Subcooling Temperature Research Articles

Enhanced Hydrate-Based Geological CO2 Capture and Sequestration as a Mitigation Strategy to Address Climate Change

Geological sequestration of CO2-rich gas as a CO2 capture and storage technique has a lower technical and cost barrier compared to industrial scale-up. In this study, we have proposed CO2 capture and storage via hydrate in geological formation within the hydrate stability zone as a novel technique to contribute to global warming mitigation strategies, including carbon capture, utilization, and storage (CCUS) and to prevent vast methane release into the atmosphere caused by hydrate melting. We have attempted to enhance total gas uptake and CO2 capture efficiency in hydrate in the presence of kinetic promoters while using diluted CO2 gas (CO2-N2 mixture). Experiments are performed using unfrozen sands within hydrate stability zone condition and in the presence of low dosage surfactant and amino acids. Hydrate formation parameters, including sub-cooling temperature, induction time, total gas uptake, and split fraction, are calculated during the single-step formation and dissociation process. The effect of sands with varying particle sizes (160–630 µm, 1400–5000 µm), low dosage promoter (500–3000 ppm) and CO2 concentration in feed gas (20–30 mol%) on formation kinetic parameters was investigated. Enhanced formation kinetics are observed in the presence of surfactant (1000–3000 ppm) and hydrophobic amino acids (3000 ppm) at 120 bar and 1 ℃ experimental conditions. We report induction time in the range of 7–170 min and CO2 split fraction (0.60–0.90) in hydrate for 120 bar initial injection pressure. CO2 split fraction can be enhanced by reducing sand particle size or increasing the CO2 mol% in incoming feed gas at given injection pressure. This study also reports that formation kinetics in a porous medium are influenced by hydrate morphology. Hydrate morphology influences gas and water migration within sediments and controls pore space or particle surface correlation with the formation kinetics within coarse sediments. This investigation demonstrates the potential application of bio-friendly amino acids as promoters to enhance CO2 capture and storage within hydrate. Sufficient contact time at gas-liquid interface and higher CO2 separation efficiency is recorded in the presence of amino acids. The findings of this study could be useful in exploring the promoter-driven pore habitat of CO2-rich hydrates in sediments to address climate change.

Implementing Superhydrophobic Surfaces within Various Condensation Environments: A Review

AbstractSteam condensation is omnipresent, and inevitable in many industrial processes. A classic example is seen within condensers that are used to liquify various gasses. In consequence, water‐vapor is eventually deposited upon the exterior of heat pipes that passively cool the gas to form a thin film of liquid. This macroscopic film is responsible for the degradation of heat transfer efficiencies. Liquid repellent surfaces can microscopically manipulate the hydrodynamics of formulating condensate to transition toward dropwise/jumping‐droplet condensation. This effect will save operating and maintenance costs of steam condensers within power‐plants, subsequently reducing emissions, operating power, and fuel consumption. However, the challenges associated with the stabilization of dropwise/jumping‐droplet condensation, especially at large subcooling temperatures and high supersaturation ratios, have prohibited the use of liquid repellent surfaces. This review aims to discuss recent improvements and understanding of dropwise/jumping‐droplet condensation surfaces on superhydrophobic surfaces. Numerous examples shall be included, showing experimental and numerical studies of various innovative properties of superhydrophobic structures. In addition, various fabrication techniques of superhydrophobic surfaces that are applicable within industrial systems are discussed. These topics are consolidated to provide guidelines for future research into dropwise/jumping‐droplet condensation on liquid repellent surfaces.

