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Guest Editorial

J. Heat Transfer. 2017;139(11):110301-110301-1. doi:10.1115/1.4037533.
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We are extremely happy to present this collection of research papers emerging from an International Workshop on New Understanding in Nanoscale/Microscale Phase Change Phenomena held in Trondheim, Norway during June 12–16, 2016. The workshop was organized by Carlos A. Dorao, Maria Fernandino, and Satish Kandlikar as conference co-chairs. It was hosted by the Thermal Two Phase Flow Laboratory of the Department of Energy and Process Engineering at the Norwegian University of Science and Technology (NTNU). It was supported by the Department of Energy and Process Engineering and the NANO2021 Program of the Research Council of Norway.

Commentary by Dr. Valentin Fuster

Review Article

J. Heat Transfer. 2017;139(11):110801-110801-7. doi:10.1115/1.4036578.

Results of lattice Boltzmann (LB) simulations of macroscale effects (heating modes, heater size, and saturation temperature) as well as microscale effects (wettability and roughness) on saturated pool boiling from superheated horizontal surfaces are summarized in this paper. These effects on pool boiling curves from natural convection through nucleate boiling to critical heat flux (CHF) and from transition boiling to film boiling are illustrated. It is found that macroscale effects have negligible influence on nucleate boiling heat transfer, and Rohsenow's correlation equation fits well with the simulated nucleate boiling heat transfer on smooth hydrophilic and hydrophobic horizontal surfaces. Both macroscale and microscale effects have important influence on critical heat flux and transition boiling heat transfer.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2017;139(11):111501-111501-11. doi:10.1115/1.4036651.

New requirements for heat exchangers offered pool boiling heat transfer on structured and coated surfaces as one of the promising methods for effective heat removal. In this study, pool boiling experiments were conducted on polyhydroxyethylmethacrylate (pHEMA)-coated surfaces to investigate the effect of surface orientation on bubble dynamics and nucleate boiling heat transfer. pHEMA coatings with thicknesses of 50, 100, and 200 nm were deposited using the initiated chemical deposition (iCVD) method. De-ionized water was used as the working fluid. Experiments were performed on horizontal and inclined surfaces (inclination angles of 10 deg, 30 deg, 50 deg, and 70 deg) under the constant heat flux (ranging from 10 to 80 kW/m2) boundary condition. Obtained results were compared to their plain surface counterparts, and heat transfer enhancements were observed. Accordingly, it was observed that the bubble departure phenomenon was affected by heat flux and wall superheat on bare silicon surfaces, while the supply path of vapor altered the bubble departure process on pHEMA-coated surfaces. Furthermore, the surface orientation played a major role on bubble dynamics and could be considered as a mechanism for fast vapor removal from surfaces. Bubble coalescence and liquid replenishment on coated surfaces had a promising effect on heat transfer coefficient enhancement on coated surfaces. For horizontal surfaces, a maximum enhancement of 25% relative to the bare surface was achieved, while the maximum enhancement was 105% for the inclined coated surface under the optimum condition. iCVD was proven to be a practical method for coating surfaces for boiling heat transfer applications due to the obtained promising results.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111502-111502-7. doi:10.1115/1.4036653.

While the role of the liquid properties, surface morphology, and operating conditions on critical heat flux (CHF) in pool boiling is well investigated, the effect of the properties of the heater material is not well understood. Previous studies indicate that the heater thickness plays an important role on the CHF phenomenon. However, beyond a certain thickness, called the asymptotic thickness, the local temperature fluctuations on the heater surface caused by the periodic bubble ebullition cycle are evened out, and the CHF is not influenced by further increasing the thickness. In the present work, data from literature and pool boiling experiments conducted in this study with seven substrates—aluminum, brass, copper, carbon steel, Monel 400, silver, and silicon—are used to determine the effect of the thermophysical property of the material on CHF for thick heaters that are used in industrial pool boiling applications. The results indicate that the product of density (ρ) and specific heat (cp) represents an important substrate property group that affects the CHF, and that the thermal conductivity is not an important parameter. A well-established force-balance-based CHF model (Kandlikar model) is modified to account for the thermal properties of the substrate. The predicted CHF values are within 15% of the experimental results.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111503-111503-10. doi:10.1115/1.4036655.

