Increase In Critical Heat Flux Research Articles

Investigation of droplet boiling on superhydrophilic CuO multiscale hierarchical structured surfaces

This work presents the experimental investigation of droplet boiling characteristics on superhydrophilic surfaces. These surfaces are prepared on a copper substrate, and the boiling characteristics are studied and compared with the bare copper surface. Two types of superhydrophilic surfaces exhibiting needle-like (CuO-1) and sheet-like (CuO-2) nanostructures are prepared by the chemical etching oxidation technique. The hierarchical surface structures are characterized by carrying out SEM and XRD analysis. We employ high-speed imaging along with temperature measurement to characterize the heat transfer phenomenon. The surface temperature varies from 100 °C to 190 °C, and the Weber number is maintained constant at 2.78, corresponding to the gentle droplet impact regime. The effectiveness in the heat transfer performance of the different surfaces is characterized by measuring the critical heat flux (CHF) and the Leidenfrost point (LFP), and it is correlated with the surface properties viz. roughness, wettability, and porosity. The superhydrophilic surface alteration reduces the contact angle by about 99.7% compared to the bare surface. This reduction in contact angle corresponds to an increase in Critical Heat Flux (CHF) by 69.95% for CuO-1 and 47.17% for CuO-2, respectively, compared to bare copper. It is also seen that the LFP is delayed on the superhydrophilic surfaces, with an increase in the LFP temperature by 16.67% and 14.28% for CuO-1 and CuO-2 coatings, respectively. The effect of structures on the atomization and formation of secondary droplets is also presented. We report the contact coefficient (kw) for superhydrophilic surfaces for the first time, which is helpful in the theoretical prediction of CHF. These results show that significant improvement in heat transfer can be obtained in spray cooling and hot spot cooling applications without resorting to sophisticated microfabrication techniques.

Pool Boiling of CNT + GO Nanomaterial–Coated Copper Substrate: An Experimental Study

Abstract Nanoparticle coating on copper substrates like carbon nanotubes (CNT) and graphene oxide (GO) is a promising method to enhance the surface properties as well as improve the boiling heat transfer characteristics. The main objective of the present investigation is to study the influence of the nanocomposite coating on the performance of pool boiling heat transfer. CNT + GO nanomaterials are coated on copper substrates via the dip coating method by varying the concentration of the nanomaterial. Morphological analysis, surface roughness, and wettability behavior of the coating are also observed. The result shows that CNT + GO increases the surface roughness of the samples, and the coated samples are superhydrophilic in nature. Compared with the uncoated sample, the coated sample shows the maximum increase in critical heat flux and heat transfer coefficient is 145.76% and 259.08%, respectively. A high-speed camera is used to study the bubble dynamics. Bubble diameter, departure frequency, and site density are also calculated and presented.

Parameter study for critical heat flux under external reactor vessel cooling condition based on a mechanism model

External reactor vessel cooling has been adopted an important method to implement In Vessel Retention severe accident mitigation strategy for many advanced passive pressurized water reactors. Critical heat flux is one of the important criteria to evaluate the success or failure of this strategy. The uncertainty of engineering factors in practical design and operation conditions may make influence on critical heat flux more or less. In this paper, a parameter study for critical heat flux was carried out based on a new critical heat flux mechanism model. This mechanism model is developed based on the mechanism of liquid film dryout, and the predicted results of this model are in good agreement with the experimental results. Five important parameters, namely, coolant subcooling degree, geometry of lower head, system pressure, flow rate and heating power shape are analyzed based on this mechanism model. The results of parameter study show that the decrease of inlet coolant temperature and lower head radius are beneficial to the increase of critical heat flux. And the critical heat flux increases with the increase of system pressure. While the change of flow rate and heating power shape have little effect on critical heat flux. Under the condition of same circumference, the critical heat flux of spherical lower head is higher than that of elliptical lower head in the middle and low inclination angle region, while the trend is opposite in high inclination angle region, but not significant.

