Homogeneous Condensation Process Research Articles

Synergistic Effect of Multi-field Force on Condensable Particulate Matter Properties and Behavior in Flue Gas: A Case Study in a Municipal Solid Waste Incineration Plant

At present, filterable particulate matter (FPM) emissions from stable source flue gases are well controlled, and the more difficult to remove condensable particulate matter (CPM) has received much attention due to its environmental hazards. In this study, we built a cold electrode electrostatic precipitator (CE-ESP) and systematically investigated the removal efficiency of CPM from municipal solid waste incineration plant flue gas by CE-ESP under three different operating modes. The results showed that hydrocarbons, esters and aromatic compounds were the main organic substances in CPM. The CE-ESP had a significant contribution to the removal of SO42–, F–, Al, Ni, Cr, and Pb from CPM. In contrast, it had a less effective removal of NO3– and Ca in CPM. The CE-ESP was effective for removing esters in CPM. The removal efficiency of CE-ESP for CPM could reach 76%, and the removal efficiency of organic substance was higher than that of inorganic substance. It is demonstrated that the combination of thermophoretic force and electric field in CE-ESP intensifies the turbulent agglomeration process, and the diffusion force generated by the concentration field synergistically enhances the interparticle coagulation and agglomeration. This promotes the homogeneous condensation process of CPM and the heterogeneous condensation of CPM and FPM, which is conducive to the reduction of condensable particulate matter in flue gas and can provide a theoretical basis for practical application in engineering.

A potential strategy of carbon dioxide separation using supersonic flows

Carbon capture and storage (CCS) is one of the most promising technologies to tackle climate challenges. The separation of carbon dioxide (CO2) is the key step to achieve the economic and technical objectives of CCS. The present study proposes a potential strategy to separate CO2 using the phase change behavior in supersonic flows, which is not only a clean process of CO2 processing but also provides an efficient way to maximum utilize the thermal energy. To this end, a condensation flow model based on the real gas thermodynamics is developed to obtain an accurate evaluation of the heat and mass transfer due to the homogeneous condensation process of CO2 in supersonic flows. The prediction accuracy of the ideal gas model and real gas model is compared, and the result shows that the real gas condensation model presents a more accurate prediction of CO2 supersonic condensation with the root mean square error (RMSE) of 0.0147. The sensitivity of the two models to inlet pressure is analyzed, which shows that the ideal gas model under-estimated the liquid fractions of CO2 condensation by 2.8% of the total mass as well as over-estimated the latent heat by 20.1% at Wilson point during the heat transfer process. The condensation performances and Wilson point characteristics of CO2 are analyzed by using the real gas model. The prediction model of the relationship between the degree of supercooling, pressure, and expansion rate at the Wilson point was established with the mean relative error of 0.176% and the relative RMSE of 2.275% respectively, which is of great help for further forecasting to obtain the regularity of known data for CO2 separation in supersonic flows.

Comparative molecular dynamics simulations of homogeneous condensation of refrigerants

Performance improvement and environmental impact are very important issues in the selection of suitable refrigerant for domestic refrigeration systems. The homogeneous condensation of two hydrocarbon (HC) refrigerants, namely, R600 (butane) and R600a (isobutane) and two hydrofluorocarbon (HFC) refrigerants i.e. R134a (1,1,1,2-tetrafluoroethane) and R143a (1,1,1-trifluoroethane) was investigated by molecular dynamics simulations. The thermophysical properties of the four refrigerants, such as the density, internal potential energy, and cell volume of the molecular systems during the condensation process are compared. The influences of the various refrigerants on the potential energy and the cluster formation are described. The results show that vapor molecules undergo a rapid phase transition to a subcooled liquid within a specific time period. In the simulations, clusters of the substances are formed initially, which then aggregate to produce the condensate liquid droplets over time. The densities of all the refrigerants at the liquid phase after condensation agree with REFPROP data. A comparative analysis of the refrigerants was performed based on the saturation ratio to investigate the time of complete condensation in the homogeneous condensation process.

