This paper provides a literature review on absorption refrigeration technology. A number of research options such as various types of absorption refrigeration systems, research on working fluids, and improvement of absorption processes are discussed.

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A review of absorption refrigeration technologies

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.

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

Recently, many urban areas of the world have experienced rapid growth of population and industrial activity raising concerns of environmental deterioration. To meet challenges associated with such rapid urbanization, it has become necessary to implement wise strategies for environmental management and planning, addressing the exclusive demands of urban zones for maintaining environmental sustainability and functioning with minimum disruption. These strategies and related public policy must be based on state-of-the-science tools for environmental forecasting, in particular, on mathematical models that accurately incorporate physical, biological, chemical and geological processes at work on urban scales. Central to such models are the mechanics of environmental fluids (air and water) and their transport and transformation characteristics. Although much progress has been made on understanding environmental flow phenomena, a myriad of issues akin to urban flow, the transport phenomena, air and water quality and health issues (epidemiology) remain to be understood and quantified. We propose to initiate a new focus area – Urban Fluid Mechanics (UFM) – tailored to research on such issues. For optimal societal impact, UFM must delve into fundamental and applied fluid flow problems of immediate utility for the development of urban public policy and environmental regulations. Such efforts often entail the use of `whole' systems approach to environmental studies, requiring careful synthesis between crosscutting areas. In this paper, a few topics in the realm of UFM are presented, the theme being the flow and air quality in urban areas. Topics such as the scales of flow, the atmospheric boundary layer, pollutants and their transport and modeling of flow and air quality are briefly reviewed, discussed and exemplified using case studies. Identification of important flow-related issues, rigorous multidisciplinary approaches to address them and articulation of results in the context of socio-political cause calebre will be the challenges faced by UFM.

Urban fluid mechanics : Air circulation and contaminant dispersion in cities

Heat transfer in rectangular microchannels

The objectives of this paper are to examine the effect of nano particles on the bubble type absorption by experiment and to find the optimal conditions to design highly effective compact absorber for NH3/H2O absorption system. The initial concentrations of NH3/H2O solution and the kinds and the concentrations of nano particles are considered as key parameters. The results show that the addition of nano particles enhances the absorption performance up to 3.21 times. Moreover, the absorption rate increases with increasing concentration of nano particles and the nano particles are more effective for lower absorption potential solution. The potential enhancement mechanism for binary nanofluid is suggested. The experimental correlations of the effective absorption ratio for each nano particles, Cu, CuO, and Al2O3, are suggested within ±10% error-band.

The effect of nano-particles on the bubble absorption performance in a binary nanofluid

In this study, experiments have been performed for water vapour absorption into 50 and 60 mass% aqueous lithium bromide solution films flowing down a vertical surface to investigate the effects of liquid diffusivity values, molecular properties of the concentrated solutions and non-absorbable gases. The experimental results for wavy films over a film Reynolds number range of 15–90 indicate larger dimensionless mass transfer rates than for strictly laminar flow when the diffusivity of water in a concentrated lithium bromide solution is less than that in a dilute solution. The complete set of results shows that the physical property data for lithium bromide solutions including the diffusivities measured by Kashiwagi are sufficient to explain mass transfer behavior.

Absorption of water vapour into falling films of aqueous lithium bromide

The results of vertical falling film experiments on the absorption of water vapor to aqueous lithium bromide solutions with an additive, 2-ethyl-1-hexanol, are reported. During the absorption, the film becomes highly turbulent. Consequently, the heat and mass transfer is significantly enhanced by turbulent mixing. In addition, the instability mechanisms are detailed. In the vicinity of water absorption, surface-tension gradients due to the lower LiBr concentration, the lower additive concentration, and the higher temperature at the interface, can favor instability of the falling film.

The interfacial turbulence in falling film absorption: effects of additives

The surface tension of an aqueous lithium bromide solution containing an active surfactant (2-ethyl-1-hexanol) was measured over the lithium bromide concentration range 40 [<=] C[sub LiBr] [<=] 60 wt % and surfactant concentration range 0 [<=] C[sub sur] [<=] 200 ppm. The Du Nouy ring method was employed to determine the surface tension.

Surface tension of aqueous lithium bromide + 2-ethyl-1-hexanol

Synoptic classification is used to identify meteorological conditions characteristic of high-pollution periods at Nogales, Arizona. Low surface winds determined by local surface cooling at night with little vertical mixing were found to be most important. This condition was simulated in a 0.79-m-square box filled with water with the lower surface made to model a 12-km-square region of the surface topography of the United States-Mexico border at Nogales. The aluminum base was cooled to induce the downslope flows. Photographs of dye initially placed on the surface at many locations were used to obtain a set of surface velocities that formed the input to the Diagnostic Wind Model (DWM). The DWM provided hourly velocity data with grids of 500- and 250-m spacings. The similarity arguments used to analyze the relationship of the physical model to the atmosphere are discussed. Although the magnitude of the wind vectors in the physical model cannot be matched to the atmosphere, the directions can be used...

Synoptic Classification and Physical Model Experiments to Guide Field Studies in Complex Terrain

Axisymmetric submerged jets of dilute solutions of poly(ethylene oxide) and polyacrylamide of high molecular weight were studied using a two-component laser Doppler velocimeter. At from 40–60 jet diameters from the source the small eddies are suppressed in the solutions studied. The behavior of the large eddies depends upon the elastic nature of the solution.

Neil S. Berman

Papers

Absorption of water vapour into falling films of aqueous lithium bromide

The interfacial turbulence in falling film absorption: effects of additives

Surface tension of aqueous lithium bromide + 2-ethyl-1-hexanol

Synoptic Classification and Physical Model Experiments to Guide Field Studies in Complex Terrain

Two‐component laser doppler velocimeter studies of submerged jets of dilute polymer solutions