Reduction In Critical Heat Flux Research Articles

Micro/nanostructuring of metal additively manufactured aluminum alloy for enhanced pool boiling of dielectric fluids

Surface micro/nanotextures have the potential to increase bubble nucleation and promote capillary liquid wicking to enhance pool boiling heat transfer significantly. Despite past studies implementing surface micro/nanofabrication strategies to enhance boiling, the role of structure length scale and morphology on boiling heat transfer coefficient (h) and critical heat flux (CHF) of low-surface-tension dielectric fluids remains unclear. In this study, we systematically tune the surface microstructures on additively and conventionally manufactured aluminum alloys (AM AlSi10Mg and Al6061) from 1 µm to 5 µm to study the influence of microstructure length scale on boiling performance. In addition, to further understand the effect of submicron structures on boiling, nanostructures of 300 nm length were generated on a plain AM surface and hierarchically incorporated atop the microstructures. Dielectric fluid, HFE-7100, was used to investigate the effects of surface morphology on pool boiling heat transfer coefficient (h) and critical heat flux (CHF). Our results show that the 5 µm microstructured AM surface, AM-H(400)E(5), attained a CHF of 19.44 W/cm2 and a maximum heat transfer coefficient (hmax) of 2.89 W/cm2·K, which represents a reduction in CHF of 28.5 % and an enhancement in hmax of 103.8 %, as compared to the plain Al6061 surface. In addition, AM-H(400)E(5) achieved a large enhancement ratio of 2.2 as compared to the plain Al6061 at a heat flux of 20 W/cm2, and the highest h value amongst all structured surfaces, indicating its cavity size is optimal for bubble nucleation. Through the systematic tuning of micro/nanostructures length scale and morphology, this study found that the micro/nanostructures of 1 µm size and below are too small to serve as bubble nucleation sites. Additionally, micro/nanostructure wickability plays a negligible role in affecting pool boiling performances of highly wetting dielectric fluids, while surface morphology plays a dominant role in bubble nucleation to enhance boiling. Lastly, high-speed immersion microscopy was performed to understand the effects of micro/nanostructures on bubble characteristics such as bubble departure diameter and growth period. In summary, this work not only reports the first micro/nanostructured AM surfaces utilizing scalable fabrication techniques for enhanced boiling, but it also demonstrates the potential of tunning the micro/nanostructure length scale to optimize the bubble nucleation site density, resulting in significantly improved pool boiling performances.

Experimental study on flow boiling CHF in annulus channel under heaving conditions using simulant fluid R134a targeting nuclear reactor applications

Floating nuclear power plants have attracted significant attention as a prospective means to deliver clean energy, particularly in geographically remote areas where conventional power infrastructure is inaccessible. The ocean environment, however, can affect the thermal–hydraulic phenomena in them. Notably, in the two-phase system, Critical Heat Flux (CHF) can change due to the periodic variation of additional forces. While a few experimental studies have been conducted on CHF under heaving motion, expanding the range of test conditions is necessary to elucidate its mechanism at the operating pressure of a Pressurized Water Reactor (PWR). Hence, the present study performed an experiment on the flow boiling CHF under heaving motion. To simulate the heaving motion of the ocean environment, a heaving motion platform was designed and constructed. It can simulate the heaving motion with a maximum acceleration of 0.6 g and a period ranging from 3 to 6 s. The CHF test loop utilized refrigerant R134a as the working fluid, and the tests were conducted under the thermal–hydraulic conditions corresponding to PWR operation conditions applying a fluid-to-fluid scaling. The test section consists of a single heater rod in the annulus channel. CHF was measured under both static and heaving motion conditions, and a comprehensive parametric study was conducted on the CHF variation due to the oscillating acceleration field. The results demonstrated that, in most cases, CHF decreased compared to the static condition. The reduction in CHF due to heaving motion could reach up to 9% in the present test configuration, and the reduction ratio was proportional to the magnitude of the heaving acceleration. In addition, the effect of heaving motion was pronounced in two distinct thermal–hydraulic regions: when the critical quality approached zero and when it exceeded 0.6. Based on the experimental observations, the CHF reduction mechanism in each region was suggested.

