•A theoretical model for bubble nucleation in liquid film boiling is proposed.•Bubble nucleation during liquid film boiling is much more difficult than that in pool boiling and flow boiling with the ...minimum size of active microcavities being several times larger.•The model is well validated by existing experimental data with theoretically predicted critical thickness for liquid film boiling is larger than 100 µm.
Many industrial applications such as electronic cooling and thermal desalination have adopted the heat dissipation technologies of capillary evaporation and liquid film boiling, with bubble nucleation being the transition criterion between them. In this paper, a theoretical model for bubble nucleation in liquid film boiling is developed, which mainly considers three aspects: transient heat conduction inside the liquid film supported with porous wick, evaporation at the liquid-vapor interface, and superheat requirement for the stable existence of a bubble nucleus. The criterion for the onset of nucleate boiling (ONB) in liquid film boiling is established, and size range of active microcavities for bubble nucleation is obtained, as well as comparisons being made with that during pool boiling and flow boiling. The results indicate that the size range of active microcavities in liquid film boiling is narrower with the minimum size being several times larger, and bubble nucleation in liquid film boiling is much more difficult. Although superhydrophilic surface with small contact angle is disadvantageous to bubble nucleation, it will decrease the minimum size of effective microcavities radius within the size scope of a few micrometers, which is beneficial for ONB in liquid film boiling. Moreover, critical thickness to trigger ONB in liquid film boiling is found to decrease with increasing contact angle, augmenting superheat and strengthening effective thermal conductivity of the wick, and its theoretical prediction is validated with experimental data in the literature. Our model offers a new avenue for understanding the ONB in liquid film boiling, and therefore can serve as a guideline for future enhancement design.
•Nanostructured and plain tubes tested under falling film-boiling conditions.•Heat transfer coefficients measured in refrigerants R-134a and R-245fa.•Critical heat flux due to departure from nucleate ...boiling identified.•Critical dryout identified.•Falling film heat transfer coefficients compared to those of pool-boiling.
Falling film evaporators offer an attractive alternative to flooded evaporators as the lower fluid charge reduces the impact of leaks to the environment and associated safety concerns. A study was conducted of saturated falling film boiling of two refrigerants on one polished, one roughened and three nanostructured copper tubes in order to evaluate the potential of nanostructures in falling film refrigerant evaporators. Tubes were individually tested, placed horizontally within a test chamber and heated by an internal water flow with refrigerant distributed over the outside of the tubes. Wilson plots were used to characterise the internal water heat transfer coefficients (HTCs). A layer-by-layer (LbL) process was used to create the first nanostructured tube by coating the outside of a tube with silica nanoparticles. A chemical bath was used to create copper oxide (CuO) protrusions on the second nanostructured tube. The third tube was coated by following a commercial process referred to as nanoFLUX. R-245fa at a saturation temperature of 20 °C and R-134a at saturation temperatures of 5 °C and 25 °C were used as refrigerants. Tests were conducted over a range of heat fluxes from 20 to 100 kW/m and refrigerant mass film flow rates per unit length from 0 to 0.13 kg/m/s, which corresponds to a film Reynolds number range of 0 to approximately 1500 to 2500, depending on the refrigerant. Heat fluxes were increased further to test whether the critical heat flux (CHF) point due to a departure from nucleate boiling (DNB) could be reached. The CuO and nanoFLUX tubes had the lowest film Reynolds numbers at which critical dryout occurred at heat fluxes near 20 kW/m2, but as the heat fluxes were increased towards 100 kW/m2, critical dryout occurred at the highest film Reynolds numbers of the tubes tested. Furthermore, in some higher heat flux cases, CHF as a result of DNB for the CuO and nanoFLUX tubes was reached before critical dryout occurred, and DNB became the limiting operational factor. The refrigerant condition that had the worst dryout performance in terms of film Reynolds number was R-134a at 25 °C, followed by R-134a at 5 °C and R245fa at 20 °C. Tests across the heat flux range and refrigerant conditions revealed that compared to the polished tube, the roughened tube had HTCs between 60 and 100% higher, the LbL tube had HTCs between 20% lower and 20% higher, the CuO tube had HTCs between 20% lower and 80% higher and the nanoFLUX tube had HTCs between 40 and 200% higher than the polished tube. The falling film enhancement ratios for the plain and nanostructured tubes were found to be of a similar order of magnitude, typically between 1.3 and 0.8.
