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Guest Editorial

J. Heat Transfer. 2017;140(1):010301-010301-1. doi:10.1115/1.4037326.
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The 5th ASME Micro/Nanoscale Heat and Mass Transfer Conference (MNHMT 2016) was held on Jan. 4–6, 2016, in Singapore. The conference was organized by the School of Mechanical and Aerospace Engineering of Nanyang Technological University and sponsored by the ASME Heat Transfer Division, Xi'an Jiaotong University, Singapore Institute of Manufacturing Technology, Quantum Technology Group (Singapore) Pte. Ltd., and Nanjing Inout E-Business Co., Ltd. This conference series is dedicated to Dr. Chang-Lin Tien (1935–2002), a world renowned scholar and a leader in higher education, whose intellect and unique vision have continued to inspire our efforts in expanding the frontiers of micro/nanoscale heat and mass transfer. The previous four conferences were hosted by National Cheng Kung University in Tainan, Taiwan (January 2008), Shanghai Jiao Tong University in Shanghai, China (December 2009), Georgia Institute of Technology in Atlanta, GA (March 2012), and University of Hong Kong in Hong Kong (December 2013).

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2017;140(1):011501-011501-11. doi:10.1115/1.4037343.

The computational fluid dynamics (CFD) modeling of boiling phenomena has remained a challenge due to numerical limitations for accurately simulating the two-phase flow and phase-change processes. In the present investigation, a CFD approach for such analysis is described using a three-dimensional (3D) volume of fluid (VOF) model coupled with a phase-change model accounting for the interfacial mass and energy transfer. This type of modeling allows the transient analysis of flow boiling mechanisms, while providing the ability to visualize in detail temperature, phase, and pressure distributions for microscale applications with affordable computational resources. Results for a plain microchannel are validated against benchmark correlations for heat transfer (HT) coefficients and pressure drop as a function of the heat flux and mass flux. Furthermore, the model is used for the assessment of two-phase cooling in microelectronics under a realistic scenario with nonuniform heat fluxes at localized regions of a silicon microchannel, relevant to the cooling layer of 3D integrated circuit (IC) architectures. Results indicate the strong effect of two-phase flow regime evolution and vapor accumulation on HT. The effects of reduced saturation pressure, subcooling, and flow arrangement are explored in order to provide insight about the underlying physics and cooling performance.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):011502-011502-8. doi:10.1115/1.4037448.

An extra high evaporating heat transfer coefficient can be obtained by thin-film evaporation. In the current investigation, a new detailed mathematical model is developed by considering the effects of bulk flow and interfacial thermal resistance on fluid flow and heat transfer in the thin-film region of an evaporating meniscus. In addition to the interfacial thermal resistance occurring at the liquid–vapor interface, the pressure difference between liquid and vapor is considered to the bulk flow effect. The results show that the bulk flow, which depends on the pressure difference between the interfacial pressure and vapor pressure, significantly affects thin-film profile, heat flux distribution, interfacial temperature, meniscus radius, mass flow rate, and average flow velocity in the evaporating thin-film region. While the interfacial thermal resistance occurring at the liquid–vapor interface affects fluid flow and heat transfer in the evaporating thin-film region, the bulk flow effect is more important than the interfacial thermal resistance.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2017;140(1):011801-011801-9. doi:10.1115/1.4037339.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilize the enhanced fluid mixing characteristics of Dean vortices and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimizing the heat exchanger design based on the parameters of maximizing heat transfer while minimizing pressure drop and unit cost. A range of cross-sectional geometries for the curved channels was compared, showing significantly higher Nusselt numbers than equivalent straight channels throughout and finding superior performance factors for square, circular, and symmetrical trapezoidal profiles. Due to the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. The variation of Nusselt number over the length of the channel for a range of different curvatures (and hence Dean numbers) is also examined, showing significantly higher heat transfer occurring in strongly curved channels, especially in areas where the generated Dean vortices are strongest. The variation in Nusselt number was found to form the shape of an “arc.” In this way, a relationship between the Dean number and the Nusselt number is characterized and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):011802-011802-9. doi:10.1115/1.4037342.