Experimental and theoretical study of a vertical tube in shell storage unit with biodegradable PCM for low temperature thermal energy storage applications

This article presents the experimental investigations of the coconut oil-based TES module for HVAC applications in the ambient and-sub ambient temperature range. To properly study this problem modular experimental module and test loop were developed. Special attention has been paid to study the physical mechanism of the melting/solidification process for natural substance (coconut oil) which has perspectives to be used in thermal energy storage systems. It has been a highlight that the melting/solidification process is much more complex for non-eutectic mixtures and there are limited analytical models witch could be used to design TES working with such substances. Due to the limit of literature data for thermophysical properties of coconut oil own experimental data has been conducted.The article presents both visual and thermal characteristics of the melting and solidification process. It has been confirmed the strong influence of natural convection phenomena at the melting process. What is more, it has been observed that in the case of coconut oil it is better to talk about appearance melting temperature than melting temperature. The experimental analysis confirmed that appearance melting temperature is a function of radial distance from the heat source. In the case of solidification, the influence of natural convection is reduced quickly due to the increasing domination of the conduction heat transfer mechanism. For the solidification temperature profiles are characteristic of the occurrence of subcooling temperature. The level of temperature drop in sub cooling is a function of radial distance from the heat source. It has been also confirmed that in case of melting as well as solidification process the shape of the phase-change profile seems to form a truncated cone (in case of melting inverted).To validate the engineering approach to vast discrepancies in the literature data own experiment has been evaluated with a simple mathematical model to verify the applicability of the literature data in system behavior prediction. Based on the best-fit approach the parameters thermal resistance of the module R = 1.3 W/K was found. Additionally the latent heat of the coconut oil of 178 kJ/kg with Gaussian distribution (σ=1.1) for melting in temperature of 25 °C and solidification at 21 °C. The proposed correlation can be easily used to optimize latent thermal energy storage cooperated with the HVAC system.

Two-phase interfacial area characteristics and transport under the transition from subcooled boiling to saturated boiling flows

This study focuses on the analysis of the transition from the subcooled boiling to saturated boiling flow. During this process, vapor generates drastically in a short developing length, which makes it difficult to predict the interfacial characteristics of two-phase flow. To investigate the characteristics of this process, experiments were performed in a vertical annulus with 19.1 mm inner diameter and 38.1 mm outer diameter. The two-phase interfacial parameters including time-averaged local vapor fraction, interfacial area concentration, and bubble interfacial velocity are obtained using high-temperature miniature four-sensor electrical conductivity probes. In this study, the conductivity probe signal is processed using the newly-developed signal processing algorithm developed for small spherical bubbles by Shen and Nakamura (2014) , instead of the algorithm used in the previous subcooled boiling experimental studies Ozar (2009) and Kumar et al. (2019), [3] for large bubbles developed by Kataoka et al. (1986). The flow conditions cover a wide range of inlet subcooling temperature at 12.9–21.2 °C, heat flux at 57.9–197.3 kW/m2, and inlet flow rate at 0.265–0.536 m/s. The vapor fractions are cross-validated with impedance void meter measurements. The state-of-art two-group interfacial area transport equations (IATE) [6], (Ozar, 2009) and (Brooks and Hibiki, 2016) is evaluated with the experimental data. The result shows that the two-group IATE gives fairly good predictions on the transition phenomena. Besides, a defect was identified in the condensation model (Park et al., 2007) used in IATE, which is the bubble boundary diameter classifying the two condensation mechanisms: heat-transfer-controlled and inertia-controlled condensation. A rigorous approach to estimate this boundary diameter is established in this study.

Subcooled flow boiling of water in a metal-foam tube: An experimental study

The present article experimentally explored heat transfer through a horizontal metal-foam tube with a constant wall heat flux during the subcooled flow boiling of water. The impacts of significant parameters, including subcooled inlet temperature (30–60 °C), mass flux (150–310 kg/m2s), and heat flux (100–230 kW/m2) were investigated on the transfer of boiling heat while measuring pressure loss and heat transfer coefficient. Inserting metal foam into a tube increased the HTC of flow boiling by a maximum of 580%. An increase of 22% was observed in the heat transfer increment divided by pressure loss defined as the thermal performance index.

