Pillar structure surfaces and surface vibrations are considered to be two individual techniques to enhance nucleate boiling. In this work, a volume of fluid (VOF) based numerical model is developed ...to study bubble growth promotion during nucleate pool boiling on a pillar structure surface under the aid of surface vibration. Three vibration modes of the pillar structure, including the horizontal vibration mode (HVM), vertical vibration mode (VVM) and angular vibration mode (AVM), are considered, and their effects on the bubble dynamics and heat transfer are discussed with various vibration amplitudes and frequencies. It is found that the pillar structure surface is favorable for bubble detachment as it constrains the bubble base radius and forces the bubble to grow more vertically compared to the plain surface. Although the HVM and AVM exert no or less vertical force component on the bubble for its detachment than the VVM does, but their horizontal force component significantly alters the bubble shape profile, breaks the force balance, and greatly increases the bubble departure frequency and heat transfer. The largest increment of liquid-vapor phase change rate is found to be 30.9% for the HVM. The bubble departure frequency and the vapor volume flux quantity increase with the pillar vibration amplitude and frequency. This work will deepen our understanding of fundamental bubble growth mechanisms as altered by vibration on the pillar structure surface for better heat transfer applications.
•The heat transfer coefficient (HTC) is improved with surface structuring.•The largest manufactured cavities were most successful at increasing the HTC.•The highest density of nucleation cavities ...resulted in the highest HTCs.•The onset of boiling, HTC and critical heat flux were not directly correlated.•The laser textured surfaces improved the HTC compared to the bare silicon surface.
Pool boiling experiments were performed using saturated double-distilled and degassed water on 10 × 10 mm silicon samples, which were (i) untreated; (ii) laser structured; or (iii) modified with etched artificial nucleation cavities. The etched silicon surfaces were fabricated with differently sized micro nucleation cavities (5–30 µm) and pitches (0.125–2 mm) to allow a comparative analysis of the fabricated surfaces. The heat transfer coefficient (HTC) comparative analysis conducted at the heat flux of 200 kW/m2 exhibits the highest enhancement of 244% during nucleate boiling on the silicon sample with etched nucleation cavities with a 30 µm diameter and a 0.125 mm pitch. The experimental results consistently show that HTC increases with decreasing the pitch and increasing the size of the nucleation cavities in the range of the experimental conditions. The superheat required for the onset of nucleate boiling and the critical heat flux were not substantially affected with the structured surfaces. However, the boiling phenomena propagates more promptly to the entire available silicon surface, when the sample is laser treated or etched compared to the reference bare silicon sample.
•This review explores passive enhancement of pool boiling using surface modification.•Examined are macrosacle, microscale, nanoscale and multiscale (hybrid-scale) techniques.•Addressed are ...performance goals of inhibiting incidence hysteresis and increasing both nucleate boiling heat transfer coefficient and CHF.•Shown is that micro and nano surface features are susceptible to blockage and changes in performance over time.
This paper provides a comprehensive review of published articles addressing passive enhancement of pool boiling using surface modification techniques. They include macroscale, microscale, and nanoscale surfaces, as well as multiscale (hybrid-scale), and hybrid-wettability techniques. Different enhancement methods are assessed in terms of underlying fluid routing mechanisms and ability to achieve three distinct heat transfer goals: eliminating incipient boiling hysteresis, increasing nucleate boiling heat transfer coefficient, and ameliorating critical heat flux (CHF), especially for inert dielectric coolants that are both highly wetting and possess relatively poor thermophysical properties. While different enhancement scales are shown to provide different degrees of success in achieving the three goals, it is shown that both microscale and nanoscale surface features are susceptible to blockage, resulting in deterioration of the enhancement over time. This review also points to scarcity of sufficiently sized databases for a given enhancement scheme in terms of fluid type, surface material, size, and orientation, enhancement shape, pattern, and scale, and operating pressure. This renders available findings less-than-adequate tools for design of practical cooling systems.
•A pressure control method is applied to control the environmental pressure of the nucleate boiling system in the molecular dynamics simulation.•Nucleate boiling processes with different system ...pressures are realized.•The bubble dynamics and interface evolution during nucleate boiling, as well as the energy barrier for nucleation, have been quantitatively explored.
Nucleate boiling as one of the most efficient heat dissipation technology is an important technology for aerospace thermal protection. However, the supersonic environment of the spacecraft brings pressure oscillation to the phase change cooling system, which affects the heat transfer efficiency of nucleate boiling. Despite molecular dynamic simulation being an effective tool for studying the microscopic mechanism of boiling, there is still a lack of relevant research on the microscopic mechanism of the influence of pressures on nucleate boiling. Therefore, we have proposed a novel pressure-control method to explore the system pressure effect on nucleate boiling using the molecular dynamics simulation method. The bubble dynamics and interface evolution during nucleate boiling under different system pressures, as well as the energy barrier for nucleation, have been illustrated and quantitatively explored. We expect that the proposed pressure-control method can be conveniently applied in the future boiling studies under complex and varied pressure environments.
