Polydisperse aerosol condensation with heat and mass conservation: I. Model description with applications to homogeneous systems

A computational solution of the coupled heat and mass transport equations for aerosols, which may be either single component or multicomponent, in a finite volume is developed under nonsteady state and nonisothermal conditions. It is capable of computing the temperature and vapor density profile around the droplets. Both mass and heat are conserved effectively in the volume, which enables this computational model to describe time-dependent, competitive, aerosol condensation and evaporation. Calculations for pure water particles and sodium nitrate particles in the presence of water vapor are performed. By comparing the particle growth for different number densities at various initial supersaturation conditions, both the initial saturations and particle number concentrations are shown to have great impact on the aerosol condensational growth. Detailed discussions on the change of bulk temperature, bulk relative humidity (RH), total surface area and volume are given.     

A method for aerosol particle removal by applying condensation phenomena

This paper describes an experimental study on a method of removing aerosol particles from air by condensing heated and humidified air. In the experiment, air, including aerosol particles, is circulated by a fan in a closed clean room system consisting of a closed vessel, a humidifier, two condensers, and an after-heater. The concentration of aerosol particles at the inlet of the closed vessel, the relative humidity, and the temperature in the system are measured for several humidification and condensation conditions. We find that the removal rate of aerosol particles in air increases when the mass flow rate of the condensate increases, by means of enlarging the temperature difference between the heating water in the humidifier and the cooling water in the condensers. We also show that this method is more effective when the temperature level in the humidifier is increased. The aerosol particle removal mechanism of this method is considered to be related to the generation of mist using aerosol particles as nuclei, inertia trapping, and the suction effect of condensation. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(6): 398–409, 1997     

Effects of atmospheric aerosol on the performance of environmentally sustainable passive air-breathing PEM fuel cells

The effects of different airborne particulate contaminants (oxalic acid, soot, ammonium sulfate, and sodium chloride) and the gaseous air impurity NO_x on the performance of a passive air-breathing proton exchange membrane fuel cell (AB-PEMFC) were investigated by introducing the pollutants into the oxidant air fed to a small AB-PEMFC module. All contamination data were corrected for variations in air temperature and relative humidity. While 10 ppmv NO_x degraded fuel cell performance by 22% within 4 h and showed partial recovery of 70% within another 11 h, the effects of the aerosol particles were considerably less severe. Only oxalic acid and ammonium sulfate exhibited significant performance degradation with upper limits of 24% 1000 h⁻¹ and 3% 1000 h⁻¹, respectively. Sodium chloride as well as soot particles did not cause any significant performance decline. Because diffusion is the main transport mechanism for aerosol particles into a passive AB-PEMFC, only a minor fraction of the total particle mass can actually deposit inside these fuel cells resulting in rather low effects even under very high aerosol loadings. Therefore, aerosol related effects seem negligible under ambient conditions.     

Coupled Conduction,Convection, Radiation Heat Transfer with Simultaneous Mass Transfer in Ice Rinks

ABSTRACT

This article presents numerical predictions of velocity, temperature, and absolute humidity distributions in an indoor ice rink with ventilation and heating. The computational fluid dynamics (CFD) simulation includes the effects of radiation between all inside surfaces of the building envelope, turbulent mixed convection, and vapor diffusion, as well as conduction through the walls and condensation on the ice. The net radiative fluxes for each element of the envelope's inside surfaces are calculated with a modified version of Gebhart's method. This modification reduces the calculation time and the memory required to store the radiation view factors for the discretized elements of the inside surfaces of the envelope. The predicted temperatures show very good agreement with measured data. The CFD code also calculates the heat fluxes toward the ice due to convection from the air, to condensation of vapor, and to radiation from the walls and ceiling. It is shown that a low emissivity ceiling reduces the sum of these fluxes and the risk of vapor condensation on the ceiling.     