Molecular dynamics simulations on homogeneous condensation of R600a refrigerant

Molecular Dynamics (MD) simulations have been employed to study the homogeneous condensation phenomenon of R600a (isobutane) refrigerant in the vapor compression refrigeration system. The molecular system consists of superheated vapor isobutane in a cuboid simulation cell with periodic boundary conditions in all directions. A more physically reliable thermostat and a barostat are applied to control the temperature and pressure of the cell containing the molecules. Based on AHRI standard 460 rating conditions, the simulations started from the initial configuration of vapor phase isobutane system, and once the equilibration is established at 361K with 1atm, the system is suddenly cooled to the ambient temperature of 308K and is set under the condensing pressure of 7.1658bar corresponding to the saturation temperature at 325K of the compound at supersaturation ratio 1.548. The density, internal potential energy, and volume changing with time of superheated vapor molecules in the homogeneous condensation process of the system are investigated. The results show that vapor molecules have underwent a rapid phase transition to the subcooled liquid at a certain critical time period. Subcritical clusters of isobutane molecules are formed in the preliminary part of the simulations, which then aggregate to generate the critical condensate nucleus over time. An examination of the radial distribution functions (RDFs) shows that the bonding and non-bonding interactions are formed in the vapor and liquid phases of isobutane. It is truly consistent with the internal potential energy and phase change properties. The computed densities of both phases agree well with the NIST standard reference thermodynamic and transport property data of the compound. A comparative study is executed through several simulations for various values of supersaturation and undersaturation ratios with different numbers of molecules to investigate the critical time of phase transition in the homogeneous condensation process.

Influences of area ratio and surface roughness on homogeneous condensation in ejector primary nozzle

Homogeneous condensation inside the ejector has been ignored by numerous researchers. In this paper, the wet steam model was utilized to study the influences of nozzle area ratio and surface roughness on homogeneous condensation in a primary nozzle. A substantial amount of latent heat is released during the homogeneous condensation process to increase static pressure and reduce Mach number. The increase of nozzle area ratio reduces condensation intensity and lowers nozzle pressure but increases liquid mass fraction. Furthermore, with the growth of surface roughness, the slightly increased dryness fraction is almost ineffective for alleviating erosion caused by tiny droplets; the slight decrease in mass flow rate will reduce ejector performance to some extent, while the sharp increase of entropy generation results in more energy wastes. Therefore, nozzle area ratio and surface roughness should be carefully designed to alleviate potential condensation and enhance ejector performance at the same time.

Incorporation of ROH (R = CH3, C2H5, 2-C3H7) into (H2O)6–: Substituent Effect on the Growth Process of the Hydrogen-Bond Network

The condensation reaction of water cluster anions, (H2O)n(-) + H2O → (H2O)n+1(-), offers a prime opportunity to explore the growth process of the hydrogen-bond network involving molecular uptake and network rearrangement. Here, by exploiting an Ar-mediated approach, we investigate the association reaction of water hexamer anions, (H2O)6(-), with ROH (R = CH3, C2H5, 2-C3H7) by mass spectrometry combined with photoelectron spectroscopy. Quantitative analysis of the product mass spectra reveals that incorporation of ROH (R = CH3, C2H5) into (H2O)6(-) occurs with a cross section of the same size as in the (H2O)6(-) + D2O condensation, but with a slightly smaller cross section for R = 2-C3H7. Coexistence of two types of isomers, high electron-binding (type I) and low electron-binding (type II) forms, is observed in all the product ROH·(H2O)6(-) species by photoelectron spectroscopic measurement. These findings, in conjunction with ab initio study of MeOH·(H2O)6(-) structures, lead us to propose a molecular uptake mechanism at play in the incorporation of ROH into the (H2O)6(-) network. This also provides complementary information on the homogeneous condensation process of pure water cluster anions.

