Thermo-fluid Fields Research Articles

Investigating dynamic characteristics and thermal-lag phenomenon in a thermal-lag engine using a CFD-mechanism dynamics model

Thermal-lag engines (TLE) are external combustion engines with a single piston that offer the potential for reduced maintenance and production costs compared to traditional Stirling engines. Despite these promising prospects, studies on the physical behavior of this engine type remain limited. In this study, three novelties are presented: (1) a novel computational fluid dynamics-mechanism dynamics (CFDMD) model with high complexity for the TLE; (2) an analysis of the work generation in relation to the phase difference between pressure and volume; and (3) an analysis of the thermal-lag phenomenon based on the full spectrum of the phase difference between temperatures and piston position. In the CFDMD model, there are two key components: a computational fluid dynamics (CFD) model that captures the temporal evolution of the three-dimensional thermo-fluid fields and a mechanism dynamics model that solves the transient dynamics of the crank-drive mechanism. The CFDMD model effectively simulates the complex interplay between the thermodynamic behavior of the working gas and the dynamic behavior of the engine mechanism. The study additionally investigates the temporal evolution of the engine speed and examines its dynamic behavior. The influence of parameters, including crank radius, heating temperature, and initial crank angle, on shaft power and brake thermal efficiency is thoroughly explored. To gain a profound understanding of the indicated work generation mechanism and the thermal-lag phenomenon, the discrete Fourier transform provides full spectra of the periodic quantities of interest generated by the CFDMD model. It is found that the numerical average engine speed at different heating temperatures and torques from the CFDMD model is in good agreement with the experimental data, with a maximum difference of 140 rpm. The shaft power decreases from 19.8 to 5.3 W, while brake thermal efficiency increases from 4.5 to 7.7% as the initial crank angle varies from −90° to 90°. The first-oscillation phase differences between pressure and volume are close to 180°, decrease as the heating capacity is raised, and dominate the swelling of the PV diagrams. Only the first harmonic oscillation of the heater temperature exhibits the thermal-lag phenomenon.

Three-Dimensional Effects of Torch Arrangement on the Thermo-Fluid Fields inside the Plasma Furnace

Numerical simulation was proposed here to investigate the three-dimensional effects of torch arrangement on the thermo-fluid fields inside the plasma furnace in a plasma medical waste treatment system. Different turbulence models were used and the simulation results were compared with previous measurement results. The effect of the torch arrangement on the flow and temperature fields was clarified by different rotary-cutting angles. It is shown that the torch arrangement affects strongly the thermo-fluid fields. An appropriate rotary-cutting angle is beneficial to safe and complete decomposition of the medical waste due to a more uniform temperature distribution inside the furnace. Besides, the life of the furnace would be greatly shortened for the case where the rotary-cutting angle is 0 ° because the thermal plasma jets converged in the central area of the molten pool and caused severe thermal shocks to the furnace throat.

Modelling of inhomogeneous mixing of plasma species in argon–steam arc discharge for broad range of operating conditions

Numerical simulation of mixing of argon- and water-plasma species in argon-steam arc discharge has been investigated in thermal plasma generator with the combined stabilization of arc by axial gas flow (argon) and water vortex. Mixing process is described by the combined diffusion coefficients method in which the coefficients describe the diffusion of argon “gas”, with respect to steam “gas”. Calculations for currents 150–600 A with 15–40 standard liters per minute (slm) of argon reveal inhomogeneous mixing of argon and oxygen-hydrogen species with the argon species prevailing near the arc axis. However, calculations for currents higher than 400 A were not straightforward and a phenomenon of premixing of argon and steam species in the upstream discharge region was predicted from modelling to obtain reasonable agreement with experimental data. Premixed argon-steam plasma has a global impact on the plasma jet parameters near the exit nozzle as well as on the overall arc performance. The results of thermo-fluid fields, species mole fractions and radiation losses from the discharge are presented and discussed. Our former calculations based on the homogeneous mixing assumption differ from the present model in temperature, enthalpy, radiation losses, and flow field. Comparison with available experiments exhibits very good qualitative and quantitative agreements for the radial temperature profiles and satisfactory agreement for the velocity profiles 2 mm downstream of the exit nozzle.

Heat Transfer Enhancement due to Acoustic Fields: A Methodological Analysis

The aim of this paper is to expose the main involved physical phenomena underlying the alteration of convective heat transfer in a heat exchanger subjected to imposed vibrations. This technique seems to have interesting features and industrial applications, such as for efficiency increases, heat transfer rate control and cleanliness action. However, a clear description and comprehension of how vibrations may alter the convective heat transfer coefficient in a heat exchanger has still not been reached due to the complexity of the involved physical mechanisms. For this reason, after a presentation and a schematization of the analyzed thermodynamic system, the fundamental alterations of the thermo-fluid dynamics fields are described. Then, the main involved physical phenomena are exposed for the three cases of gaseous, monophasic liquid and boiling liquid mediums. Finally, on the basis of the characteristics of these described phenomena, some considerations and indications of general validity are presented.

