Experimental study on the pool boiling CHF enhancement using magnetite-water nanofluids

This paper described the effects of a magnetite-water nanofluid (MWNF) on the critical heat flux (CHF) enhancement using an Ni–Cr wire in pool boiling. All experiments were performed at a saturated condition under atmospheric pressure. The CHF values between the MWNF and the other nanofluids with several volume concentrations were compared to evaluate the effect of the MWNF on the CHF enhancement. The CHF values of the MWNF were enhanced from approximately 170% to 240% of pure water as the nanoparticle concentration increased. In addition, the CHF for the MWNF showed the highest value among the evaluated nanofluids. In this paper, three methods were introduced to elucidate the mechanism underlying CHF enhancement. First, scanning electron microscope (SEM) images were obtained to explain the CHF enhancement mechanism due to the deposited nanoparticles, which is related to the surface wettability of the heating surface during the pool boiling. Second, bubble formation in pool boiling was analyzed using image processing to demonstrate the relationship between bubble dynamics and CHF enhancement. Finally, the magnetic field was analytically calculated using the Biot–Savart law to evaluate the effects of the magnetic field on the CHF.     

Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids

To investigate the characteristics of CHF (Critical Heat Flux) enhancement using nano-fluids, pool boiling CHF experiments of two water-based nano-fluids with titania and alumina nanoparticles were performed using electrically heated metal wires under atmospheric pressure. The results showed that the water-based nano-fluids significantly enhanced CHF compared to that of pure water. SEM (Scanning Electron Microscopy) observation subsequent to the pool boiling experiments revealed that a lot of nanoparticles were deposited on heating surface during pool boiling of nano-fluids. In order to investigate the role of the nanoparticle surface coating on CHF enhancement of nano-fluids, pool boiling CHF of pure water was measured using a nanoparticle-coated heater prepared by pool boiling of nano-fluids on a bare heater. It was found that pool boiling of pure water on the naonoparticle-coated heater sufficiently achieved the CHF enhancement of nano-fluids. It is supposed that CHF enhancement in pool boiling of nano-fluids is mainly caused by the nanoparticle coating of the heating surface.     

Critical heat flux enhancement in pool boiling using alumina nanofluids

The pool boiling characteristics of dilute dispersions of alumina nanoparticles in water were studied. Consistent with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly changes surface texture of the heater wire surface which could be the reason for improvement in the CHF value. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20301     

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.     

池沸腾临界热流密度及临界波长的实验研究

分别在光滑及波形结构的铜表面上对水和乙醇进行饱和池沸腾实验,观测了临界热流密度(CHF)下临界波长的变化趋势,并分析了表面结构对沸腾传热系数及CHF的影响。实验验证了光滑表面上,临界波长随工质的不同而变化,继而影响CHF,其实验值与经典的临界波长及临界热流密度理论一致。而粗糙表面上的乙醇沸腾实验进一步发现,波形结构可以减小临界波长,从而有效提高CHF,其影响规律与相关文献的理论模型较为符合。     

CFD modeling for nucleate pool boiling of nanofluids

Boiling is one of the most important processes in almost every industrial heat exchanger arrangement. The present study examines the role played by nanofluids in increasing the heat transfer rate which could improve process efficiency as well as operational cost. The setup consists of a stainless steel vertical cylinder pressure vessel having a horizontal heating rod made of stainless steel submerged in a pool of working fluid (distilled water, alumina/water nanofluid of variable concentration). Simulations were carried out using a 2D geometrical domain in order to calculate values of heat transfer coefficient for different constant heat flux applied on the heater at atmospheric as well as sub atmospheric pressures. For the simulations, a transient Eulerian multiphase involving boiling model was used along with various sub-models involving drag, lift, heat and mass transfer models. The simulated results for the value of heat transfer coefficient were compared and validated from the experimental results.     

A nucleate boiling limitation model for the prediction of pool boiling CHF

A nucleate boiling limitation model which is applicable to the heat transfer prediction in the nucleate boiling region and the CHF was proposed for a pool boiling. The present model was developed based on the direct observations of the physical boiling phenomena. The predicted boiling curves for the nucleate boiling region agree well with both the vertical and the horizontal surface data for all the contact angles. The predicted CHF for the vertical surface also agrees well with the experimental data, but the present model underpredicts the CHF by about 30% for the horizontal surface data.     

Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux

The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids.     

