Flux Transfer Research Articles

Small-scale Field-aligned Currents in the Magnetopause Boundary Layer

Based on high-resolution measurements from the Magnetospheric Multiscale mission from 2015 May to 2018 June, we statistically investigate the properties of small-scale field-aligned currents (SFACs) in the magnetopause boundary layer. A total of 2235 SFACs are successfully identified. The durations of SFACs mainly fall between 0.2 and 0.3 s. Over 90% of SFACs have a width of less than 1 ion inertia length and are primarily distributed from 5 to 25 electron inertia lengths, implying that the SFACs belong to the kinetic-scale current layer. The main carriers of SFACs are electrons, and over 70% of SFACs exhibit net energy dissipation (i.e., J · E ′ > 0) with the majority of energy dissipation taking place in the parallel direction. SFACs are widely distributed spatially, and the occurrence rate of SFACs is higher in the boundary layer closer to the magnetosphere. Additionally, less than half of the total SFACs are identified in well-known structures, including the magnetic reconnection region, flux transfer event, Kelvin–Helmholtz vortex, and exhaust region, and 54% of the SFACs are in the “others” unknown structures. These results improve our comprehension of the current system at the magnetopause and the roles of SFACs in the coupling between the solar wind and magnetosphere.

Microfluidic controllable production and morphology-independent phase-change heat transfer of boehmite nanofluids

Enhanced boiling heat transfer through nanofluids represents a significant approach to improving the cooling performance of electronic devices. However, the preparation of nanofluids poses challenges regarding production efficiency, morphology control, and structural stability. In addition, the underlying mechanism of flow boiling heat transfer enhancement of nanofluids, particularly changes in vapor behavior, remains a limited understanding. This study presents a facile and straightforward microfluidic strategy for the controllable synthesis of nanofluids toward efficient heat transfer enhancement. The microfluidic flow synthesis approach effectively enhances mixing efficiency at various flow rates, enabling continuous and high-throughput synthesis of boehmite nanofluids with different morphologies. The synthesized nanofluids exhibit long-term stability (>50 days) at room temperature. Both rod- and sphere-shaped nanofluids under equivalent concentrations can significantly improve critical heat flux (CHF) and maximum heat transfer coefficient (HTC) across various operating parameters, but with similar enhancement characteristics. Analysis of the period of bubble growth and burst, bubble size distributions, number of bubbles, and vapor phase fractions reveals that rod- and sphere-shaped nanofluids could shorten the period, enhance surface wetting, increase nucleation sites, and reduce bubble size efficiently and similarly. Furthermore, the effect of the concentration of nanofluid on phase-change heat transfer performance is systematically explored. 0.01 wt% nanofluid behaves the most remarkable enhancement, CHF and maximum HTC are improved by 24 % and 14.4 %, respectively. These findings not only shed new light on the controllable synthesis of functional nanomaterials but also provide important guidelines for the rational design of nanofluids for efficient two-phase cooling of electronic devices.

Enhanced flow boiling heat transfer in narrow gap with different widths using different-mode-interacting boiling

The concept of different-mode-interacting boiling (DMIB) utilizes the areas of two different thermal conductance materials by expanding the wetted area over the regions of their dry areas. This enhances the critical heat flux (CHF) and heat transfer coefficient (HTC) in a wide channel (e.g., Wch = 80 mm) to provide adequate cooling in previous study. In this study, the effectiveness of DMIB in enhancing the CHF and HTC was investigated when using a channel width (Wch = 10 mm) equal to that of the heating surface, aiming to provide better efficiency for both uniform surface (US) and nonuniform surfaces (NUSs) with material widths (W = 0.5 and 1.0 mm) in narrow gaps. Experiments were performed with a heating surface of 10 mm2 using subcooled deionized water at different temperatures ΔTsub = 2.0, 10, 20, and 30 K, inlet velocities v = 0.1, 0.2, and 0.4 m/s (corresponding to mass fluxes: 100, 200, and 400 kg/m2s), and gap sizes h = 1.0, 2.0, and 5.0 mm. The results revealed that the heat transfer performance of the NUSs was significantly enhanced compared to the US for both Wch. However, both the CHF and HTC values were higher with Wch = 80 mm than with Wch = 10 mm owing to the expansion of coalesced bubbles in the flow direction, sustained nucleate boiling regime, and effective liquid rewetting during the intermittent nucleate boiling-partial dryout near the CHF owing to the DMIB. The maximum CHF and CHF enhancement ratios for Wch = 10 mm were 3.59 MW/m2 and 52.2 %, respectively, whereas for Wch = 80 mm, they were 3.66 MW/m2 and 89 %, respectively. In addition, the HTC and HTC enhancement ratio for Wch = 10 mm were 1261 kW/m2K and a factor of 7, respectively, whereas those for Wch = 80 mm were 2026.9 kW/m2K and a factor of 12, respectively. In contrast, only a minimal difference existed in the heat transfer performance of Wch = 10 mm compared to that at Wch = 80 mm; therefore, Wch = 10 mm can be considered as having a better cooling efficiency per channel width due to the minimal pumping power and significant CHF and HTC enhancement.

