Re-entrant Cavity Research Articles

Experimental investigations into flow boiling heat transfer in ribbed micro-channels with rectangular reentrant cavities

Reentrant cavities are fabricated at the side walls of ribs and bottom walls of grooves to enhance the flow boiling heat transfer in rectangular-ribbed micro-channels. To reveal the geometrical effect of reentrant cavities, two different reentrant cavities, i.e. RRMB-1 (ribbed micro-channel with narrow reentrant cavities) and RRMB-2 (ribbed micro-channel with wide reentrant cavities), are designed and the RM micro-channel (i.e. smooth ribbed micro-channel without reentrant cavities) is manufactured as the baseline case. The flow boiling heat transfer performance in the micro-channels is experimentally investigated at four mass fluxes and a range of heat fluxes. The flow regimes are visualized during the flow boiling process. The influences of reentrant cavities on the heat transfer coefficient, critical heat flux, pressure drop, outlet temperature fluctuation and performance evaluation criterion of three different micro-channels are analyzed and compared. The results showed that the capillary effect in the reentrant cavities results in a 19.5% increase in the heat transfer coefficient, and a 23.4% increase in the critical heat flux for the RRMB-1 at a low mass flux. The pressure drop in the RRMB-1 micro-channel is slightly higher than that in the RM micro-channel in the single-phase region, while it is opposite at the high heat flux condition. The critical heat flux in the RRMB-2 micro-channel is lower than the RRMB-1 micro-channel by 34.5%, but the pressure drop in the RRMB-2 micro-channel is higher than the RRMB-1 micro-channel by 50.3% in the two-phase region.

Highly efficient fabrication of reentrant microchannels with micro serrated pin fins using a micro staggered multi-edge ball-end milling tool in a single process

Microchannels with micro pin fins and reentrant cavities can increase the heat dissipation area and enhance heat transfer, which are promising for high-performance microchannel heat sinks for heat dissipation of high-heat-flux devices. Nevertheless, their fabrication is time-consuming and cost-inefficient for conventional methods. To this aim, we in this study developed a novel micro staggered multi-edge ball end milling tool (SMBMT) to fabricate a unique type of reentrant microchannels with micro serrated pin fins (RMSPF) in a single process. The formation feasibility of the RMSPF was demonstrated, and they were of narrow exit slots with a width of 500 μm on the top, reentrant circular cavities with a diameter of 800 μm at the bottom, and micro serrated pin fins with a width of about 52 μm and a height of 35 μm on the wall surface of reentrant cavities. More microscale serrated pin fins with much smaller sizes than the micro cutting edges of the SMBMT were obtained due to the staggered arrangement and overlapping effect of the multiple micro cutting edges. A geometrical model of the SMBMT with discrete multiple cutting edges was developed by considering the structure of the RMSPF. The formation process mechanism of RMSPF and its chip formation process was investigated with both experiments and finite element (FE) simulations. Compared to conventional micro ball end milling tool (CBM) with continuous cutting edges, the SMBMT suppressed the burr formation inside reentrant microchannels and improved the surface quality, and reduced the cutting force by up to 53 %. The enhanced cutting performance of SMBMT can be attributed to that the multiple discrete cutting edges of SMBMT effectively decreased the contact area of tool-workpiece and the friction between cutting tool and chips. This study offered a highly efficient method to fabricate microchannels with surface microstructures in a single micromilling process, which provided valuable insights for the development of high-performance microchannel heat sinks in a wide range of application areas.

Equivalent circuit analysis of a higher order mode coaxial reentrant cavity

An equivalent circuit analysis has been proposed for co-axial reentrant cavities which have been used extensively in Multiple-beam Klystron (MBK). Formulation for the equivalent capacitance and inductance of coaxial reentrant cavities (single and double reentrant cavity) have been obtained and used for the calculation of cavity parameters, i.e. resonant frequency and characteristic shunt impedance. The unloaded quality factor of the cavities has been obtained using Wheeler’s incremental inductance rule. Simulations of coaxial reentrant cavities have been carried out using CST studio and simulation results have been compared with the results obtained from the proposed analysis. A good agreement between the results has been obtained.

