Visualization study of the effects of nanoparticles surface deposition on convective flow boiling CHF from a short heated wall

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. Changing flow velocity from 0 m/s to 4 m/s, the water boiling on nanoparticles-coated heater was conducted and CHF increased at a given velocity. To understand clearly the mechanism of flow boiling CHF enhancement in nanofluid, the visualization of the nucleate boiling and CHF phenomenon was conducted using the high-speed video camera. It was found that the boiling heat transfer on the nanoparticles-coated heater was lower than that on bare heater, which induced the different flow regime at same heat flux. The different wetting zone on bare and nanoparticles-coated heaters was observed by visualization study. Based the wetting zone fraction, there was brief that the nucleate boiling fraction on heater would be related with the surface wettability. A new concept of flow boiling model was proposed based on the wetting zone fraction. Finally, the effect of nanoparticles deposition layer on the heater was interpreted with the physical mechanisms to increase CHF.     

Boiling heat transfer characteristics of nanofluids jet impingement on a plate surface

The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface was experimentally investigated. The experimental results were compared with those from water. The quantificational effects of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux (CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the surface.     

Boiling characteristics in small vertical tubes with closed bottom for nanofluids and nanoparticle-suspensions

An experimental study was carried out to understand the nucleate boiling characteristics and the critical heat flux (CHF) of water, the water based nanofluids and the water based nanoparticle-suspensions in vertical small heated tubes with a closed bottom. Here, the nanofluids consisted of the base liquid, the CuO nanoparticles and the surfactant. The nanoparticle-suspensions consisted of the base liquid and CuO nanoparticles. The surfactant was sodium dodecyl benzene sulfate. The study focused on the influence of the nanoparticles and surfactant on the nucleate boiling characteristics and the CHF. The experimental results indicated that the nanoparticle concentrations of the nanofluids and nanoparticle-suspensions in the tubes do not change during the boiling processes; the nanoparticles in the evaporated liquid are totally carried away by the steam. The boiling heat transfer rates of nanofluids are poorer than that of the base liquid. However, the boiling heat transfer rates of nanoparticle-suspensions are better than that of the base liquid. Comparing with the base liquid, the CHF of the nanofluids and the nanoparticle-suspensions is higher. The CHF is only related to nanoparticle mass concentration when the tube length and the tube diameter are fixed. The experiment confirm that there is a thin nanoparticle coating layer on the heated surface after the nanofluids boiling test but there is no coating layer on the heated surface after the nanoparticle-suspensions boiling test. This coating layer is the main reason that deteriorates the boiling heat transfer rates of nanofluids. An empirical correlation was proposed for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom.     

Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface

An experimental study was performed to understand the nucleate boiling heat transfer of water–CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations. The experimental apparatus is a miniature flat heat pipe (MFHP) with micro-grooved heat transfer surface of its evaporator. The experimental results indicate that the operating pressure has great influence on the nucleate boiling characteristics in the MFHP evaporator. The heat transfer coefficient and the critical heat flux (CHF) of nanofluids increase greatly with decreasing pressure as compared with those of water. The heat transfer coefficient and the CHF of nanofluids can increase about 25% and 50%, respectively, at atmospheric pressure whereas about 100% and 150%, respectively, at the pressure of 7.4 kPa. Nanoparticle mass concentration also has significant influence on the boiling heat transfer and the CHF of nanofluids. The heat transfer coefficient and the CHF increase slowly with the increase of the nanoparticle mass concentration at low concentration conditions. However, when the nanoparticle mass concentration is over 1.0 wt%, the CHF enhancement is close to a constant number and the heat transfer coefficient deteriorates. There exists an optimum mass concentration for nanofluids which corresponds to the maximum heat transfer enhancement and this optimum mass concentration is 1.0 wt% at all test pressures. The experiment confirmed that the boiling heat transfer characteristics of the MFHP evaporator can evidently be strengthened by using water/CuO nanofluids.     

