Experimental investigation on evaporation of urea‐water‐solution droplet for SCR applications

The evaporation behavior of urea‐water‐solution (UWS) droplet was investigated for application to urea‐selective catalytic reduction (SCR) systems. A number of experiments were performed with single UWS droplet suspended on the tip of a fine quartz fiber. To cover the temperature range of real‐world diesel exhausts, droplet ambient temperature was regulated from 373 to 873 K using an electrical furnace. As a result of this study, UWS droplet revealed different evaporation characteristics depending on its ambient temperature. At high temperatures, it showed quite complicated behaviors such as bubble formation, distortion, and partial rupture after a linear D²‐law period. However, as temperature decreases, these phenomena became weak and finally disappeared. Also, droplet diminishment coefficients were extracted from transient evaporation histories for various ambient temperatures, which yields a quantitative evaluation on evaporation characteristics of UWS droplet as well as provides valuable empirical data required for modeling or simulation works on urea‐SCR systems. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Film boiling of discrete droplets of mixtures of coal and water on a horizontal brass surface

The effect of coal on the evaporation of water from mixtures of coal and water was studied in a Leidenfrost-type experiment. Pulverized bituminuous coal was mixed with water in volume concentrations of 14% and 26%, with corresponding total droplet volumes of 0.052 cm³ and 0.055 cm³, respectively. Evaporation times were recorded by videotape over a range of plate temperatures from just above the Leidenfrost point of water to 670°K. The test surface used in all the experiments was brass, and the pressure was kept constant at atmospheric.Three stages in the evaporation lifetime of the coal/water droplets were observed: (1) evaporation of levitated sessile shaped drops; (2) a regime in wResults showed that evaporation times of water were lower in the presence of coal than for a pure water droplet containing the same volume of water as     

Effect of crystallization kinetics on the properties of spray dried microparticles

A droplet chain technique was used to study the influence of the crystallization process on the morphology of spray dried microparticles. A piezoceramic dispenser produced a chain of monodisperse solution droplets with an initial diameter in the range of 60–80 µm. Aqueous solutions of sodium nitrate were prepared in concentrations ranging from 5 mg/ml to 5?10^?5 mg/ml. The solution droplets were injected into a laminar flow with gas temperatures varying from 25 to 150°C, affecting the droplet temperature and the evaporation rate, accordingly. Dried particles with diameters between 0.3 and 18 µm were collected. The properties of the collected microparticles were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization. The model accounted for the dependence of the diffusion coefficient of sodium nitrate in water on droplet viscosity. The viscosity trend for sodium nitrate solutions was determined by studying the relaxation time observed during coalescence of two aqueous sodium nitrate droplets levitated in optical tweezers. The combination of theoretical derivations and experimental results showed that longer time available for crystallization correlates with larger crystal size and higher degrees of crystallinity in the final microparticles.

© 2016 American Association for Aerosol Research     

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

This work is aimed to develop a multicomponent evaporation model for droplets of urea‐water solution (UWS) and a thermal decomposition model of urea for automotive exhausts by using the selective catalytic reduction systems. In the multicomponent evaporation model, the influence of urea on the UWS evaporation is taken into account using a nonrandom two‐liquid activity model. The thermal decomposition model is based on a semidetailed kinetic scheme accounting not only for the production of ammonia (NH₃) and isocyanic acid but also for the formation of heavier solid by‐products (biuret, cyanuric acid, and ammelide). This kinetics model has been validated against gaseous data as well as solid‐phase concentration profiles obtained by Lundstroem et al. (2009) and Schaber et al. (2004). Both models have been implemented in IFP‐C3D industrial software to simulate UWS droplet evaporation and decomposition as well as the formation of solid by‐products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas‐phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Convective Drying Kinetics of Single Droplets of Aqueous Glucose

The convective drying kinetics of single droplets of aqueous glucose was measured using a single droplet drying rig. The effects of air temperature and velocity were evaluated. It was found that the droplet of aqueous glucose shrank uniformly, retaining a nearly spherical shape during drying. The normalized volume (d/d₀)³ of the droplet decreased linearly with its moisture content. A constant-drying-rate-like period occurred when the moisture content of the droplet was higher than an amount of about 1.0 kg kg^?1 dry solid. The diameter of the droplet decreased sharply due to the evaporation of water, while its temperature remained at a wet-bulb-like temperature in this period. When the moisture content of the droplet was lower than the above-mentioned value, the drying transferred to a falling-drying-rate-period, during which the temperature of the droplet rose quickly and approached the air temperature as drying continued. The effect of air temperature on the drying of single droplets of aqueous glucose was more pronounced when compared with that of air velocity.     

