Vapor absorption by LiBr aqueous solution in vertical smooth tubes

Heat and mass transfer in a falling film vertical in-tube absorber was studied experimentally with LiBr aqueous solution. The presented results include the effect of solution flow rate, solution subcooling and cooling water temperature on the absorption in a smooth copper tube 16.05 mm I.D. and 400 mm long. The experimental data in the previous report for a 1200-mm-long tube was also re-examined and compared. It was demonstrated by the observation of the flow in the tube that the break down of the liquid film into rivulets leads to deterioration of heat and mass transfer at lower film Reynolds number or in longer tubes. An attempt to evaluate physically acceptable heat and mass transfer coefficients that are defined with estimated temperature and concentration at the vapor–liquid interface was also presented.     

Heat and mass transfer in vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems

A model is developed for calculation of simultaneous heat and mass transfer processes in vertical bubble absorbers as used for ammonia-water absorption refrigeration systems. Some preliminary experiments have been performed in an absorber without heat removal. The results from these experiments are compared with the literature and give a first indication about the methods for prediction of the absorption process. Experiments have also been performed with simultaneous heat removal. The internal diameters of the absorbers tested were 10.0, 15.3, and 20.5 mm. The mass transfer coefficients resulting from these experiments are correlated by a modified Sherwood relation. An interative procedure is presented which allows design of vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems.     

A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water

An absorber is a major component in the absorption refrigeration systems, and its performance greatly affects the overall system performance. In this study, both the numerical and experimental analyses in the absorption process of a bubble mode absorber were performed. Gas was injected into the bottom of the absorber at a constant solution flow rate. The region of gas absorption was estimated by both numerical and experimental analyses. A higher gas flow rate increases the region of gas absorption. As the temperature and concentration of the input solution decrease, the region of gas absorption decreases. In addition, the absorption performance of the countercurrent flow was superior to that of cocurrent. Mathematical modeling equations were derived from the material balance for the gas and liquid phases based on neglecting the heat and mass transfer of water from liquid to gas phase. A comparison of the model simulation and experimental results shows similar values. This means that this numerical model can be applied for design of a bubble mode absorber.     

A critical review of models of coupled heat and mass transfer in falling-film absorption

In absorption space-conditioning systems, the performance of the absorber is critical to the overall system performance, size, and first-cost. The objective of this paper is to provide a comprehensive review of the significant efforts that researchers have made to mathematically model the coupled heat and mass transfer phenomena that occur during falling-film absorption. A detailed review of the governing equations, boundary conditions, assumptions, solution methods, results, and validation of these investigations is presented. This review excludes experimental work in this area, the effect of additives, and the effect of non-absorbable gases. It is shown that most work found in the literature has focused on the particularly simplified case of absorption in laminar vertical films of water-lithium bromide. Fewer researchers have considered the important situations of wavy films, turbulent films, and films on horizontal tubes. Investigations of the ammonia-water fluid pair have been generally more empirical in nature and/or restricted to vertical laminar films. This review is used to highlight key areas which need attention such as film and vapor hydrodynamics, especially the non-periodicity, instability, and recirculatory motion of waves in the vertical wall case and droplets and waves in the horizontal tube case. Also the potential interaction of the heat and mass transfer process on the film hydrodynamics, surface wetting, heat transfer in the vapor phase, and common simplifications to the governing equations should all be considered carefully. Finally, emphasis must be placed on experimental validation of the local conditions and transfer processes within the absorber, not just overall transport values.     

Numerical design of ammonia bubble absorber applying binary nanofluids and surfactants

The objectives of this paper are to analyze the combined heat and mass transfer characteristics for the ammonia bubble absorption process and to study the effects of binary nanofluids and surfactants on the absorber size. The ammonia bubble absorbers applying binary nanofluids and surfactants are designed and parametric analyses are performed. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case are applied in the numerical model. The values of the effective absorption ratio are decided from the previous experimental correlations. The kinds and the concentrations of nano-particles and surfactants are considered as the key parameters. The considered surfactants are 2-ethyl-1-hexanol (2E1H), n-octanol, and 2-octanol and nano-particles are copper (Cu), copper oxide (CuO), and alumina (Al₂O₃). The results show that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. In order to reach 16.5% ammonia solution under the considered conditions, for example, the addition of surfactants (2E1H, 700 ppm) can reduce the size of absorber up to 63.0%, while the application of binary nanofluids (Cu, 1000 ppm) can reduce it up to 54.4%. In addition, it is found that the effect of mass transfer resistance is more dominant than that of heat transfer resistance. That is, the enhancement of mass transfer performance is more effective than that of heat transfer performance.     

