EVAPORATION, DROP SIZE DISTRIBUTION AND ENTROPY GENERATION CHARACTERISTICS OF A LIQUID SPRAY CONTAINING DISSOLVED SOLIDS IN A CONVECTIVE MEDIUM

Theoretical studies have been made through a unified nondimenslonal numerical model to evaluate the evaporation, drop size distribution and entropy generation characteristics of an atomized spray of liquid containing dissolved solids in a uniform stream of gas at high temperature. The influence of pertinent input parameters, namely, the initial concentration, initial Reynolds number of the spray and the ratio of ambient to initial drop temperature on the evaporation, drop size distribution and entropy generation histories of the spray with its down stream distance have been established.     

EVAPORATION,DROP SIZE DISTRIBUTION AND ENTROPY GENERATION CHARACTERISTICS OF A LIQUID SPRAY CONTAINING DISSOLVED SOLIDS IN A CONVECTIVE MEDIUM

Theoretical studies have been made through a unified nondimenslonal numerical model to evaluate the evaporation, drop size distribution and entropy generation characteristics of an atomized spray of liquid containing dissolved solids in a uniform stream of gas at high temperature. The influence of pertinent input parameters, namely, the initial concentration, initial Reynolds number of the spray and the ratio of ambient to initial drop temperature on the evaporation, drop size distribution and entropy generation histories of the spray with its down stream distance have been established.     

采用填料强化的鼓泡塔直接接触换热性能研究

以正戊烷-水为物系,进行了采用填料强化的鼓泡塔直接接触蒸发换热实验。实验采用顺流操作,考察了分散相流量、温差以及分布器孔径对体积换热系数和汽化高度的影响。实验得出：在戊烷流量为23.868 L/h,分布器孔径为2.5 mm时加填料的鼓泡塔的体积换热系数约为未加填料的2倍;在一定的操作条件下,加填料的鼓泡塔中汽化高度随分散相流量和分布器孔径的增大而增加,随温差的增大而减小;加填料的鼓泡塔中体积换热系数随分散相流量的增加而增加,随分布器孔径的增大而减小,与温差成负幂指数关系。     

蒸发式过冷水制冰中单个水滴的蒸发过冷特性

为分析蒸发式过冷水制冰中单个水滴在此低温低湿空气环境中的蒸发特性,建立了水滴蒸发过冷过程的数理模型。通过悬挂水滴实验与模拟结果的对比,验证了模型的有效性。因此利用该数学模型预测微小直径水滴的蒸发特性是可行的。通过模拟计算获得了水滴初始直径、初始水温、空气温度、空气含湿量和空气流速对水滴蒸发过冷过程的影响。结果表明,水滴初始直径越小、温度越低或空气流速越大,水滴的冷却速率就越大,达到稳态时的过冷时间就越短。另外,通过降低空气温度或含湿量不仅提高了水滴的冷却速率,而且增加了水滴达到稳态时的过冷度。通过水滴蒸发过冷特性的分析,可为制冰系统的优化设计及提高系统制冰效率提供理论参考。     

Three-dimensional numerical study of heat and mass transfer in fluidized beds with spray nozzle

The numerical computations of temperature and concentration distributions inside a fluidized bed with spray injection in three-dimensions are presented. A continuum model, based on rigorous mass and energy balance equations developed from Nagaiah et al., is used for the three-dimensional simulations. The three-dimensional model equation for nozzle spray is reformulated in comparison to Heinrich. For solving the non-linear partial differential equations with boundary conditions a finite element method is used for space discretization and an implicit Euler method is used for time discretization.The time-dependent behavior of the air humidity, air temperature, degree of wetting, liquid film temperature and particle temperature is presented using two different sets of experimental data. The presented numerical results are validated with the experimental results. Finally, the parallel numerical results are presented using the domain decomposition methods.     

Numerical simulation of in-flight particle oxidation during thermal spraying

Metal powders are widely used for thermal spray coating to improve wear, corrosion and temperature resistance of products. The high thermal profiles endured for sprayed particles give rise to oxidation on the surface of metal powders. Metallic oxides are brittle and undermine the performance of coated products. To understand the growth of oxide layers on in-flight metal powders, an oxidation model is implemented into the Lagrangian formula of particle tracking. The numerical simulation is achieved for a 3D combusting gas flow generated by a high velocity oxygen fuel (HVOF) thermal spray gun. The results are able to demonstrate the correlation between in-flight particle oxidation and operation parameters.     

An empirical model for estimating evaporative hydrocarbon emissions from canister-equipped vehicles

An empirical model for the estimation of evaporative hydrocarbon emissions from canister-equipped vehicles has been developed from experimental data obtained from repeated tests on a single carbon canister. The model is able to predict canister adsorption and desorption behaviour under different loading and purging conditions and it calculates canister weight and breakthrough emissions for various experimental variables including temperature, mixture concentration and mass flow rate. The model enables the simulation of a full evaporative emission Sealed Housing for Evaporative Determination (SHED) test procedure taking into account vehicle-specific estimations of fuel vapour generation and permeation. Results from measurements conducted on a vehicle equipped with the same canister were used to calibrate the model. Model results were in good agreement with measured evaporative emissions from a number of vehicles.     