Cavity-magnon-polariton spectroscopy of strongly hybridized electro-nuclear spin excitations in LiHoF4

We first present a formalism that incorporates the input-output formalism and the linear response theory to employ cavity-magnon-polariton coupling as a spectroscopic tool for investigating strongly hybridized electro-nuclear spin excitations. A microscopic relation between the generalized susceptibility and the scattering parameter |S11|\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|S_{11}|$$\\end{document} in strongly hybridized cavity-magnon-polariton systems has been derived without resorting to semi-classical approximations. The formalism is then applied to both analyze and simulate a specific systems comprising a model quantum Ising magnet (LiHoF4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {LiHoF}_4$$\\end{document}) and a high-finesse 3D re-entrant cavity resonator. Quantitative information on the electro-nuclear spin states in LiHoF4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {LiHoF}_4$$\\end{document} is extracted, and the experimental observations across a broad parameter range were numerically reproduced, including an external magnetic field traversing a quantum critical point. The method potentially opens a new avenue not only for further studies on the quantum phase transition in LiHoF4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {LiHoF}_4$$\\end{document} but also for a wide range of complex magnetic systems.

Oscillation-Based Active Sensor on SIW Reentrant Cavity Resonator for Self-Sustained Ultralow Concentration Detection to Saline Solution

Oscillation-Based Active Sensor on SIW Reentrant Cavity Resonator for Self-Sustained Ultralow Concentration Detection to Saline Solution

Dew points for hydrogen-rich (hydrogen + propane) and (hydrogen + n-butane) mixtures determined with a microwave re-entrant cavity resonator

Dew-point measurements were performed for hydrogen-rich binary mixtures with hydrocarbon mole fractions of 0.09995 C3H8, 0.1499 C3H8, 0.0513 n-C4H10, and 0.09888 n-C4H10 using a modified microwave re-entrant cavity apparatus. Isochoric measurements were conducted in the temperature range of (256 to 286) K with pressures up to 6.8 MPa and 2.0 MPa for mixtures containing propane and n-butane, respectively. The combined expanded uncertainties (k = 2) in dew-point temperature and pressure were estimated to be between (0.35 and 2.0) K and (0.0011 and 0.030) MPa. The agreement with predictions of the GERG-2008 equation of state is within 5.5%. However, better agreement with predicted values was achieved using a cubic equation recently developed in-house. Moreover, experiments with short- and long-term exposure of pressure sensors to hydrogen have shown that pressure measurement, and thus the uncertainty, can be affected; further investigation on this matter is therefore inevitable to improve hydrogen thermophysical property data.

Heat transfer performance of microchannels with different reentrant cavities based on field synergy principle and entropy production analysis

The performance analysis of three types of microchannels with reentrant cavities were investigated in this study. The flow and heat transfer characteristics were analyzed according to the field synergy principle and entropy production. The results indicated that as the Reynolds number ranged from 200 to 1000, the Darcy friction factor of microchannels with reentrant cavities decreased sharply and then slowly. Among microchannels examined, a microchannel with circular reentrant cavities exhibited the smallest Darcy friction factor in the Reynolds number range of 200–1000. The Nusselt number of microchannels with reentrant cavities increased with the Reynolds number range of 200–1000. Among the microchannels, the microchannel with circular reentrant cavities had the largest Nusselt coefficient, from 6.07 to 10.29. According to the principle of field synergy, the heat transfer field of microchannel with circular reentrant cavities had the smallest synergy angle (α), from 85.1°to 83.2°, and the best field synergy. According to an analysis of the entropy production, the entropy production caused by heat transfer in microchannels with reentrant cavities gradually decreased as Reynolds number range from 200 to 1000. The microchannel with circular reentrant cavities had minimal entropy production caused by heat transfer; thus, it had the best heat transfer performance. The flow entropy production of microchannels with reentrant cavities increased with the Reynolds number. Among the microchannels, the microchannel with circular reentrant cavities had the smallest entropy production caused by flow. The present study aims to provide alternative ways to evaluate flow and heat transfer performance of microchannels.

