HEAT AND MASS TRANSFER DURING DRYING OF GRANULAR PRODUCTS BY COMBINED CONVECTION AND CONDUCTION

Abstract

Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with an imposed hot air flow and a conductive heat flux at the wall.

The model equations are numerically solved using a finite volume method. The numerical simulation gives the time and space evolution of temperature when the lateral area of the cylinder is heated by a constant density flux and a constant temperature. The influence of different parameters (essentially the ratio of heat flux to the heat capacity of flow, and the dryer geometry) on the relative moisture content and the drying time is examined.     

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.     

ANALYSIS OF HEAT AND MASS FLUXES DURING MICROWAVE DRYING

Abstract

The kinetics of combined convective and micro-wave drying presents universally more than the two habitual drying phases. Superimposed to a general heating of the product, a brutal and short acceleration of the drying rate raises all flux densities at a very important level. This is particularly evident with the class of product we used in this study : non porous polymeric gels.

Moreover, an unusual correlation between the kinetics and the surface temperature around the micro-wave specific phase might appear : unexpectedly, the evaporation flux density decreases when the surface temperature increases noticeably.

In order to elucidate this apparent complexity and understand the underlying physical phenomena, we analysed term by term all fluxes according to their origin. The splitting was achieved through mathematical modelling and partially validated by the recording of temperatures and mass fluxes.

Usually with convective drying, all rate changes are governed by the state o f the product : hygroscopicity and porosity development. Here in the case of combined micro-wave drying, all accidents on the drying curves are determined by the heat generation and the heat transfer situation. The constant rate period is also governed by the balance of heat fluxes. The accelerated period corresponds to a heat wave arriving to the surface. Most of the time, the product gives out heat to the surrounding air : this heat is lost for the drying but this cooling effect may he used for the protection of the sample.     

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.     

Convective Drying of Wastewater Sludges: Influence of Air Temperature, Superficial Velocity, and Humidity on the Kinetics

The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 3³ factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.     

生物质型煤热风干燥特性及模型研究

为降低生物质型煤干燥成本,提高干燥效率,研究热风干燥风速和温度对热风干燥特性的影响,拟合了生物质型煤在不同热风干燥条件下水分随时间变化的模拟曲线。结果表明:生物质型煤与多数多孔介质类似,干燥过程可分为加速干燥、恒速干燥、降速干燥3个阶段,其中恒速干燥阶段的干燥时间约25 min;热风温度越高,风速越大,生物质型煤的干燥速率越大,干燥时间越短,干燥时的裂纹率也越高。当干燥温度180℃,风速1.2 m/s时,生物质型煤热风干燥效果较好,干燥热效率最高为48.34%。通过对不同温度、风速条件下的生物质型煤干燥试验数据与常用干燥模型进行拟合分析,发现Sabbet干燥模型拟合度最好,当干燥温度180℃,风速1.2 m/s时相关性系数为0.997,二者相关性显著,因此Sabbet干燥模型可较好地反映生物质型煤在不同温度、风速下的干燥特性。     

SIMULATION OF THE HEAT AND MOISTURE TRANSFER PROCESS DURING DRYING IN DEEP GRAIN BEDS

Grain drying is a simultaneous heat and moisture transfer problem. The modelling of such a problem is of significance in understanding and controlling the drying process. In the present study, a mathematical model for coupled heat and moisture transfer problem is presented. The model consists of four partial differential equations for mass balance, heat balance, heat transfer and drying rate. A simple finite difference method is used to solve the equations. The method shows good flexibility in choosing time and space steps which enable the simulation of long term grain drying/cooling processes. A deep barley bed is used as an example of grain beds in the current simulation. The results are verified against experimental data taken from literature. The analysis of the effects of operating conditions on the temperature and moisture content within the bed is also carried out     

Convective Drying of Wastewater Sludges: Influence of Air Temperature,Superficial Velocity,and Humidity on the Kinetics

Abstract

The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 3³ factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.     

CONVECTIVE DRYING OF AGRICULTURAL PRODUCTS. EFFECT OF CONTINUOUS AND STEPWISE CHANGE IN DRYING AIR TEMPERATURE

Samples of banana were dried in a two-stage heat pump dryer capable of producing stepwise control of the inlet drying air temperature while keeping absolute humidity constant. Two stepwise air temperature profiles were tested. The incremental temperature step change in temperature of the drying air about the mean air temperature of 30 °C was 5 °C. The total drying time for each temperature-time profile was about 300 minutes. The drying kinetics and color change of the products dried under these stepwise variation of the inlet air temperature were measured and compared with constant air temperature drying. The stepwise air temperature variation was found to yield better quality product in terms of color of the dried product. Further, it was found that by employing a step-down temperature profile, it was possible to reduce the drying time to reach the desired moisture content.     

