Falling Film Absorption Process Research Articles

Modeling and experimental verification of the enhancement of TiO2 nanofluid on ammonia falling film absorption process

The energy efficiency of absorption refrigeration system is restricted by the absorption rate of absorber. Nanofluid is widespread applied to enhance the absorber performance, because the boundary thickness is reduced by the random Brownie motion of the contained nanoparticles. To describe the enhancement of nanoparticles simply and accurately, a mathematical model is built based on the ammonia solution falling film absorber. The momentum exchange and properties modifications caused by nanoparticles are considered in the governing equation. Meanwhile, an experimental facility of ammonia absorption refrigeration system is established, and the nanofluid is served as a circulating solution. The nanofluid is comprised of TiO2 nanoparticles and ammonia solution. Compared with the measurement of experiment, the ammonia absorbed rate of absorber predicted by the mathematical model has a maximum relative error of 8.9%. Furthermore, the absorption effects of the falling film absorber in different inlet conditions are analyzed. The result indicates the enhancement of nanoparticles is more significant with lower mass flow rate or poorer ammonia mass fraction.

Theoretical analysis of the enhancement effect of ultrasonic oscillation on NH3H2O falling film absorption process

Absorption refrigeration can utilize low grade heat, but the application is limited by the huge system size and low energy efficiency. The mass transfer area of absorber occupies more than 50% of the whole system. The improvement absorption effect of the absorber is significant to reduce the system size and improve the energy efficiency ratio. A method to enhance the mass transfer in an ammonia-water falling absorber by implementing ultrasonic oscillators is proposed in this paper. A mathematical model of the absorber with the installing of oscillator is established. The solution of the model shows that the total mass transfer coefficient and the ammonia absorbed rate are increased by 22.53% and 20.25%, respectively, when the sound intensity at the surface of oscillator is 20 W/m2. The enhancement effect increases with oscillation output power, the ammonia absorbed rate increases by 31.2% when the sound intensity at the surface of oscillator is 50 W/m2.

Experimental study of falling film absorption with potassium formate - Water as working pair

As an essential operation in refrigeration and chemical engineering, gas or vapor absorption with falling liquid film is usually coupled with complex fluid flow patterns and heat & mass transfer processes. Experimental studies are necessary for practical application, especially when non-traditional working pairs are adopted. In this paper, KCOOH/Water was used as the working pair to conduct the experimental study of the falling film absorption process. The results showed that waviness increased the mass transfer rate. The waviness intensity (δ/δ‾) and the average mass transfer rate increased first and then decreased with flow rate at Reynolds number ranging from 15 to 95, up to 0.92 and 3.5 g m−2s−1. The heat transfer flux increased with the flow rate in the test range, and absorption rates were sensitive to the cooling water temperature.

Heat and mass transfer in laminar falling film absorption: A compact analytical model

Absorption process plays a significant role in numerous industrial applications. Several analytical solutions have been proposed for the absorption process to calculate heat and mass transfer rates in falling film absorbers. However, the existing analytical solutions are either complex or do not consider the initial and boundary conditions at the entrance region and/or are not accurate enough. In this study, a new analytical solution for non-volatile absorbents is presented using the Laplace transform method by proposing a new velocity profile for falling films. Moreover, for the first time, compact relationships are proposed to calculate the heat and mass transfer rates in the falling film absorption process for four common absorber geometries, including: (i) inclined plates; (ii) tube bundles absorbers; (iii) vertical tubes; and (iv) coil type absorbers. The proposed compact relationships are validated with experimental data collected by others with reasonable accuracy. Furthermore, a parametric study is conducted to investigate the impact of input parameters on the absorption rate of falling film on an inclined plate. It is observed that the concentration of inlet solution is the most effective parameter and the solution mass flow rate has the least impact.

Numerical analysis of the enhancement of ammonia-water falling film absorption process by atomized equipment

As a typical absorption device, falling film absorber has a significant effect on the performance of the ammonia water absorption refrigeration system. In this paper, a novel absorber with atomizer installed at the bottom of the absorber is proposed. The atomizer is introduced to increase the effective mass transfer area between ammonia vapor and the solution. A theoretical model in established and simulation investigation is carried out to obtain the absorption enhancement by the atomizer. The result indicates that the enhancement is 10.5% with the 1 mm initial droplet diameter in the design point of this paper, and it can reach 34.2% when the initial droplet diameter is reduced to 0.2 mm. Furthermore, the optimization of enhancement effect by other droplet parameters has been analyzed.