垂直管内TFE/NMP降膜吸收过程中热质传递试验研究

作为一种新型的吸收式制冷工质时-TFE/NMP（2，2，2-trifluo-roethanol/N-methylpyrolidone，中文名：三氟乙醇/氮甲基吡咯烷酮），因其良好的工作特性而被国际制冷界所重视，但有关吸收式制冷/热泵系统运行中的一个重要环节-TFE/NMP降膜吸收过程中的传热、传质现象却有人进行过研究。在国家自然科学基金的资助下，我们建立了单根管吸试验台以研究TFE/NMP降膜吸收过程中热、质传递规律。在不同TFE/NMP溶液流量和不同冷却水流量条件下，测得两组试验数据，对试验数据进行处理并对其数据结果加以分析后，得出垂直管内TFE/NMP降膜吸收过程中热量和质量传递规律的一些特性。     

R134a-DMF垂直管内鼓泡吸收特性研究

为研究垂直管内R134a-DMF(二甲基甲酰胺)鼓泡吸收过程的热、质传递特性,本文搭建了垂直管内鼓泡吸收实验测试装置,构建了管内R134a被R134a-DMF混合溶液鼓泡吸收过程的热、质传递数学模型.进一步通过模型分析了当吸收压力为0.35 MPa,蒸气入口温度为5℃,稀溶液入口质量流量为12.0 kg/h时,吸收过程...     

Mass transfer correlation of NH3–H2O bubble absorption

The objectives of this paper are to study the effect of key parameters on absorption performance and to develop an experimental correlation of mass transfer coefficient for ammonia–water bubble absorption. The orifice diameter, liquid concentration and vapor velocity are considered as the key parameters. This study successfully visualized the bubble behavior and measured the volumetric diameter of bubbles during the bubble absorption process. The bubble absorption is grouped into two processes, bubble growth (process I) and bubble disappearance (process II), respectively. The following conclusions were drawn from the present study. A new experimental correlation for the volumetric bubble diameter was proposed with ±15% error band, which could be applied to calculate the mass transfer coefficient. The mass transfer coefficient increased with a decrease of the liquid concentration. In process II, the mass transfer coefficient increased with an increase of the Galileo number. Finally, experimental correlations of mass transfer coefficient were developed for processes I and II with ±18% error bands.     

Effect of fin spacing on the heat transfer and pressure drop of a two-row plate fin and tube heat exchanger

The analogy between heat and mass transfer (using the naphthalene sublimation technique to measure the mass transfer coefficient) was used to investigate the heat transfer capabilities of various two-row plate fin and tube heat exchanger configurations. Average transfer coefficients were determined from measurements of the mass transferred in an analogical system consisting of a pair of naphthalene plates and an array of spacer discs. The analogical system modelled a typical heat exchanger flow passage. Special attention was given to the effect of fin spacing on heat transfer capabilities. A physical interpretation of the experimental data has been given and new conclusions have been drawn. The pressure drop of the heat exchanger configurations has also been investigated.     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

Enhanced heat and mass transfer in alternating structure of tubes and longitudinal trough mesh packing in lithium bromide solution absorber

For enhancing the vapour absorption in LiBr solution systems, a novel absorber with tube and mesh packing alternating structure is designed and investigated. Stainless steel mesh screens are folded as the longitudinal trough mesh packing, and inserted to the gaps of horizontal tubes to make the absorbent flow through the tube and mesh packing regimes successively, thus forming an alternating heat and mass transfer absorption process. Experimental investigation is conducted to characterize the absorption performance of the absorption bodies of this alternating structure and conventional horizontal coils. The results show that the average mass transfer rate and cooling load are increased by 17.2% and 6.23% respectively, which confirms that the alternating structures can promote the absorption. The mesh packing provides extended absorption area, slows down the flow and well mingles the solution, which are all beneficial for vapour absorption.     

The role of surfactant adsorption rate in heat and mass transfer enhancement in absorption heat pumps

The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H₂O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.     

Frost formation and heat transfer on circular cylinders in cross-flow

When humid air comes into contact with a surface whose temperature is below the dew point of water vapour in air and also below the freezing point, frost deposition takes place over the surface. Previous studies indicate that the heat transfer rate increases at the initial stages of deposition since the rough frost surface acts as a finned one. As the frost thickens, however, the insulating effect of the frost layer predominates resulting in a reduction in the heat transfer rate. This paper presents a transient model to predict the frosting process over a circular cylinder in a cross-flow of humid air. the transfer parameters are computed employing a numerical solution of the momentum, energy and diffusion boundary-layer equations along with the continuity equation, using a finite difference scheme. Empirical correlations for thermal conductivity and density are utilized for closure purposes. Model results are compared with existing experimental data and with numerical data of previous investigators and are found to agree well in the applicable temperature and humidity ranges of the frost density and conductivity correlations.     