Biochar-mediated synergistic promotion of DIET and IHT: Kinetic and thermodynamic insights into propionate two-step syntrophic methanogenesis

Biochar can effectively enhance the thermophilic anaerobic degradation of propionate. Nevertheless, previous studies frequently ascribed this enhancement to direct interspecies electron transfer (DIET) without considering the underlying kinetics and thermodynamics involved. Additionally, quantitative evaluation of biochar’s impact on interspecies electron transfer (IET) has been limited. In this study, we developed and optimized models for analyzing thermodynamics and electron transfer flux to elucidate the mechanism by which biochar enhances the two-step syntrophic methanogenesis of propionate. Findings revealed that biochar primarily facilitated DIET (1.78 × 10−4 A), leveraging its redox capacity. Additionally, it synergistically enhanced interspecies hydrogen transfer (4.08 × 10−10 A), significantly improving IET efficiency. This synergistic effect led to accelerated oxidative degradation rates of propionate (39.2 %) and intermediate acetate (13.5 %), expanding the thermodynamic windows for their syntrophic methanogenesis (221 % and 103 %, respectively). Accordingly, biochar increased the maximum rate of propionate methanation (71.9 %) and reduced the lag phase time (51.1 %). Moreover, biochar promoted the specific growth rates of acetogenic bacteria and methanogens while significantly increasing the abundance of Pelotomaculum, Methanoculleus, and Methanosarcina. These findings provide new insights and theoretical support for the use of biochar in promoting anaerobic digestion and facilitating IET during the process.

Interfacial relaxation of droplets caused by mass transfer in a pulsed column

AbstractThe method developed in our previous work (Wang et al., AIChE J. 2020; 66(8): e16257), was used to study the influence of the mass‐transfer direction and flux on the dynamic interfacial tension (DIFT) in a pulsed column. The breakup frequency was the highest near the entrance of the dispersed droplet, corresponding to the lowest interfacial tension and highest mass transfer flux. The decrease in the interfacial tension was proportional to the square root of mass‐transfer flux, and this difference is defined as a new parameter, named interfacial relaxation. When the mass‐transfer direction was changed from the dispersed phase to continuous phase to the opposite direction, the proportion of binary breakage was decreased from ~80% to ~40% and the interfacial relaxation was more sensitive to the mass‐transfer flux. A DIFT model was finally established based on the interfacial relaxation parameter, which predicted the experimental results with an error of ±10%.

Flux transfer circuits breaking conventional limit in transfer coefficient based on a negative inductance of a π-junction

We have demonstrated transfer coefficients breaking the conventional limit in flux transfer circuits (FTCs) by introducing a π-phase-shifted Josephson junction (π-junction), where the FTCs include an input/output inductor. According to the current-phase relationship of a π-junction, the π-junction behaves as an inductor with intrinsically negative kinetic inductance. When a single-π-junction superconducting quantum interference device (π-SQUID) in which a geometric inductor is placed in parallel with the π-junction is formed, a current flowing on the inductor, that is, the internal flux is increased against an input current or an input flux supplied externally to the π-SQUID in case that the π-SQUID shows no hysteresis in characteristics of internal-external flux. The FTC under investigation (π-FTC) is composed of two identical π-SQUIDs sharing a π-junction. The magnitude of the internal flux exceeds that of the external flux in the π-SQUID near zero external flux. Using this effect, the transfer coefficients are expected to be increased in the π-FTCs. Numerical analysis for π-FTCs reveals that the transfer coefficients exceed the conventional limit in a wide range of input currents corresponding to the input flux, although the negative kinetic inductance depends on the magnitude of the input. We made several π-FTCs for critical currents of the π-junctions of 50 πA and 60 πA. The output flux was measured by constructing a flux-locked loop. The experimentally obtained ratios of the transfer coefficients of the π-FTCs to the coefficient of the conventional FTC made on the same chip agree with the numerical results, which supports the negative kinetic inductances cause the increased coefficients breaking the conventional limit. Because the transfer coefficient is almost independent of input currents, we believe that the π-FTCs are applicable for strengthening not only couplings used in quantum annealers or SQUID sensors but also couplings used in superconductor digital circuits.

