SIMULATED DEHYDRATION OF WEDGE-SHAPED SPECIMENS IN TURBULENT FLOW OF SUPERHEATED STEAM AND AIR

Numerical methods were used to study the effectiveness of superheated steam, humid air and dry air as dehydrating media in turbulent flow for the drying of wedge-shaped specimens. The Couette flow assumption was used to formulate the coupled heat and mass transfer in the turbulent boundary layer. Superheated steam was found to have highest evaporation rates at free stream temperatures above the inversion point.     

SIMULATED DEHYDRATION OF WEDGE-SHAPED SPECIMENS IN TURBULENT FLOW OF SUPERHEATED STEAM AND AIR

Numerical methods were used to study the effectiveness of superheated steam, humid air and dry air as dehydrating media in turbulent flow for the drying of wedge-shaped specimens. The Couette flow assumption was used to formulate the coupled heat and mass transfer in the turbulent boundary layer. Superheated steam was found to have highest evaporation rates at free stream temperatures above the inversion point.     

Superheated Steam Drying of a Single Particle in an Impinging Stream Dryer

Impinging stream contactors provide a novel configuration for drying and/or chemical reactions of particulates, pastes or suspensions which can be dispersed in a flowing gas stream. Essentially they consist of one or more highly turbulent “impingement” zones formed by collision of two opposing jets (OJ) in a confied channel or duct. The objective of this paper is to present computational fluid dynamic predictions for two-dimensional turbulent opposing jets over a range of nozzle-to-nozzle separations and jet Reynolds numbers for the simulation of single particle drying in these systems using superheated steam. A number of different turbulence models were tested ( e.g. high Reynolds, Lam-Bremhorst, Launder and Sharma models etc.). Predictions are performed in two distinct parts. In the first part a power law, finite volume method based on the “SIMPLEC” algorithm is used to solve the momentum and energy conservation equations for air in OJ systems in order to gain insight into their     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

ABSTRACT

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

THERMAL PERFORMANCE AND EVALUATION OF SIPERHEATED STEM DRYING SYSTEM USING SOLAR-ASSISTED ABSORPTION HEAT TRANSFORMER

This paper summarizes the main results of our study carried out under the priority-area research “Energy Conversion and Utilization with High Efficiency” supported by the grant-in-aid of scientific research from the Ministry of Education, Science and Culture of Japan. In the research we focused on applications of solar energy utilization for industrial drying systems. In this paper a new concept of a closed drying system with superheated steam provided from a temperature boosting type heat pump (absorption type heat transformer) was proposed. The heat transformer is driven directly by heat from a solar collector. The performance for different three drying periods and a number of factors affecting it were predicted by a computer simulation. Furthermore the heat and mass transfer characteristics of water evaporation into superheated steam stream by radiative and convective heating were indicated experimentally. It was concluded that the system would be useful for industries where high temperature (over 100°C) drying is required.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

A Model for the Relative Velocity of a Particle in an Impingement Dryer: Application to Heat Transfer

Abstract

A simplified mathematical model for the relative gas-particle motion in a confined jet impingement dryer is developed. Model predictions based on an unsteady momentum balance are in good agreement with the observed cycling motion of a spherical particle. The model is applied to coriander seeds submerged in a flow field of superheated steam. It is found that relative motion occurs in unsteady turbulent regime, and that steady settling velocity of particles is never achieved. Model results are applied to correlate experimental heat transfer data of an impingement dryer. Experimental Nu numbers compare fairly well with correlations for fluidized systems.     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

ABSTRACT

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

A Model for the Relative Velocity of a Particle in an Impingement Dryer: Application to Heat Transfer

A simplified mathematical model for the relative gas-particle motion in a confined jet impingement dryer is developed. Model predictions based on an unsteady momentum balance are in good agreement with the observed cycling motion of a spherical particle. The model is applied to coriander seeds submerged in a flow field of superheated steam. It is found that relative motion occurs in unsteady turbulent regime, and that steady settling velocity of particles is never achieved. Model results are applied to correlate experimental heat transfer data of an impingement dryer. Experimental Nu numbers compare fairly well with correlations for fluidized systems.     

