Search

High-temperature phase relations in the La2O3–TiO2–Nb2O5 ternary system were investigated at 1290 °C using SEM, EDS and XRD, and a subsolidus phase equilibrium diagram was constructed. It was demonstrated that the addition of the La1/3NbO3 to the La2O3:3TiO2 mixture completely stabilizes a perovskite-La2/3TiO3 compound, which is otherwise not stable at stoichiometric composition. On the other hand, a mixture La2O3:TiO2 =1:3 dissolves in La1/3NbO3 up to 15 mol% TiO2. Moreover, a newly defined composition of compound La30Ti16Nb4O87 formed in the La2O3-rich part of the system and a solid solution La3Ti5Nb10O39.5–La2Ti2Nb8O27 were proposed. In the binary subsystem LaTiNbO6-La0.462Ti0.386Nb0.614O3, the components exhibit opposite temperature dependence of resonant frequency in the microwave frequency range. Thus, the dielectric properties of the prepared ceramics can be tailored by varying the component concentration and thermally stable dielectric properties can be achieved for a composition with a LaTiNbO6/La0.462Ti0.386Nb0.614O3 molar ratio of 0.27.

Self-rewetting fluids are commonly used in heat pipes, but their performance is not well understood in pool boiling. Here, we performed boiling of n-butanol-water mixtures on thin titanium foil and utilized synchronous high-speed IR and video cameras to capture transient temperature fields and bubble dynamics. At the onset of nucleate boiling, self-rewetting fluids offered higher nucleation site density and smaller bubble diameters compared to water. However, with increasing heat flux, a significant depletion of n-butanol was created in the vicinity of nucleation sites, which caused periodic temporal fluctuations in nucleation temperature and bubble evaporation energy. Average surface temperature and bubble departure diameter for self-rewetting fluids even exceeded the values of pure water. This shows that Marangoni effects cannot overcome limited mass diffusion of more volatile component towards nucleation sites in pool boiling on flat horizontal heaters. Based on our observations, the use of a self-rewetting fluid will not increase the heat transfer coefficient when compared to pure water, which is also known for conventional mixtures with negative temperature-dependent surface tension. Our experiments offer a step towards better understanding nucleate boiling of complex mixtures and a possibility for indirect evaluation of concentration variations based on synchronous high-speed video and IR thermography.

Nucleate pool boiling experiments were performed on plain and five laser-textured stainless-steel foils using saturated pure water, 100% ethanol, 0.4% and 4.2% mole fraction ethanol – water mixtures. All laser-textured samples contained untreated, smooth 0.5 mm wide regions and intermediate textured surfaces, that differ in the width of the laser patterned regions (from 0.5 mm to 2.5 mm). For smooth surfaces, we measured significant decreases in average heat transfer coefficients (HTC) and increases in bubble activation temperatures in comparison with the laser-textured surfaces for all the tested working fluids. Significant enhancement in HTC (280%) on a textured heating surface with 2.5-mm-wide laser pattern was recorded using pure water. For pure ethanol, the highest enhancement of 268% was achieved on a heating surface with 1.5-mm-wide laser pattern. The highest enhancement of HTC for the tested binary mixtures was obtained using 2.0-mm wide-laser-textured regions (HTC improved by 235% and 279% for the 0.4% and 4.2% mixtures, respectively). Our results indicate that laser texturing can significantly improve boiling performance when the intervals of the laser-textured patterns are close to the capillary lengths of the tested fluids.

This paper investigates transient temperature fields of thin metal foil heaters recorded with high-speed infra-red thermography during saturated pool boiling of water at atmospheric pressure. Three different thin heaters were tested: stainless steel (SS316), titanium (Ti) and copper with oxidized superhydrophilic surface (CuO). Experimentally acquired temperature fields were analysed using statistical approach. Samples with lower values of wall-temperature standard deviation and skewness and high values of kurtosis are found to perform better during nucleate boiling regime by providing higher heat transfer coefficient. The same samples had narrower heater-wall temperature distributions with higher peaks. The stainless steel sample exhibited bimodal temperature distributions, which resulted from occurrence of bubbles with relatively large contact diameters and long growth times. In addition to that, it was confirmed that wall temperature distributions are effective tool to distinguish between active nucleation sites and non-active single-phase convection area and also to qualify different bubble evolution cycles.