A mass-preserving interface-correction level set/ghost fluid method for modeling of three-dimensional boiling flows

We present an efficient method for the direct numerical simulation of three-dimensional (3D) boiling flows. The liquid-vapor interface dynamics is captured using an interface-correction level set method, modified to account for the interface heat and mass transfer due to phase change. State-of-the-art computational techniques, such as the fast pressure-correction and ghost-fluid methods, are implemented to accurately solve the coupled thermo-fluid problem involving large density contrasts and jump conditions. The solver is thoroughly validated against four benchmark cases with increasing complexity, which show better mass conservation properties than traditional level set methods, thus allowing for coarser grid resolutions and lower computational costs. We further demonstrate our method by simulating two realistic 3D boiling flows in greater details. In the first case, a saturated film boiling of water vapor at near critical conditions over a horizontal hot flat plate is considered. The results are analyzed by comparing the transient evolution of the interface morphology, temperature distribution, space and time averaged Nusselt numbers obtained from numerical simulations with the semi-empirical correlation of Berenson and existing numerical literature. In the second case, we simulate the condensation and buoyancy-driven motion of a single spherical water vapor bubble at different subcooled liquid temperatures and saturation pressures. We find opposite trends of the condensation rate and the bubble rising velocity when the degree of subcooling is increased, and an increase of the condensation rate at lower saturation pressures, due to variation of the thermophysical properties.

Experimental Study on Heat Transfer and Flow Characteristics in Subcooled Flow Boiling in a Microchannel

The current study presents an experimental investigation of heat transfer and flow characteristic for subcooled flow boiling of deionized water in the microchannel heat sink. The test section consisted of a single microchannel having 300μm wide nominal dimensions and 300μm height (hydraulic diameter of 300μm). The test section formed of oxygen-free copper with 72mm length and 12mm width. Experimental operation conditions spanned the heat flux (78-800) kW/m2, mass flux (1700 and 2100) kg/m2.s at 31˚C subcooled inlet temperature. The boiling heat transfer coefficient is measured and compared with existing correlations. Also, the experimental pressure drop is measured and compared with microscale pressure drop correlations. The results showed that higher mass flux leads to higher boiling heat transfer coefficient, and the dominant mechanism is convective boiling. Also, the experimental pressure drop decrease with increasing heat flux in a single-phase region while it increases in a two-phase region. Comparing the experimental results in the experimental condition range, showed that an existing correlation provides a satisfactory prediction of heat transfer coefficient and pressure drop.

Condensation Enhancement on Hydrophobic Surfaces Using Electrophoretic Method and Hybrid Paint Coating

Condensation heat transfer on stainless steel tube utilizing superhydrophobic coatings was investigated. The electrophoretic deposition and spraying methods were employed to coat the tubes’ outer surface. The mixture that was synthesized for spray coating was a hybrid paint. It incorporated polyurethane matrix and a colloidal suspension containing organic nanoparticles. The hybrid paint had a proper adhesion to the substrate which caused more durability compared to the electrophoretic coating. The agglomeration of nanoparticles in the hybrid paint caused the formation of particles with larger size compared to the particles in the electrophoretic coating. Consequently, contact angle hysteresis over electrophoretic coating was much lesser than spraying method. Dropwise condensation regime was observed over both the coatings as a result of hydrophobicity. Nonetheless, the condensation heat flux declined over the hybrid paint at high subcooling temperatures due to transition to the flooding condensation regime. This was due to the accumulation of ultra-small droplets as a result of high nucleation sites. The continuation of droplets sweeping was hindered by sticking to the substrate. The dropwise condensation was maintained over the electrophoretic coating and the transition to the film-wise condensation happened at larger wall subcooling temperature. The ability of the electrophoretic coating in shedding droplets efficiently was caused by higher hydrophobicity, high roughness density, and minimum droplet pinning.

One-dimensional analysis of the convergent-divergent motive nozzle for the two-phase ejector: Effect of the operating and design parameters

Two-phase ejectors are used in the refrigeration cycles to decrease the throttling losses and improve the performance. The subject of this paper is the motive nozzle which is the critical component of the ejector since the pressure distribution throughout the motive nozzle affects the secondary fluid to be entrained into the ejector. A simplified version of the one of the previously established one-dimensional (1-D) converging-diverging motive nozzle models is developed in this paper to calculate the pressure, temperature, velocity, and Mach number distributions. 1-D model of the nozzle is established based on the conservation equations of mass, momentum, and energy and the equation of state under steady, frictional, and adiabatic flow assumptions with the homogeneous equilibrium condition. Maximum differences of the pressure drop throughout the nozzle between the literature data and the calculated results are around 6% and 8% for CO2 and R134a nozzles, respectively. The main objective is investigating the effects of the subcooling temperature difference, inlet pressure (condenser temperature or pressure), mass flow rate, and motive nozzle outlet diameter for R134a, R1234yf, and R1234ze(E). Parametric comparisons and evaluations are used to lead the motive nozzle designs for the further numerical and experimental studies including these new generation refrigerants for R134a replacement.