Experimental investigation of heat transfer performance in gas-atomized spray cooling

Spray cooling has substantial potential for wide application, given its unique characteristics, including high heat transfer coefficient, absence of contact thermal resistance, and the elimination of cooling hysteresis. In this study, a visualized experimental system featuring a gas-liquid two-phase cyclonic nozzle was constructed to study both the heat transfer and fluid morphology features pertaining to the wall. With the increase of wall superheat, a progressive transition across four distinct stages of heat transfer – liquid film, stream flow, droplet flow, and mist flow can be found. Of these, the droplet flow stage outperforms others in terms of heat transfer performance and is thus advocated as the preferred mode of application. In response to increasing spray pressure, the droplets generated on the wall during the droplet flow stage undergo a reduction in size, yet become more densely distributed. A notable increase in critical heat flux by 59.5 % is observed when spray pressure is elevated from 0.2 to 0.4 MPa. While raising the spray flow rate leaves the heat transfer performance during the liquid film stage largely unchanged, it effects a significant augmentation in critical heat flux. Our experimental results ultimately obtain the optimal operating parameters that facilitate the wall surface's retention in the droplet flow stage. These experimental results are poised to contribute meaningfully to the engineering practices associated with spray cooling applications.

Enhanced liquid replenishment for pool boiling heat transfer and its CHF mechanism on patterned freeze-casted surfaces

In the current investigation, the pool boiling heat transfer process of patterned porous surfaces, as well as a smooth surface, in ethanol was examined from an academic perspective. The freeze casting (also known as ice-templating) processes was used to fabricate the structured surfaces. It was discovered that freeze-casted coatings significantly improved the performance of boiling heat transfer in comparison to the smooth surface (SMS). The most remarkable increase in Critical Heat Flux (CHF) was achieved by checkerboard porous surface(CHECKERBOARD), with an improvement of 81.6% relative to that of SMS. The maximum Heat Transfer Coefficient (HTC) enhancement was obtained by full porous surface (FULLY-COVERED), with an HTC value 215.8% higher than that of the SMS. For the semicircular porous surface (SEMICIRCULAR), striped porous surface (STRIPED), annular porous surface (ANNULAR) and checkerboard porous surface(CHECKERBOARD) with 50% coating coverage, CHECKERBOARD demonstrated the best heat transfer performance, with CHF and HTC increased by 32.3% and 48.9%, respectively, compared to the SEMICIRCULAR surface. The mechanism of liquid supply at the CHF of the patterned porous surfaces was investigated through phenomenological observations of the boiling phenomena. A modified model, accounting for the coalesced bubble departure frequency and capillary wicking effects, was proposed for CHF prediction. It is noteworthy that the fluid replenishment considers both vertical and lateral replenishment of porous coatings. The CHF data from this study, along with existing literature, were utilized to validate the model, and the predicted results exhibited good agreement with the experimental data, with errors within ±8%.

Pool boiling on a biphilic surface where bubbles can move horizontally on the surface

In pool boiling on a horizontal surface, the flow of vapor is mainly the departure of vapor bubbles from the boiling surface. If the surface is composed of specially laid-out areas with varied wettabilities, vapor can move on the surface and out of the boiling region. In the present work, a silicon surface with such a function is fabricated using laser ablation and silanization grafting methods, and pool boiling on the surface is experimentally studied. On the biphilic surface, the boiling region is super-hydrophilic (SHI), and two opposite sides of the SHI region each is in connection to a wedge-shaped SHO (super-hydrophobic) track. The two sides of the SHO track are bounded by HO (hydrophobic) areas. Tests with air bubbles underwater show that the SHO tracks enable the directional spontaneous movement of the air bubbles on the surface, and the driving force for the natural motion is analyzed. Boiling heat transfer on the surface is compared with a SHI surface without the SHO tracks. The biphilic surface shows noticeable enhancement of heat transfer for low heat fluxes (< ∼150W/cm2) but reduces heat transfer for high heat fluxes in nucleate boiling. In addition, the biphilic surface extends the heat flux and surface temperature ranges of nucleate boiling, showing ∼14% increase of critical heat flux. The effects on boiling heat transfer are related to the observations of vapor transport by the SHO tracks.

Influence of heater size on critical heat flux in flat microchannels with intense localized heating

Experiments were carried out on flow boiling in mini- and micro- channels under local heating for different heater size. It was found that with an increase in the channel height in the range of 0.2–3.7 mm, the intensity of heat transfer and the critical heat flux increase significantly. For a given flow rate and channel height, the critical heat flux on the 3x3 mm2 heater exceeds the heat flux on the 10x10 mm2 heater, and the difference increases with increasing channel height. Boiling curves were also obtained.