The boiling performance of functionalized hybrid aluminum surfaces was experimentally investigated for water and self-rewetting mixtures of water and 1-butanol. Firstly, microstructured surfaces were produced via chemical etching in hydrochloric acid and the effect of the etching time on the surface morphology was evaluated. An etching time of 5 min was found to result in pitting corrosion and produced weakly hydrophilic microstructured surfaces with many microcavities. Observed cavity-mouth diameters between 3.6 and 32 μm are optimal for efficient nucleation and provided a superior boiling performance. Longer etching times of 10 and 15 min resulted in uniform corrosion and produced superhydrophilic surfaces with a micropeak structure, which lacked microcavities for efficient nucleation. In the second stage, hybrid surfaces combining lower surface energy and a modified surface microstructure were created by hydrophobization of etched aluminum surfaces using a silane agent. Hydrophobized surfaces were found to improve boiling heat transfer and their boiling curves exhibited a significantly lower superheat. Significant heat transfer enhancement was observed for hybrid microcavity surfaces with a low surface energy. These surfaces provided an early transition into nucleate boiling and promoted bubble nucleation. For a hydrophobized microcavity surface, heat transfer coefficients of up to 305 kW m−2 K−1 were recorded and an enhancement of 488% relative to the untreated reference surface was observed. The boiling of self-rewetting fluids on functionalized surfaces was also investigated, but a synergistic effect of developed surfaces and a self-rewetting working fluid was not observed. An improved critical heat flux was only obtained for the untreated surface, while a lower critical heat flux and lower heat transfer coefficients were measured on functionalized surfaces, whose properties were already tailored to promote nucleate boiling.

Functionalized interfaces enhancing phase-change processes have immense applicability in thermal management. Here, a methodology for fabrication of surfaces enabling extreme boiling heat transfer performance is demonstrated, combining direct nanosecond laser texturing and chemical vapor deposition of a hydrophobic fluorinated silane. Multiple strategies of laser texturing are explored on aluminum with subsequent nanoscale hydrophobization. Both superhydrophilic and superhydrophobic surfaces with laser-engineered microcavities exhibit significant enhancement of the pool boiling heat transfer. Surfaces with superhydrophobic microcavities allow for enhancements of a heat transfer coefficient of over 500%. Larger microcavities with a mean diameter of 4.2 μm, achieved using equidistant laser scanning separation, induce an early transition into the favorable nucleate boiling regime, while smaller microcavities with a mean diameter of 2.8 μm, achieved using variable separation, provide superior performance at high heat fluxes. The enhanced boiling performance confirms that the Wenzel wetting regime is possible during boiling on apparently superhydrophobic surfaces. A notable critical heat flux enhancement is demonstrated on superhydrophobic surfaces with an engineered microstructure showing definitively the importance and concomitant effect of both the surface wettability and topography for enhanced boiling. The fast, low-cost, and repeatable fabrication process has great potential for advanced thermal management applications.

Two different quaternary ammonium cations, methyltriethyl- (MTEA) and tetraethylammonium cations (TEA) were used as templates in the synthesis of pure-silica as well as aluminosilicate ZSM-12 (MTW-type) frameworks. The distribution of the template cations in the 12-membered rings channels in the 1-dimensional framework topology was studied; thus the as-prepared products were characterized by means of X-ray powder diffraction, Raman, transmission FTIR, solid-state NMR spectroscopy, thermogravimetric and elemental analyses and SEM. It was shown that in pure-silica (PS) ZSM-12, TEA cations are well ordered - a superstructure with three-times longer b edge (in comparison to unit cell of empty framework) along the channel is formed, which can be seen by virtue of a few additional peaks in the X-ray powder pattern. Herein we describe that its aluminosilicate counterpart with TEA also contains ordered TEA cations and is isostructural to PS-ZSM-12. Conversely, in both pure-silica and aluminosilicate ZSM-12 frameworks with MTEA, the cations are disordered and no superstructure is formed.

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.