Numerical investigation of rod bundle thermal–hydraulic behavior during reflood transients using COBRA-TF

Accurate prediction of the reflood transients during LOCA has long been a challenging task. One reason for this is the substantial cost incurred to perform large-scale reflood tests, which are formidable in experimental expenditure and intensive in technology as well as program management. Another reason preventing a comprehensive understanding of reflood transients is the high-level complexity involved in the two-phase flow mass and heat transfer processes, which makes the measurement extremely difficult and inefficient. The situation has been significantly improved with the design and operation of the Nuclear Regulatory Commission (NRC)/Pennsylvania State University (PSU) Rod Bundle Heat Transfer (RBHT) test facility. A variety of advanced instrumentations were developed and used at this facility and very high-resolution data has been obtained, especially for the liquid droplet field. In the current study taking advantage of the unique NRC/PSU RBHT data, an extensive and comprehensive code evaluation and validation is carried out using the thermal–hydraulic sub-channel analysis code COBRA-TF. The system parametric effects investigated include: the system pressure, inlet liquid subcooling temperature, inlet flooding rate and rod bundle power. A variety of thermal–hydraulic quantities predicted by the numerical code are evaluated including: the quench front propagation, cladding, spacer grid and vapor temperature variations, two-phase pressure drop, liquid droplet velocity and coolant void fraction. In addition, the prediction errors are presented for each of the quantities in great detail in order to have a comprehensive understanding of the code performance. In general, COBRA-TF agrees relatively well with the experimental data in terms of the overall quench front propagation, only predicting slightly earlier quench. However, under low inlet subcooling and high flooding rate conditions, significant discrepancies are observed. The comparison with rod bundle thermal–hydraulic parameters indicates that COBRA-TF is able to predict the cladding temperature before quench well within a 15% range. The spacer grid and vapor temperature predictions are within 20% error. While the overall droplet velocity prediction is found to be within 30% error, COBRA-TF is able to capture the decreasing trend for droplet velocity during reflood. In addition, the prediction of the dispersed flow film boiling void fraction shows that the code always under-predicts the void fraction. While the most prediction discrepancy is found to be the two-phase flow pressure drop after quench, which involves more than 50% under-prediction for the bulk liquid boiling regime. The present study clarifies the effect of different system parameters on the various two-phase flow quantities during reflood. The results obtained provide answers for many existing code modeling issues and thus can be instructive and useful for future model development and code upgrading.

Dropwise condensation from moist air over a hydrophobic metallic substrate

Moist air condensation is of significant scientific and societal interest especially for harvesting potable water from atmospheric air. Dropwise condensation of water vapor present in moist air is preferred when compared to filmwise condensation because of high transport coefficients that augment condensation rate of the distillate. The efficacy and sustainability of dropwise condensation depends on various operating parameters, for instance, temperature of moist air, relative humidity, subcooling of the substrate, thermophysical properties of moist air and physico-chemical properties of the condensing substrate. In this context, a mathematical model of dropwise condensation of moist air over a textured surface has been developed in the present work. Jointly, an experimental study of moist air condensation over a specially prepared superhydrophobic surface has been carried out for model validation. By suitably deriving the correction factor for the interfacial resistance to account for the presence of noncondensable gases, good agreement has been obtained between the model and experiments. Thereafter, simulations have been performed over a range of conditions from the viewpoint of maximizing the condensate output. Results of the study show that a vertical substrate with a high equilibrium contact angle and low contact angle hysteresis increases drainage of water drops. Hence, there is an increase in the water productivity of the surface. In addition, the condensation rate increases with increase in relative humidity, degree of subcooling and saturation temperature of moist air. The results of the study can be utilized towards designing devices for water harvesting from the atmosphere.

