Capillary Length Research Articles

Study on the parallel pressure differential type gas laminar flow sensing technique

Parallel pressure differential (PPD) type laminar flow sensing technique was invented several years ago to reduce nonlinear effect in a traditional laminar flow element (LFE). In this paper, the internal flow of each branch in a PPD LFE is numerically simulated for gas flow. The results show that the relative deviation of the pressure drops of the two branches in a PPD LFE is within ±0.05% as inlet mass flow being the same, indicating that the flow resistance characteristics of the two branches are consistent, which means that the hypothesis of a same flow rate for the two branches in a real PPD LFE is tenable. There is little difference, ±0.01%, in the local pressure losses of the two upstream capillaries outlet flows, which can be ignored in a real measurement, further verifying that the theoretical analysis of the PPD principle is reliable. Capillary length effect in a PPD LFE is also examined. The bigger capillary length, the higher measurement precision can be achieved for a certain length range. For instance, it is suggested that the length of the short components should not be shorter than the laminar flow dimensionless entrance length defined by Xe (Le/d/Re, where Le is the entrance length) = 0.035, for flow measurement uncertainty within ±1.0%. The simulation and experiment results of gas flow show that the suitable value of Kexp is 1, and in the flow range of (0.0256–5.2985) m3/h measurement error of a PPD LFE is within ±0.8% only with expansion correction, indicating that the PPD laminar flow measurement technique is suitable for the gas flow.

Experiments and modeling of boiling heat transfer on hybrid-wettability surfaces

This paper studies pool boiling heat transfer on the hybrid-wettability surfaces, i.e., hydrophobic dot/stripe patterns fabricated on a superhydrophilic substrate. It is shown that the nucleate boiling heat transfer coefficient for the hybrid surface is improved compared to both the substrate and plain copper reference, but the critical heat flux (CHF) on the enhanced surface is very complex. The pattern-to-substrate contact angle difference is also concerned: CHF for the hybrid surface increases remarkably with increasing the contact angle difference, but the nucleate boiling heat transfer coefficient declines. Combining the results for both dot and stripe patterns, it is revealed that CHF on the hybrid surfaces is closely associated with the pattern-to-surface area ratio and the pattern-to-pattern spacing: it declines generally with increasing the area ratio; its dependence on the pattern spacing is minor at large area ratios; however, at small area ratios, the pattern spacing plays an increasingly important role because it dominates both vapor-liquid instabilities and surface rewetting, and the optimal pattern spacing for maximal pool boiling heat transfer enhancement can be estimated using the capillary length. A modified theoretical model is proposed to predicting CHF on both homogenous- and hybrid-wettability surfaces.

CFD Analysis of the Optimization of Length of Capillary Tube for a Vapor Compression Refrigeration System

Abstract: The capillary tube is commonly employed in refrigerant flow control systems. As a result, the capillary tube's performance is optimal for good refrigerant flow. Many scholars concluded performance utilising experimental, theoretical, and analysis-based methods. This paper examines the flow analysis of a refrigerant within a capillary tube under adiabatic flow circumstances. For a given mass flow rate, the suggested model can predict flow characteristics in adiabatic capillary tubes. In the current work, R-134a refrigerant has been replaced by R600a refrigerant as a working fluid inside the capillary tube, and the capillary tube design has been modified by altering length and diameter, which were obtained from reputable literature. The analysis is carried out using the ANSYS CFX 16.2 software. The results show thatutilising a small diameter and a long length (R600a refrigerant flow) is superior to the present helical capillary tube. The most appropriate helical coiled design with a diameter of 0.8 mm and a length of 3 m is proposed. Keywords: Capillary Tube, Condenser, Refrigeration effect, CFD.

Transmittance of transparent horizontal and tilted windows supporting large non-absorbing pendant droplets