Numerical investigations of heat transfer augmentation in a microchannel heat sink with different shaped periodic reentrant cavities on channel sidewalls

Microchannel heat sink is widely applied in high-density heat flux devices for its high efficiency in heat dissipation. However, the most effective geometrical configuration of microchannels for best heat transfer augmentation with least pump power penalty is still unknown. In this research, the heat transfer augmentation in a microchannel heat sink with different shaped periodic reentrant cavities on channel sidewalls are numerically studied. Three different kinds of channel structures are introduced, including convex quarter-circular, concave quarter-circular, and inclined shaped reentrant cavities. The comprehensive performances of the proposed microchannels are compared with that of the smooth straight channel, with respect to the pressure drop, friction coefficient, Nusselt number, and performance factor. The results indicate that the comprehensive performance of the microchannel heat sink with periodic reentrant cavities is superior to the smooth straight channel, because its reentrant cavities can strengthen the mixing of fluids and lower the pressure loss. The proposed microchannels can enhance the heat transfer by 1.7 times, reduce the pressure drop by 3.7%–22.4% at Re 648.2, with the heat transfer entropy production decreased by 15.9%–48.2%. The inclined shaped microchannel has the best overall performance factor among the three proposed channels, which achieves up to 1.59 at Re 648.2.

Insect-inspired breathing interfaces: investigating robustness of coating-free gas entrapping microtextured surfaces under pressure cycles

Numerous natural and engineering scenarios necessitate the entrapment of air pockets or bubbles on submerged surfaces. Current technologies for bubble entrapment rely on perfluorocarbon coatings, limiting their sustainability. Herein, we investigated the efficacy of doubly reentrant cavity architecture towards realizing gas-entrapping microtextured surfaces under static and dynamic pressure cycling. The effects of positive (>1 atm), negative (<1 atm), and positive–negative cycles on the stability the gas entrapment inside individual doubly reentrant cavities were studied across a range of pressures, ramp rates, intercycle intervals, and water-column heights. Remarkably, the fate of the trapped air under pressure cycling fell into either of the following regimes: the bubble (i) monotonically depleted (unstable), (ii) remained indefinitely stable (stable), or (iii) started growing (bubble growth). This hitherto unrealized richness of underwater bubble dynamics should guide the development of coating-free technologies and help us understand the curious lives of air-breathing aquatic and marine insects.

Effect of oil concentration, viscosity, and surface tension on tube temperature and heat transfer coefficients of R1234ze(E)/oil spray evaporation on an enhanced tube bundle

Shell-and-tube evaporators are widely used in desalination, refrigeration, and petrochemical industries. The heat transfer process in spray-type evaporators combines the droplets impingement at the top rows of the bundles and falling film evaporation for the bottom rows, with a transitional region in the center of the bundle. Spray evaporators are a promising alternative to flooded-type shell and tube heat exchangers because they run with significantly lower refrigerant charge amounts.This study presents new data for the spray evaporation of low-GWP refrigerant/oil mixtures on an enhanced tube bundle. The experiments involved R134a (HFC) and R1234ze(E) (HFO) paired with synthetic Polyolester (POE68 and POE220) lubricants. The tests utilized a boiling enhanced tube (re-entrant cavity tube) and were conducted at saturation temperatures of −6.7 °C, 2.2 °C and 10 °C (20°F, 36°F and 50°F), with heat flux varying from 3 to 35 kW/m2 and oil circulation ratio (OCR) ranging from 0.4 % to 2 %. With the addition of oil, the bundle heat transfer coefficient (HTC) of refrigerant/oil mixtures decreased by 20–50 % at an OCR of 1.2 % and a heat flux of 13.5 kW/m2, across the tested saturation temperatures. The average wall superheat across the bundle exhibited at least a 48 % increase for all the refrigerant/oil mixtures tested at an OCR of 1.2 %. The viscosity of the refrigerant/oil mixture showed a limited influence on heat transfer compared to surface tension, mainly because the oil concentrations were quite small. Oil promoted a partial dry-out of the tubes, especially at the bottom of the bundle. Foaming was observed and increased with heat flux and oil concentration. Foam sheets on the tube bundle created an additional barrier for the liquid refrigerant to reach the tube heat transfer surfaces and thus decreased the overall bundle heat transfer coefficient.

