Heat transfer in pool boiling of ammonia/water mixture

The nonazeotropic binary mixtures such as, methanol/water, ethanol/water and ammonia/water, have variable boiling and dew points, depending on the combination of substance and those mass fraction. It is expected to have a higher performance as a result of decreasing the thermodynamically irreversible loss, when there is a relevant mass fraction. Therefore, ammonia/water mixture is expected to use as working fluid in small temperature difference power generation cycles and absorption refrigeration cycles. However, few experiments were carried out for measuring heat transfer coefficient for ammonia/water mixture in the world. An experimental study has been carried out to measure boiling heat transfer coefficient of an ammonia/water mixture on a horizontal heated surface at low pressure of 0.2, 0.4 and 0.7 MPa and at low mass fraction of 0 < C < 0.27 and at high pressure 0.7, 1.0 and 1.5 MPa and at mass fraction of 0.5 < C < 1.0 and at heat flux under critical heat flux the heat transfer coefficient are compared with existing correlations prediction and a revised correlation can be proposed to predict them well.     

A parametric analysis of moisture migration in an unsaturated porous slab caused by convective heat and mass transfer

An analysis is performed which describes the approach to a quasisteady state of heat and moisture migration in an unsaturated porous slab where one surface is impermeable to heat and mass transfer whereas convective heat and mass transfer occur at the other surface. The initial temperature and moisture content are uniform. The surface atx=0 is suddenly exposed to a gas stream at different temperature and with relative humidity. The conservation equations describing the heat and moisture migration are developed and then simplified with the assumption that the properties can be considered to be constant. The difference between the initial and the wet bulb temperature is used to make the equations dimensionless. In this way, the dimensionless temperature profiles are a function of only one parameter - a modified Biot number, and the dimensionless moisture profiles are functions of four parameters. The numerical results are presented in the form of temperature and moisture profiles as well as temperature and moisture gradients at the surface as a function of those parameters. The heat and moisture transfer at the surface as well as the time for the approach to the quasisteady state can be obtained from these results.

---

Eine parametrische Analyse der Feuchtigkeitswanderung in einer nichtsaturierten porösen Wand erzeugt durch konvektiven Wärme- und Stoffübergang

Zusammenfassung Eine analytische Untersuchung beschreibt den Übergang vom Anfangszum quasistationären Endzustand (im ersten Trocknungsabschnitt) der Wärme- und Stoffwanderung in einer ungesättigten porösen Wand, deren eine Oberfläche für Wärme und Stoff undurchlässig ist, während an der anderen Oberfläche konvektiver Wärme- und Stoffaustausch herrscht. Die Temperatur und der Feuchtigkeitsgehalt in der Wand sind ursprünglich konstant. Die Oberfläche beix = 0 ist plötzlich einem Gasstrom von verschiedener Temperatur und einer relativen Feuchtigkeit ausgesetzt.Die Erhaltungsgleichungen, die die Wärme- und Feuchtigkeitswanderung beschreiben, werden dadurch vereinfacht, daß die Stoffwerte als konstant angesehen werden. Der Unterschied zwischen der ursprünglichen und der Kühlgrenztemperatur wird benutzt um die Gleichungen dimensionslos zu machen. Die dimensionslosen Temperaturprofile sind dann eine Funktion eines einzigen Parameters - einer zweckmäßig definierten Biotzahl. Die Feuchtigkeitsprofile sind durch vier Parameter bestimmt. Numerische Ergebnisse sind als Profile und als Gradienten der Temperatur und der Feuchtigkeit an der Oberflächex = 0 dargestellt. Der Wärme- und Feuchtigkeitsübergang an der Oberfläche sowie die zur Erreichung des quasistationären Endzustandes benötigte Zeit können damit bestimmt werden.