This study investigates the effect of experimental setup design factors on pool boiling CHF, compares spatial temperature gradient calculation methods and analyzes the uncertainty of heat flux and surface superheat. Reported CHF values on smooth copper surfaces, measured for saturated pool boiling of water at atmospheric pressure on flat horizontal samples, are highly scattered, which cannot be explained solely by the measurement uncertainty or the randomness of the boiling process. CHF data for 54 experiments from 47 publications is analyzed using regression analysis and ANOVA to determine which experimental setup design factors influence the CHF value. Methods for estimating the axial temperature gradient in a heating stem are compared using the Monte Carlo method and analytical nonlinear gradients. Heat flux values calculated using temperature measurements in a cylindrical copper heating stem together with either constant or temperature-dependent thermal conductivity and various temperature gradient calculation methods are compared. Overall heat flux and surface superheat measurement uncertainties are analyzed and the impact of contributing uncertainties including that of the thermal conductivity, temperature measurement and distance between thermocouples is reported.

Microporous triclinic AlPO4-34, known as APO-Tric, serves as an excellent water adsorbent in thermal energy storage, especially for low temperature thermochemical energy storage. Increased water adsorption capacity of thermochemical material usually leads to higher thermal energy storage capacity, thus offering improved performance of the adsorbent. The main disadvantage of aluminophosphate-based TCM materials is their high cost due to the use of expensive organic templates acting as structure directing agents. Using ionic liquids as low cost solvents with associated structure directing role can increase the availability of these water adsorbents for TES applications. Here, a green synthesis of APO-Tric crystals at elevated and ambient pressure by using 1-ethyl-3-methyl imidazolium bromide ionic liquid is presented. Large 200 µm romboid shaped monocrystals were obtained at 200 °C after 6 days. The structure of APO-Tric and the presence of 1,3-dimetylimidazolium cation in the micropores were determined by single crystal XRD at room temperature and 150 K. Water sorption capacity of APO-Tric prepared by ionothermal synthesis at elevated pressure increased in comparison to the material obtained at hydrothermal synthesis most probably due to additional structural defects obtained after calcination. The reuse of exhausted ionic liquid was also confirmed, which adds to the reduction of toxicity and cost production of the aluminophosphate synthesis.

Boiling is one of the most effective heat transfer mechanisms and has a great potential in applications where superior thermal energy management capability is required. During nucleate boiling the fluid vaporises on a surface heated above the fluid’s saturation temperature and the latent heat of vaporisation allows dissipating large heat flux. The boiling performance is usually represented by a boiling curve that plots the spatio-temporal averaged heat flux versus the wall superheat. Here, we show a new approach to evaluating the process on the basis of wall-temperature distributions, calculated from spatio-temporal thermographs of the boiling surface. Instead of a single data point on the boiling curve, the presented distributions provide spectra of information, such as the maximum, minimum and mean wall superheat and the standard deviation of the wall temperature. These parameters are all associated with the nucleation frequencies and the nucleation site densities. The experimental results from various state-of-the-art surfaces suggest that the distributions reveal the stability of the boiling process, which opens a new path towards a better understanding and the development of heat transfer technology.

The submicron characterization of transient heat-transfer processes at solid–liquid interfaces is of great importance in many areas of science and engineering. This paper reports on a technique that allows for the transient thermal imaging of the temperature field underneath a growing bubble during nucleate boiling with submicron spatial resolution. The boiling experiments were performed on a temperature-sensitive, erbium-doped, heavy-metal glass, Er:ZBLALiP, used as a robust sensing material for the non-invasive, transient temperature measurements. These measurements were made by analyzing the intensity variations of the fluorescence emission. The thermal imaging of an active nucleation site was performed by utilizing high-resolution, fluorescence microscopy, which enabled a maximum spatial resolution of 370 nm/pixel. The high-speed acquisition above 400 fps ensured sampling of individual bubble-nucleation events. Our transient measurements clearly revealed temperature variations underneath the growing bubble, as well as a measurable bubble-departure frequency under saturated conditions. These encouraging results suggest the need for a systematic use of the corresponding fluorescence technique on enhanced boiling surfaces in order to define the local heat-transfer characteristics and to gain a better understanding of the underlying physical processes.

