Possible Mechanisms for Thermal Conductivity Enhancement in Nanofluids

This study discusses the merits of various physical mechanisms that are responsible for enhancing the heat transfer in nanofluids. Experimental studies have cemented the claim that ‘seeding’ liquids with nanoparticles can increase the thermal conductivity of the nanofluid by up to 40% for metallic and oxide nanoparticles dispersed in a base liquid. Experiments have also shown that the rise in conductivity of the nanofluid is highly dependent on the size and concentration of the nanoparticles. On the theoretical side, traditional models like Maxwell or Hamilton-Crosser models cannot explain this unusually high heat transfer. Several mechanisms have been postulated in the literature such as Brownian motion, thermal diffusion in nanoparticles and thermal interaction of nanoparticles with the surrounding fluid, the formation of an ordered liquid layer on the surface of the nanoparticle and microconvection. This study concentrates on 3 possible mechanisms: Brownian dynamics, microconvection and lattice vibration of nanoparticles in the fluid. By considering two nanofluids, copper particles dispersed in ethylene glycol, and silica in water, it is determined that translational Brownian motion of the nanoparticles, presence of an interparticle potential and the microconvection heat transfer are mechanisms that play only a smaller role in the enhancement of thermal conductivity. On the other hand, the lattice vibrations, determined by molecular dynamics simulations show a great deal of promise in increasing the thermal conductivity by as much as 23%. In a simplistic sense, the lattice vibration can be regarded as a means to simulate the phononic transport from solid to liquid at the interface.

Visualization of Boiling in Inclined Rectangular Narrow Long Channels

During the Three-Mile Island Unit 2 (TMI-2) accident, the lower part of the reactor pressure vessel had been overheated and then rather rapidly cooled down, as it was later found out in a vessel investigation project. These findings triggered a great deal of investigations to determine the critical heat flux (CHF) in narrow channels. Experiments were conducted to determine the CHF on a long downward heated rectangular narrow channel by changing the orientation of a copper crevice (5×105 mm2) type heater assembly. The test heater was placed in a demineralized, saturated water pool at atmospheric pressure. This work aims also to investigate the general boiling phenomena and the triggering mechanism for the CHF in the narrow channel through visualization of the bubble behavior in the vicinity of CHF. The test parameters include the channel size of 5 mm and the surface orientation angles from the downward facing position (180°) to the vertical position (90°). It was found that the CHF decreases as the surface inclination angle increases and as the gap size decreases. It was also shown that there exists a transition angle at which the CHF changes with a rapid slope, and that the inclination angle affects the bubble layer and the bubble discharge from the narrow gap.

On the Application of Macro-Pipe Two-Phase Heat Transfer Correlations and Flow Regime Maps to Mini-Channels

The present study is aimed at evaluating the ability of conventional “macro-pipe” correlations and regime transitions to predict the two-phase thermofluid characteristics of mini-channel cold plates. Use is made of the Taitel-Dukler flow regime maps, seven classical heat transfer coefficient correlations and two dryout predictions. The vast majority of the mini-channel two-phase heat-transfer data, taken from the literature, is predicted to fall in the annular regime, in agreement with the reported observations. A characteristic heat transfer coefficient locus has been identified, with a positive slope following the transition from Intermittent to Annular flow and a negative slope following the onset of partial dryout at higher qualities. While the classical two-phase heat transfer correlations are generally capable of providing good agreement with the low-quality annular flow data the quality at which partial dryout occurs and the ensuing heat transfer rates are not predictable by the available macro-pipe correlations.