A new mechanistic model for pool boiling CHF on horizontal surfaces

This work proposes a new mechanistic model for predicting the critical heat flux (CHF) in horizontal pool boiling systems. It is postulated that when the vapor momentum flux is sufficient to lift the liquid macrolayer from the heating surface, wetting is no longer feasible, and a transition from nucleate to film boiling occurs. This is the same mechanism that has found success in predicting CHF in flow boiling systems. An experimental investigation of CHF with pentane, hexane, and FC-72 in saturated horizontal pool boiling with chamber pressures of 150, 300, and 450 kPa provides evidence that the new model captures the variation of CHF with pressure reasonably well compared with other well known models. The new model is also compared with existing data from the literature over a reduced pressure range of 2 × 10^?5–2 × 10^?1. The mean deviation between the predicted and measured CHF is typically within 20% over the parameter space covered.     

Effect of surface particle interactions during pool boiling of nanofluids

The pool boiling characteristics of nanofluids is affected by the relative magnitudes of the average surface roughness and the average particle diameter. In the present work, an attempt has been made to study the interactions between the nanoparticles and the heater surface. The experimental methodology accounts for the transient nature of the boiling phenomena. The boiling curves of electro-stabilized Al₂O₃ water-based nanofluids at different concentrations on smooth and rough heaters and the burn-out heat flux have been obtained experimentally. Extensive surface profile characterization has been done using non-intrusive optical measurements and atomic force microscopy. A measure of the surface wettability has been obtained by determining the advancing contact angle. These results give an insight into the relative magnitudes of dominance of the prevalent mechanisms under different experimental conditions. Boiling on nanoparticle coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.     

Subcooled flow boiling CHF enhancement with porous surface coatings

The effect of micro/nanoporous inside surface coated vertical tubes on CHF was determined during water flow boiling at atmospheric pressure. CHF was measured for smooth and three different coated tubes, at mass fluxes (100–300 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Greater CHF enhancement was found with microporous coatings than with nanoporous coatings. Al₂O₃ microporous coatings with particle size <10 μm and coatings thickness of 50 μm showed the best CHF enhancement. Maximum increase in CHF was about 25% for microporous Al₂O₃. A wettability test was performed to study an increase of CHF with microporous coated surfaces.     

Effect of open microchannel geometry on pool boiling enhancement

Pool boiling enhancement through surface modification has garnered the attention of many researchers for extending the limits of heat flux dissipation. Simulated copper chips were used in a pool boiling setup with water boiling at atmospheric pressure. Heat transfer performance of different microchanneled surfaces was compared to that of a plain surface. The results of the study showed that the mechanism at work for the bubble dynamics was the ability of the surface to pull liquid through the channels to induce heat transfer. Geometrical trends were observed from the study as well with the wider and deeper channels and thinner finned surfaces showing the best heat transfer results. Without reaching the critical heat flux condition, the best performing chip dissipated a heat flux of 244 W/cm² corresponding to a record heat transfer coefficient of 269 kW/m² K.     

Photographic study of pool boiling CHF from a downward-facing convex surface

Heat transfer measurements and photographic studies are performed to capture the detailed evolution of the liquid–vapor interface near critical heat flux (CHF) for a 90-degree downward-facing convex surface. The test surface, with a width of 3.2 mm and a 102.6-mm radius, consists of a series of nine heaters that dissipate equal power. Instrumentation within each heater facilitates localized heat flux and temperature measurements along the convex surface, and transparent front and back windows enable optical access to a fairly two-dimensional liquid–vapor interface. Near CHF, vapor behavior along the convex surface is cyclical, repeatedly forming a stratified vapor layer at the bottom of the convex surface, which stretches as more vapor is generated, and then flows upwards along from the surface. Subsequently, heaters at the bottom of the convex surface, followed by the other heaters, are wetted with liquid before the nucleation/coalescence/stratification/release process is repeated. This study proves that despite the pronounced thickening of the vapor layer as it propagates upwards along the convex surface, CHF always commences on the bottom of the surface.     

Flow boiling CHF enhancement with surfactant solutions under atmospheric pressure

Surfactant effect on CHF (critical heat flux) was determined during water flow boiling at atmospheric pressure in closed loop filled with solution of tri-sodium phosphate (TSP, Na₃PO₄ · 12H₂O). TSP was added to the containment sump water to adjust pH level during accident in nuclear power plants. CHF was measured for four different water surfactant solutions in vertical tubes, at different mass fluxes (100–500 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Surfactant solutions (0.05–0.2%) at low mass flux (～100 kg/m² s) showed the best CHF enhancement. CHF was decreased at high mass flux (500 kg/m² s) compared to the reference plain water data. Maximum increase in CHF was about 48% as compared to the reference data. Surfactant caused a decrease in contact angle associated with an increase of CHF from surfactant addition.     

Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids

Although the effects of fluid properties, pressure, and subcooling, as well as heater geometry, on the pool boiling critical heat flux, or CHF, are relatively well established, explanations for the surface property effects remain controversial. Proposed formulations, embodying the dependence of CHF on the product of the heater thermal effusivity and thickness are described and compared with available data. A composite correlation for pool boiling CHF, accounting for the conduction and hydrodynamic limits, as well as the effects of pressure, subcooling, and length, is proposed. This effusivity-based correlation is found to predict a broad range of pool boiling CHF data for dielectric liquids, for thermal effusivity values between 0.2 and 120, pressure from 100 to 450 kPa, and subcoolings from 0 to 75 K, with a standard deviation of 12.5%.     

Investigations on heat transfer enhancement in pool boiling with water-CuO nano-fluids

The main focus of the present work is to investigate Critical Heat Flux (CHF) enhancement using CuO nanofluid relative to CHF of pure water. To estimate the effect of nanoparticles on the CHF, pool boiling CHF values were measured for various volume concentrations of CuO nanofluid and compared with pure water. CHF enhancement of 130% was recorded at 0.2 % by volume of CuO nano-fluids. Surface roughness of the heater surface exposed to three measured heating cycles indicated surface modifications at different volume concentrations of nanofluid. SEM image of the heater surface revealed porous layer build up, which is thought to be the reason for CHF enhancement.     

Sorption and agglutination phenomenon of nanofluids on a plain heating surface during pool boiling

The pool nucleate boiling heat transfer experiments of water (H₂O) based and alcohol (C₂H₅OH) based nanofluids and nanoparticles-suspensions on the plain heated copper surface were carried out. The study was focused on the sorption and agglutination phenomenon of nanofluids on a heated surface. The nanofluids consisted of the base liquid, the nanoparticles and the surfactant. The nanoparticles-suspensions consisted of the base liquid and nanoparticles. The both liquids of water and alcohol and both nanoparticles of CuO and SiO₂ were used. The surfactant was sodium dodecyl benzene sulphate (SDBS). The experimental results show that for nanofluids, the agglutination phenomenon occurred on the heated surface when the wall temperature was over 112 °C and steady nucleated boiling experiment could not be carried out. The reason was that an unsteady porous agglutination layer was formed on the heated surface. However, for nanoparticles-suspensions, no agglutination phenomenon occurred on the heating surface and the steady boiling could be carried out in the whole nucleate boiling region. For the both of alcohol based nanofluids and nano-suspensions, no agglutination phenomenon occurred on the heating surface and steady nucleate boiling experiment could be carried out in the whole nucleate boiling region whose wall temperature did not exceed 112 °C. The boiling heat transfer characteristics of the nanofluids and nanoparticles-suspensions are somewhat poor compared with that of the base fluids, since the decrease of the active nucleate cavities on the heating surface with a very thin nanoparticles sorption layer. The very thin nanoparticles sorption layer also caused a decrease in the solid–liquid contact angle on the heating surface which leaded to an increase of the critical heat flux (CHF).     

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.     

Flow boiling CHF in microgravity

Poor understanding of flow boiling in microgravity has recently emerged as a key obstacle to the development of many types of power generation and life support systems intended for space exploration. This study examines flow boiling CHF in microgravity that was achieved in parabolic flight experiments with FC-72 onboard NASA’s KC-135 turbojet. At high heat fluxes, bubbles quickly coalesced into fairly large vapor patches along the heated wall. As CHF was approached, these patches grew in length and formed a wavy vapor layer that propagated along the wall, permitting liquid access only in the wave troughs. CHF was triggered by separation of the liquid-vapor interface from the wall due to intense vapor effusion in the troughs. This behavior is consistent with, and accurately predicted by the Interfacial Lift-off CHF Model. It is shown that at low velocities CHF in microgravity is significantly smaller than in horizontal flow on earth. CHF differences between the two environments decreased with increasing velocity, culminating in virtual convergence at about 1.5 m/s. This proves it is possible to design inertia-dominated systems by maintaining flow velocities above the convergence limit. Such systems allow data, correlations, and/or models developed on earth to be safely implemented in space systems.