•Three-dimensional symmetric phase-change lattice Boltzmann model is built up to investigate sessile droplet boiling on micro-pillar array surface in the nucleate boiling regime.•Effects of ...micro-pillar size on bubble evolution behaviors accompanied with droplet boiling morphologies, manifested in bubble nucleation, growth, coalescence and rupture are studied.•Quantitatively assessed include nucleation activation time, droplet total evaporation time, isolated bubble nuclei density, bubble size and substrate heat flux.•Temperature, confined space and fluid flow are discussed as key factors affecting the preferential activation location and the distribution of nucleation sites.•Understanding of droplet/bubble two-phase dynamics gives some insights to optimizing droplet nucleate boiling heat transfer performance by manipulating surface properties.
Despite spray cooling in the form of droplet boiling on a textured surface being a very promising phase-change heat dissipating method, the understanding of droplet/bubble two-phase dynamics in the nucleate boiling is extraordinarily limited. In this study, we report sessile droplet boiling on micro-pillar array surface in the nucleate boiling regime using a three-dimensional lattice Boltzmann model comprehensively. Effects of micro-pillar size on bubble behaviors inside droplet are discussed in detail, covering bubble nucleation, growth, coalescence, and rupture. For the micro-pillars with large side length or small spacing, nucleation sites are activated around micro-pillar top surface. The preferential activation location of nucleation sites is determined by temperature, confined space and fluid flow. In bubble growth stage, the variation of bubble radius with time follows the square root law, being consistent with previous experiments. Bubbles merge into a large central bubble beneath droplet for the short micro-pillars while into a vapor layer for the long micro-pillars. Emergence of large central bubble prolongs droplet lifetime but deteriorates heat transfer. In addition, increasing micro-pillar side length or decreasing micro-pillar height can delay activation of nucleation sites.
•Pool boiling of refrigerant-oil mixture on modified surfaces were explored.•Foaming behavior of refrigerant-oil mixture on modified surfaces was visualized.•Mechanisms by which wettability, ...structure and oil effect boiling were revealed.•Impact of microstructure on heat transfer is greater than that of wettability.•The addition of oil promotes foaming and induces tornado-shaped bubble group.
As a crucial component of refrigeration and heat pump system, the evaporator performance directly affects system efficiency. Due to the lubricating oil in the compressor circulating into the evaporator, studying the nucleate boiling heat transfer (NBHT) of refrigerant-oil mixture is of greater practical and guiding significance. Surface modification technology has opened up a new avenue for enhancing NBHT of refrigerant-oil mixture. Therefore, experimental study was conducted on NBHT characteristics of R134a-POE mixture and R1234ze(E)-POE mixture on smooth surface, initial laser-ablated surface, stabilized laser-ablated surface, machined surface, and composite processed surface. The effects of oil concentration (ω), surface microstructure and surface wettability on NBHT of refrigerant were discussed and analyzed, and the underlying heat transfer mechanisms were elucidated. The research indicates that for NBHT of refrigerant-oil mixture, surface microstructure has a greater impact than surface wettability. To enhance heat transfer coefficient (HTC), it is crucial to establish a surface pattern comprising alternating regions of high and low nucleation sites to mitigate bubble interaction and coalescence. Once this objective is attained, enhancing surface wettability can be contemplated. The addition of oil promotes foaming and induces tornado-shaped bubble group, which facilitate heat transfer. However, the rise in mixture viscosity and surface tension exerts a more pronounced adverse effect on heat transfer, outweighing the positive impacts of foaming and tornado-shaped bubble group. Consequently, the presence of oil diminishes HTC. And as ω and heat flux increase, HTC further decreases. Additionally, the increase in vaporization nuclei on modified surface leads to a higher local ω, the increase in ω further weakens the enhanced heat transfer effect of modified surface, and thus narrows the difference in HTC among various modified surfaces. The research results aim to uncover the mechanisms by which surface wettability, surface microstructure, and lubricating oil affect NBHT of refrigerant.