Effects of Interfacial Interactions Between Fluid and Solid Wall on Microscale Flows

This work discusses the interfacial effects on flow and heat transfer at micro/nano scale. Different from bulk cases where interfaces can be simply treated as a boundary, the interfacial effects are not limited to the interface at microscale but extend into a significant, even the whole domain of the flow and heat transfer field when the characteristic size of the domain is close to the mean free path (MFP) of fluid particles. Most of microscale flow phenomena result from interfacial interactions. Any changes in the interactions between the fluid and solid wall particles could affect the flow and heat transfer characteristics, such as flow and temperature profiles, friction coefficient. The interactions depend on many parameters, such as the force between fluid and solid wall particles, microstructure of interfaces. The flow and heat transfer features does not only depend on the fluid itself, but also on the interaction with the solid wall because the interface impact can go deep inside the flow. Same fluid, same channel shape but different wall materials could have different flow characters.

High Performance and Sub-Ambient Silicon Microchannel Cooling

High performance single-phase Si microchannel coolers have been designed and characterized in single chip modules in a laboratory environment using either water at 22°C or a fluorinated fluid at temperatures between 20 and −40°C as the coolant. Compared to our previous work, key performance improvements were achieved through reduced channel pitch (from 75 to 60 microns), thinned channel bases (from 425 to 200 microns of Si), improved thermal interface materials, and a thinned thermal test chip (from 725 to 400 microns of Si). With multiple heat exchanger zones and 60 micron pitch microchannels with a water flow rate of 1.25 lpm, an average unit thermal resistance of 15.9 C-mm2/W between the chip surface and the inlet cooling water was demonstrated for a Si microchannel cooler attached to a chip with Ag epoxy. Replacing the Ag epoxy layer with an In solder layer reduced the unit thermal resistance to 12.0 C-mm2/W. Using a fluorinated fluid with an inlet temperature of −30°C and 60 micron pitch microchannels with an Ag epoxy thermal interface layer, the average unit thermal resistance was 25.6 C-mm2/W. This fell to 22.6 C-mm2/W with an In thermal interface layer. Cooling >500 W/cm2 was demonstrated with water. Using a fluorinated fluid with an inlet temperature of −30°C, a chip with a power density of 270 W/cm2 was cooled to an average chip surface temperature of 35°C. Results using both water and a fluorinated fluid are presented for a range of Si microchannel designs with a channel pitch from 60 to 100 microns.

Improvement of Micro-PIV Resolution by Selective Seeding

A novel seeding method, permitting high out-of-plane resolution and instantaneous (time-resolved) velocity field measurements using a standard Microscale Particle Image Velocimetry (micro-PIV) setup, is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. The generated particle sheet defines the depth and position of the measurement plane, independently of the details of the optical setup. Therefore, for low magnification objectives in particular, considerable improvement in the measurement depth is possible. Selectively seeded micro-PIV (SeS-PIV) is applied to a microchannel flow, and the measured instantaneous velocity fields are in excellent agreement with the theoretical solution for the flowfield. The currently presented measurements have a depth-wise resolution 20% below the estimated optical measurement depth of the micro-PIV system. In principle, a measurement depth corresponding to the diameter of the tracer particles may be achieved.

Study of the Combined Contributions of Porous Coverings and Mixture Surface Energies in Enhancing Pool Boiling

Factors as the boiling fluid surface tension and the characteristics of the solid surface where the heat transfer takes place could be modulated for increasing the boiling heat flux. In this work was observed the increase in the boiling convective heat-transfer coefficient (h) from the participation of: (a) the use of a binary mixture at its critical micelle concentration (16% w/w ethanol-water); (b) the addition of the surfactant sodium-lauryl-sulfate (SLS) to this aqueous mixture; and (c) the use of a porous covering fabricated from stainless steel bands with void volume 0.25, pore diameter 0.8 mm and covering thickness 8 mm. The sequence of results allowed the calculation of the relative participation of these factors in h (and the related values of excess temperature), for power supply from 100 to 1000 W on the same heater cartridge for all the experiments. For boiling water on the bare heater, hmax bare heater = 8.27 W/cm2 K; for boiling water on the porous covering, hmax covering = 19.36 W/cm2 K; the boiling of the water-ethanol (16%) mixture on the porous covering produced hmax covering+cmc = 31.72 W/cm2 K; and the binary mixture with 100 ppm of SLS, hmax covering+cmc+surfactant = 38.07 W/cm2 K. Considering this value of hmax covering+cmc+surfactant as the sum of the contributions, the relative participation of the mechanical forces breaking the escaping bubbles through the covering is 29.13%; the surface energies associated to the formation of micelle structures 32.47%; and the surface energies from the surfactant 16.67%. Thus, the search of enhancing heat transfer should consider the boiling mixture composition as well as the porous covering design. A comparison of the results obtained with the covering developed in this work with some coverings developed in a previous work reveals that the geometry of the covering material could be the base for heat transfer enhancement.

