Interphase heat transfer during bulk condensation in the flow of vapor–gas mixture

The investigation of heat transfer between a single droplet and a vapor–gas mixture at different Knudsen numbers of growing droplet is presented. The influence of the interphase heat transfer on the behavior of macroparameters and the distribution function of droplets was studied using the results obtained for bulk condensation of vapor–gas mixture flow in a nozzle. A comparison of results obtained within the frames of general formulation and ones following from the certain simplifying assumptions on the droplets temperature was carried out for the free molecular regime of droplets growth.     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

A numerical investigation of the evaporation process of a liquid droplet impinging onto a hot substrate

A numerical investigation of the evaporation process of n-heptane and water liquid droplets impinging onto a hot substrate is presented. Three different temperatures are investigated, covering flow regimes below and above Leidenfrost temperature. The Navier–Stokes equations expressing the flow distribution of the liquid and gas phases, coupled with the Volume of Fluid Method (VOF) for tracking the liquid–gas interface, are solved numerically using the finite volume methodology. Both two-dimensional axisymmetric and fully three-dimensional domains are utilized. An evaporation model coupled with the VOF methodology predicts the vapor blanket height between the evaporating droplet and the substrate, for cases with substrate temperature above the Leidenfrost point, and the formation of vapor bubbles in the region of nucleate boiling regime. The results are compared with available experimental data indicating the outcome of the impingement and the droplet shape during the impingement process, while additional information for the droplet evaporation rate and the temperature and vapor concentration fields is provided by the computational model.     

Numerical simulation of liquid fuel sprays evolution and the subsequent vapor/air-mixture formation in a duct with a 90°-bend

Kerosene sprays evolution and droplets vaporization and subsequent mixing of the resulting vapor with air in a 90°-bended duct are studied numerically. The role played by the bend on the aforementioned processes is highlighted by comparing the bend results with results in a straight duct that has cross-sectional dimensions and overall length as those of the bended duct. Different case studies are considered. The droplet/wall interaction regime adopted is the reflection regime. The droplet injection and wall temperature conditions are selected in such a way that this choice of droplet/wall interaction regime is justified.     

A Methodology to Simulate the Impact of Tube Fouling on Steam Generator Performance With a Thermal-Hydraulic Code

ABSTRACT

Corrosion product deposits in the secondary side of nuclear plant steam generators may result in tube fouling. Tube fouling is a deposit that is influential for the heat exchanges between the primary and the secondary circuits. It may cause a steam pressure decrease and a power reduction. This paper presents a methodology to simulate the impact of tube fouling on steam generator performances. Simulations are performed with ThermoHYdraulique des Composants, which is Electricité de France reference code for the three-dimensional (3D) modeling of two-phase thermal-hydraulic flows in whole nuclear components such as steam generators. Tube fouling induces an additional thermal resistance on tubes. This resistance is supposed to correspond to the conductive resistance of a dense deposit by using the Maxwell model for a continuous solid phase with inclusions. As fouling deposit thicknesses are not uniformly distributed on the tube bundle, several thermal resistance distributions are investigated. In most cases, tube fouling concentrated in the hot leg is the most influential distribution. Nevertheless, for a large amount of deposits, tube fouling uniformly distributed in both hot and cold legs becomes more influential. This simulation series is an initial step. The strategy to improve the thermal resistance model is discussed.     

Thermal and catalytic decomposition of aqueous ethylene glycol mixtures by film boiling

Aqueous ethylene glycol (EG) mixtures are decomposed by film boiling at near saturation temperatures on a horizontal tube in a stagnant pool containing up to 20% (volume) water. The reactor volume is the vapor layer that blankets the tube in the film boiling regime. Chemical reactions are promoted within the vapor film by the tube temperatures while the bulk liquid is close to its bubble point temperature. Experiments are carried out on bare tubes and tubes coated with nickel and platinum catalysts to show the effects involved.Results show that chemical conversion of the hydrocarbon vapors produces primarily CO and H₂. Product yields (flow rates) are enhanced on a catalyst, with an 80%EG/20%water mixture (volume percent) showing three to four times higher product yields compared to a bare tube. Platinum coatings showed slightly higher yields than nickel coatings.Diluting ethylene glycol with water decreases the overall chemical reactivity owing to preferential vaporization of water that enriches the film with steam. The presence of steam in the vapor film appears to reduce carbon deposition or “coking” on the tube when enrichment by steam is significant: deposits were observed for pure EG and 90%EG/10%water mixtures but not for 80%EG/20%water mixtures.     