管内非饱和蒸发与水膜分布实验研究

针对现有水平管束非饱和蒸发冷却装置的不足和缺陷,提出了一种管内非饱和蒸发冷却装置。通过冷膜实验,找出最合理的换热管高度,以及管内风速随风机电压的变化规律。通过恒壁温实验,比对光管和加有螺旋线换热管的对流传热系数,螺旋线的强化传热最优参数,并综合冷膜实验和恒壁温实验的结果,确定了最佳喷淋水量、最佳操作电压等操作参数。     

双液滴碰撞行为及调控机制的研究进展

双液滴碰撞行为广泛存在于雨滴形成、燃油喷雾、喷雾冷却、喷墨印刷、农药喷洒等自然现象与工业应用过程中,其碰撞结果会受到液滴参数及气相环境等因素的综合影响,研究双液滴的碰撞行为规律及调控机制一直是该领域的热点。结合目前双液滴碰撞的实验进展和数值模型,将围绕着碰撞行为的主控因素与调控机制展开综述,具体介绍了碰撞参数、液滴理化性质、气相环境等因素对液滴碰撞行为的影响规律与调控结果,并展望了液滴碰撞理论及应用的发展趋势和方向。     

蒸发式冷凝器管外水膜流动实验研究

研究蒸发式冷凝器传热传质性能时,涉及到管外喷淋水在管表面的分布形式和喷淋水与空气间的对流传热传质等问题。文内建立了蒸发式冷凝器管外水膜流动的可视化测试实验平台,采用高速摄像仪分析了喷嘴对管外水膜分布的影响,同时测试了时间、风量、水量、高度等参数对水膜温度分布的影响。结果表明,喷嘴B能获得更好的水膜分布效果,蒸发式冷凝器有一个最佳喷淋水量,其换热过程主要是由循环冷却水的显热换热和蒸发换热交替占主导作用构成的,在稳定操作条件下运行一段时间后水盘中的水温能保持较好的恒定。     

Modelling of a moving bed furnace for the production of uranium tetrafluoride. Part 2: Application of the model

The French nuclear fuel making route uses, prior to enrichment, uranium tetrafluoride UF₄ obtained from the reduction, followed by hydrofluorination of uranium trioxide UO₃. These two steps are carried out in a specific reactor known as a moving bed furnace. We developed a steady-state numerical model of the moving bed furnace, described in Part 1. In the Part 2, calculation results for a reference set of operating parameters of the furnace are presented in term of temperature, reaction rates, solid and gas compositions. Results analysis enlightens the detail of the furnace behaviour in its different zones. Unknown features have been revealed, such as thermodynamic limitation of the hydrofluorination reaction in the hot core of the moving bed. A sensibility study of various operating parameters shows how some can influence the UF₄ quality and underlines the strong coupling between the different zones of the furnace. Finally, the model is applied to define an optimal temperature progression in the furnace and suggests geometrical modifications. Besides, the validity of using the law of additive reaction times for calculating the reaction rates in such a reactor model has been checked for the first time against a numerical grain model.     

Micro-climatic environmental conditions inside a greenhousewith a built-in solar distillation system

A numerical study was carried out to investigate the micro-climatic environmental conditions inside a greenhouse-distillation system, self-sufficient for irrigating water. The greenhouse consists of the planting cavity, circulating air channels and roof solar distiller for the production of a rather modest rate of irrigating water. A turbulent, steady-state flow, energy and humidity concentration equations were solved using the numerical code FLUENT 6.1. Velocity vectors, steam function, isotherms and temperature and humidity distribution inside the greenhouse present the resultant: micro-climatic environmental conditions. The results are presented for hot days where cold and humid air (from the evaporative cooler) enters the greenhouse from one side and is circulated through the partially porous cavity (representing the plants) and flows through air flow channels and leaves from a vertical thermal chimney. The results show that, with the selected inlet flow conditions, the flow velocity, temperature, and relative humidity can be within the comfort values for plant growth. The effect of some important environmental, design, and operational parameters on greenhouse micro-climatic conditions has also been highlighted.     