Understanding the dynamic properties of a plate heat exchanger plays an essential role when developing smart control algorithms for modern energy-efficient district heating and cooling applications. In this work, the dynamic behaviour of a counterflow plate heat exchanger is studied in order to understand its transient responses due to inlet temperature disturbances at different fluid flow configurations. For this purpose, a suitable theoretical model has been proposed. The temperature transients are evaluated numerically by solving a lumped-parameter system representing the derived 1D model of the studied plate heat exchanger. The numerical results revealed that different fluid flow configurations considerably influence the transient temperature responses as well as the overall temperature drop and heat transfer rate when following the cold down process of a fluid travelling from the inlet to the outlet of the plate heat exchanger hot fluid side. The predicted transients are experimentally verified by conducting a series of systematic tests using infrared thermography technology. The analysis shows that the proposed model is in good agreement with the experiments. The results of the thermal imaging measurements also provide a more in-depth insight into the temperature distribution and its transient front propagation along the fluid flow channels. In future studies, such an experimental approach is valuable to identify the critical temperature zones when predicting the thermal fatigue life-time of brazed plate heat exchangers and as well to extend the proposed theoretical model to include the 2D local heat transfer coefficient distribution obtained by the IR thermography.

In this study, the optimal surface pattern of low and high wettability regions for enhanced boiling heat transfer is investigated using aluminum superbiphilic surfaces. Samples are fabricated by combining chemical vapor deposition of a fluorinated silane to turn them superhydrophobic and nanosecond laser texturing to render selected areas superhydrophilic. Triangular lattice pattern of superhydrophobic circular spots is utilized with spot diameters between 0.25 mm and 1.0 mm and pitch values of 0.5–2.5 mm. Pool boiling heat transfer performance of superbiphilic surfaces is evaluated using saturated water at atmospheric pressure. A strong wettability contrast is shown to be important in ensuring high heat transfer performance of wettability-patterned surfaces. Highest heat transfer performance is achieved using 0.5 mm diameter spots with a spot pitch of 1 mm and a corresponding superhydrophobic area fraction of approx. 23%. The optimal pitch value will provide a high density of potentially active nucleation sites but still allow for the development of the thermal boundary layer thus not inhibiting the activation of neighboring spots. The size of (super)hydrophobic spots appears not to have a major influence on the boiling performance when using the optimal spot pitch. The developed superbiphilic surfaces increase the CHF and provide greatly enhanced heat transfer coefficients especially at medium and high heat fluxes, making them suitable especially for high-heat-flux applications.

This study investigates critical heat flux (CHF) enhancement in pool boiling of FC-72 under saturated and subcooled conditions using laser-textured stainless-steel surfaces in combination with a hydrophobic coating. Multiple types of surfaces with capillary-length-spaced circular spots of different diameters are prepared with the spots being either untreated, hydrophobized or laser pretreated and hydrophobized. Pool boiling measurements are performed at atmospheric pressure on electrically heated ribbons. The CHF versus subcooling behavior is modeled using an established linear equation, while a modification into a second-degree polynomial is also tested to better account for nonlinear behavior of some surfaces. All developed surfaces are found to increase the CHF with enhancements ranging from 36 to 73% at saturation and 34–65% at 12 K of subcooling. Surfaces with mixed regions of low and high wettability generally exhibited the highest CHF enhancement at a 31% coverage with low wettability areas. Samples with laser-pretreated and hydrophobized spots exhibited the highest overall performance at saturation but a less steep trend of CHF versus liquid subcooling increase and CHF increase with increased superhydrophobic area ratio, indicating a different enhancement mechanism with hydrophobized microcavities presumably serving as preferential nucleation sites. Modeling of the CHF versus subcooling trend showed that (i) a second-degree polynomial equation is more appropriate for some surface types and (ii) the obtained constants differ significantly from surface to surface. Evaluation of a universal model based on all datapoints obtained within this study showed inadequate performance with error margins of ±13.5%. Consequently, the use of individual models for each type of enhanced surfaces with an overall accuracy of ±5.9% is recommended.

This study presents the application of hydrophobic polydimethylsiloxane-silica coating used for the development of biphilic surfaces that are designed to enhance the heat transfer during boiling. Surface analyses showed that this coating exhibits a high hydrophobicity due to its hierarchical structure and the use of hydrophobic polymer. An appropriate thermal treatment leads to the oxidation of the methyl groups and a formation of silicon oxide and silicon carbide that result in a wettability transition from hydrophobic to superhydrophilic. On this basis, we manufactured hydrophobic/superhydrophilic patterns on stainless-steel foils using a pulsed Nd:YAG laser. The uniform, superhydrophilic surface exhibited a 350% larger critical heat flux (CHF) than bare stainless-steel foil. High-speed IR thermography revealed that the increased wettability reduced the bubble contact diameter, allowed a higher density of active nucleation sites, and delayed the dry-out. The biphilic surfaces with differently sized hydrophobic spots exhibited the highest heat transfer coefficients, with an up to 200% higher CHF compared to the bare stainless steel. Smaller hydrophobic spots reduced the bubble diameter and increased the nucleation frequency. However, surfaces with larger hydrophobic regions promoted boiling incipience and exhibited higher heat transfer coefficients at low heat fluxes. These results suggest that the optimal biphilic pattern could only be determined for a particular operating point. Our data provide a new insight into the complex phenomena of nucleate pool boiling on chemically and mechanically heterogeneous surfaces.