Hydrophilicity degradation and steam-induced rewetting during capillary-fed boiling

Capillary-fed evaporation/boiling is a crucial process that occurs within two-phase heat transfer devices. Herein, we delve into the capillary-fed boiling performance of a grooved wick with an upper width of 200 μm and a depth of 150 μm fabricated by ultrafast laser micromachining and observe a substantially 51.8% reduction in critical heat flux (CHF), from 145.0 ± 3.3 W/cm2 to 70.1 ± 2.9 W/cm2, following five boiling cycles. Comprehensive experimental investigations are conducted to gain insights into the underlying mechanisms. Our analyses confirm that the diminished CHF arises from the hydrophilicity degradation of the evaporator, attributed to the surface adsorption of airborne organics after the dry-out. The superhydrophilic evaporator becomes highly hydrophobic with static contact angles (CAs) greater than 140° after five boiling cycles. Liquid wicking in subsequent boiling tests initiates through a steam-induced rewetting process. Moreover, we present compelling evidence that the undesired hydrophilicity degradation is effectively mitigated on nanoporous surfaces. Consequently, a microgroove-nanoparticle composite wick successfully retains its original boiling performance during repeated boiling tests. These findings enhance the understanding of capillary-fed boiling performance and offer promising avenues for the design of high-performance wicks tailored for ultrathin two-phase heat transfer devices.

A biosurfactant as prospective additive for pool boiling heat transfer enhancement

Compared to chemical or synthetic surfactants, biosurfactants have been less explored in heat transfer applications. In the present work heat transfer performance of a biosurfactant (Rhamnolipid) solution in pool boiling experiments has been reported. Pool boiling experiments have been performed on a smooth copper surface with aqueous Rhamnolipid solutions at different concentrations and results have been compared with that of DI water. Surface tension and viscosity of newly developed surfactant solutions have been experimentally measured. Critical micelles concentration (CMC) of aqueous Rhamnolipid solution has been observed at 200 ppm. Significant enhancement of heat transfer coefficient is observed with Rhamnolipid solution at CMC value. It reports 200% enhancement in heat transfer coefficient compared to that with pure water. Active nucleation sites and less coalescence of growing of bubbles contribute for heat transfer enhancement in surfactant solutions. Like other surfactants critical heat flux (CHF) value of Rhamnolipid solution at CMC is observed less than that of pure water. However, reduction in CHF (23%) of present surfactant solution is less than the reduction in CHF (≈50%) of other surfactants. Present Rhamnolipids solution shows better heat transfer performance and higher CHF value compared to SDS surfactant. Moreover, it shows better heat transfer performance in comparison to other biodegradable surfactants reported in the literature.

An experimental investigation on the effect of gravitational orientation on flow boiling performance in different channel sizes ranges from minichannels to microchannels

Flow boiling experiments were conducted in this work to investigate the effect of gravitational orientation on flow boiling performance in channels with different hydraulic diameters (Dh) ranging from minichannels to microchannels. Four different channel sizes (Dh ~ 100 μm, 300 μm, 500 μm and 1000 μm) were experimented through four different orientations such as horizontal upward facing (HU), horizontal downward facing (HD), vertical up flow (VUF) and vertical down flow (VDF) with mass flux ranging from 100 to 900 kg/m2 s. Critical heat flux (CHF), heat transfer coefficient, pressure drop and flow patterns data were determined for different channel sizes with different orientations and mass fluxes. The experimental results were also compared with the different criteria reported in the literature regarding the existence of gravity force in minichannels and microchannels. From the experimental analysis it was clear that for channel sizes (Dh) of 100 and 300 μm, the gravity force was not so dominant, but in the case of 300 μm channel, VDF reported degraded heat transfer performance. For channel sizes (Dh) of 500 and 1000 μm, the gravity force was dominant. Reversed bubble flow was observed for these channel sizes in VDF orientation trial. The HD orientation also shows a reduction in critical heat flux and heat transfer coefficient for these channel sizes. VUF orientation was found to be the best for all channel specimens irrespective of channel size. The buoyancy assisted bubble movement along the length of the channel will help in easy bubble removal and flushing. The comparison with different criteria shows that only Bond number (Bo) can able to predict the existence of gravity force in different channel size ranges.

