21 research outputs found

    Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow

    Get PDF
    The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBr-H2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data. During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.Peer ReviewedPostprint (author's final draft

    Control strategy approach based on the operational results of a small capacity direct air -cooled LiBr -Water absorption chiller

    Get PDF
    The scope of this paper is to give a short overview of the state of the art regarding control strategies, identify the role of different operating conditions, and provide useful suggestions for the design and operation of a solar assisted absorption cooling system, in line to the European regulation as well as its directly related directives. The operation of a solar absorption cooling system under real conditions is subjected to various limitations regarding its ability to satisfy the required cooling demand, as well as to avoid certain internal conditions which would lead to problematic situations and jeopardize the smooth operation of the system - such as solution crystallization and water freezing. Thus, it is very important to define and refine new control operating strategies, from an internal and external perspective. Several control strategies are discussed, altogether with a new fuzzy logic approach, which shall be experimentally validated as future actions, due to its highly promising capability.Peer ReviewedPostprint (published version

    Numerical and experimental study of absorption of H2O vapor in wavy falling film of LiBr aqueous solution in vertical tubes and in presence of non-absorbables

    Get PDF
    One of the main reasons for the discrepancies between theoretical predictions of absorption phenomena made by mathematical models when they are compared against experimental results under real conditions, are the presence of non-absorbable gases. It is well known that these non-absorbable gases inside the shell of the absorption chiller are produced mainly for two reasons: (i) air leakages (Oxygen-Nitrogen); (ii) gases produced by corrosion (Hydrogen). In order to evaluate the influence of the presence of non-absorbable gases, an experimental set-up which reproduces vapor absorption in a vertical falling film has been designed and built with a mass spectrometer analyzer. Parallelly, a mathematical model of falling film absorption of H2O by LiBr aqueous solution that considers the influence of non-absorbable gases has been implemented. The model is semi-empirical, based on Navier–Stokes equations together with energy and mass species simplified under the boundary layer hypotheses. Several experimental tests have been performed to determine the influence of the air concentration in the absorption performance. Moreover, a comparison of numerical results against experimental data has been performed under different working conditions with reasonable agreement.Peer ReviewedPostprint (author's final draft

    Experimental evaluation of a pre-industrial air-cooled LiBr-H2O small capacity absorption machine

    Get PDF
    The paper studies thermal design and describes the experimental set-up of a domestic-scale prototype experimental cooling system based on a 7kW of nominal capacity single-stage small LiBr-H2O air-cooled absorption machine. The paper illustrates the characteristics based on a methodical procedure for the design and sizing of the small capacity air-cooled absorption machine.Peer ReviewedPreprin

    Simulaton of absorption of H2O in falling film of LiBr aqueous in vertical tubes in wavy regime

    No full text
    A mathematical model of falling film absorption of H2O by LiBr aqueous solutions which considers wavy regimes has been implemented. The model is semi-empirical, based on Navier Stokes equations together with energy and mass species simplified under the boundary layer hypotheses. The coupled equations are solved by means of finite difference method in a step by step procedure. The wavy profile is introduced by solving Free Surface Deflection Equation in each grid step, the smooth film thickness is recalculated in function of the amplitude value of the wave profile and the mass absorbed. In order to validate the model developed, a comparison is performed with a simple model based on empirical information for heat and mass transfer coefficients for laminar smooth a wavy regimes. Numerical results are compared between smooth thickness film and wavy profile.Peer ReviewedPostprint (published version

    Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow

    No full text
    The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBr-H2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data. During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.Peer Reviewe

    Modelling of absorption of H2O vapor in falling film of lbr aqueous solution in vertical tubes with presence of non-condensables

    No full text
    One of the main reasons of the discrepancies between theoretical predictions made by models of absorbers of H2O-LiBr absorption chillers when they are compared with experimental results under real conditions is the presence of non-condensables gases. These non-condensables gases are inside the shell of the absorption chiller mainly for two reasons: i) air leakages (Oxygen-Nitrogen); ii) gases produced by corrosion (Hydrogen). A mathematical model of falling film absorption of H2O by LiBr aqueous solutions which considers the influence of non-condensable gases has been implemented. The model is semi-empirical, based on Navier Stokes equations together with energy and mass species simplified under the boundary layer hypotheses. Under such conditions, the differential system of equations in partial derivatives, becomes parabolic and could be solved by means of finite difference method in a step by step procedure. Detailed heat and mass transfer balances are applied at the interface to specify the boundary conditions between liquid and gas phases. In order to calculate gradient of air at the interface, the penetration theory is applied in order to avoid a detailed calculation of the gas phase. Numerically the presence of air in the interface results in a pressure drop and consequently in a reduction in heat and mass transfer rates.Peer ReviewedPostprint (published version

    Control strategy approach based on the operational results of a small capacity direct air -cooled LiBr -Water absorption chiller

    No full text
    The scope of this paper is to give a short overview of the state of the art regarding control strategies, identify the role of different operating conditions, and provide useful suggestions for the design and operation of a solar assisted absorption cooling system, in line to the European regulation as well as its directly related directives. The operation of a solar absorption cooling system under real conditions is subjected to various limitations regarding its ability to satisfy the required cooling demand, as well as to avoid certain internal conditions which would lead to problematic situations and jeopardize the smooth operation of the system - such as solution crystallization and water freezing. Thus, it is very important to define and refine new control operating strategies, from an internal and external perspective. Several control strategies are discussed, altogether with a new fuzzy logic approach, which shall be experimentally validated as future actions, due to its highly promising capability.Peer Reviewe

    Experimental and numerical investigation of H2O vapor absorption processes in falling film of LiBr aqueous solution in vertical tubes

    No full text
    The LiBr-H2O absorption systems are used mainly in large cooling capacity applications (industry, large buildings, etc.), therefore require water from cooling towers to reject heat. However, if middle and low capacity are required (commercial and residential systems), absorption machines should be air-cooled in order to become competitive [1-4]. The absorber represents a major critical component in absorption systems and one of the key issues, in it is the combined heat and mass transfer in the absorption process. For this reason the development of mathematical models for the simulation and experimental data for the validation are always useful tools for the design and improvement of falling film vertical absorbers. A testing device has been designed and built for reproducing absorption phenomena in vertical tubes with the primary objective to obtain experimental data in LiBr-H2O vertical absorbers. The versatility of the experiment allows to obtain a wide range of data.Peer ReviewedPostprint (published version

    Simulaton of absorption of H2O in falling film of LiBr aqueous in vertical tubes in wavy regime

    No full text
    A mathematical model of falling film absorption of H2O by LiBr aqueous solutions which considers wavy regimes has been implemented. The model is semi-empirical, based on Navier Stokes equations together with energy and mass species simplified under the boundary layer hypotheses. The coupled equations are solved by means of finite difference method in a step by step procedure. The wavy profile is introduced by solving Free Surface Deflection Equation in each grid step, the smooth film thickness is recalculated in function of the amplitude value of the wave profile and the mass absorbed. In order to validate the model developed, a comparison is performed with a simple model based on empirical information for heat and mass transfer coefficients for laminar smooth a wavy regimes. Numerical results are compared between smooth thickness film and wavy profile.Peer Reviewe
    corecore