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Guest Editorial  
Chang Kyoung Choi, Kenneth Kihm and David Pratt
J. Heat Transfer   doi: 10.1115/1.4036799
The 23rd Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 2016 International Mechanical Engineering Congress and Exhibition (IMECE) held in Phoenix Convention Center, Phoenix, Arizona, on November 11 ~ 17, 2016. Ten entries were submitted into the Photogallery sessions and the peer-reviewed evaluation conducted by both the participants and selected HTD K-22 Technical Committee members has identified the seven final entries for publication in this ASME Journal of Heat Transfer August issue of 2017. The purpose of publishing these entries is to draw attention to the innovative features of visualization techniques and aesthetic qualities of heat and mass transport phenomena. This Photogallery issue covers multiscale visualizations from atomic to meter scales. To focus on visualization images and schematics for each entry, the text is kept to a minimum and further details should be found directly from the authors. Our wish is that the journal readers enjoy viewing these collections, acquire knowledge of the state-of-the-art visualization techniques, and also promote their participation in the future Photogallery sessions in the SHTC and ASME-IMECE conferences.
TOPICS: Heat transfer, Visualization, Transport phenomena, Mechanical engineering, Heat
research-article  
Abulimiti Aili, Qiaoyu Ge and TieJun Zhang
J. Heat Transfer   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 nanostructures 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 nanostructures density leads to a subsequent increase in the nucleation density of water droplets because of the decreased energy barrier of nucleation in cavities between the nanostructures. Specifically, we observed droplets aligning in regions with denser nanostructures. And, the droplet number density and average volume of the aligned droplets in these regions were larger than in surrounding areas. However, the nucleation in cavities caused the initial pinning of droplets base into the nanostructures, forming a balloon-like, slightly elongated droplet morphology. The dewetting transition from the pinned Wenzel state to the unpinned Cassie state was predicted by quantifying the aspect ratio of the height and diameter of droplets ranging from 3 µm to 30 µm. Moreover, the coalescence-jumping of condensate droplets was followed by a new cycle of droplet nucleation in a linear manner on an emptied region. These findings offer guidelines for designing enhanced superhydrophobic surfaces for water and energy applications.
TOPICS: Drops, Nucleation (Physics), Nanostructures , Water, Density, Cavities, Cycles, Shapes, Condensation, Condensed matter, Design
research-article  
Jian-Jun Shu, Qi-Wen Wang and Ioan Pop
J. Heat Transfer   doi: 10.1115/1.4036727
The problem of steady mixed convection boundary-layer flow on a cooled vertical circular cylinder embedded in a fluid-saturated porous medium is studied. Here, we evaluate the flow and heat transfer characteristics numerically for various values of the governing parameters and demonstrate the existence of dual solutions beyond a critical point.
TOPICS: Porous materials, Mixed convection, Flow (Dynamics), Heat transfer, Fluids, Boundary layers, Circular cylinders
research-article  
D. Wang, D. Ewing and C. Y. Ching
J. Heat Transfer   doi: 10.1115/1.4036728
The local mass transfer over dissolving surfaces was measured at pipe Reynolds number of 50,000, 100,000 and 200,000. Tests were run at multiple time periods for each Reynolds number using 203 mm diameter test sections that had gypsum linings dissolving to water in a closed flow loop at a Schmidt number of 1200. The local mass transfer was calculated from the decrease in thickness of the gypsum lining that was measured using X-ray computed tomography (CT) scans. The range of Sherwood numbers for the developing roughness in the pipe was in good agreement with previous studies. The mass transfer enhancement (Sh/Shs) was dependent on both the height (ep−v) and spacing (λstr) of the roughness scallops. For the developing roughness, two periods of mass transfer were present: (i) an initial period of rapid increase in enhancement when the density of scallops increase till the surface is spatially saturated with the scallops and (ii) a slower period of increase in enhancement beyond this point, where the streamwise spacing is approximately constant and the roughness height grows more rapidly. The mass transfer enhancement was found to correlate well with the parameter (ep−v/λstr)0.2, with a weak dependence on Reynolds number.
