A numerical model for combined heat and mass transfer in a laminar liquid falling film with simplified hydrodynamics

Mittermaier, M.; Schulze, Peter; Ziegler, Felix

doi:10.1016/j.ijheatmasstransfer.2013.11.075

Cited by 51 publications

(21 citation statements)

References 12 publications

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“…According to Figure A, the diffusion coefficient of water in the LiBr‐H 2 O reported by Kim et al agrees well (differences are below 20%) with the Mittermaier et al correlation, which is the expression used in the present simulation.…”

Section: Microchannel Absorber Modelsupporting

confidence: 88%

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

2018

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Summary In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5‐cm length and 9.5 cm3 in volume provided with a porous membrane is presented for 3 different solution‐refrigerant pairs: LiBr‐H2O, LiCl‐H2O, and LiNO3‐NH3. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625 kW/m3 for the LiNO3‐NH3, 552 kW/m3 for the LiBr‐H2O, and 318 kW/m3 for the LiCl‐H2O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO3‐NH3 and the LiBr‐H2O solutions provide the largest cooling effects. The LiNO3‐NH3 can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr‐H2O system. The lowest cooling effect and coefficient of performance are found for the LiCl‐H2O solution, but this mixture allows the use of lower temperature heating sources (below 70°C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.

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Section: Microchannel Absorber Modelsupporting

confidence: 88%

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

2018

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“…The use of Sherwood and Schmidt dimensionless numbers to obtain the solution mass transfer coefficient requires knowledge of the diffusion coefficient. In this study, it was corrected by temperature using the equation described by Mittermaier et al [26] …”

Section: Mass Transfermentioning

confidence: 99%

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

Venegas

Vega

García-Hernando

et al. 2016

Energy

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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.

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“…Based on data measured by Löwer (1960), property functions have been derived by polynomial regression for specific heat capacity, viscosity, thermal conductivity, density, saturation temperature and mass concentration. The equations are used in order to determine these properties at the local conditions and provided in Mittermaier et al (2014). Please note that other than stated in there, the temperature T has to be inserted in Kelvin.…”

Section: Property Datamentioning

confidence: 99%

“…A detailed analysis regarding the influence of variable properties is provided in Mittermaier et al (2014). When the thermophysical properties are allowed to vary with mass fraction and temperature throughout the process, the mass transfer rate during desorption improves slightly while it decreases somewhat during absorption.…”

Section: Variable Thermophysical Propertiesmentioning

confidence: 99%

Theoretical evaluation of absorption and desorption processes under typical conditions for chillers and heat transformers

Mittermaier

Ziegler

2015

International Journal of Refrigeration

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Highlights • Comparison of absorption and desorption for absorption chillers & heat transformers• Absorption chiller: thin film leads to higher transfer rate during desorption • Heat transformer: favourable properties lead to higher transfer rate for absorption • Various assumptions have been evaluated to substantiate the effects Abstract A comparison of absorption and desorption is conducted using a detailed model describing heat and mass transfer. First, the influences of various assumptions have been evaluated. Second, typical conditions for both absorption chillers and heat transformers have been defined. The performance of absorption and desorption processes have been analysed for a flow length of 0.1m. In an absorption chiller, during desorption, the viscosity is lowered and the mass diffusivity is increased. These circumstances cause a 46% higher transfer rate as compared to absorption. Thus, the overall performance of the process is determined by the absorber component. In a heat transformer, during absorption at an elevated pressure and temperature level, the viscosity is lower and mass diffusivity is higher as compared to desorption. Therefore, the transfer rate of during absorption is 10% higher as compared to desorption. Hence, the desorber performance is somewhat more influential to the overall system performance. Nomenclature c pHeat capacityLongitudinal velocity (m s −1 )x Longitudinal coordinate, streamwise (m) y Transversal coordinate (m) Greek letters   Mass flow per unit length (kg m −1 s −1 ) sor h  Heat of sorption (J kg −1 ) δ Film thickness (m) λ Thermal conductivity (W m −1 K −1 ) μ Dynamic viscosity (Pa s) ξ Mass fraction (−) Page 2 of 25 ρ Density (kg m −3 ) Subscripts 0 Inlet condition abs Absorption avg Average A Absorbate e.g. water in liquid state des Desorption eq Equilibrium if Interface S Solution of absorbent e.g. LiBr − H 2 O V Vapour e.g. steam W Wall

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A numerical model for combined heat and mass transfer in a laminar liquid falling film with simplified hydrodynamics

Cited by 51 publications

References 12 publications

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

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

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

Theoretical evaluation of absorption and desorption processes under typical conditions for chillers and heat transformers

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A numerical model for combined heat and mass transfer in a laminar liquid falling film with simplified hydrodynamics

Cited by 51 publications

References 12 publications

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

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

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

Theoretical evaluation of absorption and desorption processes under typical conditions for chillers and heat transformers

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

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