Heat and mass transfer characteristics of a constrained thin-film ammonia-water bubble absorber

A study of absorption of ammonia vapour bubbles into a constrained thin-film of ammonia-water solution is presented. A large-aspect-ratio microchannel constrains the thickness of the weak solution film and ammonia vapour bubbles are injected from a porous wall. A counter flowing coolant in a minichannel removes the generated heat of absorption. Experiments and a simple one-dimensional numerical model are used to characterize the absorber performance at a nominal system pressure of 6.2 bar absolute. Effect of varying the mass flow rate of the weak solution, vapour flow rate, solution inlet temperature, and coolant inlet temperature on absorption heat and mass transfer rates and exit subcooling are discussed. Two absorber channel geometries, each of 600 μm nominal depth, are considered: 1) a smooth-wall channel, and 2) a stepped-wall channel that has 2-mm deep trenches across the width of a channel wall. Results indicate that the reduction in coolant inlet temperature significantly enhances the mass transfer rates in both absorber geometries. While the stepped-wall geometry exhibits higher mass transfer rates at lower coolant inlet temperatures of 30 °C and 40 °C, the smooth-wall channel shows higher mass transfer rates at the highest coolant inlet temperature of 58 °C. Both absorption limited and residence time limited conditions are observed with variation of weak solution flow rate at fixed vapour flow rates.     

Experimental report on the reliability of ammonia-water absorption chillers

The analysis of a large amount of data on the failures of absorption refrigeration machines of the ammonia-water type was carried out. The data derive from the records of a 10-year maintenance contract under which every machine was installed. The records were computerized for the period 1989–1992, and they refer to sales starting from 1980 (i.e. to 73 778 machines). The failures are classified, identifying the most serious (generator and solution pump). The rate of early failures is rather high, but the failures are generally minor and easily repaired. The production quality was not constant, with a very bad year in 1984. The failure rate is evaluated for the first 10 years of life of the equipment. In general terms, one machine in two does not require any intervention during that period.     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

An ammonia-water absorption refrigerator with a large temperature lift for combined heating and cooling

The COP (Coefficient of Performance) of an ammonia-water absorption refrigerator can be significantly improved by incorporating a mixing column and a second absorber which is combined with a two-stage refrigerant expansion. At an internal temperature lift of 85 K (−20°C to + 65°C), these modifications result in a 50% higher COP for cooling—assuming ideal components. With real components, a 40% improvement may be achieved. The proposed cycle was realised in a laboratory test plant with a refrigeration capacity of 10 kW at −20°C evaporator temperature. This machine was operated at a solution temperature up to 225°C and a condenser temperature of 65°C corresponding to a pressure up to 3.0 MPa. COP versus temperature lift and load behaviour was tested. A COP of 0.38 was achieved at a temperature lift of 85 K. An efficient high-temperature lift cycle like that described in this paper may find applications for deep freezing, as a topping cycle to achieve triple-effect performance, or as a device to produce simultaneous heating and cooling.     

Combined momentum, heat and mass transfer in vertical slug flow absorbers

Numerical solutions have been obtained for the system of equations of momentum, heat and mass transfer describing the absorption of a refrigerant vapour from a Taylor bubble into the refrigerant-absorbent solution film around the bubble. The numerical results are compared with Nusselt's solution of the energy equation and with the penetration theory solution of the mass diffusion variation. Experimental data have been collected in vertical tubular absorbers in the slug flow region with the systems ammonia-lithium nitrate and ammonia-sodium thiocyanate. Four different absorber tubes have been tested with internal diameters of 10, 15, 20, and 25 mm. These data are compared with the numerical and theoretical results. The effect of the bubble nose on mass transfer is studied. Typical temperature profiles during the absorption process in absorption cooling/heating systems are shown.     

Thermodynamic analysis of R 21-DMF vapour absorption refrigeration systems for solar energy applications

The concentration-enthalpy chart for R 21-DMF refrigerant-absorbent combination is presented for the temperature range of interest for solar-driven cooling applications. A thermodynamic analysis of the vapour absorption refrigeration system (VARS) working with R 21-DMF is made to study the theoretical performance characteristics. At evaporator temperatures lower than about 0°C, R 21-DMF has a distinct advantage over R 22-DMF systems in being able to operate at lower generator temperatures. Also R 21-DMF systems perform better than R 22-DMETEG systems.     