Condensation of steam in silicon microchannels

A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm² at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.     

Dropwise Condensation Enhancement Using a Wettability Gradient

ABSTRACT

This paper deals with the modeling of dropwise condensation process on a wettability gradient surface. The proposed heat transfer model explicitly takes into account the mechanical nonequilibrium on the periphery of the droplet due to the surface-energy gradient and the contact-angle hysteresis. The model aims to predict the dynamic behavior of a droplet placed on a wettability gradient surface regarding the temperature difference between the wall and the saturated vapor. A comprehensive analysis of all the contributing thermal resistances is proposed. The influences of contact angle, temperature difference, and other representative parameters on a single droplet on a horizontal surface are also discussed. The results indicate that a wettability gradient can cause a reduction of the mean size of the droplets on the condensing surface and thus enhance significantly the heat transfer rate.     

Fouling in a Steam Cracker Convection Section Part 1: A Hybrid CFD-1D Model to Obtain Accurate Tube Wall Temperature Profiles

Abstract

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections.     

Laminar droplet flow in combined hydrodynamically and thermally developing region of circular tubes

Forced convective heat transfer to laminar droplet flow in the combined hydrodynamically and thermally developing region of a circular tube is studied numerically for constant heat flux conditions. The saturated liquid droplets in the vapor flow are considered as equivalent heat sinks distributed in the superheated vapor stream. Numerical calculations are performed for the variations of droplet size, mean vapor velocity, and the local Nusselt number in the streamwise direction until the single-phase fully developed condition is reached. The important roles of the liquid droplets and the developing vapor velocity on the forced convective heat transfer to droplet flow in the combined entrance region of a circular tube are clearly demonstrated.     

Post-dryout heat transfer to Freon in a vertical tube at high subcritical pressures

Studies of the post-dryout heat transfer were made based on experimental data of the heat transfer to Freon 22 flowing upward in a vertical, round tube at nigh subcritical pressures (reduced pressures of 0.68–0.92). A conventional theoretical model failed in part to reproduce the measured wall temperature. A nondimensional parameter Kn was introduced to a model for the post-dryout regime to take account of the thermodynamic nonequilibrium between the vapor and the liquid droplets. A correlation of Kn was developed and a method using this correlation was successful in predicting the wall temperature.     

hiTRAN® Wire Matrix Inserts in Fouling Applications

Fouling characteristics are dictated largely by the properties of the thermal and hydrodynamic boundary layers. As a result, fouling mitigation strategies must take into account the conditions in this region. hiTRAN wire matrix tube inserts are a useful tool in altering the conditions near the tube wall, especially in the laminar and transition flow regions. This review article considers particle image velocimetry and laser doppler velocimetry measurements, which were employed in order to show the hydrodynamic differences between plain tubes and those containing inserts. Measurements indicate that the wall shear rate in tubes containing hiTRAN inserts operating in the laminar flow regime is similar to that for plain bore tubes operating in the turbulent flow regime. Moreover, the increased tube-side heat transfer coefficient that results from the reduction of the thermal boundary layer allows operation with smaller Effective Mean Temperature Differences (EMTDs). This enables the designer to reduce the tube wall temperature to a level below the fouling threshold temperature, e.g., to combat crude oil fouling. The results from the laser analyses into the hydrodynamic boundary layer are backed up by recent research data investigating the effect of hiTRAN inserts on sedimentation and particulate fouling. The thickness of the fouling layer was measured by applying a combination of photographic and laser measurement techniques. The results are compared to plain tube data and are reported as a function of both flow rate and hiTRAN insert packing density. The impact of altering the hydrodynamic and thermal conditions near to the wall is subsequently demonstrated for different fouling mechanisms. Studies of the impact of hiTRAN inserts on biological and chemical reaction fouling in crude oil processing are also reviewed. A better understanding of the threshold shear rates and wall temperatures for different fouling mechanisms is required for any study into the impact of fouling. Combining this knowledge with the principles outlined in this article clearly emphasizes the benefit of using hiTRAN wire matrix inserts as a powerful tool to mitigate fouling.     