A numerical study of soot aggregate formation in a laminar coflow diffusion flame

Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate np is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, np is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity, OH mole fraction, and C₂H₂ mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence on soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method.     

Comparison between single-phase and two-phases CFD modeling of laminar forced convection flow of nanofluids in a circular tube under constant heat flux

In this article, forced convection heat transfer with laminar and developed flow for water-Al₂O₃ nanofluid inside a circular tube under constant heat flux from the wall was numerically investigated using computational fluid dynamics method. Both single and two-phase models are accomplished for either constant or temperature dependent properties. For this study nanofluids with size particles equal to 100 nm and particle concentrations of 1 and 4 wt% were used. It is observed that the nanoparticles when dispersed in base fluid such as water enhance the convective heat transfer coefficient. The Nusselt number and heat transfer coefficient of nanofluids were obtained for different nanoparticle concentrations and various Reynolds numbers. Heat transfer was enhanced by increasing the concentration of nanoparticles in nanofluid and Reynolds number. Also, a correlation based on the dimensionless numbers was obtained for the prediction the Nusselt number. The modeling results showed that the predicted values were in very good agreement with reference experimental data.     

Aerosol deposition in ventilation ducts

This paper examines the relationship between air flow and deposition rate of aerosol particles in ventilation systems. Measurements were carried out to study the flow of aerosol particles in a small-scale heating, ventilation and air-conditioning (HVAC) system fitted with different types of fittings. Aerosol particles were injected into the inlet of the HVAC system and concentrations of the aerosol particles were measured downstream. The deposition rate of the particles was determined by measuring the concentrations of aerosol particles at the inlet and outlet of the fitting. The flow of aerosol particles in ducts was simulated using the computational fluid dynamics (CFD) package code FLUENT and results were compared with data from experiments. Initial equations were formulated between deposition ratio and air flow for different fittings. Predicted results were in close agreement with measured results.     

Air movement in naturally-ventilated buildings

The air movement and the distribution of CO₂ in naturally ventilated office room and an atrium is investigated using computational fluid dynamics. The results show that natural ventilation is capable of achieving acceptable CO₂ levels. Adequate comfort levels could also be achieved for a typical UK summer climate in both types of buildings. Both wind-driven and buoyancy-driven flows are considered.     

Simplification of analytical models and incorporation with CFD for the performance predication of closed‐wet cooling towers

Simplified analytical models are developed for evaluating the thermal performance of closed‐wet cooling towers (CWCTs) for use with chilled ceilings in cooling of buildings. Two methods of simplification are used with regard to the temperature of spray water inside the tower. The results obtained from these models for a prototype cooling tower are very close to experimental measurements. The thermal performance of the cooling tower is evaluated under nominal conditions. The results show that the maximum difference in the calculated cooling water heat or air sensible heat between the two simplified methods and a general computational model is less than 3%. The analytical model distribution of the sensible heat along the tower is then incorporated with computational fluid dynamics (CFD) to assess the thermal performance of the tower. It is found that CFD results agree well with the analytical results when the air flow is simulated with air supply from the bottom of the tower, which represents a uniform air flow. CFD shows the importance of the uniform distribution of air and spray water to achieve optimum design. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat transfer and pressure drop during HFC refrigerant saturated vapour condensation inside a brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC refrigerants 236fa, 134a and 410A saturated vapour condensation inside a brazed plate heat exchanger: the effects of saturation temperature (pressure), refrigerant mass flux and fluid properties are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature (pressure) and great sensitivity to refrigerant mass flux and fluid properties. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m²s that corresponds to an equivalent Reynolds number around 1600–1700. At low refrigerant mass flux (G_r < 20 kg/m²s) the heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [20] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (G_r > 20 kg/m²s) the heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [21] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 25–30% increase for a doubling of the refrigerant mass flux.The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on mass flux.HFC-410A shows heat transfer coefficients similar to HFC-134a and 10% higher than HFC-236fa together with frictional pressure drops 40-50% lower than HFC-134a and 50–60% lower than HFC-236fa.     