Study on CHF characteristics in narrow rectangular channel under complex motion condition

In present work, the critical heat flux (CHF) characteristics in narrow rectangular channel were theoretically investigated under complex motion condition. A three-field analysis program was developed combining with a unified form of additional forces caused by motions, and the program was validated by CHF experiment data with a better prediction accuracy. The CHF characteristics of rectangular channel were evaluated based on two types of situations: (1) oscillating inlet flow and (2) constant inlet flow with different motions including heaving, rolling and the coupled motion of heaving and rolling. The results show that the oscillation of the inlet flow has a great influence on the CHF, the increases in the amplitude and period of the inlet flow oscillation cause the dryout to occur in advance. When the inlet flow is fixed, various motion conditions have little effect on CHF for high mass flux, and the reduction of CHF does not exceed 5%. However, the CHF is obviously reduced for the low mass flux due to the significant oscillation or thinning effect of the outlet liquid film caused by motions.

Design, fabrication and nucleate pool-boiling heat transfer performance of hybrid micro-nano scale 2-D modulated porous surfaces

Future technological products are increasingly becoming compact, integrated and multi-functional with higher heat loads in confined spaces. This has impelled the quest for improved heat management solutions to ensure the devices maintain their full performance as designed. Due to the associated latent heat, nucleate pool-boiling heat transfer (NBHT) systems are proven to dissipate the high heat flux at low-temperature gradients. NBHT systems with nano-scale and micro-scale porous structured surfaces are reported to possess enhanced heat transfer coefficient (HTC) and critical heat flux (CHF). The potential synergistic benefits of combining these two unique structures into an integrated surface coating fabrication technology have been a key question of interest. This study aimed to investigate the effect of uniformly coated nanoparticles on 2-D modulated micro-porous surfaces in NBHT. The NBHT performance of four different surface types; (1) flat untreated copper surface (UnCu surface), (2) nanocoated copper surface (NPC surface), (3) 2-D modulated micro-porous surface (MMPC surface), and (4) hybrid micro/nano-scale 2-D modulated porous coated surface (HMPC surface) are investigated. The UnCu surface and MMPC surface are employed for uniform nanoparticle coating to fabricate NPC surface and HMPC surface, respectively. Morphological characteristics of the surfaces were observed using the scanning electron microscopy (SEM). According to the results, the NPC surface and MMPC surface showed an enhanced NBHT performance by 81.6% and 50.6% improvement in CHF and 130.3% and 45% enhancement in HTC, respectively. However, as compared to the MMPC surface, the NBHT performance of the HMPC surface decreased. The average reduction in CHF and HTC of the HMPC surface was 16.8% and 24%, respectively. The reduction in the NBHT performance of HMPC is explained by the deterioration of the capillary wicking characteristics of the MMPC surface.

Confinement Effects on FKE-774 Critical Heat Flux in Buoyancy-Driven Microgap Channels

Critical heat flux (CHF) of the fluoroketone fluid FKE-774 in vertical microgap channels is explored, with a focus on submillimeter spacings. Experiments were conducted using a 20 mm × 20 mm heated aluminum surface. Microgap channel spacings were decreased down to 0.3 mm, providing channel aspect ratios (height/spacing) as high as 67. In the limit where channel spacing is large, CHF was found to be 140 kW/m2 for saturated boiling at atmospheric pressure. A reduction in CHF of 55% was observed for the largest channel aspect ratio investigated. Results for degradation of the CHF limit with decreasing microgap spacing are compared to a correlation available in the literature and show a roughly hyperbolic dependence on channel aspect ratio (height/spacing) for aspect ratios larger than 10.