The effects of the parameters of a refrigeration system working with R600a on the non-equilibrium subcooled two-phase flow of the refrigerant

Abstract Household refrigerators built with vapor compression refrigeration systems use capillary tubes as expansion devices because of their low cost and simple structure. To prevent decreasing mass flow of the refrigerant and ensure the stability of the system, the refrigeration system is designed so that the refrigerant enters the capillary tube in a subcooled liquid state rather than a saturated two-phase state. However, previous researchers have reported that a two-phase flow entered the capillary in a refrigeration system running with R600a even though the refrigerant was under the subcooled temperature condition. The observed two-phase flow was in a thermodynamic non-equilibrium state, which is a mixture of subcooled liquid and subcooled vapor. Experiments were performed to investigate the effects of the refrigeration system's operating conditions on the contents of the non-equilibrium vapor phase of R600a. Experimental results demonstrated that the void fraction was changed by the system parameters in addition to the pressure and temperature. The void fraction of the non-equilibrium R600a two-phase mixture at the condenser outlet was reduced with increases in degree of subcooling, refrigerant mass flow rate, flow resistance of capillary tube, and decrease in thermal resistance on the secondary side of the condenser. The parameter that caused the biggest change in void fraction was the flow resistance of capillary tube (which was changed by a metering valve in front of capillary tube) while the effects of changes in pressure and refrigerant charge were small.

Assessing the Alkyl Chain Effect of Ammonium Hydroxides Ionic Liquids on the Kinetics of Pure Methane and Carbon Dioxide Hydrates

In this study, four ammonium hydroxide ionic liquids (AHILs) with varying alkyl chains were evaluated for their kinetic hydrate inhibition (KHI) impact on pure carbon dioxide (CO2) and methane (CH4) gas hydrate systems. The constant cooling technique was used to determine the induction time, the initial rate of hydrate formation, and the amount of gas uptake for CH4-AHILs and CO2-AHILs systems at 8.0 and 3.50 MPa, respectively, at 1 wt.% aqueous AHILs solutions. In addition, the effect of hydrate formation sub-cooling temperature on the performance of the AHILs was conducted at experimental temperatures 274.0 and 277.0 K. The tested AHILs kinetically inhibited both CH4 and CO2 hydrates at the studied sub-cooling temperatures by delaying the hydrate induction time and reducing the initial rate of hydrate formation and gas uptake. The hydrate inhibition performance of AHILs increases with increasing alkyl chain length, due to the better surface adsorption on the hydrate crystal surface with alkyl chain length enhancement. TPrAOH efficiently inhibited the induction time of both CH4 and CO2 hydrate with an average inhibition percentage of 50% and 84%, respectively. Tetramethylammonium Hydroxide (TMAOH) and Tetrabutylammonium Hydroxide (TBAOH) best reduced CH4 and CO2 total uptake on average, with TMAOH and Tetraethylammonium Hydroxide (TEAOH) suitably reducing the average initial rate of CH4 and CO2 hydrate formation, respectively. The findings in this study could provide a roadmap for the potential use of AHILs as KHI inhibitors, especially in offshore environs.

Application of general multilevel factorial design approach in forced convection and subcooled flow boiling heat transfer to CuO/water nanofluids

In this research, heat transfer in forced convection and subcooled flow boiling to CuO/water nanofluids was experimentally and theoretically investigated. The effects of various parameters comprising subcooling temperature, nanoparticle concentration, heat flux, and fluid flow rate on the heat transfer performance of the system were studied. The general multilevel factorial design approach was applied in order to design the experiments and statistically analyze the results. Results showed that all the aforementioned factors were statistically significant. A reduced quadratic model was presented to predict the heat transfer coefficient. The suggested model was in good agreement with the experimental data. It was found the heat transfer coefficient increase as the heat flux and fluid flow rate increase while the trend was reversed for the effect of subcooling temperature. Raising the nanoparticle concentration initially deteriorated the heat transfer coefficient while a further increase in concentration enhanced the heat transfer coefficient.

