Selective laser melting (SLM) is a promising manufacturing method which enables the production of complex structured components from base metal powders. With the development of SLM, the possibility of fabricating functional heat transfer devices such as heat pipes and heat sinks using this technique has also gained significant interest in the recent years. In this paper, the possibilities of producing microstructured surfaces using SLM to promote nucleate pool boiling heat transfer were explored. The SLM facility (SLM 250 HL by SLM Solutions GmbH) at the Future of Manufacturing Laboratory 1 of Singapore Centre for 3D Printing (SC3DP) in Nanyang Technological University (NTU), Singapore was employed for the fabrication of the surfaces. The machine is comprised of a Gaussian distributed Yb:YAG laser with maximum power of 400 W and laser beam spot size of 80 μm which melts and fuses the AlSi10Mg base powder of distribution size 20 μm to 63 μm layer-by-layer to develop three-dimensional structures. In total, four 1 cm × 1 cm microstructured surfaces were produced; namely micro-cavity surface, micro-fin surface, micro-sized rectangular channel (MRC) surface and micro-sized square channel (MSC) surface. Saturated pool boiling experiments were conducted on these surfaces in a water-cooled thermosyphon with FC-72 as the coolant fluid under atmospheric condition. In comparison with a plain surface, the MRC and MSC surfaces exhibited marginal improvements in the average heat transfer coefficient whilst more significant enhancements of up to 51.2% were demonstrated with the micro-cavity and micro-fin surfaces. At low heat fluxes (< 7 W/cm 2 ), minimal differences in heat transfer performances between the microstructured surfaces and plain surface were observed. For increased heat fluxes, incremental enhancements in the heat transfer coefficients were observed for the micro-cavity and micro-fin surfaces as compared to the plain surface. The highest enhancement in the heat transfer coefficient over the plain surface was determined to be 63.5% for the micro-fin surface at the heat flux of 17.9 W/cm 2 and it was also observed that the heat transfer coefficient of micro-fin surface is consistently higher that of other microstructured surfaces for the range of heat fluxes tested. In addition, higher critical heat fluxes were also achieved with all microstructured surfaces as compared to the plain surface with the highest CHF of 46.2 W/cm 2 for the micro-fin and MRC surface. Visual observations suggest that the enhancement in heat transfer from the microstructured surfaces is likely to be due to the increased bubble nucleation sites created from the extended surfaces and the artificial cavities. In summary, these results indicate the promising use of SLM to produce surface features that will enhance pool boiling heat transfer.

Both theoretical and experimental research on CO 2 physical characteristics and pool boiling heat transfer are taken in this paper. It is analyzed that CO 2 pool boiling heat transfer outside the screwed and smooth tube based on experimental study. It is resulted with the relationship between heat transfer coefficient, evaporating pressure and the heat flux. It is also indicated the enhancement factors of three screwed tube compared with smooth tube. The results provides a basis for promoting CO 2 heat transfer enhancement, and also provides a theoretical support in engineering.

Boiling heat transfer is widely used in industry and aerospace, and it can be enhanced by surface structure treatment. Here, two types of Micro-Nano bi-porous copper surfaces (MNBPCS) were prepared by hydrogen bubble template method and then sintered in reducing atmosphere. The effect of surface morphology on the saturated pool boiling of ultrapure water was investigated. Results show that, both NMBPCS have superior heat transfer performance to the plain copper surface. When the heat flux is 100W/cm 2 , the wall superheat of the two MNBPCS are about 7 and 9 °C lower than the plain copper surface respective. When the heat flux is lower than 130W/cm 2 , the wall superheat of the mono-layer MNBPCS is lower than that of the multi-layer one, because the bubbles formed on the mono-layer MNBPCS can departure more easily than those on the multi-layer one. When the heat flux is higher than 130W/cm 2 , the multi-layer MNBPCS has lower wall superheat than that of the mono-layer one, own to its better liquid accommodation from the morphology structure. Significant hysteresis phenomenon was only found on the Multi-layer MNBPCS. Its wall superheat keeps almost the same at about 13°C for its bottom layer structure with smaller cave diameter, when the heat flux is higher than 75W/cm 2 . The CHF of each MNBPCS is higher than 200W/cm 2 , and the multi-layer one is higher than the mono-layer one own to its better liquid accommodation from the morphology structure.

