Vertically Aligned Carbon Nanotube (CNT) Arrays are promising to use as advanced thermal interface material. While possessing high thermal conductivity for an individual tube, carbon nanotube array based thermal interface materials (TIMs) fell short of expectations due to poor CNTs-target surface contacts. Investigations suggested that the overall resistance can be potentially reduced to less than 1 m 2 -K/MW by increasing the number of tubes to target surface contacts. This paper use chromium/gold/indium assisted thermal pressure-bonding to enhance contacts. A CNT array with 12.7% areal density was bonded to an experimental glass surface with 2-μm indium bonding layer and 10 nm-chromium/150 nm-gold adhesion layers under pressure of 196 KPa and temperature of 350 °C. Phase sensitive photothermal reflectance method was used for thermal measurement. The overall resistance, including CNTs-glass contact resistance and effective CNT array thermal resistance, is 1.1 m 2 -K/MW ± 27%. Although the contact resistance was reduced to 0.39 m 2 -K/MW ± 15%, the effective thermal conductivity of the post-bonded 80 μm long CNTs was 114 W/m-K ± 22%, which was lower than the expected lower bound of the thermal conductivity of 12.7% filled CNT array. It was suggested that the deformation of CNT array after mechanical bonding reduced its performance.

Spray quenching is widely used in industrial applications. In atomized spray quenching (ASQ), water and air are supplied to the nozzle at a certain flow rate and pressure to produce a full cone spray consisting of discrete droplets. Impingement density of spray i.e. coolant mass flow per unit area per second is considered to be the most influential parameter for heat transfer. Impingement density varies with radius and so as the heat flux. Water quality is altered by adding five different salts i.e. NaCl, Na 2 SO 4 , NaHCO 3 , Na 2 CO 3 , and MgSO 4 in de-ionized water with various concentrations. On the other hand, a surfactant Ethoxylated ester, which is commonly added in cooling water in cast houses of metals, is added to pure water in different concentrations i.e 50, 100, 200 and 500ppm. A circular disc made of Nickel of thickness 2mm is heated to 600°C and sprayed on one side by atomized spray and the temperature distribution with respect to time is measured using Infrared camera on the other side of the disc. By this IR thermography, transient temperature measurement can be done within the window of 320×80 pixels with a minimum pixel real distance of 1mm on the sheet surface. Frequency of measurement is 150Hz. Since the temperature measurement and cooling sides are opposite at 2mm thickness apart, inverse heat conduction problem is solved by applying finite element method for calculating temperature and heat flux on the quenched side of metal sheet with respect to space and time. It has been observed that increasing the concentration of salts increase the leidenfrost point and shortens the film boiling regime. While addition of surfactants decrease the leidenfrost point and prolong the film boiling regime. Maximum heat flux position is considered as the wetting front position. There is an abrupt variation of heat flux at wetting front position due to the change of boiling phenomenon. Wetting front velocity has been compared for salt solutions, surfactant and de-ionized or pure water.

Thermal transport by phonons in water/carbon nanotube (CNT) composite systems is investigated using molecular dynamics (MD) simulation. We calculate the spectral energy density of empty and water-filled CNTs and use it to extract the mode-specific phonon group velocities, relaxation times, and thermal conductivities. The total thermal conductivity predicted from the spectral energy density is consistent with what we predict using a direct application of the Fourier law in an MD simulation. The number of atoms and simulation runtime required to predict the spectral energy density, however, are both at least one order-of-magnitude smaller.

The application of nanofluids in real systems could lead to smaller, more compact heat exchangers and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional cooling systems. In this work, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5 wt%, 1 wt%, and 3 wt%) were investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found to be linear whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but significant enhancement in the heat transfer coefficient was recorded for 1800 < Re > 2800 which is attributed to an earlier transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and increased temperature.