TOPICS: Mass transfer, Reynolds number, Surface roughness, Pipes, Gypsum, Density, Flow (Dynamics), Computerized tomography, Water, Linings (Textiles)
research-article  
Christian Helcig, Stefan aus der Wiesche and Igor V. Shevchuk
J. Heat Transfer   doi: 10.1115/1.4036729
Convective heat transfer in rotating disk systems is of great importance in many engineering applications. Despite the high practical relevance, there have been only a small number of experimental investigations regarding the influence of the Prandtl number larger than unity. Ever since Dorfman's pioneering work more than 50 years ago, various analytical works about the heat transfer of a rotating disk have been published. However, this study is a novelty by presenting measurements of the laminar convective heat transfer over a free rotating disk for a wide range of the Prandtl number up to Pr = 5000. The accuracy of the employed experimental apparatus was assessed by heat transfer measurements in air, for which reliable literature data are widely available. Natural convection effects and temperature dependent physical properties have been taken into consideration using the property-ratio method. The experimental results are in excellent agreement with analytical self-similar solutions and the theoretical correlation of Lin and Lin. The applicability of frequently used heat transfer correlations is assessed by the means of the new experimental data.
TOPICS: Convection, Prandtl number, Rotating Disks, Heat transfer, Temperature, Engineering systems and industry applications, Natural convection
Discussion  
Asterios Pantokratoras
J. Heat Transfer   doi: 10.1115/1.4036733
The present comment concerns some doubtful results in the above paper.
TOPICS: Magnetohydrodynamics, Heat transfer, Fluids, Convection, Temperature gradient
research-article  
Nataporn Korprasertsak and Thananchai Leephakpreeda
J. Heat Transfer   doi: 10.1115/1.4036734
In this paper, determination of convective heat transfer coefficient under actual convection processes is proposed by using thermoelectric modules. According to the Newton's law of cooling, the rate of heat flow through fluid is determined with the convective heat transfer coefficient, which is usually derived as a constant parameter from empirical correlations for a given flow condition. In fact, the convection heat transfer coefficient is a temperature-dependent parameter under variant operating conditions. Also, it strongly depends upon speed of fluid, which changes with respect to time. Analytical and experimental results show the effectiveness of the proposed thermoelectric modules in determining the convective heat transfer coefficient for air in real time implementation. The methodology can be generalized in further developing determination of convective heat transfer coefficients for other fluids.
TOPICS: Convection, Fluids, Flow (Dynamics), Heat, Temperature, Cooling
research-article  
Guanchen Li and Michael R. von Spakovsky
J. Heat Transfer   doi: 10.1115/1.4036735
Conventional first-principle approaches for studying nonequilibrium processes depend on the mechanics of individual particles or quantum states and as a result, require many details of the mechanical features of the system to arrive at a macroscopic property. In contrast, thermodynamics, which has been successful in the stable equilibrium realm, provides an approach for determining macroscopic properties without the mechanical details. Nonetheless, this phenomenological approach is not generally applicable to a nonequilibrium process except in the near-equilibrium realm and under the local equilibrium and continuum assumptions, both of which limit its ability to describe nonequilibrium phenomena. Furthermore, predicting the thermodynamic features of a nonequilibrium process (of entropy generation) across all scales is difficult. To address these drawbacks, steepest-entropy-ascent quantum thermodynamics (SEAQT) can be used. It provides a first-principle thermodynamic-ensemble based approach applicable to the entire nonequilibrium realm even that far-from-equilibrium and does so with a single kinematics and dynamics, which crosses all temporal and spatial scales. Based on prior developments by the authors, SEAQT is used here to study the heat and mass diffusion of indistinguishable particles. The study focuses on the thermodynamic features of far-from-equilibrium state evolution, which is separated from the specific mechanics of individual particle interactions. Results for nonequilibrium size (volume) and concentration effects on the evolutionary state trajectory are presented for the case of high temperature and low particle concentration, which, however, do not impact the generality of the theory and will in future studies be relaxed.
TOPICS: Thermodynamics, Heat, Diffusion (Physics), Entropy, Identical particles (Quantum mechanics), Equilibrium (Physics), Particulate matter, Kinematics, Dynamics (Mechanics), Trajectories (Physics), High temperature
research-article  
Shashank Natesh, Eric Truong, Vinod Narayanan and Sushil H. Bhavnani
J. Heat Transfer   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 in the span-wise 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 degree asymmetry. Drainage pathways are varied in density as non-dimensional drainage pathways per meter depth ranging from 133 to 400. Experiments are performed at varied wall subcooling temperatures from 1-10°C. Results of the asymmetric ratchet are compared against a control test surface with 45 - 45 degree 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.
