Liquid Temperature Profiles Research Articles

Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

Abstract In this article, the industrial process of CO2 capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO2 loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO2 concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO2 absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO2 mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO2 absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.

Numerical investigations on the coupling of the one-group interfacial area transport equation and subcooled boiling models for nuclear safety applications

The one-group interfacial area transport equation (IATE) was coupled to a wall heat flux partitioning model in the framework of two-phase Eulerian approach using the OpenFOAM CFD code for better prediction of sub-cooled boiling flow. The IATE was modified to include the effect of bubble nucleation at the wall and condensation in the bulk region that governs the non-uniform bubble size distribution. To validate the capability of the newly developed OpenFOAM solver, it has been used to simulate the upwards sub-cooled boiling bubbly flow in the DEBORA test facility. Predictions of the gas volume fraction, gas velocity, bubble Sauter mean diameter and liquid temperature profiles were in a good agreement with the experimental data. Simulation results of the DEBORA experiment achieved with the MUSIG model implemented in the ANSYS CFX code in other work have been compared to the simulation results by the IATE model implemented in OpenFOAM to test the competence of the one-group IATE to provide good prediction of subcooled boiling flow parameters. Both approaches were found to provide compatible results.

Thermohydraulic characteristics of a multi-string direct-contact heat exchanger

Direct-contact heat exchangers that involve energy exchange between gas and liquid streams have a variety of applications, including waste heat recovery, thermoelectric power plant cooling, and thermal desalination. Direct-contact heat exchangers are appealing as they may help mitigate potential corrosion, fouling, and scaling of solid surfaces and enhance heat transfer effectiveness. In this study, we experimentally investigate the thermohydraulic characteristics of an economic light-weight direct-contact heat exchanger that incorporates an array of strings of diameter of the order of 0.1–1 mm to sustain flows of thin liquid films. We constructed a 1.6 m-tall prototype heat exchanger with an array of as many as 112 vertically aligned strings. Thin films of a non-evaporating liquid are flown down the strings by gravity and exchange thermal energy with a counterflowing gas stream. We obtained axial liquid temperature profiles and frictional loss in the gas stream for different combinations of liquid and gas flow rates and two different string pitches. Numerical simulation is also performed to help interpret and indirectly validate our experimental results. The overall, gas-side, and liquid-side heat transfer coefficients extracted from the experimentally measured temperature profiles are examined to evaluate the impact of instability in liquid film flows and inter-bead spacing. The applicability of the Reynolds analogy is also assessed using the measured gas-stream pressure drops and air-side heat transfer coefficients. The present study helps improve our understanding of heat transfer and gas-stream pressure drop in string-based direct-contact heat exchangers and provides an experimental database to help systematically optimize their design.

Investigating the performance and thermodynamic efficiency of CO2 reactive absorption – A solvent comparison study

The capture of the CO2 released in the atmosphere by various sources is a key issue included in all environmental related European new legislation and roadmaps. Among Carbon Capture Systems (CCS), post-combustion reactive absorption is one of the most efficient and easily deployable methods. In specific, amine aqueous solvents ensure high absorption efficiency due to their reactivity with CO2. Monoethanolamine (MEA) and a mixture of Monoethanolamine and Piperazine, 30 MEA/18 PZ % wt., were found to achieve very satisfying levels of CO2 capture. For these solvents, a simulation of the entire CO2 capture unit was performed in Aspen Plus software. For the MEA case, a parametric analysis was performed to determine the impact of the absorber inlet liquid temperature, gas and liquid flow rates on CO2 mole fraction, reboiler heat duty and stripper temperature profile. The absorber inlet liquid temperature was found to be the most important factor influencing the absorption performance as well as the reboiler heat duty required for regeneration. The absorber inlet gas and liquid flow rates had also impact on the liquid temperature profiles in the absorber and on the energy cost. Comparing the two solvents, the MDEA/PZ solvents captured 99% of the CO2 of the flue gas while MEA only 80% for the conditions of the simulation. For the reference system with MEA, overall exergy efficiency was calculated considering the carbon capture unit as an integrated thermodynamic system. The overall exergy efficiency of the MEA system was calculated 23.8%, while the respective value for the MDEA/PZ system was calculated 26.1%. Evaluating the two main columns of the system in terms of exergy efficiency, the absorber showed low values of efficiency for both solvents, in specific 16.3% for MEA and 22.6% for MDEA/PZ, while the desorber had higher efficiencies, 27.8% for MEA and 34.3% for MDEA/PZ. In general, MDEA/PZ system was characterized by higher thermodynamic efficiencies both in system and individual column level. Furthermore, the analysis for the MDEA/PZ system resulted to a 5% lower reboiler heat duty than in the MEA case.