Experimental study of the effects of temperature variation on droplet size and wetness fraction in a low-pressure model steam turbine

The three-stage low-pressure model steam turbine at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) was used to study the impact of three different steam inlet temperatures on the homogeneous condensation process and the resulting wetness topology. The droplet spectrum as well as the particle number concentration were measured in front of the last stage using an optical-pneumatic probe. At design load, condensation starts inside the stator of the second stage. A change in the steam inlet temperature is able to shift the location of condensation onset within the blade row up- or downstream and even into adjoining blade passages, which leads to significantly different local droplet sizes and wetness fractions due to different local expansion rates. The measured results are compared to steady three-dimensional computational fluid dynamics calculations. The predicted nucleation zones could be largely confirmed by the measurements. Although the trend of measured and calculated droplet size across the span is satisfactory, there are considerable differences between the measured and computed droplet spectrum and wetness fractions.

Shock wave/boundary layer interaction with heat addition by nonequilibrium phase transition

This investigation deals with the interaction of a turbulent boundary layer and a normal shock wave caused by supercritical heat addition in homogeneously condensing flows. The method used is a combination of analytical and numerical procedures, which is based on the perturbation method of Bohning & Zierep for the shock/boundary layer interaction in adiabatic flow. The homogeneous condensation process is described by the classical nucleation theory of Volmer together with the molecular droplet growth law of Hertz and Knudsen. The cooling rate, the nucleation rate, the surface-averaged droplet radius and the condensate mass fraction are analyzed along several streamlines in the boundary layer. The strength of the shock wave caused by supercritical heat addition in the outer flow field weakens and ultimately vanishes near the boundary edge by the influence of viscous effects. Comparison is made of the integral boundary layer variables and the friction coefficient in adiabatic flow with those in diabatic flow.

Compressible turbulent boundary layers with heat addition by homogeneous condensation

Homogeneous condensation in transonic viscous flow is computed using a combination of analytical and numerical methods. The procedure is based on the perturbance method for the shock/boundary-layer interaction of Bohning and Zierep for adiabatic flow. In this paper, the model was extended for applications to boundarylayer flows with internal nonequilibrium heat addition. For the simulation of the homogeneous condensation process we use the classical nucleation theory of Volmer and the molecular Hertz-Knudsen droplet growth law. One momentum equation of the Navier-Stokes equations is solved by a time-stepping procedure combined with a mixing length turbulence model. Objective in view is the analysis of the macroscopic phenomena of nonequilibrium heat addition from homogeneous condensation of water vapor in transonic flows with high velocity and temperature gradients normal to the main flow direction. In detail, the cooling rate, the nucleation rate, the droplet radius, and the condensate mass fraction in the boundary layer are discussed. Approaching the wall surface and in comparison to the inviscid diabatic flow just outside of the boundary layer the nucleation process and the deposition of droplets, i.e. the condensate fraction contours, are shifted downstream, continuously or stepwise, depending on the pressure gradients at the boundary-layer edge. From the comparison of the integral boundary layer variables we conclude that the friction coefficients change only moderately, whereas the displacement and momentum thickness are found to be more sensitive to the homogeneous condensation onset.

Nonadiabatic Compressible Flow Of Humid AirAround Airfoils

The heat addition by the phase transition of water vapor in humid air, which changes the properties of compressible flows around airfoils, is investigated by numerical simulations. The computations are based on a recent small-disturbance approach by Rusak and Lee [11]. This approach describes the heat release by the nonequilibrium and homogeneous condensation process using the classical nucleation and droplet growth models. The nonlinear interaction between the heat transfer by the condensation, the kinetic energy of the compressible flow, and the thermal properties of the flow is explored and the similarity parameters which govern the problem are presented. The results of the calculations show good agreement with available simulations using the Euler equations. The similarity rules are used to suggest various flow cases which have a sufficiently close behavior of both the flow dynamics and the condensation properties. It is shown that nonadiabatic compressible flows of humid air may be applied to augment the aerodynamic performances of airfoils used in fluid machinery.