Mass transfer characteristics of a structured packing for ammonia rectification in ammonia–water absorption refrigeration systems

In ammonia–water absorption refrigeration systems a purification process of the vapours produced in the generator is required. One type of equipment to carry out the purification process is a packed column. However, detailed experimental studies at the normal operating conditions found in ammonia–water absorption refrigeration systems have not been found. An experimental facility has been designed and built to study the ammonia–water rectification in packed columns. Experimental tests have been performed at the normal operating conditions found in the high-pressure stage of a small power ammonia–water absorption refrigeration system. In this paper, the experimental set-up is described and experimental results of the height equivalent to a theoretical plate (HETP) and the volumetric mass transfer coefficient of a rectifying section with the Sulzer BX packing are presented. The HETP values and the experimental mass transfer coefficients are compared with different data and correlations proposed in the literature; it has been found that the differences are appreciable.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

Mass transfer from a particle in a shear flow with surface reactions

Summary The steady mass transfer from a spherical particle (drop, bubble or solid particle) in a shear flow with chemical surface reactions is considered. For the complete range of Peclet numbers the numerical solution for the problem is obtained. The approximate formulae for the average rate of mass transfer are suggested. The comparison between the numerical results for the rate of transfer and those which are given by the approximate formulae is discussed.     

Coupled heat and mass transfer during nonisothermal absorption by falling droplet with internal circulation

We considered mass and heat transfer during nonisothermal absorption of a gas by a falling droplet with internal circulation. Gas phase is assumed to be free of inert admixtures and mass transfer is liquid phase controlled. Mass flux is directed from a gaseous phase to a droplet, and the interfacial shear stress causes a fluid flow inside the droplet. Droplet deformation under the influence of interface shear stress is neglected. Absorbate accumulation and temperature increase in the bulk of liquid phase are taken into account. The problem is solved in the approximations of a thin concentration and temperature boundary layers in the liquid phase. The thermodynamic parameters of the system are assumed constant. The system of transient partial parabolic differential equations of convective diffusion and energy balance with time-dependent boundary conditions is solved by combining the similarity transformation method with Duhamel's theorem, and the solution is obtained in a form of Volterra integral equation of the second kind which is solved numerically. Theoretical results are compared with available experimental data for water vapor absorption by falling droplets of aqueous solution of LiBr.     

Mathematical model of heat and mass transfer processes in evaporative condensers

This study presents a new mathematical model of heat and mass transfer processes in evaporative condensers. The model consists of four ordinary differential equations with their boundary conditions and some associated algebraic equations. The model was formulated for steady-state heat and mass transfer conditions. A simulation computer program based on the model was written. It was devised for heat calculations in condensers built from bare tubes. The quality of the model was calculated by comparing the results obtained by running the program with experimental results achieved by other authors. The computed results show a good degree of conformity with experimental results. The differences are less than 20% (but in one case, 30%). The computer program may be used to determine heat performance of evaporative condensers of horizontal in-line and staggered bundle systems (if S_q > 2d_z).     

Experimental investigations on boiling of n-pentane across a horizontal tube bundle: two-phase flow and heat transfer characteristicsEtude expérimentale de l'ébullition de n-pentane à travers un faisceau de tubes horizontal: écoulement diphasique et caractéristiques de transfert de chaleur

This paper presents the heat transfer characteristics obtained from an experimental investigation on flow boiling of n-pentane across a horizontal tube bundle. The tubes are plain with an outside diameter of 19.05 mm and the bundle arrangement is inverse staggered with a pitch to diameter ratio of 1.33. The test conditions consist of reduced pressure between 0.006 and 0.015, mass velocity from 14 to 44 kg/m²s, heat flux up to 60 kW/m² and vapor quality up to 60%. The convective evaporation is found to have a significant effect on the heat transfer coefficient, coexisting with nucleate boiling. An asymptotic model allows the prediction of the heat transfer data with a fitted value of n=1.5. A strong mass velocity effect is observed for the enhancement factor, implying that the correlations available from the literature for the convective evaporation will fail in predicting the present data. This effect decreases as the mass velocity increases.     

An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube

Experimental results of local heat transfer coefficients for the boiling of working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in a smooth tube with a small diameter (5.4 mm) are presented. The experiments have been performed in the following ranges: for the inlet pressure from 65.7 kPa to 82.2 kPa, for the heat flux from 2500 to 3300 W m⁻², and for the mass velocity of the working fluid from 11.90 to 15.99 kg m⁻² s⁻¹). The quantitative estimation in reduction of the heat transfer coefficient of the wetted surface in the evaporator at a high oil concentration in the mixture is examined. The influence of heat flux and mass velocities on the values of the local heat transfer coefficients is analyzed. The equation for the modelling of the local heat transfer coefficient for boiling of an isobutane/compressor oil solution flow in the tube is suggested.