Re-entrant Cavity Research Articles

A Novel Differential Capacitive Humidity Sensor on SIW Re-Entrant Cavity Microwave Resonators With PEDOT:PSS Film

This work presents a differential humidity sensor by longitudinally stacking the sensing and referencing substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for relative humidity (RH) detection. Three different technologies, such as air-filled substrate, metal grid holes (METGH) and sensitive material, are integrated to enhance the humidity sensing response. Firstly, introduction of air-filled substrate and METGH can effectively improve the flowability of the humid air, and can reduce the response time. Secondly, PEDOT:PSS is used as a humidity sensitive (HS) material and deposited in the highly concentrated E-field distribution area of sensing SIW RECR, and the modulation of the dielectric constant of HS material by humid air can achieve high humidity sensitivity. Furthermore, the proposed differential structure itself can suppress common-mode perturbations to prevent temperature cross sensitivity. Several different sensors are fabricated and a series of experiments are performed to investigate the dependence of humidity sensing response on the device structure and the deposited amount of HS material. When a specific amount of HS material is deposited in the HS area, excellent humidity sensing performances such as high sensitivity, low hysteresis and fast response can be successfully achieved. Among them, the sensor <inline-formula> <tex-math notation="LaTeX">$\text{B}_{{1}}$ </tex-math></inline-formula> (300-uL HS material and with METGH) demonstrates the best humidity sensing performance in the range of 32.8&#x0025; to 97.3&#x0025; RH, and shows high sensitivity of 209.3 kHz/&#x0025;RH and low hysteresis of 0.592&#x0025; RH, and the corresponding response time is reduced to 1/6 of that without METGH. Moreover, the sensor <inline-formula> <tex-math notation="LaTeX">$\text{B}_{{1}}$ </tex-math></inline-formula> has excellent suppression on the temperature variation.

Pool boiling heat transfer enhancement of R134a, R32, and R600a using reentrant cavity surfaces

ABSTRACT The nucleated pool boiling heat transfer experiments for plain and five different reentrant cavity copper tube surfaces in saturated R134a, R32, and R600a have been carried out. The experiments are carried for heat fluxes ranging from 6.92 to 51.71 kW/m2 at saturation temperatures of 7°C and 10°C. A significant increase in the boiling heat transfer coefficient has been observed for all tube surfaces with the reentrant cavity. The nucleate boiling heat transfer coefficient for R134a, R32, and R600a is 1.2–2.8 times more than that for plain tube surfaces. A correlation for reentrant cavity surfaces has also been developed.

A resonant cavity system for exposing cell cultures to intense pulsed RF fields

The IEEE and ICNIRP had specified a maximum permissible exposure for instantaneous peak electric field of 100 kV/m. However, no rationale was given for this limit. A novel exposure system was designed through a detailed process of analytical analysis, numerical modelling and prototype testing. The system consists of a cylindrical re-entrant resonant cavity that can achieve an electric field strength of more than 100 kV/m with an input power of 200 W. The working of the system was evaluated in simulation and experiment in terms of scattering parameters, electric field distributions and specific absorption rate. The system was then used to carry out in-vitro exposures of a human lymphoid cell line (GG0257) to a 1195 MHz signal at 53 dBm peak power and a pulse width of 550 ns at a range of interpulse intervals to identify heating-induced changes in cell viability. The proposed system offers high Q value of 5920 in unloaded condition which was reduced to 57 when loaded with 12 ml of cell culture but still offering 67 kV/m of the field intensity. Using the system for the exposure of GG0257 cells lasting 18 min, interpulse intervals of 11 μs or less caused a reduction in the number of viable cells and a corresponding increase in necrotic cells. For a shorter exposure duration of 6 min, the reduction in cell viability was seen at interpulse intervals of 5.5 μs or less. The designed exposure system is well capable of handling high intensity electric fields. Temperature measurements with a fibre optic probe and temperature sensitive labels showed that changes in viability were associated with temperature increases above 46 °C. This novel exposure system is an efficient means to investigate the possible relationship between peak field intensity and biological effects to provide a rationale behind the maximum exposure limit of 100 kV/m.

Conductivity measurement using 3D printed re-entrant cavity resonator

A technique for measuring effective conductivity of conductor materials using 3D printed re-entrant cavity resonator is proposed. An analytical formula for the extraction of the effective conductivity has been derived in relation to energy stored in the volume of the cavity geometry. A method of resonant cavity characterisation of material based on microwave losses is utilised for the measurements. The approach offers a simplified analytical method and also supports the measurements of sample with arbitrary thickness. Samples produced from three different manufacturing processes of computer numerical control (CNC) and 3D printing, made of aluminium, copper and stainless steel were measured to demonstrate the method. The 3D printed and copper coated polymer sample is considered as reference material for the measurements. The measured results have shown that the copper coated polymer sample have similar conductivity with that CNC copper. This signifies the good finishing, low surface roughness and quality of copper coating used in 3D printed polymer device.