Microstructured uniformly and non-uniformly wettable surfaces were created on 25-μm-thin stainless steel foils by laser texturing using a marking nanosecond Nd:YAG laser (λ = 1064 nm) and utilizing various laser fluences and scan line separations. High-speed photography and high-speed IR thermography were used to investigate nucleate boiling heat transfer on the microstructured surfaces. The most pronounced results were obtained on a surface with non-uniform microstructure and non-uniform wettability. The obtained results show up to a 110% higher heat transfer coefficients and 20–40 times higher nucleation site densities compared to the untextured surface. We show that the number of active nucleation sites is significantly increased in the vicinity of microcavities that appeared in areas with the smallest (10 μm) scan line separation. Furthermore, this confirms the predictions of nucleation criteria and proves that straightforward, cost-effective nanosecond laser texturing allows the production of cavities with diameters of up to a few micrometers and surfaces with non-uniform wettability. Additionally, this opens up important possibilities for a more deterministic control over the complex boiling process.

This investigation used laser-processed 25-μm-thick stainless steel foils as heaters in pool boiling experiments under subcooled and saturated conditions at atmospheric pressure. Boling surfaces were modified by a nanosecond fiber laser. In most cases, laser-textured parts on boiling surfaces were spaced apart by a capillary length of water (2.5 mm) and had different shapes and arrangements. Multi-scale micro-cavities (with diameters ranging from 0.2 to 10 μm) on the laser-textured areas of the surfaces provided potential active nucleation sites. The highest heat flux measured before the burnout was observed on the fully treated sample; this heat flux was a factor of 3.7 greater than that of the untreated sample. The sample with hexagonally arranged textured circular shapes with a diameter of 2.0 mm provided a more than 4-fold higher heat transfer coefficient compared to the untreated sample. All of the laser-textured boiling surfaces showed enhanced pool boiling heat transfer performance in comparison to the untreated surface. The optimal spacing between the laser-textured regions was experimentally found to be equal to the capillary length of the working fluid. Our results demonstrate that laser texturing has strong potential for producing patterned surfaces for engineering applications of boiling heat transfer.

Pool boiling experiments of water and ethanol-water binary mixtures were conducted on smooth and laser textured stainless steel foils. High-speed IR thermography was used to measure transient temperature field during boiling in order to determine nucleation frequencies, nucleation site densities, bubble activation temperatures, wall-temperature distributions and average superheats as well as heat transfer coefficients. Saturated pool boiling experiments were conducted at atmospheric pressure over a heat flux range of 5–250 kW m−2 for pure water and ethanol-water mixtures (1% and 10% m/m). For both mixtures and both types of surfaces we measured significant decrease in average heat transfer coefficient and increase in bubble activation temperatures in comparison to pure water. However, laser textured surface in average provided around 60% higher nucleation frequency and more than 100% higher nucleation site density compared to smooth surface for both of the tested binary mixtures. Consequentially, heat transfer coefficient was enhanced for more than 30%. Our results show that laser textured surfaces can improve boiling performance for water and ethanol-water mixtures, but at the same time the addition of ethanol reduces heat transfer coefficient despite the enhancement of nucleation site density and nucleation frequency. This is also in agreement with available experimental data and existing theoretical models.

Nucleate water pool boiling at high heat flux was investigated on the 25 µm titanium and stainless-steel heaters at atmospheric pressure. A high-speed IR thermographic camera was applied to measure the rapidly changing transient temperature field, which served as input data for calculating the transient local heat flux distributions by solving a 3-dimensional (3-D) inverse heat conduction problem (IHCP). A phenomenon of hot spot was observed at the irregular active bubble site characterized by a longer waiting time and a higher activation temperature compared to a regular active nucleation site. The results show that the temperature of the hot spot can significantly exceed the temperature of the heater in its surroundings and remains present on the boiling surface even after the bubble departure. The calculations have shown a strong reduction of the local heat flux at the spot, which represented a potential for the beginning of the boiling crisis.