•Heat transfer mechanisms in thin horizontal liquid layers have been investigated.•The effect of the shape and thermal conductivity of 2D-modulated porous coatings on heat transfer has been ...studied.•The relationship between relative pressures and liquid layer heights is presented in detail.•The effect of 2D-modulated porous coatings on crisis phenomena and transients has been described.
Investigation results on heat transfer during evaporation and boiling in thin horizontal liquid layers on 2D modulated capillary-porous coatings with a sinusoidal profile in a wide range of relative pressures and layer heights are presented in this paper. Coatings were made on a 3D laser printer using the technology of additive selective laser sintering (SLS). One coating was made of bronze with a modulation wavelength equal to the Laplace constant of the working fluid and two coatings were made of stainless steel and bronze with a doubled wavelength. The experimental data obtained on the coatings were compared with each other and with the values obtained by evaporation and boiling of n-dodecane on a smooth surface under the same conditions. It is shown that in the region of low relative pressures in liquid layers with a height less than the Laplace constant, heat transfer intensification achieved on a sample made of a material with low thermal conductivity (stainless steel) was five times higher as compared to a smooth surface. In the range of relative pressures, when nucleate boiling was observed in the layers, the temperature difference reached on the bronze coatings was approximately 3–4 times less than on a smooth surface. The temperature difference for the stainless steel coating changed little with an increase in the heat flux. In pre-crisis conditions, the temperature difference during boiling on the stainless steel coating was less than on the bronze coatings in the entire range of relative pressures and layer heights. The highest critical heat fluxes were obtained on a bronze coating with a modulation wavelength equal to the Laplace constant.
•Single-phase liquid and nucleate boiling regimes, and critical heat flux for spray cooling are reviewed.•Discussed are dominant mechanisms, data trends, and predictive correlations and models.•Also ...discussed are enhancement schemes, including surface modifications, additives to the liquid, and use of nanofluids.•It shows future work that must be conducted using many fluids and broad ranges of operating conditions.
This study is the first part of a two-part review of spray cooling, which addresses the relatively high-flux, low-temperature mechanisms and predictive tools associated with the single-phase liquid cooling and nucleate boiling regimes, as well as critical heat flux (CHF). The second part will be focused on the relatively high-temperature transition boiling and film boiling regimes, and the Leidenfrost point, which are encountered in quenching of metal alloy parts. In this part, key spray hydrodynamic parameters influencing heat transfer performance are identified, including volumetric flux, mean droplet diameter, and mean droplet velocity. This is followed by detailed identification of dominant mechanisms, data trends, correlations, and predictive models recommended by different research teams. Also discussed are spray cooling enhancement schemes, including micro and macro modifications to the surface itself, additives to the liquid, and use of nanofluids. Overall, contradictory findings point to a need for future experimental work that must be conducted systematically using many fluids with vastly different thermophysical properties, and broad ranges of operating conditions. There is also a need for further research to investigate parameters that influence CHF, including dissolved gas, spray inclination angle, and interaction between neighboring sprays when using multi-nozzle arrays to cool large surfaces.
•Flow boiling is studied inside silicon microchannels (400 × 400 µm).•A smooth channel and a channel with microstructured sidewalls have been fabricated.•R134a and HFE-7000 are investigated as ...boiling fluids at 30 °C temperature.•Microstructured (MS) walls promote a reduction of wall superheat at ONB.•Heat transfer enhancement in the MS channel due to the presence of more nucleation sites.
Flow boiling heat transfer is investigated in a single silicon microchannel with microstructured sidewalls using R134a (at 0.19 reduced pressure) and HFE-7000 (at 0.035 reduced pressure) as working fluids. The channel is 51 mm long with a 400 μm x 400 μm square cross section. Sidewall microstructures in the form of triangular cavities for the entire height of the channel were introduced during the fabrication process. By including the cavity pattern in the photolithography mask design, high accuracy and reproducibility is ensured during the fabrication of the test sample. A smooth channel with the same geometry but without sidewall microstructures was also fabricated. Flow boiling experimental results are presented for both channel configurations. For R134a at mass velocity G = 800 kg m−2 s−1, the comparison of the boiling curves shows that: in the channel with the microstructures, the wall superheat at the onset of nucleate boiling (ONB) is reduced from 21 K to 6 K as compared to the smooth channel. An increase of the flow boiling heat transfer coefficient up to 40% is found in the microstructured channel, which must be due to the increased number of active nucleation sites. Heat transfer results, flow visualizations and comparison between R134a and HFE-7000 data provide a clearly evidence of the active role of bubble nucleation during flow boiling in microchannels. These results are of fundamental importance in the design of next generation thermal management systems for electronics.