The Effect of Surface Geometry of Solid Wall on the Collapse of a Cavitation Bubble

Abstract The objective of this paper is to reveal the influence of different surface geometric conditions on the dynamic behavior characteristics of a laser-induced bubble collapse. A high-speed camera system was used to record the oscillation process of the laser-induced bubble on plane solid walls with different roughness and a wall containing reentrant cavities full of water or gas. The focus is on the quantitative analysis of the morphological characteristics of the cavitation bubble near the solid wall under different surface forms during the first two oscillation periods. The results show that the dimensionless ratio γ, defined as the distance from the center of the bubble to the wall divided by the maximum radius of the bubble, has a great influence on the change of the cavitation shape in the direction of the vertical wall. Different surface geometries without gas in our cases have no significant effect on the collapse time of cavitation bubbles. While for the surface containing gas, the direction of movement of the bubble accompanying the microjet will greatly change during the collapse of the cavitation bubble, and the collapse time seems to be independent of the dimensionless ratio γ. These achievements shed the light for engineering to avoid the damage of the microjet caused by designing suitable surface geometry.

On the direction of the re-entrant jet and the limiting cavity flow configurations

In this paper, we investigate an old classical free streamline problem, namely, the two-dimensional re-entrant jet cavity flow past an obstacle. It is well known that for the re-entrant jet model, the direction of the jet is a free parameter that can be specified arbitrarily. To fix this uncertainty, we make a complementary conjecture: the direction should be chosen so that the mean kinetic energy of the remote part of the jet is minimal. Considering cavity flows over an oblique flat plate as an example, we show numerically that the direction is almost opposite to the incident flow. In addition, we present an analytical confirmation of this conclusion that is independent of the obstacle shape. Further, considering again the oblique flat plate as an example, we give an answer to the following question: What happens when the angle of attack tends to zero and the cavity number is finite? We demonstrate that for the limiting configurations, the re-entrant jet vanishes, and the limit is a free-surface flow with a symmetric bubble above the plate and two stagnation points on the lower side of the plate. For this limit we construct a simple exact analytical solution.

HEAT-TRANSFER ENHANCEMENT FOR THE POOL BOILING OF QUASI-AZEOTROPIC REFRIGERANT MIXTURE (R-410A) ON THE RE-ENTRANT CAVITY SURFACES

The present experiments are carried out to analyze the heat-transfer mechanism and study the effects of cavity mouth size of re-entrant cavity surfaces on boiling heat transfer in R410A quasi-azeotropic refrigerant mixtures at 7 and 10° C saturation temperature. The experiments are carried out for heat fluxes ranging from 10.7 to 79.7 kW/m2. An analysis of experimental data shows that the reentrant cavities with the mouth size of 0.51, 0.41, 0.31, 0.25, and 0.21 mm, and 0.11 mm can be advantageously used to enhance the heat-transfer coefficient of plain tubes. The nucleate boiling heat-transfer coefficients for R410A are 1.5-2.3 times more than plain tube surfaces. Finally, based on experimental data, correlations have also been developed for the prediction of heat-transfer coefficient during the pool boiling of saturated R410A over re-entrant cavity tubes.

A Microwave Displacement Sensor Based on SIW Double Reentrant Cavity with Ring Gaps

A Microwave Displacement Sensor Based on SIW Double Reentrant Cavity with Ring Gaps

Precise determination of R134a boiling bundle effect on a column of reentrant cavity tubes

The boiling bundle effect was investigated on a column of reentrant cavity tubes to reveal the heat transfer coefficient (HTC) variation for a tube which is affected by other tubes. All experimental data were obtained on the specific tube, which is the distinct difference between the adopted homologous method and the routine method. Two columns of eight horizontal tubes were submerged in R134a liquid, and each tube was specifically heated by an ethylene glycol–water mixture. The saturation temperature was 6.0 °C, the heat flux ranged from 10.0 kW/m2 to 28.0 kW/m2, and the row number ranged from 1 to 7. The tube length in the test section was 500 mm, and the pitch–diameter ratio was 1.32. Results showed that the uncertainty of the bundle effect using the homologous method was less than ± 4.4%, which decreased by at least 80% compared with that using the routine method. The values were from 1 to 1.32, indicating that the upward bubbles from the lower heated tubes positively influenced the boiling HTC. Moreover, the bundle effect increased with the increase in row number and decreased with the increase in heat flux. Correlations of the single-tube HTC and the bundle effect with vapor film Reynolds number and effective row number were established to predict the experimental data. The current study can offer a reference for further investigations of film or convective boiling on tube bundles.

