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     

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.     

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     

Two‐step consecutive reaction model and kinetic parameters relevant to the decomposition of Chinese forest fuels

Decomposition of some Chinese forest fuels has been studied by means of nonisothermal thermogravimetric analysis in oxidative and inert atmosphere at low heating rates. A comparison between thermogravimetric curves obtained in air and nitrogen shows that the existence of oxygen enhances the decomposition rate and changes the mechanisms of thermal degradation. After the water evaporation, two well‐defined decomposition stages have been observed in thermogravimetric curves obtained in air, which correspond to oxidative degradation of main components and oxidation of char formed. A kinetic model named “two‐step consecutive reaction model” is developed to describe the thermal degradation process of these materials in air, and there is a good agreement between the experimental and calculated thermogravimetric and derivative thermogravimetric curves. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 571–576, 2006     

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     

等压似三元相图在尿素加热分解工序上的应用

本文是新编《尿素工学》一书的一些补充。叙述了用等压似三元相图计算尿素工艺中减压加热分解工序中的甲铵分解率，总氨蒸出率以及分解后的气液组成的方法。提供的算图可作为传统的水溶液全循环法各种分解压力和加热温度条件下上述各参数计算之用。     

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.     

Evaporation of Multicomponent Droplets

ABSTRACT

The evaporation of multicomponent drops having diameters on the order 10 /am was investigated using electrodynamic levitation and light-scattering measurements. Multicomponent drops representative of diesel fuels and binary drops were studied. The binary system consisted of the halogenated compounds 1-iododocane and 1-bromotetradecane, and the model fuel drops were composed of hexadecane, 1-methyl naphthalene, and cis- and trans-decahydronaphthalene (decalin). The effect of an additive, tertiary butyl peroxide, on the evaporation of the multicomponent system was explored, and some data are reported for the additive ethyl hexyl nitrate. The additives were found to have negligible effect on the evaporation rate. The measured evaporation rates are compared with a theoretical analysis based upon diffusion-controlled evaporation using the UNIFAC (Reid et al., 1987) model for liquid phase activity coefficients. The model was found to be in good agreement with experiment.     

Nano‐Computed Tomographic Measurements of Partially Decomposed Ammonium Perchlorate Particles

High performance solid rocket motors typically contain ammonium perchlorate (AP) particles as the oxidizer. Ammonium perchlorate provides good performance, but thermal decomposition leads to safety concerns for handling and storing solid propellant. Computed tomography is shown to allow for visualization of the AP decomposition process, providing in‐situ, quantitative data. The current work demonstrates the use of nano‐computed tomography (nano‐CT) scanning to elucidate aspects of AP decomposition by studying partially decomposed 400 μm diameter AP particles after isothermal heating at 200 °C. Data provides insight into shape and location of the pores. Analysis shows that the porosity developed within the particle begins approximately 15 μm below the particle surface and moves inward as well as outward towards the surface as the heating time increases. No substantial heating time dependence was observed on the pore size distribution. The size and location distributions of decomposition sites forming below the AP particle surface were quantified for the first time. Comparisons to previous work are discussed.     

Experimental investigation of the evaporation behavior of urea-water-solution droplets exposed to a hot air stream

The behavior of droplets of urea-water-solution (UWS) evaporating under the influence of a hot stream of air was investigated experimentally, under temperatures ranging from 100°C to 400°C. The droplets were suspended on a glass microfiber to minimize the influence of heat conduction, through the fiber, on the evaporation rate of the droplet. The flow rate of air, under all experimental conditions, was measured and these data were used to estimate the average velocity of air around the droplet. Experiments were also conducted on droplets of pure water and the results were compared. The initial mass fraction of urea, in the solution, did not appear to have a significant effect on the evaporation constant, but it did affect a few essential aspects of the evaporation behavior. The evaporation of water droplets was in accordance with the d² law at all temperatures, whereas the evaporation of UWS droplets was ambient-temperature dependent.     

