线与试验结果吻合很好.在此基础上,进一步研究了模型中不可凝结气体质量分数以及不同进口湍动能和湍流耗散率对空泡形成和发展的影响,确定了模型中不可凝结气体质量分数、进口湍动能和湍流耗散率的合理取值.应用空化流模型对非定常空化流动进行了数值模拟,数值模拟结果清晰地反映了翼型表面空化云的初生、成长、脱落和溃灭的全过程,并指出反向射流是引起空化云脱落的重要原因.非定常计算得到的斯特劳哈数与试验相吻合,进一步验证了该模型在空化流数值计算中的可靠性.

基于配流盘表面形貌的柱塞泵空化现象研究

针对轴向柱塞泵配流副内的空化现象,建立了考虑配流盘表面形貌的柱塞泵配流副空化模型,分析了压力、转速和配流盘表面形貌对轴向柱塞泵配流副内空化现象的影响。首先,由于机加工零部件表面具备分形特征,采用了分形理论模拟配流盘表面形貌,在此基础上建立了轴向柱塞泵配流副空化模型;然后,采用有限差分法和松弛迭代法求解了数学模型,得到了符合精度要求的数值解,从而得出了配流副油膜压力分布特征,在此基础上分析了配流盘表面容易发生空化的位置;最后,通过改变配流副的压力、转速和分形参数,得到了不同的压力、缸体转速和表面形貌对配流副空化现象的影响。研究结果表明:配流盘表面易在高压和低压的过渡区域发生局部空化,当配流副排油腔压力达到30 MPa,且配流盘表面较粗糙时,配流盘局部气体体积分数达到92%左右;采用提高配流盘表面的加工精度,降低其表面粗糙度的方法,可以降低配流盘表面产生空化现象的严重程度。

Validation of dynamic cavitation model for unsteady cavitating flow on NACA66

Unsteady cavitating flow is extremely complicated and brings more serious damages and unignorable problems compared with steady cavitating flow.CFD has become a practical way to model cavitation;however,the popularly used full cavitation model cannot reflect the pressure-change that the bubble experiences during its life path in the highly unsteady flow like cloud cavitating.Thus a dynamic cavitation model(DCM)is proposed and it has been considered to have not only the first-order pressure effects but also zero-order effect and can provide greater insight into the physical process of bubble producing,developing and collapsing compared to the traditional cavitation model.DCM has already been validated for steady cavitating flow,and the results were reported.Furthermore,DCM is designed and supposed to be more accurate and efficient in modeling unsteady cavitating flow,which is also the purpose of this paper.The basic characteristic of the unsteady cavitating flow,such as the vapor volume fraction distribution and the evolution of pressure amplitude and frequency at different locations of the hydrofoil,are carefully studied to validate DCM.It is found that not only these characteristics mentioned above accord well with the experimental results,but also some detailed transient flow information is depicted,including the re-entrant jet flow that caused the shedding of the cavity,and the phenomenon of two-peak pressure fluctuation in the vicinity of the cavity closure in a cycle.The numerical results validate the capability of DCM for the application of modeling the complicated unsteady cavitating flow.

A Rayleigh-Plesset based transport model for cryogenic fluid cavitating flow computations

The present article focuses on modeling issues to simulate cryogenic fluid cavitating flows.A revised cavitation model,in which the thermal effect is considered,is derivated and established based on Kubota model.Cavitating flow computations are conducted around an axisymmetric ogive and a 2D quarter caliber hydrofoil in liquid nitrogen implementing the revised model and Kubota model coupled with energy equation and dynamically updating the fluid physical properties,respecitively.The results show that the revised cavitation model can better describe the mass transport process in the cavitation process in cryogenic fluids.Compared with Kubota model,the revised model can reflect the observed"frosty"appearance within the cavity.The cavity length becomes shorter and it can capture the temperature and pressure depressions more consistently in the cavitating region,particularly at the rear of the cavity.The evaporation rate decreases,and while the magnitude of the condensation rate becomes larger because of the thermal effect terms in the revised model compared with the results obtained by the Kubota model.

