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     

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     

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.     

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     

Maximum Spreading of Urea Water Solution during Drop Impingement

Droplet impingement of urea water solution (UWS) is a common source for liquid film and solid deposits formed in the tailpipe of diesel engines. In order to better understand and predict wetting phenomena on the tailpipe wall, this study focuses on droplet spreading dynamics of urea water solution. Impingement of single droplets is investigated under defined conditions by high‐speed imaging using shadowgraphy technique. The experimental studies are complemented by numerical simulations with a phase‐field method. Computational results are in good agreement with experimental data for the advancing phase of spreading and the maximum and terminal spreading radius, whereas for the receding phase notable differences occur. For the maximum spreading radius, an empirical correlation derived for glycerol‐water‐ethanol mixtures is found to be valid for millimeter‐sized UWS droplets as well. A numerical simulation for a much smaller droplet however indicates that this correlation is not valid for the tiny droplets of UWS sprays in technical applications.     

Bivariate population balance model of ethanol‐fueled spray combustors

We present a bivariate population balance‐based formulation of the performance of well‐mixed adiabatic combustors fed by ethanol (EtOH)‐containing sprays of prescribed droplet size distribution (DSD) and composition. Our historically interesting example is the fuel‐cooled V‐2 chemical rocket—using 75 wt % EtOH + H₂O solution, and oxidizer O₂(L). Of special interest are the predicted combustion “intensity” (GW/m³) and efficiency (EtOH fraction vaporized) at each ratio of combustor mean residence time to feed‐droplet characteristic vaporization time. Our formulation exploits a quasi‐steady, gas‐diffusion‐controlled individual droplet evaporation rate law, and the method‐of‐characteristics to solve the associated first‐order population balance partial differential equation governing the joint distribution function n(m₁, m₂) of the fuel spray exiting such a chamber, where m₁ = EtOH mass/droplet, and m₂ = H₂O mass/droplet. Besides the combustor efficiency and intensity, this bivariate distribution function enables predictions of corresponding unconditional DSD, and the joint distribution function(diam., droplet temperature)—perhaps measurable. Our numerically exact formulation/results also provide valuable test cases for convenient approximate methods (bivariate moment and spectral/weighted residual) to predict these “correlated” bivariate distribution functions in more complex situations. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Prediction of turbulent gas‐solid flow in a duct with a 90° bend using an Eulerian‐Lagrangian approach

A dilute, particle‐laden turbulent flow in a square cross‐sectioned duct with a 90° bend is modeled using a three‐dimensional Eulerian‐Lagrangian approach. Predictions are based on a second‐moment turbulence closure, with particles simulated using a Lagrangian particle tracking technique, coupled to a particle‐wall interaction algorithm and a random Fourier series method used to model particle dispersion. The performance of the model is tested for a gas‐solid flow in a horizontal‐to‐vertical duct, with predictions showing good agreement with experimental data. In particular, the consistent use of anisotropic and fully three‐dimensional approaches throughout yields predictions that result in fluctuating particle velocities in acceptable agreement with data. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

In situ FT‐IR‐ATR studies on the structure development of polyurethane‐urea systems

This article reports the application of Fourier Transform Infrared‐Attenuated Total Reflectance (FTIR‐ATR) technique for investigation of in situ polymerization of polyurethane‐urea elastomers. Formulations comprising of diisocyanates, a polyether diol and a diamine based chain extender have been used in this study. The diisocyanates used were 4,4′‐diphenylmethane diisocyanate and toluene diisocyanates. The polyether diol and diamine used were propyleneglycol block‐PEO‐b‐PPO and 4‐(4‐(2‐(4‐(4‐amino‐2‐(trifluoromethyl) phenoxy) phenyl) propan‐2‐yl) phenoxy)‐3‐(trifluoromethyl) benzeneamine, respectively. These reactants were mixed and placed on the ATR cell, and then the infrared spectra were recorded at an interval of 1.75 s while continuously heating. The polyurethane‐urea formation was monitored by the decay in the intensity of isocyanate band at 2258 to 2261 cm^?1. As the polymerization progressed, new peaks appeared at wavenumbers of 1711 cm^?1, 1697 cm^?1, and 1655 cm^?1. These peaks correspond to the urethane carbonyl, hydrogen bonded urethane, and urea carbonyl groups, respectively. It was found that with the progress of the reaction, the shift in the peaks at 1655 and 1697 cm^?1 occurs gradually. This shift in peaks is attributed to the hydrogen bonding. The hydrogen bonding and hence the shift in the peak is a cumulative effect of three phenomena namely: (1) degree of polymerization, (2) macro and microphase separation, and (3) temperature effect. A rationale is discussed to deconvolute these three effects. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Decomposition kinetics and recycle of binary hydrogen‐tetrahydrofuran clathrate hydrate

