A simple model to predict the performance of a H2O–LiBr absorber operating with a microporous membrane

Abstract

This is a postprint version of the following published document:Venegas, M.; Vega, M.; García-Hernando, N.; Ruiz-Rivas, U. (2016) A microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller with the aim of reducing the size of this cooling technology. The simulation of the heat and mass transfer between the solution and the vapour phase in a H 2 O LiBr absorber using porous fibres is considered. Heat and mass transfer processes are modelled by means of selected correlations and data gathered from the open literature. This new model is applied for the simulation of the absorber under typical operating conditions of absorption cooling chillers. Absorption rate, heat and mass transfer co efficients, solution concentration, temperatures of the working fluids and pressure potential along the absorption channels are calculated. For the case considered in this study, the absorber channels are of 5 cm length, offering a maximum ratio between cooling capacity of the chiller and absorber volume of 1090 kW/m 3 . This ratio is higher than twice the usual values found in falling film absorbers using conventional circular tubes. The mean absolute error between the model results and the experimental data gathered from the open literature is 8.5%, showing the capability of the model to predict the per formance of membrane based absorbers.

Modeling and performance analysis of an absorption chiller with a microchannel membrane-based absorber using LiBr-H2 O, LiCl-H2 O, and LiNO3 -NH3

“…In the following paragraphs, an overview of the modeling approach is provided. For a detailed discussion of the complete procedure and the correlations employed, the reader is referred to our previous publication (Venegas et al) because the same modeling approach is used in the present work.…”

“…The heat transfer coefficients ( h ) appearing in the overall coefficients U s_cw and U s_v in Equations and are estimated using heat transfer correlations taken from the literature. In the present study, we use the heat transfer correlations of Lee and Garimella for the thermal entrance region and Shah and London for fully developed flow, as described in Venegas et al These correlations take the form shown in Equations and , respectively, where the Nusselt number for the fully developed flow is a constant, for the aspect ratio of the channels fixed with the dimensions specified in Section . …”

“…The overall mass transfer resistance between the vapor and bulk solution ( R ov ) includes the resistance to diffusion through the solution boundary layer ( R b ) and the resistance to diffusion of vapor through the membrane active layer ( R m ). The same approach presented by Ali and applied in Venegas et al is followed here to obtain the overall mass transfer resistance: …”

“…Isfahani and Moghaddam tested an absorber using the LiBr‐H 2 O solution and a super hydrophobic nanofibrous membrane obtaining an absorption rate of approximately 0.006 kg/m 2 s, using channels of 100 μm thick and a flow velocity of 5 mm/s. Venegas et al developed a simple model to predict the performance of a LiBr‐H 2 O microchannel absorber operating with a microporous membrane. Recently, Asfand et al present a simulation of a membrane‐based absorber with a quaternary salt system (LiBr + LiI + LiNO 3 + LiCl) and a ternary mixture of alkali nitrates (LiNO 3 + KNO 3 + NaNO 3 ) with water as refrigerant using the ANSYS/FLUENT code.…”

“…Our approach differs from the work of Asfand et al in that the mixtures evaluated and the dimensions of the microchannels used are different. Further, in the present work, a simplified model (described in Venegas et al) is used, which is less time consuming than the numerical codes and needs less hypotheses to simulate the system. The considered absorbent‐refrigerant pairs are LiCl‐H 2 O and LiNO 3 ‐NH 3 , which have already been used in conventional absorbers, but not in membrane‐based absorbers.…”