管内进口段对流换热过程不可逆性的数值模拟

The irreversibility due to laminar forced convection in the entrance region of a duct was studied by means of numerical simulation. Local entropy generation distributions and total entropy generation were obtained based on the entropy generation equation. The effect of Reynolds number on the irreversibility was studied for constant wall temperature and heat flux. The results showed that the irreversibility in the entrance region was different from that in the fully developed region. Reynolds number had a strong effect on the irreversibility in the entrance region. The entropy generation caused by temperature difference was relatively dominant over that caused by viscous flow. The irreversibility at constant wall temperature was different from that at constant wall heat flux.     

紧凑通道内CO2/润滑油混合物沸腾换热特性研究

为了测试润滑油对二氧化碳流动沸腾换热特性的影响,对外径6 mm、内径4 mm紧凑通道内的CO₂/润滑油混合物的换热进行实验研究。实验工况为质量流量2.74～5.61 kg·h^-1,饱和温度-4～8℃,热通量3.2～5 kW·m^-2,油浓度0～6%。结果表明：润滑油浓度越大,CO₂的局部传热系数越小;含1.5%油浓度相对于无油工况下平均传热系数下降了约42.4%; 传热系数随热通量、饱和温度的升高而增加,干涸后随着质量流量的增加传热系数增加;干涸随油浓度的增加、热通量的减小、饱和温度的升高、质量流量的增加而延迟;干涸特性对传热系数有显著影响,干涸阶段占整个换热过程的35.4%。     

Method for the Design of a Contact Dryer-Application to Sludge Treatment in Thin Film Boiling

A method, based on the analysis of heat transfer, is developed for the design of contact dryers in thin film boiling for sludge treatment. An experimental device was built in order to estimate the heat flux density exchanged between a hot metallic plate and the drying sample. This specific device is made of a thick copper plate heated at a temperature above 100°C, on which the sludge is coated. The contact temperature and the heat flux density are estimated using a temperature sensor inside the copper block, and an inverse conduction method (Beck's sequential function specification method). The corresponding drying curve is deduced from an energy balance. Sludge drying was studied on an electrically heated laboratory continuous drum dryer. A simple steady state model of the drum dryer was developed by taking into account the combined effects of internal and external transfers. This model is validated by comparison with drying curves obtained from sampling and simulation. In this case, the relative importance of internal and external heat transfers is highlighted.     

Microwave-Vacuum Drying of Wood: Model Formulation and Verification

Based on the mechanism of moisture and heat transfer in wood during microwave-vacuum drying (MVD), a one-dimensional mathematical model to describe the process of wood MVD was established and verified by experiments in this research. The results showed that the process of MVD of wood experienced three distinct periods: (1) accelerating rate with rapid warming-up drying period, (2) a constant temperature and constant rate drying period, and (3) a heating-up with falling rate drying period. Compared with conventional hot air drying, the total drying process is almost governed by a constant rate period in vacuum-microwave drying of wood. The predicted temperature and moisture content in wood match well with the experimental data, the square of the relevant coefficient of the values of simulation and test is above 0.9, and the simulation precision of the change rule of the moisture is higher than that of the temperature.     

边界条件对颗粒-流体对流传热的影响

对单个球形颗粒与周围流体的对流传热进行数值模拟,考察了等温边界、等热流量边界和流固耦合边界条件的影响。结果表明,流固耦合边界和等温边界所得时均面积加权努塞尔数与经验公式计算结果基本一致,等热流量边界模拟结果大于其它两种边界条件结果。时间平均局部面积加权努塞尔数的分布表明,当流动稳定且不发生分离时,努塞尔数从前滞点到后滞点逐渐减小;当出现非稳态涡旋时,努塞尔数从前滞点到分离角附近逐渐减小并出现最小值,后逐渐增大直至后滞点。     

Numerical Analysis of Drying Characteristics of Wet Material Immersed in Fluidized Bed of Inert Particles

The drying characteristics of a relatively large wet material immersed in a fluidized bed of inert particles were examined theoretically. Calculations of heat transfer and evaporation of water on the material surface were included in a discrete element method–computational fluid dynamics (DEM-CFD) simulation. Only the preheating and constant drying rate periods were considered in the simulation.

Our simulation made it possible to calculate the relation between the behavior of the fluidizing particles and the drying characteristics. The drying rate increased with an increase in the number of fluidizing particles in contact with the material and the number of fluidizing particles changed with time; that is, the drying rate fluctuated. When the drying rate increased, the fluid temperature decreased and the vapor content of the fluid increased around the material; that is, the temperature and vapor content of the fluid were associated with the behavior of the fluidizing particles.     