Mathematical Modelling of Thermal Stratification in a Cryogenic Propellant Tank

Cryogenic tanks used for space applications are filled with sub-cooled cryogenic propellants, whose liquid-vapor interface remains undisturbed for long periods of time prior to launch. During this period, substantial amount of heat leaks into the tank from external sources such as solar and ambient convective fluxes, even though the tank is well insulated. This results in thermal stratification near the liquid vapour interface. A transient, two-phase, thermodynamic model of stratification in a cryogenic tank is developed, considering propellant boundary layer flow due to natural convection close to tank wall. Continuity, momentum, energy and mass transfer equations are solved using finite difference-based formulations of SINDA/FLUINT simulator. The analytical model is validated with test results reported in literature. Subsequently, studies are carried out to investigate the effect of liquid sub-cooling in propellant tank on stratified mass and liquid temperature profile. The study shows that sub-cooling of cryogenic tank leads to significant increase in stratified mass.

Simulation of continuous packed bed reactive distillation column for the esterification process using activity based kinetic model

A mathematical model for the continuous packed bed reactive distillation process of esterification of acetic acid with methanol is developed. The kinetic rate equation, which plays a major role for the performance of reactive distillation and it is the part of model, is required for the liquid phase reversible esterification reaction. The mineral sulphuric acid is used as the catalyst. The kinetic experiments are carried out under different temperatures in the range of 305.15 to 333.15 K and catalyst concentrations in the range of 0.1267 mole H+/lit to 0.6537 mole H+/lit. From that experimental data the kinetic model is developed and the same is used for the simulation of reactive distillation process. Equilibrium stage model, in which the vapour and the liquid leaving a stage are assumed to be in equilibrium with each other, has been used for the simulation of reactive distillation process by incorporating our kinetic model. Conversion of acetic acid as function of reflux ratio and reboiler ratio has been predicted. The liquid composition and temperature profiles versus stage number have been also predicted. Finally, the optimum operating conditions obtained from the simulation results for high pure methyl acetate by reactive distillation process.

Analytical solutions to the Stefan problem with internal heat generation

A first-order, ordinary differential equation modeling the Stefan problem (solid–liquid phase change) with internal heat generation in a plane wall is derived and the solutions are compared to the results of a computational fluid dynamics analysis. The internal heat generation term makes the governing equations non-homogeneous so the principle of superposition is used to separate the transient from steady-state portions of the heat equation, which are then solved separately. There is excellent agreement between the solutions to the differential equation and the CFD results for the movement of both the solidification and melting fronts. The solid and liquid temperature profiles show a distinct difference in slope along the interface early in the phase change process. As time increases, the changes in slope decrease and the temperature profiles become parabolic. The system reaches steady-state faster for larger Stefan numbers and inversely, the time to steady-state increases as the Stefan number decreases.

Modelling of high pressure CO2 absorption using PZ+AMP blended solution in a packed absorption column

Abstract A rate-based steady-state model for CO 2 absorption into a PZ+AMP blended solution is presented by taking into account the column hydraulics, mass transfer resistances and chemical reactions. The simulation results were compared with the experimental results of CO 2 absorption by a PZ+AMP blended solution in a packed absorption column at low and high CO 2 partial pressure conditions. The model predicts CO 2 concentration, amine concentration, the chemical enhancement factor, and liquid temperature profiles along the column. The model was in good agreement to predict the CO 2 concentration profiles along the column at low CO 2 partial pressure. However, it was found that the model needs to be corrected by introducing a correction factor for overall volumetric mass transfer coefficient ( K g a e ) for the simulation of CO 2 concentration profiles along the column at high CO 2 partial pressure conditions in the range of 404–1616 kPa.

