Liquid flow rate effects during partial evaporation in a falling film micro contactor

The focus of this study is the investigation of the effect of liquid flow rate on partial evaporation, enhanced by convective nitrogen flow, in a falling film micro contactor. Experiments are performed at different flow rates and for a certain heating liquid temperature. The temperatures of the gas and liquid phases are measured at their exit points, and the evaporation rate is determined while the channels are monitored with a microscope.The channels do not appear fully wetted at low liquid flow rates, and the liquid temperature is relatively low. The bad wetting is attributed to liquid flow maldistribution that can cause temperature gradients and subsequently enhances the maldistribution by uneven evaporation. A heat transfer model is developed and solved for different numbers of wetted channels. According to the modelling results the low liquid temperatures, can be fully explained by liquid flow maldistribution and channel drying.These wetting effects are minimized by restricting the liquid flow at its exit point and increasing the amount of liquid maintained at the bottom of the contactor. This improves the temperature profile and prevents drying, as it is possible that capillary forces tend to drive liquid upwards when dry spots occur.     

Mechanistic Modeling of Pollutant Removal,Temperature, and Evaporation in Chemical Air Scrubbers

Chemical air scrubbers reduce the concentration of water‐soluble components such as ammonia from the outgoing ventilation air through absorption in water, followed by chemical conversions and removal of the end products. A mechanistic model for a countercurrent air scrubber was set up. Mass balances for ammonia, hydrogen sulfide, nitrous oxide, and methane were implemented, as well as the water mass balance and heat balances. The model was validated against experimental data from a conventional fattening pig housing facility. The effect of influent characteristics, design parameters, and control handles on the removal efficiency, the temperature profile, and the water evaporation rate were investigated through simulation. The model was able to describe the behavior of a countercurrent chemical air scrubber.     

流化床气化炉内煤颗粒的升温过程及影响

根据热传递理论,对夹套高温烟气加热的流化床气化炉内煤颗粒的升温过程进行了分析与计算,结果表明,颗粒的总体升温速率越高,颗粒升至床温所需的时间就越少;提高流化速度,有利于颗粒的升温,但是影响较小;颗粒粒径对颗粒的升温影响很大,随着颗粒粒径的减小,煤颗粒达到床温所需时间急剧下降。当粒径小于1mm,床温为700～1100℃时,颗粒内部能够在1s左右达到等温,3～5s后颗粒温度能升至99%床温。     

高温热管换热器的应用前景

根据高温热管换热器的结构特点列举了几项高温热管换热器成功应用的实例 ,并对其应用前景及目前存在的问题作一简要分析 ,提出几点建议     

Influences of heat transport on the determination of reaction rates using the temperature scanning plug flow reactor

The temperature scanning plug flow reactor enables the investigation of reactions over a wide temperature range with a small time exposure. The experimental data that are gained by carrying out a temperature scanning experiment can be used to calculate the reaction rate of each component of the reactant mixture. The temperature scanning experiment has to be carried out in a certain procedure that has to be fulfilled in order to be able to apply the underlying theory. The influence of the heat transfer coefficients on the determination of the reaction rates was investigated. The ammonia synthesis was taken as an example.     

原油蒸馏过程换热器支出出口温度平衡控制

设计了某原油蒸馏过程换热器支路出口温度平衡控制系统 ,经实际运行检验 ,控制效果很好。     

Investigation of Temperature Distribution and Heat Transfer in Fluidized Bed Using a Combined CFD-DEM Model

Fluidized beds are widely used in many industries because they are effective for both mixing and drying. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on complex phenomena without physically disturbing the flows. However, most studies have focused on the aerodynamics of the particles. In this study, a combined computational fluid dynamics (CFD)-DEM model, which allows prediction of gas and particle temperature profiles and heat transfer coefficients in a two-dimensional fluidized bed, was developed. The predicted results were compared with the experimental results at the superficial gas velocities of 2.04, 2.22, and 2.41 m/s and at the controlled inlet temperature of 343 K. Based on the comparison between the predicted and experimental results, it was found that the developed model performed adequately in predicting the gas temperature profiles, and the predicted particle temperature profiles were higher than the experimental data. The predicted heat transfer coefficient was slightly higher than the experimental data. However, the predicted and experimental results had a similar trend in which the heat transfer coefficient increased as a function of an increase in superficial gas velocity. In addition, the minimum fluidization velocity predicted by the developed model agreed well with the experimental data. Such predictions can provide essential information on temperature and heat transfer coefficients inside the fluidized bed for design and scale-up.     