Water spray cooling is an important technology which has been used in a variety of engineering applications for cooling of materials from high-temperature nominally up to 900°C, especially in steelmaking processes and heat treatment in hot metals. The effects of cooling water temperature on spray cooling are significant for hot steel plate cooling applications. The local heat flux measurements are introduced by a novel experimental technique in which test block assemblies with cartridge heaters and thermocouples are used to measure the heat flux distribution on the surface of hot steel plate as a function of heat flux gauge. The spray is produced from a fullcone nozzle and experiments are performed at fixed water impact density of G and fixed nozzle-to-target spacing. The results show that effects of water temperature on forced boiling heat transfer characteristics are presented for five different water temperatures between 5 to 45°C. The local heat flux curves and heat transfer coefficients are also provided to a benchmark data for the actual spray cooling of hot steel plate cooling applications.

With the advancement in microfabrication technologies the size of electronic devises keeps decreasing. At the same time high density of circuits and the directly related heat generation requires simultaneous increase in the performance of cooling systems to keep the electronics operating at optimum temperatures. Liquid cooling incorporating microchannel networks as the heat sinks is known to be a promising approach to this heat transfer problem because of its plausible manufacturability at the chip level. However, even the upper range of heat fluxes achieved in such microchannel heat sinks will not meet the increasing demands of the electronics industry. We present herein a novel experimental study of convective heat transfer in serpentine microchannels with segmented liquid-liquid emulsions. It is demonstrated that this concept yields significant heat transfer enhancement in micro channel heat sinks compared to traditional single phase liquid cooling. The flow of water and mineral oil droplets is examined experimentally in microchannels with cross section 100 by 100 μm and flow rates up to 130 μl/min. The study is focused on the investigation of the local temperature of the liquid and on the influence of the oil droplets on the temperature distribution and heat transfer. Laser Induced Fluorescence (LIF) is employed to measure the temperature of the flow fields with and without droplets, and PIV measurements are conducted to determine the velocity field. The range of flow conditions which led to a stable segmented flow with enhancement of heat transport was identified. The increase of the oil fraction was accompanied by an increase of the recirculation zone in the transverse direction to the flow and by higher velocity gradients inside the zones thus resulting in enhanced mixing of water across the channel. The LIF measurements indicate that for the same total flow rate, the local temperature of the water region separating the droplets will be higher than in the case of single phase liquid flow. For segmented flow, up to a four-fold increase of the Nusselt number compared to pure water flow was observed.

A carbon nanofiber material, consisting of bottomless graphene cups inside on each other in a line, like a set of soft-drink cups, has been discovered to have the potential to conduct heat ballistically over a long distance. Its longitudinal heat transport ability had been forecast to be extremely poor due to the weak van der Waals force operating between the graphene cups, but our measurements using nano thermal sensor showed that its thermal conductivity is much higher than that along the c-axis of bulk graphite. This unexpected result can be understood by its similarity to a one-dimensional (1D) harmonic-chain where no phonon is scattered even for an infinite length. The current graphene-based nanofiber resembles this type of “superconductive” chain due to the huge difference between the stiff covalent bonding in each cup and the weak inter-cup interaction. A non-equilibrium molecular dynamics simulation is conducted to explore the phonon transport in this fiber. The simulation results show that the thermal conductivity varies with the fiber length in a power law fashion with an exponent as large as 0.7. The calculated phonon density of states and atomic motions indicate that a low-frequency quasi-1D oscillation occurs there. Our investigations show that treating the current nanofiber as a 1D chain with three-dimensional oscillations explains well why this material has the most effective ballistic phonon transport ever observed.

Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per density. At high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the phenomena. In macro tubes, large numbers of researches have been conducted to investigate the liquid film thickness. However, in micro tubes, quantitative information for the annular liquid film thickness is still limited. In the present study, annular liquid film thickness is measured using a confocal method, which is used in the previous study [1, 2]. Glass tubes with inner diameters of 0.3, 0.5 and 1.0 mm are used. Degassed water and FC40 are used as working fluids, and the total mass flux is varied from G = 100 to 500 kg/m 2 s. Liquid film thickness is measured by laser confocal displacement meter (LCDM), and the liquid-gas interface profile is observed by a high-speed camera. Mean liquid film thickness is then plotted against quality for different flow rates and tube diameters. Mean thickness data is compared with the smooth annular film model of Revellin et al. [3]. Annular film model predictions overestimated the experimental values especially at low quality. It is considered that this overestimation is attributed to the disturbances caused by the interface ripples.

We developed a measurement technique that can quantitatively map out the dopant density profile of a silicon integrated-circuit device. This method obtains the quantitative doping density profile by simultaneously carrying out local heating, temperature sensing, and thermoelectric voltage measurement at the tip of a diamond thermocouple probe. This probe, which is the key component of the proposed scheme, is fabricated through a nano-fabrication technique that makes use of boron-doped diamond film that can resist stress up to 10 Gpa, which is necessary for stable electric contact with silicon samples. The tip and cantilever of the probe are made of B-doped diamond by means of the silicon lost-mold technique that guarantees a sharper tip apex than that of a diamond-coated probe. A gold-chromium thermocouple junction is integrated at the tip apex for simultaneous heating and sensing. The size of the thermocouple is about 500 nm and the radius of the tip apex is less than 50 nm. The measurement technique is demonstrated by measuring the thermopower distribution across a silicon p-n junction and the result is compared with the theoretical values.

Heat transfer enhancement of carbon-nano fibers (CNF’s) attached on a wall surface within a micro-channel is investigated in this paper using a three-dimensional numerical method. Carbon-nanofibers, also known as graphite nanofibers, can be grown by catalytic decomposition of certain hydrocarbon at a metal surface such as iron, cobalt, nickel and some of their alloys. Typical sizes of CNF’s vary between 2 and 100 nm, with lengths ranging from 5 to 100 μm. Experimental research has shown that the presence of carbon-nano fibers grown on a surface of a fine metallic structure can enhance heat transfer by 50% [2]. These fibers influence the fluid flow, and enlarge the heat exchanging surface. The enhancement depends very much on the carbon-nano fibers density and on the structure of the carbon-nano fibers itself. This numerical study is giving directions in optimizing this new material. A random generation growth model has been developed to generate a stochastic structure of the CNF layer. Next to this a 3D Lattice Boltzmann model has been developed to simulate the heat transfer in a micro-channel flow with the surface covered with CNF’s. The 3D Lattice Boltzmann model has been verified on microchannel flow with heat transfer. Results of the conjugate heat transfer (including CNF’s at the wall) will be presented. The influence of carbon-nano fibers density and their structure on the heat transfer coefficient through the carbon-nano fibers layer is determined.

In recent years considerable attention has been paid to the study of microscale flow and heat transfer with phase change and chemical reactions. This article reviews the patterns of the microscale two-phase gas-liquid flow, the statistical parameters of slug flow and capillary phenomena in annular flow for a rectangular microchannel. The evaporative and condensing heat transfer model for the curved liquid microfilm in microchannel and near contact line is developed and discussed. The influence of forced convection, nucleate boiling and thin film evaporation on microscale flow boiling heat transfer is reviewed and analyzed. The model of forced boiling heat transfer in microchannel is developed and compared with the existing experimental data. The mechanism and patterns of microscale explosive evaporation in the MEMS system is determined at high external heat flux density and the acousto-thermal model of the explosive evaporation is considered. The results of calculations are compared with the experimental data. The peculiarities of heat and mass transfer in a micro channel with surface catalytic reactions producing the hydrogen are presented. The kinetics of sequence of chemical reactions at nanoscale catalyst under conditions of significant nonuniformity of temperature and species concentration fields is considered.