Pool boiling heat transfer of ammonia outside enhanced tubes with various fin structures

Pool boiling heat transfer of ammonia on a smooth tube and three enhanced tubes was investigated at the saturation pressure ranging from 6.15 to 11.7 bar, which corresponds to the saturation temperature from 10 to 30 °C. The test tube with a length of 796 mm was immersed in the liquid pool of ammonia inside a cylindrical chamber having an internal diameter of 254.4 mm, and convection or pool boiling heat transfer outside the test tube was generated by flowing hot water through the tube. Three enhanced tubes of a low-finned tube (Low-fin), a low-finned tube with oblique cuttings (Turbo-C), and a finned tube having re-entrant cavities (Turbo-E) were prepared to examine the pool boiling enhancement on various finned tubes. Boiling hysteresis on the smooth tube was observed between when the heat flux increases and when the heat flux decreases, which resulted in the deviation of the heat transfer coefficients from the previous correlations when the heat flux decreases. A suitable correlation was suggested to compensate for the deviation of the heat transfer coefficients in case of decreasing the heat flux by modifying the Stephan-Abdelsalam correlation. Three enhanced tubes exhibited considerable enhancement in the heat transfer coefficients compared with the smooth tube, and boiling performance on the Turbo-E tube having re-entrant cavities was highly affected by the variation of the bubble diameter according to the saturation pressure. Enhancement factors of the enhanced tubes were obtained from the ratio of thermal conductance on the enhanced tube to that on the smooth tube.

Low frequency, 100–600 MHz, searches with axion cavity haloscopes

We investigate reentrant and dielectric loaded cavities for the purpose of extending the range of axion cavity haloscopes to lower masses, below the range where the Axion Dark Matter experiment (ADMX) has already searched. Reentrant and dielectric loaded cavities were simulated numerically to calculate and optimize their form factors and quality factors. A prototype reentrant cavity was built and its measured properties were compared with the simulations. We estimate the sensitivity of axion dark matter searches using reentrant and dielectric loaded cavities inserted in the existing ADMX magnet at the University of Washington and a large magnet being installed at Fermilab. Published by the American Physical Society 2024

Pool boiling heat transfer of ammonia outside a single tube with fin structures: Hysteresis phenomena and boiling enhancement

Pool boiling heat transfer of ammonia outside a single tube was investigated at three saturation pressures of 6.15, 8.57, and 11.7 bar (saturation temperature of 10, 20, and 30 °C) at the heat fluxes ranging from 1.2 to 72.9 kW/m2. Hysteresis in a boiling curve and boiling enhancement were examined on both the smooth tube with machined roughness and two enhanced tubes with low fins and re-entrant cavities. All three tubes have outermost diameters from 15.68 to 15.87 mm and the same length of 796 mm. A large hysteresis between increasing the heat flux and decreasing the heat flux was observed on the smooth tube, and the hysteresis increased as the saturation pressure decreased. The boiling hysteresis of ammonia was reasonably explained from the thermodynamic analysis based on Clausius-Clapeyron and Young-Laplace equations. Heat transfer coefficients on the enhanced tubes were increased by 47% to 105% (Low-fin) and 103% to 159% (Turbo-E) compared with the smooth tube at the saturation pressure of 8.57 bar. However, the hysteresis itself was increased on the enhanced tubes owing to the wall temperature decrease along the fin. Hydrophobic coating and the enhanced tube having re-entrant cavities with cavity mouths of a few micrometers were suggested to mitigate the hysteresis phenomena in the pool boiling of ammonia.

