Energy Transport Mechanisms in Nanofluids and Its Applications

Nanofluid is a solid-liquid mixture consisting of solid nanoparticles or nanofibers with sizes typically of 1–100 nm suspended in liquid. Thermal conductivity and heat transfer performance of nanofluids is superior to those of the original pure carrier fluids because the suspended nanoparticles remarkably improve energy exchange capability of the suspensions. In the present paper, the investigations efforts cover microscopic and mesoscaled approachs for the heat transfer enhancement mechanism of the nanofluid, flow and heat transfer mechanism and the relevant control methods of the magnetic fluid by suspending magnetic nanoparticles in base fluids, and some applications of nanofluid on a variety of thermal systems in order to understand energy transfer mechanism of nanofluids and guide future applications of nanofluids to thermal engineering.

Investigating the Effect of Solid Boundaries on the Gas Molecular Mean-Free-Path

A key parameter for micro-gas-flows, the mean free path, is investigated in this paper. The mean free path is used in various models for predicting micro gas flows, both in the governing equations and their boundary conditions. The conventional definition of the mean free path is based on the assumption that only binary collisions occur and is commonly described using the macroscopic quantities density, viscosity and temperature. In this paper we compare the prediction by this definition of the mean free paths for helium, neon and argon gases under standard temperature and pressure conditions, with the mean free paths achieved by measurements of individual molecules using the numerical simulation technique of molecular dynamics. Our simulation using molecular dynamics consists of a cube with six periodic boundary conditions, allowing us to simulate an unconfined gas “package”. Although, the size of this package is important, since its impact on computational cost is considerable, it is also important to have enough simulated molecules to average data from. We find that the molecular dynamics method using 20520 simulated molecules yields results that are within 1% accuracy from the conventional definition of the mean free paths for neon and argon and within 2.5% for helium. We can also conclude that the normal approximation of only considering binary collisions is seemingly adequate for these gases under standard temperature and pressure conditions. We introduce a single planar wall and two parallel planar walls to the simulated gas of neon and record the mean free paths at various distances to the walls. It is found that the mean free paths affected by molecular collisions with the walls corresponds well with theoretical models up to Knudsen numbers of 0.2.

Experimental Investigation of Critical Heat Flux in Microchannels for Flow-Field Probes

Critical heat flux (CHF) of water in circular stainless steel microchannels with inner diameters ranging from ∼127μm to ∼254 μm was investigated. Forty-five CHF data points were acquired over mass velocities ranging from 1,200 kg/m2s to 53,000 kg/m2s, heated lengths from 2 cm to 8 cm, and exit qualities from −0.2 to 0.15. Most of the exit qualities fell below 0.1. It was found that CHF conditions were more dependent on mass velocity and heated length than on exit thermal condition. The results were also compared to six CHF correlations, with a mean average error ranging from 22% to 261.8%. A new correlation was proposed to better predict the critical heat flux data under the thermal-hydraulic conditions studied in this investigation. In developing the correlation, 319 data points were added from two previous studies.

Switching Time of Electrowetting-Based Devices

The switching time is important factor for enhancing performance of electrowetting-based devices, such as electronic display, liquid lens, and RF switches. These devices require fast spreading, receding time of droplet. In this work, we suggested an analytical method to predict the switching time. We considered the case of a liquid droplet on an insulator-coated electrode surrounded by air. The properties of droplet such as volume, density, viscosity, and interfacial tension affects the switching time. First we explored the influence of drop volume on switching time. To predict the switching time, the dynamics of a droplet are modeled by using the domain perturbation method. It is shown that the overall dynamics is governed by P2 shape mode. Based on theoretical analysis, we introduced a parameter which represents the switching time. We compared the theoretical prediction with experimental results, and then discussed the effect of contact-line hysteresis.

