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.

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%.

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.

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.

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.