Fouling Management of Thermal Cracking Units

ABSTRACT

Visbreakers and other thermal cracking units are thermal process units in crude oil refineries that upgrade heavy petroleum, usually residual oils produced from atmospheric or vacuum distillation of crude oil. The associated process streams of these units consist of heavy hydrocarbons with very high viscosities and impurities, resulting in fouling of the heat exchangers used to cool or heat these streams. This paper presents a practical fouling analysis for thermal cracking units in a refinery in Germany. Fouling management at this refinery was initiated as part of the refinery energy-saving program. Following similar analysis of the refinery's crude preheat trains, heat exchanger networks associated in the thermal cracking units were modeled by entering the plant monitoring data, network topology, and heat exchanger geometries into a commercial heat exchanger network simulator, SmartPM. Fouling behaviors of vacuum residue streams and thermal cracker residue streams were identified and quantified. Both chemical reaction fouling and particulate fouling mechanisms were identified to be responsible for the fouling in these streams. Dynamic fouling models were fitted and used to predict fouling of these heavy petroleum streams, which fouled on both the shell and tube sides of the shell-and-tube heat exchangers.     

海水淡化系统水平管降膜蒸发器传热系数研究

针对海水淡化系统水平管降膜蒸发器,总结和分析管内冷凝侧与管外蒸发侧的换热系数关联式,比较管内径、入口蒸汽流速、蒸汽冷凝温度、出口蒸汽干度对管内蒸汽冷凝侧换热系数的影响;研究传热温差以及喷淋密度对管外蒸发侧换热系数的影响。结合不同的污垢系数,进行了总传热系数的影响因素分析,为海水淡化系统的工程设计提供依据。     

有限流沿燃烧室内喷雾撞壁过程的数值研究

作者应用STAR—CD软件对有限流沿燃烧室内喷雾撞壁过程进行了数值研究，发现在活塞接近上止点时，因为缸内温度较高，所以在喷雾到达燃烧室壁面时，大部分液滴已经蒸发，这样基本上没有液滴与壁面的相互作用，所以撞壁模型的选取对计算结果的影响不大。同时也验证了限流沿可以使燃油从壁面剥离。     

汽油单液滴撞击不同壁面试验研究

为明确汽油单液滴撞壁特性,设计了单液滴撞壁系统,分析了汽油液滴撞壁现象及不同壁面对汽油液滴撞壁结果转捩的影响。研究表明:汽油液滴撞击干壁面时在壁面粘附铺展成一层附壁油膜,附壁油膜促使液滴再次撞壁时发生皇冠射流飞溅现象。附壁油膜越薄,飞溅越剧烈,飞溅持续时间越短。相比硅油膜动力黏度,硅油膜厚度对汽油液滴撞壁后形态演变过程影响更大。汽油液滴撞击硅油膜会稀释硅油膜,稀释前后射流分别为碗状射流、皇冠状射流。随着稀释程度增加,皇冠状射流的二次液滴数量增加,二次液滴中含有壁面硅油组分。无量纲时间τ1时,Rioboo模型能较好地预测铺展因子变化规律,但若超过此时间则Rioboo模型预测不准。     

伞喷油雾与热多孔介质相互作用的数值模拟

为深入认识多孔介质发动机中均匀混合气的形成,用改进的KIVA-3V详细模拟了伞喷油雾与热多孔介质之间的相互作用．在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型．为检测数值模型的合理性,在Senda等人的实验条件下进行了数值计算．油束碰壁后油滴和油蒸气分布的数值计算结果与实验结果吻合较好．在简化多孔介质结构的基础上和不同的环境压力及喷雾锥角时,模拟了伞喷油雾与热多孔介质的碰撞过程．计算结果表明,伞喷油雾的喷雾锥角及空间压力对油滴在多孔介质中的分布有着很大的影响,在多孔介质厚度一定时,通过调节这些参数,能够形成均匀混合气．     