The influence of a vertical chevron corrugated plate on wetting and thermal performance of a detachable plate-type falling film absorber

The absorption system utilizing LiBr/H2O as working fluids plays an increasingly important role in district heating. However, the heat transfer of the existing systems is mainly through the shell-and-tube structure, resulting in the system’s vast land occupation. A detachable plate-type falling film absorber is proposed to address the problem. The type and geometry of the plate utilized in this system are designed in this paper. The vertical chevron corrugated plate is proposed to improve the wetting performance in the negative pressure channel and enhance the heat transfer performance of the water in the positive pressure channel. The thermal performance experiment of the system with this plate is carried out under the absorption-evaporation condition. Under the rated condition, the overall heat transfer coefficient of absorption process rises to 0.983 kW·m−2·K−1, and the mass flux of vapor absorbed rises to 1.9 × 10−3 kg·m−2·s−1. Moreover, the overall heat transfer coefficient of the plate falling film absorption process is split, and the empirical correlations of convective heat transfer on the water side and the solution side are fitted respectively. The thermal performance improvement further improves the feasibility of miniaturization of the absorption system.

Match property analysis of falling film absorption process

A one-dimensional three-fluid heat transfer model is presented here to analyze coupled heat and mass transfer process in falling film absorption process. In this model, mass transfer process is converted to its heat transfer equivalent process through the heat and mass transfer analogy. The analytic solutions can be gained by solving the equations in the model for the match property analysis. Thermal resistance of heat transfer process is defined based on the entransy dissipation theory. Conditions of matched flow and matched inlet parameter are deducted. Then thermal resistance is decompounded into thermal resistance caused by finite heat and mass transfer capability Ra, thermal resistance caused by unmatched flow Rf and thermal resistance caused by unmatched inlet-parameter Rp. The properties of Ra, Rf and Rp are quantitatively analyzed. Match property analysis method presented here provides a quantitatively optimization criteria of the absorber, and a new perspective to understand absorption process.

Heat and Mass Transfer Enhancement for Falling Film Absorption Process in Vertical Plate Absorber by Adding Copper Nanoparticles

This paper examined the effect of nanoparticles on the absorption of vapor into a liquid film of lithium bromide aqueous solution flowing down over a cooled vertical channel. For this, we realized a two-dimensional code which solves the governing equations using Comsol Multiphysics. In this model, the cooling water flows countercurrent to a solution of concentrated lithium bromide mixed with the copper nanoparticles. After the validation of the model, the effects of parameters such as Reynolds number, solid volume fraction of copper nanoparticles, inlet concentration and inlet temperature of solution on the performance of the absorption are presented and discussed. The important results indicate that the mass and heat transfer in binary nanofluids are enhanced more than that in base fluid and the efficiency of the nanofluid becomes higher to that of the base fluid for a lower Reynolds number and the inlet concentration and for a higher inlet temperature of solution.

Experimental study on enhancement of falling film absorption process by adding various nanoparticles

Absorption air conditioning system containing no chlorofluorocarbons is considered environmentally friendly and a competitive alternative to conventional vapor compression air conditioning system. One of the most commonly used refrigeration working pair is LiBr/H2O. This paper experimentally studies the falling film absorption of LiBr/H2O with various kinds of nanoparticles dispersed, such as Cu, Al2O3 and CNT. The results show that the variety, mass fraction and size of nanoparticles, as well as solution flow rate are considered as the key parameters. The smaller the nanoparticles size, the better the effective absorption ratio. The larger the nanoparticles mass fraction, the better the effective absorption ratio. The best enhancement is achieved by adding Cu nanoparticles for all the studied nanoparticles species. The effective absorption ratios for all the studied cases are higher than 1.0, which proves that the dispersed nanoparticles have positive effects on the absorption process.

Analytical and Numerical Solution of Conjugate Heat and Mass Transfer in Falling Film Absorption Process

Investigation of heat and mass transfer at nonisothermal absorption on the falling liquid film surrounded by pure vapour is presented. The analytical method for entrance region and numerical method for solving the problem are proposed. The analytical solution is obtained for the entry region, where temperature and concentration perturbations are near the interface. Then, the analytical solution is used as an inlet condition for further numerical solution by the finite-difference method. The key dimensionless criteria characterizing heat and mass transfer at absorption on the falling liquid film are defined.

