Significant Critical Heat Flux Enhancement Research Articles

Effect of gamma irradiation on the critical heat flux of sintered nano-coated surfaces

The electrophoretic deposition (EPD) technique was used to manufacture uniform SiO2, Al2O3, ZrO2, and TiO2 thin film coatings on thin steel plates (1.5 mm by 90 mm) to study heat transport properties when in contact with water at boiling temperature. To find the thickness and uniformity of the coatings, scanning electron microscope (SEM) images were made. The captured images showed that the coating thickness is uniformly increased by the EPD method. To investigate the effect of sintering, the coated plates were placed in a furnace at various temperatures. To investigate the effect of gamma irradiation on the critical heat flux (CHF) of the coated surfaces, the test specimens were irradiated in a gamma cell with different dose rates, and then BET, SEM, and EDS analyses were performed. Significant enhancement in CHF, for the hydrophilic surfaces, can be achieved by the EPD method. The results showed that the CHF for the sintered nano-coated surfaces, SiO2, Al2O3, ZrO2, and TiO2, increase by 24%, 23%, 19%, and 10%, compared to those of unsintered nanocoated surfaces, respectively. It was observed that irradiation significantly decreases the maximum pore diameter while it increases the porosity, pore surface area, and pore volume. The CHF of the nano-coated surfaces irradiated at 300 kGy increased by 12%, 8%, 5%, and 12% for SiO2, Al2O3, ZrO2, and TiO2, respectively. Nevertheless, the effect of gamma irradiation on the sintered surfaces was insignificant.

Experimental study on the CHF enhancement effect of nanofluids on the oxidized low carbon steel surface

Studies about using nanofluids to enhance the Critical Heat Flux (CHF) of In-vessel Retention (IVR) strategy in the third-generation reactor have been conducted extensively and show a significant CHF enhancement effect. However, low carbon steel SA508 used in the reactor vessel is easy to oxidize and the oxidation can lead to changes in the surface properties which may affect the CHF enhancement effect of nanofluids. In this study, pool boiling CHF experiments with low carbon steel SA508 surfaces were conducted in distilled water and nanofluids under different boiling time to investigate the CHF enhancement effect of nanofluids under low carbon steel surface oxidization condition. CHF in distilled water increases rapidly with boiling time due to the rapid surface oxidation during the boiling and the increase ratio can be nearly 2 due to the surface oxidation. CHF in nanofluids is stable and independent of boiling time. The difference between CHF in nanofluids and CHF in distilled water decrease to 17% under the longest boiling time conditions due to the surface oxidation. The deposition layer of nanoparticles on the surface leads to the capillary wicking and decrease in the nucleation site and thus, CHF is enhanced. This study is of great significance for exploring the actual effect of nanofluids on the CHF enhancement of IVR strategy.

Pool Boiling Heat Transfer and CHF enhancement of Aqueous Al2O3 and CuO Nanofluid

In this study, experiments are performed to understand the critical heat flux (CHF) enhancement using nanofluids, with special emphasis on pool boiling. Al2O3 and CuOnanopowder were purchased from Sisco Research Laboratories (SRL) having particle size of ∼25 nm and ∼40 nm respectively. The critical heat flux measurement at atmospheric pressure for three different concentrations (0.01gm/lt, 0.1gm/lt and 1gm/lt) of Al2O3 and CuOnano particles suspension in water was taken and pool boiling experiment was carried out at atmospheric pressure using 0.111mm Nichrome wire submerged in each solution. The significant enhancement in CHF has been recorded with each of the nano solutions as compared to the pure water. With increasing the concentration of thenanoparticles the CHF was found to increase as compared to deionised water.

Effect of heater orientation on pool boiling heat transfer from sintered copper microporous coating in saturated water