This study establishes that the deviation of large pendant droplets from an ideal cap-shape due to gravity can have significant and complex impacts on the normal-hemispherical and directional-hemispherical transmittances of light through horizontal and tilted transparent windows. First, the shape of pendant droplets larger than the capillary length was predicted numerically by balancing gravitational and surface tension forces for various droplet volumes, contact angles, and window tilt angles. Then, light transfer through windows supporting such numerically generated droplets was simulated using the Monte Carlo ray-tracing method. The window transmittance for large droplets was found to be nearly independent of droplet spatial arrangement and size distribution for relatively narrow size distributions. Furthermore, the droplets could be assumed to be cap-shaped in predicting the normal-hemispherical transmittance for droplet volumes V< 10 µL and contact angles θc<θcr where θcr is the critical angle for total internal reflection at the droplet/air interface. However, for larger droplets and/or contact angles, assuming droplets to be cap-shaped caused the transmittance to be overestimated by as much as 37% for horizontal windows. This was due to gravity-induced deformation of the droplet shape resulting in increased reflection at the droplet/air interface. For tilted windows, the droplet deformation caused the normal-hemispherical transmittance to increase with increasing droplet volume and window tilt angle. For both horizontal and tilted windows, transmittance decreased linearly with increasing droplet surface area coverage. These results and numerical tools can be used to design energy efficient solar stills, greenhouses, and covered photobioreactors, for example.

An analytical study of lipid-oligonucleotide aggregation properties

Lipid-oligonucleotides (LON) attract great interest as supramolecular scaffolds to improve the intracellular delivery of nucleic acids. Analytical characterization of LON assemblies is critical to formulation development, understanding in-vivo performance, as well as quality control. For this study, we selected LONs featuring different modifications on both oligonucleotide (with or without a G4 prone sequence) and lipid (mono or bis-alkyl chain covalently attached to the oligonucleotide sequence). Size exclusion chromatography (SEC) and, for the first time, capillary electrophoresis (CE) were investigated to study LON supramolecular self-assemblies. Results were correlated to those obtained with conventional physico-chemical characterization techniques i.e. gel electrophoresis, dynamic light scattering, and circular dichroism. In SEC, a separation between LON monomers and micelles was achieved in 5min on a TSK-gel G3000PW column at 70°C with 100% water, as mobile phase. CE conditions were optimized using a fused-silica capillary length of 10.0cm effective length at 15°C. Different background electrolytes were tested by varying the nature and the concentration of salts added. A sodium tetraborate buffer with 75mM NaCl appeared suitable to promote LON assembly. CE offers benefits to LON micelle analysis in terms of speed of analysis, high resolution, and low quantity of sample injected. Moreover, CE provides an appropriate tool to assess the impact of media of biological relevance on LON self-assembly. In this work, the key role of lipophilic tails and the formation of tetramolecular G-quadruplexes on the stability of LON micelles was confirmed.

Loss with ageing but preservation of frontal\xa0cortical capillary pericytes in post-stroke dementia, vascular dementia and Alzheimer\u2019s disease

Cerebral pericytes are an integral component of the neurovascular unit, which governs the blood–brain barrier. There is paucity of knowledge on cortical pericytes across different dementias. We quantified cortical pericytes in capillaries in 124 post-mortem brains from subjects with post-stroke dementia (PSD), vascular dementia (VaD), Alzheimer’s disease (AD) and AD-VaD (Mixed) and, post-stroke non-demented (PSND) stroke survivors as well as normal ageing controls. Collagen 4 (COL4)-positive nucleated pericyte soma were identified as protrusions on capillaries of the frontal cortex. The COL4-positive somata or nodule-like cell bodies were also verified by platelet derived growth factor receptor-β (PDGFR-β) immunohistochemistry. The mean (± SEM) pericyte somata in frontal cortical capillaries in normal young controls (46–65 years of age) was estimated as 5.2 ± 0.2 per mm capillary length. This number was reduced by 45% in older controls (> 78 years) to 2.9 ± 0.1 per mm capillary length (P < 0.001). We further found that the numbers of pericyte cell bodies per COL4 mm2 area or per mm capillary length were not decreased but rather preserved or increased in PSD, AD and Mixed dementia groups compared to similar age older controls (P < 0.01). Consistent with this, we noted that capillary length densities identified by the endothelial marker glucose transporter 1 or COL4 were not different across the dementias compared to older controls. There was a negative correlation with age (P < 0.001) suggesting fewer pericyte somata in older age, although the % COL4 immunoreactive capillary area was increased in older controls compared to young controls. Using a proven reliable method to quantify COL4-positive nucleated pericytes, our observations demonstrate ageing related loss but mostly preserved pericytes in the frontal cortex of vascular and AD dementias. We suggest there is differential regulation of capillary pericytes in the frontal lobe between the cortex and white matter in ageing-related dementias.