Integrated microchannel cooling in a three dimensional integrated circuit: A thermal management

Microchannel cooling is a promising technology for solving the three-dimensional integrated circuit thermal problems. However, the relationship between the microchannel cooling parameters and thermal behavior of the three dimensional integrated circuit is complex and difficult to understand. In this paper, we perform a detailed evaluation of the influence of the microchannel structure and the parameters of the cooling liquid on steady-state temperature profiles. The results presented in this paper are expected to aid in the development of thermal design guidelines for three dimensional integrated circuit with microchannel cooling.

냉각수 및 전기 가열 방식에 따른 요소수 해동 특성에 관한 수치해석 연구

A Urea-SCR(Selective Catalytic Reactor) system, which converts nitrogen oxides into nitrogen and water in the presence of a reducing agent, creates a major exhaust gas aftertreatment system for NOx reduction among other compounds. With regard to vehicle applications, a urea solution was chosen based on its eutectic composition of a 32.5  urea-water solution. An important advantage of this eutectic composition is that its melting point of -11.7 is sufficiently low to avoid solidification in cold °C environments. However, the storage tanks must be heated separately in case of low ambient temperature levels to ensure a sufficient amount of liquid is available during scheduled start ups. In this study, therefore, a numerical investigation of three-dimensional unsteady heating problems analyzed to understand the melting processes and heat transfer characteristics including liquid volume fraction, temperature distributions, and temperature profiles. The investigations were performed using Fluent 6.3 commercial software that modeled coolant and electric heater models based on a urea solution. It is shown that the melting performance with the electric heater is higher than a coolant heater and is more efficient. Corresponding Author, manykim@jbnu.ac.kr 2016 The Korean Society of Mechanical Engineers C

USING MULTI-WALL CARBON NANOTUBE (MWCNT) BASED NANOFLUID IN THE HEAT PIPE TO GET BETTER THERMAL PERFORMANCE

In this study, thermal performance of a cylindrical heat pipe is investigated numerically. Three different types of water based nanofluids, namely (Al2O3+Water), (Diamond+Water) and (Multi-Wall Carbon Nano tube (MWCNT) +Water) have been used. The influence of using the simple nanofluids and MWCNT nanofluid on the heat pipe characteristics such as liquid velocity, pressure profile, temperature profile, thermal resistance and heat transfer coefficient of heat pipe have been studied. A new correlation developed by Bakhshan and Saljooghi, for viscosity of nanofluids has been implemented. The results show good agreement with the available analytical and experimental data. Also, the MWCNT based nanofluid has lower thermal resistance, higher heat transfer coefficient and lower temperature difference between evaporator and condenser sections, so it has good thermal specifications as a working fluid for use in heat pipes. The prepared code has capability for parametric studies also.

Using pulse shaping to control temporal strain development and solidification cracking in pulsed laser welding of 6082 aluminum alloys

High-speed observation of visible and infrared radiation was performed to measure the molten pool geometry, velocity of the solid–liquid interface and temperature profile during laser spot welding of aluminum. Hot cracking occurred at a late stage of solidification for the investigated laser pulse shapes. Hot cracking could be minimized by using a pulse shape with two distinct power levels and a final cooling slope to shut down the laser power. The drop of the laser power from the first to the second power level led to a high cooling rate and high interface velocity at the beginning of solidification. This drop in temperature and molten pool diameter released strains originating from thermal contraction and solidification shrinkage at the beginning of solidification, where spot welding is expected to have a higher ductility. After this initial high solidification rate, low interface velocities were observed during solidification within the second power level. The final solidification rate increased again as a function of the cooling time of the last laser pulse section. This type of solidification process was also found in metallographic microstructures with greater dendritic structures during solidification within the second power level. The strain release at the beginning of solidification minimized residual strains for the remaining solidification, so that crack-free and full penetration bead-on-plate seam welding with overlapping spot welds was possible.

Experimental and computational investigation of vertical downflow condensation

This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.