Comparative Investigation of Thermal Decomposition of Various Modifications of Hexanitrohexaazaisowurtzitane (CL‐20)

The thermal decomposition kinetics of different polymorphs of CL‐20 (α, γ and ε) has been investigated by thermogravimetry, IR spectroscopy and optical and electronic microscopy. The reactions proceed with self‐acceleration and can be described by a kinetic law of first order with autocatalysis. Already at the earliest stages of decomposition (≤1%) phase transitions take place from αγ and from εγ. For this reason the observed decomposition is related to the decomposition of γ‐CL‐20. On the other hand, the kinetics of decomposition depends on the initial polymorphic state, so that the thermal decomposition increases in the series: α<γ<ε. Experiments with different samples of α‐CL‐20 demonstrate that different rates of decomposition are observed for the same polymorph depending on the mean size and the size distribution of the crystals and their morphological features. In some cases the thermal stability of α‐CL‐20 can be increased by previous annealing. It is concluded that the thermal decomposition of CL‐20 is purely a solid‐state process. Microscopical and spectroscopical analysis of the condensed CL‐20 decomposition product (formed after prolonged heating at high temperature) show that it has a network structure and consists mainly of carbon and nitrogen.     

Thermal Decomposition Process of HMX

The thermal decomposition of HMX has been investigated using thermoanalytical techniques and infrared spectroscopic study at both above and below its melting point. The weight loss phenomenon that occurs as the temperature is elevated at a constant heating rate has been clearly separated into four elementary processes which are induction period, sublimation, first order solid phase reaction, and highly exothermic liquid phase reaction by plotting them against the logarithm of the heating rate versus the reciprocal temperature. Hydroxymethyl formamide has been shown to be a major product of the liquid phase decomposition, which suggests that the decomposition of HMX in the liquid phase should be initiated by the N-N bond scission but not by the C-N bond scission.     

Thermogravimetric analysis of polystyrene: Influence of sample weight and heating rate on thermal and kinetic parameters

This article studies the influence of the heating rate and sample weight on the thermal decomposition of polystyrene (first-order kinetics). For this purpose, the kinetic parameters (i.e., frequency factor and activation energy), variables at the maximum decomposition rate (such as conversion, reaction rate, and temperature), as well as some characteristic temperatures have been determined for a series of experiments where the heating rate varies (0.5–11.5 K/min) and also, the sample weight (6.0–25.1 mg). Some mathematical equations have been developed that allow: (1) evaluation of the activation energy of thermal decomposition by different ways and comparing the results obtained; (2) relating different parameters between themselves, such as the heating rate with the temperature at the maximum decomposition rate or the frequency factor with the heating rate and sample weight. Finally, some theoretical explanations of the variation of thermal and kinetic parameters have been proposed. © 1996 John Wiley & Sons, Inc.     

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.     

The effect of microencapsulation with nitrocellulose on thermal properties of sodium azide particles

Based on the coacervation principle a solvent/non-solvent method has been used for microencapsulation of sodium azide (NaN₃) with fibrous nitrocellulose (NC). Scanning electron microscopy (SEM) was employed to examine the coating morphology. The thermal behavior of solid samples has been studied by means of thermogravimetry (TG) and differential scanning calorimetry (DSC). The results of TG–DTA analysis revealed that the main thermal degradation for the pure NC and NaN₃ occurs in the temperature ranges of 192–220 and 415–420 °C, respectively. The effects of some parameters, such as NC to NaN₃ weight ratio and volume and addition time of non-solvent, on coating quality and thermal properties have been investigated by SEM and thermal methods. The results of these experiments showed that the decomposition temperature of most stabilized coated sodium azide is about 50 °C higher than that of the pure sample. The DSC experiments were conducted to study the influence of the heating rate (5, 10, 15 and 20 °C/min) on the thermal decomposition processes of the pure NC, coated and pure NaN₃ samples. The results revealed that, as the heating rate was increased, decomposition temperature of the compounds was increased. Also, the kinetic parameters such as activation energy and frequency factor of the decomposition processes were obtained from the DSC data by non-isothermal methods proposed by ASTM E696 and Ozawa. Our finding showed that coated NaN₃ has lower decomposition rate with respect to the pure one.     