Multimaterial 3D-printed contactless conductivity/laser-induced fluorescence dual-detection cell for capillary electrophoresis

利用多材料3D打印技术研制了用于毛细管电泳(CE)的二合一检测池,实现了电容耦合非接触电导(C4D)与共聚焦激光诱导荧光(LIF)两种检测方法在毛细管柱上同一位置同时检测。3D打印的检测池采用了导电的复合聚乳酸(PLA)材料制作C4D的屏蔽层,采用普通的绝缘PLA材料支撑C4D金属管电极并隔离屏蔽层。两根金属管电极通过“打印-暂停-打印”的方式嵌入到检测池中,两电极被2 mm厚的导电屏蔽层隔开,在屏蔽层中有一直径为1 mm的圆形通孔用于LIF检测。该检测池与带流通式进样接口的自组装CE系统联用,用于同时检测无机离子和异硫氰酸荧光素(FITC)标记的氨基酸。研究优化了C4D激励信号频率与电泳缓冲液浓度,选用的电泳缓冲溶液为10 mmol/L 3-吗啉丙烷-1-磺酸(MOPS)与10 mmol/L二(2-羟乙基)亚氨基三(羟甲基)甲烷(Bis-Tris)的混合溶液,选用C4D激励频率为77 kHz。二合一检测池应用于内径为25 μm的毛细管时,C 4D对Na+、K+和Li+的检出限分别为2.2、2.0和2.6 μmol/L; LIF对荧光素和FITC的检出限分别为7.6和1.7 nmol/L。两种检测方法的相对标准偏差在0.3%至4.5%之间(n=3),工作曲线的相关系数r 2≥0.9904。采用3D打印技术可以在实验室内实现复杂结构的制作,降低了制作的成本,且便于方法的推广和改进。

INFERRING CAPILLARY PRESSURE CURVE FROM 2D ROCK IMAGES BASED ON FRACTAL THEORY IN LOW-PERMEABILITY SANDSTONE: A NEW INTEGRATED APPROACH

Reliable capillary pressure data are required for reservoir simulation and fluid flow characterization in porous media. The capillary pressure is commonly measured in the laboratory, which is costly, challenging, and accompanied by measurement uncertainties, especially for low-permeability core samples. Besides laboratory measurements, two-dimensional (2D) rock images reveal another prospect to obtain capillary pressure curves. This paper presents a new integrated approach combining image processing and fractal theory to infer the capillary pressure curve from 2D rock images in low-permeability sandstone. Our approach’s unique feature is its new representation of the pore structure based on information extracted from 2D cross-sections using image processing techniques (i.e. image segmentation and watershed partitioning). Furthermore, we derived an innovative analytical fractal model to calculate the capillary pressure from the newly proposed pore system representation. A new tortuous length equation is introduced to eliminate the developed fractal models’ dependency on the straight capillary length. The pore fractal dimension is computed using the box-counting method from the processed 2D image. The tortuosity fractal dimension is obtained from solving the developed fractal equations of porosity and permeability with the corresponding laboratory measurements. Additionally, a procedure for inferring capillary pressure from multiple cross-sections is proposed. The good accuracy in predicting capillary pressure for five low-permeability sandstone core samples demonstrates the developed approach’s robustness.

Monitoring of circulating amino acids in patients with pancreatic cancer and cancer cachexia using capillary electrophoresis and contactless conductivity detection.

Branched chain amino acids (BCAAs), alanine and glutamine are determined in human plasma by capillary electrophoresis with contactless conductivity detection (CE/C4 D). The baseline separation of five amino acids from other plasma components is achieved on the short capillary effective length of 18cm in 3.2mol/L acetic acid with addition of 13% v/v methanol as background electrolyte. Migration times range from 2.01min for valine to 2.84min for glutamine, and LODs for untreated plasma are in the interval 0.7-0.9μmol/L. Sample treatment is based on the addition of acetonitrile to only 15μL of plasma and supernatant is directly subjected to CE/C4 D. Circulating amino acids are measured in patients with pancreatic cancer and cancer cachexia during oral glucose tolerance test. It is shown that patients with pancreatic cancer and cancer cachexia syndrome exhibit low basal circulating BCAAs and glutamine levels and loss of their insulin-dependent suppression.

