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Ba Y.,Xian Jiaotong University | Ba Y.,Los Alamos National Laboratory | Kang Q.,Los Alamos National Laboratory | Liu H.,Xian Jiaotong University | And 3 more authors.
Journal of Computational Science | Year: 2016

In this study, the dynamical behavior of a droplet on topologically structured surface is investigated by using a three-dimensional color-gradient lattice Boltzmann model. A wetting boundary condition is proposed to model fluid-surface interactions, which is advantageous to improve the accuracy of the simulation and suppress spurious velocities at the contact line. The model is validated by the droplet partial wetting test and reproduction of the Cassie and Wenzel states. A series of simulations are conducted to investigate the behavior of a droplet when subjected to a shear flow. It is found that in Cassie state, the droplet undergoes a transition from stationary, to slipping and finally to detachment states as the capillary number increases, while in Wenzel state, the last state changes to the breakup state. The critical capillary number, above which the droplet slipping occurs, is small for the Cassie droplet, but is significantly enhanced for the Wenzel droplet due to the increased contact angle hysteresis. In Cassie state, the receding contact angle nearly equals the prediction by the Cassie relation, and the advancing contact angle is close to 180°, leading to a small contact angle hysteresis. In Wenzel state, however, the contact angle hysteresis is extremely large (around 100°). Finally, high droplet mobility can be easily achieved for Cassie droplets, whereas in Wenzel state, extremely low droplet mobility is identified. © 2016 Elsevier B.V. Source

Ba Y.,Xian Jiaotong University | Liu H.,Xian Jiaotong University | Sun J.,Xian Jiaotong University | Sun J.,Collaborative Innovation Center for Advance Aerospace Engine | Zheng R.,Fuzhou University
International Journal of Heat and Mass Transfer | Year: 2015

Asymmetric T-junctions have recently emerged as a promising tool in microfluidics. However, previous studies of the droplet formation mechanism are largely limited to symmetric T-junctions. In this work, the droplet formation in universal T-junctions, including both symmetric and asymmetric T-junctions, is investigated by a three-dimensional color-gradient lattice Boltzmann model. A three-dimensional color-conserving boundary condition is developed to model fluid-surface interactions, which suppresses the spurious velocities near the contact lines and improves numerical accuracy. Model verification is conducted by the partial wetting test and the droplet formation in a symmetric T-junction. Then, an in-depth study is performed for universal T-junctions. In both symmetric and asymmetric T-junctions, the droplet length is linearly dependent on flow rate ratio at low capillary number, and the droplet formation successively undergoes squeezing, dripping and jetting regimes as the capillary number increases. By investigating the local pressure and velocity field, we find that the upstream pressure and viscous force respectively dominates the droplet formation in squeezing and dripping regimes. In squeezing regime, the pressure fluctuates significantly, and the fluctuation amplitude and frequency decrease with the channel width ratio; while in dripping regime, the pressure fluctuation is negligibly small, and the viscous force is found to decrease with the channel width ratio. Consequently, the droplet size increases with the channel width ratio in both regimes. In addition, the viscosity ratio and surface wettability are found to be influential to the formation regime, droplet shape and size for various channel width ratios and capillary numbers, and play important roles in droplet formation. ©Elsevier Ltd. All rights reserved. Source

Zheng R.,Xian Jiaotong University | Liu H.,University of Strathclyde | Sun J.,Xian Jiaotong University | Sun J.,Collaborative Innovation Center for Advance Aerospace Engine | Ba Y.,Xian Jiaotong University
Physica A: Statistical Mechanics and its Applications | Year: 2014

The Cassie-Baxter model is widely used to predict the apparent contact angles on textured super-hydrophobic surfaces. However, it has been challenged by some recent studies, since it does not consider contact angle hysteresis and surface structure characteristics near the contact line. The present study is to investigate the contact angle hysteresis on striped textured surfaces, and its elimination through vibrating the substrate. The two-phase flow is simulated by a recently proposed lattice Boltzmann model for high-density-ratio flows. Droplet evolutions under various initial contact angles are simulated, and it is found that different contact angles exist for the same textured surface. The importance of the contact line structure for droplet pinning is underlined via a study of droplet behavior on a composite substrate, with striped textured structure inside and flat structure outside. A "stick-jump" motion is found for the advancing contact line on the striped textured surface. Due to hysteresis, the contact angles after advancing are not consistent with the Cassie-Baxter model. The stable equilibrium is obtained through properly vibrating the substrate, and the resulted contact angles are consistent with Cassie's predictions. © 2014 Elsevier Inc. All rights reserved. Source