Some Considerations In Simulation Of Superheated Steam Drying Of Softwood Lumber

ABSTRACT

A mathematical model for high-temperature drying of softwood lumber with moist air has been modified and extended to simulate wood drying with superheated steam. In the simulation, differences between the two types of drying are considered, these include: external heat and mass transfer processes and calculation of equilibrium moisture content. The external mass transfer coefficient in the perheated steam drying was found to be much higher than that in the moist air drying, however, the heat ransfer coefficients for these two cases were of the same order. The predicted drying curves and wood temperatures from the superheated steam drying model were compared with experimental data and there was close agreement. Further studies will apply the model to development of commercial drying schedules for wood drying with superheated steam.     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

Abstract

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

Impingement heat transfer under a confined slot jet. Part I: Effect of surface throughflow

Local and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a permeable surface at which there may be throughflow. Local Nusselt number was obtained using a unique porous sensor designed for measurement of local heat transfer at a permeable surface which is subjected to rapidly and widely varying heat transfer. Measurements were performed for a wide range of jet Reynolds number and throughflow rates. Convective heat transfer coefficients was found to be enhanced by throughflow, and the enhancement factor in terms of Stanton number to be independent of jet Reynolds number and of extent of heat transfer area.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

HEAT TRANSFER FROM A HORIZONTAL BUBBLING SURFACE TO AN OVERLYING WATER POOL†

An experimenl was performed to study the effects of bubbling from a circular, horizontal, fiat plate on the heat transfer to an overlying water pool. The plate had drilled orifices through which nitrogen was injected into an overlying pool of water at atmospheric pressure. For “deep” pools, the heat transfer coefficient was found to increase only about 20% over a range of superficial gas velocities from 0.6 to 8.5cm/s, A turbulent heat transfer model developed by Konsetov was found to agree well with the data from this experiment. This model and the experimental data suggest that under certain conditions the heat transfer coefficient is similar for both horizontal and vertical surfaces. These conditions are, that the bubbles only contribute to the stirring action in the pool and only when the bubbling pool is considered “deep”. When the pool height fell below 60% of its diameter, the heat transfer coefficient decreased almost linearly with pool height. This suggests that there occurs a reduction in the turbulent fluctuations or in the characteristic length scale based on the average turbulent eddy size as the pool height decreases.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

A mathematical model for drum drying of black liquor slurry using superheated steam impinging jets

A mathematical model is developed and tested for drum drying of an aqueous slurry subjected to impinging jets of superheated steam. The heat and mass transfer within the slurry film was modeled as a one dimensional, pseudo unsteady diffusion problem while the external convective heat transfer rate was obtained by solving the steady full conservation equations of mass, momentum and energy numerically in two and three dimensions. A modified low Reynolds number version of the k-? turbulence model was selected after a careful evaluation of the predictive performance of various k-? models for impingement flow and heat transfer. The computed heat transfer one-dimensional diffusion model for the slurry film. Agreement between experimental data obtained using a black liquor slurry and results of the model was found to be satisfactory.     

基于传热分区的直流蒸汽发生器换热性能仿真

针对直流蒸汽发生器二次侧复杂的流动与换热过程, 为准确研究其内热力参数空间分布规律, 将其分为预热段、过冷沸腾段、饱和核态沸腾段、强制对流蒸发段、缺液段、过热段, 基于分布参数法建立了直流蒸汽发生器一维均相流数学模型, 并自主开发了可实现自动分段计算的直流蒸汽发生器MATLAB仿真程序。仿真结果表明, 基于传热分区的模型能准确预测不同工况下壁温飞升的幅度、位置及各传热区域传热管的长度, 得到直流蒸汽发生器工作恶劣区域。所得结果可为预防直流蒸汽发生器超温爆管等事故提供理论支持。     

压力旋流式喷嘴喷淋液膜区换热过程的数值模拟

利用Fluent软件对压力旋流式喷嘴的内外流场进行了数值模拟，以等效的二维网格模型模拟圆周对称的三维流动，多相流和湍流模型分别采用VOF和雷诺应力模型。在两种条件下，对喷嘴流场进行模拟：①气相为空气，不发生相间热质传递；②气相为饱和水蒸气，发生相间热质传递。相变模型采取Fluent中内嵌的Lee模型。将数值模拟结果同实验结果进行对比，并以数值模拟的数据对喷嘴内外流场展开分析。模拟结果显示，由于液相在喷嘴旋流室内的螺旋运动，导致喷嘴内部形成“空气芯”，液相速度在喷嘴旋流室与收缩段的连接处变化剧烈；另外，发生相间热质传递条件下，流场的压力要整体稍低且速度场的速度最大值更大；液膜的传热系数沿液膜流动方向不断减小；因气相冷凝使得液膜厚度更大，液膜破碎长度也因蒸气冷凝而变得更长。