Frost formation on fin-and-tube heat exchangers. Part I—Modeling of frost formation on fin-and-tube heat exchangers

In this study, the heat and mass transfer characteristics of heat exchangers during frost formation process are analyzed numerically. Unsteady heat and mass transfer coefficients of the air side, heat transfer coefficient of the refrigerant side, air-frost layer interface temperature, the surface efficiency of the heat exchanger and the mass flow rate of the frost accumulated on the heat exchanger surface are calculated. The total conductivity (UA) and pressure drop of the heat exchanger are reported for different air inlet temperature, relative humidity, air mass flow rate and the refrigerant temperature.     

Surface tension driven flows in shallow layers due to heat or mass transfer to the free surface

In a number of physical systems a surface tension driven flow is established in a shallow layer of liquid as a result of heat or mass transfer to the free surface. Such transfer processes often produce a thin temperature or concentration boundary layer near the free surface. We have considered the relatively simple situation when this occurs in shallow two-dimensional channel flow under steady conditions. It is shown that the properties of the boundary layer can be obtained by solving a sequence of parabolic partial differential equations and that the shape of the free surface results from the solution of an integral equation. The simple case of uniform surface transfer has been considered, but the analysis developed can be extended to more complex situations.     

Prediction of in-tube condensation heat transfer characteristics of binary refrigerant mixtures

This study presents a prediction model for the condensation heat transfer characteristics of binary zeotropic refrigerant mixtures inside horizontal smooth tubes. In this model, both the vapor-side and liquid-side mass transfers are considered, and the high flux mass transfer correction factor is used to evaluate mass transfer coefficients. The model was applied to the binary zeotropic refrigerant mixture R134a/R123, which has a large temperature glide. Calculation results showed that the heat transfer degradation of R134a/R123 due to gradients in the mass fraction and temperature is considerable, and depends on the mass fraction of the more volatile component and the vapor mass quality of the refrigerant mixture. By comparison with experimental data, incorporating the present finite mass transfer model for the liquid film side into the calculation algorithm was shown to reasonably well predict the condensation heat transfer coefficients of binary refrigerant mixtures with the mean deviation of about 10.3%. In the present calculations, however, it was also found that the high flux mass transfer correction factor had only a slight effect on the condensation heat transfer.     

水冷却塔传质过程理论模型研究

现有描述冷动塔填料表面传热传质过程的理论模型都与一个经验K值有关,由于以往的研究者都将这个系数表示成水气流量的函数,没有正确地提示揭示传质过程的本质,本文采用液膜蒸发理论,将K值有示成湿空气温差及压差的函数,通过对某些已知实验条件的实验结果进行验算,发现该模型能正确的描述冷动填料表面的传质过程,采用基于液膜蒸发理论的新模型将改善冷动塔的设计方法。     

船舶围壁空气层对舱室热环境的影响

采用频域回归法（FDRM）计算4种船舶围壁的动态传热特性,并分析空气层对船舶围壁动态传热及舱室热环境的影响。计算结果表明：空气层对船舶围壁的传热有明显的阻隔作用,且空气层越厚阻隔作用越明显;船舶围壁空气层与保温层的排列次序对围壁传热有一定的影响,空气层在外、保温层在内的排列方式隔热效果更好,内壁温度波动小。     

Mixed convection in micropolar fluid flow over a horizontal plate with surface mass transfer

Combined free and forced convection in the boundary layer flow of a micropolar fluid over a horizontal surface is studied. Buoyancy effects on the flow and temperature fields are discussed. The influence of uniform mass transfer from the surface is also considered. Wall friction and heat transfer results are presented for various cases representing the relative effects of blowing or suction as compared to the combined effects of buoyancy and mass transfer.     

Heat and mass transfer through a vapor film in view of the motion of liquid-vapor interface and the rise of temperature at the interface

Unsteady-state processes of heat and mass transfer in a vapor film formed during the interaction of a highly heated body with a cold liquid are investigated using a numerical solution of the kinetic Boltzmann equation. Two options of formulation of the problem are treated, which are more complex compared to those treated in the previous study, namely, (a) the liquid surface moves at a constant velocity, and the liquid temperature is constant; and (b) the liquid surface is stationary, and the liquid is heated (heat is transferred by heat conduction alone). It is demonstrated that, in analyzing various applications (for example, problems on the evolution of vapor cavities under conditions of film boiling), the state of vapor in the film may be taken to be quasi-steady-state; it may be described by steady-state kinetic relations for which the effect of mass flux may be ignored.     