Nucleate boiling efficiency can be significantly enhanced using the nanostructured graphene surface. The wettability of the graphene nanoplatelets (GNPs) coated surface is tunable using thermal ...curing process. The uncured GNPs surface is hydrophobic in nature while the cured GNPs surface manifests a hydrophilic characteristic and the latter performs better than the former in the nucleate boiling regime. Ultimately the ultrafast water permeation property of the cured GNPs enhances the efficiency of nucleate pool boiling, leading to a maximum enhancement of 151% in the boiling heat transfer coefficient and 154% in the vapor mass flow rate. The boiling performance of the uncured GNPs surface is also enhanced despite its hydrophobicity. Hence, the enhancement in boiling is not solely attributed to the surface wettability. The enhancement is attributed to the fascinating ultrafast water permeation property of graphene on top of its nanoporous structure. The unimpeded fast water transport within the nanochannel-network of GNPs facilitates the fast absorption of latent heat of vaporization by the water molecules, leading to a substantial increase in the nucleation, growth and departure of vapor bubbles. We propose a new explanation on the role of graphene coating on nucleate pool boiling enhancement. This study provides important insights into the effects of ultrafast water permeation property of graphene on the nucleate boiling heat transfer.
•Experimental investigation on the thermal characteristics of two phase were conducted.•The results indicate that qONB is not influenced directly by the system pressure.•It is revealed that bubble ...detaching diameter is an important factor.
The thermal characteristics of two-phase water were experimentally studied under 0.5–5.0 MPa in the 1.0 mm × 50 mm and 2.5 mm × 50 mm narrow rectangular channels. The experimental results about the onset of nucleate boiling (ONB) and the two-phase heat transfer demonstrate the followings. At the onset of nucleate boiling, the wall superheat, inlet subcooling and pressure were discussed. It seemed the inlet subcooling and pressure to have the same effect on ONB. The influence factors of two-phase heat transfer characteristics were investigated in 1.0 mm and 2.5 mm narrow rectangular channels. It is revealed that bubble detaching diameter is an important factor affecting two-phase heat transfer in the narrow rectangular channels.
•Thermal-hydraulic performance of the PHX was investigated for the ORC application.•Test was conducted at low R-245fa mass flux, and below 100°C heat source temperature.•Flow boiling heat transfer ...was mainly dominated by nucleate boiling mechanism.•Overall heat transfer coefficient strongly depended on R-245fa side heat transfer.
In this study, the experimental investigation of a plate heat exchanger was conducted, which was used as an organic Rankine cycle evaporator. The heat exchanger performance was investigated at the low mass flux and moderate evaporation temperature ranges, as it was designed for a cascade heat utilization in a fourth-generation district heating and cooling system. The experiments were conducted under various operating conditions by changing the R-245fa mass flux, evaporation pressure, R-245fa inlet temperature, heat source inlet temperature, and heat source mass flux. Moreover, the heat transfer and pressure drop mechanisms were thoroughly investigated with an internal process analysis. The two-phase heat transfer coefficient exhibited a strong dependency on the heat flux, indicating that flow boiling heat transfer was mainly dominated by the nucleate boiling mechanism. When both the evaporation pressure and R-245fa mass flux were increased, the single-phase heat transfer accounted for a significantly large portion of the total heat transfer, leading to the rapid decrease in the heat transfer rate. The port and elevation pressure drops together accounted for 27–47% of the total pressure drop due to the low R-245fa mass flux range. With the heat source side variation, the two-phase heat transfer coefficient was more affected by the heat source inlet temperature than the heat source mass flux owing to the increase in the excess temperature. Additionally, the overall heat transfer coefficient strongly depended on the R-245fa side heat transfer coefficient, because the R-245fa had lower heat transfer coefficients than the heat source, even in the two-phase region.