Numerical Simulations of Gas-Liquid Boiling Flows Using OpenFOAM

The present work is aimed at development of an experimentally verified computational model based on OpenFOAM for the simu- lations of gas-liquid boiling flows. The open source CFD code OpenFOAM was developed further to include k-E turbulence model, different inter-phase coupling forces and energy transport accompanied with phase change. The most commonly used wall heat flux partitioning model, proposed by Kurul and Podowski 1, was implemented in the OpenFOAM to simulate two-phase boiling. The predicted axial and radial vapor volume fraction distribution, liquid temperature distribution and wall temperature profiles were compared with the DEBORA measurements 2. A good agreement between the predictions and measurements was observed. The experimentally verified model was used to understand the effects of various interfacial closures e.g. lift force, turbulent dispersion force and wall force, on the vapor volume fraction distribution. The effects of bubble induced turbulence and nucleation site density on the hydrodynamics of the gas-liquid boiling flows were studied. Such an experimentally verified open source CFD code will be useful to simulate boiling under different regimes and in complex geometries.

Using of Multiwall Carbon Nanotube Based Nanofluid in the Heat Pipe to Get Better Thermal Performance

Thermal performance of a cylindrical heat pipe is investigated numerically. Three different types of water based nanofluids, namely, Al2O3 + Water, Diamond + Water, and Multi-Wall Carbon Nano tube (MWCNT) + Water, have been used. The influence of using the simple nanofluids and MWCNT nanofluid on the heat pipe characteristics such as liquid velocity, pressure profile, temperature profile, thermal resistance, and heat transfer coefficient of heat pipe has been studied. A new correlation developed by Bakhshan and Saljooghi (2014) for viscosity of nanofluids has been implemented. The results show, a good agreement with the available analytical and experimental data. Also the results show, that the MWCNT based nanofluid has lower thermal resistance, higher heat transfer coefficient, and lower temperature difference between evaporator and condenser sections, so it has good thermal specifications as a working fluid for use in heat pipes. The prepared code has capability for parametric studies also.

Conjugate heat transfer during viscous liquid movement in the open cavity, considering its cooling through outer boundary of back surface

Numerical simulation was carried out for nonisothermal incompressible viscous liquid in the open rectangular cavity, considering the heat loss through the channel bottom outer boundary. Liquid flow pattern and temperature profiles were obtained for solid and liquid phases. Influence of model parameters on heat carrier motion pattern and cavity cooling mode has been investigated. The interest in studies of convective flow in different heat transfer conditions and for various cavities is caused by a wide applied value of the problem. Large amount of heating devices is being used in power plants and energy systems, production processes of a various complexity level. The necessity of studying these processes is determined by development of such energy-intensive indistries as power engineering, metallurgy, chemical industry and many other (1-5). Experimental development of technologies always assumes a high probability of selecting not the best process conditions both for productivity and quality, and for implementation costs. Therefore, there arises a necessity to study theoretical hydrodynamic regularities and heat transfer during heat carrier motion in limited space. This article considers nonsteady interaction of liquid with the open cavity (Fig. 1). Process of molten metal movement in the channel is being investigated with consideration to heat exchange with its walls. Thermal and physical characteristics of the molten metal and cavity walls are identical. The purpose of this work is to investigate the hydrodynamics and conjugate heat transfer during movement of nonisothermal incompressible viscous liquid in the rectangular-type cavity in conditions of the channel's outer boundary cooling. Similar problems have been solved before (6-8) for thermal insulation conditions of the cavity outer boundaries. Study of the described process was done using a mathematical model based on the Navier-Stokes equations in the variables vortex-stream function, the energy equation, and the heat-conduction equation for the cavity material with corresponding initial and boundary conditions + U X + V Y = 1

One-dimensional Stefan problem formulation for solidification of nanostructure-enhanced phase change materials (NePCM)

The effects of rejecting nanoparticles at the solid–liquid interface of a colloid on the development of the solid–liquid interface, temperature and concentration profiles are investigated. The Rubinstein problem will be used in the current study which admits a closed-form analytical solution. A model NePCM based on cyclohexane as a solvent mixed with copper nanoparticles (diameters of 7, 5 and 2nm) with various initial volume fractions of the nanoparticles will be considered. The rejection rate of the particles will be controlled through the segregation coefficient (1 being the case of no rejection of particles, 0.1, 0.01, and 0.001). It was found that for the case of no particle rejection, the expected expedited movement of the solid–liquid interface with respect to the pure cyclohexane as the volume fraction of the particles increases is not always guaranteed for the same cold side surface temperature. However, for most cases with particle rejection considered the solid–liquid interface is decelerated with respect to pure cyclohexane as the volume of the nanoparticles is increased, and this deceleration is more pronounced as the particle size decreases. This deceleration is attributed to solidification with the rejection of the particles switched from diffusion- to solute-controlled solidification, and also due to the development of constitutionally-supercooled liquid in front of the solid–liquid interface. The maximum value of the concentration at the solid–liquid interface decreases as the concentration of the particles is increased; however, the value of the interface temperature is decreased as the concentration of the particles is increased. It was found that the transition segregation coefficient is the non-dimensional parameter that controls the transition from diffusion- to solute-controlled solidification, which is increased with the increase of the particle’s volume fraction and with the decrease of the particle size.