Numerical analysis of drying characteristics of frozen material immersed in fluidized bed at low temperature under reduced pressure

A spherical frozen material was dried in a fluidized bed of inert particles at a low temperature (lower than the melting point of water) under reduced pressure. To evaluate the drying characteristics of the frozen material, the heat and mass transfers in the material during the drying process were calculated using one-dimensional differential equations. The fitting parameters (accommodation coefficient of internal sublimation and heat transfer coefficient at the surface of the material) were determined to fit the calculation results and the experimental data. Drying characteristics, such as the distributions of moisture content and temperature in the material during drying, were calculated. Operational conditions, such as bed temperature, humidity, and heat transfer coefficient (gas velocity) at the surface of the material were varied in the calculation, and the effects on the end time of drying were estimated. Sublimation in the interior of the material governs the drying characteristics. The dry region in the material became resistant to heat transfer. The calculation results are reasonable for expressing the drying characteristics of freeze-drying, that is, our calculation method can be used to estimate the drying characteristics of frozen material in a fluidized bed.     

肋板式吸附床传热性能的数值模拟

设计了新型肋板式结构吸附床,建立了吸附床在非第一类边界条件的三维传热传质数学模型.应用计算流体动力学中有限控制体积方法对吸附床内传热特性进行了模拟计算,并通过研究吸附床内温度随时间的动态变化,分析了换热流体参数和吸附剂导热系数变化对床层传热特性的影响.计算结果表明,吸附剂的导热系数,换热流体流速、温度对床层传热特性影响显著且肋板式吸附床传热效率高,吸附床内温度分布均匀.     

Semianalytic method for heat transfer calculation in the liquid film under conditions of a constant heat flux on the wall

For calculating the heat transfer in the free-falling liquid film, a semianalytic method is offered in which the temperature field in the liquid is presented as a series of basis functions which satisfies boundary conditions. The proposed method is demonstrated by the example of the problem regarding the film heating under conditions of the constant heat flux on the wall taking into account the heat transfer on the interfacial surface. The analytical solution is derived for the thermal initial section, on which the liquid heating occurs in a thin layer near the wall. Calculations using the proposed method are well agreed with the numerical solution obtained by the finite-difference method and with experimental data.     

Heat transfer under conditions of plane potential flow past obstacles

An exact solution is given of the equation of convective heat transfer (mass transfer) under conditions of plane potential flow of incompressible liquid along a wall which has a bend in the form of a half-ellipse to present an obstacle to incident flow. The temperature of the obstacle surface is arbitrary, and the temperature of the rest of the wall coincides with that of incident flow. Particular cases of the constant temperature of the obstacle and of the sine distribution of its surface temperature are treated.     

Evaluation of heat and mass transfer coefficients in a gauze-type structured packing air dehumidifier operating with liquid desiccant

In this paper, high performance packing, namely, structured packing that has good heat and mass transfer characteristics, is proposed for dehumidification of air using liquid desiccants and for regeneration of liquid desiccants. In order to design a structured packing tower for liquid desiccant — air contacting operations, heat and mass transfer coefficients for each phase are required. This paper is concerned with the interface transfer of heat and mass when air is brought into contact with the liquid desiccant solution. A theoretical study of evaluating heat and mass coefficients in an air-desiccant contact system employing three liquid desiccants, namely calcium chloride, lithium chloride, and a mixture of 50% calcium chloride and 50% lithium chloride (called cost effective liquid desiccant, CELD) is investigated. Moreover, air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid. The findings for the three liquid desiccants are compared and discussed.     