TOPICS: Drainage, Condensed matter, Density, Condensation, Visualization, Subcooling, Geometry, Steady state, Fluids, Temperature, Wetting, Brass (Metal)
research-article  
Md Mahamudur Rahman and Matthew McCarthy
J. Heat Transfer   doi: 10.1115/1.4036693
While significant enhancements in pool boiling critical heat flux (CHF) and heat transfer coefficient (HTC) have been demonstrated using structured surfaces, fundamental questions remain about the nature of the enhancements 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 and four different copper oxide nanostructured coatings. Additionally, twenty different multiscale structures were fabricated coinciding with each permutation of the various microchannels and nanostructures. The nanostructured coatings were observed to increase CHF via surface wicking, but decrease HTC due to the suppression of nucleation. 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 W/m2K, 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 form 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.
TOPICS: Copper, Boiling, Critical heat flux, Microchannels, Coatings, Nanostructures , Nucleation (Physics), Heat transfer coefficients, Pool boiling, Dynamics (Mechanics), Heat, Flux (Metallurgy), Bubbles
research-article  
Chien-Yuh Yang and Kun-Chieh Liao
J. Heat Transfer   doi: 10.1115/1.4036694
This paper provides an experimental investigation of heat transfer performance and pressure drop of supercritical carbon dioxide cooling in microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm x 0.5 mm cross section was used as the test section. The temperature drops of supercritical CO2 cooled inside the test section was controlled at 2, 4 and 8 oC separately for each test to investigate the effect of properties 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 convention correlations. However, while the test conditions near the critical point, the difference between the present test results and the prediction values is very high. Both heat transfer and pressure drop were strongly affected by the ranges of temperature cooled in the test section while they were near the critical point conditions. Since there is a drastic peak of the properties change near the critical point, if we use the properties integrated but not averaged from inlet to the exit temperatures, we may obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of super critical carbon dioxide in microchannel are indeed similar to these at normal conditions if its properties were appropriately evaluated.
TOPICS: Heat transfer, Microchannels, Supercritical carbon dioxide, Temperature, Pressure drop, Cooling, Aluminum tube, Heat exchangers, Carbon dioxide, Friction
research-article  
Arvind Jaikumar, Aniket Rishi, Anju Gupta and Satish G. Kandlikar
J. Heat Transfer   doi: 10.1115/1.4036695
Enhanced pool boiling heat transfer, with simultaneous increase in both 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 different techniques associated with generating enhancement structures 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. Specific microscale morphologies were generated with copper and Graphene oxide coatings 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 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 record CHF of 2.2 MW/m2 (1.8X over a plain surface) with an HTC of 155 kW/m2°C (~2.4X over a plain surface) was obtained. Furthermore, close correlations between the boiling performance and the microscale surface morphology in these three categories have been identified.
TOPICS: Copper, Oxide coatings, Graphene, Pool boiling, Microscale devices, Critical heat flux, Nucleation (Physics), Boiling, Density, Heat transfer, Surface roughness, Electrodeposition, Bubbles, Performance characterization, Cavities, Printing, Water, Heat transfer coefficients, System efficiency
research-article  
Chukwudi Azih and Metin I. Yaras
J. Heat Transfer   doi: 10.1115/1.4036689
A key subject of interest for technologies that involve flows of fluids at the supercritical thermodynamic state is the development of prediction methods that capture the fluid dynamics and convection heat transfer at this state. Due to the elevated temperatures and pressures associated with certain working fluids at this thermodynamic state, surrogate fluids are often used as substitutes for performing experiments during the design stages of prototype development. The success of this approach depends on the development of similarity criteria or fluid-to-fluid models. Similarity criteria for mixed-convection heat transfer in supercritical fluids are proposed based on a set of non-dimensional dynamic similarity parameters and state-space parameters developed through our current understanding of the physical mechanisms that affect heat transfer in fluids at this state. The proposed similarity criteria are successfully validated using data from ducted flows of supercritical fluids with configurations having upstream, downstream, or wall-normal oriented gravitational acceleration.
TOPICS: Flow (Dynamics), Supercritical fluids, Heat transfer, Mixed convection, Modeling, Fluids, Engineering prototypes, Convection, Design, Temperature, Gravitational acceleration, Fluid dynamics
Technical Brief  
Rabha Khatyr, Jaafar Khalid-Naciri and Ali Il Idrissi
J. Heat Transfer   doi: 10.1115/1.4036690
The asymptotic behaviour of laminar forced convection in a circular duct for a Herschel- Bulkley fluid with constant properties is analysed. The viscous dissipation effects and the axial heat conduction in the fluid are considered. The asymptotic bulk and mixing temperature field, and the asymptotic value of the bulk and mixing Nusselt number are determined for every boundary condition enabling a fully developed region. In particular, it is proved that whenever the wall heat flux tends to zero, the asymptotic Nusselt number is zero. The comparison with other existing solutions in the literature for Newtonian and non-Newtonian cases are analysed.