In this study, six benchmark experiments are conducted on bubbles at different growth stages to evaluate the assumptions of the existing microchannel flow boiling heat transfer models/hypothesis. The results show that the bubble ebullition process triggers a spike in the local surface heat flux due to the thin film evaporation and transient conduction heat transfer mechanisms. This enhancement in the surface heat flux is limited to a very small area at the bubble–surface contact region at the nucleation site limiting the overall heat transfer contribution of the bubble ebullition process. The contribution of these two mechanisms of heat transfer increases as the bubble–surface contact area becomes larger. As the bubbles length increases, the time period of activation of the microlayer evaporation mechanism substantially increases while that of the transient conduction mechanism remains relatively unchanged. When the microchannel is mostly occupied by bubbles, the thin film evaporation mechanism becomes the dominant heat transfer mode. The results clearly indicate that single-phase heat transfer mechanism active at surface regions not covered by bubbles is governed by the laminar flow theory (for the test conditions presented here). In essence, a measureable enhancement effect in the liquid phase due to bubbles growth and flow has not been observed. A comparison with the existing microchannel flow boiling models suggests that the three-zone flow boiling model can qualitatively describe the heat transfer events observed in this experiment but fails to accurately predict the magnitude of the heat transfer mechanisms.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111504-111504-6. doi:10.1115/1.4036678.

Graphene has been investigated due to its mechanical, optical, and electrical properties. Graphene's effect on the heat transfer coefficient (HTC) and critical heat flux (CHF) in boiling applications has also been studied because of its unique structure and properties. Methods for coating graphene oxide (GO) now include spin, spray, and dip coating. In this work, graphene oxide coatings are spray coated on to a copper surface to investigate the effect of pressure on pool boiling performance. For example, at a heat flux of 30 W/cm2, the HTC increase of the GO-coated surface was 126.8% at atmospheric pressure and 51.5% at 45 psig (308 kPa). For both surfaces, the HTC increases with increasing pressure. However, the rate of increase is not the same for both surfaces. Observations of bubble departure showed that bubbles departing from the graphene oxide surface were significantly smaller than that of the copper surface even though the contact angle was similar. The change in bubble departure diameter is due to pinning from micro- and nanostructures in the graphene oxide coating or nonhomogeneous wettability. Condensation experiments at 40% relative humidity on both the plain copper surface and the graphene oxide coated surface show that water droplets forming on both surfaces are significantly different in size and shape despite the similar contact angle of the two surfaces.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111505-111505-9. doi:10.1115/1.4036681.

This investigation experimentally examines the role of interface capture on the transport and deposition of colloidal material in evaporating droplets. It finds that deposition patterns cannot be characterized by the ratio of interface velocity to particle diffusion rate alone when the two effects are of the same order. Instead, the ratio of radial velocity to particle diffusion rate should also be considered. Ring depositions are formed when the ratio of radial velocity to the particle diffusion rate is greater than the ratio of interface velocity to diffusion. Conversely, uniform depositions occur when the ratio of radial velocity to diffusion is smaller than the ratio of interface velocity to diffusion. Transitional depositions with a ring structure and nonuniform central deposition are observed when these ratios are similar in magnitude. Since both ratios are scaled by diffusion rate, it is possible to characterize the depositions here using a ratio of interface velocity to radial velocity. Uniform patterns form when interface velocity is greater than radial velocity and ring patterns form when radial velocity is larger. However, Marangoni effects are small and Derjaguin, Landau, Verwey, and Overbeek (DLVO) forces repel particles from the surface in these cases. Further research is required to determine if these conclusions can be extended or modified to describe deposition patterns when particles are subjected to appreciable Marangoni recirculation and attractive DLVO forces.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111506-111506-10. doi:10.1115/1.4036682.