Boiling induced nanoparticle coating and its effect on pool boiling heat transfer on a vertical cylindrical surface using CuO nanofluids

Experiments were performed to study boiling induced nanoparticle coating and its influence on pool boiling heat transfer using low concentrations of CuO- nanofluid in distilled water at atmospheric pressure. To investigate the effect of the nanoparticle coated surface on pool boiling performance, two different concentrations of CuO nanofluids (0.1 and 0.5?g/l) were chosen and tests were conducted on a clean heater surface in nanofluid and nanoparticle coated surface in pure water. For the bare heater tested in CuO nanofluid, CHF was enhanced by 35.83 and 41.68?% respectively at 0.1 and 0.5?g/l concentration of nanofluid. For the nanoparticle coated heater surface obtained by boiling induced coating using 0.1 and 0.5?g/l concentration of nanofluid and tested in pure water, CHF was enhanced by 29.38 and 37.53?% respectively. Based on the experimental investigations it can be concluded that nanoparticle coating can also be a potential substitute for enhancing the heat transfer in pure water. Transient behaviour of nanofluid was studied by keeping heat flux constant at 1,000 and 1,500?kW/m² for 90?min in 0.5?g/l concentration. The boiling curve shifted to the right indicating degradation in boiling heat transfer due to prolonged exposure of heater surface to nanofluid. Experimental outcome indicated that pool boiling performance of nanofluid could be a strong function of time and applied heat flux. The longer the duration of exposure of the heater surface, the higher will be the degradation in heat transfer.     

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m^?2, mass fluxes range of 353–1,059 kg m^?2 s^?1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.     

Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids

The pool boiling critical heat flux (CHF) performance of aqueous nanofluids was investigated using various nanoparticles of TiO₂, Al₂O₃, and SiO₂. The usage of a nanofluid as a working fluid can significantly enhance the pool boiling CHF, which was found to be strongly dependent on the kind of nanoparticle, as well as its concentration. A nanoparticle surface coating was observed on the heating surface after the experiment. The CHF of pure water on the nanoparticle-coated surface was higher than that of nanofluids for all cases. This revealed that the cause of the CHF enhancement using nanofluids is due to the fact that the heater surface is modified by the nanoparticle deposition. The mechanism of CHF enhancement due to the nanoparticle coating was discussed, relating it to surface wettability, surface roughness, and maximum capillary wicking height of the nanoparticle-coated surface.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Convective heat transfer of nanofluids with correlations

To investigate the convective heat transfer of nanofluids, experiments were performed using silver–water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver–water nanofluid, with ±10% agreement between experiments and prediction.     

Turbulent forced convection heat transfer of non-Newtonian nanofluids

Forced convection heat transfer of non-Newtonian nanofluids in a circular tube with constant wall temperature under turbulent flow conditions was investigated experimentally. Three types of nanofluids were prepared by dispersing homogeneously γ-Al₂O₃, TiO₂ and CuO nanoparticles into the base fluid. An aqueous solution of carboxymethyl cellulose (CMC) was used as the base fluid. Nanofluids as well as the base fluid show shear-thinning (pseudoplastic) rheological behavior. Results indicate that the convective heat transfer coefficient of nanofluids is higher than that of the base fluid. The enhancement of the convective heat transfer coefficient increases with an increase in the Peclet number and the nanoparticle concentration. The increase in the convective heat transfer coefficient of nanofluids is greater than the increase that would be observed considering strictly the increase in the effective thermal conductivity of nanofluids. Experimental data were compared to heat transfer coefficients predicted using available correlations for purely viscous non-Newtonian fluids. Results show poor agreement between experimental and predicted values. New correlation was proposed to predict successfully Nusselt numbers of non-Newtonian nanofluids as a function of Reynolds and Prandtl numbers.     