Separate effects of surface roughness, wettability, and porosity on the boiling critical heat flux

The separate effects of surface wettability, porosity, and roughness on the critical heat flux (CHF) of water were examined using engineered surfaces. Values explored were 0, 5, 10, and 15 μm for Rz (roughness), <5°, ∼75°, and >110° for static contact angle (wettability), and 0 and 50% for pore volume fraction. The porous hydrophilic surface enhanced CHF by 50%–60%, while the porous hydrophobic surface resulted in a reduction of CHF by 97%. Wettability had little effect on the smooth non-porous surface CHF. Surface roughness (Ra, Rq, Rz) had no effect on CHF within the limit of this database.

Effects of Hypervapotron Geometry on Thermalhydraulic Performance

Plasma disruptions and edge localized modes can result in transient heat fluxes as high as 5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on portions of a tokamak reactor first wall (FW). To accommodate these heat loads, the FW will likely use water-cooled hypervapotron heatsinks to enhance the heat transfer. In this article, we present the results of a computational fluid dynamics (CFD) study using 70 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C inlet water at 2.7 MPa to investigate the tooth height and backchannel depth of 50-mm-wide hypervapotrons with 6-mm-pitch and 3-mm side slots. We compare a popular design with 4-mm-high teeth and a 5-mm backchannel to a more optimal case with 2-mm-high teeth and a 3-mm backchannel under nominal heat loads (0.5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) on a 100-mm-heated length and under single-phase flow conditions. Better heat transfer in the latter case and the smaller backchannel permit a factor of two reduction in the required mass flow while maintaining the same beryllium armor surface temperatures near 130 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The shallow teeth and smaller backchannel allow the 40 fingers in a typical panel to flow in parallel and simplify the water circuit. A comparison of the two hypervapotron designs during off-normal loading (5.0 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and two-phase flow then follows. The design with 2-mm teeth has a 3.5% higher beryllium surface temperature of 648 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and reduces the critical heat flux (CHF) by ~2%. Hypervapotron width also plays a role in heat transfer and CHF. CFD results for 36 and 70 mm wide hypervapotrons compared to the 50-mm case reveal similar thermal performance at low heat flux, but a reduction in CHF with increasing width. This study highlights the necessary compromise between design margin during transient events, effective heat transfer under nominal conditions, limitations on finger width, and the simplicity needed in the water circuit design.

Comparison of CHF measurements in horizontal and vertical tubes cooled with R-134a

An experimental study of the critical heat flux (CHF) in horizontal and vertical tubes cooled with R-134a has been completed. The investigated ranges of flow parameters in R-134a were outlet pressures of 1.31, 1.67 and 2.03 MPa (8, 10 and 12 MPa in water-equivalent values), mass flux from 500 to 3000 kg m −2 s −1 (700–4300 kg m −2 s −1 in water-equivalent values), and critical quality from −0.1 to +0.9. The wide range of qualities was achieved using tubes of different heated lengths and two-phase flow at the test-section inlet. The R-134a CHF data obtained in the vertical orientation agreed with the R-134a-equivalent CHF values from the water-based CHF look-up table. The effect of orientation on CHF was found to depend on mass flux, quality and pressure, as well as the limiting critical quality. This effect is strong at low mass fluxes, but disappears at high mass fluxes. At qualities higher than the limiting critical qualities, the CHF in horizontal flow can be greater than the corresponding value in vertical flow at the same critical quality conditions. A maximum reduction in CHF due to flow stratification was observed at qualities between the limiting critical qualities for horizontal and vertical flows. The orientation effect on CHF appears to be much stronger for R-134a than for water flow at the same critical quality, equivalent mass flux (based on vertical flow fluid-to-fluid modeling relationships) and density ratio. This behavior is primarily due to the larger density difference between liquid and vapor and the lower vapor velocity in R-134a.