Surface effects on Sub-cooled pool boiling for smooth and laser-ablated silicon surfaces

Smooth and laser-ablated silicon surfaces with wettability ranging from SHI (super-hydrophilic) to SHO (super-hydrophobic) are tested in subcooled pool boiling of water. The experiment focuses on the observation of bubble phenomena using a high-speed camera and the measurement of surface temperature using an infrared camera. The formation and departure of vapor bubbles from the surfaces are found to vary significantly with the surface wettability. Nucleate boiling curves show that both the heat transfer coefficient and the critical heat flux increase with the surface wettability changing from SHO to SHI. For the hydrophilic surfaces (smooth and textured), the heat transfer coefficient increases with the heat flux. For the hydrophobic surfaces (smooth and textured), the heat transfer coefficient decreases with the increasing heat flux. The effect of the surface wettability is quantitatively assessed using the surface-fluid factor. It is found that the factor's value increases with the surface wettability changing from SHI to SHO. The surface effect for the hydrophilic surfaces remains relatively constant over the nucleate boiling range, whereas for the hydrophobic surfaces the effect shows significant dependency on the heat flux. These findings are also revealed from the boiling tests on a biphilic surface consisting of a SHO region and a SHI region.

Recommendation for prediction of pool boiling heat transfer of various liquids on microstructured surfaces

Recommendations for prediction heat transfer coefficients and critical heat flux according to available in the literature experimental data of heat transfer and critical heat flux for boiling different liquids on microstructured surfaces made by deformed cutting method are obtained. In the work, on available in the literature sources of experimental data on heat transfer and critical heat flux at boiling of various liquids on the microstructured surfaces made by the deforming cutting method, recommendations for prediction of heat transfer coefficients and critical heat fluxes are received. Microstructured surfaces allow to intensify the heat transfer is 1.1 to 6 times. Due to the variable wettability of microstructured surfaces elements, critical heat flux increase before 4 times. The proposed criteria equations allow predicting heat transfer coefficients with an error of 30%, and critical heat fluxes with an error of 30-35%. In order to improve the accuracy of forecasting, the possibility of using an artificial neural network model for generalizing heat transfer coefficients is shown. Forecasting using an artificial neural network model allows us to determine the heat transfer coefficients with an error of ±20%.The equations are of interest for designing cooling systems for microelectronic devices, heat and mass transfer devices, boiling zones of heat pipes and thermosyphons, etc.

Enhancement of flow boiling heat transfer using heterogeneous wettability patterned surfaces with varying inter-spacing

This study experimentally investigated the influence of heterogeneous wettability-patterned surfaces with varying inter-spacing on flow boiling heat transfer characteristics. The test surfaces consisted of three fluorooctyltrichlorosilane hydrophobic-patterned array structures having a triangle, inverted triangle, and circular shape on a SiO2- hydrophilic substrate, with an inter-spacing of 0.75 or 1 mm. The working fluid was deionized water, and the Reynolds number was 6,000 at atmospheric pressure. Among the test surfaces, varying the inter-spacing between neighboring hydrophobic patterns slightly enhanced the heat transfer coefficient (HTC) due to changing the bubble characteristics. In terms of the shape effect of hydrophobic patterns, the heterogeneous wettability-patterned surfaces dominated the overall flow boiling heat transfer performance showing a significant increase in critical heat flux (CHF) compared to the Si surface, by 40–43%. In addition, all of the wettability test surfaces showed a markedly higher heat transfer coefficient than the Si surface, by 35–163%. This experiment is explained by analyzing the relationship between bubble lift forces and the various hydrophobic-patterned shapes in a horizontal flow channel, in an attempt to better understand flow boiling heat transfer and optimize the pattern design.

Application peculiarities of the high-temperature fluids containing nanoparticles in gas-tube boilers

The paper considers the application peculiarities of the high-temperature fluids containing nanoparticles in gas-tube boilers. The classification of nanofluids obtaining methods is presented, one-step and two-stepmethods for preparing the product are considered. The main thermophysical properties of materials are described. The values of the thermal conductivity coefficient for some materials that are usually used as the base ones for mixing liquids and nanoparticles are represented. The mechanisms of heat transfer in nanofluids in the forced and free convection and boiling processes determining the gas-tube boiler efficiency are considered. Criteria dependences of calculating the heat transfer in the free and forced convection and boiling when nanofluids flowing through the heat generators are proposed. It is proved that when adding nanoparticles to the base liquid, an increase in the heat transfer coefficient will be observed if its thermal conductivity is increased. Besides, the convective heat transfer is affected by the density and viscosity of the nanofluid. When the liquid boils, the critical heat flux increases, and the determining factor of the heat transfer coefficient is the particles concentration. The represented data are prerequisites of developing the efficient gas-tube boiler with a high-temperature fluid.