On the base of modern probability approach closed equation for probability density function of coordinates and velocities of two particles in turbulent flow is obtained. The system of equations for balance of mass, averaged velocities and intensities of turbulent chaotic motion of particles with account of correlated motion of particles are deduced. The closed expressions for intensity of relative chaotic motion between particles are obtained on the base of probability density function of particles displacement with correlation effects. Spectral presentation of second velocity moments of gas phase is used for calculation of intensity of particles relative chaotic motion. Boundary condition taking into account coefficients of new particle formation and momentum restitution during two particles collision is found. Formula for calculation of turbulent coagulation kernel of particles in gravity field is gain. Influence of cloud turbulence and turbulence in a pipe flow on intensity of droplets coagulation is studied. Strong effects of relative turbulent diffusion between droplets, droplets inertia and droplets gravitational settling on intensity of coagulation are found out. Connection between internal structure of turbulence type and coagulation rate is illustrated. The calculation results are compared with data of large eddy simulations. The results of calculation intensity of droplets relative motion in atmospheric conditions are presented.

This study is concerned with pyroelectric energy conversion to directly convert waste heat into electricity. The pyroelectric effect refers to the flow of charges to or from the surface of a material upon heating or cooling. A prototypical pyroelectric energy converter was designed, built, and tested. It performed the Olsen cycle consisting of two isothermal and two isovoltage processes in the charge-voltage diagram. Co-polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films sandwiched between metallic electrodes were used as the pyroelectric elements. Their temperature oscillation, charge, and voltage along with the overall heat input and output were measured experimentally. Then, the electrical power generated and the energy efficiency of the device were computed. The effects of channel width, frequency, and stroke length on temperature swing, heat input, and energy and power densities were investigated. Reducing the channel width and increasing the stroke length had the largest effect on device performance. A maximum energy density of 130 J/L of P(VDF-TrFE) was achieved at 0.061 Hz frequency with temperature oscillating between 69.3 and 87.6°C. Furthermore, a maximum power density of 10.7 W/L of P(VDF-TrFE) was obtained at 0.12 Hz between 70.5 and 85.3°C. In both cases, the voltages in the Olsen cycle were 923 and 1732 V imposed on a 45.7 microns thick 60/40 P(VDF-TrFE) films. To the best of our knowledge, this is the largest energy density achieved by any pyroelectric energy converter using P(VDF-TrFE). It also matches performances reported in the literature for more expensive lead zirconate stannum titanate ceramic films operated at higher temperatures between 110 and 185°C.

Experiments have been conducted into the unidirectional freezing of an aqueous solution of winter flounder antifreeze protein 0.02mm thick. It is confirmed that the instantaneous temperature field can be measured with a near-infrared camera. It is found that the difference between the conduction heat flux of pure water near the interface and that of ice is approximately equal to the heat flux for solidification, which is the product of ice density, interface velocity and the latent heat of fusion. The sum of the conduction heat flux of protein solution near the front edge of the serrated interface and the heat flux for solidification is approximately equal to the conduction heat flux of ice. On the other hand, the sum of the conduction heat flux of protein solution near the bottom edge of the serrated interface and the heat flux for solidification is much higher than the conduction heat flux of ice.

Molecular dynamics (MD) simulations are carried out to calculate the surface tension of bubbles formed in a metastable Lennard-Jones (LJ) fluid. The calculated normal and transverse pressure components are used to compute a surface tension which is compared to the surface tension computed from the Young-Laplace equation. Curvature effects on surface tension are investigated by performing various sized simulations ranging from 6,912 to 256,000 LJ particles. Density profiles, pressures, and calculated surface tension are shown to have a strong dependence on the choice of the interaction cutoff radius. A cutoff radius of 8σ, significantly larger than that commonly used in the literature, is recommended for accurate calculations in liquid-vapor systems.