냉각수 순환 가열 방식을 이용한 요소수의 동결 및 해동 특성에 관한 수치적 연구

Urea-SCR은 동절기, 북유럽과 북미지역과 같은 -20℃ 이하의 환경에서 요소수가 동결되는 문제점을 해결해야 한다. 따라서 이러한 요소수 저장탱크에 해동 시스템을 적용하여 시동 초기, 요소수를 적정 시간 내 분사하기 위한 기술의 확보가 필요하다. 본 연구에서는 저장탱크 내 요소수의 동결현상과 냉각수 순환 가열방식을 적용한 해동현상에 대하여 상용 소프트웨어인 Fluent 6.3을 이용하여 3차원 비정상상태 수치해석을 수행하였다. 이를 통하여 요소수의 동결 및 해동과정 중 나타난 온도분포, 상경계면, 그리고 액상분율을 분석하여 열전달 특성을 고찰하였다. 결론적으로 요소수의 동결은 저장탱크 벽면으로부터 중심부로 이루어졌으며, 해동현상은 순환 파이프와 인접할수록 요소수의 상변화가 빠르게 진행하였다. 또한, 냉각수의 70℃, 200ℓ/h 조건에서 1ℓ의 액상 요소수를 얻는데 약 190초의 시간이 필요하였다.

Flow patterns and heat transfer in a square cross-section micro condenser working at low mass flux

Flow patterns and heat transfer were investigated during condensation of n-pentane in an air-cooled square cross-section micro condenser. The test section consisted of a borosilicate square microchannel, of inner and outer edges 553 μm and 675 μm, respectively, and of a length 208 mm. The transparency of the microchannel walls allowed the visualization of the phases distribution and the different condensation flow regimes. Different mass velocities ranging between 3 and 15 kg m−2 s−1 were imposed. Three main flow regimes were identified: annular regime, intermittent regime, and spherical bubbles regime. A specific experimental procedure was developed, based on bubbles tracking, in order to determine accurately the hydraulic and thermal parameters profiles in the isolated bubbles zone, such as the time-averaged void fraction profile α¯(z) and the time-averaged vapour quality profile x¯(z), according to the axial position in the microchannel. Thanks to energy balance, the time-averaged liquid temperature profile T¯l(z) in the isolated bubbles zone was determined. A significant temperature difference between the liquid and vapour phases was highlighted. Therefore, the latent and total heat fluxes released in this zone were quantified and compared to each other. Besides, the relationship between the void fraction and the vapour quality in the spherical bubbles zone was determined and compared to existing void fraction models. Finally, the bubbles detachment frequency was determined. A relationship between this frequency and the mass velocity was proposed.

Rate based modeling for CO2 absorption using monoethanolamine solution in a hollow fiber membrane contactor

This work presents a rate based model to estimate the performance of CO2 absorption by monoethanolamine (MEA) solution using a hollow fiber membrane contactor. The predicted CO2 fluxes were validated with the experimental data of both polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes from the literature. The simulation results of the PTFE membrane showed that the CO2 flux could be predicted by the rate based model with a deviation less than 4% for a non-wetted mode of operation. For the PVDF membrane, the model predicted the CO2 fluxes fairly well under partially-wetted mode of operation. The simulated concentration profiles in both the gas and liquid phases, as well as the liquid temperature profile of PTFE membrane, corresponded well with the experimental data. A study of parameter sensitivity analysis revealed that the proposed model is most sensitive to Henry's constant and when the reaction regime between CO2 and MEA changes from intermediate fast-instantaneous to instantaneous, when the membrane is 20% wetted. A comparison between the CO2 fluxes of the non-wetted and partially wetted modes showed that the increase of CO2 flux in the non-wetted mode with an increasing gas composition and liquid temperature is more significant than that of the partially wetted mode.