Critical Heat Flux Enhancement Mechanism Research Articles

Acoustic sensing for investigating critical heat flux enhancement during pool boiling on electrodeposited copper foams

Nano-engineered surfaces have shown great promise in improving boiling heat flux and heat transfer coefficient for thermal management. Electrodeposited copper foams have attracted lots of scientific and engineering interest owing to their multi-tier structures. In this study, hierarchical copper foams with ultra-wicking properties were fabricated using cathodic deposition at different pH, including both acidic (pH 0) and basic (pH 10 and 12) solutions. Pendant droplet wicking tests and pool boiling experiments were performed to characterize the wicking flux and the critical heat flux (CHF) of the fabricated copper foams, respectively. The CHF enhancement mechanism in hierarchical copper foams was investigated with high-speed optical imaging and acoustic emission (AE) sensing during both the wicking and boiling tests. The results show that the wicking flux of the electrodeposited copper foams increases with the pH due to smaller dendrites and crystallites. Boiling CHF is also higher on the base-deposited copper foams (pH 10 and pH 12) than on the acid-deposited foams (pH 0). Consistent acoustic signatures are observed between wicking and boiling, i.e., smaller AE amplitudes are observed at higher wicking flux and higher CHF, which supports wicking-based CHF enhancement theories. A spike in the AE amplitude is observed at CHF during water boiling on copper foam surfaces due to the rapid growth of the vapor layer blanketing the boiling surface. During the nucleate boiling regime, the AE amplitude decreases with the heat flux in the low heat flux stage but increases with the heat flux in the high heat flux stage. This relationship is found to be due to the interplay of bubble size and bubble count in the stress field that bubbles exert on the heater surface. By analyzing the maximum bubble size, bubble count, and the power and dominant frequency of vapor fraction at varying heat fluxes from high-speed videos, our work shows no obvious effect of the surface structures on the dynamics of the apparent liquid–vapor interface. Combining the optical and acoustic analysis, it is concluded that the key role that surface structures play in CHF enhancement is improving capillary wicking underneath the bubbles rather than affecting the dynamics of the apparent liquid–vapor interfaces. This study not only explores the relationship between electrochemical control and wicking properties but also demonstrates the role of wicking in structure-enhanced boiling CHF through acoustic emission analysis.

Experimental investigation on critical heat flux enhancement by mixing-vanes spacer grid in a vertical annular geometry

In this paper, critical heat flux (CHF) experiment was carried out in a vertical single-rod channel using R-134a as working fluid to study the effect of mixing-vanes spacer grid (MVSG) on CHF enhancement. The single-rod channel had a rectangular geometry with a uniformly heated rod centrally installed. The test conditions were pressure from 1.8 to 2.7 MPa, mass fluxes from 600 to 2100 kg·m−2·s−1 and subcooling temperature from 10 to 40 ℃, which was determined by Katto’s fluid-to-fluid modeling method with reference to the operation condition of Pressurized Water Reactors (PWRs). The effects of flow parameter and downstream distance on CHF enhancement by MVSG were investigated. In addition, visualization study on flow regime during CHF occurrence and bubble behavior when passing though MVSG was observed by a high-speed camera, which contributed to improving better understanding on the mechanism of CHF enhancement by MVSG.

CHF enhancement of downward-facing saturated pool boiling on the SCGS-modified surfaces with multi-scale conical pin fin structures