Boiling heat transfer enhancement via compound effect of the EHD effect and micro-structured surfaces has been experimentally and analytically investigated. A fluorinated dielectric liquid was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. Micro-structured surfaces, which are mainly used for cutting tools, were developed with diamond particles using electro-deposition technique. Four different particle diameters were prepared: 5, 10, 15, and a mixture of 5 and 1.5 μm. The surface containing a mixture of 5 and 1.5 μm provided a 7 K reduction in the incipient superheat and a 26 % increase in the critical heat flux compared with those of a non-coated surface. Upon application of a −5 kV/mm electric filed to the micro structured surface (a mixture of 5 and 1.5 μm particles), a heat flux of 70.2 W/cm 2 at a superheat of 21.7K was obtained. The experimental data correlated well with previous theoretical analysis.

Graphene oxide (GO) can be deposited on a heater surface to produce nanoscale structures that enhance the thermal limit of the heater. GO has shown a very unique feature showing CHF enhancement without wettability improvement in terms of apparent advancing contact angle unlike any other typical nanoparticles. Many studies have analyzed the reason for the enhanced critical heat flux (CHF) of the heater. Three major models have been used for studying the enhanced CHF of the heater in many heat transfer studies: wettability of surface (contact angle), Rayleigh–Taylor instability wavelength, and thermal activity. In this study, scanning electron microscopy (SEM) images and the contact angle were taken to explain the enhanced wettability of a heater surface in a GO nanofluid. GO is composed of carbon and oxygen. This material has good affinity with water because of its polarity. In an electric field, GO reduces into reduced graphene oxide (RGO). This chemical reduction on the surface may be one factor that enhances the CHF. To examine how a GO nanofluid can enhance CHF to more than twice its original magnitude, a wettability model was applied that uses the contact angle of the nichrome wire heater surface after the CHF experiment. The reason why the wettability model could not completely account for the CHF enhancement in a GO nanofluid was also determined. The reduction of GO was considered to explain CHF enhancement.

In this work, thermophysical properties and pool boiling characteristics of water-in-polyalphaolefin (PAO) nanoemulsion fluids and their dependence on water concentration have been investigated experimentally. Water-in-PAO nanoemulsion fluids are formed via self-assembly with surfactant sodium sullfosuccinate (AOT). Thermal conductivity of the fluids is found to increase with water concentration, as expected from the Maxwell equation. However, the measured thermal conductivity increase is very moderate, e.g., a 16% increase for 8.6Vol. %. Unlike thermal conductivity, the dynamic viscosity of these nanoemulsion fluids first increases with water concentration, reaches a maximum, and then decreases. This trend could be attributed to the attractive forces among water droplets. The boiling behavior of these nanoemulsion fluids has been altered due to water nanodroplets. Adding water nanodroplets can lower the nanoemulsion’s boiling point compared to the pure PAO. Another interesting phenomenon observed is that pool boiling of nanoemulsion fluids randomly follows two different curves when the water concentration is in the range of 5.3 Vol. % to 7.8 Vol. %. The mechanism underlying this phenomenon is not understood yet, but it may be related to the evolution of microstructures in the water-in-PAO nanoemulsion fluids.

This paper quantifies the influence of Al 2 O 3 nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a Turbo-BII-HP boiling surface. An Al 2 O 3 nanolubricant (a lubricant containing dispersed nano-size particles) was made by suspending nominally 10 nm diameter Al 2 O 3 particles in a synthetic polyolester to roughly a 1.0% volume fraction. The nanoparticles caused, on average, a 12% degradation in the boiling heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for the three lubricant mass fractions that were tested. The degradation was nearly constant for heat fluxes between 20 kW/m 2 and 120 kW/m 2 . It was speculated that the boiling heat transfer degradation was primarily due to a combination of (1) film boiling in the reentrant cavity rendering the nucleate boiling enhancement mechanism of the nanoparticles ineffective and (2) a reduction in bubble frequency due to the increased surface wetting as caused by the nanoparticles. In addition, these degradation factors might be mitigated with increased nanoparticle loading.

With regard to potential application in pressurized water reactors (PWRs), a nanoporous heated surface was tested in pool boiling of an aqueous solution of boric acid (H 3 BO 3 ), or borated water (1% volume concentration). The effect of different system pressures and different surface orientations on pool boiling heat transfer was studied. The nanoporous surface consisted of a coating of alumina nanoparticles applied on a 1 cm 2 flat copper surface through nanofluid boiling. An uncoated surface in borated water was similarly tested, and due to boric acid deposition, the boiling heat transfer (BHT) degraded and the critical heat flux (CHF) enhanced relative to pure water. Also, the possibility of transient pool boiling behavior of borated water was investigated but none was detected. For all pressures and orientations, the nanoporous surface further enhanced the CHF of borated water to the CHF produced by the nanoporous surface in pure water. Over the nanoporous surface, the CHF of borated water was increasingly better with decreasing pressure, than that over the plain surface. However, BHT degraded slightly further. Boric acid deposition over the nanoporous surface was believed to be the source of this BHT degradation, but played no apparent role in the further CHF enhancement.