Decomposition kinetics and recycle of hydrogen–tetrahydrofuran (H₂–THF) clathrate hydrates were investigated with a pressure decay method at temperatures from 265.1 to 273.2 K, at initial pressures from 3.1 to 8.0 MPa, and at stoichiometric THF hydrate concentrations for particle sizes between 250 and 1000 μm. The decomposition was modeled as a two‐step process consisting of H₂ diffusion in the hydrate phase and desorption from the hydrate cage. The adsorption process occurred at roughly two to three times faster than the desorption process, whereas the diffusion process during formation was slightly higher (ca. 20%) than that during decomposition. Successive formation and decomposition cycles showed that occupancy seemed to decrease only slightly with cycling and that there were no large changes in hydrate structure due to cycling. Results provide evidence that the formation and decomposition of H₂ clathrate hydrates occur reversibly and that H₂ clathrate hydrates can be recycled with pressure. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Microwave combination heating: Coupled electromagnetics‐ multiphase porous media modeling and MRI experimentation

The work includes development of a multiphase porous media model and magnetic resonance imaging (MRI) experiments to study microwave combination heating. Combination of electromagnetic, convective and radiant heating was considered. The material being heated was modeled as a hygroscopic porous medium with different phases: solid matrix, water and gas, and included pressure driven flow, binary diffusion and phase change. The three‐dimensional transport model was fully coupled with electromagnetics to include the effect of variable properties. MRI was used to obtain spatial temperature and moisture distributions to validate the model. The model demonstrated that high and low moisture materials behave differently under different combinations of heating and general guidelines for combining heating modes were obtained. Low moisture materials can be heated effectively using higher microwave power which is not possible in high moisture material. Cycling of microwave was found to be useful in distribution of excessive volumetric heat generated by microwaves. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Computational Models for Simulating Multicomponent Aerosol Evaporation in the Upper Respiratory Airways

An effective model for predicting multicomponent aerosol evaporation in the upper respiratory system that is capable of estimating the vaporization of individual components is needed for accurate dosimetry and toxicology analyses. In this study, the performance of evaporation models for multicomponent droplets over a range of volatilities is evaluated based on comparisons to available experimental results for conditions similar to aerosols in the upper respiratory tract. Models considered include a semiempirical correlation approach as well as resolved-volume computational simulations of single and multicomponent aerosol evaporations to test the effects of variable gas-phase properties, surface blowing velocity, and internal droplet temperature gradients. Of the parameters assessed, concentration-dependent gas-phase specific heat had the largest effect on evaporation and should be taken into consideration for respiratory aerosols that contain high volatility species, such as n-heptane, at significant concentrations. For heavier droplet components or conditions below body temperatures, semiempirical estimates were shown to be appropriate for respiratory aerosol conditions. In order to reduce the number of equations and properties required for complex mixtures, a resolved-volume evaporation model was used to identify a twelve-component surrogate representation of potentially toxic JP-8 fuel based on comparisons to experimentally reported droplet evaporation data. Due to the relatively slow evaporation rate of JP-8 aerosols, results indicate that a semiempirical evaporation model in conjunction with the identified surrogate mixture provide a computationally efficient method for computing droplet evaporation that can track individual toxic markers. However, semiempirical methodologies are in need of further development to effectively compute the evaporation of other higher volatility aerosols for which variable gas-phase specific heat does play a significant role.     