---

Nomenclature Bi ^* modified Biot number, Eq. (30) - c _c composite specific heat (solid + fluid) - c _pa specific heat of air at constant pressure - C _t defined by Eq. (15) - D ^* moisture diffusion coefficient due to temperature gradient - D _c vapor diffusion coefficient through porous medium - h convective heat transfer coefficient - h _m convective mass transfer coefficient - i enthalpy - i _vl heat of evaporation - j diffusive mean flux - k thermal conductivity - k _c composite thermal conductivity (including vapor diffusion) - K moisture diffusion coefficient due to moisture content gradient - K moisture diffusion coefficient due to suction potential gradient - m _a mass flux into air space - P_l,P ₂ dimensionless parameters, Eqs. (46), (47) - q heat flux - t temperature - t _wb wet bulb temperature - w vapor mass fraction (mass of vapor/mass of air) - W moisture content (mass of moisture/mass; of dry medium) - x coordinate Greek symbols _c composite thermal diffusivity (including vapor diffusion) - suction potential - relative humidity - density - time Indices a air - c composite (solid + fluid) - d dry - i initial - l liquid - matrix potential - 0 atx = 0 - s saturated - v vapor     

Experimental investigation in pool boiling heat transfer of ammonia/water mixture and heat transfer correlations

The nucleate pool boiling heat transfer coefficient of ammonia/water mixture was investigated on a cylindrical heated surface at low pressure of 4-8 bar and at low mass fraction of 0 < x_NH3 < 0.3 and at different heat flux. The effect of mass fraction, heat flux and pressure on boiling heat transfer coefficient was studied. The results indicate that the heat transfer coefficient in the mixture decreases with increase in ammonia mass fraction, increases with increase in heat flux and pressure in the investigated range. The measured heat transfer coefficient was compared with existing correlations. The experimental data were predicted with an accuracy of ±20% by the correlation of Calus&Rice, correlation of Stephan-Koorner and Inoue-Monde correlation for ammonia/water mixture in the investigated range of low ammonia mass fraction. The empirical constant of the first two correlations is modified by fitting the correlation to the present experimental data. The modified Calus&Rice correlation predicts the present experimental data with an accuracy of ±18% and the modified Stephan-Koorner correlation with an accuracy of ±16%.     

Numerical modeling of incline plate LiBr absorber

Among major components of LiBr–H₂O absorption chillers is the absorber, which has a direct effect on the chillier size and whose characteristics have significant effects on the overall efficiency of absorption machines. In this article, heat and mass transfer process in absorption of refrigerant vapor into a lithium bromide solution of water-cooled incline plate absorber in the Reynolds number range of 5 < Re < 150 is performed numerically. The boundary layer assumptions are used for the mass, momentum and energy transport equations and the fully implicit finite difference method is employed to solve the governing equations. Dependence of lithium bromide aqueous properties to the temperature and concentration is employed as well as dependence of film thickness to vapor absorption. An analysis for linear distribution of wall temperature condition carries out to investigate the reliability of the present numerical method through comparing with previous investigation. The effect of plate angle on heat and mass transfer parameters is investigated and the results show that absorption mass flux and heat and mass transfer coefficient increase as the angle of the plate increase. The main parameters of absorber design, namely Nusselt and Sherwood numbers, are correlated as a function of Reynolds Number and the plate angle.     