2D thermal modeling of a solid oxide electrolyzer cell (SOEC) for syngas production by H2O/CO2 co-electrolysis

Solid oxide fuel cells (SOFCs) can be operated in a reversed mode as electrolyzer cells for electrolysis of H₂O and CO₂. In this paper, a 2D thermal model is developed to study the heat/mass transfer and chemical/electrochemical reactions in a solid oxide electrolyzer cell (SOEC) for H₂O/CO₂ co-electrolysis. The model is based on 3 sub-models: a computational fluid dynamics (CFD) model describing the fluid flow and heat/mass transfer; an electrochemical model relating the current density and operating potential; and a chemical model describing the reversible water gas shift reaction (WGSR) and reversible methanation reaction. It is found that reversible methanation and reforming reactions are not favored in H₂O/CO₂ co-electrolysis. For comparison, the reversible WGSR can significantly influence the co-electrolysis behavior. The effects of inlet temperature and inlet gas composition on H₂O/CO₂ co-electrolysis are simulated and discussed.     

Conjugate heat and mass transfer in a hollow fiber membrane module for liquid desiccant air dehumidification: A free surface model approach

Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.     

Optimized mass flux ratio of double-flow solar air heater with EHD

The numerical study of laminar forced convection inside double-flow solar air heater with electrohydrodynamic technique is investigated by finite difference method. The electric field is generated by the wire electrodes charged with DC high voltage. The mathematical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It can be perceived that augmented heat transfer with presence of an electric field increases with the supplied voltage but decreases with the total mass flux. The optimized mass flux ratio is expressed incorporating with concerning parameter comprising of the electrode arrangement, the number of electrodes, the total heat flux at an absorbing plate, and the channel geometry.     

Test of turbulence models for natural convection in an open cubic tilted cavity

A comparison was made between six turbulence models and experimental temperature profiles for the turbulent natural convection in a tilted open cubic cavity. The experimental setup consists of a cubic cavity of 1 m by side with one vertical wall receiving a constant and uniform heat flux, whereas the remaining walls are thermally insulated. The thermal fluid is air and the aperture is facing the heated wall. The temperature profiles were obtained at different heights and depths and each one consists of 10 positions inside the cavity. A commercial computational fluid dynamic software was used for the simulation and different turbulence models of k-ε_t and k-ω families were evaluated against experimental data. The lowest absolute average percentage difference for the experimental and numerical temperature profiles was for the rk-ε_t model and the highest was for the sk-ω model.     

A fast and simple numerical model for a deeply buried underground tunnel in heating and cooling applications

Underground tunnels used in underground constructions serve as huge ventilation pipes that conduct outside fresh air into a cavern. Predicting the heat and mass transfer is critically important in order to exploit the relatively constant underground soil temperature for heat transfer, and to ensure sufficient ventilation for occupational safety. This paper presents a numerical model developed to describe the simultaneous heat transfer between air and the tunnel surface, taking into account the condensation phenomena inside the tunnel. The soil surrounding the tunnel is treated as an equivalent long annulus and divided into several cross-section slices. With appropriate assumptions, a set of discrete numerical equations and its solution is proposed. The developed model is validated against field measurements which showed good agreement between the simulated results and measurement data. The model is then applied to an underground tunnel operating for a ten-year-period.     

Hydrogen–hydrocarbon turbulent non-premixed flame structure

In this study, the structure of turbulent non-premixed CH₄-H₂/air flames is analyzed with a special emphasis on mixing and air entrainment. The amount of H₂ in the fuel mixture varies under constant volumetric fuel flow. Mixing is described by mixture fraction and its variance while air entrainment is characterized by the ratio of gas mass flow to fuel mass flow at the inlet section. The flow field and the chemistry are coupled by the flamelet assumption. Mixture fraction and its variance are transported by the computational fluid dynamics (CFD) code. The slow chemistry aspect of NO_x is handled by solving an additional transport equation with a source term derived from flamelet library.     

