Heat and mass transfer during gas hydrate formation at the surface of rising slugs with small admixtures of inert gases

A model of gas hydrate formation on the surface of a slug of forming hydrate gas with inert gas admixture rising in a channel filled with liquid is suggested. Detailed analysis of the case of small concentrations of the inert admixtures in the gaseous phase is presented. Effect of heat released during gas hydrate formation on the rate of gas hydrate formation is investigated. Analytical formula for the rate of gas hydrate formation as a function of the concentration of the inert admixtures in the gaseous phase is derived. It is shown that an isothermal model of gas hydrate formation in the presence of inert gases underestimates a rate of gas hydrate formation in comparison with the nonisothermal model.     

Simulation of binary vapor condensation in the presence of an inert gas - a sequel

Three approximate methods of calculating multicomponent mass transfer rates due to Toor-Stewart-Prober, Taylor-Smith and Burghardt-Krupickza are compared to an exact method due to Krishna-Standart. All four methods are based on a film model of mass transfer. It is shown that the three approximate methods are exact if all binary diffusion coefficients are equal $\text{and}$ the film thickness is a constant. In applications to binary vapor condensation in the presence of an inert gas, for example, where empirical methods of estimating mass tranfer coefficients must be used, it is shown that the method of Burghardt and Krupiczka is $\text{not}$ in agreement with the other methods even when the binary diffusion coefficients are all equal. The magnitude of the discrepancy is not, however, large except when the mole fraction of inert species is small.     

惰性气体在活塞浇铸中的应用

随着对活塞内在质量要求的不断提高,活塞的铸造要求越来越高。如何解决活塞浇铸中氧化夹杂的产生是一关键问题。通过惰性气体的应用,在浇铸过程中形成保护氛围,减少氧化夹杂的产生,降低活塞因氧化夹杂产生的缺陷而导致的报废。     

Turbulent film condensation in the presence of non-condensable gases over a horizontal tube

A numerical model is presented for studying turbulent film condensation in the presence of non-condensable gases over a horizontal tube. Inertia, pressure gradient are included in this analysis, and the influence of turbulence in the proposed two-phase model is considered. The numerical results demonstrate that a very small bulk concentration of non-condensable gas reduces the heat transfer coefficient and film thickness considerably. The local heat flux and film thickness increase as tube surface temperature decreases at any bulk concentration of non-condensable gas. Moreover, inlet velocity increases as film thickness decreases and heat flux increases, a numerical result in agreement with that obtained by Nusselt. Numerical results indicate that average dimensionless heat transfer coefficients are in good agreement with theoretical and experimental data.     

Temperatures and the condensate heat resistance in dropwise condensation of multicomponent mixtures with inert gases

The temperature variations occurring in dropwise condensation at condenser plates of a compact, polymer heat exchanger are studied using instantaneous infrared temperature field recordings. An averaging procedure in time and an assessment of extreme values is proposed and carried out. With the results, the heat resistance of the condensate is quantified. It is found that mixing and convection in the condensate, caused by coalescence and drainage of drops, reduces the condensate heat resistance by a factor 4 as compared with purely conductive heat transfer. This reduction is comparable, both in nature and in magnitude, to the effect of enhanced mixing due to turbulence in the liquid film of filmwise condensation. A second condensable species has been added to the gas mixture in order to study the contribution of Marangoni convection due to concentration gradients to the condensate heat transfer resistance. No contribution is found.     

Conjugate Heat and Mass Transfer from a Drying Rectangular Cylinder in Confined Air Flow

Heat and mass transfer from a porous body subject to convective drying is investigated numerically based on Luikov's equations. The air flow is assumed incompressible, two-dimensional, laminar, confined in a channel, and parallel to the rectangular-shaped solid. The finite-volume method is used and the computed temporal and spatial variations of moisture content, temperature, concentration, and flow parameters for two different flow rates are analyzed. Two flow configurations are studied: with and without a flow divider upstream of the cylinder in an attempt to eliminate the presence of separation zones and study their effect on drying. It was found that such effects may greatly affect the drying process, along with frontal area stagnation and the thickness of the body.     

Approximate equations for film condensation in the presence of non-condensable gases

Non-condensable gases greatly influence vapor condensation, resulting in a substantial reduction in the condensation heat transfer coefficient. Although extensive analytical and numerical investigations of condensation heat transfer in the presence of non-condensable gases have been done, most of the solutions are quite complicated. Based on a thermodynamics analysis, when the vapor is not close to its critical state and the mass fraction of the non-condensable gas in the main stream is less than 0.1, an equation which relates the vapor/gas-liquid interface parameters and the main stream parameters was developed in the present work. For forced convection film condensation heat transfer on the outside surface of a horizontal tube, the present equation combining with an existing analytical solution as well as a heat transfer correlation given by previous investigators, gives the heat flux and the interfacial parameters of the water vapor-air mixture. The results show that the predicted heat flux is in good agreement with experimental data available in the literature and that even a small amount of air substantially reduces the heat flux. An algebraic equation set is given to calculate free convection film condensation on a vertical flat surface, which associates the interfacial and main stream parameters, an integral solution and an analytical solution given by previous investigators. The calculated results are in good agreement with experimental data in the literature.     