Influence of nanoparticle surface coating on pool boiling

The purpose of this work is to study the effects of nanostructured surface coatings on boiling heat transfer and CHF. Boiling experiments are performed on a 100 μm diameter platinum wire immersed in saturated water or pentane at 1 bar. Nanostructured surface coating is obtained by deposition of charged γ-Fe₂O₃ nanoparticles (average diameter of 10 nm) on the platinum wire. Two different processes are compared: vigorous boiling and electrophoresis.The deposition of nanoparticles onto the heated surface induces a significant increase of the boiling critical heat flux (CHF) related to the increase of wettability. It also induces a decrease of the heat transfer coefficient when the wire is entirely covered with nanoparticles. The critical heat flux enhancement depends on the wettability of the fluid compared with the bare heater. Different physical mechanisms are also studied to explain the evolution of the characteristic parameters of the boiling on nanostructured surfaces.     

Recent achievements in the thermal hydraulics of high heat flux components in fusion reactors

Some components of fusion thermonuclear reactors, such as divertors, plasma limiters, or first-wall armor, are believed to be subjected to operating conditions characterized by extremely high thermal loads. It is therefore necessary to remove from the surface of these components very high heat fluxes, ranging from 2 to 60 MW/m². Water subcooled flow boiling, under conditions of high mass flux, high liquid subcooling, and small to intermediate channel diameter, can accomodate these very high heat fluxes. Further enhancement of the upper limit of cooling, the critical heat flux (CHF), can be obtained by making use of turbulence promoters such as twisted tapes and coiled wires even if coupled with a relevant increase in pressure drop. An overview is presented of recent achievements obtained in water subcooled flow boiling CHF under operating conditions of interest to the thermal hydraulic design of fusion reactors. Observed basic parametric trends—CHF as a function of mass flux, pressure, subcooling, and channel geometry—are outlined, together with findings on the use of CHF enhancement techniques. From experiments it was seen that water subcooled flow boiling allows CHF conditions as high as 228 MW/m² to be achieved under extreme geometric and thermal hydraulic conditions. On the other hand, design and engineering boundary conditions limit variation in these conditions, and a suitable compromise has not yet been reached. Predictive tools are presented for the evaluation of subcooled flow boiling CHF both in straight tubes and with twisted tapes, and are assessed with reference to recent available experimental data.

Although several indications for practical applications can be found in recent achievements, a full understanding of the basic mechanisms of heat transfer and CHF in subcooled flow boiling has not yet been achieved. Future research to overcome the present lack of knowledge in this field is suggested.     

Effects of nano-fluid and surfaces with nano structure on the increase of CHF

Critical heat flux (CHF) has necessitated inconvenient compromises between economy and safety in most industries related to thermal systems. Recent development of nanotechnology has enabled synthesis of nano-sized particles and development of new heat transfer fluids with suspended nano-sized particles, i.e., nanofluids. When nanofluids were used in boiling heat transfer cooling, anomalous increase of CHF was reported. Subsequently, nanoparticle deposition on the boiling surface was revealed to contribute to CHF enhancement. Research on surface characteristics determined that three major characteristics affect CHF: wettability, liquid spreadability and multi-scale geometry. We fabricated artificially modified surfaces with arrays of octagonal micro-posts, or ZnO nanorods, or both, and measured their performance in enhancing CHF. The presence of three major characteristics enhanced CHF most.     

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m² s, heat flux 120–270 kW/m², vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.     

Visualization of boiling phenomena in inclined rectangular gap

An experimental study was performed to investigate the pool boiling critical heat flux (CHF) in one-dimensional inclined rectangular channels by changing the orientation of a copper test heater assembly. In a pool of saturated water under the atmospheric pressure, the test parameters included the gap sizes of 1, 2, 5, and 10 mm, and the surface orientation angles from the downward-facing position (180°) to the vertical position (90°). Tests were conducted on the basis of the visualization of boiling phenomena in the narrowly confined channel and open periphery utilizing a high-speed digital camera. To prevent the heat loss from the water pool and copper test heater, a state-of-the-art vacuum pumping technique was introduced. It was observed that the CHF generally decreased as the surface inclination angle increased and as the gap size decreased. In the downward-facing position (180°), however, the vapor movement was enhanced by the gap structure, which produced the opposing result; that is, the CHF increased as the gap size decreased. Phenomenological characteristics regarding the interfacial instability of vapor layer were addressed in terms of visualization approaching the CHF. It was found that there exists a transition angle, around which the CHF changes with a rapid slope.     