On the validity of two-flux and four-flux models for light scattering in translucent layers: angular distribution of internally reflected light at the interfaces.

Optical characterization and appearance prediction of translucent materials are required in many fields of engineering such as computer graphics, dental restorations or 3D printing technologies. In the case of strongly scattering materials, flux transfer models like the Kubelka-Munk model (2-flux) or the Maheu's 4-flux model have been successfully used to this aim for decades. However, they lead to inaccurate prediction of the color variations of translucent objects of different thicknesses. Indeed, as they rely on the assumption of lambertian fluxes at any depth within the material, they fail to model the internal reflectance at the interfaces, penalizing the accuracy of the optical parameter extraction. The aim of this paper is to investigate the impact of translucency on light angular distribution and corresponding internal reflectances by the mean of the radiative transfer equation, which describes more rigorously the impact of scattering on light propagation. It turns out that the light angular distribution at the bordering interfaces is often far from being lambertian, and that the internal reflectance may vary significantly according to the layer's thickness, refractive index, scattering and absorption coefficients and scattering anisotropy. This work enables to better understand the impact of scattering within a translucent layer and also invites to revisit the well-known Saunderson correction used in 2- or 4-flux models.

Direct Detection of Ongoing Magnetic Reconnection at Mercury's High‐Latitude Magnetopause

AbstractAn ongoing magnetic reconnection event was detected in the Mercury's high latitude magnetopause during a northward interplanetary magnetic field. The reconnection X‐line region was revealed in the Mercury's magnetopause based on the encountered flux ropes ejected away from this region both planetward and tailward. A series of magnetic flux ropes, known as flux transfer event shower were observed tailward of this X‐line region. These flux ropes were probably expanding and deflected as they were ejected away tailward from the X‐line region. Large‐amplitude variations in all three components of the magnetic field and a few small‐scale flux ropes were observed inside the X‐line region, which could be the seed of the flux rope shower at the magnetopause. The observations suggest that magnetic reconnection is highly dynamic and persistent in Mercury's magnetosphere.

Impacts of micro- to nano-sized carbon supplements on mixed and archaea-free halophilic cultures when used for bioenergy recovery from saline wastewater

Due to the growing interest in enhancing dark fermentation and anaerobic digestion via carbon-based additives to exploit different lucrative synergies, process-based optimizations and mechanistic analyses are crucial. Herein, we studied the impacts of carbon supplements on halophilic microbial cultures as a process intensification way towards bioenergy recovery from saline wastewater. Three additives, i.e., powdered activated carbon, multiwall carbon nanotubes and graphene, were examined. Two sets of parallel batch experiments were conducted using archaea-free (for dark fermentation) and mixed (for anaerobic digestion) halophilic cultures at different supplement doses. Adding 30 mg/L of graphene and 100 mg/L of powdered activated carbon exhibited the maximum improvements in dark fermentation (H2-production) and anaerobic digestion (biogas production), i.e., ∼28% and ∼27% over the control, respectively. This underscored the dose-dependent impact of different-sized carbon amendments. Both the electron transport system activity and extracellular polymeric substances were improved with all amendments, indicating a stimulated electron transfer flux. However, distinct bacterial compositions were identified when using mixed and archaea-free cultures along with indicators for direct interspecies electron transfer establishment in the presence of archaea. Beyond this, such findings also supported the expanded role of conductive additives on H2-producing bacteria, including Clostridium. According to a techno-economic analysis, powdered activated carbon emerged as the best for optimizing bioenergy recovery (in terms of H2 or biogas) from saline wastewater, providing a solid basis for relevant applications.