流化床内多组分颗粒传质过程模拟

为了揭示活性颗粒的传质特性,采用Euler-Euler双流体模型,并结合气泡介尺度曳力模型和多组分传质模型,利用计算流体动力学(CFD)软件对存在惰性颗粒情况下多组分颗粒流化床内水蒸气吸附过程进行了数值模拟,得到流化床内固体颗粒体积分数和水蒸气质量分数分布,验证了气泡介尺度曳力模型的合理性,分析了入口表观气体速度、水蒸...     

Design of CO2 absorption plant for recovery of CO2 from flue gases of gas turbine

The ongoing human-induced emission of carbon dioxide (CO₂) threatens to change the earth's climate. A major factor in global warming is CO₂ emission from thermal power plants, which burn fossil fuels. One possible way of decreasing CO₂ emissions is to apply CO₂ removal, which involves recovering of CO₂ from energy conversion processes. This study is focused on recovery of CO₂ from gas turbine exhaust of Sarkhun gas refinery power station. The purpose of this study is to recover the CO₂ with minimum energy requirement. Many of CO₂ recovery processes from flue gases have been studied. Among all CO₂ recovery processes which were studied, absorption process was selected as the optimum one, due to low CO₂ concentration in flue gas. The design parameters considered in this regard, are: selection of suitable solvent, solvent concentration, solvent circulation rate, reboiler and condenser duty and number of stages in absorber and stripper columns. In the design of this unit, amine solvent such as, diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), and monoethanolamine (MEA) were considered and the effect of main parameters on the absorption and stripping columns is presented. Some results with simultaneous changing of the design variables have been obtained. The results show that DGA is the best solvent with minimum energy requirement for recovery of CO₂ from flue gases at atmospheric pressure.     

Growth and evaporation of aerosol particles in the presence of adsorbable gases

Influence of adsorption of noncondensable molecules on the surface of phase transition on the rate of growth (evaporation) of aerosol particles (drops) is investigated theoretically. Combined manifestation of the effects that influence the rate of the growth of particles in the opposite directions is considered: decrease in surface tension on adsorption of a foreign gas and blocking of the surface of phase transition by the molecules adsorbed. The effect of the adsorption of the molecules of a noncondensable (in particular, buffer) gas on the rate of homogeneous nucleation and also on the growth of particles in chemical deposition is discussed.     

流化床扬析速率常数的三相传质模型

本文基于上气泡集合理论建立的流化床三相传质模型,模拟细粉在流化床中的浓度分布,并以此对流化床设计,运行中的重要参数－细粉扬析速率常数Ｋ进行了计算与讨论,计算值与试验结果有良好的一致性。     

Entropy generation in condensation in the presence of high concentrations of noncondensable gases

The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.     

Mass transfer of viscous gas flow over porous stretching/shrinking sheet in the presence of slip and mass transpiration

The effect of the mass transpiration parameter on the viscous gas flow past a porous stretching/shrinking sheet in the presence of Navier's slip is investigated, and also, the mass transfer characteristics are examined. The physical flow problem executes the Navier–Stokes and the mass equation, which forms the system of nonlinear partial differential equations. These are transformed via similarity variables into a system of ordinary differential equations. The slip flow model of the total mass transfer on the moving sheet is modeled by introducing gas slip velocity. The total mass transfer on the moving sheet is modeled by inducing slip models of first and second order. Further, the suction which induces the slip velocity as opposed to the surface movement is examined. The mass suction-induced slip forces the adjacent gases to flow in the reverse direction to sheet movement. Thus, the solution space expands with the slip-induced suction and sheet movement. In the mass injection case, the induced slip increases the effect of the fluid flow for sheet movement. Upon all previous flow models, the present investigation is significant due as it investigates the mass transfer of viscous gasses flow past a porous medium in the presence of slip and mass transpiration.     

Numerical Simulation of Heat and Mass Transfer during Nanosecond Laser Chemical Vapor Deposition on a Particle Surface

Chemical vapor deposition of TiN on a spherical particle surface subjected to nanosecond laser heating is simulated numerically here. A thermal model is developed to describe the heat transfer within the particle, chemical reaction on the particle surface, and mass transfer in the gases. The heat conduction and mass diffusion equations are discretized using the finite volume method with fully implicit schemes, and solved with tridiagonal matrix method. Temporal distributions of particle surface temperature and deposited film thickness resulting from multiple-pulse irradiation are analyzed for a wide range of parameters including laser fluence, pulse repetition frequency, pulse width, initial particle temperature, particle radius, and total pressure in the reaction chamber.     

Effect of thermal diffusion on absorption during dissolution of short gas plugs

We study a nonisothermal gas absorption from a rising short gas plug using an approximation of the infinite dilution of absorbate and taking into account the effect of thermal diffusion on the rates of heat and mass transfer. Short contact time solutions of energy and mass conservation equations are obtained by a similarity method. It is shown that for absorption of ammonia and hydrogen chloride by water and water vapor by aqueous solution of LiBr neglecting effect of thermal diffusion results in overestimating the rate of mass transfer during absorption.     

Numerical Simulation of Heat and Mass Transfer during Absorption of Hydrogen in Metal Hydride Tank

The present study is concerned with cooling a metal hydride tank during the charging process by using a water jacket and fins. Results indicate that the effect of the water jacket becomes more significant over time. Variation of the Reynolds number has no influence on the charging time in the turbulent regime while changing the flow regime from laminar to turbulent improves the results slightly. Furthermore, adding fins on the cooling jacket enhances the heat transfer significantly through better removal of the heat from the central region of the metal bed. Hence, the charging time was significantly reduced.