Focusing the latent heat release in 3D phase field simulations of dendritic crystal growth

We investigate a family of phase field models for simulating dendritic growth of a pure supercooled substance. The central object of interest is the reaction term in the Allen–Cahn equation, which is responsible for the spatial distribution of latent heat release during solidification. In this context, several existing forms of the reaction term are analyzed. Inspired by the known conclusions of matched asymptotic analysis, we propose a new variant (the ‘ΣP1-P’ model) that is simple enough to allow mathematical and numerical analysis and robust enough to be applicable to solidification under very large supercooling. The important component of the model (the Σ limiter) can also be incorporated into the original models to extend the range of their applicability. The individual models are tested in a number of numerical simulations focusing on mesh-dependence and model parameter settings. When the phase interface thickness is kept large with respect to the microscopic capillary length to make numerical computations feasible, the parameters of the Σ limiter can be tuned to improve agreement with previous models. The results obtained using the ΣP1-P model exhibit a good quantitative agreement with experimental data from rapid solidification of nickel melts.

Enhancement of household refrigerator energy efficiency by studying the effect of refrigerant charge and capillary tube length

Experiments were carried out initially by considering pure R134a and hydrocarbon refrigerant mixtures such as (R290/R600a) (HCM1 44/56, HCM2 50/50, HCM3 54/46, HCM4 64/36, and HCM5 74/26 wt %). Tests are conducted at the atmospheric temperature of 30°C in a domestic refrigerator system. The performance parameters such as pull-down time, desired effect, power consumption, and running cost of the system are to be analysed at different evaporator temperatures, the mass of refrigerant and varying length of capillary tubes. To evaluate the refrigeration effect, Power consumption and COP of the domestic refrigerator at various freezer temperatures (–9°C, –12°C &amp; –15°C) were selected. Results report that out of all the alternative mixtures the amount of energy input was less consumed in the case of HCM1 at a minimal expansion length of 6.3 mm. In the case of HCM5, the least energy was consumed at a capillary length of 5.24 mm whereas in the case of R134a it was at 3.3 mm.

다양한 액체에 떠 있는 부피가 일정한 버블의 형상 비교

In this study, an accurate numerical method has been implemented to compute the shape of floating bubbles. Using the developed code, we have compared and analyzed how the shape of bubbles with a constant volume is changed when floating on six different liquids. Except when the volume of the bubble is very small or large, it has been confirmed that the shape and the rising height from the free surface were significantly changed according to liquids or capillary lengths. In addition, it has been found that the smaller the capillary length, the higher the bubbles were floating on. Since the shape of the isovolumetric bubble differs considerably depending on the liquid, the bursting of the bubble is expected to change accordingly.

Aluminium Recycling in Single- and Multiple-Capillary Laboratory Electrolysis Cells

This work is a contribution to the approach for Al purification and extraction from scrap using the thin-layer multiple-capillary molten salt electrochemical system. The single- and multiple-capillary cells were designed and used to study the kinetics of aluminium reduction in LiF–AlF3 and equimolar NaCl–KCl with 10 wt.% AlF3 addition at 720–850 °C. The cathodic process on the vertical liquid aluminium electrode in NaCl–KCl (+10 wt.% AlF3) in the 2.5 mm length capillary had mixed kinetics with signs of both diffusion and chemical reaction control. The apparent mass transport coefficient changed from 5.6∙10−3 cm.s−1 to 13.1∙10−3 cm.s−1 in the mentioned temperature range. The dependence between the mass transport coefficient and temperature follows an Arrhenius-type behaviour with an activation energy equal to 60.5 kJ.mol−1. In the multiple-capillary laboratory electrolysis cell, galvanostatic electrolysis in a 64LiF–36AlF3 melt showed that the electrochemical refinery can be performed at a current density of 1 A.cm−2 or higher with a total voltage drop of around 2.0 V and specific energy consumption of about 6–7 kWh.kg−1. The resistance fluctuated between 0.9 and 1.4 Ω during the electrolysis depending on the current density. Thin-layer aluminium recycling and refinery seems to be a promising approach capable of producing high-purity aluminium with low specific energy consumption.

