Stabilizer effect on CHF and boiling heat transfer coefficient of alumina/water nanofluids

We measured the critical heat flux (CHF) and boiling heat transfer coefficient (BHTC) of water-based Al₂O₃ (alumina) nanofluids. To elucidate the stabilizer effect on CHF and BHTC of alumina/water nanofluids, a polyvinyl alcohol (PVA) was used as a stabilizer. The plate copper heater (10 × 10 mm²) is used as the boiling surface and the concentration of alumina nanoparticle varies 0–0.1 vol.%. The results show that the BHTC of the nanofluids becomes lower than that of the base fluid as the concentration of nanoparticles increases while CHF of it becomes higher. It is found that the increase of CHF is directly proportional to the effective boiling surface area and the reduction of BHTC is mainly attributed to the blocking of the active nucleation cavity and the increase of the conduction resistance by the nanoparticle deposition on the boiling surface.     

Experimental investigation of the effect of particle deposition on pool boiling of nanofluids

Research on pool boiling of nanofluids has shown contradicting trends in the heat transfer coefficient (HTC). Such trends have been attributed, in part, to nanoparticle deposition on the heater surface. An experimental investigation of the transient nature of nanoparticle deposition and its effect on the HTC of pool boiling of nanofluids at various concentrations has been carried out. Pool boiling experiments have been conducted on a horizontal flat copper surface for alumina (40–50 nm) water based nanofluids at concentrations of 0.01, 0.1 and 0.5 vol.%. Nanofluids boiling experiments have been followed by pure water boiling experiments on the same nanoparticle-deposited (NPD) surfaces. This technique has been employed in order to separate the effect of nanoparticle deposition from the effect of nanofluids properties on the HTC. Contrary to what was expected, boiling of pure water on the NPD surface produced using the highest concentration nanofluid resulted in the highest HTC. A closer look at the nature of the NPD surfaces explained such trend. A new approach using a transient surface factor in Rohsenow correlation has been proposed to account for the transient nature of nanoparticle deposition. The applicability of such approach at different concentrations has been investigated.     

Surface effects of ribbon heaters on critical heat flux in nanofluid pool boiling

This paper deals with a study of enhanced critical heat flux (CHF) and burnout heat flux (BHF) in pool boiling of water with suspended silica nanoparticles using Nichrome wires and ribbons. Previously the current authors and other researchers have reported three-digit percentage increase in critical heat flux in silica nanofluids. This study investigates the effect of various heater surface dimensions, cross-sectional shapes as well as surface modifications on pool boiling heat transfer characteristics of water and water-based nanofluids. Our data suggest that the CHF and BHF decrease as heater surface area increases. For concentrations from 0.1 vol% to 2 vol%, the deposition of the particles on the wire allows high heat transfer through inter-agglomerate pores, resulting in a nearly 3-fold increase in burnout heat flux at very low concentrations. The nanoparticle deposition plays a major role through variation in porosity. The CHF enhancement is non-monotonic with respect to concentration. As the concentration is increased, the CHF and BHF decrease prior to increasing again at higher concentrations. Results show a maximum of 270% CHF enhancement for ribbon-type heaters. The surface morphology of the heater was investigated using SEM and EDS analyses, and it was inferred that the 2 vol% concentration deposition coating had higher porosity and rate of deposition compared with 0.2 vol% case.     

Pool boiling characteristics of nanofluid on flat plate based on heater surface analysis

Nucleate pool boiling of Al₂O₃ based aqueous nanofluid on flat plate heater has been studied experimentally. For boiling of nanofluid (< 0.1 vol.%) on heating surface with ratio of average surface roughness to average diameter of particles much less than unity when boiling continue to CHF, the heat transfer coefficient of nanofluid boiling reduces while critical heat flux (CHF) increases. CHF enhancement increased with volume fraction of nanoparticles. Atomic force microscope (AFM) images from boiling surface showed that after boiling of nanofluid the surface roughness increases or decreases depending on initial condition of heater surface. Changes in boiling surface topology during different regions of boiling, wettability and thermal resistance of heater surface owing to nanoparticles deposition cause to variations in nanofluids boiling performance.     

