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.     

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.     

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%.     

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.     

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.     

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.     

Pool boiling characteristics of low concentration nanofluids

The pool boiling behavior of low concentration nanofluids (?1 g/l) was experimentally studied over a flat heater at 1 atm. Boiling of nanofluids produces a thin nanoparticle film, on the heater surface, which in turn is believed to increase the critical heat flux. The present study also indicates that the nanoparticle deposition results in transient characteristics in the nucleate boiling heat transfer. Finally, this study investigates possible causes responsible for the deposition of nanoparticle on the heater surface. Experimental evidence shows that microlayer evaporation, during nanofluid boiling, is responsible for the nanoparticle coating formed on the heater surfaces.     

Critical heat flux on flow boiling of methanol–water mixtures in a diverging microchannel with artificial cavities

This study constitutes an experimental investigation into the convective boiling heat transfer and critical heat flux (CHF) of methanol–water mixtures in a diverging microchannel with artificial cavities. Flow visualization shows that bubbles are generally nucleated at both the artificial cavities and side walls of the channel. This confirms the proper functioning of such artificial cavities. Consequently, the wall superheat of the onset nucleate boiling is significantly reduced. Experimental results show that the boiling heat transfer and CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. The CHF increases with an increase in mass flux at the same molar fraction. On the other hand, the CHF increases slightly from x_m = 0 to 0.3, and then decreases rapidly from x_m = 0.3 to 1 at the same mass flux. The maximum CHF is reached at x_m = 0.3, particularly for a mass flux of 175 kg/m² s, due to the Marangoni effect. Flow visualization confirms that the Marangoni effect helps a region with a liquid film breakup persist to a higher heat flux, and therefore a higher CHF. Moreover, a new empirical correlation involving the Marangoni effect for the CHF on the flow boiling of methanol–water mixtures is developed. The present correlation prediction shows excellent agreement with the experimental data, and further confirms that the present correlation may predict the Marangoni effect on the CHF for the convective boiling heat transfer of binary mixtures.     

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.     

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite measuring 10 mm × 10 mm is investigated, and results are compared with those for smooth copper (Cu) of the same dimensions. Although liquid is out-gassed for hours before performing the pool boiling experiments, air entrapped in re-entrant type cavities, ranging in size from tens to hundreds of microns, not only enhanced the nucleate boiling heat transfer and the critical heat flux (CHF), but also, the mixing by the released tiny air bubbles from the porous graphite prior to boiling incipience enhanced the natural convection heat transfer by ∼19%. No temperature excursion is associated with the nucleate boiling on porous graphite, which ensues at very low surface superheat of 0.5–0.8 K. Conversely, the temperature overshoot at incipient boiling on Cu is as much as 39.2, 36.6, 34.1 and 32.8 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. Nucleate boiling ensues on Cu at a surface superheat of 11.9, 10.9, 9.5 and 7.5 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. The saturation nucleate boiling heat flux on porous graphite is 1700% higher than that on Cu at a surface superheat of ∼10 K and decreases exponentially with increased superheat to ∼60% higher near CHF. The CHF values of HFE-7100 on porous graphite of 31.8, 45.1, 55.9 and 66.4 W/cm² in 0 (saturation), 10, 20 and 30 K subcooled boiling, are 60% higher and the corresponding superheats are 25% lower than those on Cu. In addition, the rate of increase in CHF with increased liquid subcooling is 50% higher than that on Cu.     

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x_m = 0–0.1, and then decreases rapidly from x_m = 0.1–1 at a given mass flux of 175 kg/m² s. The maximum CHF is reached at x_m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.     