Flow boiling heat transfer characteristics and correlation development of R290/R600a mixtures in an internally threaded tube

Excessive hydrofluorocarbons (HFCs) usage significantly contributes to the global warming. Therefore, there is a widespread shift toward natural refrigerants with low global warming potential (GWP). Among these alternative refrigerants, R290, R600a, and their mixtures have gained considerable attention. However, their flammability limits the charge. To decrease the equipment volume and reduce the refrigerant charge, internally threaded tubes have been introduced to enhance the heat transfer. Therefore, in this study, flow boiling experiments were conducted on R290, R600a, and mixtures inside an internally threaded tube, obtaining heat transfer data under a pressure of 0.216–0.416 MPa, heat flux of 10.5–73.0 kW m−2, mass flux of 70–190 kg m−2 s−1. To quantitatively discern the dominant heat transfer mechanism, a dimensionless number Nz was introduced. In the nucleate boiling dominant region, an increase in the heat flux markedly enhances the heat transfer, with pressure and mass flux playing minor roles. Furthermore, the heat transfer deterioration trend of the mixtures aligns with the variation trends of concentration slip and boiling range. In the convective heat transfer dominant region, a rise in the mass flux significantly increases the heat transfer coefficient, while the pressure and heat flux have minor effects. Additionally, at high R290 mole fraction and vapor quality, high surface tension maintains a continuous liquid film, enhancing the heat transfer. Furthermore, new heat transfer correlations for flow boiling inside the internally threaded tubes were established, which exhibit a mean relative deviation of −2.30 % and 8.80 % for the obtained experimental results and collected data, respectively.

Investigation of flow boiling critical heat flux and heat transfer within a horizontally oriented channel with one-sided heating at three levels of subcooled inlet

The aim of this study is to investigate the impact heater orientation and inlet conditions on heat transfer mechanisms during flow boiling, inside horizontally oriented rectangular channel using PF-5060. The experimental setup allows both upward and downward-facing single-sided heating configurations. Various inlet mass flux are examined, along with different levels of inlet subcooling. The impact of gravity is studied using flow visualization. During downward facing heating, vapor accumulates along the copper heater wall due to buoyancy effects resulting in notably low CHF and heat transfer values. In contrast, with upward facing heating, buoyancy assists in extraction of vapor from the copper heater wall, facilitating increased liquid contact and higher CHF and heat transfer values. CHF values begin to converge at high inlet mass flux for both orientations, which can be attributed to inertia dominating gravity. Inlet subcooling also influences CHF, with highly subcooled conditions yielding higher CHF. The impact of orientation and subcooled inlet on CHF and heat transfer coefficients are captured. Experimental CHF data was used to validate the Hydrodynamic Instability-Based Model for CHF prediction demonstrating a Mean Absolute Error of 10.8%. Effectively considering gravity forces, heated wall orientation, and flow regimes, the model demonstrates a proficiency in its comprehensive approach.

Vorticity dynamics at partial-slip boundaries

In this paper we discuss the dynamics of vorticity at partial-slip boundaries. We consider the total vector circulation, which includes both the total vorticity of the fluid and the slip velocity at the boundary (the interface vortex sheet). The generation of vector circulation is an inviscid process, which does not depend on either viscosity or the slip length at the boundary. Vector circulation is generated by the inviscid relative acceleration between the fluid and the solid, due to either tangential pressure gradients or tangential acceleration of the partial-slip wall. While the slip length does not affect the creation of vector circulation, it governs how vector circulation is distributed between the total vorticity of the fluid and the interface vortex sheet. Specifically, the partial-slip boundary condition prescribes the ratio between boundary vorticity and the strength of the interface vortex sheet, and the viscous boundary flux transfers vector circulation between the interface vortex sheet and the fluid interior to maintain this condition. The interaction between a vortex ring and a partial-slip wall is examined to highlight various aspects of this formulation. For the head-on collision, the quantity of vector circulation diffused into the fluid as secondary vorticity increases as the slip length is decreased, resulting in a stronger secondary vortex and increased rebound of the vortex ring. For the oblique interaction, the extent to which the vortex ring connects to the boundary increases as the slip length is increased.

Dayside Pc2 Waves Associated With Flux Transfer Events in a 3D Hybrid‐Vlasov Simulation

AbstractFlux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation.