Nanofluid stabilizes and enhances convective boiling heat transfer in a single microchannel

The flow boiling heat transfer in a single microchannel was investigated with pure water and nanofluid as the working fluids. The microchannel had a size of 7500 × 100 × 250 μm, which was formed by two pyrex glasses and a silicon wafer. A platinum film with a length of 3500 μm and a width of 80 μm was deposited at the bottom channel surface, acting as the heater and temperature sensor. The nanofluid had a low weight concentration of 0.2%, consisting of de-ionized water and 40 nm Al₂O₃ nanoparticles. The nanoparticle deposition phenomenon was not observed. The boiling flow displays chaotic behavior due to the random bubble coalescence and breakup in the milliseconds timescale at moderate heat fluxes for pure water. The flow instability with large oscillation amplitudes and long cycle periods was observed with further increases in heat fluxes. The flow patterns are switched between the elongated bubbles and isolated miniature bubbles in the timescale of 100 s. It is found that nanofluid significantly mitigate the flow instability without nanoparticle deposition effect. The boiling flow is always stable or quasi-stable with significantly reduced pressure drop and enhanced heat transfer. Miniature bubbles are the major flow pattern in the microchannel. Elongated bubbles temporarily appear in the milliseconds timescale but isolated miniature bubbles will occupy the channel shortly. The decreased surface tension force acting on the bubble accounts for the smaller bubble size before the bubble departure. The inhibition of the dry patch development by the structural disjoining pressure, and the enlarged percentage of liquid film evaporation heat transfer region with nanoparticles, may account for the heat transfer enhancement compared to pure water.     

Pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating

We study the pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating. Nanofluids are the mixture of de-ionized water and Al₂O₃ particles with 0.1%, 0.2%, 0.5% and 1.0% weight concentrations. The microheater is a platinum surface by 50 × 20 μm. Three types of bubble dynamics were identified. The first type of bubble dynamics is for the boiling in pure water, referring to a sharp microheater temperature increase once a new pulse cycle begins, followed by a continuous temperature increase during the pulse duration stage. Large bubble is observed on the microheater surface and it does not disappear during the pulse off stage. The second type of bubble dynamics is for the nanofluids with 0.1% and 0.2% weight concentrations. The microheater surface temperature has a sharp increase at the start of a new pulse cycle, followed by a slight decrease during the pulse duration stage. Miniature bubble has oscillation movement along the microheater length direction, and it disappears during the pulse off stage. The third type of bubble dynamics occurs at the nanofluid weight concentration of 0.5% and 1.0%. The bubble behavior is similar to that in pure water, but the microheater temperatures are much lower than that in pure water. A structural disjoining pressure causes the smaller contact area between the dry vapor and the heater surface, decreasing the surface tension effect and resulting in the easy departure of miniature bubbles for the 0.1% and 0.2% nanofluid weight concentrations. For the 0.5% weight concentration of nanofluids, coalescence of nanoparticles to form larger particles is responsible for the large bubble formation on the heater surface. The microlayer evaporation heat transfer and the heat transfer mechanisms during the bubble departure process account for the higher heat transfer coefficients for the 0.1% and 0.2% nanofluid weight concentrations. The shortened dry area between the bubble and the heater surface, and the additional thin nanofluid liquid film evaporation heat transfer, account for the higher heat transfer coefficient for the 0.5% nanofluid weight concentration, compared with the pure water runs.     

Boiling enhancement by TiO2 nanoparticle deposition

TiO₂ nanoparticle-coated nickel wires were produced by electrical heating in various nanofluid concentrations ranging from 0.01 to 1 wt.% with various processing heat fluxes from 0 to 1000 kW/m². The experimental results demonstrated up to 82.7% enhancement on critical heat flux (CHF) in condition of coated nickel wire (processed in 1 wt.% with 1000 kW/m²) boiling in pure water. The contact angle measurement revealed that the hydrophilic porous coating formed by vigorous vaporization of TiO₂ nanofluid in nucleate boiling regime enormously modified the wettability of heating surface consequently improving the CHF. Besides, it is evident that the coverage of nanoparticle deposition tended to become more complete as concentration and processing heat flux increased based on SEM and EDS analysis. The nanoparticles dispersed in base fluid exhibited little effect on CHF enhancement and could even hinder the percentage of CHF augmentation from boosting, which demonstrated that one could enhance CHF by using only small amount of nanoparticles just adequate to form surface coatings instead of preparing working fluid with great bulk. However, according to the boiling curves in all cases of coated nickel wires, it is supposed that the nucleate boiling heat transfer coefficient deteriorates as a result of thermal resistance resulted from the occurrence of nanoparticle deposition. In summary, the coated porous structure of nanoparticles leads to enhance CHF and to decrease boiling heat transfer coefficient.     