结霜工况下空温式翅片管气化器传热试验研究

为深入掌握低温液体在空温式翅片管汽化器内的气化情况以及其与翅片管表面霜层生长的相互影响规律,以液氮为介质进行了低温液体在空温式翅片管气化器内的气化试验。通过热电偶和刻度带分别对翅片管上不同位置的温度和霜层厚度进行了测量,并分析了翅片管表面霜层的生长规律及翅片管内低温液体的流动特性。结果表明:气化器表面结霜过程受冷表面温度影响较大,冷表面温度越低,结霜速率越大,霜层越厚。结霜工况下的气化器工作状态分预冷和稳态两种工作状态。预冷工作状态低温液体进入气化器后迅速气化,其过程包含气液两相和单气相两个换热段。稳态工作状态低温液体在气化器内气化经历单液相、气液两相、单气相三个换热段,单液相段翅片管表面结霜最为严重,单气相段翅片管表面无霜晶形成。因此认为,可通过分状态分段设计空温式翅片管气化器从而减弱结霜对翅片管传热的影响,提高气化器换热效率。     

Numerical Investigation of Bubble Induced Marangoni Convection: Some Aspects of Bubble Geometry

Thermocapillary or Marangoni convection is a surface tension driven flow that occurs when a gas–liquid or vapor–liquid interface is subjected to a temperature gradient. In the past, the contribution to local heat transfer arising from Marangoni convection has been overlooked as insignificant since under earth gravity it is overshadowed by buoyant convection. This study numerically investigates some aspects of bubble size and shape on local wall heat transfer resulting from Marangoni convection about individual bubbles on a heated wall immersed in a liquid silicone oil layer (Pr = 110) of depth 5 mm. It was found that increasing bubble volume causes an increase in the area over which Marangoni convection has affect. Heat transfer therefore increases with bubble size. Over the effective area, the surface averaged hot wall heat transfer is not affected greatly by bubble shape. The surface averaged heat transfer over the effective area on both the hot and cold walls is affected dramatically by bubble size, but the increase is more profound on the cold wall.     

Heat Transfer between a Gas-Droplet Medium and a High-Temperature Surface

A one-dimensional problem is treated, which is associated with heat conduction under conditions of contact between a finely divided gas-droplet medium with a highly heated surface when the temperature of the gas phase exceeds significantly the temperature of the liquid phase. Criterional equations are obtained for the calculation of the coefficient of heat transfer from a surface to a gas-droplet medium. Estimates are made of the time of complete evaporation of droplets and of the thickness of vapor-gas layer in the vicinity of the surface. The obtained results may be used to investigate the processes of cooling a metal.     

Temperature and Moisture Distributions Inside a Layer of Moist Material Placed on a Hot Isothermal Surface

The problem of contact drying of a layer of moist material placed on a hot isothermal surface is considered. A system of heat and mass transfer equations and equations for the initial and boundary conditions are proposed. Luikov's theoretical approach is utilized, but all relevant thermophysical coefficients are considered to be fully dependent on the temperature and moisture content inside the material during the process. The model is solved numerically. Results for temperature and moisture distributions inside the layer of the material obtained using this model are compared to results obtained by solving a model with constant thermophysical coefficients in the governing equations, formed by using the same type of heat and mass transfer equations and equations for the initial and boundary conditions. The process is also examined experimentally, which provided results that enable verification of the proposed models.     

翅片结构对换热器结霜特性影响的实验研究

本文对空气源热泵的翅片管换热器表面霜层生长特性进行实验研究,通过红外热像仪对霜层表面温度进行测量,并用热电偶直接测量装置进行校核.分析了平片、波纹片、条缝片3种翅片类型及翅片节距对霜层厚度、结霜量、换热量的影响,并用霜层-湿空气界面条件等作为传热及传质驱动力分析了霜层生长规律.实验结果表明,波纹片及条缝片翅片换热器界面...     

Rheological and thermal properties of the surface layer of liquids

A grid model of the surface liquid layer at the boundary with the gas and its rheological and thermophysical properties (temperature dependence of the surface tension and surface heat capacity) have been described. It has been obtained that the mole heat capacity of the surface layer is nearly the same (5.6 J/(mole·K)) for many liquid metals. It has been shown that for calculation of the rheological properties it is expedient to use the Kelvin model. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 5, pp. 160–163, September–October, 2005.     

Heat transfer on a flat plate immersed in a laminar flow of a heated liquid

The case of one-sided heat transfer to a flat plate immersed in a flow of heated liquid is examined. Equations are obtained for the liquid temperature distribution in the boundary layer, and for the heat flux from the liquid to the wall.