Novel surface treatment technology was developed in the University of Paderborn in partnership with MiCryon Technik GmbH, Quedlinburg, Germany. The technology allows creation of micro-pin structures, which enhance the boiling heat transfer up to 18 times in comparison with a smooth surface, and provide an independency of the surface superheat on the applied heat flux [1–6]. The micro pins as basic structure elements can be created with the diameters of 0.1 μm to 25 μm at pins density ranging up to 10 9 pins/cm 2 . Such pin structures are created by electro-deposition of metallic ions on the basic surface. The microstructure provides for the very first time in thermal engineering a possibility to adjust the available length of the three phase line (TPL) on demand, correspondingly tailoring the shape of a boiling curve. The TPL, formed by the micro pins piercing the vapor-liquid interface, acts as an extremely efficient heat sink, providing high heat transfer coefficients and the constancy of the wall superheat at heat fluxes up to 125 kW/m 2 . Present article delivers the summary on boiling experiments performed with the novel microstructure, and reveals the quantitative dependencies of the heat transfer enhancement rate on the TPL length, having the pressure and the liquid type as further parameters. A newly discovered phenomenon of the vapor bubble chains formation on microstructured surfaces is discussed as well. Experiments were conducted with the refrigerants R134a, R141b and the fluorocarbon liquid FC-3284 at pressures, ranging between 0.5 bar and 9 bar.

The past decades have witnessed the diverse applications of boiling heat transfer enhancement in the removal of high density heat flux released by electronic components or power devices. People have developed many enhanced surfaces to obtain the highest heat transfer coefficient in nucleate boiling or to raise the CHF. In the boiling arena bubbling and nucleation site density play core parts, and hence it is crucial to correlate them quantitatively with surface structure and heat transfer conditions. For example one can determine by that correlation the best arrangement of boiling cavities for a given heat flux. However, the bubbling is highly influenced by inter-bubble actions. It has been found that the interactions can considerably change the bubble’s size, frequency and spatial distribution. The interactions are needed to be taken accounts of for a good correlation. Researchers tried to formulate the interactions as a single function of the inter-site spacing but have obtained contradictory conclusions, as suggests that they depend also on other parameters. In the present study we conducted a saturated boiling heat transfer experiment to investigate the interactions with respects to both the inter-site spacing and the wall thermal conductivity. The test section was fabricated by both copper and stainless steel, whose surface has two cylindrical artificial cavities of 50μm in diameter. It was heated with a uniform heat flux. The results show that both the bubble diameter D b and frequency f are functions of the inter-cavity distance s , but they vary in different manners in the copper and the stainless steel surfaces. In the copper surface, we observed evident enhancement of the boiling heat transfer at 1> S >0.4 and a slight inhibitive effect at 1.6> S >1, where S = s/D b . On the contrary the two nucleate sites in the stainless steel surface interfere with each other giving rise to evident suppression of boiling heat transfer at 1.6> S >0.65 and only slight enhancement at 0.65> S >0.3. Note that the copper’s thermal conductivity is 22 times larger than the stainless steel. Numerical simulation has revealed that the temperature variation beneath the copper cavities is much less than the stainless steel, which partly explains the differences in our experimental results. It is suggested that modeling the bubble interactions should take accounts of not only the distance-to-diameter ratio but also the fluid and wall properties.

Magnetic nanoparticles have been used in clinical and animal studies to generate localized heating for tumor treatments when the particles are subjected to an external alternating magnetic field. One approach to deliver the nanoparticles is via directly injecting the nanoparticles in the extracellular space of the tumor. Its advantage is that multiple-site injections can be exploited to cover the entire target region in the case of an irregularly shaped tumor. Currently since most tissue is opaque, the detailed information of the nanoparticle spreading after the injection can not be visualized directly and it is often quantified by indirect methods such as temperature measurements to inversely determine the distribution. In this study, we use a high-resolution microCT imaging system to investigate the nanoparticle concentration distribution in a tissue equivalent agarose gel. The preliminary results are promising to obtain a 3-D distribution of the ferrofluid in tissue. The local density variations induced by the nanoparticles in the vicinity of the injection site can be detected and analyzed by the microCT system. Experiments are performed to study how the injection amount, gel concentration, and nanoparticle concentration in the ferrofluid affect nanoparticle spreading in the gel. The obtained quantified information is important for future studies of temperature elevations in opaque tumor to design optimized treatment protocols.

