ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

ADIABATIC SORPTION OF DILUTE SINGLE ADSORBATE FROM AN INERT GAS—EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass (k) and heat (h) transfer coefficients on the column dynamics for Type II adsorption system were analyzed by numerically solving the conservation equations for a single sorbate. Langmuir equilibrium and the linear driving force model for mass transfer were assumed

The transfer zone is larger in an adiabatic column than the corresponding zone in an isothermal column even for a small rise in column temperature. Initially proportionate pattern transfer zones are formed in both the adiabatic and the isothermal columns. As the column length increases these profiles approach to the constant pattern limit. Column adiabaticity slows down the rate of approach to constant pattern limit. On the other hand, higher adsorbate concentration and high k enhance this rate. The symmetry of the breakthrough curve may be distorted in an adiabatic column. The effects of k and h on the size of the mass transfer zone are interchangeable. Consequently, estimation of k from experimental breakthrough data by assuming thermal equilibrium between gas and solid phases may be erroneous.     

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。     

The simultaneous adsorption of carbon dioxide and water vapour by fixed beds of molecular sieves

In an isothermal fixed bed absorption system for constant inlet flowrate of inert gas and concentrations of two adsorbates the more strongly adsorbed component will displace the weaker one and effect its breakthrough characteristics. A small scale technique was used to study this phenomenon for the adsorption of water vapour and carbon dioxide on 4A type synthetic zeolites with helium as the carrier gas. Single adsorbate equilibria and rate data determined with the binary mixtures were then used to calculate the results for the ternary one. Partial differential equations were derived to describe the mass transfer of each adsorbate based on pore diffusion in the adsorbent as a major rate controlling mechanism. With a simplified binary Langmuir equilibrium to describe the effect of water on the adsorption of carbon dioxide, and assuming that the effective pore diffusion coefficient of one adsorbate was not affected by the presence of the other, accurate predictions of the concentration transient were obtained. Four zones are formed in the bed with the ternary mixture resulting from the desorption of carbon dioxide by water, the concentrations of the former rising above the inlet value. An enhanced rate of adsorbed phase transport of water was found, additional to that expected from macropore diffusion, suggesting that surface or capillary flow of the adsorbate occurred.     

涡产生器结构参数对管片式换热器局部传热特性的影响

通过萘升华传质/传热比拟实验,研究了涡产生器攻角θ、形状角β及跨局Δy对顺排圆管管片式板芯传热特性的影响.结果表明,涡产生器可显著改善圆管尾部回流区的换热;对给定的Re数,Nu数随攻角的增大而增大;适当减小形状角可以扩大涡旋的影响区域,从而使换热增强;当Re数较小时,适当减小跨距会更有效地改善圆管尾部回流区的换热.     

空气-水/吸湿溶液热湿传递特性分析（Ⅱ）传质特性分区

选取典型状态点计算了空气与水或溶液进口状态相对位置不同时的热湿处理结果,依据传质效果的不同及全热换热与传质方向的异同,将空气与溶液的热湿传递过程在焓湿图上划分为A～D区,空气与水的过程分为A～C区,分析了各分区中的热湿传递特点及获得较优传质效果的流型,确定了各分区的界线。利用可及处理区域和热湿传递的分区结果分析和优化设计空气的加湿过程和溶液的再生过程,指出应将热量投入给水或溶液来获得更好的加湿效果或再生效果。     

A Quasi‐Steady State Shell and Shrinking Core Approach to the Drying of Porous Particles and an Example of Parameter Identification

A quasi‐steady state shell and shrinking core approach which recognizes heat and mass transfer resistances in both the gas and particle phases for drying of a porous particle is proposed. A mean field model (constant properties) using this approach was embedded in a spreadsheet combined with a genetic algorithm for parameter identification to provide an easy means of characterizing the drying process from drying data. In drying, assuming a mean field, four major parameters are typically unknown: two related to the process (heat and mass transfer coefficients) and two which incorporate porous particle properties (shell thermal conductivity and vapour diffusivity). It is shown how these four parameters may be determined from experimental drying data. The model was applied to data for spouted bed drying of rice. For the particular case studied, external heat transfer was found to be the controlling mode, although resistance to moisture diffusion within the particle is important. The approach presented admits of future refinements to improve its scope and utility.     