Undersizing of droplets from a vented nebulizer caused by aerosol heating during transit through an anderson impactor

The use of Anderson cascade impaction to measure droplet sizes from a vented nebulizer connected directly to the impactor was examined by performing two sets of impactor measurements. In the first set, the impactor was operated in room temperature air. In the second set, the impactor was immersed in a cooled water bath at the same temperature as the aerosol exiting the nebulizer (10°C). Normal saline was nebulized in five Pari LC Jet+nebulizers driven by a single Pulmo-Aide compressor, operating under ambient conditions of 36% RH and 22°C. Impaction was done once steady temperatures were reached. When the impactor was operated in room temperature air, the air travelling through the impactor warmed from 10°C at the entrance to the impactor to room temperature at the exit of the impactor. This heating resulted in significant droplet shrinkage due to humidification by evaporation from the droplets, since the impactor immersed in the cooled water bath gave an MMAD that was on average 70% larger than was obtained when the impactor was operated in room air (3.4 μm vs 2.1 μm). These results emphasize the need for caution when using impactors to measure nebulized hygroscopic aerosols, since even if these aerosols enter the impactor in vapor pressure equilibrium with their surrounding air, significant size changes can occur during transit through the impactor if the temperature of the aerosol differs significantly from that surrounding the impactor.     

Modeling the depleting mechanism of urea‐water‐solution droplet for automotive selective catalytic reduction systems

The current work aims to develop a reliable theoretical model capable of simulating the depletion process of urea‐water‐solution (UWS) droplets injected in a hot exhaust stream as experienced in an automotive urea‐based selective catalytic reduction system. A modified multicomponent vaporization model is presented and implemented in the current study to simulate the behavior of UWS droplet in heated environment. Although water depletion is modeled as a vaporization process, urea depletion is modeled using two different approaches: (i) vaporization and (ii) direct thermal decomposition. The suitability of both depletion approaches is assessed in the current study by comparison with experimental data of the decay of a single UWS droplet in a quiescent heated environment. The decay rate of UWS droplet is accurately predicted with the multicomponent vaporization model. The possibility of internal gasification is demonstrated. Based on the complex decomposition behavior of urea, the current study proposes a decomposition mechanism for UWS droplet. The suitability of implementing the rapid mixing approach is assessed through comparison with the diffusion limit approach at various operating conditions and initial UWS droplet sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Investigation on UWS evaporation for vehicle SCR applications

Urea water solution (UWS) droplet evaporation characteristics directly affect the conversion and distribution of NH₃ in urea based selective catalytic reduction (SCR) system. The UWS droplet temperature is very difficult to be measured directly. Whereas, this piece of research work involves the measurement of droplet temperature by an Omega‐K type thermocouple of 127 µm diameter. According to the temperature changes of the droplet, the evaporation process can be divided into four steps. Droplets heat and mass transfer processes are derived theoretically at high exhaust temperature. The UWS droplet has been placed in a continuous observation test system to investigate its diameter and temperature variations in the aforementioned four steps. The results shown that, this unique method of four steps analysis has more explicitly and better described the UWS evaporation process, hence establishing the basis for the subsequent detailed simulation and monitoring. © 2015 American Institute of Chemical Engineers AIChE J, 62: 880–890, 2016     

Evaporation of Pure Droplets in the Convective Regime Under High Mass Flux

An analytical expression analogous to the D² law was introduced, based on a mass transfer consideration, to describe the evaporation of pure droplets in the convective regime based on the Ranz-Marshall-type correlation for the Sh number. Comparison was made to single droplet drying experiments of pure water. It was found that the effect of mass transfer coefficient depression is significant under the high mass flux condition from the droplet. An expression was obtained to approximate the total time required for complete evaporation of a pure droplet. This expression can be used as a quick computation tool for more fundamental studies on droplet evaporation under controlled ambient conditions.     