Simulation of sprinkler—hot layer interaction using a field model

This paper reports a study of the interaction of a sprinkler water spray with the fire-induced hot layer using the field modelling technique. Data obtained in the large test room of the recent Swedish experiments reported by Ingason and Olsson (1992) are used to validate the results. The problem is divided into a gas phase and a liquid phase. For the gas phase, the set of conservation equations for mass, momentum and enthalpy of air flow induced by the fire is solved numerically using the Pressure Implicit Splitting Operator (PISO) algorithm. For the liquid phase, the sprinkler water spray is described by a number of droplets with initial velocity and diameter calculated by empirical expressions for the nozzle at different operating water pressures and flow rates. The trajectory of each droplet is calculated by solving the equation of motions, by including the dragging and heat transfer with the hot layer. The water droplet is assumed to be non-evaporating and only the source terms in the gas momentum and enthalpy equations of the air flow included the interaction effects with water droplets, i.e. the ‘Particle-Source-in-Cell’ method. The predicted results include the gas flow, temperature and smoke concentration field; the shape of the water spray; and some relevant macroscopic parameters such as amount of convective cooling, drag-to-buoyancy ratio, etc. The average smoke layer temperature and the smoke layer interface height are also calculated. The effect of the mean droplet size on those parameters is illustrated. Finally, a comparison of the water density received at floor level in cases with and without the fire is made.     

Simulation of residual stress in thick thermal barrier coating (TTBC) during thermal shock: A response surface-finite element modeling

The current study demonstrates a well-designed response surface methodology (RSM), based on the generated dataset of finite element method (FEM) to establish an integrated model for simulation of residual stress distribution in a thick thermal barrier coating (TTBC). In this study, typical TTBCs were applied on Hastelloy X Nickel-based superalloy using air plasma spray technique followed by thermal cycling. The recorded stress data of Raman spectroscopy was employed to verify the proposed FEM model. A relatively good agreement was obtained between predicted residual stresses and measured ones. Verified FEM model was used to carry out the parametric studies to evaluate the effects of such various parameters as interface amplitude, wavelength, thermally grown oxide thickness and preheating temperature on the stress distribution in the TTBC during the thermal cycling. The computed data were subsequently used for the development of RSM model. In conclusion, experimentally verified numerical data was used to construct a statistical model based on RSM and successfully used to predict the residual stress distribution field in TTBC during thermal cycling. The obtained results of hybrid FEM- RSM model were in acceptable conformity with Raman spectroscopy measurements.     

Diesel nozzle geometry influence on spray liquid-phase fuel penetration in evaporative conditions

An experimental study of real multi-hole Diesel nozzles is performed under current DI Diesel engines operating conditions. The aim of the investigation is to study the influence of orifice geometry on the flow at the nozzle exit and to analyse its effect on the spray in evaporative conditions. Special attention is taken in the study of the influence of cavitation on the orifice internal flow and spray development. The spray liquid-phase fuel penetration has been characterized. The visualization was made in a wide optical access engine for different operating conditions. From the measurements, the dependencies of nozzle geometry, injection conditions and ambient conditions on liquid-phase length were studied and analyzed. A model for liquid-phase fuel penetration in diesel sprays based on nozzle flow parameters has been proposed and validated.     

The use of a simplified mathematical model for prediction of burn out of non-uniform sprays

A simple one-dimensional mathematical model is developed which is used to calculate the burning rate of a spray of droplets with an initial Rosin—Rammler size distribution. The model is based on experimental observations of the influence of temperature, partial pressure of oxygen, and droplet size on the burning rate of a single droplet. In addition, balance equations are employed to determine the time dependent parameters of the rate equation. The model is used to show the significance of various input parameters on the combustion rate (i.e. evaporation rate) of the spray. The validity of the model is examined by comparing predicted changes in combustion efficiency with those obtained previously from a single tubular turbojet combustor operating with liquid iso-octane over a range of inlet oxygen concentrations.     

Concentrating dilute sulfuric acid by spray evaporator

Large quantities of dilute spent sulfuric acid are released in many chemical processes. Recovering the dilute acid is not only profitable to the manufacturer but also imperative to environmental protection. This paper proposes a spray evaporator with a Venturi-type nozzle to concentrate the dilute sulfuric acid. Both hot air and dilute acid flow concurrently upwards through the nozzle. Water involved in the droplets is vaporized in the chamber and the dilute acid is concentrated. The bench-scale experimental results show that the dilute acid with initial concentration 18 wt% can be easily concentrated to 40–75 wt%. The measured parameters, such as concentration of outlet sulfuric acid, outlet air temperature and total pressure drop, are in accordance with those estimated from a mathematical model incorporating momentum, mass and heat transfer between the acid and air. The model is also applied to simulate the performance of the concentrator, including variations of droplet diameter, droplet velocity, droplet temperature, air temperature, air absolute humidity as well as pressure drop along the concentrator.     

Drying of Liquid Feedstocks in a Spout‐Fluid‐Bed with Draft‐Tube Submerged in Inert Solids: Hydrodynamics and Drying Performance

A spout‐fluid bed with draft tube submerged in a bed of polypropylene beads was used for drying maltodextrin solutions. The hydrodynamics of the dryer were studied by determining the annular air flow vertical profile at different spouting velocities, using an additional air flow rate through the annulus equivalent to 0.5 U_mf. The drying performance of the dryer was studied through the determination of several dryer response parameters (product moisture, evaporative capacity and volumetric evaporative capacity). These parameters were compared with those obtained in a conventional spouted bed with inert solids and a spray dryer.