New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes

A new flow boiling heat transfer model and a new flow pattern map based on the flow boiling heat transfer mechanisms for horizontal tubes have been developed specifically for CO₂. Firstly, a nucleate boiling heat transfer correlation incorporating the effects of reduced pressure and heat flux at low vapor qualities has been proposed for CO₂. Secondly, a nucleate boiling heat transfer suppression factor correlation incorporating liquid film thickness and tube diameters has been proposed based on the flow boiling heat transfer mechanisms so as to capture the trends in the flow boiling heat transfer data. In addition, a dryout inception correlation has been developed. Accordingly, the heat transfer correlation in the dryout region has been modified. In the new flow pattern map, an intermittent flow to annular flow transition criterion and an annular flow to dryout region transition criterion have been proposed based on the changes in the flow boiling heat transfer trends. The flow boiling heat transfer model predicts 75.5% of all the CO₂ database within ±30%. The flow boiling heat transfer model and the flow pattern map are applicable to a wide range of conditions: tube diameters (equivalent diameters for non-circular channels) from 0.8 to 10 mm, mass velocities from 170 to 570 kg/m² s, heat fluxes from 5 to 32 kW/m² and saturation temperatures from −28 to 25 °C (reduced pressures from 0.21 to 0.87).     

Boiling heat transfer in rectangular microchannels with reentrant cavities

This paper investigates flow boiling of water in microchannels with a hydraulic diameter of 227 μm possessing 7.5 μm wide reentrant cavities on the sidewalls. Average two-phase heat transfer coefficients and CHF conditions have been obtained over a range of effective heat fluxes (28–445 W/cm²) and mass velocities (41–302 kg/m² s). High Boiling number and Reynolds number have been found to promote convective boiling, while Nucleate Boiling dominated at low Reynolds number and Boiling number. A criterion for the transition between nucleate and convective boiling has been provided. Existing correlations did not provide satisfactory agreement with the heat transfer coefficient but did predict CHF conditions well.     

Turbulent forced convection effectiveness of alumina–water nanofluid in a circular tube with elevated inlet fluid temperatures: An experimental study

The present study aims to explore experimentally the influence of elevated inlet fluid temperature on the turbulent forced convective heat transfer effectiveness of using alumina–water nanofluid over pure water in an iso-flux heated horizontal circular tube at a fixed heating power. A copper circular pipe of inner diameter 3.4 mm was used in the forced convection experiments undertaken for the pertinent parameters in the following ranges: the inlet fluid temperature, T_in = 25 °C, 37 °C and 50 °C; the Reynolds number, Re_bf = 3000–13,000; the mass fraction of the alumina nanoparticles in the water-based nanofluid formulated, ω_np = 0, 2, 5, and 10 wt.%; and the heating flux, q_o^″ = 57.8–63.1 kW/m². The experimental results clearly indicate that the turbulent forced convection heat transfer effectiveness of the alumina–water nanofluid over that of the pure water can be further uplifted by elevating its inlet temperature entering the circular tube well above the ambient, thereby manifesting its potential as an effective warm functional coolant. Specifically, an increase in the averaged heat transfer enhancement of more than 44% arises for the nanofluid of ω_np = 2 wt.% as the inlet fluid temperature is increased from 25 °C to 50 °C.     

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.     

Enhancing the onset of pool boiling by wettability modification on nanometrically smooth surfaces

Pool boiling experiments are performed with degassed water on highly smooth surfaces of two different wettabilities: hydrophilic and hydrophobic cases. Boiling curves and visual observations on the boiling have been performed. The onset of nucleate boiling (ONB) has been measured and the influence of the wettability has been quantified. As the inherent mean roughness of the glass substrates was lower than one nanometer it was possible to show the sole effect of the wettability. No hysteresis in the boiling curve was observed for both cases. The ONB was observed after 3.5 °C superheat on the hydrophobic case and the heat transfer coefficient (HTC) changed suddenly from the one of a convection regime (1.5 kW/m² K) to the one of a nucleate boiling regime (4 kW/m² K). On the contrary for the hydrophilic case, despite superheat above 37 °C and presence of boiling, the HTC was kept as the one of the convection regime.     