A hybrid RANS/LES model for simulating time-dependent cloud cavitating flow around a NACA66 hydrofoil

Cloud cavitating flow is highly turbulent and dominated by coherent large-scale anisotropic vortical structures. For the numer- ical investigation of such a class of flow, large eddy simulation （LES） is a reliable method but it is computationally extremely costly in engineering applications. An efficient approach to reduce the computational cost is to combine Reynolds-averaged Navier-Stokes （RANS） equations with LES used only in the parts of interest, such as massively separated flow regions. A new hybrid RANS/LES model, the modified filter-based method （FBM）, is proposed in the present study which can perform RANS or LES depending on the numerical resolution. Compared to the original FBM, the new method has three modifications： the state-of-the-art shear stress transport （SST） model replaces the k-c model as a baseline RANS model. A shielding function is introduced to obviate the switch from RANS to LES occurring inside the boundary layer. An appropriate threshold controlling the switch from RANS to LES is added to achieve an optimal predictive accuracy. The new model is assessed for its predictive capability of highly unsteady cavitating flows in a typical case of cloud cavitation around a NACA66 hydrofoil. The new mod- el results are compared with data obtained from the Smagorinsky LES and SST model based on the same homogeneous Zwart cavitation model. It is found that the modified FBM method has significant advantages over SST model in all aspects of pre- dicted instantaneous and mean flow field, and its predictive accuracy is comparable to the Smagorinsky LES model even using a much coarser grid in the simulations.

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.

Heat Transfer Optimization for Air-Mist Cooling between a Stack of Parallel Plates

A theoretical model is developed to predict the upper limit heat transfer between a stack of parallel plates subject to multiphase cooling by air-mist flow.The model predicts the optimal separation distance between the plates based on the development of the boundary layers for small and large separation distances,and for dilute mist conditions.Simulation results show the optimal separation distance to be strongly dependent on the liquid-to-air mass flow rate loading ratio,and reach a limit for a critical loading.For these dilute spray conditions,complete evaporation of the droplets takes place.Simulation results also show the optimal separation distance decreases with the increase in the mist flow rate.The proposed theoretical model shall lead to a better understanding of the design of fins spacing in heat exchangers where multiphase spray cooling is used.

Eco-compensation in Guanting Reservoir Watershed Based on Spatiotemporal Variations of Water Yield and Purification Services

Guanting Reservoir（GR） is one of the most important water sources for Beijing and neighboring regions.Due to water pollution,it was withdrawn from the system to supply Beijing drinking water;however,after a thorough treatment process,GR was made a reserve water source since 2007.To develop a comprehensive and quantitative analysis of water yield and purification services in the GR watershed,this study selected two time periods：the period when GR was withdrawn from the system supplying local drinking water and the period that it has been designated a reserve water source.The In VEST model was used to evaluate the quantities of water yields,and total nitrogen and total phosphorus outputs from 1995 to 2010 Additionally,the spatiotemporal variations of water yield services and water quality purification services in the GR watershed were analyzed.The results showed that water yield services in the GR watershed first weakened and then became stronger,but weakened overall during the years 1995 to 2010.Water yield capacity in the basin decreased from 1.89×10^9 m3 in 1995 to 1.43×10^9 m3 in 2010（a drop of 24.0% in total）.Water quality purification services also showed the same tendency.Total nitrogen output decreased from 4028.7 t in 1995 to 3611.4 t in 2010,while total phosphorus decreased from 379.7 t in 1995 to 354.0 t in 2010.Nitrogen and phosphorus purification services were enhanced by 10.4% and 6.8%,respectively.Changes in the climate and land use were the main factors which lead to the changes in the water yield service in the GR watershed.Policies intended to protect water resource have matched the varying trends of water quality purification services during different periods.On one hand,the research results provide a foundation to identify key fields for eco-compensation in the Guanting Reservoir basin.On another hand,the ecosystem service value will increase on the basis of eco-compensation criteria through setting the scenarios of returning farmland to forest and ecological protection.This method directly reflects increases in ecosystem service values that have occurred since measures to protect the ecological environment have been implemented.This method is more persuasive and feasible than using eco-compensation criteria based on regional ecosystem service values determined by land use/coverage type.It can provide a new way to assess eco-compensation in the Guanting Reservoir basin and other regions.

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.