Heat Removal Augmentation in Steam Generating Channels with Swirled and Transit Flows

The paper presents the results of investigations of the effect of flow swirling and of the combined effect of swirling and transit flow on the heat removal rate and critical heat flux (CHF) in steam generating channels of annular type. It is established that a significant reduction in CHF and in the rate of heat removal occurs on convex heat transfer surfaces. Using swirled and transit flows allows one to considerably enhance the heat removal rate and CHF on convex heat transfer surfaces. Pressure losses and heat removal rate are shown to be dependent on the swirled-to-transit flow ratio. With an optimal relation between these flows it is possible to develop and construct highly efficient steam generating devices in which pressure losses differ only slightly from those in smooth channels.

Scaling parameter of CHF under oscillatory flow conditions

Critical heat flux (CHF) is reduced by flow oscillations. The reduction of CHF is significantly influenced by flow oscillation period and amplitude, heat capacity of test tube, and mean inlet mass flux. A scaling parameter of the temperature response of the tube wall was derived based on a lumped-parameter model of the tube wall's heat capacity. When this scaling parameter was applied to CHF under flow oscillations, the experimental data were successfully correlated. © 1999 Scripta Technica, Heat Trans Asian Res, 28(6): 541–550, 1999

Scaling Parameter of CHF under Oscillatory Flow Condition.

Critical heat flux (CHF) was reduced by flow oscillations, and the reduction of CHF was significantly influenced by the flow oscillation period, amplitude, heat capacity of test tube and mean inlet mass flux. A scaling parameter of temperature response of tube wall was derived based on a lumped-parameter model of tube wall heat capacity. Applying this scaling parameter to the CHF data under flow oscillation, the experimental data of CHF was successfully correlated.

Forced convection boiling from a nonflush simulated electronic chip

An experimental investigation has been undertaken to determine the effect of heated surface height on forced convective boiling. An inert fluorocarbon, FC-72 (3M Industrial Chemical Products Division) is circulated through a vertical rectangular channel at velocities of 1-4 m/s and subcoolings of 20 and 35°C. Results for five surface heights, as measured relative to the flow channel wall, were obtained. These were 0.127-mm recessed, 0.229-, 0.457-, and 0.635-mm protruded and flush with the flow channel wall. A reduction in critical heat flux (CHF) occurred at low velocities, while an increase occurred at higher velocities for the protruded cases. A reduction of CHF occurred at all velocities for the recessed condition. Additional results in the velocity range of 5-7 m/s are presented for the flush condition. This data shows that for velocities greater than 4 m/s, CHF becomes weakly dependent on the Weber number. Weak dependence on Weber number also implies a direct proportionality to velocity and a weak dependence on heated length.

Transition flow boiling heat transfer to water in a vertical annulus

Transition boiling heat transfer coefficients for water at 25–30 psia flowing upward at low velocities have been obtained Hot mercury, flowing on the inside of a tube with a 0.54 in. o.d. served as the heat source. Water flowed in the annular space between the heat source and an outer glass tube having a 1 in. dia. Thermocouples placed at several elevations within the mercury stream allowed the rate of heat transfer to be determined. The heat transfer coefficients appear consistent with other transition boiling data providing an appropriate allowance is made for the reduction in critical heat flux at high void fractions.

Waveform, Confinement and Heater Thickness Effects on the Critical Heat Flux from Electrically-Heated Thin Strips

Three parameters affecting the critical heat flux from vertical, electrically heated, thin (0.051-0.254 mm) silver strips in R-113 (CC12F-CC1F2) are discussed. The first is the reduction in critical heat flux (up to 32%) with decreasing ribbon thickness due to a lateral thermal conduction effect. The second is a reduced RMS heat flux for an AC sinusoidal waveform. The third is the reduced critical heat flux (up to 40%) from a vertical ribbon when confined between parallel plates, with no apparent dependence on ribbon length.