The paper describes results from an experimental study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling at atmospheric pressure with polished smooth boiling surface. A micro scaled electrode with slits for bubbles to come out was designed in order to create non uniform high electric field strength and to produce electrohydrodynamics (EHD) convection with the application of dc voltage. The application of high electric field strongly enhanced the heat flux and the heat transfer coefficient. From observations of the behavior of bubbles over the electrode and the boiling surface condition, the instability between the liquid and the vapor increased the heat flux, the heat transfer coefficient and the CHF.

Microbubble emission boiling (MEB) has been recognized for a potential phenomenon to be applied to devices such as next-generation heat exchanger for cooling small but high-power CPU and for long-duration exploring in the space/deep ocean; the MEB realizes higher heat flux than the critical heat flux known as ‘CHF’ accompanying with blowing out microbubbles smaller than 100 μm from the heated surface under highly subcooled condition. Invaluable information on characteristics of heat transfer and occurring condition of the MEB has been indicated by several research groups. Little knowledge on the mechanism of the MEB, however, has been accumulated; one of the reasons must be an immense complexity of the phenomenon of the boiling itself with interaction among solid-liquid-gas phases. In the present study, the authors focus on the condensing process of a vapor bubble in order to comprehend the mechanism of the microbubble formation and emitting processes in the MEB. In order to extract an interaction between the vapor and the liquid from a complex boiling phenomenon, a series of experiments of injection of superheated vapor to a subcooled pool are carried out. Comparison of the condensing process of the vapor and microbubble formation is made between the present vapor-injection experiment and conventional on-wire pool boiling experiment.

An experimental study of the pool boiling two-phase heat transfer on a sintered Cu microparticle porous structure module surface is conducted. Enhanced heat transfer capacity of this module surface has been reported, and the boiling characteristics have been investigated. The bubble dynamics and nucleate size distribution have been compared to the theoretical predictions, and the speculated mechanisms have been discussed.

Although boiling process has been a major subject of research for several decades, its physics still remain unclear and require further investigation. This study aims at highlighting the effects of the surface wettability on pool boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20 to 110° by modifying nanoscale surface topography and chemistry. The experimental results obtained disagree with the predictions of the classical models. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer. In this approach, we introduce the concept of macro- and micro-contact angles to explain the observed phenomenon.

The present paper reports a new series of experimental data of CHF (critical heat flux) of pool boiling on cylinders. Platinum wires of 30 mm in length are simultaneously used as heaters and thermometers. Their diameters are 0.1, 0.06, and 0.025 mm, respectively. FC-72 and acetone are used as working fluids. The range of the subcooling is from 0 K to about 50 K. The gaps between CHF and MHF (minimum heat flux) become narrower and narrower with the decrease of the heater diameter. But it exists even in the saturated pool boiling on the smallest wire in the present study. The dependence of CHF on the subcooling in acetone differs from that in FC-72 though the data locate in the similar range of Bond number in the two different kinds of working fluids. It indicates that interactions between the influences of the subcooling and size on CHF will be important for the small Bond number, and that there may exist some other parameters, which may be material-dependant, in addition to the Bond number that play important roles in the CHF phenomenon with small Bond number.

Accurate prediction of critical heat flux (CHF) in microchannels and small diameter tubes is of great interest in estimating the safe operational limits of cooling systems employing flow boiling. Scale analysis is applied to identify the relevant forces leading to the CHF condition. Using these forces, a local parameter model is developed to predict the flow boiling CHF. The theoretical model is an extension of an earlier pool boiling CHF model incorporating a force balance among the evaporation momentum, surface tension, inertia, and viscous forces. Weber number, capillary number, and a new non-dimensional group K 2 , representing the ratio of evaporation momentum to surface tension forces, emerged as main groups in quantifying the narrow channel effects on CHF. The constants in the model were calculated from the available experimental data. The mean error with ten data sets is 19.7 percent, with 76 percent data falling within ±30% error band, and 93 percent within ±50% error band. Evaluating individualized set of constants for each data set resulted in mean errors of less than 10 percent for all data sets. The success of the model indicates that flow boiling CHF can be modeled as a local phenomenon and the scale analysis is able to reveal important information regarding fundamental mechanisms leading to the CHF condition. The final equations resulting from this model are given by Eqs. (18–22) along with the transition criteria given by Eq. (28).