Modeling and simulation of the injection of urea-water-solution for automotive SCR DeNOx-systems

The evaporation of water from a single droplet of urea water solution is investigated theoretically by a Rapid Mixing model and a Diffusion Limit model, which also considers droplet motion and variable properties of the solution. The Rapid Mixing model is then implemented into the commercial CFD code Fire 8.3 from AVL Corp. Therein, the urea water droplets are treated with Lagrangian particle tracking. The evaporation model is extended for droplet boiling and thermal decomposition of urea. CFD simulations of a SCR DeNOx-system are compared to experimental data to determine the kinetic parameters of the urea decomposition. The numerical model allows to simulate SCR exhaust system configurations to predict conversion and local distribution of the reducing agent.     

Three‐dimensional direct simulation of a droplet impacting onto a solid sphere with low‐impact energy

In this paper, a numerical model is developed for direct simulation of droplet impinging onto a spherical surface on a fixed Eulerian mesh. The model couples the level‐set method and the interfacial cell immersed boundary method to the single‐fluid formulation of the Navier–Stokes equations which are solved by a finite‐volume projection technique. Moving contact lines are modelled here with a simple static contact angle model. The model is shown to converge, and to agree with previous work in the literature. The model is then applied to investigate the impact behaviour of a droplet onto solid sphere of different diameters at low Weber number and low Reynolds number. The simulation results show that the droplet used in present study seems to deposit on different spherical surfaces through oscillating. The simulated results also suggest that the impacted‐sphere size has a significant effect on the impact dynamics of the droplet. A local breakage phenomenon may be found in the centre of the droplet collision with a smaller sphere during the first recoiling stage. A regime map is then established to provide quantitative analysis for the breakage mode of the current impacting process.     

Metastable boundary conditions of water‐in‐oil emulsions in the hydrate formation region

A stepwise pressurization method was proposed for determining the metastable boundary conditions of water‐in‐oil emulsions in the hydrate formation region. The metastable boundary pressures of four water‐in‐n‐octane emulsions in the presence of methane gas were determined at four specified temperatures. The experimental results show that the metastable boundary pressures increase with decreasing water droplet sizes. A thermodynamic model was developed for calculating the metastable boundary conditions of a water‐in‐oil emulsion in which assuming that the collapse of a metastable emulsion requires the formation of a stable hydrate film with a critical thickness on the surfaces of water droplets. The model was used to correlate the experimental data and determine the critical thickness of the hydrate film. It was demonstrated that the calculated results were in good agreement with the experimental data. The determined critical thickness is at nanoscale, ranging from 14 to 40 nm, which decreases with decreasing water droplet sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Synthesis of Carbon‐Rich Hybrid Foam from GAP‐Modified Polyurethane

4,4′‐Diisocyanato diphenylmethane (MDI)‐based polyurethanes melt and start to burn at 150–200 °C. Mainly H₂O, CO₂, CO, HCN, and N₂ are formed. The new modified polyurethane shows a different pyrolysis behavior. GAP‐diol (glycidyl azide polymer), which was used as a modifying agent, is a well‐known energetic binder with a high burning velocity and a very low adiabatic flame temperature. The modified polyurethane starts to burn at approximately 190 °C because of the emitted burnable gases, but it does not melt. The PU foam shrinks slightly and a black, solid, carbon‐rich hybrid foam remains. TGA and EGA‐FTIR revealed a three‐step decomposition mechanism of pure GAP‐diol, the isocyanate‐GAP‐diol, and PU‐GAP‐diol formulations. The first decomposition step is caused by an exothermic reaction of the azido group of the GAP‐diol. This decomposition reaction is independent of the oxygen content in the atmosphere. In the range of 190–240 °C the azido group spontaneously decomposes to nitrogen and ammonia. This decomposition is assumed to take place partly via the intermediate hydrogen azide that decomposes spontaneously to nitrogen and ammonia in the range of 190–240 °C. The second decomposition step was attributed to the depolymerization of the urethane and bisubstituted urea groups. The third decomposition step in the range of 500–750 °C was attributed to the carbonization process of the polymer backbone, which yielded solid, carbon‐rich hybrid foams at 900 °C. In air, the second and the third decomposition step shifted to lower temperatures while no solid carbon hybrid foam was left. Samples of PU‐GAP‐diol, which were not heated by a temperature program but ignited by a bunsen burner, formed a similar carbon‐rich hybrid foam. It was therefore concluded that the decomposition products of the hydrogen azide, ammonia and mainly nitrogen act as an inert atmosphere. FTIR, solid‐state ¹³C‐NMR, XRD, and heat conductivity measurements revealed a high content of sp²‐hybridized, aromatic structures in the hybrid foam. The carbon‐rich foam shows a considerable hardness coupled with high temperature resistance and large specific surface area of 2.1 m²⋅g⁻¹.     