Experimental investigation of forced convective heat transfer coefficient in nanofluids of Al2O3/EG and CuO/EG in a double pipe and plate heat exchangers under turbulent flow

Nanofluid is the term applied to a suspension of solid, nanometer-sized particles in conventional fluids; the most prominent features of such fluids include enhanced heat characteristics, such as convective heat transfer coefficient, in comparison to the base fluid without considerable alterations in physical and chemical properties. In this study, nanofluids of aluminum oxide and copper oxide were prepared in ethylene glycol separately. The effect of forced convective heat transfer coefficient in turbulent flow was calculated using a double pipe and plate heat exchangers. Furthermore, we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. We also evaluated the effects of particle concentration and operating temperature on the forced convective heat transfer coefficient of the nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the base fluid, ranging from 2% to 50%. Moreover, the results indicate that with increasing nanoparticles concentration and nanofluid temperature, the convective heat transfer coefficient of nanofluid increases. Our experiments revealed that in lower temperatures, the theoretical and experimental findings coincide; however, in higher temperatures and with increased concentrations of the nanoparticles in ethylene glycol, the two set of results tend to have growing discrepancies.     

Laminar convective heat transfer of non-Newtonian nanofluids with constant wall temperature

Nanofluids are obtained by dispersing homogeneously nanoparticles into a base fluid. Nanofluids often exhibit higher heat transfer rate in comparison with the base fluid. In the present study, forced convection heat transfer under laminar flow conditions was investigated experimentally for three types of non-Newtonian nanofluids in a circular tube with constant wall temperature. CMC solution was used as the base fluid and γ-Al₂O₃, TiO₂ and CuO nanoparticles were homogeneously dispersed to create nanodispersions of different concentrations. Nanofluids as well as the base fluid show shear thinning (pseudoplastic) rheological behavior. Results show that the presence of nanoparticles increases the convective heat transfer of the nanodispersions in comparison with the base fluid. The convective heat transfer enhancement is more significant when both the Peclet number and the nanoparticle concentration are increased. The increase in convective heat transfer is higher than the increase caused by the augmentation of the effective thermal conductivity.     

Pool boiling heat transfer performance of Newtonian nanofluids

Experimental measurements were carried out on the boiling heat transfer characteristics of γ-Al₂O₃/water and SnO₂/water Newtonian nanofluids. Nanofluids are liquid suspensions containing nanoparticles with sizes smaller than 100 nm. In this research, suspensions with different concentrations of γ-Al₂O₃ and SnO₂ nanoparticles in water were studied under nucleate pool boiling heat transfer conditions. Results show that nanofluids possess noticeably higher boiling heat transfer coefficients than the base fluid. The boiling heat transfer coefficients depend on the type and concentration of nanoparticles.     

Effect of ionic additive on pool boiling critical heat flux of titania/water nanofluids

TiO₂/water nanofluids were prepared and tested to investigate the effects of an ionic additive (i.e., nitric acid in this study) on the critical heat flux (CHF) behavior in pool boiling. Experimental results showed that the ionic additive improved the dispersion stability but reduced the CHF increase in the nanofluid. The additive affected the self-assembled nanoparticle structures formed on the heater surfaces by creating a more uniform and smoother structure, thus diminishing the CHF enhancement in nanofluids.     

Nucleate boiling heat transfer in aqueous solutions with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60 °C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001%, 0.001%, 0.01%, and 0.05%. For the dispersion of CNTs, polyvinyl pyrrolidone polymer is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime. On the other hand, critical heat flux of the aqueous solution is enhanced greatly showing up to 200% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat fluxes and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results in.