Heat and mass transfer characteristics of simulated high moisture flue gases

The heat transfer process occurring in a condensing heat exchanger where noncondensible gases are dominant in volume is different from the condensation heat transfer of the water vapor containing small amount of noncondensible gases. In the process the mass transfer due to the vapor condensation contributes an important part to the total heat transfer. In this paper, the Colburn-Hougen method is introduced to analyze the heat and mass transfer process when the water vapor entrained in a gas stream condenses into water on the tube wall. The major influential factors of the convective-condensation heat transfer coefficient are found as follows: the partial pressure of the vapor p _v , the temperature of the outer tube wall T _w , the mixture temperature T _g , Re and Pr. A new dimensionless number Ch, which is defined as condensation factor, has been proposed by dimensional analysis. In order to determine the relevant constants and investigate the convection-condensation heat and mass transfer characteristics of the condensing heat exchanger of a gas fired condensing boiler, a single row plain tube heat exchanger is designed, and experiments have been conducted with vapor-air mixture used to simulate flue gases. The experimental results show that the convection-condensation heat transfer coefficient is 1.52 times higher than that of the forced convection without condensation. Based on the experimental data, the normalized formula for convention-condensation heat transfer coefficient is obtained. A heat transfer area m² - Ch condensation factor - c _p specific heat at constant pressure, J/(kg·K) - G mass flux Kg/(m²·s) - h heat transfer coefficient W/(m²·K) - J J-factor - Nu Nusselt number - pa pressure - Pr Prandtl number - Q heat transfer rate - q heat flux W/m² - r latent heat, kJ/kg - Re Reynolds number - Sc Schmidt number - T temperature, C or K - heat conductivity m W/(m·K) - density, kg·m³ - g gas - h moistened hot air - i interface - v vapor - w water     

Heat and mass transfer near the stagnation point with injection and suction of various gases through the body surface

Numerical solutions of the equations of the laminar boundary layer in the vicinity of the stagnation point of an axisymmetric blunted body with injection of single-component gases into a homogeneous external stream are obtained and generalized. More than 30 different pairs of gases are investigated. The heat and mass transfer in a multicomponent laminar boundary layer with the injection of a gas mixture, and also with simultaneous injection and suction of different gases through the body surface, is analyzed. An approximate method is proposed for calculating the heat and mass transfer in a laminar boundary layer.Notation density - T temperature - J enthalpy - M molecular weight - c_i mass concentration - x_i molar concentration - viscosity coefficient - heat conductivity - D_ij binary diffusion coefficient - D_i generalized diffusion coefficient - V_i diffusion velocity - q convective heat flux - surface friction - G over-all mass flow rate through the surface - G_i flow rate of the i-th component through the surface - /c_p heat transfer coefficient - _i mass transfer coefficient of the i-th component - _q injection coefficient for heat transfer (2. 7) - injection coefficient for mass transfer (2. 7) - , are the parameters of the intermolecular interaction potential function - /c _p =q/(J _e -J _wo), _i = (pc _i Vi) _w /(c _iw -c _ie ) (pc _i V _f) _w /(c _iw –c _ie ) The author wishes to thank V. S. Dranichkin, M. V. Gusev, and A. I. Noikin, who assisted with the computer calculations and the analysis of the computer results.     

Effect of pressure on mass absorption in an ammonia–water absorption system

Absorption phenomenon of ammonia vapor into ammonia water solution has been investigated experimentally, by inserting superheated ammonia vapor into a test cell containing a stagnant pool of ammonia water solution. Before commencing the experiment, the pressure in the test cell corresponds to the equilibrium vapor of the ammonia–water system at room temperature. When the valve is opened, mechanical equilibrium is established quickly and the pressure in the test cell becomes equal to that of the ammonia vapor cylinder. The difference between the initial pressure in the vapor cylinder and the initial pressure in the test cell is found to have a major influence on the absorption rate. The main objective of this study is to investigate the effect of this initial pressure difference on the absorption rate of ammonia vapor. A correlation which gives the total absorbed mass of ammonia as a function of the initial concentration, the initial pressure difference and time is derived. In addition the absorbed mass at no pressure difference could be estimated from the absorbed mass at initial pressure difference.     

Effect of surfactant concentration on saturated flow boiling in vertical narrow annular channels

Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8–16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100–1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m² s and heat flux from 40 to 200 kW/m². Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C₀, reaches a maximum close to the value of C/C₀ = 1 (where C₀ = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C₀ = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations.     