Microchannel heat exchangers offer the potential for high heat transfer coefficients; however, implementation challenges must be addressed to realize this potential. Maldistribution of phases among the microchannels and the changing phase velocities associated with phase change present design challenges. Flow maldistribution and oscillatory instabilities can affect transfer rates and pressure drops. In condensers, evaporators, absorbers, and desorbers, changing phase velocities can change prevailing flow regimes from favorable to unfavorable. Geometries with serpentine passages containing pin fins can be configured to maintain favorable flow regimes throughout the component for phase-change heat and mass transfer applications. Due to the possibility of continuous redistribution of the flow across the pin fins along the flow direction, maldistribution can also be reduced. These features enable high heat transfer coefficients, thereby achieving considerable compactness. The characteristics of two-phase flow through a serpentine passage with micro-pin fin arrays with diameter 350 μm and height 406 μm are investigated. An air–water mixture is used to represent two-phase flow through the serpentine test section, and flow features are investigated using high-speed photography. Improved flow distribution is observed in the serpentine geometry. Distinct flow regimes, different from those observed in microchannels, are also established. Void fraction and interfacial area along the length of the serpentine passages are compared with the corresponding values for microchannels. A model developed for the two-phase frictional pressure drops across this serpentine micro-pin fin geometry predicts experimental values with a mean absolute error (MAE) of 7.16%.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;140(1):012001-012001-4. doi:10.1115/1.4037327.

Ultrafast cooling is the key to successful cell vitrification cryopreservation of lower concentration cryoprotective solution. This research develops a cell cryopreservation methodology which utilizes thin film evaporation and achieves vitrification of relatively low concentration cryoprotectant with an ultrafast cooling rate. Experimental results show that the average cooling rate of dimethylsulfoxide (DMSO) cryoprotective solution reaches 150,000 °C/min in a temperature range from 10 °C to −180 °C. The ultrafast cooling rate can remarkably improve the vitrification tendencies of the cryoprotective solution. This methodology opens the possibility for more successful cell vitrification cryopreservation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):012002-012002-9. doi:10.1115/1.4037341.

Slug oscillations and heat transfer performance in the start-up stage of oscillating heat pipes (OHPs) with different surface wetting characteristics were investigated experimentally. The inner surfaces of the OHPs were superhydrophilic surface, hydrophilic surface, copper, hydrophobic surface, and superhydrophobic surface, respectively. There was a thin liquid film between the vapor bubble and the surface in the hydrophilic OHP which was different from hydrophobic OHP. Results showed that start-up performance was improved in hydrophilic OHP due to the low flow resistance and deteriorated in hydrophobic OHP as opposed to the copper OHP. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. Compared with the copper OHP, start-up time and start-up temperature were reduced by 100 s and 3.32–4.41 °C in the hydrophilic OHP at the start-up stage. Slug oscillation frequency and temperature oscillation amplitude increased with heat input; however, slug oscillation amplitude increased first and then decreased with heat input. Compared with the copper OHP, with the increasing of 0–57% in slug oscillation amplitude and 0–100% in slug oscillation frequency, the thermal performance was enhanced by 0–67% in the hydrophilic OHP. Although the slug oscillation frequency in the superhydrophobic OHP was higher than that in the copper OHP, with the decreasing of 0–70% in the slug oscillation amplitude, the thermal resistance in superhydrophobic OHP was significantly increased and was 1.5–5 times higher than that in the copper OHP.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):012003-012003-10. doi:10.1115/1.4037392.