The three-dimensional (3D) stacking of integrated circuits (ICs), and emergent microelectronic technologies require low-profile cooling solutions for the removal of relatively high heat fluxes. The flow boiling of dielectric refrigerants represents a feasible alternative to such applications by providing compatibility with the electrical interconnections, relatively uniform temperature profiles, and higher heat transfer coefficients than those obtained with single phase-cooling. Despite important experimental evidence in this area has been recently reported in the literature, the modeling of such has remained in basic and limited forms due to the associated complexities with the physics of two-phase flow with phase-change. In an effort to expand the studied possibilities for the modeling of flow boiling, the present investigation compares two different phase-tracking methods for the analysis of such phenomena: the volume of fluid (VOF) and the coupled level set—volume of fluid (CLSVOF) techniques. These interface tracking and reconstruction techniques are coupled with a phase change model that accounts for the mass and energy transfer source terms to the governing equations. The geometric domain is constituted by a silicon microgap 175 μm high with a substrate thickness of 50 μm, and populated with circular pin fins of 150 μm diameter, where the heat conduction is simultaneously solved with temperature dependent properties. The flow boiling regimes and their spatial and temporal evolution are compared between both methods by maintaining the operating conditions. Results indicate that both methods provide a good capability to predict major two-phase flow regimes observed in experimental studies with these types of arrangements. However, the CLSVOF offers a sharper interface reconstruction than the standard VOF method by predicting bubble nucleation and departure mechanisms more closely to experimental observations.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111507-111507-15. doi:10.1115/1.4036708.

Condensation of a highly wetting fluid on a horizontal surface with asymmetric millimeter-sized ratchets and periodically located film drainage pathways (DPs) in the spanwise direction is characterized. The hypothesis to be tested is whether the geometry would result in a net steady-state preferential drainage of the condensate film. Experiments are performed using PF5060 on a brass surface with ratchets of 3 mm pitch and 75–15 deg asymmetry. Drainage pathways are varied in density as nondimensional drainage pathways per meter depth ranging from 133 to 400. Experiments are performed at varied wall subcooling temperatures from 1 to 10 °C. Results of the asymmetric ratchet are compared against a control test surface with 45–45 deg symmetric ratchets. Both global and film visualization experiments are performed to characterize the differences in condensation between the symmetric and asymmetric surfaces. Global mass collection results indicate that all characterized asymmetric ratchet surfaces exhibit a net directional drainage of condensate while the symmetric control surface exhibited no preferential drainage. Among the asymmetric ratchets, the total mass flux rate increase with decrease in drainage pathway density, while the net mass flux rate increased with pathway density. Visualization of the condensate film was performed to explain the trends in net drainage with subcooling for different drainage pathway densities. For small drainage path density surfaces, a two-dimensional analytical model was developed to further characterize the effect of ratchet angle and Bond number on the net preferential drainage.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111508-111508-9. doi:10.1115/1.4036693.

Boiling heat transfer can be substantially altered with the addition of surface structures. While significant enhancements in critical heat flux (CHF) and heat transfer coefficient (HTC) have been demonstrated using this approach, fundamental questions remain about the nature of enhancement and the role of structure length scale. This work presents a systematic investigation of structures from 100's of nanometers to several millimeters. Specifically, copper substrates were fabricated with five different microchannel geometries (characteristic lengths of 300 μm to 3 mm) and four different copper oxide nanostructured coatings (characteristic lengths of 50 nm to 50 μm). Additionally, twenty different multiscale structures were fabricated coinciding with each permutation of the various microchannels and nanostructures. Each surface was tested up to CHF during pool boiling of saturated water at atmospheric conditions. The nanostructured coatings were observed to increase CHF via surface wicking, consistent with existing models, but decrease HTC due to the suppression of the nucleation process. The microchannels were observed to increase both CHF and HTC, generally outperforming the nanostructured coatings. The multiscale surfaces exhibited superior performance, with CHF and HTC values as high as 313 W/cm2 and 461 kW/m2 K, respectively. Most importantly, multiscale surfaces were observed to exhibit the individual enhancement mechanisms seen from each length scale, namely, increased nucleation and bubble dynamics from the microchannels and wicking-enhanced CHF from the nanostructures. Additionally, two of the surfaces tested here exhibited uncharacteristically high HTC values due to a decreasing wall superheat at increasing heat fluxes. While the potential mechanisms producing this counterintuitive behavior are discussed, further research is needed to definitively determine its cause.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111509-111509-11. doi:10.1115/1.4036695.