Numerical and experimental analysis of the single droplet evaporation in a ultrasonic spray pyrolysis device

The present study deals with the numerical analysis of the water droplet evaporation in the carrier gas inside an ultrasonic spray pyrolysis (USP) device. Droplet evaporation is studied through numerical computational fluid dynamics simulation using Ansys Fluent version 16.1 software. The governing equations for mass, momentum, and energy contain source terms for the effects of droplet evaporation. The results are provided as time dependent evaporation rate, temperature and diameter of droplet. Additional experimental evaporation of HAuCl₄ solution droplets with temperatures of 80, 100 and 120°C was performed on a USP device. The obtained dried particles of gold chloride were characterized with TEM and analysed for their size and shapes to determine the effect of evaporation rate on the dried particle morphology. This provides insight into selecting optimal parameters for gold nanoparticle synthesis with HAuCl₄ in USP, for targeted sizes and shapes of the nanoparticles.     

Effect of moisture on the oxidation behavior of ZrB2

Oxidation studies of ZrB₂ were performed under wet air and dry air conditions at 1200°C, 1400°C, and 1500°C for 1, 4, and 10 h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO₂), in addition to boria (B₂O₃) produced in dry air, which increased the evaporation rate of B₂O₃. Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (ie, rate controlled by diffusion through boria) to linear (ie, underlying ZrB₂ is directly exposed to the oxidizing environment) at shorter times and lower temperatures.     

Bicomponent droplets evaporation: Temperature measurements and modelling

Development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to reducing the production of green-house gases and to meeting air quality standards for other emissions. In non-premixed combustion systems, vaporization of the droplets and mixing of the vapour with the surrounding oxidizer control the overall rate of energy release. Droplet vaporization is affected by the nature of the liquid petroleum that contains species having extremely different volatilities. A fine understanding of multi-component droplet vaporization is therefore an important issue to optimize the combustion systems. This paper presents the application of a recently developed technique to measure the temperature of bi-component droplets. Based on the three-color laser-induced fluorescence (LIF) technique, droplet temperatures can be measured regardless of the composition. The method requires adding a small amount of a fluorescent organic dye to the fuel which is composed of ethyl-alcohol and acetone. The accuracy of the measurement is about ±1.3 °C. In this study, the emphasis is placed on the evaporation of the binary mixture in a hot air plume, the temperature of which (around 650 °C) largely exceeds the boiling point of the liquid components. An extensive study of the influence of the initial composition and droplet diameter is carried out. Finally, the experimental results are compared to an evaporation model based on the discrete components approach.     

Mechanisms of Drying of Skin-Forming Materials. II.Droplets of Heat Sensitive Materials

ABSTRACT

Drying studies were carried out on single droplets of gelatin solution, typifying a skin forming, heat-sensitive material. Each droplet was suspended from a novel rotating glass nozzle which enabled direct measurement of its weight and temperature during drying. Observations using this appartatus successfully detected the decrease in core temperature and increase in drying rate resulting from granule dissolution at 30 °C.

The study covered air temperatures in the range 19 °C–175 °C. Scanning Electron Microscopy was used to examine the dried crusts. The resistance of the skin to vapour diffusion was significantly increased with an increase in the air temperature. At air temperature > 60 °C the dried particle lost the structure and properties of the original powder. The optimum drying conditions to avoid this were predicted for this type of skin forming-material.     

Modeling and simulation of urea-water-solution droplet evaporation and thermolysis processes for SCR systems

A reliable mathematical model of urea-water-solution(UWS) droplet evaporation and thermolysis is developed.The well known Abramzon–Sirignano evaporation model is corrected by introducing an adjustment coefficient considering the different evaporation behaviors of UWS droplet at different ambient temperatures. A semidetailed kinetic scheme of urea thermolysis is developed based on Ebrahimian's work. Sequentially, the evaporation characteristics, decomposition efficiency of a single UWS droplet and deposit formation are simulated. As a result, the relation of evaporation time, relative velocity, exhaust temperature and droplet initial diameter is presented. Synchronously, it indicates that temperature is the decisive factor for urea thermolysis. Different temperatures result in different deposit components, and deposit yield is significantly influenced by temperature and decomposition time. The current work can provide guidance for designing urea injection strategy of SCR systems.     

Surface temperature of acoustically levitated water microdroplets measured using infra-red thermography

A 58 kHz acoustic levitator was fitted with an infra-red thermography camera to examine the drying behaviour of water microdroplets at various drying-air temperatures. The evaporation rate was greater with larger initial droplet size at otherwise identical drying-air temperature (T_da). Measurement of droplet aspect ratio indicate that this is caused by differing acoustic field strengths. The measured droplet surface temperature in dry air showed no dependence on initial droplet size, but deviated from the wet bulb and also from the droplet temperature predicted by acoustic levitation theory. The degree of deviation of drying rate from that predicted by the d²-law using the wet bulb was dependent on T_da. Use of measured droplet surface temperature instead of the wet bulb gave, however, good agreement with the d²-law in dry air. No substantial effects of acoustic field streaming on drying rate could therefore be seen, even at the sound pressure levels of 106-165 dB used. Interpretation of evaporation rates of acoustically-levitated droplets requires therefore the measured droplet surface temperature.     