•Effect of POE lubricant (1–10%) on pool boiling heat transfer of R-134a refrigerant is investigated.•Tests were performed with smooth tube with heat flux ranging from 10 to 90 kW/m2.•The bubble size ...is reduced and the bubble density is increased appreciably with the oil addition.•The heat transfer coefficient (HTC) may be increased or impaired depending on oil concentration.•The influence of visocoity amid POEA-68 and POEA-170 on HTC is relatively small..
The present study examines the influence of polyolester lubricant oil (POEA-68 and POEA-170) with R-134a on the nucleate pool boiling performance for a horizontal smooth tube. The heat flux ranges from 10 kW/m2 to 90 kW/m2 and the lubricant mass fraction varied from 1% to 10% at a saturation temperature of 10 °C, 0 °C and, −6 °C, respectively. The experimental results show that the heat transfer coefficient (HTC) of the pure refrigerant is increased with an increase in saturation temperature. But the presence of lubricant oil alters the HTC considerably. The addition of 3% POEA-68 lubricant shows the highest 29.3% HTC enhancement relative to pure refrigerant at a supplied heat flux of 70 kW/m2 and at a saturation temperature of 0 °C. Whereas for R-134a/POEA-170 lubricant mixture at 3% of mass fraction, the highest enhancement in HTC was around 26% at a supplied heat flux of 70 kW/m2 and −6 °C saturation temperature. The bubble size is reduced and the bubble density is increased appreciably with the addition of POE oil. It was observed that the HTC of lubricant-refrigerant mixtures differs appreciably with the variation of lubricant oi mass fraction (ω). The HTC first decreases at a very low mass fraction (ω≤1%), followed by an appreciable increase in the range of 1% ≤ ω ≤ 3%, and finally reveals a continuous decline for the higher mass fraction. The dramatic decrease in HTC for lubricant-refrigerant mixture is associated with mass transfer resistance and a significant increase in surface tension.
•Recent advancements in enhanced microchannels and their fabrication methods are reviewed•Four types of enhanced microchannels and their enhancement performance have been identified•Typical ...fabrication methods for enhanced microchannels has been discussed•Insights and recommendations on future directions of enhanced microchannels have been proposed
Rapid increase in heat fluxes within a small area in microelectronic, defense, energy, solar and medical components has spurred an urgent need for two-phase microchannel heat sinks due to their large heat transfer area to volume ratios, compact heat sink size, and high heat transfer coefficient (HTC). Nevertheless, underlying problems of large wall superheat for onset of nucleate boiling, inherent flow instability and low values of critical heat flux of flow boiling in conventional solid parallel microchannels pose severe challenges for practical applications of microchannel heat sinks in high heat flux dissipations. To address the above issue, numerous efforts have been taken to the design and fabrication of enhanced microchannels for flow boiling enhancement of two-phase microchannel heat sinks in recent years. To overview this subject, recent advancements in flow boiling enhancement and fabrication of enhanced microchannels are comprehensively reviewed in this paper. To the best knowledge of the authors’, it is the first time to present the advancements of enhanced microchannels from the fabrication perspective, which is critical for the application and commercialization of enhanced microchannel heat sinks. The enhanced microchannels are classified into flow disruption structures, reentrant cavity structures, porous structures, and nanostructures. Emphasis is on the flow boiling enhancement performance of these enhanced microchannels in microchannel heat sinks. Subsequently, typical fabrication methods for enhanced microchannels are summarized together with the discussion of their advantages and disadvantages, such as etching, micro-mechanical cutting, micro electrical discharge machining, laser processing, sintering, chemical vapor deposition and 3D printing. Finally, the challenges and future research directions of enhanced microchannels are reasonably clarified.
Review of drop impact on heated walls Liang, Gangtao; Mudawar, Issam
International journal of heat and mass transfer,
March 2017, 2017-03-00, 20170301, Letnik:
106
Journal Article
Recenzirano
•Drop impact is reviewed in terms of heat transfer mechanisms and predictive tools.•It is segregated into film evaporation, nucleate, transition and film boiling.•Inconsistencies are found regarding ...impact process and ensuing heat transfer.•Recommendations are provided for future research.
This paper provides a comprehensive review of published literatures concerning the fluid mechanics and heat transfer mechanisms of liquid drop impact on a heated wall. The review is divided into four parts, each centered on one of the main heat transfer regimes: film evaporation, nucleate boiling, transition boiling, and film boiling. Each of these regimes is discussed in detail in terms of available depictions of drop deformation and/or breakup, proposed heat transfer mechanisms, predictive correlations and/or models. It is shown that understanding the underlying physics for each heat transfer regime is highly dependent on the experimental methods that investigators have adopted, and broadness of available databases in terms of liquid type, drop size and momentum, impact angle, and wall material and surface roughness. Despite significant advances in experimental, theoretical and computational research in understanding the interfacial behavior of the drop from the moment of impact, there are many inconsistencies concerning some of the most important aspects of the impact process and ensuing heat transfer, especially in regards to critical heat flux, transition boiling, and the Leidenfrost point. This review is concluded with recommendations concerning future work that is needed to address poorly understood and/or contradictory issues.