Boiling time effect on CHF enhancement in pool boiling of nanofluids

Using TiO₂–water nanofluids as the test liquid, pool boiling experiments were carried out to investigate the dependence of the nucleate boiling heat transfer, surface wettability and critical heat flux (CHF) on the boiling time in nanofluids. In the experiments performed at sufficiently high nanoparticle concentrations, the boiling heat transfer first degraded, then improved, and finally reached an equilibrium state. It was hence supposed that the present nanofluids had competing effects to deteriorate and enhance the nucleate boiling heat transfer. As for the surface wettability and CHF, the static contact angle asymptotically decreased whilst the CHF asymptotically increased with an increase in the boiling time. The maximum CHF enhancement measured in the present experiments was 91%, and strong correlation was found between the contact angle and the CHF. Although the boiling time needed to achieve the maximum CHF enhancement was less than a minute at high particle concentrations, a longer time of the order of 1 h was necessary at the lowest particle concentration tested in this work. This experimental result indicated that sufficient attention should be paid to the boiling time effect particularly in industrial applications of nanofluids to emergency cooling.     

Nucleate boiling heat transfer enhancement for water and FC-72 on titanium oxide and silicon oxide surfaces

An experimental study was performed to investigate the nucleate boiling and critical heat flux (CHF) of water and FC-72 dielectric liquid on hydrophilic titanium oxide (TiO₂) nanoparticle modified surface. A 1 cm² copper heater with 1 μm thick TiO₂ coating was utilized in saturated pool boiling tests with water and highly-wetting FC-72, and its performance was compared to that of a smooth surface. Results showed that TiO₂ coated surface increased CHF by 50.4% and 38.2% for water and FC-72, respectively, and therefore indicated that boiling performance enhancement depends on the level of wettability improvement. A silicon oxide (SiO₂) coated surface, exhibiting similar surface topology, was tested to isolate the roughness related enhancement from the overall enhancement. Data confirmed that hydrophilicity of TiO₂ coated surface provides an additional mechanism for boiling enhancement.     

Reconstruction of local heat fluxes in pool boiling experiments along the entire boiling curve from high resolution transient temperature measurements

In this paper, we consider a transient inverse heat conduction problem (IHCP) defined on an irregular three-dimensional (3D) domain in pool boiling experiments. Heat input to a circular copper heater of 35 mm diameter and 7 mm thickness is provided by a resistance heating foil pressed to the bottom of the heater. The heat flux at the inaccessible boiling side is estimated from a number of temperature readings in the heater volume. These temperatures are measured by some high-resolution microthermocouples, which are mounted 3.6 μm below the surface in the test heater. The IHCP is formulated as a mathematical optimization problem and solved by the conjugate gradient (CG) method. The arising partial differential equations (PDEs) are solved using the software package DROPS. A simulation case study is used to validate the performance of the solution approach. Finally, we apply the solution approach to the IHCP in pool boiling experiments. The procedure enables the reconstruction of local instantaneous heat flux distribution on the heater surface at different locations along the boiling curve.     

Pool boiling characteristics of multiwalled carbon nanotube (CNT) based nanofluids over a flat plate heater

This paper is mainly concerned about the pool boiling heat transfer behavior of multi-walled carbon nanotubes (CNTs) suspension in pure water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS). Three different concentrations of 0.25%, 0.5% and 1.0% by volume of CNT dispersed with water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS) were prepared and boiling experiments were conducted over a stainless steel flat plate heater of size 30 mm² and 0.44 mm thickness. The test results exhibit that the addition of carbon nanotubes increases boiling heat transfer coefficients of the base fluids. At a given heat flux of 500 kW/m², the enhancement of heat transfer coefficient was found to be 1.5, 2.6 and 3.0 times of water corresponding to 0.25%, 0.5% and 1.0% concentration of CNT by volume in water, respectively. In water–CNT–surfactant nanofluid, it was found that 0.5% of CNT concentration gives the highest enhancement of 1.7 compared with water. In both water and water–surfactant base fluids, it was observed that the enhancement factor for 0.25% of CNT first increases up to the heat flux of 66 kW/m² and then decreases for higher heat fluxes. Further, the overall heat transfer coefficient enhancement in the water–CNT nanofluids is approximately two times higher than that in the water–CNT–surfactant nanofluids. With increasing heat flux, however, the enhancement was concealed due to vigorous bubble generation for both water–CNT and water–CNT–surfactant nanofluids. Foaming was also observed over the liquid-free surface in water–CNT–surfactant nanofluids during the investigation. No fouling over the test-section surface was observed after experimentation.     