Sub-solidus phase relations in the ternary CaO–La2O3–TiO2 system at 1400 °C in air were determined. The multi-phase samples were prepared by a solid-state reaction method, whereas the single-phase samples for the structure analysis of selected solid solutions were prepared by a wet-precipitation method in order to provide good homogeneity of the starting mixtures. The phases in the prepared samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The oxides form seven ternary compounds in the equilibrium state, many solid solutions (which extend across a broad concentration region), and a large, single-phase area based on the CaTiO3 solid solution. The structures of several new phases – solid solutions on the tie lines CaTiO3–Ca3La4Ti3O15 and La2TiO5–Ca3La4Ti3O15 – were determined in detail.

A comprehensive understanding of the transient heat- and mass-transfer processes on the submicron scale requires the development of novel, non-invasive, temperature-measurement techniques. Here, we present a fluorescence-based method for the non-invasive characterization of transient temperature fields on a submicron scale using a temperature-sensitive erbium/ytterbium co-doped transparent glass–ceramic. This inorganic material is more stable against degradation and photobleaching than traditional organic dyes. Transient heat-conduction experiments were performed on an Er:GPF1Yb0.5Er glass–ceramic sample, which was simultaneously used as a temperature sensor. The temperatures were determined by measuring the fluorescence spectrum and by measuring the intensity variations of the emission spectrum in the temperature range between 25 and 150 °C. A spatial resolution of 400 nm was achieved across an approximately 200 μm × 200 μm field of view at an imaging frequency of 7 Hz. The conducted experiments demonstrate that Er-doped glass–ceramics permit non-invasive and transient thermal imaging and can be adopted for the analysis of heat-transfer processes on submicron scales.

This article presents a new method for the control of a gas-sand conical fluidized bed with a wide size distribution of particles. A two-step procedure was developed and experimentally tested. The minimum velocity for full fluidization is determined by a new statistically based method and then the fully fluidized state is initiated and controlled. It is proven that the characteristics of the pressure drop vs. fluidization velocity remain practically the same for different bed heights and over a wide size distribution of particles. The method analyses the recorded pressure drops with a histogram. The minimum velocity for full fluidization is determined as the smallest fluidization velocity from the histogram bin with the highest density. Finally, it is also proved that the fluidization velocity is a reliable parameter for controlling the fluidization. The method was used to control fully fluidized beds at the minimum gas velocity in a pilot FICFB gasificator.

Nucleate boiling enables effective cooling and heat transfer at low temperature differences between a heated surface and the surrounding fluid. It is utilized in many applications, ranging from large power plants to small microelectronics. To enhance the boiling process by minimization of the surface temperature and increase the maximum attainable heat flux, several approaches for surface modifications were recently developed. However, each of them has at least one important drawback, including challenging and expensive production, mechanical and/or thermal instability or problematic scale-up. Herein, a straightforward, robust and flexible method using a nanosecond fiber laser for production of surfaces with multi-scale micro-cavities (with diameters ranging from 0.2 to 10 μm) is developed. Examination of these surfaces in two very contrasting fluids - water, which is polar, has high surface tension and high latent heat of vaporization; and non-polar, dielectric tetradecafluorohexane (FC-72) with low surface tension and much lower latent heat - confirms that such surfaces enable enhanced heat transfer and controlled boiling in combination with diverse fluids. This demonstration suggests that the developed method has the potential to overcome the current limitations for further miniaturization of microelectronic devices and to increase performance and safety in high heat flux systems.

Thickness-sensitive, spectrally selective paints based on a silane treatment of pigments were prepared with different pigment- volume concentrations. The critical pigment-volume concentration was determined by means of electrochemical impedance spectroscopy, while the pigment particle size distribution was determined with ultrasound spectroscopy. The selectivity versus thickness relation of a paint with a near-critical pigment-volume concentration was studied spectroscopically through performance criteria. Its nonlinearity was shown to be related to the surface topography. This relation was further supported by hydrophobicity measurements. Heat-gathering tests in a simulated solar collector supported the spectroscopic determination of an optimal dry-film thickness.