The trend towards a better understanding of the fundamentals of nucleate boiling in re-entrant cavities is supported by the variation of the heating surface’s characteristics and the identification of parameters influencing the heat transfer at enhanced tubes. The optimized surface of enhanced evaporator tubes supports the bubble formation by providing stable nucleation sites, which are cavities that trapped the necessary amount of vapor to generate the next bubble. The optimal size of the cavities for bubble formation depends on various thermodynamic properties of the fluid and the wall material. The knowledge of these physical mechanisms is important for the further optimization. The influence of micro- and macrostructures on the overall heat transfer coefficient is investigated with the refrigerant R134a and the hydrocarbon propane (R290) boiling in a wide range of reduced pressures (p* = p s /p c = 0.03 to 0.5) and heat fluxes (0.05 to 100 kW/m 2 ). The measurements are carried out using a standard apparatus and a horizontally positioned, electrically heated surface with various wall materials. Two different materials — copper and mild steel — with the same surface preparation by polishing are investigated. Furthermore, heat transfer measurements are carried out on a plain mild steel tube and on an industrially manufactured surface of the GEWA-PB type. The polished surfaces demonstrate a deterministic microstructure, the roughness parameters depends strongly on the measurement direction. The heat transfer coefficient as function of the heat flux of the polished copper tube can be described by the correlation of the VDI Heat Atlas, while the mild steel surface differ from former investigations due to the deep re-entrant cavities remaining from the drawn surface. The onset of boiling is nearly the same of both materials because of these cavities on the mild steel surface. As presented in the recent years, the heat transfer of nucleate boiling at tubes with subsurface channels can be divided into different domains, each influenced by different parameters like wettability, the product of vapor density and evaporation enthalpy. The identification of parameters influencing the bubble formation is done by heat transfer measurements, high-speed-video recording and photographic documentation. The experimental results of this work are compared to results of the polished surfaces. The heat transfer coefficient increases drastically for the enhanced tube, especially for beginning nucleation. The same α-q-relationship as on plain tubes is observed for higher pressures and heat fluxes but for three times higher values of the heat transfer coefficient α.

In order to investigate the effect of n-decanol, a kind of alcohol surfactants, on the absorption of water vapor into lithium bromide aqueous solution, this study focused on microscopic structure of the liquid-vapor interface of the electrolyte solution, the impact of n-decanol molecules on the interfacial properties and the absorption dynamic process employing the method of molecular dynamics simulation. The liquid-vapor configuration of lithium bromide aqueous solution added with four concentration of n-decanol can be analyzed by examining the density profile, the radial distribution functions and orientational order parameter. The computed results revealed that n-decanol molecules tended to adsorb at the interface with the methyl group pointing into the vapor phase and hydrophilic hydroxyl group pointing into the liquid phase which do much help to form a hydrogen bond network with water, and the tendency of this kind of preferred orientation became distinct with the increase of the amount of n-decanol. The hydrocarbon chains of n-decanol molecules were inclined to close to stay upright near the interface while the monolayer of n-decanol came into being near the interface. Ions were repelled from the surface. The direct interactions between hydroxyl hydrogen of n-decanol and anion exist, and there are much stronger electrostatic interactions between oxygen of n-decanol and cation. The dynamic process of the absorption of water into aqueous electrolyte solution with or without n-decanol was explored by molecular dynamics simulation under non equilibrium conditions. The simulation results showed that in comparison to the lithium bromide aqueous solution without n-decanol, the electrolyte aqueous solution with n-decanol can absorb more water molecules distinctly for 100 ps.