Transitional Flow and Related Transport Phenomena in Complex Microchannels

In microchannels with typical dimensions from 10 μm to few hundreds μm, the flow is dominated by viscous forces, leading often to laminar flow conditions. At the entrance or in bends and curves, where the flow is accelerated or changes its direction, inertial forces generate transverse flow velocities. Due to continuity, compensating transverse velocity components generate vortex pairs, such as Dean flow in circular bends. The flow is still laminar, steady, and shows no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions, often in larger channels (100 μm to few mm) or for higher flow rates, is called transitional flow. That embraces the first occurrence of pulsating vortices, period doubling of vortex pairs, flow bifurcation, and regularly fluctuating wake flow or vortex shedding. With increased flow velocity, this process leads to chaotic flow phenomena being first evidence of turbulence. This paper describes the transitional flow characteristics in single channel elements such as bends and T-junction as well as around fins and posts in channels. These elements are used to augment the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of the flow characteristics is fundamental for the understanding of transport phenomena. Additionally, this knowledge can be used to design successful microstructured devices for various applications by knowing how to generate and control vortices in microchannels. Concepts from chaotic advection are presented here to describe vortex flow and related transport characteristics. Though recent advances has shed new light on transport phenomena in complex channel structures, many issues are still unknown and huge potential is hidden in optimized channel devices.

Experimental Study of Evaporation Heat Transfer Characteristics and Pressure Drops of R410A and R134a in a Multiport Mini-Channel

The evaporation heat transfer coefficients and pressure drops of R-410A and R-134a flowing through a horizontal-aluminium rectangular multiport mini-channel having a hydraulic diameter of 3.48 mm are experimentally investigated. The test runs are done at refrigerant mass fluxes ranging between 200 and 400 kg/m2s. The heat fluxes are between 5 and 14.25 kW/m2, and refrigerant saturation temperatures are between 10 and 30 °C. The effects of the refrigerant vapour quality, mass flux, saturation temperature and imposed heat flux on the measured heat transfer coefficient and pressure drop are investigated. The experimental data show that in the same conditions, the heat transfer coefficients of R-410A are about 20–50% higher than those of R-134a, whereas the pressure drops of R-410A are around 50–100% lower than those of R-134a. The new correlations for the evaporation heat transfer coefficient and pressure drop of R-410A and R-134a in a multiport mini-channel are proposed for practical applications.

Thermal Bubble Dynamics Under the Effect of Acoustic Vibration

The effect of acoustic field on the dynamics of micro thermal bubble is investigated in this paper. The micro thermal bubbles were generated by a micro heater which was fabricated by standard Micro-Electro-Mechanical-System (MEMS) technology and integrated into a mini chamber. The acoustic field formed in the mini chamber was generated by a piezoelectric plate which was adhered on the top side of the chamber’s wall. The dynamics and related heat transfer induced by the micro heater generated vapor bubble with and without the existing of acoustic field were characterized by a high speed photograph system and a micro temperature sensor. Through the experiments, it was found that in two different conditions, the temperature changing induced by the micro heater generated vapor bubble was significantly different. From the analysis of the high speed photograph results, the acoustic force induced micro thermal bubble movements, such as forcibly removing, collapsing and sweeping, were the main effects of acoustic enhanced boiling heat transfer. The experimental results and theoretical analysis were helpful for understanding of the mechanisms of acoustic enhanced boiling heat transfer and development of novel micro cooling devices.

Micro/Meso Scale Modelling of Two-Phase Flow on Functional Surfaces: A Numerical Simulation of Water Droplets on Natural Hydrophobic Surfaces With Micro Roughness

A micro/meso scale modelling of two-phase droplets move on hydrophilic/hydrophobic surfaces with micro roughness is reported. The physical model is basically of two-phase flow interacting with the surfaces of different hydrophobicity or wettability. Numerical modelling based on the lattice Boltzmann method (LBM) is developed and applied to the computational calculation and simulation. The LBM modelling deals with surface tension dominated behaviour of water droplets in air spreading on a hydrophilic surface with hydrophobic strips of different sizes and contact angles under different physical and interfacial conditions, and aims to find quantitative data and physical conditions of the biomimetic approaches. The current LBM can be applied to simulate two-phase fluids with large density ratio (up to 1000), and meanwhile deal with interactions between a fluid-fluid interface and a partial wetting wall. In the simulation, the interactions between the fluid-fluid interface and the partial wetting wall with different hydrophobic strips such as single strip, intersecting stripes, and alternating & parallel stripes, of different sizes and contact angles are considered and tested numerically; the phenomena of droplets spreading and breaking up, and the effect of hydrophobic strips on the surface wettability or self-cleaning characteristics are simulated, reported and discussed.