A new method for the synthesis of Ce 0.80 Sm 0.20 O 1.90 is presented. A drying of a solution of ethylene glycol and stoichiometric amounts of the acetates Ce(C 2 H 3 O 2 ) 3 · x H 2 O and Sm(C 2 H 3 O 2 ) 3 · x H 2 O resulted in a highly crystalline product of [Ce 0.80 Sm 0.20 (C 2 H 3 O 2 ) 3 (C 2 H 6 O 2 )] n , a new chained polymeric coordination compound. With the calcination of this precursor at 900 °C and sintering at 1150 °C, pure Ce 0.80 Sm 0.20 O 1.90 , was obtained with the desired homogeneity and density. This paper reports elemental (CHN and ICP-MS) and single crystal X-ray structure analyses of the precursor [Ce 0.80 Sm 0.20 (C 2 H 3 O 2 ) 3 (C 2 H 6 O 2 )] n . Its thermal decomposition to Ce 0.80 Sm 0.20 O 1.90 has been studied via HT-XRD and TG–DTA analysis. The structure and composition of Ce 0.80 Sm 0.20 O 1.90 was also confirmed by powder XRD (Rietveld refinement).

The purpose of this study was to evaluate the effect of Y-TZP ceramic surface functionalization with a nano-structured alumina coating on bond strength of the resin modified glass ionomer dental cement. A total of 160 disc-shaped specimens were produced and randomly divided into two groups of 80. Half of the discs in each group received an alumina coating which was fabricated by exploiting the hydrolysis of aluminum nitride (AlN) powder. The shear bond strengths of the resin-modified glass ionomer cement FUJI+ (GC Japan) and the composite resin luting agent RelyX Unicem (3M ESPE, USA) were then studied for the coated and uncoated surfaces The SEM analyses revealed that the application of an alumina coating to the Y-TZP ceramics created a highly retentive surface for bonding. The bond strengths for the coated groups in both cements were significantly higher than the uncoated groups.

Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 μm thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1 ms and 40 μm, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions.

Experiments were performed to study the spatio-temporal temperature variation underneath growing bubbles on a thin platinum heating foil in saturated and subcooled nucleate pool boiling of water at atmospheric pressure. The transient wall temperature distributions were recorded with spatial resolution of 40 μm by a high-speed infrared camera at intervals of 1 ms, synchronised with a high-speed video camera to record bubble motion. Examples are presented of the transient distributions of wall temperature, heat flux and heat transfer coefficient underneath bubbles growing with the fast and slow bubble detachment mechanisms in saturated and subcooled pool boiling. Comments are made on the evidence for and against particular mechanisms of heat transfer.

Correlations for nucleate boiling heat transfer should be improved, or in the long term possibly be replaced, by the development of mechanistic simulations that include the non-uniform spacing and variable characteristics of the nucleation sites and non-linear interactions between the sites. This paper discusses the interactions that should be included in simulations and some lessons from a first attempt to validate a particular simulation against experimental spatio-temporal data for wall temperature. Input data for nucleation site positions and characteristics are a particular problem and the prospects for obtaining this data from measurements that are independent of boiling are discussed.

The influence of the surface of horizontally and vertically oriented ribbon heaters made of stainless steel 302 or steel 1010 on pool boiling CHF in FC-72 or H2O was studied. The ribbon was 50 mm long, 5 mm high and 76.4 μm thick. Surface treatment was performed with different sandpapers and etching in diluted acid. In the centre line average Ra range from 0.02 to 1.5 μm , the pool boiling CHF on stainless steel 302 ribbon heaters in FC-72 increased from 87.6 to 115.4 kW/m 2 and in H2O it increased from 410.8 to 499.7 kW/m 2 , while for steel 1010 ribbon heaters the CHF increased in H2O from 309.7 to 443.9 kW/m 2 . On an etched heater surface, pool boiling CHF in H2O on both stainless steel 302 and steel 1010 increased by 51% with respect to a sanded surface with the same Ra. It appears that a multiparametric heating surface characteristic and the surface chemistry parameters need to be included in the study.

A technique of activating nucleation sites by heating them with laser beams has been developed to enable direct measurements of interactions between active nucleation sites. Interactions between two, three and four simultaneously active nucleation sites in various arrangements on 25 μm thick copper and titanium foil in saturated water pool boiling were studied experimentally. The experiments were performed with heated spot diameters ranging from 1.66 to 5.23 mm and heat fluxes up to 560 kW/m2. The interaction between two laser activated nucleation sites occurs as a net decrease of activity of both sites. It is possible that the activity of both decreases simultaneously, or the activity of one increases, while the activity of the other nucleation site decreases simultaneously. Similar behaviour was also observed in the experiments involving three and four active nucleation sites. In the extreme case, one nucleation site can deactivate the other. Local surface characteristics can play an important role in the interaction between the nucleation sites.