Value of nail fold video capillaroscopy and carotid intima media thickness in assessment of micro and macro-vascular disease in systemic sclerosis patients

Aim of the work: To assess micro and macro-vascular involvement in systemic sclerosis (SSc) patients and their possible association with variable disease parameters using non invasive imaging techniques. Patients and Methods: Thirty (SSc) patients and twenty four controls were enrolled. Skin thickness was assessed using modified Rodnan skin score (mRss). Nailfold videocapillarscopy (NVC) was employed to detect microangiopathy and carotid intima media thickness (IMT) was assessed by power Doppler ultrasound. Results: Patients were 96.7% females and 3.3% males, mean age was 41.6 ± 11.5 years, with mean disease duration 4.7 ± 3 years. 73.3% had limited form while 26.7% had diffuse form. The mean mRss was 18.6 ± 8.5. Microangiopathic patterns detected by NVC were: early, active and late (36.65%), (26.7%) and (36.65%) respectively, and structural alterations included: scar (70%), large capillaries (60%), hemorrhage (41.4%), scanty (36.7%), branched (20%), mega capillaries (16.7%), and tall (10%), but neither crossed nor ramified capillaries were detected. The mean IMT was higher in patients (1 ± 0.2 mm) compared to controls (0.7 ± 0.09 mm) (p < 0.001). A significant inverse correlation was found between high density lipoprotein (HDL) with capillary length (r = −0.27, p = 0.05), arterial and venous loop diameters (r = −0.29,p = 0.03)(r = −0.41, p = 0.003) respectively. On linear regression for prediction of HDL, only the relation to venous loop was significant (p = 0.03). Atherosclerosis on carotid doppler showed significant association with arterial loop diameter (p = 0.04), venous loop diameter (p = 0.02) and capillary length (p = 0.001). Conclusion: Micro- and macro-vascular disease in SSc are different entities that do not simultaneously exist in every patient, although endothelial dysfunction can eventually lead to both, and should be meticulously evaluated.

A novel condenser with offset strip fins on a mini channel flat tube for reducing the energy consumption of a household refrigerator

The aim of the study is to reduce the amount of refrigerant and energy consumption of a household refrigerator by conserving the desired temperature inside the fresh food and freezer compartments. In this study, two novel condensers with different offset strip fin configurations on a mini-channel flat tube are used instead of a conventional wire-on-tube condenser. The impact of this novel application on the performance of the refrigerator is investigated experimentally by following the norms defined by IEC 62552:2015. The tests are conducted in a wide range of refrigerant charges (48–64 g) with three different capillary lengths (2800, 3300, and 3800 mm). Alternative couples of refrigerant charge and capillary length for the novel condensers are determined to decrease the daily energy consumption under the level of the original system with the wire-on-tube condenser. One of the aims of the study is achieved by 5.7% of energy-saving when one of the novel condensers is coupled with a 3300 mm length capillary tube and 50 g of refrigerant. The other aim of the study related with the environmental effect is achieved by reducing 10.7% of the refrigerant amount with the application of the novel condenser with offset strip fins on a mini channel flat tube.

A FRACTAL MODEL FOR PREDICTING THE EFFECTIVE THERMAL CONDUCTIVITY OF ROUGHENED POROUS MEDIA WITH MICROSCALE EFFECT

The effective thermal conductivity (ETC) of roughened porous media (RPM) is of interest in a number of applications of heat transfer. In this work, a fractal analytical model for the ETC of RPM with microscale effect is proposed. The proposed fractal model is expressed in terms of relative roughness, the molecular mean free path, porosity, fractal dimensions (pore area fractal dimension and tortuosity fractal dimension), maximum pore diameter, capillary straight length, and the thermal conductivity of the solid matrix and gas. It is observed that the dimensionless ETC of RPM decreases with increasing relative roughness, pore area fractal dimension and tortuosity fractal dimension. Besides, it is found that the dimensionless ETC of RPM increases with thermal conductivity ratio of gas phase over solid phase. In addition, it is found that the dimensionless ETC of RPM is slightly dependent on the relative roughness and tortuosity fractal dimension when [Formula: see text]. The determined dimensionless ETC of RPM is in good agreement with experimental data and existing models reported in the literature. With the proposed fractal model, the physical mechanisms of heat transport through RPM with microscale effect are better elucidated. Every parameter in the fractal analytical model has clear physical meaning, with no empirical constant.

Hollow-glass-microsphere-assisted half-circle interference for hydrostatic pressure measurement with high sensitivity

We propose and demonstrate a half-circle interferometer using a hollow glass microsphere (HGM) resonator. The half-circle interference is induced by a mismatch between the fundamental mode in the HGM and the modes in the capillary wall. The theoretical model is verified by comparing the simulated and experimental results. The variation in capillary length induced by the axial pressure contributes the most to the half-circle interference, which features a device with a high hydrostatic pressure sensitivity of -1.099 nm/kPa. This device shows potential as a hydrostatic pressure sensor owing to its stability, high sensitivity, and robustness.