Experimental study of bubble flow behavior during flow instability under uniform and non-uniform transverse heat distribution

Experiments are conducted to study bubble flow behavior during the instability of subcooled boiling under uniform and non-uniform transverse heating. The non-uniform heat distribution introduces non-uniform bubble generation and condensation rates on the heated surface, which is different from the uniform heating. These bubble generation and condensation characteristics introduce a non-uniform local pressure distribution in the transverse direction, which creates an extra non-uniform pressure on the flowing bubbles. Therefore, different bubble flow behavior can be observed between uniform and non-uniform heating conditions. In the uniform heating, bubble velocity fluctuations are low, and the bubbles travel straight along the axial direction. In the non-uniform heating, more fluctuation in the bubble velocity occurs at low mass flow rate and high subcooled inlet temperatures, and reverse flow is observed. Additionally, the bubbles show a zigzag trajectory when they pass through the channel, which indicates the existence of cross flow in the transverse direction.

Heat transfer in a liquid under focused ultrasonic field

The ultrasonic heat transfer enhancement has a strong dependence on the sound intensity. The higher the local sound intensity is, the better the heat transfer enhancement achieves. Therefore, a focused ultrasonic field is proposed in cavities with elliptical cross-section based on interference criterion and standing wave criterion. To design a cavity with focused ultrasonic field, sound source and heat source are purposely located at the two focus points where sound intensity could be reinforced. There is a compromise between the value of the standing wave coefficient and the time-averaged sound pressure. The profile of heat transfer coefficient is found to be spatially consistent with the simulated time-averaged sound pressure distribution. The cavities with focused ultrasonic field always present larger heat transfer coefficient of natural convection heat transfer than the ordinary ones and the cavity with standing wave coefficient k2 = 8 presents better natural convection heat transfer performance than the cavity with standing wave coefficient k2 = 6. For the sub-cooled boiling heat transfer, bubble plays an important role on the ultrasonic heat transfer enhancement effect. At larger heat flux or smaller sub-cooled temperature, bubbles gather together to form bubble cloud that presents large thermal resistance. The cavitation collapse can disperse the bubble cloud.

Cyclic stability of lithium nitrate trihydrate in plate fin heat exchangers

Latent heat thermal energy storage systems based on hydrated salts are effective solutions for thermal management of transient thermal systems. Hydrated salts have relatively large specific and volumetric latent heat compared to paraffin phase change material. However, hydrated salts are not ubiquitous in energy storage systems because they exhibit inconsistent sub-cooling temperatures and tend to phase segregate due to incongruent melting which leads to degradation in the energy storage capacity. Here we demonstrate high stability of a congruently melting salt hydrate - lithium nitrate trihydrate, in a vacuum sealed plate-fin heat exchanger with water as heat transfer medium cycling between 40 °C and 20 °C, over 500 heating and cooling cycles. The specific latent heat of lithium nitrate trihydrate remained stable at 294 ± 37 J·g−1 and the sub-cooling temperatures increased only by 0.27 °C, demonstrating its stability in operational thermal energy storage systems.

Experimental and numerical study on filmwise condensation of pure propane and propane/methane mixture

The condensation process of pure propane and propane/methane mixture on the vertical plate were experimentally and numerically researched to clarify the condensation characteristics at the first stage of natural gas liquification. In this work, a visualization experimental system was designed to study the flow pattern and heat transfer characteristics of propane vapor condensation on the vertical plate. The condensate film on the plate is wave laminar flow on the experimental conditions of liquid film Reynolds number from 59 to 321. With the increase of the liquid film Reynolds number, the flow patterns turn from straight isolated waves (about 8-10 mm length) to dense and short waves, which enhances the condensation heat transfer coefficient. At the same time, a 2-D numerical model was proposed to reveal the condensation mechanism of propane and propane/methane mixture with different wall subcooled temperature (5-40 K) and methane mole fraction (80%-95%). The numerical results agree well with the experimental data. And the results show that 80% methane reduces the liquid film thickness approximately by half comparing with the pure propane. The increase of methane mole fraction could reduce the liquid film thickness, but its contribution to the heat transfer is negligible compared with the mass diffusion resistance caused by the gas boundary layer. The heat transfer coefficient decreases beyond ninety percent compared with pure propane at the methane mole fraction of 80–95%.