The bubble dynamics of ethanol vapor bubbles growing, coalescing and condensing in a subcooled ethanol liquid pool were investigated experimentally and numerically for a range of subcoolings and heating rates. The bubbles were generated from an artificial pair of nucleation sites made of microscale tubes mounted flush with the bottom surface of the liquid pool with the vapor supplied by a vapor generator. Observations of the bubble generation with a high speed camera show the various coalescence modes with no coalescence at low heating rates and high subcoolings and horizontal and/or vertical coalescence depending on the heating rate and subcooling. At very low subcoolings, the bubbles grew quite large with various types of coalescence. The numerical results using solutions of the Navier-Stokes equations with the VOF model and using a simplified one dimensional model also describe the bubble dynamics and the conditions for coalescence. The numerical results suggest that the condensation rate at the interface is probably much higher than predicted by the model due to significant convection in the liquid pool or due to significant disturbance of the interface by the vapor jet entering the bubble.

The need to operate thermal systems, including those in nuclear power plants, below the critical heat flux (CHF) requires inconvenient compromises between economy and safety. This has led to much research aimed at enhancing the CHF. CHF enhancement was recently noticed in nanofluids where the heating surface was fouled by nanoparticles. We attempted to imitate the micro/nano multi-scale structure of nanoparticle fouling by surface modification. In this study, surface treatment was used to change the surface wettability of a Zircaloy-4 heater. A pool boiling apparatus was developed based on the results of the surface treatment, and an experimental study was performed to investigate the increase in CHF.

This paper quantifies the influence of diamond nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. Nanofluids are liquids that contain dispersed nano-size particles. A lubricant based nanofluid (nanolubricant) was made by suspending 10 nm diameter diamond particles in a synthetic ester to roughly a 2.6% volume fraction. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) as large as 129% for the best performing tests. A similar enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 91% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in boiling heat transfer degradation of approximately 19% for the best performing tests. It was speculated that the poor quality of the nanolubricant suspension caused the performance of the (99.5/0.5), and the (98/2) nanolubricant mixtures to decay over time to, on average, 36% and 76% of the of pure R134a/polyolester performance, respectively. Thermal conductivity and viscosity measurements and a refrigerant\lubricant mixture pool-boiling model were used to suggest that increases in thermal conductivity and lubricant viscosity are mainly responsible for the heat transfer enhancement due to nanoparticles. Particle size measurements were used to suggest that particle agglomeration induced a lack of performance repeatability for the (99.5/0.5) and the (98/2) mixtures. From the results of the present study, it is speculated that if a good dispersion of nanoparticles in the lubricant is not obtained, then the agglomerated nanoparticles will not provide interaction with bubbles, which is favorable for heat transfer. Further research with nanolubricants and refrigerants are required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.

This study shows the transient characteristics of the pool boiling curves using nanofluid as the boiling fluid. This time-dependency is in sharp contrast to a unique steady-state pool boiling curve that is typically obtained for a pure fluid. Past researches on nanofluids have provided several interesting information about the thermal characteristics for this potentially promising cooling fluid. Results from these studies have shown some extraordinary critical heat flux (CHF) values and thermal conductivity enhancement that is yet to be explained by existing theories and correlations. The nature of the pool boiling curve for a nanofluid is dependent on the nanoparticle concentration in the base fluid. Higher concentration nanofluids show a perceptible degradation in the boiling heat transfer coefficient but have exhibited an enhanced CHF value (up to ∼80%) when compared to the CHF value of the base fluid (water). Another key observation has been in the significant deposition of nanoparticles on the heater surface. This fouling of the heater surface by nanoparticles is widely viewed as a main contributor that modifies the pool boiling curve of the base liquid. The deposition of the nanoparticles on the heater surface is dynamic and this in turn makes the nanofluid pool boiling curve exhibit transient characteristics.

In recent years nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions.

The purpose of this work is to describe nucleate and transition boiling occurring in an inclined narrow space (0° ≤ φ ≤ 35°) between a heated upward-facing copper disk and an unheated surface for saturated n-pentane. The distance between the heated surface and the confinement plate is enclosed between 200 μm ≤ s ≤ 90 mm. The heat flux and the wall temperature are determined by mean of an inverse heat conduction method. The instantaneous volume fraction of the vapour phase within the confined space is measured using capacitive sensors. The influence of the confinement and the inclination angle on the boiling curves and the flow patterns are analysed. The evolution of void fraction with the heat flux increase, the reduction of the gap size and the inclination of the heated surface are determined. Boiling becomes unsteady for given heat flux and inclination angle ranges. The boiling cycles are characterized by periodic liquid feeding and two-phase structure expulsion from the confined space. The cycle’s frequency increase with the heat flux and the inclination angle increase is analyzed. The conditions of unsteady boiling apparition are summarized in unsteady boiling pattern maps.