Comparison study of liquid replenishing impacts on critical heat flux and heat transfer coefficient of nucleate pool boiling on multiscale modulated porous structures

The critical heat flux (CHF) and heat transfer coefficient of de-ionized (DI) water pool boiling have been experimentally studied on a plain surface, one uniform thick porous structure, two modulated porous structures and two hybrid modulated porous structures. The modulated porous structure design has a porous base of 0.55 mm thick with four 3 mm diameter porous pillars of 3.6 mm high on the top of the base. The microparticle size combinations of porous base and porous pillars are uniform 250 μm, uniform 400 μm, 250 μm for base and 400 μm for pillars, and 400 μm for base and 250 μm for pillars. Both the CHF and heat transfer coefficient are significantly improved by the modulated porous. The boiling curves for different kinds of porous structures and a plain surface are compared and analyzed. Hydrodynamic instability for the two-phase change heat transfer has been delayed by the porous pillars which dramatically enhances the CHF. The highest pool boiling heat flux occurring on the modulated porous structures has a value of 450 W/cm², over three times of the CHF on a plain surface. Additionally, the highest heat transfer coefficient also reaches a value of 20 W/cm² K, three times of that on a plain copper surface. The study also demonstrates that the horizontal liquid replenishing is equally important as the vertical liquid replenishing for the enhancement of heat transfer coefficient and CHF improvement in nucleate pool boiling.     

Effects of high subcooling on two-phase spray cooling and critical heat flux

Experiments were performed with FC-77 using three full-cone spray nozzles to assess the influence of subcooling on spray performance and critical heat flux (CHF) from a 1.0 × 1.0 cm² test surface. The relatively high boiling point of FC-77 (97 °C at one atmosphere) enabled testing at relatively high levels of subcooling. Increasing the subcooling delayed the onset of boiling but decreased the slope of the nucleate boiling region of the spray boiling curve. The enhancement in CHF was relatively mild at low subcooling and more appreciable at high subcooling. CHF was enhanced by about a 100% when subcooling was increased from 22 to 70 °C, reaching values as high as 349 W/cm². The FC-77 data were combined with prior spray CHF data from several studies into a broad CHF database encompassing different nozzles, fluids, flow rates, spray orientations, and subcoolings. The entire CHF database was used to modify the effect of subcooling in a previous CHF correlation that was developed for relatively low subcoolings. The modified correlation shows excellent predictive capability.     

Heat transfer characteristics during subcooled flow boiling of a kerosene kind hydrocarbon fuel in a 1 mm diameter channel

The subcooled flow boiling heat transfer characteristics of a kerosene kind hydrocarbon fuel were investigated in an electrically heated horizontal tube with an inner diameter of 1.0 mm, in the range of heat flux: 20–1500 kW/m², fluid temperature: 25–400 °C, mass flux: 1260–2160 kg/m² s, and pressure: 0.25–2.5 MPa. It was proposed that nucleate boiling heat transfer mechanism is dominant, as the heat transfer performance is dependent on heat flux imposed on the channel, rather than the fuel flow rate. It was found that the wall temperatures along the test section kept constant during the fully developed subcooled boiling (FDSB) of the non-azeotropic hydrocarbon fuel. After the onset of nucleate boiling, the temperature differences between inner wall and bulk fluid begin to decrease with the increase of heat flux. Experimental results show that the complicated boiling heat transfer behavior of hydrocarbon fuel is profoundly affected by the pressure and heat flux, especially by fuel subcooling. A correlation of heat transfer coefficients varying with heat fluxes and fuel subcooling was curve fitted. Excellent agreement is obtained between the predicted values and the experimental data.     

Natural convection in a rectangular enclosure containing an oval-shaped heat source and filled with Fe3O4/water nanofluid

This work concerns with the study of natural convection heat transfer in rectangular cavities with an inside oval-shaped heat source filled with Fe₃O₄/water nanofluid. The finite element method is employed to solve the governing equations for this problem. Average Nusselt numbers are presented for a wide range of Rayleigh number (10³ ≤ Ra ≤ 10⁵), volume fraction of nanoparticles (0 ≤ ϕ ≤ 14%), and four different size and shapes of the heat source. Depending on concentration of the nanoparticle, geometry of the heat source, and the value of Rayleigh number different behaviors are monitored for average Nusselt numbers. Configuration of the heat source dictates a significant change on the behavior of the average Nusselt number, while addition of the nanoparticles has a negative effect on the magnitude of Nusselt number for this problem.