Prototype superfluid gravitational wave detector

We study a cross-shaped cavity filled with superfluid $^4$He as a prototype resonant-mass gravitational wave detector. Using a membrane and a re-entrant microwave cavity as a sensitive optomechanical transducer, we were able to observe the thermally excited high-$Q$ acoustic modes of the helium at 20 mK temperature and achieved a strain sensitivity of $8 \times 10^{-19}$ Hz$^{-1/2}$ to gravitational waves. To facilitate the broadband detection of continuous gravitational waves, we tune the kilohertz-scale mechanical resonance frequencies up to 173 Hz/bar by pressurizing the helium. With reasonable improvements, this architecture will enable the search for GWs in the 1-30 kHz range, relevant for a number of astrophysical sources both within and beyond the Standard Model.

A Miniaturized Differential Microwave Microfluidic Sensor With High Decoupling

This letter presents a miniaturized differential microfluidic sensor by longitudinally integrating two substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for characterization of dielectric liquid materials. The mutual coupling between these two RECRs is prevented by the stacked metal wall between them. The feedline implemented by a microstrip-to-stripline-to-SIW transformer in the stacked metal wall is proposed to excite these two RECRs simultaneously. The dependence of complex permittivity of calibrated samples on the resonance frequency shift and quality factor of the sensor is used to build a predictive model of complex permittivity of liquid. The ability of this sensor to accurately measure liquid complex permittivity is verified by practical measurements.

Effect of stagger angle on capillary performance of microgroove structures with reentrant cavities

Effect of stagger angle on capillary performance of microgroove structures with reentrant cavities

Effect of wettability on nucleate pool boiling heat transfer of a low surface tension fluid outside horizontal finned tubes

Nucleate pool boiling heat transfer of a low surface tension fluid (R134a) is tested outside the surfaces with polymer coating. Nanoscale Parylene films are coated on the surfaces with Plasma Enhanced Chemical Vapor Deposition(PECVD) method. The thickness of the coating is about 10 nm. Heat flow rate intensity varies in the scope of 15–150 kW/m2. Saturation temperatures are 6, 10 and 16 °C, respectively. According the investigation, for the smooth tube, it is found that the roughness of the surface reduced after the coating and the pool boiling heat transfer performance is also deteriorated. The boiling heat transfer can be enhanced to a certain degree for two finned structures. For the saw-teeth tube, the heat transfer coefficient(HTC) can be 1.1–1.3 times higher than that without coating. For the reentrant cavity tube, at lower heat flux, the effect of coating on the performance is insignificant. While, the heat transfer can be enhanced up to a factor of 1.2 at higher heat flux. In general, the effect of the Parylene polymer coating on the pool boiling of the low surface tension fluid R134a are insignificant at the moderate heat flux

Surface structure repels water droplets at room temperature, ice at colder temperatures

Designing a doubly reentrant cavity structure that achieves promising hydrophobicity and icephobicity

Effect of Beam tunnels on Resonant Frequency of Cylindrical Reentrant Cavity

Analytical formulations for the resonant frequency of a reentrant cavity for klystron are available in the literature only for such cavities having a single beam-tunnel. An improved analytical formulation has been proposed in this paper for the calculation of cavity gap-capacitance of reentrant cavities having single and multiple beam-tunnels and its effects on the resonant frequency are studied. The results obtained through analysis have been validated against those obtained from the 3D electromagnetic field simulations and measurements. The proposed analytical formulation provides good estimation of resonant frequency of cavity with single and multiple beam-tunnels.

Droplet impact dynamics and heat transfer on nanostructured doubly reentrant cavity under freezing temperature

Icephobic materials have wide applications for economic reasons as well as for risk reduction of ice accretion on an airframe. However, the mechanism behind the effect of structure parameters on droplet wetting dynamics and heat transfer on a doubly reentrant cavity is still elusive. This paper reports droplet impact dynamics and heat transfer on a set of doubly reentrant cavity surfaces by changing linewidth, microstructure height, and solid fraction under different surface temperatures and droplet impact velocities. It was found that the ratio of pitch distance and microstructure height is the most important parameter to control droplet dynamics and heat transfer. The surface with a small ratio (P/H &amp;lt; 1) of pitch distance (P) and microstructure height (H) has the best performance, which can successfully repel the droplet even when We = 1000 under −20 °C. The process of the liquid penetration was theoretically studied, and it was found that the temperature of the air inside largely increased due to droplet impact. The smaller the ratio, the higher the temperature increased. The increased air temperature restrains the ice nucleation rate and reduces the viscosity of water to make it easier to be drained out and therefore achieve icephobicity. When the kinetic energy of droplets is sufficient high, the compressed air temperature is so high that an expanding bubble will be generated at the center to make the liquid depart from the surface in a significantly shorter time named “doubly recoil” state, for the liquid in this state is recoiled from both inside and outside.