In this study, we investigate the heater orientation effects at inclination angles from 0° (horizontal upward) to 180° (horizontal downward) on pool boiling heat transfer of saturated water using a durable high-temperature thermally-conductive microporous coating (HTCMC) created by sintering copper powder of an average particle size of 67μm onto a 1cm×1cm plain copper surface with ∼300μm thickness. The HTCMC surface showed a critical heat flux (CHF) of ∼2MW/m2 and a maximum nucleate boiling heat transfer (NBHT) coefficient of ∼400kW/m2K, which are 2 and 8 times higher than those of a plain copper surface, respectively, at the upward horizontal inclination angle. The current experimental results showed that the CHF values of the HTCMC were maintained as ∼2MW/m2 at upward inclination angles from 0° to 90°, whereas the CHF values decreased as the inclination angle changed from 90o to 180o, at which point the values were ∼1.4MW/m2. The reduction at 180° is due to individual bubbles merging and forming larger bubbles by blockage from the surface. This increases the vapor residence time on the surface and prevents liquid access to the heated surface at downward inclination angles.However, it is noted that the CHF value of the HTCMC at 180° is ∼4.5 times higher than that of plain copper surface (0.3MW/m2 at 180°). HTCMC also shows a noticeably higher CHF value at 180° compared to nanocoating or other enhanced surfaces. This significant CHF enhancement is believed to be due to a large number of small feed jet bubbles created from porous structures, postponing the dryout caused by forming large bubbles on the heated surface.

Saturated pool boiling heat transfer from vertically oriented silicon surfaces modified with foam-like hexagonal boron nitride nanomaterials

In this work, the ability of a foam-like hierarchical hexagonal boron nitride (h-BN) nanomaterial to act as a scalable surface modifier for improved pool boiling performance is investigated. The h-BN foam samples were grown using an atmospheric pressure chemical vapor deposition process onto the surfaces of open-cell nickel foam followed by selective etching. Saturated pool boiling experiments were conducted in degassed deionized water for unaltered silicon surfaces, silicon surfaces modified with planar or foam-like h-BN materials plus adhesion layer, and a silicon sample with only the adhesion layer present. Results from the unaltered silicon surface obtained in this study were found to be in excellent agreement with previous studies. For the planar h-BN control sample the observed critical heat flux (CHF) occurred at 24.2±2.4W/cm2 at wall superheat of 28.0±2.1K, a factor of three lower CHF than for the unaltered Si surface. CHF for h-BN foam samples occurred at 69.1±6.9W/cm2 at wall superheat of 25.0±2.1K and 87.9±8.8W/cm2 at wall superheat of 24.9±2.1K. These CHF results are comparable to the bare silicon values but significantly enhanced compared to the adhesion layer only and planar h-BN samples, thereby indicating foam-like h-BN nanomaterials have the potential to facilitate significant CHF enhancement but that means of integration is critical to determining overall performance. Comparison of boiling curves reveals the h-BN foam samples demonstrated higher heat transfer coefficient at moderate and high heat flux relative to bare silicon, with CHF being reached at ∼30% lower wall superheat.

Flow boiling heat transfer enhancement on copper surface using Fe doped Al2O3–TiO2 composite coatings

In the present work, flow boiling experiments were conducted to study the effect of spray pyrolyzed Fe doped Al2O3–TiO2 composite coatings over the copper heater blocks on critical heat flux (CHF) and boiling heat transfer coefficient. Heat transfer studies were conducted in a mini-channel of overall dimension 30mm×20mm×0.4mm using de-mineralized water as the working fluid. Each coated sample was tested for two mass fluxes to explore the heat transfer performance. The effect of Fe addition on wettability and porosity of the coated surfaces were measured using the static contact angle metre and the atomic force microscope (AFM), and their effect on flow boiling heat transfer were investigated. A significant enhancement in CHF and boiling heat transfer coefficient were observed on all coated samples compared to sand blasted copper surface. A maximum enhancement of 52.39% and 44.11% in the CHF and heat transfer coefficient were observed for 7.2% Fe doped TiO2–Al2O3 for a mass flux of 88kg/m2s.

Flow boiling phenomena in a single annular flow regime in microchannels (II): Reduced pressure drop and enhanced critical heat flux

In Part II of this study, we report that pressure drop was reduced by approximately 48% and critical heat flux (CHF) was increased by approximately 300% in SiNW microchannels compared to these in smooth wall microchannels. The hydraulic characteristics of the single annular flow were systematically investigated to reveal the mechanisms responsible for the reduced pressure drop and enhanced CHF. In the single annular regime, the liquid and vapor flows were nearly fully separated during the entire flow boiling process (i.e., from the onset of nucleate boiling to the CHF conditions). Moreover, the entrainment droplets were reduced by flattening the profile of the liquid–vapor interfaces using the high capillary pressure generated by SiNWs. These two factors, i.e., flow separation and reduced entrainment droplets, lead to a dramatic reduction of frictional pressure drop. The separation of liquid and vapor flows as well as the improved global and local liquid supply result in a significant CHF enhancement without using inlet restrictors (IR). Reynolds number based the vapor flow at the exit ranged from 0.1 to 2100.