Hydrodynamic effects of the elliptical spacer filament on the flow and mass transfer in a desalination membrane channel

This study numerically investigates the impacts of an elliptical spacer on the flow and mass transfer within a desalination membrane channel. We consider the effects of the fluid Reynolds number, the elliptical spacer position yin/H, the flow attack angle θ and the aspect ratio B of spacer on the fluid flow, concentration polarization, and permeation flux. Our findings reveal that the elliptical spacers can improve both the mass transfer and permeation flux compared with the traditional circular spacers. Placing the spacer near the membrane surface effectively disrupts the concentration boundary layer, thereby amplifying the local concentration polarization surrounding the spacer. A single recirculation vortex behind the spacer induces numerous large vortices around the membrane, resulting in a significant increment in freshwater production. By changing θ and B, the system's stability can be reduced, potentially leading to a 16.0% increment in permeation flux when compared with the traditional circular spacers. Furthermore, the drag and lift coefficients acting on the elliptical spacer are greater than those of circular spacers, which is the main reason for the transition of flow condition. Our findings provide a reference for the design of the efficient structures in reverse osmosis membranes.

Characterization of Magnetic Flux Contents for Two Flux Transfer Events From Both In Situ and Ionospheric Observations

AbstractFlux transfer events (FTEs) are a type of magnetospheric phenomena that exhibit distinctive observational signatures from the in situ spacecraft measurements. They are generally believed to possess a magnetic field configuration of a magnetic flux rope and formed through magnetic reconnection at the dayside magnetopause, sometimes accompanied with enhanced plasma convection in the ionosphere. We examine two FTE intervals under the condition of southward interplanetary magnetic field (IMF) with a dawn‐dusk component. We apply the Grad‐Shafranov (GS) reconstruction method to the in situ measurements by the Magnetospheric Multiscale (MMS) spacecraft to derive the magnetic flux contents associated with the FTE flux ropes. In particular, given a cylindrical magnetic flux rope configuration derived from the GS reconstruction, the magnetic flux content can be characterized by both the toroidal (axial) and poloidal fluxes. We then estimate the amount of magnetic flux (i.e., the reconnection flux) encompassed by the area “opened” in the ionosphere, based on the ground‐based Super Dual Auroral Radar Network (SuperDARN) observations. We find that for event 1, the FTE flux rope is oriented in the approximate dawn‐dusk direction, and the amount of its total poloidal magnetic flux falls within the range of the corresponding reconnection flux. For event 2, the FTE flux rope is oriented in the north‐south direction. Both the FTE flux and the reconnection flux have greater uncertainty. We provide a detailed description about a formation scenario of sequential magnetic reconnection between adjacent field lines based on the FTE flux rope configurations from our results.

Boiling mechanism of biphilic surfaces based on Helmholtz instability and Taylor instability

This study demonstrated that combination hydrophilic copper and hydrophobic nano-silver surfaces simultaneously enhanced the critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling heat transfer (BHT) with deionized water as the employed working fluid. To further investigate the influence of geometric factors on heat transfer, this study designed three patterns denoted as squares/stripes/networks, ranging from 50 μm to 3000 μm. The effects of pattern size d, spacing p, and pitch ratio p/d on heat transfer results were obtained. Based on bubble visualization, the study further analyzed the reasons for inflection points in the boiling curve and the mechanisms caused by porous hydrophobic coating. Finally, the study proposed a numerical model suitable for predicting CHF of biphilic surfaces based on Helmholtz instability and Taylor instability. The model began with vapor columns escaping toward far-field and the fluid supplied to the heating surface. Concrete model derivation and adjustments are categorized according to the relationship between pattern size and bubble detachment diameter. For surfaces where the bubble detachment diameter exceeds the pattern size, this study originated from the perspective of Helmholtz instability on the vapor column surface. The resulting model is anticipated to quantitatively solve for the former and modulate the wavelength of Taylor instability on biphilic surfaces. Regarding surfaces with bubble detachment diameters smaller than the pattern size, this research begins with the existence time of the micro-liquid layer. The derived model validates the predominant role of p/d in the CHF values on micron-scale biphilic surfaces.

Turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers with wall disturbances

Hypersonic turbulence over disturbed walls can be encountered when the fuselage of the vehicles is ablated or installed with transpiration-cooling porous media, resulting in the variation of the wall heat transfer that complicates its accurate predictions. In this paper, we study the turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers at the free-stream Mach number of 6.0 subject to wall disturbances with different wall temperatures, with our focus on the compressibility effects. We found that the temperature fluctuation intensity and the turbulent heat flux are enhanced in the outer region, which is attributed to the higher mean temperature gradient induced by the wall disturbances. The solenoidal and dilatational components of the turbulent heat flux, obtained by the Helmholtz decomposition that splits the velocity fluctuations into the solenoidal and dilatational components, are of opposite signs, with the latter smaller in magnitude but showing the tendency of mitigating the former. The integration formula that decomposes the wall heat transfer into various physical processes reveals that the viscous dissipation generates the wall heat transfer, while the turbulent heat flux, work of pressure on dilatation and the mean flow convection transport it towards the free stream. The contribution of the dilatational motions to the viscous dissipation and turbulent heat flux is negligible compared with that of the solenoidal components.

An improved agglomerate model with adsorption isotherm for oxygen transport in CCLs with low Pt loading

Understanding of oxygen transport in cathode catalyst layers with low Pt loading is of great importance to the proton exchange membrane fuel cells. The resistance to oxygen transport within the catalyst layers (CCLs) increases significantly as the Pt loading decreases. Thus, it is necessary to develop an accurate continuum-scale model for the oxygen transport and reaction in catalyst layers with low Pt loading so as to guide the optimization of catalyst layers. To address this issue, a modified agglomerate model is developed in the present study. In the model, the CCLs are conceptualized as packings of agglomerate particles composed of the carbon, ionomer, and catalyst. Each of agglomerate particle is covered by a thin layer of ionomer. The main features of our model are two folds: (1) adsorption isotherm is employed instead of Henry's law, commonly used the previous agglomerate models, to depict the oxygen adsorption at the surface of ionomer; (2) non-uniform oxygen transfer flux across the ionomer layer surrounded each agglomerate is considered. The results predicted by our developed model are in good agreement with the experimental data for the polarization curve and the variations of the oxygen transport resistance with the Pt loadings. Especially, our model is superior to the previous agglomerate models in regarding to the variation of oxygen transport resistance with mass fraction of Pt in the catalyst. The modified agglomerate model developed in this work provides an effective tool for investigation of oxygen transport and reaction in CCLs with low Pt loading.

Effects of Orbit Raising and Deorbiting in Source-Sink Evolutionary Models

The sustainability of the low-Earth-orbit (LEO) environment is threatened by the growing number of anthropogenic space objects planned to be launched in the coming years. This paper investigates the evolution of objects residing in LEO through the MIT Orbital Capacity Assessment Tool (MOCAT), an evolutionary multishell, multispecies source-sink model. The proposed novelty considers the flow of objects crossing multiple shells during orbit raising and deorbiting maneuvers, modeled through the secular variation of the semimajor axis under a low-thrust continuous applied control. To this aim, a higher-fidelity MOCAT version, including active satellites, derelicts, debris, and rocket bodies, has been developed and used. The results demonstrate that incorporating orbit transfer fluxes into the model results in a higher number of collisions, which leads to a greater quantity of debris and poses a greater threat to the safety of LEO.

ENHANCEMENT EVALUATION CRITERIA FOR POOL BOILING ENHANCEMENT STRUCTURES IN ELECTRONICS COOLING: CHF ENHANCEMENT RATIO (ER-CHF) AND ENHANCEMENT INDEX (EI)

Extensive research shows the necessity of efficient cooling systems to enable electronic components to operate at high performance levels for a sustained period. While conventional methods have served the cooling needs so far, rising computational power, energy efficiency, and sustainability requirements call for improved techniques. The literature shows the effectiveness of two-phase systems in cooling electronic components like microprocessors. The literature further describes various enhancement mechanisms to elevate the critical heat flux (CHF) and heat transfer coefficient (HTC) in these systems. While a high CHF is desired, having a high HTC is equally important to keep the operating temperatures below a permissible limit. The present article summarizes enhancement structures found in the literature suitable for electronic cooling to provide this dual enhancement in CHF and HTC. New enhancement evaluation criteria that also consider the surface temperature limit imposed by the electronic components are introduced. The CHF enhancement ratio (ER<sub>CHF</sub>) represents the ratio of CHF for enhancement structures to the CHF for a plain surface, and the enhancement index (EI) represents the ratio of wall superheat at CHF with the enhanced structures to the wall superheat at its respective CHF condition for a plain surface. It is desirable to have a high value of ER<sub>CHF</sub> coupled with a low value of EI (lower the better), preferably below 1.0.