Song P.,Xianning University | Song P.,Collaborative Innovation Center for Advance Aerospace Engine | Sun J.,Xianning University | Sun J.,Collaborative Innovation Center for Advance Aerospace Engine
Journal of Mechanical Science and Technology | Year: 2015

Transonic axial flow fan has relatively high blade tip speed and produces higher pressure ratio than the subsonic. However, considerable losses are brought about by the shock waves close to blade tip and over part of span, leading to deteriorated overall efficiency and operating flow range. The present study is to mitigate shock wave and reduce losses through simultaneous variation of blade sectional profiles and their stacking line in blade design. Both sectional profiles and stacking line are varied simultaneously to provide more flexible blade shape tuning. To achieve a best blade shape and produce maximum performance gains, a global optimization method is incorporated in the blade shape design. It includes an improved CCEA (cooperative co-evolution algorithm) optimizer and one-stage Expected Improvement (EI) based adaptively updated Kriging surrogate model. The former has divided the high-dimension optimization problems into readily solved low-dimension ones, while the later has enabled the optimizer to jump out of from the local optima and search the solution towards the global optima. The optimization is conducted for Rotor67 at design condition with a single workstation, and considerable overall efficiency and pressure ratio gains are simultaneously obtained, while the flow range is also extended. This is supported by the significantly improved flow behavior in the optimized blade passages, where the chordwise shock wave is mitigated, leading to an increase in overall efficiency; the spanwise static pressure distribution is improved evidently and this improves the overall pressure ratio. © 2015, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg. Source

Wang K.,Xian Jiaotong University | Sun J.,Xian Jiaotong University | Sun J.,Collaborative Innovation Center for Advance Aerospace Engine | Song P.,Xian Jiaotong University | Song P.,Collaborative Innovation Center for Advance Aerospace Engine
Cryogenics | Year: 2015

A cryogenic liquid turbine expander is developed as a replacement for traditional Joule-Thomson valves used in the cryogenic systems for the purpose of energy saving. An experimental study was conducted to evaluate the performance of the turbine expander and is the subject of this paper. The test rig comprises a closed-loop liquefied nitrogen system, cryogenic liquid turbine expander unit, and its auxiliary and measuring systems. The test operating parameters of the turbine expander are determined on the basis of flow similarity rules. Pre-cooling of the liquid nitrogen system is first performed, and then the tests are conducted at different flow rates and speed ratios. The turbine expander flow rate, inlet and outlet pressure and temperature, rotational speed and shaft torque were measured. Experimental results and their uncertainties were analyzed and discussed. The following are demonstrated: (1) For both test cases, turbine expander peak isentropic efficiency is respectively 78.8% and 68.4% obtained at 89.6% and 92% of the design flow rate. The large uncertainties in isentropic efficiency are caused by the large enthalpy variations subjected to small measurement uncertainties in temperature and pressure. (2) Total efficiency and hydraulic efficiency of the turbine expander are obtained. They are essentially the same, since both include flow-related effects and also bearing losses. Comparisons of total efficiency and hydraulic efficiency were used to justify measurement uncertainties of different quantities, since the former involves the measured mass flow rate and enthalpy drop (being dependant on inlet and outlet temperature and pressure), while the latter involves the actual shaft power, volume flow rate, and inlet and outlet pressure. (3) Losses in flow passages and the shaft-bearing system have been inferred based on the measured turbine expander total efficiency, isentropic efficiency, and mechanical efficiency, which are respectively 57.6-74.8%, 62.1-78.8% and 89.5-96.4%. Uncertainty analysis is conducted for experimental isentropic efficiency, hydraulic efficiency, and total efficiency. The hydraulic efficiency seems to be the best measure for assessing the performance of cryogenic liquid turbine expander. (4) Isentropic efficiency versus speed ratio is obtained from the experimental data. The experimental isentropic efficiency increases with the speed ratio, and it reaches 78.8% at the largest experimental speed ratio. A higher efficiency would be achieved if the speed ratio could reach a larger value. This provides some guidance for an optimal operation of the turbine expander in the future. © 2015 Elsevier Ltd. Source

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