Comparative study of the effect of hybrid nanoparticle on the thermal performance of cylindrical screen mesh heat pipe

The thermal performance of a cylindrical screen mesh heat pipe with hybrid nanofluid was experimentally investigated. The hybrid nanofluid was prepared by mixing Al2O3 and CuO nanoparticles with deionised water. The heat pipe was fabricated with straight copper tube of dimensions 300 mm length, 12.5 mm outer diameter and 1 mm thickness. The wick structure in the heat pipe was created by a three layer copper screen mesh of 100 mesh size. The heat input to the heat pipe was varied from 50 W to 250 W in five equal steps. The heat pipe was tested with three hybrid nanofluids made with combinations of Al2O3 and CuO nanoparticle in DI water (Al2O3 75%–CuO 25%, Al2O3 50%–CuO 50% and Al2O3 25%–CuO 75%). The tested hybrid nanofluids were made with 0.1% volume concentration of Al2O3 and CuO nanoparticle combination in deionised water. The results of the investigation showed that for the maximum heat load of 250 W considered in this work, the thermal resistance of the hybrid nanofluid with combination, Al2O3 25%–CuO 75%, showed 44.25% reduction compared to deionised water. The reduction in thermal resistance is due to the formation of porous coating on the wick surface which increases the wettability and surface roughness thereby creating more nucleation sites as seen in the SEM images. From the experimental investigation, it was observed that hybrid nanofluids are alternative to the conventional working fluids in heat pipes for electronic cooling applications.     

Pool boiling of perfluorocarbon mixtures on silicon surfaces

The need for higher pool boiling critical heat flux (CHF) in electronic cooling applications has turned attention to the use of binary mixtures of dielectric liquids. The available literature demonstrates that the addition of a liquid with higher saturation temperature, higher molecular weight, higher viscosity and higher surface tension can lead to significant enhancement of CHF, beyond what can be achieved through changes in pressure, liquid subcooling, and the product of surface effusivity and heater thickness. The current study focuses on extending the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10%, 15%, and 20% of FC-40 by weight, a pressure range of between 1 and 3 atm, and fluid temperature from 22 to 45 °C, leading to high subcooling conditions. It is found that peak heat flux can be increased to as high as 56.8 W/cm² compared to 25.2 W/cm² for pure FC-72 at 3 atm and 22 °C. It is believed that the increase in the mixture latent heat of evaporation and surface tension, accompanying the depletion of the lower boiling point fluid in the wall region plays the major role in enhancing the critical heat flux for binary mixtures.     

Preparation and pool boiling characteristics of copper nanofluids over a flat plate heater

The copper nanoparticles of average size of 10 nm have been prepared by the sputtering method and characterized through atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The pool boiling heat transfer characteristics of 0.25%, 0.5% and 1.0% by weight concentrations of copper nanoparticles has been studied. Different copper based nanofluids were prepared in both, distilled water and distilled water with 9.0 wt% of sodium lauryl sulphate anionic surfactant (SDS). The pool boiling heat transfer data were acquired for the boiling of nanofluids over a 30 mm square and 0.44 mm thick stainless steel plate heater. The experimental results show that for the critical heat flux of pure water is 80% higher than that of water–surfactant fluid. Also, it was found that the critical heat flux for 0.25%, 0.5% and 1.0% concentrations of copper nanoparticles in copper–water nanofluids are 25%, 40% and 48% higher than that of pure water. But in the case of copper–water with surfactant nanofluids comparing with pure water, the CHF decreases to 75%, 68%, and 62% for respective concentrations of copper nanoparticles. The heat transfer coefficient decreases with increase of nanoparticles concentration in both water–copper and water–copper with surfactant nanofluids.     