UNSTEADY MHD HEAT AND MASS TRANSFER BY MIXED CONVECTION FLOW IN THE FORWARD STAGNATION REGION OF A ROTATING SPHERE AT DIFFERENT WALL CONDITIONS

This study is focused on the problem of MHD heat and mass transfer by mixed convection flow in the forward stagnation region of a rotating sphere in the presence of heat generation and chemical reaction effects. The surface of the sphere is maintained at constant fluid temperature and species concentration. The governing equations of the problem are converted into ordinary differential equations by using suitable similarity transformations. Two cases are considered, namely, constant wall temperature and mass (CWTM) and constant heat and mass fluxes (CHMF). The obtained self-similar equations for both cases are solved numerically using an efficient iterative implicit finite-difference method. The numerical results are compared with previously published results on special cases of the problem and found to be in excellent agreement. The obtained results are displayed graphically to illustrate the influence of the different physical parameters on the velocity components in x- and y-directions, temperature, and concentration profiles as well as the local surface shear stresses and local heat and mass transfer coefficients.     

Characterization of stoichiometric adsorption complexes in H-ZSM-5 using microcalorimetry

We have used microcalorimetry to examine the stoichiometric adsorption complexes formed by ammonia, pyridine, and isopropylamine at the A1 sites in H-ZSM-5. The results show that the heats of adsorption for each adsorbate are constant up to a coverage of one molecule /Al, implying that all of the acid sites in H-ZSM-5 are equivalent in strength. The adsorption energies of the complexes formed by ammonia, pyridine, and isopropylamine were found to be 150, 200, and 240 kJ/mol, respectively. Comparison of these values with a simple potential-energy model which assumes that the heat of adsorption scales linearly with gas phase proton affinities suggests that proton transfer dominates the interaction between the adsorbate and the acid site, but that other factors must also be included.     

A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.     

Theoretical prediction of temperature profiles in an adsorbent fixed-bed

The object of this paper is the theoretical study of the heat transfer in an adsorbent fixed-bed when the adsorption equilibrium is an irreversible one, the adsorption rate being limited by internal diffusion in particles. Then equations of the thermal wave put into a dimensionless form are proposed and discussed.It appears that the heat losses and the initial concentration of adsorbate in the fluid phase are important parameters for the heat transfer and that the thermal wave can grow before or behind the concentration front.     

Influence of natural convection on mass transfer rates for the electrolysis of ferricyanide ions

Results are presented for local and overall current densities at a cylindrical cathode in an unsupported solution of potassium ferro- and ferricyanide under conditions that mass transfer is controlling. The applicability of laminar boundary layer theory to describe natural convection is tested by examining the circumferential variation of the local mass transfer rate. At large electrolyte concentrations two types of instabilities are observed. One involves unsteady flow at the rear of the cylinder and the other both at the front and the rear. The magnitude of current densities at the cathode is predicted within 6 per cent by laminar natural convection theory if the density difference at the cathode is calculated by the appropriate procedures developed by Wilke, Eisenberg and Tobias.     

大粗糙度横肋管摩擦阻力与传热特性

在凝结换热实验台上 ,采用恒壁温法和恒热流法对油品在大粗糙度横肋管中的层流与湍流摩擦阻力与传热特性进行了实验研究 .实验过程中Re为 5 0～ 6 2 0 0 ,Pr为 75～ 2 6 0 ,管子粗糙高度为 5mm ,导程为 30mm .实验结果表明 ,由于实验管特殊几何结构 (大粗糙度 )和油品物性参数的相互作用 ,使其表现出与以水或空气为工质的常用强化管不同的流动传热规律 ,并且实验方法对实验结果的影响不大 .根据实验结果 ,提出了层流区和湍流区油品量纲 1摩擦系数及传热系数的关联式 ,并将实验数据与Ravigururajan和Bergles关联式的计算值进行了比较 .研究结果可用于大粗糙度横肋管换热器的设计及运行参数优化 .     

Heat transfer coefficient in a high pressure tubular reactor for ethylene polymerization

Heat transfer in tubular reactors for the high pressure polymerization of ethylene is very complex, since these tubular reactors are usually divided into several zones that exhibit different flow patterns and critical fouling behavior. The correct estimation of the overall heat transfer coefficient along the reactor axial distance is a major issue when assessing the predictive capabilities of a mathematical model for the process. In general, previous models employed either constant heat transfer coefficients or the usual correlations for the Nusselt number. Neither of these two approaches is accurate enough to allow a correct prediction of the reactor behavior with respect to temperature profiles and product molecular properties. The present work performs a more comprehensive estimation of the heat transfer coefficient in these reactors. At a first stage the overall heat transfer coefficients were estimated by using approapriate energy balances and a good set of experimental data. Then, a predictive model was proposed for the overall heat transfer coefficient. All flow regimes, as well as fouling effects, were taken into account, and the parameter estimation was based on temperature profiles obtained from an industrial reactor. The temperature profiles, conversions, pressures and molecular properties calculated by means of the experimentally fitted heat transfer coefficients or with the predictive model showed good agreement with plant data.     