Nanoparticle (NP) aggregation can not only change the unique properties of NPs but also affect NP transport and membrane penetration behavior in biological systems. Coarse-grained (CG) molecular dynamics (MD) simulations were performed in this work to investigate the aggregation behavior of NPs with different properties in ionic solutions under different temperature conditions. Four types of NPs and NP aggregates were modeled to analyze the effects of NP aggregation on NP translocation across the cell membrane at different temperatures. Hydrophilic modification and surface charge modification inhibited NP aggregation, whereas stronger hydrophobicity and higher temperature resulted in a higher degree of NP aggregation and a denser structure of NP aggregates. The final aggregation percentage of hydrophobic NPs in the NaCl solution at 37 °C is 87.5%, while that of hydrophilic NPs is 0%, and the time required for hydrophobic NPs to reach 85% aggregation percentage at 42 °C is 6 ns, while it is 9.2 ns at 25 °C. The counterions in the solution weakened the effect of surface charge modification, thereby realizing good dispersity. High temperature could promote the NP membrane penetration for the same NP, while it also could enhance the NP aggregation which would increase the difficulty in NP translocation across cell membrane, especially for the hydrophobic NPs. Therefore, suitable surface modification of NPs and temperature control should be comprehensively considered in promoting NP membrane penetration in biomedical applications.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;140(1):012401-012401-10. doi:10.1115/1.4037396.

In the present study, an experimental investigation has been carried out to analyze the heat transfer characteristics of CuO–water nanofluids jet on a hot surface. A rectangular stainless steel foil (AISI-304, 0.15 mm thick) used as the test surface is electrically heated to obtain the required initial temperature (500 °C). The distribution of surface heat flux on the target surface is evaluated from the recorded thermal images during transient cooling. The effect of nanoparticle concentration and Reynolds number of the nanofluids on the heat transfer characteristics is studied. Tests are performed for varied range of Reynolds number (5000 ≤ Re ≤ 12,000), two different CuO–water nanofluids concentration (Ф = 0.15%, 0.6%) and two different nozzle to plate distance (l/d = 6, 12). The enhancement in Nusselt number for CuO–water nanofluids was found to be 14% and 90%, for nanofluids concentration of Ф = 0.15% and Ф = 0.60%, respectively, compared to pure water. The test surface characteristics after nanofluids jet impingement are studied using scanning electron microscope (SEM). Based on the investigation, a correlation among various parameters, namely, Reynolds number (Re), Prandtl number (Pr), nozzle to plate distance (l/d), and Nusselt number (Nu), is presented.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):012402-012402-10. doi:10.1115/1.4037397.

Particulate fouling and particle deposition at elevated temperature are crucial issues in microchannel heat exchangers. In this work, a microfluidic system was designed to examine the hydrodynamic effects on the deposition of microparticles in a microchannel flow, which simulate particle deposits in microscale heat exchangers. The deposition rates of microparticles were measured in two typical types of flow, a steady flow and a pulsatile flow. Under a given elevated solution temperature and electrolyte concentration of the particle dispersion in the tested flow rate range, the dimensionless particle deposition rate (Sherwood number) was found to decrease with the Reynolds number of the steady flow and reach a plateau for the Reynolds number beyond 0.091. Based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, a mass transport model was developed with considering temperature dependence of the particle deposition at elevated temperatures. The modeling results can reasonably capture our experimental observations. Moreover, the experimental results of the pulsatile flow revealed that the particle deposition rate in the microchannel can be mitigated by increasing the frequency of pulsation within a low-frequency region. Our findings are expected to provide a better understanding of thermally driven particulate fouling as well as to provide useful information for design and operation of microchannel heat exchangers.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):012403-012403-7. doi:10.1115/1.4037363.

The recently confirmed violation of the no-slip boundary condition in the flow of small-molecule liquids through microchannels and nanochannels has technological implications such as friction reduction. However, for significant friction reduction at low cost, the microchannel wall needs to be chemically inhomogeneous. The direct fluid dynamic consequence of this requirement is a spatial variation in the local degree of liquid slippage. In this work, the pressure-driven flow in a channel with periodically patterned slippage on the channel walls is studied using a spectrally accurate semi-analytical approach based on Fourier decomposition. The method puts no restrictions on the pitch (or wavelength) and amplitude of the pattern. The predicted effective slip length in the limits of small pattern amplitude and thick channels is found to be consistent with previously published results. The effective degree of slippage decreases with the patterning amplitude. Finer microchannels and longer pattern wavelengths promote slippage.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):012404-012404-8. doi:10.1115/1.4037398.

