The Correlation of Coupled Heat and Mass Transfer Experimental Data for Vertical Falling Film Absorption

Abstract

Abstract Absorption chillers are gaining global acceptance as quality comfort cooling systems. These machines are the central chilling plants and the supply for comfort cooling for many large commercial buildings. Virtually all absorption chillers use lithium bromide (LiBr) and water as the absorption fluids. Water is the refrigerant. Research has shown LiBr to be one of the best absorption working fluids because it has a high affinity for water, releases water vapor at relatively low temperatures, and has a boiling point much higher than that of water. The heart of the chiller is the absorber, where a process of simultaneous heat and mass transfer occurs as the refrigerant water vapor is absorbed into a falling film of aqueous LiBr. The more water vapor absorbed into the falling film, the larger the chiller’s capacity for supporting comfort cooling. Improving the performance of the absorber leads directly to efficiency gains for the chiller. The design of an absorber is very empirical and requires experimental data. Yet design data and correlations are sparse in the open literature. The experimental data available to date have been derived at LiBr concentrations ranging from 0.30 to 0.60 mass fraction. No literature data are readily available for the design operating conditions of 0.62 and 0.64 mass fraction of LiBr and absorber pressures of 0.7 and 1.0 kPa. Experiments were conducted on an internally cooled smooth tube 0.01905 m in outside diameter and 1.53 m in length. Tests were conducted with no heat and mass transfer additive. The data, for testing at 0.62 and 0.64 mass fraction of LiBr, were scaled and correlated into both Nusselt (Nu) and Sherwood (Sh) formulations. The average absolute error in the Nusselt correlation is about ±3.5% of the Nu number reduced from the experimental data. The Sherwood correlation is about ±5% of the reduced Sh data. Data from the open literature were reduced to the authors’ Nu and Sh formulations and were within 5% of the correlations developed in the present study. Hence, this study provides correlations for the complex heat and mass transfer process that is validated against extensive experimental data. The study therefore contains useful information for the design of a vertical column absorber operating with no heat and mass transfer additive.