Progress Toward Simulating Departure from Nucleate Boiling at High-Pressure Applications with Selected Wall Boiling Closures

Recently, a Eulerian-based two-fluid computational fluid dynamics (CFD) framework with a wall heat flux partitioning approach has been intensively investigated for departure from nucleate boiling (DNB) simulation under the U.S. Department of Energy–funded Consortium for Advanced Simulation of Light Water Reactors (CASL) program. Understanding of the DNB characteristics over a range of pressurized water reactor–like operating conditions and accurate prediction of boiling crisis in the nuclear power system have been grand challenges because of the large impact of DNB on reactor safety and operational economics. The ultimate goal of this task in the CASL program is to introduce a robust multiphase CFD–based DNB modeling framework that is capable of characterizing an entire boiling history in which the wall boiling mode experiences the following through multiple stages of heat transfer mode: (1) single-phase convective heat transfer, (2) nucleate boiling heat transfer, and (3) identification of the departure of nucleate boiling. To validate the CASL boiling model, we have benchmarked simulated DNB over three different flow channel configurations (pipe flow, 5 × 5 fuel bundle with mixing vane tests, and 5 × 5 fuel bundle without mixing vane tests) against experimental measurements, and the validation result with open literature is reported. The DNB detection criteria in the simulation are checked by monitoring the peak wall temperature, wall dryout factor, and net energy balance. In addition to the DNB performance test, some preliminary sensitivity results on closure model selection are reported to address the prediction capability of local void profile against measurements. The boiling simulation tested in this study exhibits a maximum deviation of 24% from the measured DNB value in a high-pressure (i.e., 138 bars) subcooled pipe flow test. The ranges of operating conditions are as follows: 1650 to 2650 kg/m2·s for mass flux and 8.5 to 96 K for subcooled inlet temperature. The deviation is even reduced to 7% when the subcooled temperature is less than 40 K. Besides accuracy, base practice guidelines for DNB detection criteria are tested by monitoring three simulation variables: (1) maximum wall temperature, (2) wall dryout factor (i.e., K-value), and (3) energy balance. Numerical robustness of DNB simulation is largely achieved in most of the validation test except for a few high subcooled test cases.

Density Maximization of One-Step Electrodeposited Copper Nanocones and Dropwise Condensation Heat-Transfer Performance Evaluation.

Currently, it is still a great challenge to obtain copper-based high-efficient dropwise condensation heat transfer (CHT) interfaces via template-free electrodepositing technologies. Here, we report that the density of template-free electrodeposited copper nanocones can maximally reach 1.5 × 106/mm2 by the synergistic control of substrate surface roughness, poly(ethylene glycol) (PEG) molecular weight, and PEG concentration. After thiol modification, the densely packed copper nanocone samples can present low-adhesive superhydrophobicity and condensate microdrop self-jumping function at ambient environment. Condensation heat and mass transfer characterizations show that the CHT coefficient of copper surfaces can maximally enhance 98% for 20 °C vapor and 51% for 40 °C vapor by in situ growth of superhydrophobic densely packed copper nanocones. Although the dropwise condensation mode can change from the jumping mode to the mixed jumping and sweeping mode and the shedding-off mode along with the increase of surface subcooling and vapor temperature, the CHT performance of the nanosample is still superior to that of the contrast flat hydrophobic surface during the whole testing range of surface subcooling. As vapor temperature increases to 80 °C, the CHT performance of the nanosample is inferior to that of the contrast sample. The CHT enhancement under low-temperature vapor should be ascribed to the enhancement of small-drop mass transfer ability caused by low-adhesive superhydrophobicity nature of nanosample surfaces. Their performance degradation mainly results from increased drop-drop drag force along with the increase of surface subcooling and vapor temperature. In sharp contrast, the CHT deterioration under high-temperature vapor should be ascribed to larger drop-surface adhesion and drop-drop drag force. The former is caused by vapor penetration, whereas the latter is caused by the dramatically increased nucleation density and growth rate of condensates. These findings would help design and develop copper-based high-efficiency condensation heat transfer interfaces.