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER IN AN AIR DUCT WITH AN INCLINED HEATING SURFACE

In this paper some experimental results showing the influence of inclination of a cylindrical heating surface immersed in an air duct on heat transfer coefficient are presented. The experiments were performed in a laboratory-scaled apparatus of square cross section with dimensions 120 ×120 mm and 1400 mm in height. Heat transfer surface was an electrical heater made of brass tube with outer diameter of 19 mm and 110 mm length. In each experiment the temperatures of the front and rear side of the heating surface, inclination angle, air velocity, inlet air temperature, and heater power were measured. It was concluded that heat transfer coefficient depends on flow conditions and angle of inclination. The maximum Nusselt number (Nu) was observed to occur about 45° inclination relative to the horizon, for the range of Reynolds numbers used in experiments. The values of heat transfercoefficients in the vertical position were very nearly the same as they were in the horizontal position. Based on the experimental results, a correlation was proposed for estimation of Nu at the desired flow velocity and inclination angle, relative to Nu at zero inclination.     

MATHEMATICAL AND PHYSICAL FOUNDATIONS OF DRYING THEORIES

ABSTRACT

In this paper a survey is given concerning to the stochastic modelling approaches in transport processes with a special emphasis on application possibilities for simultaneous heat and mass transfer in drying.

First, the mostly used classical modelling methods for drying are discussed which lead to a linear parabolic type of PDE systems supposing constant (state-independent) conductivity coefficients. Powerful discretisation methods are shown for their solution.

Basic principles of variational calculus are discussed then with an attention on direct methods. As a simple application a first-order approximation example is formed, and the solution of the system equation is presented. It is also shown, that the thermodynamical state-dependence of the conductivity coefficients has a crucial influence on the flow pattern of the coupled heat and mass transfer, which is particularly obvious in the cases, when the so-called percolative phase transitions take place. It effects a discrete change of the conductivity coefficients and their probabilities as well. An illustration is shown for percolative phase transition. Describing statistical properties of percolative     

纳米管阵列表面流动沸腾传热特性的实验研究

利用阳极氧化法在钛换热管内表面制备出了一层管阵列结构纳米薄膜。以该纳米管阵列表面管为传热元件,蒸馏水为工质,采用管外电加热竖管强制循环工艺,在恒质量流速下考察了纳米管阵列表面管的流动沸腾传热性能。在实验的基础上,得出了纳米管阵列表面管流动沸腾传热系数关联式。实验结果表明,与光滑表面管相比,流动沸腾传热温差降低了30%～55%,在没有增加阻力的情况下,沸腾传热系数提高了1.5～2.2倍。     

二元混合物降膜蒸发的数值模拟

提出了二元混合物在垂直圆管中降膜蒸发的传热传质耦合数学模型及其相应的数值解法 .模型考虑了液膜内传质对传热的影响 .计算表明传质对传热有明显影响 ,忽略传质的影响对预测膜平均传热系数会带来较大误差 .     

HEAT AND MASS TRANSFER IN THE ADSORBENT BED OF AN ADSORPTION HEAT PUMP

The heat and mass transfer equations governing an adsorbent bed in an adsorption heat pump and the mass balance equation for the adsorbent particles in the adsorbent bed were solved numerically to simulate the cycle of a basic adsorption heat pump, which includes isobaric adsorption, isosteric heating, isobaric desorption, and isosteric cooling processes. The finite difference method was used to solve the set of governing equations, which are highly nonlinear and coupled. The pressures of the evaporator and condenser were 2 and 20 kPa, respectively, and the regeneration temperature of the bed was 403 K. Changes in the temperature, adsorptive pressure, and adsorbate concentration in the adsorbent bed at different steps of the cycle were determined. The basic simulated cycle is presented in a Clausius-Clapeyron diagram, which illustrates the changes in average pressure and temperature of the adsorbent bed throughout the cycle. The results of the simulation indicated that the most time-consuming processes in the adsorption heat pump cycle were isobaric adsorption and isobaric desorption. The high thermal resistance of the bed slows down heat transfer, prolonging adsorption and desorption processes.