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Chengzhi Hu
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Journal Articles
Journal:
Journal of Heat Transfer
Article Type: Photo Gallery
J. Heat Transfer. March 2020, 142(3): 030907.
Paper No: HT-20-1065
Published Online: February 5, 2020
Abstract
In addition to the increase of thermal conductivity, heat transfer enhancement mechanism for nanofluids also includes the changes of the flow characteristics, therefore it is needed to take an in-depth research on nanofluids flow characteristics. In this paper, the flow characteristics of H2O and SiO2-H2O nanofluids in a rectangular convex channel (channel composed of continuous staggered rectangular convex platform) at the Reynolds numbers 2300, 2500, 3000 and 4000 are studied by the quantitative PIV method (Fig. 1a). The rectangular convex channel (Fig. 1b) has periodic perturbation effect on the fluid flow, so that the flow direction is changed for several times, and vortexes are generated, which makes turbulence enhanced. In this way, flow is in the intense turbulent state under a low flow rate. Results show that the flow fields becomes more chaotic by the addition of nanoparticles (Fig. 2 and 3). Both the number and the size of vortices increase observably. The vorticity of nanofluids is also enhanced compared with H2O, and with the increase of Reynolds number, the increased ratio in the vorticity magnitude is getting higher (Fig. 4). At different Reynolds number, the pressure loss of nanofluids increases by 2.27%, 2.23%, 1.5% and 14.7%, respectively. As shown the flow resistance does not increase significantly compared to base fluids, especially at low Reynolds number. It can be concluded that the interaction between nanoparticles and the basic fluid strengthens the flow field disturbance, which is benefit to the heat transfer of nanofluids.
Proceedings Papers
Proc. ASME. HT2019, ASME 2019 Heat Transfer Summer Conference, V001T12A002, July 14–17, 2019
Paper No: HT2019-3729
Abstract
To reveal the effect of superhydrophobic rough surface on the friction properties, molecular dynamics simulations are used to study the friction properties of Couette flow. In particular, the influence of load on the flow properties is considered in this work. Results show that there is a critical load ( P crit ), and the friction-reduction properties of superhydrophobic surfaces with stripes are only presented when the load is smaller than the P crit . With the decrease in the distance between stripes, the P crit is increased. Under a low load, the friction force is increased with increasing the distance between stripes. However, under high load condition we observe an opposite trend. The height of stripe has little impacts on the P crit .
Proceedings Papers
Proc. ASME. HT2019, ASME 2019 Heat Transfer Summer Conference, V001T16A001, July 14–17, 2019
Paper No: HT2019-3730
Abstract
In addition to the increase of thermal conductivity, heat transferring for nanofluids strengthening mechanism also includes the changes of the flow characteristics, therefore it is needed to take an in-depth research on nanofluids flow characteristics. However previous visualization experiment do not quantitatively analyze the change of flow characteristics after nano-particles is added, do not reveal the mechanism of nanofluids changing the characteristics of the fluid in intense turbulent flow condition. Therefore in this paper, by means of particle image velocimetry, quantitatively study SiO 2 -water nanofluids flow characteristics in intense turbulent flow condition and analyze the influence of SiO 2 -water nanofluids on turbulent flow energy by measuring the pressure drop caused by fluid flowing through the channel. Fluid flow through rectangular convex channel (channel composed of continuous staggered rectangular convex platform) to obtain the steady intense turbulent flow. The rectangular convex channel makes the fluid flow through obstacles for several times, so that the flow direction changes for several times, and vortexes are generated in the local scope, which makes turbulence enhance and increases minor loss. In this way, flow can be in the intense turbulent state under a low flow rate, which meets the experiment requirement and is convenient to compare the influence of nano-particles on flow resistance and energy loss. The experiment takes the quantitative PIV experimental research on pure water and the volume fraction of 0.5% SiO 2 -water nanofluids respectively in the Reynolds number is 2300, 2500, 3000, 4000. Through the experiment, we can obtain nanofluids turbulent flow condition fluctuating velocity, turbulence kinetic, energy loss and so on, and the fluid flow velocity vector, streamline and vorticity graph. Through the quantitative comparison of the spiral numbers, the vorticity distribution, and energy loss, analyze strengthening effect and influence of flow resistance on basic fluid after adding nanoparticles.