EFFECT OF SOLUBLE ADDITIVES, BORIC ACID (H3BO3) AND SALT (NaCl), IN POOL BOILING HEAT TRANSFER

The effects on pool boiling heat transfer of aqueous solutions of boric acid (H₃BO₃) and sodium chloride (NaCl) as working fluids have been studied. Borated and NaCl water were prepared by dissolving 0.5~5% volume concentration of boric acid and NaCl in distilled-deionized water. The pool boiling tests were conducted using 1 x 1 ㎠ flat heaters at 1 atm. The critical heat flux (CHF) dramatically increased compared to boiling pure water. At the end of boiling tests it was observed that particles of boric acid and NaCl had deposited and formed a coating on the heater surface. The CHF enhancement and surface modification during boiling tests were very similar to those obtained from boiling with nanofluids. Additional experiments were carried out to investigate the reliability of the additives deposition in pure water. The boric acid and NaCl coatings disappeared after repeated boiling tests on the same surface due to the soluble nature of the coatings, thus CHF enhancement no longer existed. These results demonstrate that not only insoluble nanoparticles but also soluble salts can be deposited during boiling process and the deposited layer is solely responsible for significant CHF enhancement.

Thermal Physics and Critical Heat Flux Characteristics of Al2O3–H2O Nanofluids

This study investigates the influence of the thermal physics of nanofluids on the critical heat flux (CHF) of nanofluids. Thermal physics tests of nanoparticle concentrations ranged from 0 to 1 g/L. Pool boiling experiments were performed using electrically heated NiCr metal wire under atmospheric pressure. The results show that there was no obvious change for viscosity and a maximum enhancement of about 5 to 7% for thermal conductivity and surface tension with the addition of nanoparticles into pure water. Consistently with other nanofluid studies, this study found that a significant enhancement in CHF could be achieved at modest nanoparticle concentrations (<0.1 g/L by Al2O3 nanoparticle concentration). Compared to the CHF of pure water, an enhancement of 113% over that of nanofluids was found. Scanning electron microscope photos showed there was a nanoparticle layer formed on the heating surface for nanofluid boiling. The bubble growth was photographed by a camera. The coating layer makes the nucleation of vapor bubbles easily formed. Thus, the addition of nanoparticles has active effects on the CHF.

Pool boiling CHF enhancement by micro/nanoscale modification of zircaloy-4 surface

Consideration of the critical heat flux (CHF) requires difficult compromises between economy and safety in many types of thermal systems, including nuclear power plants. Much research has been directed towards enhancing the CHF, and many recent studies have revealed that the significant CHF enhancement in nanofluids is due to surface deposition of nanoparticles. The surface deposition of nanoparticles influenced various surface characteristics. This fact indicated that the surface wettability is a key parameter for CHF enhancement and so is the surface morphology. In this study, surface wettability of zircaloy-4 used as cladding material of fuel rods in nuclear power plants was modified using surface treatment technique (i.e. anodization). Pool boiling experiments of distilled water on the prepared surfaces was conducted at atmospheric and saturated conditions to examine effects of the surface modification on CHF. The experimental results showed that CHF of zircaloy-4 can be significantly enhanced by the improvement in surface wettability using the surface modification, but only the wettability effect cannot explain the CHF increase on the treated zircaloy-4 surfaces completely. It was found that below a critical value of contact angle (10°), micro/nanostructures created by the surface treatment increased spreadability of liquid on the surface, which could lead to further increase in CHF even beyond the prediction caused only by the wettability improvement. These micro/nanostructures with multiscale on heated surface induced more significant CHF enhancement than it based on the wettability effect, due to liquid spreadability.

Alumina Nanoparticles Enhance the Flow Boiling Critical Heat Flux of Water at Low Pressure

Many studies have shown that addition of nanosized particles to water enhances the critical heat flux (CHF) in pool boiling. The resulting colloidal dispersions are known in the literature as nanofluids. However, for most potential applications of nanofluids the situation of interest is flow boiling. This technical note presents first-of-a-kind data for flow boiling CHF in nanofluids. It is shown that a significant CHF enhancement (up to ∼30%) can be achieved with as little as 0.01% by volume concentration of alumina nanoparticles in flow experiments at atmospheric pressure, low subcooling (&lt;20°C), and relatively high mass flux (⩾1000kg∕m2s).