Cryogenic and Fluidic Ways Lead to Low Cost Micro/Nano Devices

Building systems as compactly as possible has been a major theme in modern science and engineering practices. However, such enthusiastic endeavor often encounters big troubles due to high cost and complexity of the process it involves. Part of the reasons comes from the methodology itself, the fabrication, designing and characterization procedure etc. Among various disciplines to making micro/nano object, those enabled from the thermal and hydrodynamic science plays a rather important role. In this article, we will illustrate a cryogenic way for realizing a group of different micro/nano devices which can be implemented as mechanical, hydraulic, electrical, or optical functional units. The basic principle of the method lies in the formation of ice crystals in small area, from which micro/nano aqueous objects or signals transmitting across them can be blocked, manipulated and analyzed. In this way, a series of micro/nano devices such as freeze tweezer, ice valve, freeze-thaw pump, electrical or optical signal switch and micro thermal analyzer etc. can be developed via a rather simple and low cost way. As examples, some latest advancement made in the authors’ lab will be reviewed. Their innovative applications in a wide variety of micro/nano engineering fields will be discussed. Further, to illustrate the low cost way to directly manufacture micro/nano objects, we will explain a bubble fabrication method whose basic principle lies in the chemical reaction occurring at the fluidic interfaces between two or more soap adjacent bubbles. A unique virtue of the bubble is that it can have a rather huge diameter however an extremely small membrane thickness, whose smallest size can even reach nano scale. Therefore, the administrated chemical reaction in the common interface of the contacting bubbles would lead to products with extremely small size. Particularly, all these results were achieved via a rather straightforward way. The bubble builds up a bridge between the macroscopic manipulation/observation and the fabrication in small world. Several typical micro structures as fabricated in the lab will be illustrated. As a flexible, easily controllable, and low cost method, the bubble fabrication can possibly be developed as a routine strategy for making micro/nano structures in the near future.

Development of Cooling System for a Large Area at High Heat Flux by Using Flow Boiling in Narrow Channels

Advance of an electronic technology has caused the increase of heat generation density for semiconductors densely integrated. Thermal management becomes more important, and a cooling system for high heat flux is required. It is extremely effective to such a demand using flow boiling heat transfer because of its high heat removal ability. To develop the cooling system for a large area at high heat flux, the cold plate structure of narrow channels with auxiliary unheated channel for additional liquid supply was devised and confirmed its validity by experiments. A large surface of 150mm in heated length and 30mm in width with grooves of an apex angle of 90 deg, 0.5mm depth and 1mm in pitch was employed. A structure of narrow rectangular heated channel between parallel plates with an unheated auxiliary channel was employed and the heat transfer characteristics were examined by using water for different combinations of gap sizes and volumetric flow rates. Five different liquid distribution modes were tested and their data were compared. The values of CHF larger than 1.9×106W/m2 for gap size of 2mm under mass velocity based on total volumetric flow rate and on the cross section area of main heated channel 720kg/m2s or 1.7×106W/m2 for gap size of 5mm under 290kg/m2s were obtained under total volumetric flow rate 4.5×10−5m3/s regardless of the liquid distribution modes. Under several conditions, the extensions of dry-patches were observed at the upstream location of the main heated channel resulting burnout not at the downstream but at the upstream. High values of CHF larger than 2×106W/m2 were obtained only for gap size of 2mm. The result indicates that higher mass velocity in the main heated channel is more effective for the increase in CHF. It was clarified that there is optimum flow rate distribution to obtain the highest values of CHF. For gap size of 2mm, high heat transfer coefficient as much as 7.4×104W/m2K were obtained at heat flux 1.5×106W/m2 under mass velocity 720kg/m2s based on total volumetric flow rate and on the cross section area of main heated channel. Also to obtain high heat transfer coefficient, it is more useful to supply the cooling liquid from the auxiliary unheated channel for additional liquid supply in the transverse direction perpendicular to the flow in the main heated channel.