Couette flow of a multicomponent ionized gas

We consider equilibrium flow of a multicomponent ionized gas between two catalytic plates of infinite length, one of which moves parallel to the other with constant velocity. The results of [1] are generalized for ionized gaseous mixtures which are in local thermodynamic equilibrium. Formulas are presented for calculating the thermal flux and the effective thermal conductivity for ambipolar diffusion.Then a special ionization case is discussed.Notation A_i chemical symbol of the i-th component - W_i projection of the molar diffusive flux vector of the i-th component on the y-axis - x_i molar concentration - H_i enthalpy - m_i molecular weight - Q_s heat of the s-th reaction - K_ps(T) equilibrium constant of the s-th reaction - W_i mass formation rate of the i-th component per unit volume - Z_i charge number - e unit charge (electron charge) - E electric field intensity - distance between the plates - N number of components - v _sl stoichiometric coefficients - density - T temperature - p pressure - u projection of average velocity on y-axis - viscosity - thermal conductivity - D_ij binary diffusion coefficient - R universal gas constant - k Boltzmann constant In conclusion, the author wishes to thank G. A. Tirskii for proposing the study and for suggestions made in the course of the investigation.     

Concentration-dependent diffusion in a composite slab with and without chemical reactions

Concentration-dependent diffusion of solute in a composite slab is investigated. The complex diffusion problem can be described by a set of nonlinear diffusion equations which is coupled to each other through the nonlinear interfacial boundary conditions. A two-layer diffusion is illustrated and the coupled nonlinear diffusion equations are conveniently solved by the orthogonal collocation method. Numerical simulation of the example reveals many interesting diffusion characteristics which are quite different from those in a single slab diffusion system.Nomenclature a _j expansion coefficient - A _i,j element of collocation matrix - B _i,j element of collocation matrix - C _a , C _b surface concentration - C _i concentration in the ith layer - D _i diffusion coefficient in the ith layer - D _i0 diffusion coefficient at very low concentration - k _i reaction rate in the ith layer - K _i dimensionless reaction rate, k _i l _i ² c _a ^m–1 /D ₁₀ - l _i thickness of the ith layer - m order of chemical reaction - n order of the orthogonal polynomial approximation - P _j–1(x _i ) orthogonal polynomial of order j - t time - x _i coordinate of the ith layer - X _i dimensionless coordinate of the ith layer, x _i/l _i - ratio of diffusion coefficient at low concentration, D ₂₀/D ₁₀ - ratio of thicknesses of layer, l ₁/l ₂ - _i dimensionless parameter in the concentration-dependent function of the ith layer - ratio of surface concentration, C _b /C _a - dimensionless time, tD ₁₀/l ₁ ² - _i dimensionless concentration in the ith layer, C _i /C _a      

Nonlinear Effects in Osmotic Volume Flows of Electrolyte Solutions through Double-Membrane System

The results of experimental study of volume osmotic flows in a double-membrane system are presented in this article. The double-membrane system consists of two membranes (M _u, M _d) oriented in horizontal planes and three identical compartments (u, m, d), containing unstirred binary or ternary ionic solutions. In this system concentrations of the solutions fulfil the following conditions C _us  = C _ds  < C _ms (s = 1 or 2). Solutions of aqueous potassium chloride or ammonia were used as binary solutions, whereas potassium chloride dissolved in aqueous ammonia solution or ammonia dissolved in aqueous potassium chloride solution were used as ternary solutions. For binary solutions, the dependencies of a volume flux (J _v) on potassium chloride or ammonia concentration (C _ms ) are linear, whereas for ternary solutions these dependencies are nonlinear. The volume flux amplification and the osmotic conductivity coefficients were calculated on the basis of experimental data. The coefficient of the volume flux amplification for ternary solutions in comparison to binary ones depends on solutes concentrations and has maximum values dependent on solutes concentrations. Similarly, the osmotic conductivity coefficient has maximal values dependent on solutes concentrations. Moreover, the thermodynamic model of the osmotic volume flux was developed and the results were interpreted within the gravitational instability category.     