A variety of industrial applications such as power generation, water distillation, and high-density cooling rely on heat transfer processes involving boiling. Enhancements to the boiling process can ...improve the energy efficiency and performance across multiple industries. Highly wetting textured surfaces have shown promise in boiling applications since capillary wicking increases the maximum heat flux that can be dissipated. Conversely, highly nonwetting textured (superhydrophobic) surfaces have been largely dismissed for these applications as they have been shown to promote formation of an insulating vapor film that greatly diminishes heat transfer efficiency. The current Letter shows that boiling from a superhydrophobic surface in an initial Wenzel state, in which the surface texture is infiltrated with liquid, results in remarkably low surface superheat with nucleate boiling sustained up to a critical heat flux typical of hydrophilic wetting surfaces, and thus upends this conventional wisdom. Two distinct boiling behaviors are demonstrated on both micro- and nanostructured superhydrophobic surfaces based on the initial wetting state. For an initial surface condition in which vapor occupies the interstices of the surface texture (Cassie-Baxter state), premature film boiling occurs, as has been commonly observed in the literature. However, if the surface texture is infiltrated with liquid (Wenzel state) prior to boiling, drastically improved thermal performance is observed; in this wetting state, the three-phase contact line is pinned during vapor bubble growth, which prevents the development of a vapor film over the surface and maintains efficient nucleate boiling behavior.
•FeCrAl- and Cr-layers on tube surfaces were fabricated using a DC magnetron sputtering technique.•NBHTC of FeCrAl layer was higher than Cr layer while CHF was enhanced similarly to each other.•CHF ...enhancement showed a good agreement with a product of spreading rate and surface roughness.
This study compares the nucleate boiling heat transfer coefficient (NBHTC) and critical heat flux (CHF) of two candidate coatings, FeCrAl and Cr, being considered for accident-tolerant fuel (ATF) cladding applications. To form an intrinsic surface roughness, the tube surfaces were initially grinded with 800 and 60 grit sandpapers, and then, FeCrAl and Cr were physically deposited using the direct current magnetron sputtering technique. The FeCrAl- and Cr-layered surfaces became hydrophilic on the lumped nanostructures and superhydrophilic on the particulate nanostructures, respectively. When subjected to the pool boiling conditions of vertically-oriented tubes, the NBHTC and CHF of the FeCrAl-layered tube increased by 24% and 34%, respectively, with the exception of the similar NBHTC generated by the 60 grit sandpaper. In contrast, the NBHTC of the Cr-layered tube decreased by 15% due to the suppressed nucleation on the particulate nanostructures, while CHF increased by 27%. The CHF enhancement was analyzed based on the liquid spreading behavior of a water droplet from the morphological changes. The capillary flow rate, which is a product of the liquid spreading rate, us, and arithmetic roughness height, Ra, resulted in a more accurate prediction of the CHF than the equilibrium contact angle. Additionally, the potential boiling performance of the FeCrAl and Cr coatings were discussed by comparing the pool boiling results of the vertical tube and horizontal plate orientations.
•Low CHF values are observed in flow boiling of water under subatmospheric pressure.•New data of CHF is obtained across flow rates, subcooling and pressure conditions.•Performance of existing models ...is shown to vary significantly.•A new model is developed and shown to capture the range of condition considered.
Critical Heat Flux (CHF) is the maximal limit of heat flux in two-phase nucleate boiling heat transfer; therefore, an understanding of CHF under a wide range of conditions is important for safe system operation. In this work, CHF experiments are conducted over a range of subatmospheric system pressures in a vertical square channel that is heated on one side. The experimental conditions cover a pressure range of 20 kPa to 108 kPa, a mass flux range of 45–190 kg/m2-s, and an inlet subcooling range of 0–14 K. Heat flux is gradually increased until an excursion of the wall temperature occurs, indicating CHF. For the experimental conditions considered, CHF increases with rising system pressure, mass flux, and inlet subcooling, although the effects of mass flux and inlet subcooling are weak. A new correlation for CHF is developed and found to predict the data with an average error of ±15.6%.
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