Numerical studies on heat and fluid flow of nanofluid in a partially heated vertical annulus

A numerical investigation on natural convective heat transfer of nanofluid (Al₂O₃+water) inside a partially heated vertical annulus of high aspect ratio (352) has been carried out. The computational fluid dynamics solver Ansys Fluent is used for simulation and results are presented for various volume fraction of nanoparticles (0‐0.04) at different heat flux values (3‐12 kW/m²). Two well‐known correlations for evaluating thermal conductivity and viscosity have been used. Thus different combinations of the available correlations have been set to form four models (I, II, III, and IV). Therefore, a detailed analysis has been executed to identify effects of thermophysical properties on heat transfer and fluid flow of nanofluids using different models. The results show enhancement in heat transfer coefficient with volume fraction of nanoparticles. Highest enhancement achieved is found to be 14.17% based on model III, while the minimum is around 7.27% based on model II. Dispersion of nanoparticles in base fluid declines the Nusselt number and Reynolds number with different rates depending on various models. A generalized correlation is proposed for Nusselt number of nanofluids in the annulus in terms of volume fraction of nanoparticles, Rayleigh number, Reynolds number, and Prandtl number.     

CFD based evaluation of polymer particles heat transfer coefficient in gas phase polymerization reactors

In this work, computational fluid dynamics (CFD) has been employed to compute convection heat transfer coefficient (h) that is the key parameter in calculation of heat transfer rate between particles and fluids in an ethylene polymerization fluidized bed reactor. In addition, the effects of various parameters such as free stream fluid velocity, particles size, and particles interactions with different configurations on heat transfer coefficient were studied. Simulation results are in agreement with common engineering knowledge of the process. The results also indicate that particle interactions, particle size and fluid velocity have more significant impacts on h.     

Heat and Mass Transfer in Adsorption-chemical Cooling with Phase Transitions in a Sorbent

The complex model of heat and mass transfer during adsorption and chemical heat conversion is presented. It includes combined effect of physical and chemical adsorption and accompanying ammonia condensation/evaporation directly inside the sorber taking into account the conjugate heat transfer between the sorbent and the heat-transfer fluid. It was found that the specific cooling power and average cooling temperature are one-valued function of a ratio of the sorber length to the mass flow of heat-transfer fluid. The use of condensation/evaporation in a sorbent can increase the temperature effect more than twice.     

Heat Exchangers as Equipment and Integrated Items in Waste and Biomass Processing

Heat recovery systems play an important role in waste to energy and biomass processing. An efficient approach that follows a recommended hierarchy in design, process as a whole (e.g., incineration) → subsystem of the process (e.g., heat recovery system) → equipment (e.g., air pre-heater), is shown. Important factors have to be taken into consideration in processes for incineration (combustion of biomass), especially available energy, specific features of hot process fluid (flue gas), type of waste/biomass, fouling, and environmental impact. A combination of intuitive design, know-how, and a sophisticated approach based on up-to-date computational tools is shown. Some novel types of heat exchangers (e.g., air preheaters for high- and low-temperature applications, heat recovery steam generators and/or heaters, and those for specific applications) that can be substituted for conventional ones are presented. An improved or even optimum design of heat exchangers requires computational support in the following areas: a simulation based on energy and mass balance, the thermal and hydraulic calculation of heat exchangers, a CFD (computational fluid dynamics) approach, optimization, and heat integration. Some examples are presented. An approach that ranges from an idea to an industrial application is demonstrated on the novel design of integrated compact equipment (combustion chamber installed inside heat exchanger) for the thermal treatment of waste gases, including heat recovery. This approach involves simulation for obtaining basic process parameters, thermal and hydraulic calculations, design of experimental facility, the manufacture of the equipment and building of this facility, operation and functionality testing, data acquisition for validating and improving the CFD model, and the utilization of feedback from industrial applications.