Proceedings Papers
Proc. ASME. MNHMT2019, ASME 2019 6th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T03A006, July 8–10, 2019
Paper No: MNHMT2019-4180
Abstract
In order to study the effect of liquid-solid interaction and surface temperature on drag reduction and heat transfer, non-equilibrium molecular dynamics simulation is performed to investigate the density profile, velocity profile, velocity slip and temperature profile of fluid by changing liquid-solid interaction factor α and surface temperature. The result shows that there is a low density layer near the surface when α is small (weak liquid-solid interaction), larger α (strong liquid-solid interaction) can induce density oscillation and solid-like layer near the surface. Velocity slip will decrease as the increases of α. It’s worth noting that for α < 0.02, the density oscillation becomes more obvious as the rises of temperature, which impairs drag reduction; For α > 0.02 , the rises of temperature will impair the oscillation, which enhances drag reduction. Due to the existence of low density layer, the heat transfer capacity is very weak when α is small, but the capacity will be enhanced as the increases of α.
Proceedings Papers
Proc. ASME. MNHMT2019, ASME 2019 6th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T04A008, July 8–10, 2019
Paper No: MNHMT2019-4164
Abstract
Molecular dynamics simulation is performed to investigate the rapid boiling of nanofluid with the variation nanoparticle wettabilities above hydrophobic surface. Four fluids are selected: base fluid (fluid 1), nanofluid with nanoparticle wettability less than (fluid 2), equal tofluid 3) and greater than (fluid 4) surface wettability. It should be noted that nanoparticle is deposited on the surface in this paper. Results show that nanofluid responds rapid boiling faster than base fluid. For fluid 4, the efficiency in heat transfer is enhanced due to the improvement of surface wettability. While for fluid 2 and 3, the surface wettability is deteriorated by the depositional nanoparticle. The heat flux is strengthened, but argon temperature and evaporation number reduce, and thus fluid 2 and 3 are not beneficial for heat transfer.
Proceedings Papers
Proc. ASME. MNHMT2019, ASME 2019 6th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T04A010, July 8–10, 2019
Paper No: MNHMT2019-4215
Abstract
The manipulation of micro- or nano-structure is a promising method to improve pool boiling heat transfer performance. However, most studies just focus on the micro- or nano-structure without considering the combination micro- and nano-structure. In this paper, we fabricated synergistic microchannel, nano-structure, and micro-nano structure surface on the nickel by different technologies. Pool boiling of DI water under saturated condition was experimentally investigated. Result shows at the wall superheat of 18 K, the heat transfer coefficient of micro-nano structure, nano-structure and synergistic micro-channel surface are 16400, 13050, and 13400 W/m 2 K higher 89%, 50%, and 54% than that of smooth surface, respectively. The improved heat transfer is attributed to active nucleation sites and capillary flow.
Proceedings Papers
Proc. ASME. HT2017, Volume 2: Heat Transfer Equipment; Heat Transfer in Multiphase Systems; Heat Transfer Under Extreme Conditions; Nanoscale Transport Phenomena; Theory and Fundamental Research in Heat Transfer; Thermophysical Properties; Transport Phenomena in Materials Processing and Manufacturing, V002T10A015, July 9–12, 2017
Paper No: HT2017-4735
Abstract
Molecular dynamics simulation was performed to investigate pool boiling heat transfer of nanofluids on rough walls. Nanoparticle movement was calculated to investigate the physical mechanisms of boiling heat transfer. The simulated system consisted of four regions: vapor argon, liquid argon, solid copper, and copper nanoparticles, and three cases were considered: base fluids (case A), nanoparticles far from the wall (case B), and nanoparticles near the wall (case C). Boiling heat transfer was enhanced by the addition of nanoparticles, and the enhancement increased with increasing heating temperature. Case C showed that nanoparticles were adsorbed on the nonevaporated film and did not move with the fluids. Thus, nanoparticles enhanced heat and energy transfer between the wall and fluids. Case B showed that nanoparticles moved randomly in the fluid area, which enhanced heat transfer within the fluid.
Proceedings Papers
Proc. ASME. MNHMT2016, Volume 1: Micro/Nanofluidics and Lab-on-a-Chip; Nanofluids; Micro/Nanoscale Interfacial Transport Phenomena; Micro/Nanoscale Boiling and Condensation Heat Transfer; Micro/Nanoscale Thermal Radiation; Micro/Nanoscale Energy Devices and Systems, V001T02A007, January 4–6, 2016
Paper No: MNHMT2016-6451
Abstract
The flow and heat transfer characteristics of nanofluids in the near-wall region were studied by non-equilibrium molecular dynamics simulation. The nanofluid model consisted of one spherical copper nanoparticle and argon atoms as base liquid. The effective thermal conductivity (ETC) of nanofluids and base fluid in shear flow fields were obtained. The ETC was increased with the increasing of shear velocity for both base fluid and nanofluids. The heat transfer enhancement of nanofluids in the shear flow field ( v ≠0) is better than that in the zero-shear flow field ( v =0). By analyzing the flow characteristics we proved that the micro-motions of nanoparticles were another mechanism responsible for the heat transfer enhancement of nanofluids in the flow field. Based on the model built in the paper, we found that the thermal properties accounted for 52%–65% heat transfer enhancement and the contribution of micro-motions is 35%–48%.