Enhancing the critical heat flux (CHF) of external reactor vessel cooling (ERVC) to achieve the strategy of in-vessel retention (IVR) has been deemed as an essential requirement for the security of modern commercial PWR nuclear power plants. A new type of multi-scale conical pin fin structures had been designed to meet this need. These conical structures of four different axial heights (1.0, 1.5, 2.0 & 2.5mm) and similar radial geometric parameters were fabricated on four heating surfaces of copper substrates by supersonic cold gas spray technique (SCGS), and the conical pin fin structures were combined with a thin layer of micro porous structures. Transient-state (quenching) and steady-state (heating) pool boiling experiments had been respectively conducted on 6 different downward-facing surfaces under different inclination angles including 5°, 30°, 45°, 60°, 75° & 90° (in vertical), which included above-mentioned four pin fin surfaces, a plain surface and a coated surface covered with a 1mm-thick layer of uniform porous coating fabricated by SCGS with copper particles. The experimental results showed that the CHF increased with the inclination angles. The CHF of all the SCGS-modified surfaces were significantly enhanced compared with those of the plain surface. The maximum CHF value and the maximum CHF enhancement belonged to the 2.0mm-height pin fin surface whose CHF values were more than 1.7 times of those of the plain surface, and up to 2.1 times in 90°. The 1.0mm pin fin surface had the greatest performance in boiling heat transfer coefficient (HTC) and its CHF-enhancing performance also greatly exceeded that of 1.0mm uniform porous coating surface. Remarkable performances of multi-scale conical pin fin structures had been experimentally confirmed, hence the safety margin of IVR-ERVC will be greatly enhanced after adopting them. In addition, the CHF enhancement mechanism of the multi-scale conical pin fin structures was analyzed.

Experimental investigations of bubble behaviors and heat transfer performance on micro/nanostructure surfaces

The multi-level hierarchical surfaces combining different characteristics of single modified surfaces such as expanded heat transfer area, nucleation site density and capillary wickability can further enhance the heat transfer performance. The pool boiling experiment of FC-72 with 35 K subcooling was conducted on the hybrid micro/nanostructure surface (NPF50-60) with nanoforest structure fabricated on the top and bottom of micro-pin-fins using the dry etching and plasma repolymerization techniques. As a comparison, experiments were also conducted on the smooth surface (S), the micro-pin-finned surface (PF50-60) and the nanoforest surface (NS). The novel phenomenon of bubble oscillation on heating surface was observed, which is considered as the result of interactions between evaporation and condensation effects. The predictions of bubble center position during oscillation from forces analysis agree well with the experimental results. In addition, the bubble jumping induced by coalescence was also observed. The results indicated that the three micro/nanostructure surfaces can significantly enhance the boiling heat transfer performance compared to the smooth surface. The lower wall superheat and greater heat transfer coefficient (HTC) with relatively large critical heat flux (CHF) were achieved on the hybrid micro/nanostructure surface for the larger cavity size of nanoforest structure. The wicking velocity of different surfaces obtained from the capillary wickability tests shows a good linear relationship with the CHF. It was concluded that the mechanism of CHF enhancement on micro/nanostructure surfaces is the liquid replenishment with capillary wickability to prevent the expansion of dry spots and maintain a higher critical heat flux.

Pool boiling heat transfer of FC-72 on pin-fin silicon surfaces with nanoparticle deposition

In the present study, two types of micro-pin–fin configurations were fabricated on silicon surfaces by a dry etching method, i.e., staggered pin fins (#1) and aligned pin fins with empty areas (#2). The micro-pin–fin surfaces were then further modified by depositing FeMn oxide nanoparticles (∼35 nm) electrostatically for 8 h and 16 h, respectively, namely #1-8h, #1-16h, #2-8h and #2-16h. Subcooled pool boiling heat transfer was experimentally studied on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased. Additionally, critical heat fluxes were also greatly improved, e.g., the critical heat flux on #1 was almost twice of that on the smooth surface. Generally, the nanoparticle deposition could further enhance pool boiling heat transfer, including the heat transfer coefficient and critical heat flux (CHF). High speed visualizations were taken to explore the mechanisms behind the heat transfer performance. The bubble behavior on the micro-pin–fin surfaces with and without nanoparticles was compared at low, moderate and high heat fluxes, respectively. The wickability of FC-72 on the test surfaces was measured, based on which, a modified CHF model was proposed to predict the experimental CHFs. Accordingly, a possible mechanism of CHF enhancement was described.