Numerical Simulation of Thermo-Electro-Magnetic Performances in Supersonic Channel Flow

A compressible magnetohydrodynamic (MHD) model composed of MHD Navier-Stokes (N-S) equations and magnetic induction equations is proposed in the present study for analyzing the magnetohydrodynamic characteristics in MHD generator and MHD accelerator channels of Magneto-Plasma-Chemical propulsion system [10∼12]. A splitting algorithm based on an alternative iteration is also developed for solving the two sets of equations [9]. As a test case, a supersonic MHD flow in a square duct was simulated. The numerical results are compared with the results computed by solving the classical N-S equations for the perfect gas flow, together with the results computed utilizing the degenerate MHD N-S equations for the same channel flow with constant applied magnetic field. The thermo-electro-magnetic performances of the test cases with constant and variable applied fields are then discussed.

Two-Phase Pressure Drop and Boiling Heat Transfer in a Single Horizontal Microchannel

With a single microchannel and a series of microheaters made with MEMS technique, two-phase pressure drop and local flow boiling heat transfer were investigated using deionized water in a single horizontal rectangular microchannel. The test microchannel has a hydraulic diameter of 100 μm and length of 40 mm. A real time observation of the flow patterns with simultaneous measurement are made possible. Tests are performed for mass fluxes of 90, 169, and 267 kg/m2s and heat fluxes of from 100 to 600 kW/m2. The experimental local flow boiling heat transfer coefficients and two-phase frictional pressure gradient are evaluated and the effects of heat flux, mass flux, and vapor qualities on flow boiling are studied. Both the evaluated experimental data are compared with existing correlations. The experimental heat transfer coefficients are nearly independent on mass flux and the vapor quality. Most of all correlations do not provide reliable heat transfer coefficients predictions with vapor quality and prediction accuracy. As for two-phase pressure drop, the measured pressure drop increases with the mass flux and heat flux. Most of all existing correlations of two-phase frictional pressure gradient do not predict the experimental data except some limited conditions.

Experimental Study on Fundamental Phenomena of Boiling by Using Heat Transfer Surface With Well-Defined Cavities Created by MEMS: The Effect of Spacing Between Cavities

Pool nucleate boiling heat transfer experiments were performed for water by using the well-controlled and -defined heat transfer surfaces. Artificial cavity(ies) was (were) created on the mirror-finished silicon plate of 0.525 mm thickness by utilizing the Micro-Electro Mechanical System (MEMS) technology. Each cavity had cylindrical shape. The diameter and the depth of the cavity were 10μm and 40μm, respectively. Experiments were performed in a range of a heat flux ∼6.0 × 104 W/m2 for distilled water. When the cavity interval was close, the horizontal and declining coalescence of bubble on the cavities were dominant. This vigorous bubble coalescence created strong convection. The heat carried by this convection took a main part in the heat transfer when cavities were close. As the cavity interval became wide, the horizontal and declining coalescence did not take place anymore. The coalescence was limited only to the vertical lift or no coalescence. In this situation, bubbles grew large on the cavities and absorbed latent heat sufficiently. Bubbles themselves took the main role of carrying heat away from the heat transfer surface when cavities were further apart.