Effect of Ball Milled and Sintered Graphene Nanoplatelets–Copper Composite Coatings on Bubble Dynamics and Pool Boiling Heat Transfer

Herein, the efficacy of graphene nanoplatelets–copper (GNP–Cu) composite coatings formed by ball milling and sintering methods in enhancing the pool boiling, i.e., phase‐change heat transfer tested for distilled water, is presented. The pool boiling performance is quantified by an increase in critical heat flux (CHF) and heat transfer coefficient (HTC) at the lower wall superheat temperature. The ball milling facilitates the draping of highly thermally conductive GNP around individual copper particles and subsequent sintering results in morphological features that further promote high wicking rates of the coatings. A variety of coatings are formed with varying GNP concentrations and copper particle sizes. Highest pool boiling performance is achieved for the 2 wt% GNP and 20 μm diameter Cu coating, with a CHF of 239 W cm−2 and HTC of 285 kW m−2 °C−1, which indicate an increment of ≈91% and ≈438%, respectively, compared with a pristine copper surface. Stretched or elongated pores observed for certain compositions of the coatings, termed as “pore pockets” in this work, contribute in higher pool boiling heat transfer due to their ability to contain and supply liquid, while acting as nucleation sites for vapor generation and escape.

Semi-Analytical Modeling of Critical Heat Flux in Convective Boiling of Nanofluids

Using two-phase flows because of their high thermal conductivity coefficients is favorable in various industries such as oil, petro chemistry, and power plants that require significant thermal conductivity coefficients in heat exchangers. On the other hand, the limiting factor in benefiting from these flows is the critical heat flux phenomenon, the onset of which must be delayed by various means. One of the methods to achieve this goal is using nanofluids. Nanofluids, due to their higher thermal conductivity in comparison with base fluids, have demonstrated the capability to delay the onset of critical heat flux phenomenon. In the present Paper, silver, copper, alumina and nickel water-based nanofluidic flows in heat exchangers are analytically studied, and the critical heat flux is assessed under different mass flux and volume fraction conditions. Reports of Look Up Table are employed for calculation of critical heat flux in the present Paper. All four mechanisms of mass transfer are simulated, and pressure variations in both dimensions are considered. Effects of mass transfer on vapor core pressure are comprehensively assessed. The results obtained indicate a directly proportional relation between mass velocity and critical heat flux. In addition, increasing the concentration of nanoparticles also increases the critical heat flux. Of the studied nanofluids, the highest critical heat flux increase was observed for a 5 vol % water–silver nanofluid with a 81.6% increase in the critical heat flux relative to pure water.

Perspectives and problematic issues in the development of heat transfer enhancement methods at boiling and evaporation

The paper presents an analysis of modern methods of heat transfer intensification and critical heat flux increase during boiling and evaporation using micro-nanostructuring of surface. Special attention is paid to the analysis of such promising methods of modifying heat-transfer surfaces as the application of combined microstructured surfaces/coatings by SLM/SLS (3-D printing), the organization of structures with contrast wettability on the micro-scale. The conclusions formulated on the basis of generalization of results of complex research on studying of heat transfer, transitional and crisis phenomena at boiling in the conditions of free convection, at evaporation and boiling in flowing down films and in thin horizontal layers of liquids at use of various types of microstructuring of heat- transfer surfaces are presented.

Visualizing near-wall two-phase flow morphology during confined and submerged jet impingement boiling to the point of critical heat flux

Confined and submerged two-phase jet impingement is a compact, low-pressure-drop solution for high heat flux dissipation from electronic components. Nucleate boiling can be sustained up to significantly higher heat fluxes during two-phase jet impingement as compared to pool boiling. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. In this study, the two-phase flow morphology of a single subcooled jet of water that impinges on a circular heat source is visualized at high speed with synchronized top and side views of the confinement gap. The impinging jet issues from a 3.75-mm-diameter orifice that is held at a height of 2 orifice diameters above a 25.4-mm-diameter heat source. The experiments are conducted at a jet Reynolds number of 15,000 and a jet inlet subcooling of 10 °C across a range of heat fluxes up to the critical heat flux. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, a regular cycle is observed of formation and collapse of vapor structures that bridge the heated surface and the orifice plate, which causes significant oscillations in the pressure drop. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl-like vapor structure that recurrently shrinks, due to vapor break-off at the edge of the orifice plate, and is again replenished due to vapor generation at the heater surface. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure as a fluid wall jet that wets the heated surface upon which discrete bubbles are generated due to boiling. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn-like regime, shearing off some liquid droplets into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater.