Crystallization Measurements Using DTA Methods: Applications to Zerodur®

Differential thermal analysis (DTA) experiments described herein apply two methodologies by which to estimate absolute nucleation rates and/or temperature dependencies using a low-expansion, commercially relevant, multicomponent glass-ceramic as a test material. In particular, the Marotta and the Ray et al. methods were applied to Zerodur® and both provided valuable insight into a relatively complicated crystallization process. Previous nucleation and crystal growth rate data were used in a comparative manner. Although the Marotta method accurately predicted the maximum nucleation temperature, the nature of the technique precluded an estimation of absolute nucleation rates. The Ray et al. technique provided reasonable nucleation rates, but failed to provide useful estimates of the number of quenched-in nuclei, in part because of a substantial "blank" contribution coming from additional, unwanted nucleation during heating and cooling stages of the DTA technique.     

高温辐射环境中液滴的沸腾效应

建立了液滴在高温对流和辐射环境中的受热和蒸发模型,结合液滴均质沸腾模型,编制了计算程序。以正十二烷液滴为例,考虑液滴的膨胀效应以及液滴与周围气流的热物性变化,数值模拟了高温辐射与对流加热下的液滴升温和蒸发过程。分析了不同对流和高温辐射条件下,液滴内部是否能够发生沸腾。研究表明,液滴在高温辐射和对流加热下,蒸发伴随热膨胀;高温热辐射加热可导致液滴内部温度高于表面温度,升温到一定程度后可达到液滴内部沸腾状态;影响液滴沸腾的因素有液滴半径、辐射温度、环境气流温度等;同时,随着液滴蒸发,高温环境中液滴的沸腾过热度逐渐增大。     

Thermogravimetric and mass-spectrometric study of the thermal decomposition of PBCT resins

The thermal decomposition of three commercial samples of carboxy-terminated polybutadiene (PBCT) resins was studied by thermogravimetric analysis (TGA) at heating rates varying from 2° to 100°C/min. Kinetic parameters of the decomposition process at different heating rates were evaluated by means of the Fuoss method.¹ The decomposition process and the activation energy values are found to be dependent on heating rate. Mass-spectrometric analysis of the decomposition products shows that the pyrolysis products of PBCT resins are mainly low molecular weight hydrocarbons: ethylene, acetylene, butadiene, propadiene, vinylcyclohexene, etc. The rates of evolution of these hydrocarbon products vary with the carboxy content of the PBCT resin. Based on this, a carbonium ion mechanism has been suggested for the thermal decomposition. The data generated from this work are of importance for a consideration of the mechanism of combustion of composite solid propellants based on PBCT binders.     

Study on the influence of ageing on thermal decomposition of double‐base propellants and prediction of their in‐use time

A comparative study of the thermal decomposition of naturally and artificially aged double‐base propellants has been carried out at five different heating rates in a dynamic nitrogen atmosphere using differential scanning calorimetry (DSC) technique. The results show that there is only one decomposition peak on DSC curves, and this decomposition has been accelerated by ageing. The influence of the heating rate on the DSC behaviour of the propellants was verified. The kinetic parameters such as activation energy and frequency factor and the thermodynamic parameters such as the enthalpy of activation, free energy of activation and critical explosion temperature for the compounds were obtained from DSC data using non‐isothermal methods proposed by Kissinger and Ozawa. On the other hand, a prediction of the in‐service period by van't Hoff's equation was performed and compared with the experimental results in order to check the constancy and validity of this method. As a result, the prediction procedure used to obtain the time–temperature profile was achieved with good accordance. Copyright © 2012 John Wiley & Sons, Ltd.     

Kinetics of non‐isothermal decomposition of basic cobalt carbonate

The kinetics of the thermal decomposition of basic cobalt carbonate was investigated under non‐isothermal heating in air media using thermogravimetric analysis (TGA). TG‐DTG curves showed that the decomposition proceeded through two well‐fined steps in air. Weight loss of the thermal decomposition of basic cobalt carbonate was in good agreement with the theoretical weight loss. The isoconversional method has been applied to the data in order to evaluate a dependence of the effective activation energy on the extent of conversion, and the possible conversion functions have been estimated through the multiple linear regression method. The average apparent activation energies of the second steps (0.4 ≤ α ≤0.9) were 96.607–144.537 kJ/mol.