Numerical study of falling film absorption process in a vertical tube absorber including Soret and Dufour effects

Vapor absorption in aqueous LiBr inside a vertical tube absorber is studied numerically. The governing equations of concentration and energy have been derived in general forms, including the species interdiffusion, Soret and Dufour effects due to combined concentration and temperature gradients during absorption process. All thermophysical properties of solution and the heat of absorption are variable, as functions of temperature and concentration in constant absorber pressure. An in-house CFD code is used to solve the governing equations. The obtained results have been validated with available experimental data and a good agreement is observed. The simulation results reveal the important role of the interdiffusion term in energy equation. Moreover, the Soret and Dufour effects result in a relatively small increase in average heat and mass fluxes at the interface.

Laplace transform solution of conjugate heat and mass transfer in falling film absorption process

In this study, the conjugate heat and mass transfer process taking place during the absorption of a refrigerant vapor into a falling liquid film is analyzed using the Laplace transform method. The Laplace transform method has been previously utilized to model the process but under a uniform velocity profile assumption. Here, a more realistic linear velocity profile is employed. The results suggest that the uniform velocity profile assumption overestimates the refrigerant concentration across the liquid film, and underestimates the temperature profile and the vapor absorption rate. Overall, the uniform velocity profile assumption can result in up to 30% error in calculating the absorption rate. Using the new model, the effect of different flow parameters on the absorption rate has been studied. The efficacy of the model is demonstrated through comparison with various experimental and numerical results reported in the literature. The analytical solution developed in the present study provides a simple, fast and accurate approach for calculating the absorption rate at different operating conditions and fluid properties.

Numerical Simulation of H2O/LiBr Falling Film Absorption Process

Absorber is a critical component of an absorption air-conditioning system. Its performance influences directly the overall efficiency and volume. In this paper, the commercial simulation software CFD-FLUENT was used to simulate the falling film absorption process of the coolant LiBr/H2O solution. This research attempts to know the effects of various Reynolds number on the absorption progress. The simulation results indicated that there was an optimized Reynolds number of liquid film which leads to an averaged maximum mass flux of 2.9×10-3kg·m-2·s-1 at the interface. The local interfacial mass transfer coefficient arrives at the maximum at the inlet, then decreases rapidly along the flow direction and approaches to a constant value gradually. It is also find that the local interfacial mass transfer coefficient increases with the increase of Reynolds number while the local heat transfer coefficient exhibits the opposite trend.

Heat and mass transfer enhancement for falling film absorption process by SiO2 binary nanofluids

The objectives of this study are to visualize the dispersion of nano-particles in binary nanofluids, to find the effect of key parameters such as SiO2 nano-particles concentration on the distribution stability in the H2O/LiBr nanofluids, and to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids. The binary nanofluids are H2O/LiBr with SiO2 nano-particle. The key parameters are the base fluid concentration of LiBr, the concentration of SiO2 nano-particles in vol%, and kinds of additives. It is found that the concentration of SiO2 nano-particles should be less than 0.01 regardless of the existence of the distribution stabilizer. It is also found that the maximum improvements of heat transfer rate and mass transfer rate reach 46.8% and 18%, respectively, when the concentration of SiO2 nano-particle is 0.005vol%.

Numerical simulation on the falling film absorption process in a counter-flow absorber

Falling liquid film is commonly employed in variety of industrial systems, such as LiBr/H 2O absorption heat pump or chiller. In this paper, the falling film absorption in aqueous lithium bromide solution was investigated numerically using CFD software package-Fluent. The practical convective boundary condition at the cooling water side was considered. The heat transfer coefficient is assumed constant, and the coolant temperature changes linearly along its flow path. The numerical results indicate that the profile of temperature is exponential and their gradients are high due to the distinct heat effect associated with the absorption at the interface and the cooling effect of coolant at the wall at small downstream distance. As the downstream distance increases, the profile of temperature is nearly linear. The absorption heat and mass fluxes reach a maximum at the inlet region and decrease at the outside of the inlet region. Specially, the effect of variable physical properties on the absorption process was considered and discussed. The prediction of the total absorption mass transfer rate is about 6.5% higher when assuming constant properties.

Heat and mass transfer enhancement of binary nanofluids for H 2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H 2O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H 2O/LiBr absorption system.