Combined convection and radiation heat transfer to thermally developing laminar droplet flow in concentric annuli

This paper presents numerical results for combined convection and radiation heat transfer to a laminar mist flow in the thermal entrance region of a concentric annulus with a heated core at constant wall temperature and an insulated outer wall. The saturated droplets in the mist flow are considered as equivalent heat sinks distributed in the superheated vapor stream. Numerical calculations are performed for the variations of droplet size, mean vapor velocity, and the local Nusselt number in the streamwise direction until the single-phase fully-developed condition is reached. The important role of the saturated droplets on combined convection and radiation heat transfer to mist flow is clearly demonstrated.

---

Kombinierte Wärmeübertragung durch Konvektion und Strahlung im thermischen Einlauf einer laminaren Tröpfchenströmung in einem konzentrischen Ringspalt

Zusammenfassung Dieser Artikel stellt numerische Ergebnisse für kombinierte Wärmeübertragung durch Konvektion und Strahlung im thermischen Einlauf einer laminaren Tröpfchenströmung in einem konzentrischen Ringraum mit beheiztem Kern bei konstanter Wandtemperatur und isolierter Außenwand dar. Die gesättigten Tröpfchen wirken als verteilte Wärmesenken im überhitzten Dampfstrom. Numerische Berechnungen werden unter Variation des Tröpfchendurchmessers, der durchschnittlichen Dampfgeschwindigkeit und der Nusselt-Zahl durchgeführt, bis eine einphasige vollausgebildete Strömung erreicht ist. Der wichtige Einfluß der gesättigten Tröpfchen auf die kombinierte Wärmeübertragung durch Konvektion und Strahlung wird klar gezeigt.

---

Nomenclature A liquid loading parameter, defined in Eq. (3) - A _d heat transfer area of droplets per unit volume of vapor - A _w heat transfer area of heated wall per unit volume of vapor - C wall superheat parameter, defined in Eq. (5) - C _p specific heat of vapor - D dimensionless droplet diameter,d/d ₀ - D _h hydraulic diameter, 2(r ₀–r _i) - d droplet diameter - d ₀ droplet diameter at thermal entrance (x=0) - E dimensionless parameter, defined in Eq. (6) - H dimensionless parameter, defined in Eq. (7) - F _w–d geometric view factor - h _d heat transfer coefficient for evaporating droplets - h _p0 heat transfer coefficient of non-evaporating droplet or solid sphere with diameter ofd ₀ - k thermal conductivity of vapor - n droplet number density (number of droplets per unit volume of vapor) - n ₀ droplet number density at thermal entrance (x=0) - Nu _x local Nusselt number, defined by Eq. (17) - Pr Prandtl number of vapor,C _p/k - Q _r radiative heat transfer to droplets (per unit volume of vapor) - q _w heat flux at the inner wall - R dimensionless radial position,r/r _i - Re Reynolds number of vapor, 2 _v V₀ r _i/ - r radial position - r _i radius of inner tube - r _o radius of outer tube - S heat sink parameter, defined in Eq. (4) - T temperature of vapor - T _m bulk mean temperature of vapor - T _s saturated temperature - T _w inner wall temperature - V mean vapor velocity - V fully-developed vapor velocity, given in Eq. (12) - V ₀ mean vapor velocity atx=0 - x axial position in thermal entrance region - X dimensionless axial position, (x/r _i)/(Re·Pr) - z ₀ flow quality atx=0 Greek symbols ₀ vapor void fraction atx=0 - ratio of radius,r _i/r₀ - _d emissivity of droplets - _w emissivity of inner heated wall - dimensionless vapor temperature, defined in Eq. (9) - _m dimensionless vapor mean temperature, given by Eq. (14) - _wi dimensionless inner wall temperature - _wo dimensionless outer wall temperature - dynamic viscosity of vapor - _l liquid density - _v vapor density - Stefan-Boltzmann constant     