Термокапиллярные структуры и разрыв нагреваемой пленки жидкости

The data of thermocapillary structures formation and breakdown of the heated liquid film flowing down on a vertical surface with the Reynolds number varied from 0.1 up to 500 are analyzed. It is shown that the interaction of waves with thermocapillary structures type A leads to an increase in critical heat flux, corresponding to the liquid film rupture, compared with literature data (regime B).

An experimental study of the nanofluid pool boiling on the aluminium surface

Nucleate boiling due to use latent heat instead of sensible heat has an extra potential for dissipating high heat flux, but it needs to control critical heat flux. On the other hand, it is well known that nucleate boiling is strongly under the influence characteristics of surface. One of the methods to control critical heat flux is to change surface characteristics by nanofluid pool boiling. In this study, nanofluids which contain aluminium oxide nanoparticles with four different levels of concentration including 0.002, 0.01, 0.05 and 0.1 vol% were used for pool boiling tests at the atmospheric pressure. Tests showed that generally to add nanoparticles will cause the critical heat flux increase and at the certain concentrations about 0.01 vol%, critical heat flux has maximum enhancement about 19%. In this research, additionally, the effects of surface parameters such as wettability, roughness and thickness of deposited nanoparticles on the critical heat flux have been analysed.

Microscale Morphology Effects of Copper–Graphene Oxide Coatings on Pool Boiling Characteristics

Enhanced pool boiling heat transfer, with simultaneous increase in critical heat flux (CHF) and heat transfer coefficient (HTC), is desired to improve overall system efficiency and reduce equipment size and cost. This paper focuses on combining graphene oxide (GO) and porous copper particles to generate microstructures based on their ability to enhance HTC, CHF, or both. Three pool boiling performance characteristics based on CHF improvements and wall superheat reductions are identified: Type I—reduction in wall superheat only, type II—increase in CHF only, and type III—increase in CHF with reduction in wall superheat at higher heat fluxes. Specific microscale morphologies were generated using (a) screen-printing and (b) electrodeposition techniques. In type-I, rapid bubble activity due to increased availability of nucleation cavities was seen to influence the reduction in the wall superheats, while no increase in CHF was noted. Roughness-augmented wettability was found to be the driving mechanism in type-II enhancement, while wicking and increased nucleation site density were responsible for the enhancement in type-III. An HTC enhancement of ∼216% in type-I and a CHF improvement of ∼70% in type-II were achieved when compared to a plain copper surface with water. In type-III enhancement, a CHF of 2.2 MW/m2 (1.8× over a plain surface) with a HTC of 155 kW/m2 °C (∼2.4× over a plain surface) was obtained. Furthermore, close correlation between the boiling performance and the microscale surface morphology in these three categories has been identified.

The experimental study of nanofluids boiling crisis on cylindrical heaters

The paper deals with the experimental study of nanofluids boiling on cylindrical heater. The studied nanofluids were prepared on the basis of distilled water and nanoparticles of silicon, aluminum and iron oxides as well as diamond. The volume concentration of nanoparticles varied from 0.05 to 1%. The nanoparticles diameter ranged from 10 to 100 nm, the diameter of the heater was varied from 0.1 to 0.3 mm. It is revealed that the use of nanofluids provides a several-fold increase of the critical heat flux. However, critical heat flux in nanofluids depends on the material and size of nanoparticles as well as on the diameter of used cylindrical heater. It is shown that the critical heat flux increases with increasing the nanoparticles size, while it decreases with increasing the heater diameter. It was revealed for the first time that the critical heat flux depends on the boiling process duration.

An experimental study of thermocapillary rupture of a locally heated horizontal liquid layer on various substrates

This article experimentally studies a thermocapillary rupture of a locally heated horizontal liquid film on substrates of different materials and different thickness. In the course of work the dependence of the critical heat flux on the initial and final film thickness was determined. Upon reaching the heat flux and film thickness at which the liquid film boils up, the film disrupts at almost constant finite thickness. The research has shown that with an increase in thermal conductivity and thickness of the substrate the critical heat flux increases. On the substrates of a material with high thermal conductivity (copper) the heat flux is redistributed on the substrate; at that the localization of heating is lost, and the rupture is not observed.