Enhanced pool boiling heat transfer, with simultaneous increase in critical heat flux (CHF) and heat transfer coefficient (HTC), is desired to improve overall system efficiency and reduce equipment size and cost. This paper focuses on combining graphene oxide (GO) and porous copper particles to generate microstructures based on their ability to enhance HTC, CHF, or both. Three pool boiling performance characteristics based on CHF improvements and wall superheat reductions are identified: Type I—reduction in wall superheat only, type II—increase in CHF only, and type III—increase in CHF with reduction in wall superheat at higher heat fluxes. Specific microscale morphologies were generated using (a) screen-printing and (b) electrodeposition techniques. In type-I, rapid bubble activity due to increased availability of nucleation cavities was seen to influence the reduction in the wall superheats, while no increase in CHF was noted. Roughness-augmented wettability was found to be the driving mechanism in type-II enhancement, while wicking and increased nucleation site density were responsible for the enhancement in type-III. An HTC enhancement of ∼216% in type-I and a CHF improvement of ∼70% in type-II were achieved when compared to a plain copper surface with water. In type-III enhancement, a CHF of 2.2 MW/m2 (1.8× over a plain surface) with a HTC of 155 kW/m2 °C (∼2.4× over a plain surface) was obtained. Furthermore, close correlation between the boiling performance and the microscale surface morphology in these three categories has been identified.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111510-111510-11. doi:10.1115/1.4037153.

In spite of the extensive work in flow boiling in small-diameter tubes, the general characteristics and dominant mechanisms remain elusive. In this study, flow boiling heat transfer of R134a inside a 5 mm I.D., smooth horizontal stainless steel pipe is experimentally studied. Local heat transfer coefficients (HTCs) were measured for heat fluxes from 3.9 to 47 kW/m2 and mass fluxes from 200 to 400 kg/m2 s at a saturation temperature of 18.6 °C. The studied cases have shown different behaviors at low and high heat fluxes. At low heat fluxes, the convective contribution looks to control the HTC, while at high heat fluxes the nucleation of vapor looks to be the dominant mechanism. Reducing the heat flux, the HTC approaches asymptotically a limit equivalent to the single-phase HTC defined in terms of the sum of the superficial liquid and vapor Reynolds numbers. A new correlation for dominant convective flow boiling is proposed and evaluated against experimental data from the literature.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):111511-111511-14. doi:10.1115/1.4037162.

Solid–fluid interfaces switching from a superhydrophilic to a superhydrophobic wetting state are desired for their ability to control and enhance phase-change heat transfer. Typically, these functional surfaces are fabricated from polymers and modify their chemistry or texture upon the application of a stimulus. For integration in relevant phase-change heat transfer applications, several challenges need to be overcome, of chemical stability, mechanical and thermal robustness, as well as large scale manufacturing. Here, we describe the design and fabrication of metallic surfaces that reversibly switch between hydrophilic and superhydrophobic states, in response to pressure and temperature stimuli. Characterization of the surfaces in pool boiling experiments verifies their thermal and mechanical robustness, and the fabrication method is scalable to large areas. During pool boiling experiments, it is experimentally demonstrated that the functional surfaces can be actively switched between a high-efficiency mode suitable at low heat flux, and a high-power mode suitable for high heat flux applications.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;139(11):112401-112401-10. doi:10.1115/1.4036763.