Water Vapor Diffusion Through Soil as Affected by Temperature and Aggregate Size

Water vapor diffusion through the soil is an important part in the total water flux in the unsaturated zone of arid or semiarid regions and has several significant agricultural and engineering applications because soil moisture contents near the surface are relatively low. Water vapor diffusing through dry soil is absorbed for both long and short terms. Long-term absorption allows more water to enter than exit the soil, as reflected in the concentration gradient. Short-term absorption leads to an apparent reduction in the diffusion rate, as reflected in the diffusion coefficient. This investigation studied the effects of soil temperature and porosity on the isothermal diffusion of water vapor through soil. The diffusion model consisted of 25.4 cm × 8.9 cm × 20.3 cm Plexiglas box divided into two compartments by a partition holding a soil reservoir. Water vapor moved from a container suspended by a spring in one compartment, through the porous medium in the center of the model, to calcium chloride in a container suspended by a spring in the other compartment. The porous materials consisted of aggregates of varying size (2–2.8, 1–2, and 0.5–1 mm) of a Fayatte silty clay loam (a fine-silty, mixed mesic Typic Hapludalf). The flow rates of water vapor were measured at temperatures of 10, 20, 30, and 40°C. Warmer temperatures increased the rate of diffusion through dry soil while reduced the amount of water absorbed by that soil. Reducing porosity slowed the rate of diffusion and increased the amount of water absorbed. The dry soil in this study absorbed from 1/8 to 2/3 of the diffusing water. Maximum absorption rates occurred with the most compact soil samples at the highest temperature, though the maximum absorption as a percentage of the diffusing water was in the compact samples at the lowest temperature. The diffusivity equation D/D ₀ = [(S – 0.1)/0.9]² fit the D/D ₀ values obtained from these data if a coefficient of 1/3 or 1/3.5 is added to correct for the time delays caused by temporary sorption of the diffusing water vapor. The data, influenced by the interaction of water vapor and soil materials, represent a diffusion rate lower than the diffusion rate that would have resulted without this interaction. Mention of trade names, proprietary products, or specific equipment is intended for reader information only and does not constitute a guarantee or warranty by the USDA-ARS nor does it imply approval of the product named to the exclusion of other products. An erratum to this article can be found at     

A tube-by-tube reduction method for simultaneous heat and mass transfer characteristics for plain fin-and-tube heat exchangers in dehumidifying conditions

This study proposed a new method, namely a tube-by-tube reduction method to analyze the performance of fin-and-tube heat exchangers having plain fin configuration under dehumidifying conditions. The mass transfer coefficients which seldom reported in the open literature, are also presented. For fully wet conditions, it is found that the reduced results for both sensible heat transfer performance and the mass transfer performance by the present method are insensitive to change of inlet humidity. Unlike those tested in fully dry condition, the sensible heat transfer performance under dehumidification is comparatively independent of fin pitch. The ratio of the heat transfer characteristic to mass transfer characteristic (h_c,o/h_d,o C_p,a) is in the range of 0.6~1.0, and the ratio is insensitive to change of fin spacing at low Reynolds number. However, a slight drop of the ratio of (h_c,o/h_d,o C_p,a) is seen with the decrease of fin spacing when the Reynolds number is sufficient high. This is associated with the more pronounced influence due to condensate removal by the vapor shear. Correlations are proposed to describe the heat and mass performance for the present plate fin configurations. These correlations can describe 89% of the Chilton Colburn j-factor of the heat transfer (j_h) within 15% and can correlate 81% of the Chilton Colburn j-factor of the mass transfer (j_m) within 20%.     