Molecular Dynamics–Continuum Hybrid Simulation for the Impingement of Droplet on a Liquid Film

A molecular dynamics–continuum hybrid method is used to study the droplet impingement process on a liquid film. The hybrid code is validated by simulating the sudden-start Couette flow and unsteady heat transfer problem. The impingement process is strongly affected by Weber number. At low Weber number, the evolution of the crown after droplet impingement is stable, while at high Weber number the secondary droplets emerge and the splash phenomenon occurs. The effect of liquid film thickness on the evolution of crown diameter is also investigated.     

Double Perforated Impingement Plate in Shell-and-Tube Heat Exchanger

This article presents a solution to a chronic problem causing repeated tube failure at shell-and-tube heat exchangers. The problem is related to the fouling process on the tubes' surface, which accumulates downstream from the impingement plate at the exchanger inlet nozzle within the first tube rows due to low velocity and vortices production. In fouling services, the suspended deposits, fouling, accumulates on the tubes' surface downstream from the impingement plate, causing under-deposit corrosion, raising the tubes' surface temperature due to lack of cooling, and accelerating fouling process. Under-deposit corrosion attacks tubes and causes repeated tube failure, costing a lot of money in terms of material, maintenance, and production losses. Normal practice of extending tube life and delaying their failure is to upgrade the tubes' metallurgy. So the article objective is to present an economical solution option through modifying the impingement plate in the shell-and-tube heat exchangers where the impingement plate is recommended by the Tubular Exchanger Manufacturers Association (TEMA). The impingement modification is to replace the solid conventional impingement plate with double spaced plates having offset holes, called double perforated impingement plates (DPIP). The objective of this work can be met by comparing the simulation of the shell-side inlet flow distribution around the conventional and modified (DPIP) impingement plates and ensuring enhancement of the flow pattern distribution at the area behind the impingement plates. Since experimental work in flow investigation is time-consuming and costly, computational fluid dynamics (CFD) Fluent software was implemented as a cost-effective helpful tool to conduct the simulation for comparison purposes. The modified impingement plate, DPIP, will destroy vortices created behind the conventional plate, retarding fouling accumulation. DPIP will enhance shell-side flow distribution downstream from the impingement plate and will stop fouling accumulation on the tubes to prevent under-deposit corrosion.     

Leidenfrost boiling of methanol droplets on hot porous/ceramic surfaces

An experimental and analytical study of film boiling methanol droplets on a porous/ceramic surface is reported. Droplet evaporation times in the wetting and film boiling regimes were measured on a polished stainless-steel surface and three ceramic/alumina surfaces of 10%, 25% and 40% porosity. It was found that the Leidenfrost temperatures increased as surface porosity increased. The Leidenfrost point of the 10% and 25% porous surfaces were nearly 100 K higher and 200 K higher, respectively, than that of the polished stainless-steel surface; methanol droplets could not be levitated on the 40% porous surface at surface temperatures as high as 620 K, which was the maximum surface temperature which could be imposed on this particular material with our apparatus. The evaporation time of liquid deposited on this surface was thus almost two orders of magnitude lower than for levitated droplets on the three other surfaces tested at the same temperature. In the Leidenfrost regime droplets evaporated faster on the porous surfaces than on the stainless-steel surface, and the evaporation time decreased with increasing surface porosity at the same surface temperature. The reduced evaporation times were thought to have their origin in a decrease of the vapor film thickness separating the droplet from the ceramic surface due to vapor absorption and flow within the ceramic material. An analysis of flow in a horizontal channel bounded by an impermeable ẇall above and a permeable wall of finite thickness below was used to model the film boiling process. The results provided a basis for correlating our evaporation time measurements.