尿素水溶液液滴的蒸发特性

在石英管式炉上通过挂滴法来观察单个尿素水溶液（urea-aqueous-solution,UAS）液滴的具体蒸发过程,比较了不同环境温度以及不同初始直径大小下液滴的蒸发特性。结果表明,尿素溶液液滴在100～1300 ℃的温度范围内呈现出了不同的蒸发行为。在较高的温度下,液滴的蒸发行为较为复杂,如气泡的产生、液滴的变形以及发生微爆的现象;但是,随着环境温度的降低,这些现象就变得非常微弱甚至消失。同时,还定量分析了稳态蒸发常数与温度、液滴初始直径之间的变化关系,发现在初始直径为2.5 mm、温度在100～600 ℃之间变化的情况下,稳态蒸发常数从0.02075 mm2/s增加到了0.23953 mm2/s,增大了10倍左右。此外,还对气流流速为0.25～1.25 m/s范围内的液滴蒸发特性作了实验研究。当液滴周围有强迫气流存在时,液滴与气体间的换热方式由导热转变为对流换热,从而增强了液滴表面的传热传质能力,促进了液滴的蒸发。     

Diesel and bio-diesel fuel deposits on a hot surface

The aim of this study is to investigate the deposition remains of fuel droplets that impinge on a hot surface of Aluminum Alloy. The fuel droplets tested are diesel fuel (DF) and palm oil based ester that refers as a bio-diesel fuel (BDF). Temperatures of Aluminum Alloy surface and impingement interval of droplets are two parameters that were considered for various conditions of experiments. The maximum evaporation point (MEP) obtained from evaporation characteristics for each fuel was used as a reference point to decide the surface temperature of the Aluminum Alloy, where deposits of fuel were observed. The impingement interval was set at 3, 5 and 8 s with various surface temperatures of 320 °C and 370 °C. To understand the effect of heat transfer on deposition development, surface temperature of deposits was measured by using infrared thermometer. The deposition development depended on a few factors such as droplet impingement interval, hot surface temperature, type of fuel, and heat transfer of fuel deposits. BDF produced rapid development of fuel deposits comparing to DF.     

电场作用下基面液滴蒸发与内部流动数值模拟

为探究电场强化基面液滴蒸发的原理,本文采用有限元方法,对外加电场作用下的固体基面上液滴的蒸发过程进行了数值模拟,对比了不同电导率液滴的蒸发过程,分析了电场、液滴蒸发速率和内部流动的影响及其成因,以及液滴在电场作用下的内部流动与液滴传热传质的关系,结果表明,电场力的作用能够显著强化液滴内部的流动,对液滴的传热传质具有促进作用。此外,本文分析了温度对电场下基面液滴蒸发及内部流动的影响,发现温度对电场、液滴内部流动及蒸发的强化作用也有着较为明显的影响：对于电导率较低的纯水液滴,当电场强度低于和高于临界值6kV/cm时,温度对电场强化液滴内部流动和蒸发的影响有所不同;对于电导率较高的盐酸液滴,温度对电场强化液滴内部流动和蒸发的影响随电场强度升高均较大。本文为发展高效静电喷雾冷却技术提供了研究基础。     

Enhanced methods for experimental investigation of single droplet drying kinetics and application to lactose/water

In this work, a suspension device is presented that can be used for the investigation of single droplet drying kinetics at temperatures from ambient to 200°C. Evaluation is hybrid, using both camera data and a water mass balance. In this way, drying kinetics can be captured for shrinking droplets and also after a constant diameter of the wet particle has been reached and during inflation/deflation periods. Results can be further enhanced by X-ray μ-CT investigation of the internal morphology of dried particles. It is proposed to read critical conditions for crust formation from a plot of evaporation flux over water mass fraction. Repeatability and accuracy of the experimental and evaluation methods were checked with pure water droplets. Moreover, the entire evaluation was conducted for aqueous solutions of lactose with various initial concentrations.