A PDMS-Assisted Back-to-Back SIW Re-Entrant Cavity Microwave Resonator Film for VOC Gas Detection

A PDMS-Assisted Back-to-Back SIW Re-Entrant Cavity Microwave Resonator Film for VOC Gas Detection

Heat transfer of uncoated and nanostructure coated commercially micro-enhanced refrigeration tubes under pool boiling conditions

The heat transfer performance of commercially produced micro-enhanced tubes with and without a nanocoating was investigated under pool boiling of saturated refrigerant. These multiscale enhancements were on the outside of 19 mm horizontal copper tubes heated by water to determine the effectiveness of this multiscale enhancement technique on industrially relevant tubes and geometry. The tubes tested were a plain tube roughened by sandpaper, a low finned GEWA-KS tube and two micro-enhanced re-entrant cavity tubes, the GEWA-B5 and EHPII. The tubes were tested in R134a at saturation temperatures of 5 °C and 25 °C across a range of heat fluxes from 20 kW/m2 to 100 kW/m2 under pool boiling conditions. The nanocoating applied to the tubes produced a forest of copper oxide nanostructures on the surface, increasing wickability of the surface. A Scanning Electron Microscopy showed that copper oxide nanocoatings coated all micro-enhanced tubes evenly without impeding the surface features or significantly blocking the re-entrant cavities. For the uncoated tubes in pool boiling, the heat transfer coefficients of the EHPII was up to 519 % greater than those of the plain roughened tube, the GEWA-B5 was up to 539 % higher than the roughened tube and the GEWA-KS was at best 64 % higher than those of the plain roughened tube. Increases in the saturation temperature to 25 °C produced minor improvements in heat transfer coefficients. The application of the copper oxide nanocoating resulted in generally decreased heat transfer performance by approximately 40 % on average compared to the uncoated tubes, with the GEWA-B5 tube the worst affected. Degradation of the heat transfer is thought on plain surfaces to be due to the flooding of nucleation sites, while re-entrant cavity style enhanced surfaces were thought to experience degraded sensible and latent heat transfer due to impeded flow in the microstructure capillary channel, as bubbles were noted to be trapped in the channels. It is recommended that the heat transfer at much higher heat flux ranges be explored for possible HTC enhancement.

From Bioinspired Topographies toward Non-Wettable Neural Implants.

The present study investigates different design strategies to produce non-wettable micropatterned surfaces. In addition to the classical method of measuring the contact angle, the non-wettability is also discussed by means of the immersion test. Inspired by non-wettable structures found in nature, the effects of features such as reentrant cavities, micropillars, and overhanging layers are studied. We show that a densely populated array of small diameter cavities exhibits superior non-wettability, with 65% of the cavities remaining intact after 24 h of full immersion in water. In addition, it is suggested that the wetting transition time is influenced by the length of the overhanging layer as well as by the number of columns within the cavity. Our findings indicate a non-wetting performance that is three times longer than previously reported in the literature for a small, densely populated design with cavities as small as 10 μm in diameter. Such properties are particularly beneficial for neural implants as they may reduce the interface between the body fluid and the solid state, thereby minimiing the inflammatory response following implantation injury. In order to assess the effectiveness of this approach in reducing the immune response induced by neural implants, further in vitro and in vivo studies will be essential.

Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures

Multiscale surface structures offer the opportunity to combine the heat transfer enhancement provided by microscale structures with the dryout benefits provided by some nanostructures, which is particularly attractive for falling film evaporators, who have to prevent dryout to ensure high heat transfer coefficients can be maintained.In this study commercially produced plain, low-finned and 3D enhanced commercially produced tubes were tested uncoated as well as coated with fibrous nanostructures that induce wicking on the surface to investigate the opportunity that multiscale enhancements might offer falling film boiling evaporators. Single tubes were heated internally by water and tested in the horizontal position with refrigerant R-134a drizzled over the outside of the tube surface at saturation temperatures of 5 and 25 °C, heat fluxes from 20 to 100 kW/m2 and film Reynolds numbers of up to 2000. The tubes tested were a roughened plain tube, a low-finned Gewa-KS tube and two different 3D enhanced tubes, an Gewa-B5 and an Turbo EHPII, that have networks of re-entrant cavities.The uncoated 3D enhanced tubes achieved the highest heat transfer coefficients, outperforming a roughened plain tube by up to 500 %. The uncoated low-finned tube had heat transfer coefficients that were up to 140 % higher than the roughened plain tube, despite having only an 80 % larger surface area, thought to be due to bubbles sliding down the channels between the fins, increasing the surface area of the bubbles in contact with the surface and thus allowing for increased microlayer evaporation.The application of the copper oxide coating benefitted the low-finned tube, with heat transfer enhancement of up to 60 %, possibly due to liquid wicked underneath the sliding bubbles and onto the fin tips. However, the nanocoating largely decreased the heat transfer of the roughened plain tube and the 3D enhanced tubes, thought to be due flooding of nucleation sites on the roughened tube and interference with the hydraulics of the re-entrant cavity network on the surface of the 3D enhanced surfaces.The low-finned tube also benefitted the most from the falling film boiling heat transfer mode compared to previous pool boiling results, with heat transfer increased by up to 80 and 460 % for the uncoated and coated low-finned tube respectively, again likely due to the bubbles sliding in the fin channels under falling film conditions.The Gewa-B5 tubes had the best dryout performance, followed by the Turbo EHPII tubes and then the low-finned Gewa-KS tube. The open pore structure of the Gewa-B5 likely aided it in dryout prevention, while the fins of the Gewa-KS are thought to have prevented lateral fluid distribution and thus worsen dryout.The dryout performance of the tubes we not markedly improved by the nanocoating, highlighting the difficulty in improving wettability on high wetting low surface tensions fluids such as refrigerants. Microscale enhancements were thus shown to have significant influence on dryout performance, while nanoscale enhancements had little effect in this study.

Bandwidth Enhancement of a V-Band Klystron with Stagger-Tuned Multiple Radial Re-Entrant Cavities.

The V-band frequencies are becoming popular due to their application potential towards secure high data rate communications. This article reports bandwidth enhancement of an 11-cavity V-band Klystron amplifier employing staggered tuning. A systematic approach is presented to stagger-tune the periodically allocated multiple cavities of the Klystron operating at 60.1 GHz. Using the three-dimensional particle-in-cell (PIC) simulation, it is shown that, employing the proposed approach, the -3 dB bandwidth of the device (with peak tuned configuration) has been increased from 165 MHz to 540 MHz, demonstrating a 260% increment. The -1 dB bandwidth of the device is estimated to be 270 MHz. The proposed approach of stagger tuning may be employed for similar devices employing multiple RF cavities to meet the requirement of wide bandwidth.

A phase-locked transit time oscillator operated with low external guiding magnetic field

A phase-locked coaxial transit time oscillator is designed for microwave sources with a small volume and low magnetic field in high-power microwave coherent combining. The oscillator achieves controlled output phase through pre-modulation of electron beams. To improve the phase-locked performance with low magnetic fields, a reentrant input cavity is used to ensure the stability and uniformity of the electric field. Electric field uniformity of the buncher and extractor is also improved. To reduce the device volume, a reflector with pre-modulation function is used to reduce the axial size of the device. Particle-in-cell simulation shows that 5 kW input power is sufficient to lock-in a 610 MW output microwave, with a conversion efficiency of 40.6% at 0.66 T. The input power ratio reaches −50.8 dB. Simulation results demonstrate that the device can still achieve phase locking with a magnetic field as low as 0.5 T.

Strong to ultrastrong coherent coupling measurements in a YIG/cavity system at room temperature

We present an experimental study of the strong to ultrastrong coupling regimes at room temperature in frequency-reconfigurable three-dimensional reentrant cavities coupled with a yttrium-iron-garnet slab. The observed coupling rate, defined as the ratio of the coupling strength to the cavity frequency of interest, ranges from 12% to 59%. We show that certain considerations must be taken into account when analyzing the polaritonic branches of a cavity spintronic device where the rf field is highly focused in the magnetic material. Our observations are in excellent agreement with electromagnetic finite-element simulations in the frequency domain.