An experimental study of heater size effect on micro bubble generation

An experimental study of the heater size effect on micro boiling is reported in detail. Using a 1.66-ms-wide heating pulse, boiling in subcooled water was investigated on a series of micron/submicron thin film Pt heaters with various feature sizes ranging from 0.5 μm to 70 μm. It was found that there existed a critical heater size (10 μm): single spherical bubble generation with heater’s feature size less than 10 μm; oblate vapor blanket on the heater surface with the size larger than 10 μm. The bubble dynamics was studied by the visualization of the bubble nucleation process with a high-speed CCD. The onset bubble nucleation temperature was measured by using each Pt heater as a resistive temperature sensor. The formation of the oblate vapor blanket was attributed to the condensation effect of the vapor outside the superheated zone. The analysis was further validated by generating spherical bubble on heater with size larger than 10 μm with a longer heating pulse.     

Numerical research of nature convective heat transfer enhancement filled with nanofluids in rectangular enclosures

Heat transfer enhancement utilizing nanofluids in a two-dimensional enclosure is investigated for various pertinent parameters. The Khanafer's model is used to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion. Transport equations are model by a stream function-vorticity formulation and are solved numerically by finite-difference approach. Based upon the numerical predictions, the effects of Rayleigh number (Ra) and aspect ratio (AR) on the flow pattern and energy transport within the thermal boundary layer are presented. The diameter of the nanoparticle d_p is taken as 10 nm in nanofluids. The buoyancy parameter is 10³ ≤ Ra ≤ 10⁶ and aspect ratios (AR) of two-dimensional enclosure are 1/2, 1, 2. Results show that increasing the buoyancy parameter and volume fraction of nanofluids cause an increase in the average heat transfer coefficient. Finally, the empirical equation was built between average Nusselt number and volume fraction.     

Effect of nanoparticle size on nucleate pool boiling heat transfer of refrigerant/oil mixture with nanoparticles

Effect of nanoparticle size on nucleate pool boiling heat transfer of refrigerant/oil mixture with nanoparticles was investigated experimentally. For the preparation of the test fluid, refrigerant R113, ester oil VG68, and Cu nanoparticles with three different average diameters of 20, 50 and 80 nm were used. Experimental conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m^?2, nanoparticle concentrations in the nanoparticles/oil suspension from 0 to 30 wt%, and nanoparticles/oil suspension concentrations from 0 to 5 wt%. The experimental results indicate that the nucleate pool boiling heat transfer coefficient of R113/oil mixture with Cu nanoparticles is enhanced by a maximum of 23.8% with the decrease of nanoparticle size from 80 to 20 nm under the present experimental conditions, and the enhancement increases with the decrease of nanoparticles/oil suspension concentration or the increase of nanoparticles concentrations in the nanoparticles/oil suspension. A general nucleate pool boiling heat transfer coefficient correlation for refrigerant/oil mixture with nanoparticles is proposed, and it agrees with 93% of the existing experimental data of refrigerant/oil mixture with nanoparticles within a deviation of ±20%.     

Microscale heterogeneous boiling on smooth surfaces—from bubble nucleation to bubble dynamics

In this investigation, boiling incipience and bubble dynamics on a microheater with a geometry of 100 μm × 100 μm fabricated with MEMS technology are evaluated using a high-speed digital camera. For the purpose of comparison with conventional boiling heat transfer, boiling incipience and bubble dynamics are also studied on a carefully selected microheater with a fabricated defect (i.e., a microcavity on the heater surface). Of industrial interest are the effects of dissolved gases on boiling incipience and bubble dynamics, which are also discussed in detail. The possible nucleation temperature (or incipience temperature) is analyzed and discussed from the perspective of the measured bulk temperature of the microheater and a 3D heat conduction numerical model. The time-resolved bubble dynamics (i.e., the bubble size evolution, interface velocity and interface acceleration) are all presented along with high-speed digital images. Based upon this investigation, it is clear that explosive boiling can take place on a smooth surface no matter how slow the heating rate, and dissolved gases have a significant influence on the incipience temperature and bubble behavior. Furthermore, this study illustrates that the classical kinetics of boiling can explain the explosive boiling occurring on a smooth surface in principle and can provide a useful guide for the design of microscale heat transfer and/or MEMS devices. Although unexpected, due to the gravitational effects, Marangoni flow on the vapor–liquid interface induced by the temperature gradient was also observed.     

Nucleate pool boiling heat transfer of TiO2–R141b nanofluids

Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO₂ nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05 vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05 vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500 kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same.