TOPICS: Fluids, Heat conduction, Energy dissipation, Forced convection, Ducts, Heat flux, Temperature, Boundary-value problems
research-article  
Mehrdad Mehrvand and Shawn A. Putnam
J. Heat Transfer   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 sub-cooled 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.
TOPICS: Thermoreflectance, Water, Microchannels, Heat transfer coefficients, Heating, Forced convection, Natural convection, Pumps, Power density, Heat flux, Fluid dynamics, Flow (Dynamics), Heat, Temperature, Cooling, Fluids, Metals, Lasers, Heat conduction, Flux (Metallurgy), Coolants, Anisotropy, Channel flow, Convection
research-article  
Xiangfei Yu, Corey Woodcock, Yingying Wang, Joel L. Plawsky and Yoav Peles
J. Heat Transfer   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/m2s. It was found that 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.
TOPICS: Flow (Dynamics), Heat transfer, Boiling, Subcooling, Microchannels, Heat transfer coefficients, Critical heat flux, Flux (Metallurgy), Fins, Pressure drop, Visualization, Fluids, Heat, Temperature, Nucleate boiling
research-article  
Nanxi Li and Amy Rachel Betz
J. Heat Transfer   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 non-homogeneous 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.
TOPICS: Copper, Boiling, Graphene, Bubbles, Pressure, Coating processes, Coatings, Oxide coatings, Sprays, Critical heat flux, Drops, Spin (Aerodynamics), Rotation, Condensation, Atmospheric pressure, Particle spin, Electrical properties, Nanostructures , Pool boiling, Shapes, Water, Heat flux, Heat transfer coefficients
research-article  
Collin T. Burkhart, Kara Maki and Michael J. Schertzer
J. Heat Transfer   doi: 10.1115/1.4036681
This investigation provides experimental evidence examining 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 the interface velocity to the particle diffusion rate alone when the two effects are of the same order. Instead, the ratio of radial velocity to the 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 non-uniform central deposition are observed for cases where the characteristic ratios are similar in magnitude. Since both ratios are scaled by diffusion rate, it is possible to characterize the deposition patterns observed 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 DLVO forces repel particles from the surface in these cases. Further research is required to determine if the conclusions here can be extended or modified to describe deposition patterns when particles are subjected to appreciable Marangoni recirculation and attractive DLVO forces.
TOPICS: Diffusion (Physics), Drops, Evaporation, Particulate matter
research-article  
Daniel Lorenzini and Yogendra Joshi
J. Heat Transfer   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 approach for 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. While important experimental evidence in this area has been recently reported in the literature, the modeling of transport has remained in basic and limited forms due to the associated complexities with the physics of two-phase flow with phase-change. 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 using temperature dependent properties. The flow boiling regimes and their spatial and temporal evolution are compared between both methods by maintaining identical operating conditions. The CLSVOF offers a sharper interface reconstruction than the standard VOF method by predicting bubble nucleation and departure mechanisms more closely in agreement with experimental observations.
TOPICS: Flow (Dynamics), Boiling, Silicon, Cooling, Fluids, Heat conduction, Flux (Metallurgy), Bubbles, Nucleation (Physics), Heat, Temperature, Energy transformation, Temperature profiles, Heat transfer coefficients, Physics, Modeling, Two-phase flow, Fins, Integrated circuits, Refrigerants
research-article  
Abdolali Khalili Sadaghiani, Ahmadreza Motezakker, Alsan Volkan Ozpinar, Gozde Ozaydin-Ince and Ali Kosar
J. Heat Transfer   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 pHEMA coated surfaces to investigate the effect of surface orientation on bubble dynamics and nucleate boiling heat transfer. pHEMA (polyhydroxyethylmethacrylate) coatings with thicknesses of 50, 100 and 200 nm were deposited using the iCVD (initiated chemical deposition) method. De-ionized water was used as the working fluid. Experiments were performed on horizontal and inclined surfaces (inclination angles of 10° ,30°, 50° and 70°) under the constant heat flux 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.
TOPICS: Heat transfer, Pool boiling, Bubbles, Dynamics (Mechanics), Vapors, Coatings, Heat flux, Heat transfer coefficients, Silicon, Water, Nucleate boiling, Heat, Fluids, Boiling, Heat exchangers, Boundary-value problems

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