A compact and high sensitive photometer based on axial multi-reflection inside metal capillary: Application to detecting nitrite of nanomolar concentration

As for photometry analysis, increasing the optical-path (OP) via multi-reflection is an effective method for improving the detection sensitivity. In this paper, a photometer based on axial multi-reflection between two end-faces of capillary was proposed and investigated. The probe light can be back and forth reflected inside the capillary, by employing apertured silver-mirrors at both ends of a metal-capillary. With designed aperture profile (i.e., a central disc-aperture and a peripheral ring-aperture) of the mirror, a 4.2-fold enhancement on OP was realized (i.e., OP is 4.2 times the capillary length). Moreover, the photometer was applied to detecting nitrite with a detection limit of 1 nM, which exhibits 8.6~15 fold improvement compared with that of previous photometers with similar capillary length and the same chromogenic reagent without sample enrichment. The photometer features compact, low cost and easy fabrication.

Simplified miniaturized analytical set-up based on molecularly imprinted polymer directly coupled to UV detection for the determination of benzoylecgonine in urine

A simple, selective, and sensitive method involving a miniaturized solid phase extraction step based on a monolithic molecularly imprinted polymer (MIP) directly coupled on-line to UV detection was developed for the determination of benzoylecgonine (BZE) in complex biological samples. Monolithic MIPs were prepared into 100 μm internal diameter fused-silica capillaries either by thermal or photopolymerization. While leading to similar selectivities with respect to BZE, photopolymerization has made it possible to produce monoliths of different lengths that can be adapted to the targeted miniaturized application. The homogeneous morphology of these monolithic MIPs was evaluated by scanning electron microscopy prior to measuring their permeability. Their selectivity was evaluated leading to imprinting factors of 2.7 ± 0.1 for BZE and 4.0 ± 0.6 for cocaine (selected as template for the MIP synthesis) with polymers resulting from three independent syntheses, showing both the high selectivity of the MIPs and the reproducibility of their synthesis. After selecting the appropriate capillary length and the set-up configuration and optimizing the extraction protocol to promote selectivity, the extraction of BZE present in human urine samples spiked at 150, 250, and 500 ng mL−1 was successfully carried out on the monolithic MIP and coupled directly on-line with UV detection. The very clean-baseline of the resulting chromatograms revealing only the peak of interest for BZE illustrated the high selectivity brought by the monolithic MIP. Limits of detection and quantification of 56.4 ng mL−1 and 188.0 ng mL−1 were achieved in urine samples, respectively. It is therefore possible to achieve analytical threshold in accordance with the legislation on BZE detection in urine without the need for an additional chromatographic separation.

Vapor stem bubbles dominate heat transfer enhancement in extremely confined boiling

Boiling has long been sought as the heat dissipation mechanism for a wide variety of compact thermal management applications owing to low-resistance heat transport, high heat flux limits, and surface isothermalization. This work aims to elucidate the thermofluidic transport mechanisms of boiling in extremely confined gaps through experimental measure of the temporal evolution of heat fluxes and surface temperatures during deionized water boiling, as well as high-speed visualization of bubble formation. The flow visualizations reveal small residual pockets of vapor, termed ‘stem bubbles’ herein, that remain on the boiling surface through a pinch-off process where vapor escapes through the edges of the confined heated region. These stem bubbles act as seeds for vapor growth in the next phase of the boiling process. These bubbles dictate the boiling performance for extremely confined boiling as defined based on a dimensionless ratio of the gap spacing to capillary length (Bo≤ 0.35 - 0.5). This conclusion is supported by the enhanced thermal response of the surface compared to nucleate boiling. Because activation of nucleation sites is not required for stem bubble boiling, phase change occurs at a reduced surface superheat at a given heat flux compared to nucleate boiling. Criteria for the dimensionless confinement gap spacing are identified to harness this improved heat transfer rate of the stem bubble boiling regime. This new understanding of boiling in extremely confined gaps offers a new direction to design compact two-phase thermal management solutions through using the unique enhancements provided by the vapor stem bubble boiling regime.