Temperature distribution within the moving bed of rotary kilns: Measurement and analysis

Inadequacies in the temperature measurement within the moving bed have hindered a thorough understanding of the processes occurring within rotary kilns. A new measuring system, consisting of thermocouple arrays, a radio-transmitter, a radio-receiver and a computer monitor is introduced in this paper. With it, the 3D temperatures within the moving bed, as well as the temperatures of the freeboard gas and the kiln wall, can be measured and saved automatically. Experiments with sand on a co-current pilot kiln demonstrated that, in the passive layer of the moving bed, the temperatures were approximately constant in the circumferential direction. In the radial direction, however, large temperature difference was observed within the bed near the feed end of the kiln, and the difference became smaller as the bed went progressed through the kiln. This temperature measuring system can be used to obtain data over a wide range of operating conditions for use in engineering design. The obtained results may give new thoughts in theoretical modeling of heat transfer within the moving bed of rotary kilns.     

Temperature control of the water gas shift reaction in microstructured reactors

Microchannel reactors offer unique possibilities for temperature control of chemical reactions due to the strong coupling of channel and wall temperatures. This may be applied to all chemical reactions which require a certain temperature profile to achieve an optimum yield. For the reformation of hydrocarbons for fuel cell applications a low CO concentration of the product gas is desired. In conventional systems, this is achieved by sequentially processing the reformate through a high and low temperature water gas shift reactor because increased temperature enlarges the reaction rate while lower temperature shifts the equilibrium to the desired small CO concentrations. However, for every gas composition arising during the reaction process an optimum temperature exists at which the reaction rate is highest. We will demonstrate that this optimum temperature profile to a good approximation can be achieved in a single step WGS reactor by controlling the temperature via cooling gas flowing in counter current to the reformate. Furthermore, the effect of water addition (steam injection) is analysed for a conventional two-step adiabatic reactor system and the possible size reduction in an integrated heat-exchanger reactor under comparable conditions is validated. Finally, the effect of diffusion limitations at various channel dimensions is investigated applying a two-dimensional model which allows a trade-off between pressure drop or respective reactor size and performance when dimensioning a real system in future.     

Development of a comprehensive free radical photopolymerization model incorporating heat and mass transfer effects in thick films

A comprehensive kinetic model describing photopolymerization is developed which allows variation of temperature, species concentrations, and light intensity through the thickness of a photopolymerized film. Heat and mass transfer effects are included, as is the generation of heat by both reaction and light absorption. In addition to initiation, propagation, and termination mechanisms, both primary radical termination and inhibition are incorporated into the model. The possible presence and diffusion of an inert solvent are also accounted for. Thus, the model is useful for examining complex polymerization kinetics and behavior in industrially and commercially important thick film photopolymerizations, such as the curing of contact lenses, dental restorative materials, photolithographic resists, and optoelectronic coatings. The comprehensive model is used to predict polymerization rate, temperature, and conversion profiles in a variety of systems. The effects of heat generation and the thermal boundary conditions are explored, with the result that heat generation in thick samples leads to greatly increased conversions approaching 100 percent. Increased temperature in these samples also may lead to the appearance of two rate maxima, with the first due to the temperature increase and the second caused by the autoacceleration process. The magnitude of the temperature increase, along with the resultant effects, is more pronounced in insulated systems.     

高温热管翅的传热性能分析

对目前国内长度最短的与最长的高温热管翅进行传热性能比较。试验结果表明,两根高温热管翅具有相似的起动温度曲线,起动温度均比理论值高出约30℃左右;理论分析发现,高温热管翅主要受到声速传热极限和携带传热极限的制约;进一步研究表明,最长高温热管翅的传热系数值约是最短高温热管翅的10倍。     

Modeling and simulation of heat and mass transfer during drying of solids with hemispherical shell geometry

A heat and mass transfer model was proposed to describe the moisture and temperature evolution during drying of solid products with hemispherical shell geometry (HSG). The dimensionless form of the model was numerically solved for both several drying conditions and values of a geometrical factor related with the inner radius of the HSG to obtain their moisture and temperature profiles. In addition, average drying kinetics were calculated from the volume integration of local moisture values. A theoretical and numerical approach was used to develop a mass transfer analogy between the proposed HSG and a simpler flat slab-shaped product. These analogies provide simple mathematical expressions for drying process simulation and estimation of diffusion coefficients in solids with the proposed geometry, and may be applicable to other mass and heat transfer operations. Furthermore, the presented procedure may be used to develop similar expressions in other non-traditional or dissection geometries.     