Experimental investigation and mechanism of critical heat flux enhancement in pool boiling heat transfer with nanofluids

In the present study, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods. Various characterizations techniques are carried out to study the in-plane crystallite size, number of layers, presence of functional groups and surface morphology. Different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are prepared by dispersing the flakes in DI water. The colloidal stability of 0.3 g/l concentration is measured after 5 days using Zetasizer and found to be stable. The rGO/water nanofluids are then used to study the effect on the enhancement of critical heat flux (CHF) in pool boiling heat transfer. Results indicate an enhancement in CHF ranging from 145 to 245 % for the tested concentrations. The mechanisms of CHF enhancement are analyzed based on surface wettability, surface roughness, and porous layer thickness. The macrolayer dryout model sufficiently supports the mechanism of CHF enhancement of thin wire with rGO deposits, which is not reported yet.

Experimental study of a universal CHF enhancement mechanism in nanofluids using hydrodynamic instability

Pool boiling tests were designed and performed to confirm the relation between the enhancement of critical heat flux (CHF) in nanofluids and the development of a hydrodynamic instability on the test heater surface from the deposition of nanoparticles during boiling. These pool boiling experiments were carried out under atmospheric pressure using a 0.49mm diameter cylindrical Ni–Cr wire as a heating element with ZnO, SiO2, SiC, Al2O3, graphene oxide and CuO nanoparticles at 0.01% volume concentration. The test fluids are distilled water and R-123. The CHF value for each nanofluid is obtained and compared with the measured Rayleigh–Taylor wavelength. Higher CHF results have shorter Rayleigh–Taylor wavelengths in all cases. We propose a modified theoretical model for hydrodynamic instability that incorporates these effects and fits experimental results.

Critical height of micro/nano structures for pool boiling heat transfer enhancement

Critical heat flux (CHF) enhancement by surface modifications has been an extensively researched area in pool boiling heat transfer. Here we report a fundamental mechanism of CHF enhancement where nano/micro ridges are fabricated on surfaces to fragment and evaporate the metastable non-evaporating/adsorbed film present at the base of a bubble in the contact line region. CHF increase of ∼125% is obtained with only ∼40% increase in surface area. An analytical model is extended to explain the CHF enhancement and to determine the average non-evaporating film thickness, which serves as the critical height for nano/micro structures for pool boiling heat transfer enhancement.

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.

Measurement of Key Pool Boiling Parameters in Nanofluids for Nuclear Applications

Nanofluids, colloidal dispersions of nanoparticles in a base fluid such as water, can afford very significant Critical Heat Flux (CHF) enhancement. Such engineered fluids potentially could be employed in reactors as advanced coolants in safety systems with significant safety and economic advantages. However, a satisfactory explanation of the CHF enhancement mechanism in nanofluids is lacking. To close this gap, we have identified the important boiling parameters to be measured. These are the properties (e.g., density, viscosity, thermal conductivity, specific heat, vaporization enthalpy, surface tension), hydrodynamic parameters (i.e., bubble size, bubble velocity, departure frequency, hot/dry spot dynamics) and surface conditions (i.e., contact angle, nucleation site density). We have also deployed a pool boiling facility in which many such parameters can be measured. The facility is equipped with a thin indium-tin-oxide heater deposited over a sapphire substrate. An infra-red high-speed camera and an optical probe are used to measure the temperature distribution on the heater and the hydrodynamics above the heater, respectively. The first data generated with this facility already provide some clue on the CHF enhancement mechanism in nanofluids. Specifically, the progression to burnout in a pure fluid (ethanol in this case) is characterized by a smoothly-shaped and steadily-expanding hot spot. By contrast, in the ethanol-based nanofluid the hot spot pulsates and the progression to burnout lasts longer, although the nanofluid CHF is higher than the pure fluid CHF. The presence of a nanoparticle deposition layer on the heater surface seems to enhance wettability and aid hot spot dissipation, thus delaying burnout.

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.