Flow of a multicomponent gas between parallel permeable surfaces

The present paper gives an exact solution of the equations describing the flow of a multicomponent gas between two parallel permeable planes, one of which moves relative to the other with constant velocity (i. e., we study a flow of the Couette type).Notation y coordinate - u, v velocity components - density - c_i mass concentration of i-th component - I_i diffusional flux of i-th component - H enthalpy - T temperature - m molecular weight - viscosity coefficient - heat conduction coefficient - c_p mixture specific heat - D_ij the binary diffusion coefficients - P Prandtl number - S_ij Schmidt number - N total number of components - n number of components in injected gas - l distance between planes Indices i, j component numbers - w applies to quantities for y=0 - ^* applies to quantities for y=l     

Critical heat fluxes in boiling of liquid nitrogen underheated to saturation point in forced-flow conditions

This paper gives the results of experimental determinations of the critical heat fluxes in the boiling of Liquid nitrogen in forced-flow conditions in the mass velocity range 2 · 10³-40 · 10³ kg/m² · sec, pressure range 29 · 10⁴–245 · 10⁴ N/m², and at underheatings corresponding to the onset of normal boiling crises.Notation q₀ critical heat flux - r heat of vaporization - i enthalpy of flow corresponding to saturation point - i enthalpy of flow corresponding to liquid temperature - surface tension - density of liquid - density of saturated vapor - C _f friction factor - W_g mass velocity - Fr_* Froude number - g acceleration due to gravity     

Numerical modelling of heat and mass transfer in adsorption solar reactor of ammonia on active carbon

 In this paper, we present a modelling of the performance of a reactor of a solar cooling machine based carbon–ammonia activated bed. Hence, for a solar radiation, measured in the Energetic Laboratory of the Faculty of Sciences in Tetouan (northern Morocco), the proposed model computes the temperature distribution, the pressure and the ammonia concentration within the activated carbon bed. The Dubinin–Radushkevich formula is used to compute the ammonia concentration distribution and the daily cycled mass necessary to produce a cooling effect for an ideal machine. The reactor is heated at a maximum temperature during the day and cool at the night. A numerical simulation is carried out employing the recorded solar radiation data measured locally and the daily ambient temperature for the typical clear days. Initially the reactor is at ambient temperature, evaporating pressure; P _ev =P _st (T _ev =0 ^∘C) and maintained at uniform concentration. It is heated successively until the threshold temperature corresponding to the condensing pressure; P _cond =P _st (T _am ) (saturation pressure at ambient temperature; in the condenser) and until a maximum temperature at a constant pressure; P _cond . The cooling of the reactor is characterised by a fall of temperature to the minimal values at night corresponding to the end of a daily cycle. We use the mass balance equations as well as energy equation to describe heat and mass transfer inside the medium of three phases. A numerical solution of the obtained non linear equations system based on the implicit finite difference method allows to know all parameters characteristic of the thermodynamic cycle and consider principally the daily evolution of temperature, ammonia concentration for divers positions inside the reactor. The tube diameter of the reactor shows the dependence of the optimum value on meteorological parameters for 1 m² of collector surface. Received on 10 January 2001     

Effect of salt on boiling heat transfer of ammonia-water mixture

Nucleate pool boiling heat transfer coefficients were determined experimentally for NH₃–H₂O, NH₃–H₂O–LiNO₃ and NH₃–H₂O–LiBr mixtures. Both the salts were effective in increasing the heat transfer coefficient of NH₃–H₂O mixture. A concentration of 10 mass% of the salts in water, produced the greatest enhancement in heat transfer coefficient at all the range of pressure, heat flux and ammonia concentration studied in this investigation. The experiments indicated that ammonia concentration also has the impact on the augmentation of heat transfer coefficient in NH₃–H₂O binary mixture by the addition of salts. For the solution of ammonia mass fraction 0.30, high concentration of LiBr gives the highest heat transfer coefficient, for ammonia mass fraction of 0.25, high concentration of LiNO₃ gives the maximum heat transfer coefficient, for ammonia mass fraction of 0.15, both the salts are equally effective in increasing the heat transfer coefficient.     