Nucleation is the first stage of phase change phenomena, including condensation on nanostructured superhydrophobic surfaces. Despite plenty of theoretical studies on the effect of nanostructure density and shape on water droplet nucleation, not many experimental investigations have been reported. Here, we show both experimentally and theoretically that a moderate increase in the nanostructure density can lead to an increase in the nucleation density of water droplets because of the decreased energy barrier of nucleation in cavities formed between the nanostructures. Specifically, we observed droplets aligned in regions with denser nanostructures. The number and average volume of the aligned droplets in these regions were larger than that of the droplets in the surrounding areas. However, nucleation in cavities subsequently caused initial pinning of the droplet base within the nanostructures, forming a balloonlike, slightly elongated droplet shape. The dewetting transition of the pinned droplets from the Wenzel state to the unpinned Cassie state was predicted by quantifying the aspect ratio of droplets ranging from 3 to 30 μm. Moreover, the coalescence-jumping of droplets was followed by a new cycle of droplet condensation in an aligned pattern in an emptied area. These findings offer guidelines for designing enhanced superhydrophobic surfaces for water and energy applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):112402-112402-13. doi:10.1115/1.4036683.

An experimental study on subcooled flow boiling with engineering fluid HFE-7000 in a microchannel fitted with piranha pin fins (PPFs) is presented. Heat fluxes of up to 735 W/cm2 were achieved and mass fluxes ranged from 618 kg/m2s to 2569 kg/m2 s. It was found that the flow boiling heat transfer was significantly enhanced with PPFs. The heat transfer coefficient with flow boiling was double the corresponding single-phase flow. Correlations for two-phase heat transfer coefficient and pressure drop in the nucleate flow boiling regime were developed based on the boiling, Weber, and Jakob numbers. The onset of nucleate boiling (ONB) and the critical heat flux (CHF) conditions were determined through visualization and was typically initiated from the last row of fins where temperatures were highest and flow rates lowest.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):112403-112403-12. doi:10.1115/1.4036691.

The demands for increasingly smaller, more capable, and higher power density technologies have heightened the need for new methods to manage and characterize extreme heat fluxes. This work presents the use of an anisotropic version of the time-domain thermoreflectance (TDTR) technique to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and subcooled channel-flow configuration. Studies focused on room temperature, single-phase, degassed water flowing at an average velocity of ≈3.5 m/s in a ≈480 μm hydraulic diameter microchannel (e.g., Re ≈ 1850), where the TDTR pump heating laser induces a local heat flux of ≈900 W/cm2 in the center of the microchannel with a hot-spot area of ≈250 μm2. By using a differential TDTR measurement approach, we show that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the single-phase convective heat transfer coefficient, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Our measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, our flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ≈1060 ± 190 kW/m2 K, where the Nusselt number correlations predict ≈900 ± 150 kW/m2 K.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(11):112404-112404-7. doi:10.1115/1.4036694.

This paper provides an experimental investigation of heat transfer and pressure drop of supercritical carbon dioxide cooling in a microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm × 0.5 mm cross section was used as the test section. The temperature drops of supercritical CO2 cooled inside the test section were controlled at 2 °C, 4 °C, and 8 °C separately for each test to investigate the effect of property change on the friction and heat transfer performance at various temperature cooling ranges near the critical point. The test results showed that while the test conditions were away from the critical point, both heat transfer and pressure drop performance agreed very well with those predicted by conventional correlations. However, for the test conditions near the critical point, the difference between those of the test results and the predicted values is very high. Both heat transfer and pressure drop were strongly affected by the ranges of temperature cooling in the test section while they were near the critical conditions. Since there is a drastic peak of the property change near the critical point, if we use the properties integrated but not averaged from inlet to the exit temperatures, we obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of supercritical carbon dioxide in microchannels with size near 0.5 mm are indeed similar to these at normal conditions if its properties are appropriately evaluated.

Commentary by Dr. Valentin Fuster

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