A surface model‐based simulation for warpage of injection‐molded parts

The mid‐plane model for warpage simulation of injection‐molded parts requires a mid‐plane mesh whose transformation is considerably time consuming. To overcome this drawback, a surface model‐based warpage simulation is presented, in which the part is represented as a perfect bonding of two half‐thickness plates with their reference surfaces at the outer boundary of the part. The plates over the surface mesh are modeled as flat shell elements, and a new triangular flat shell element is developed which combines an Assumed Natural DEviatoric Strain (ANDES) based membrane component and a Refined Nonconforming Element Method (RNEM) based bending component. The bonding is accomplished by multipoint constraints and a Lagrange multiplier based elimination method is proposed for constraint application. The results show that compared with some popular shell elements, ANSYS, Moldflow and the experiments, the presented model exhibits a high performance in computation accuracy. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Harnstoffhydrolyse für die selektive katalytische Reduktion von NOx: Vergleich der Flüssig‐ und Gasphasenzersetzung

In order to reduce the NO_x concentration in car exhausts usually the selective catalytic reduction with ammonia is used. However, to avoid the transport of ammonia in vehicles urea is applied as NH₃ precursor. Controlled urea decomposition before the injection into the exhaust gas system itself may be accomplished by the use of a separate reactor. Urea decomposition to ammonia in the liquid phase under pressure in a heated reactor was compared to its decomposition in the gas phase. In the liquid phase, higher conversion rates relative to the reactor volume were realized than in the gas phase. Catalysts which showed high activity in the gas phase influenced the decomposition in the liquid phase only slightly.     

Structure‐Based Design of New KSP‐Eg5 Inhibitors Assisted by a Targeted Multicomponent Reaction

An integrated multidisciplinary approach that combined structure‐based drug design, multicomponent reaction synthetic approaches and functional characterization in enzymatic and cell assays led to the discovery of new kinesin spindle protein (KSP) inhibitors with antiproliferative activity. A focused library of new benzimidazoles obtained by a Ugi+Boc removal/cyclization reaction sequence generated low‐micromolar‐range KSP inhibitors as promising anticancer prototypes. The design and functional studies of the new chemotypes were assessed by computational modeling and molecular biology techniques. The most active compounds— 20 (IC₅₀=1.49 μM , EC₅₀=3.63 μM ) and 22 (IC₅₀=1.37 μM , EC₅₀=6.90 μM )—were synthesized with high efficiency by taking advantage of the multicomponent reactions.     

Steady‐state behavior of liquid fuel hydrazine decomposition in packed bed

A general theoretical model is presented to analyze the steady‐state decomposition process of liquid monopropellants in packed beds for thruster systems. Additionally, an experiment studying the decomposition of liquid hydrazine in a packed bed is used to validate this model. The liquid droplet evaporation rate is determined through calculating the gas‐liquid mass transfer for the mixture temperatures lower than the liquid propellant boiling point and solving the gas‐liquid or liquid‐solid heat transfer equations at the temperature exceeding the boiling point. The process of liquid propellant decomposition in packed beds are simulated based on the Naive–Stokes equation for the mixture model integrated with the developed liquid evaporation rate, in which both the heterogeneous catalytic reaction coupled with the diffusion of reactants in the pore of catalyst, and the homogenous decomposition reactions are considered. The calculated results for the axial distribution of the temperature are in good agreement with the experimental data. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1064–1080, 2015