Effects of Material Symmetry on the Coefficients of Transport in Anisotropic Porous Media

The objective of this article is to highlight certain features of a number of coefficients that appear in models of phenomena of transport in anisotropic porous media, especially the coefficient of dispersion the second-rank tensor D _ij , and the dispersivity coefficient, the fourth-rank tensor a _ijkl , that appear in models of solute transport. Although we shall focus on the transport of mass of a dissolved chemical species in a fluid phase that occupies the void space, or part of it, the same discussion is also applicable to transport coefficients that appear in models that describe the advective mass flux of a fluid and the diffusive transport of other extensive quantities, like heat. The case of coupled processes, e.g. the simultaneous transport of heat and mass of a chemical species, are also considered. The entire discussion will be at the macroscopic level, at which a porous medium domain is visualized as a homogenized continuum.     

A numerical study of the non-absorbable effects on the falling liquid film absorption

A numerical study for the simultaneous heat and mass transfer in a falling liquid film absorption process with the presence of non-absorbable gases is presented. Water vapor mixed with air as the non-absorbables being absorbed into a falling smooth aqueous lithium chloride film flow was chosen as the model problem for the study. The finite difference numerical calculation was proceeded by marching downward from the top end, owing to the parabolic type energy and concentration equations for both liquid and gas phases. The results indicate that the local non-absorbable gas concentration is much higher at the gas-liquid interface than that in the ambient, hence the local vapor pressure is lowered there such that the absorption driving potential of the vapor pressure difference is reduced. The resulting reduction of the absorption rate due to the presence of the non-absorbables suggests that its effect must be carefully considered in the application of absorption heat pump design. The present study can provide some useful information for this purpose.

---

Numerische Studie über die Einwirkung von nicht absorbierbaren Stoffen auf die fallende Flüssigkeitsfilmabsorption

Zusammenfassung Hier wurde eine numerische Studie der Wärmeund Stoffübertragung in einem Absorptionsprozeß eines fallenden Flüssigkeitsfilms in Anwesenheit von nicht absorbierbaren Gasen dargestellt. Ein Wasserdampf-Luft-Gemisch, das in Anwesenheit von nicht absorbierbaren Gasen von einer fallenden glatten flüssigen Lithium-Chlorid-Filmströmung absorbiert wird, wurde als das Modellproblem für diese Studie gewählt. Die numerische Berechnung mit dem Finite Differenzenverfahren wurde schrittweise vom obersten Ende nach unten durchgeführt. Die Berechnung bezieht sich auf den parabolischen Typ der Energie- und Konzentrationsgleichungen für die Flüssigkeits- und Gasphasen. Die Ergebnisse weisen darauf hin, daß die lokale nicht absorbierbare Gaskonzentration bei der Gasflüssigkeitsphase sehr viel höher ist als in der Umgebung. Weiter ist der lokale Dampfdruck so erniedrigt worden, daß sich das Absorptionsbewegungspotential des Dampfdruckunterschiedes reduziert. Die resultierende Reduzierung der Absorptionsrate, die auf die Anwesenheit der nicht absorbierbaren Stoffe zurückzuführen ist, verlangt eine sorgfältige Einbeziehung ihere Einflüsse auf die Gestaltung der Absorptionswärmepumpen. Diese Arbeit kann einige nützliche Informationen für diesen Zweck geben.

---

Nomenclature C absorbent concentration in weight fraction of salt - C _a nonabsorbables concentration in molar fraction - C _a at inlet and infinity - C _in C at film inlet - c _p specific heat of liquid - c _{p g} specific heat of gas - D species diffusivity for LiCl-H₂O - D _g species diffusivity for air-water vapor - g gravity - h _o film thickness - H _a heat of absorption - k thermal conductivity of liquid - k _g thermal conductivity of gas - L transformation constant - water vapor mass absorption rate - P _v water vapor pressure - Re film Reynolds number=V ₀ h ₀/ - T film temperature - T _in film temperature at inlet - T _g gas temperature - T _w wall temperature - T gas temperature at inlet and infinity - u velocity inx-direction - U =1.5V ₀ - V ₀ mean film velocity=g h ₀ ² /3µ - x coordinate parallel to the wall - y coordinate normal to the wall in the film region - y _g coordinate normal to the wall in the gas region - transformedy _g Greek letters dynamic viscosity - liquid density - _g gas density