莫来石——刚玉质高温陶瓷辊棒的研制

通过对国外几种高温陶瓷辊棒组成，显微结构与性能的分析，研究了辊棒的化学－－矿物为微结构与性能的关系。采用国产高纯原料及自行研制的高压液压挤 和全自动、大容积高温梭式窑，研制成功了高温力学性能、抗热震性能优良的高温陶瓷辊棒，其主要技术性能达到国外同类产品水平，可以满足烧成温度在１２００￣１３００℃的高温辊道窑的使用要求。     

固定管板式换热器的温度场数值分析

在对实际结构进行合理简化的基础上,以影响流动和传热的主要结构建立了某固定管板式换热器温度场数值计算模型,采用分段模拟、整体综合的方法,利用CFD软件Fluent对该换热器在正常操作工况下的流动与传热情况进行了数值模拟,得到了计算流道上有关各个构件的壁温场分布,并把主要结构CFD数值计算的结果与实测温度数据进行了对比。结果表明,CFD模拟模型数值分析得到的温度数据与实测数据相符,说明温度场的数值模拟分析方法及其流动条件的假定是符合实际的,计算参数选择是合理可行的。有关固定管板换热器中管束、管板和壳体的温度梯度变化情况的分析表明,尽管在它们的轴向、周向和径向都存在温度梯度,但是温度梯度变化最大的方向是轴向,这意味着轴向将产生最大热应力。     

基于自适应模糊PID的水泥窑分解炉温控系统研究

对非线性大滞后等特殊的系统,存在常规PID控制器控制效果不甚理想的问题,为此针对水泥窑分解炉温度控制系统,提出一种参数自适应模糊PID控制策略,并进行了仿真研究。结果表明:该控制系统响应速度快,调节时间短,控制精度高,控制效果优于传统的PID控制器。     

Utilization of a Radial Temperature Model for Heat Conduction within Spherical Particles to Model Granular Assemblies

A heat transfer (DEM) model for application in the particle based discrete element simulation method is presented. It utilizes an analytical solution of the heat diffusion equation for a solid spherical particle to obtain temporal and radial solutions of the temperature distributions within the particles. This radial temperature model avoids the shortcomings of the usual assumption of spatially uniform temperature profiles in particles. The concept is designed to minimize computing power and memory requirement in order to allow the computation of granular assemblies consisting of a large number of particles. Results obtained for a particle subject to transient convective boundary conditions are compared with a Crank‐Nicholson implicit scheme as numerical reference solution. A first implementation of the radial temperature model in a discrete element code reveals the additional computational cost as negligible compared to the demands of contact identification and force calculation.     

The effect of design parameters on temperature of a heated microsystem

This paper demonstrates the effect of microfluidic system design on temperature profiles of the whole system and more specifically of the microfluidics. Computational fluid dynamics models were used to estimate the effect of some parameters on the temperature of a microsystem with a relatively small heater. The system consisted of a Poly-Ether-Ether-Ketone (PEEK) chipholder with a heater, and a glass chip, surrounded by air. The material and design of the chipholder had a dominant effect on the temperature. Bringing the heater from 22 to 80 ° C resulted in a temperature gradient of over 40 ° C over the length of the chip at fluid level. Due to slow heating, quick switching of temperature is not possible. The steady state temperature profile at fluid level can be changed by adapting the geometry and material of the chipholder. By including the system’s thermal properties in microfluidic system design desired temperature profiles can be obtained. Computer models, such as described in this paper can be used to design a system which thermal behaviour matches the process requirements for the intended use.     

The effects of power characteristics on the heat transfer process in various types of motionless mixing devices

The main objective of this paper is to present the measurement results of the pressure drop and the heat transfer coefficient in the various types of static mixers. The experimental investigations are provided for the explanation of the influence of hydrodynamic conditions on the heat transfer enhancement for different static mixers. Based on the analysis of the experimental database and theoretical considerations, original formulas are proposed for the determination of the power consumption and the heat transfer in various types of motionless mixing devices. In this paper a new criterion is also defined which takes into consideration both the heat transfer process and hydrodynamic conditions. This criterion may be successfully applied to the selection of a static mixer for the heat transfer problems.     

Oxidant control and air-oxy switching concepts for CFB furnace operation

Oxy combustion in circulating fluidized bed (CFB) boilers was investigated in this paper. Oxy combustion is a carbon capture and storage technology, which uses oxygen and recirculated flue gas (RFG) instead of air as an oxidant. Air and oxy combustion were compared through physical considerations and simulations, focusing on process dynamics, transients and control. The oxidant specific heat capacity and density are elevated in oxy combustion, which leads to slower temperature dynamics. Flue gas recirculation introduces internal feedback dynamics to the process. The possibility to adjust the RFG and oxygen flows separately gives an additional degree of freedom for control. In the simulations, “direct” and “sequenced” switches between air- and oxy-firing were compared. Fast “direct” switches with simultaneous ramping of all inputs should be preferred due to the resulting smooth temperature responses. If these process input changes are unfeasible, the fuel should be altered after the gaseous flows (“sequenced” method).