Liquid film boiling on plain and structured tubular surfaces with and without hydrophobic coating

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m−2) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m−2. Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux.

A Modified SIW Re-Entrant Microfluidic Microwave Sensor for Characterizing Complex Permittivity of Liquids

In this paper, a modified substrate integrated waveguide (SIW) re-entrant microwave sensor is implemented by an annular slot loaded SIW re-entrant cavity resonator (RECR) to evaluate complex permittivity of liquid under test (LUT). The fringe electric field (E-Field) around metal post of the SIW RECR is suppressed by loading the plane disk. And then, by loading a narrow annular slot, the corresponding fringe non-linear capacitor is transferred as an annular capacitor which has linear response to real permittivity of LUT and is connected in parallel with the gap parallel plate capacitor of the RECR, and then the corresponding physical equivalent circuit model is accurately established. The dependence of the sensitivity of the proposed sensor on the dimension of the annular slot is discussed. The unique quantitative function relationship, instead of a mathematical fitted one, between the resonant frequency and the real permittivity of LUT is obtained to characterize the real permittivity predictive model by reasonably neglecting the ratio term of annular-slot capacitance to LUT effective capacitance. Additionally, an improved algorithm for more accurately extracting imaginary permittivity is proposed. A prototype of the proposed sensor is fabricated and measured. The predictive model of the complex permittivity is calibrated by using of methanol-water mixtures with known complex permittivity and is further verified by ethanol-water mixtures. The result of experiment verification, with the maximum measurement errors of real and imaginary permittivity of 4.07% and 7.79%, demonstrates that the complex permittivity of the liquid can be accurately characterized by using the proposed sensor.

Effects of materials and microstructures on pool boiling of saturated water from metallic surfaces

Pool boiling experiments using horizontal, flat, and upwards facing aluminum, stainless steel, and copper, heating surfaces are conducted in distilled water. The wettability, measured by the static contact angle (CA), and the critical heat flux (CHF) are found to vary significantly among the three surfaces. The Boehmite treated aluminum, stainless steel, and copper surfaces have a CHF of 1850, 1375, and 1015 kW/m2, respectively. Powder Injection Molding (PIM) and High-Temperature Conductive Microporous Coating (Cu-HTCMC) are also applied on the copper heating surface to create various microstructures, and their pool boiling performances are compared to that of a plain copper surface. The PIM and Cu-HTCMC reduce the contact angle and increase the CHF value. However, the Cu-HTCMC causes an increase in CHF well beyond the expected value, and it is believed to be due to the porous coating structures and reentrant type cavities within the microporous coating.

A Folded Substrate Integrated Waveguide Re-Entrant Cavity for Full Characterization of Magneto-Dielectric Powder Materials

In this study, a folded substrate integrated waveguide (FSIW) re-entrant cavity sensor (RECS) is proposed for full characterization of magnetic-dielectric powder materials. In the re-entrant cavity, the electric field (E-field) and magnetic field (H-field) are localized at two different regions for detecting complex permittivity and complex permeability independently. Due to the extremely weak H-field in the high-intensity E-field region of the re-entrant cavity, the impact of the high magnetic loss of magneto-dielectric (MD) materials on the permittivity measurement is ignored, which simplifies the algorithm for extracting the complex permittivity. The simulated results demonstrate a good electromagnetic separation performance of the FSIW RECS, characterized by the very low cross-sensitivity-ratios ${S}_{\textit {fm}2}{/}{S}_{\textit {fe}}$ of 0.0289 and ${S}_{\textit {Qm}2}{/}{S}_{\textit {Qe}}$ of 0.0035. Folding technology is used to enhance the sensitivity for extracting the complex permittivity and realize the miniaturization of the designed sensor. To accurately characterize the MD materials with the high magnetic loss tangent up to 0.7, the weak-coupling feeding method is employed to improve the Q-factor of this sensor, and magnetic loss tangent is taken into account in the extraction of the real part of the permeability. The prototype of the FSIW RECS was fabricated and experimentally verified by measuring the complex permittivity and permeability of several powder materials. The measured relative permittivity and permeability of powder samples are in good agreement with the referred ones, and the corresponding maximum relative errors are 3.79% and 4.44%, respectively.