Numerical study of transient heat transfer performance for molten salt receiver tube

The heat transfer experimental system and mathematical model of receiver tube are established to analyze the transient heat transfer performance of molten salt with heat flux of outer wall sudden change, molten salt velocity sharp reduction, heat flux of outer wall and molten salt velocity sharp reduction at the same time. The results show that when the heat flow on the outer wall of the tube changes suddenly (sudden increase or decrease), the temperature of the molten salt in the center of the tube at the inlet section of the heat absorption tube changes less, but the temperature of the tube wall changes faster. When the molten salt velocity sharp reduction, the outlet temperature of molten salt and the outer wall temperature increase with time, but the difference of outer wall and inner wall temperature decreases at first and then increases, when t≥16.0 s, each temperature and temperature difference reach steady state. When the heat flux on the outer wall of the heat absorption tube and the molten salt flow rate in the tube are halved at the same time, the temperature of the molten salt in the center and the outlet of the tube rises first and then decreases with the passage of time. After the steady state, the two molten salt temperatures maintain a constant value and are compared with the transient state. The corresponding molten salt temperature is close to initial starting temperature. The difference of outer wall and inner wall temperature after transient stability is proportional to the heat flux of outer wall, but which is independent of molten salt velocity. The outlet temperature equation of molten salt in receiver tube after transient stability is obtained, which provides a theoretical basis for the outlet temperature control of molten salt in receiver during the transient heat transfer process.     

熔盐吸热管瞬态传热特性的数值研究

建立熔盐吸热管瞬态传热的实验台和数值计算模型,分析管外壁热通量突变、熔盐流速突减、外壁热通量和熔盐流速同时突减对吸热管瞬态传热特性的影响规律,结果表明：管外壁热通量突变（突增或突减）时,吸热管入口段的管中心熔盐温度变化较小,但其管壁温度变化较快。管内熔盐流速突减时,熔盐出口温度和管外壁温度均随时间的推移逐渐增大,而管外壁与管内壁温差随时间的推移先降低后升高,t≥16.0 s,各温度和温差基本稳定。吸热管外壁热通量和管内熔盐流速同时减半时,管中心及出口熔盐温度均随时间的推移先升高后降低,稳态后两处熔盐温度保持定值且与瞬态开始前对应熔盐温度接近。吸热管瞬态稳定后的管外壁与管内壁温差和管外壁热通量变化呈正比,与熔盐流速变化无关。得到瞬态稳定后吸热管熔盐出口温度表达式,为瞬态传热过程中吸热器熔盐出口温度控制提供理论依据。     

膜式水冷壁结构燃磷塔的开发与传质传热的数值模拟（Ⅱ）传热特性影响因素分析

利用已建立的数学模型考察了燃磷量、过剩空气系数、壁面温度、辐射吸收系数对新型燃磷塔的最高燃烧温度、烟气排放温度、壁面最大热流和平均热流的影响.模拟结果表明,无论过剩空气系数固定还是空气流量固定,随燃磷量的增大,最高燃烧温度、烟气出口温度、壁面平均热流和最大热流都呈上升趋势.在燃磷量不变情况下,最高燃烧温度、壁面最大热流和平均热流均随过剩空气系数的增大而减小,但烟气出口温度缓慢增加.壁面温度对最高燃烧温度、壁面最大热流和平均热流影响不明显,但烟气出口温度随壁面温度的升高而增加较大.辐射吸收系数对最高燃烧温度和壁面最大热流影响很大,随辐射吸收系数的增大,最高燃烧温度和烟气出口温度会下降,而壁面最大热流和平均热流会上升.     

A rationally based model applicable for heat pump tumble dryer

The present study proposes a rationally based heat pump clothes dryer model which is capable of handling transient behaviors of both air-side and refrigerant side. The model can take into account the geometrical variation of the heat exchangers. Yet the proposed model is free from extra constraints that were normally imposed by previous studies. The simulation shows that there exist three stages in a typical drying process, including a preheat stage, followed by a constant evaporation stage, and finally a falling drying rate period. The calculation indicates that the variation of the clothes temperature and the system suction/discharge pressure show an appreciable rise in the first stage, remain nearly unchanged at the second stage, and increase again at the final stage of drying process. Also, the corresponding maximum specific moisture extraction rate (SMER) peaks at the end of the second stage while the corresponding coefficient of performance (COP) drops continuously. The simulation also indicates that increasing the air volumetric flowrate yields a lower discharge/suction pressure, a higher COP value, and a shorter drying time. The results also suggested that a good strategy to maximize COP is initiated by a low volumetric rate during the preheat stage, increasing it during the second stage and lowering it to an intermediate flowrate at the end of the drying period. Increasing heat exchanger size, either by increasing heat exchanger width or the number of tube row, results in a lower suction/discharge pressure and a shorter drying time. Although both approaches adopt larger surface area to promote overall performance, it is found that increasing the width of heat exchanger yields more effective results.     

- VISCOELASTIC STRAIN-STRESS AND HEAT/MOISTURE TRANSFER -

The theoretical analysis was developed to predict the deformation characteristic of formed clay during a drying process in a ceramic production. The three dimensional strain-stress distribution as well as heat and moisture transfer in a slab shape of clay were simultaneously analyzed by a finite element method. Linear viscoelasticity was assumed for the strain-stress analysis to account the effect of creep. The calculated result agreed well with an experimental result performed for a slab clay heated by hot air flow during the preheating and constant drying rate periods. A large tensional stress, which may generate a crack, was observed initially around the surface area. It was also found that the time behavior of the volume change of the formed clay is significantly influenced on the drying conditions and/or the drying rate.