Heat and mass transfer coefficients of falling-film absorption process

This paper deals with the method to calculate heat and mass transfer coefficients of falling film absorption process over vertical tube or plate type surface employed in absorption refrigeration system. The conventional log mean temperature/concentration difference method is criticized for lack of physical rationality, and for incorrect results from the calculation. A new method based on a simplified model is proposed and demonstrated by numerical simulations and comparison analysis.

Non-absorbable gas effects on heat and mass transfer in falling film absorption

Film absorption involves simultaneous heat and mass transfer in the gas-liquid system. While the non-absorbable gas does not participate directly in the absorption process, its presence does affect the overall heat and mass transfer. An experimental study was performed to investigate the heat and mass transfer characteristics of LiBr-H₂O solution flowing over 6-row horizontal tubes with the water vapor absorption in the presence of non-absorbable gases. The volumetric concentration of non-absorbable gas, air, was varied from 0.17 to 10.0%. The combined effects of the solution flow rate and its concentration on the heat and mass transfer coefficients were also examined. The presence of 2% volumetric concentration of air resulted in a 25% reduction in the Nusselt number and 41 % reduction in the Sherwood number. Optimum film Reynolds number was found to exist at which the heat and mass transfer reach their maximum value independent of air contents. Reduced Nusselt and Sherwood numbers, defined as the ratio of Nusselt and Sherwood numbers at given non-absorbable gas content to that with pure water vapor, were correlated to account for the reduction in the heat and mass transfer due to non-absorbable gases in a falling film absorption process.

A simple model for falling film absorption on vertical tubes in the presence of non-absorbables

Most water–lithium bromide (LiBr) absorption chillers have a purge system to remove non-absorbable gases that cause a reduction in cooling capacity. Generally, the non-absorbables are originated in corrosion/passivation processes inside the machine, but leaks can also be a source of concern. However, since leaks must be corrected immediately to avoid machine deterioration, this study is mostly aimed at the non-absorbables evolved during operation. This paper analyses the effect of inlet non-absorbable air concentration, outlet purge velocity, absorber pressure and cooling water temperature on the falling film absorption process inside a vertical tube absorber, based on a simple transport coefficient model. This model consists of three ordinary differential equations solved with as method for initial-value problem, and a set of auxiliary equations. The study shows that the effect of non-absorbables can be significant, and furthermore provides a quantitative framework to aid in purge design. The nominal working conditions in this study were a solution Reynolds number of 100, an absorber pressure of 1.3 kPa, a cooling water temperature of 35 °C and an inlet solution concentration of 62% LiBr by weight. The results indicate that a minimum vapour velocity is required to sweep the non-absorbables along the absorber towards the purge, thus preventing reduced absorption fluxes. At a cooling water temperature of 35 °C, an inlet air concentration of 20% (by mole) resulted in a 61% reduction in mass absorption flux.

Combined Heat and Mass Transfer Under Different Inlet Subcooling Modes During NH3-H2O Falling Film Absorption Process

Experiments were conducted for ammonia-water falling film absorption in a plate heat exchanger with offset strip fins. The objectives of this paper were to analyze combined heat and mass transfer during the ammonia-water absorption process under different inlet subcooling modes, and to obtain heat transfer coefficients (Nusselt number). This paper examined the effects of the inlet subcooling modes, the inlet concentration difference, liquid Reynolds number, and vapor Reynolds number on the heat transfer performance. Inlet liquid concentrations were set at 0, 5, 10, and 15 percent in mass of ammonia, while inlet vapor concentration ranged from 64.7 to 83.6 percent. Experiments were conducted in three ways according to the inlet subcooling conditions, i.e., Case A Tv>Tl, Case B Tv∼Tl, and Case C Tv<Tl. In Case A, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption was confirmed in the experiments, which resulted in a lower absorption performance. The heat transfer coefficient increased as the inlet subcooling increased in all cases. The effect of inlet subcooling on heat transfer performance was more significant in Case A than in Cases B and C. The inlet subcooling had more significant effect on the heat transfer performance than the inlet concentration difference. Nusselt number increased as liquid and vapor Reynolds numbers increased. The vapor velocity should be maximized to increase absorption performance in cocurrent ammonia-water absorption process. The parametric analysis provides fundamental understandings of the ammonia-water absorption process, and thus gives a guideline for heat exchanger compactness in ammonia-water absorption systems.