Comparative simulation and investigation of ammonia-water: absorption cycles for heat pump applications

Several recent programs in absorption research have focused on technology for domestic heating and cooling utilizing natural gas. In residential and small commercial size applications, ammonia-water cycles offer the possibility of a gas-fired heat pump for both winter heating and summer cooling, at better year-round COPS than currently available by various alternatives. Several cycles have been considered for this purpose, ranging from the simplest single effect to the GAX (Generator-Absorber heat eXchange) with its different variations. Detailed calculations of ammonia-water systems are rather difficult, particularly in complex cycles such as the GAX. This may be the reason that relatively few simulation studies have been published to date that give more than design point performance. The objective of the present study has been a comprehensive investigation of various ammonia-water cycles, with operating conditions and different design parameters varied over a wide range to compare their performance. To this end, a modular simulation program (ABSIM) was employed, which makes it possible to simulate absorption cycles in varying configurations. The cycles investigated increase in complexity from the basic single-effect cycle, through the same cycle with an added precooler, through an added solution-cooled absorber and solution-heated generator, to the GAX and branched GAX cycles with different types of branching. Each cycle was formed on the basis of the previous one by adding one or two components at each stage, resulting in increasing complexity rewarded by improved performance. This kind of investigation enabled determination at each stage of the influence of the added components on the cycle. The results of the investigation show cooling CON ranging from about 0.5 for the simplest single-effect cycle to about 1.0 for the GAX cycle.     

Sputtering deposition and characterization of tandem absorbers for photo-thermal systems operating at mid temperature

In this paper the first results of a work concerned with the development of TiAlON selective solar absorbers with high solar absorptance and low thermal emittance are presented. These absorbers are thought for solar collectors application at operational temperature of about 300 °C. Solar absorber tandems, consisting of an absorbing TiAlON compound sputtered on a TiC buffer layer deposited on a metal (Cu) substrate, have been produced in two ways: by changing the nitrogen flux percentage in (Ar + N₂ + O₂) mixture at fixed thickness (300 nm) and by varying the absorber thickness at fixed nitrogen percentage (5%) in the sputtering gas. The optical response in the UV-IR range is discussed. The tuning of the nitrogen flux allows to change the metal elements ratio (Ti/Al) in the oxynitride layer, varying the reflectance intensity both in the IR and visible range. The thickness variation permits to shift the cut-off wavelength of the transition from short wavelength high absorptance to long wavelength low emittance. The tuning of these two parameters (nitrogen flux and absorber thickness) enables to control the optical response of the samples to make them suitable for photo-thermal device application.     

CO_2载冷剂氨双级压缩制冷系统及其性能分析

针对氨制冷系统在实际应用中存在安全隐患的问题,总结提高系统安全性的方法和途径,介绍以CO2为载冷剂的半封闭式氨制冷系统的组成及其应用。对以CO2为载冷剂的氨双级压缩制冷系统性能进行理论计算与对比分析,结果表明：在蒸发温度为-50~-30℃范围内,与NH3/CO2复叠式制冷系统相比,CO2载冷剂氨双级压缩制冷系统的EER约高5.7%~10.8%。     

溴化锂吸收式制冷系统溶液热交换器的传热性能研究

以板式溶液热交换器为例，建立物理数学模型。通过对模型的求解，得到溶液热交换器在不同流速下的传热特性。计算结果表明，提高溶液热交换器中的溶液流速可以提高其传热系数，但会导致溶液的进出口温差减小，使溶液间换热不充分。设计溶液热交换器时可根据设计要求与文中计算结果选择不同的溶液流速。     

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.     

Application of air cooled condensers in European supermarket refrigeration systems

The application of air cooled condensers in European supermarket refrigeration systems is discussed. Compressors are usually hermetic or open types with capacities in the 5 kW range. Advantages over water-cooled systems are lower initial and operating costs and easier maintenance. There are, however, a number of problems, primarily sensitivity to ambient air temperatures which causes high condensing temperatures in hot summer weather and consequently a substantial increase in energy demand. Problems can also arise when the air temperature falls below 0°C; the condensing pressure must be maintained by fan speed control, air flow control or reduction of active heat transfer surface by flooding.     

垂直管吸收器内泡式吸收过程的数值研究

建立垂直管吸收器管内泡式吸收过程中传热传质的数学物理模型，对其泡式吸收过程进行数值研究，获得泡式吸收方式的一些传热传质特性，为吸收器的优化设计提供一定理论指导。     

Advances and future aspects of heat transfer relating to refrigeration and heat pumps

The 16th International Congress of Refrigeration and the 7th International Heat Transfer Conference demonstrated that the exponential growth of the study of heat transfer will continue for at least a few more years. The papers to be quoted are mainly from these conferences. Even restricting the field to only refrigeration and heat pumps will still make it necessary to concentrate on a few special topics.     