Journal Articles
Article Type: Research-Article
J. Nanotechnol. Eng. Med. November 2014, 5(4): 041001.
Paper No: NANO-14-1047
Published Online: October 17, 2014
Abstract
Limitations in energy crisis and environment protection promote the development of engine lubricants. By friction machine and AVL diesel engine bench, the present investigation studies the tribological properties and dynamic performance of diesel engine with lubricants of commercial quality dispersed with different mass concentrations of nanodiamond particles. Reverse dragging process tests and mapping characteristics tests were brought in the bench test. Additionally, investigations were conducted using viscometer, thermal conductivity meter, scanning electron microscopy (SEM), and transmission electron microscope (TEM) to interpret the possible influence mechanisms of tribology and thermal conduction with nanodiamond particles. The friction machine experimental results show that lubricants dispersed with nanodiamond particles exhibit good friction-reduction and antiwear properties. The engine bench tests indicate that it has a desirable effect on engine performance, decreasing the mechanical loss while increasing fuel economy.
Proceedings Papers
Proc. ASME. MNHMT2013, ASME 2013 4th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T02A004, December 11–14, 2013
Paper No: MNHMT2013-22024
Abstract
Understanding how the nanoparticles influence flow behavior of nanofluids is important for revealing mechanism of heat transfer enhancement by using nanofluids. The aim of this work was to study the microscopic change in base fluid and micro-motion of nanoparticles due to Brownian motion by molecular dynamics simulation. The present work established shearing flow simulation models considering different shapes of nanoparticles. Velocity distribution and number density distribution of fluid, and angular velocity components and translational velocity components of nanoparticles were statistically analyzed. The results of velocity distribution and number density distribution showed that adding nanoparticles reduces flow boundary layer and causes uneven distribution of mass; and the results for angular velocity components and translational velocity components of nanoparticles showed that nanoparticles rotate fast in the fluid, and vibrate irregularly. The present study suggests that adding nanoparticles causes microscopic change for base fluid including reducing thickness of flow boundary layer and uneven density distribution in fluid. In addition, the micro-motions of nanoparticles including rotation and vibration due to Brownian motion strengthen micro-flow effect and momentum transfer in nanofluids. Furthermore, by comparing motion behaviors of nanoparticles in different shapes the present work reveals that shapes of nanoparticles influence deeply flow behavior of nanofluids.
Proceedings Papers
Proc. ASME. MNHMT2013, ASME 2013 4th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T02A001, December 11–14, 2013
Paper No: MNHMT2013-22008
Abstract
In order to reveal the mechanisms of heat transfer enhancement in nanofluids from the flow characteristics, this paper firstly used LES (Large eddy simulation)–Lagrange method to simulate the turbulent flow of nanofluids through a straight circular tube. It has been observed that nanoparticles would move up and down and sideways besides main flowing. The turbulent characteristics of nanofluids have been changed greatly in comparison with pure water: the turbulent intensity and Reynolds stress are enhanced obviously; there are more vortexes in the flow field. These flow characteristics of nanofluids can effectively strengthen the transport of momentum, mass and energy, which is the main reason for heat transfer enhancement in nanofluids. It is also found that nanofluids containing smaller diameter nanoparticles have higher turbulent intensity and flow activity. The flow characteristics of nanofluids are sensitive to the changes of smaller diameter nanoparticle size. While using different nanoparticle materials, the flow characteristics of nanofluids have a little change. At last, to verify the aforesaid views, the flow behaviors of nanofluids in the near wall region and main flow region have been simulated by molecular dynamics.
Proceedings Papers
Proc. ASME. IMECE2009, Volume 2: Biomedical and Biotechnology Engineering, 361-366, November 13–19, 2009
Paper No: IMECE2009-10432
Abstract
For the purpose of realizing the noninvasive exploration of gastrointestinal tract, a novel magnetic propulsion system is proposed, which includes a patient support, a magnet assembly with two groups of permanent magnets positioned oppositely, and a magnet support. The proposed approach exploits permanent magnet and coupling movement of multi-axis components to generate quasi-static magnetic field for controlling the position, orientation, and movement of a self-propelled robotic endoscope in the gastrointestinal tract. By driving the five coupling axes, the proposed magnetic propulsion system is capable of steering the capsule endoscope through the intestinal tract in multi-directions of 2D space. Experiments in simulated intestinal tract are conducted to demonstrate controlled translation, rotation, and rototranslation of capsule endoscope. Finite Element Method is used to analyze navigation system’s mechanical properties and the distributions of magnetic field. The proposed technique has great potential of enabling the application of controlled magnetic navigation in the field of capsule endoscopy.