The absorption air-conditioning system is a low-power-consumption and low-noise system and is also good at balancing the electricity peak-valley system. It can be driven by low-grade energy, such as solar energy and industrial exhaust heat. The nanofluids, which possess the superior thermophysical properties, exhibit a great potential in enhancing heat and mass transfer. In this paper, nanofluids of H2O/LiBr with Fe3O4 nanoparticles were introduced into absorption air conditioning system. The effects of critical parameters, such as the flow rate of H2O/LiBr nanofluids, nanoparticle size and mass fraction, on the falling film absorption were investigated. The H2O/LiBr nanofluids with Fe3O4 nanoparticle mass fractions of 0.01 wt %, 0.05 wt % and 0.1 wt %, and nanoparticle sizes of 20 nm, 50 nm and 100 nm were tested. The results imply that the vapor absorption rate could be improved by adding the nanoparticles to H2O/LiBr solution. The smaller the nanoparticle size, the greater the enhancement of the heat and mass transfer. The absorption enhancement ratio increases sharply at first by increasing the nanoparticle mass fraction within a range of relatively low mass fraction and then exhibits a slow growing even reducing trends with increasing the mass fraction further. For Fe3O4 nanoparticle mass fraction of 0.05 wt % and nanoparticle size of 20 nm, the maximum mass transfer enhancement ratio is achieved about 2.28 at the flow rate of 100 L h−1. Meanwhile, a fitting formula of mass transfer enhancement ratio for Fe3O4 nanofluids has been improved.

Commentary by Dr. Valentin Fuster

Research Papers: Thermal Systems

J. Heat Transfer. 2017;140(1):012801-012801-4. doi:10.1115/1.4037337.

To design a better adsorption performance in a novel magnetic adsorption device used for hemodialysis (HD), the mechanical properties of magnetic absorbents trapped inside a two-phase system are studied in this paper. A gradient magnetic coil field is assumed to produce the magnetic driving force that balances other hydraulic forces for the adsorbents. For this field, a related winding equation for the solenoid coil is obtained in our previous work; and a complement practical form of the winding equation is derived in this paper. Case studies are also described in this paper to explore the design aspects of the field.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2017;140(1):014501-014501-5. doi:10.1115/1.4037340.

In this study, micro-encapsulated phase change material (microPCM) was successfully synthesized by emulsion polymerization method, using magnesium sulfate heptahydrate (MSH) as core material and urea resin (UR) as shell material. The surface morphologies and particle size distributions of the microPCM were tested by scanning electron microscopy (SEM) and laser particle size analyzer. The chemical structure of microPCM was analyzed by Fourier-transform infrared spectroscopy (FTIR). The thermal properties were investigated by differential scanning calorimetry (DSC) and thermal conductivity coefficient instrument, respectively.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(1):014502-014502-6. doi:10.1115/1.4037394.

Open-cell metal foams exhibit distinctive advantages in fluid control and heat transfer enhancement in thermal and chemical engineering. The thermofluidic transport characteristics at pore scale such as topological microstructure and morphological appearance significantly affect fluid flow and conjugated heat transfer in open-cell metal foams, important for practically designed applications. The present study employed an idealized tetrakaidecahedron unit cell (UC) model to numerically investigate the transport properties and conjugated heat transfer in highly porous open-cell metal foams (porosity—0.95). The effects of foam ligaments and nodes (size and cross-sectional shape) on thermal conduction, fluid flow, and conjugated heat transfer were particularly studied. Good agreement was found between the present predictions and the results in open literature. The effective thermal conductivity was found to decrease with increasing node-size-to-ligament ratio, while the permeability and volume-averaged Nusselt number were increased. This indicated that the effects of node size and shape upon thermofluidic transport need to be considered for open-cell metal foams having high porosities.

Commentary by Dr. Valentin Fuster

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