Generalized ideal reference cycle for regenerative refrigeration: Part 2. Adiabatic systems

A generalized ideal reference cycle for regenerative refrigeration systems is discussed. The cycle has adiabatic processes of compression and expansion. Cyclic heat addition to and rejection from the cycle occur at constant pressure or constant volume immediately preceding the regenerative thermal processes. These are assumed to occur partly at constant volume, partly at constant pressure. Equations are developed for cycle work, heat lifted, coefficient of performance and the pressure ratio: mean effective/maximum cycle pressures. Various special cases are deduced from the generalized analysis including the pseudo-Stirling and pseudo-Ericsson cycles.Results are presented for the Stirling and pseudo-Stirling cycles to illustrate the effect of volume compression ratio and regenerative effectiveness on the cycle Coefficient of Performance. The pressure ratio p_mean/p_max for the same two cycles are compared to a function of the volume compression ratio.     

Frozen storage, refrigeration equipment and freezing systems in the seafood industry

The status of frozen storage, refrigeration equipment and freezing systems in the seafood industry is considered in relation to the importance of seafood in the diet and the need to increase supply and provide transport and storage. Preservation methods include chilling, chilling in a controlled atmosphere, and freezing. The best methods of cold storage, followed finally by thawing are discussed.     

太阳能吸收式制冷-海水淡化联产系统在别墅的应用

设计太阳能吸收式制冷-海水淡化联产系统,以大连、深圳、上海3个沿海城市为例,选取一栋别墅建筑作为研究对象,利用Matlab对系统容量匹配和经济性进行比较。结果表明:对于同一栋别墅,太阳能集热器面积由建筑物所在地区的建筑冷负荷确定,大连最小,深圳其次,上海最大;淡水产量与建筑冷负荷呈正比关系;联产系统在上海初投资最高,在深圳运行维护费用最高,在大连投资回收期最长;相比太阳能吸收式制冷系统,太阳能吸收式制冷-海水淡化联产系统的投资回收期可缩短9.4%。     

Attempt of integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator: Experience and results

The integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator as heat source was analyzed theoretically and experimentally. Operation parameters of the fridge were determined by parametric fitting experimental data using a thermodynamic model of the cycle leading to a working pressure of 25 bar, weak and strong solution concentrations of 15% and 30% respectively and a COP of ∼0.18. As sold, the fridge can operate with a reduction in the electric power supply of up to 40%. Results showed that the Parabolic Concentrator (PC) used can deliver temperatures above 200 °C for almost 6 h with an average useful power delivery and efficiency of 530 W and 26% respectively. During laboratory test runs it was possible to hold the refrigeration cycle running in a very unstable regime despite the inappropriate heat distribution in the generator unit due to the lower temperature heat source used (220 °C). However, outdoor testing with the sun as energy source proved unsuccessful due to the higher heat losses that occur at the pipes connecting the PC with the fridge.     

Heat and mass transfer effects on flow past an impulsively started vertical plate

Summary An exact solution to the problem of lfow past an impulsively started infinite vertical plate in the presence of uniform heat and mass flux at the plate is presented by the Laplace-transform technique. The velocity, the temperature and the concentration profiles are shown graphically. The rate of heat transfer, the skin-friction, and the Sherwood number are also shown on graphs. The effect of different parameters like Grashof number, mass Grashof number, Prandtl number, and Schmidt number are discussed.List of symbols C species concentration near the plate - C species concentration in the fluid far away from the plate - C dimensionless concentration - C _p specific heat at constant pressure - D mass diffusion coefficient - g acceleration due to gravity - Gr thermal Grashof number - Gc mass Grashof number - j mass flux per unit area at the plate - K thermal conductivity of the fluid - Nu Nusselt number - Pr Prandtl number - q heat flux per unit area at the plate - Sc Schmidt number - t time - t dimensionless time - T temperature of the fluid near the plate - T temperature of the fluid far away from the plate - T _w temperature of the plate - u velocity of the fluid in thex-direction - u ₀ velocity of the plate - u dimensionless velocity - x coordinate axis along the plate - y coordinate axis normal to the plate - y dimensionless coordinate axis normal to the plate - volumetric coefficient of thermal expansion - ^* volumetric coefficient of expansion with concentration - coefficient of viscosity - kinematic viscosity - density - skin-friction - dimensionless skin-friction - dimensionless temperature - er fc complementary error function - similarity parameter     

太阳能沼气驱动吸收式制冷空调的仿真研究

因全球能源危机和环境污染日益严峻,加上建筑空调需求量的增长和电力成本增加,考虑到吸收式制冷具有无比的优越性,提出了将可再生能源—太阳能和沼气联合驱动吸收式制冷空调的思想,建立了此系统主要性能参数的数学模型,按照三种不同的运行方案,运用matlab/simulink软件分别建立其仿真模块并对主要性能参数进行仿真计算,仿真结果够很好的反映太阳能沼气吸收式制冷空调运行的特点。为可再生能源应用于建筑空调的研究与应用、缓解化石能源供给的危机和保护环境提出了有意义的参考。