The purpose of Critical heat flux (CHF) experiments was to determine the role of various types and thickness of enhanced coated surface on a horizontal, vertically oriented ribbon heaters made of Ti and Steel 1010 of different thickness. Saturated pool boiling in FC-72 at atmospheric pressure was used in the experiment. The microstructure and surface topography are important factors in pool boiling CHF. In conditions of highly wetting FC-72 and increased roughness, CHF increased by 6 to 12%. However, CHF increased by 29% with greater topographic unevenness of the surface and lower roughness, which was obtained by etching Steel 1010 in H2SO4 acid. CHF also increased when the content of metals and metal oxides particles in the coating were increased. The CHF ratio of enhanced coated surface to a ribbon heater featuring a standard surface finish is up to 2.3. In addition, the asymptotic CHF of these uncoated heaters was considerably exceeded.

The technique of laser heating a thin metal foil was used to experimentally observe local phenomena in pool boiling CHF on a small flat surface. The experiments were performed with saturated water boiling on 25 μm thick copper and titanium foils with heating surface diameters ranging from 2.27 to 9 mm. The number of simultaneously active nucleation sites depends on the heat flux and the size and material of the laser heated surface. A reduction in the heating surface diameter DH moves the transition between one and several simultaneously active nucleation sites towards higher heat fluxes. CHF for small heating surfaces is lower than that for entirely heated surfaces. The variation of the ratio of bubble/vapour mass diameter to the heating surface diameter, DB/DH, with DH is similar to the variation of CHF with DH. CHF is significantly influenced by the number of simultaneously active nucleation sites. The influence of heating of the surrounding surface on decreasing DH, which was observed using two laser beams with different diameters was more intense at higher DH on Cu foil than on Ti foil.

There are many aspects of critical heat flux (CHF) that still require clarification. Particularly important is the role that the heater-side factors play in CHF. The saturated pool boiling CHF on horizontally suspended, vertically oriented, ribbon heaters in FC-72 fluid was measured. On the basis of comprehensive experimental results (50 data sets) and data sets available in the literature, the effects of ribbon thermal properties and thickness were determined. A practical criterion is proposed for sizing noninsulated ribbon heaters without significant conductance/capacitance effects.

A method is presented for the prediction of performance limit in two-phase closed thermosyphon based on the thermodynamical corresponding states principle. Molecular weight, critical temperature, critical pressure and acentric factor are needed instead of thermodynamic and transport properties of fluid at a given temperature. Developed corresponding states correlation for maximum heat flux is found to be in fairly good agreement with the 436 experimental data for 12 nonpolar and polar fluids with fill ratio greater than 0.4 in vertical two-phase closed thermosyphons with an inner diameter of 3–34 mm and the ratio of the evaporator length to its diameter with values of up to 325.

For cookers with oven to be interesting for the market, they need to exhibit a high energy efficiency. On the basis of an analysis of individual influences on the energy efficiency of ovens, several solutions were proposed for improving oven operation. An analytical solution was made for describing the nonstationary temperature field within a test brick with convection at a constant heat transfer coefficient and a continuously variable temperature of the surrounding fluid using Green's functions. The influences of door sealing, vapor outlet slit, additional insulation, the oven regulation regime, door glazing and irradiation within the oven were experimentally analyzed. Because of the proposed improvements, the analyzed mass produced oven was moved to the A class of energy efficiency according to standard EN 50304:2001.

The influence of decreasing the distance between artificially activated nucleation sites and a naturally activated nucleation site on their activity was studied. A naturally activated nucleation site was activated during saturated water pool boiling at atmospheric pressure using electrical or laser heating of a relatively large surface of 25 μm thick copper or titanium foil. The interaction between nucleation sites located at various distances was observed during their simultaneous activation with a laser heated spot, which could be moved along the surface. The results of measurements indicate that the influence of active nucleation sites on each other's activity are mutual. Moving a laser heated spot along the surface was used to show the influence of location on the activity of nucleation sites.