Effect Of Turbulence Modelling In Numerical Analysis Of Melting Process In An Induction Furnace

AbstractIn this paper, the velocity field and turbulence effects that occur inside a crucible of a typical induction furnace were investigated. In the first part of this work, a free surface shape of the liquid metal was measured in a ceramic crucible. Then a numerical model of aluminium melting process was developed. It took into account coupling of electromagnetic and thermofluid fields that was performed using commercial codes. In the next step, the sensitivity analysis of turbulence modelling in the liquid domain was performed. The obtained numerical results were compared with the measurement data. The performed analysis can be treated as a preliminary approach for more complex mathematical modelling for the melting process optimisation in crucible induction furnaces of different types.

Detailed Numerical Simulation of Single-Walled Carbon Nanotube Synthesis in a Radio-Frequency Induction Thermal Plasma System

2D axisymmetric numerical calculations are conducted to model the thermo-fluid fields and chemical reactions leading to the formation of SWCNTs in an RF plasma system. A modified version of the SWCNT “reduced” chemical model is used to estimate the formation of SWCNT in an RF plasma system for the first time. The “reduced” model incorporates 14 species and 36 chemical reactions to predict the formation of metal and carbon clusters and SWCNTs. By combing the chemistry model into the RF plasma CFD code, the formation and development of carbon and metal catalyst clusters and their reactions which produce SWCNTs are shown. The chemistry model is shown to under-predict the yield rate of SWCNT. In order to better predict the yield rate, a sensitivity analysis is performed to modify the dominant reaction rates. The modified model predicts the yield of SWCNTs correctly within the range reported experimentally. However, more studies should be conducted to validate the accuracy of the model for different operating conditions.

Experimental and numerical investigation of a micro-CHP flameless unit

A numerical and experimental investigation of a system for the micro-cogeneration of heat and power, based on a Stirling cycle and equipped with a flameless burner, is carried out with the purpose of evaluating the system performances with hydrogen-enriched fuels. The numerical model of the combustion chamber gave significant insight concerning the analysis and interpretation of the experimental measurements; however it required considerable efforts, especially regarding the definition of proper boundary conditions. In particular a subroutine was developed in order to couple the oxidation process to the overall operation of the micro-cogeneration unit. This procedure was proved to perform satisfactory, providing values of the heat source for the Stirling cycle that lead to expected thermal efficiencies in agreement with those indicated in the literature for similar systems. The importance of a proper turbulence-chemistry interaction treatment and rather detailed kinetic schemes to capture flameless combustion was also assessed. A simple NO formation mechanism based on the thermal and prompt routes was found to provide NO emissions in relatively good agreement with experimental observations when applied on thermo-fluid dynamic fields obtained from detailed oxidation schemes.

Numerical modelling of plasma spray process

Reproduction of the coating quality in the plasma spraying is tough task. To overcome this problem, it is necessary to understand the behaviour of the arc inside the torch, plasma jet and particles in a plasma jet. Various experimental and modelling works have been carried out on these topics. In this article, a few of our new results of plasma arc inside the torch are presented and some of our simulated results of plasma jets and particle behaviour in a plasma jet are summarized. Electro-thermal efficiencies predicted using two different models are close to the measured one. The effect of atmosphere, where the plasma jet is issued, on temperature field is stronger than on velocity field. The influence of the carrier gas and particle loading on the plasma jet thermo-fluid fields is discussed. The effects of particle loading and turbulence modulation on the particle velocity and temperature are clarified. Turbulence modulation does not affect the particle dispersion significantly.

Comparisons Between Two Different Three-Dimensional Arc Plasma Torch Simulations

Several numerical models have been developed to study the characteristics of an arc inside the nontransferred plasma torch. A few of them have considered complete geometry of cathode and anode nozzle (type I) whereas others have considered only anode nozzle with cathode tip (type II). In this work, a three-dimensional model is developed to simulate Ar-N2 arc in type I and type II geometries. Various combinations of the arc length and arc core radius are predicted for the torch power that corresponds to given gas flow rate and current. Various combinations of the same and minimum entropy production for all cases could not be predicted in type II geometry. The difference between velocities predicted in both geometries is larger than that between temperatures. Three-dimensional effect in the plasma jet thermo-fluid fields demises along the axial direction. Torch efficiencies and arc voltages predicted in both geometries are comparable with measurements.

Numerical investigation on thermofluid fields induced by pen-like atmospheric non-thermal plasma torch with array type: A preliminary study

The pen-like plasma torch device, which was developed in 2002, has very simple structure and has been successfully applied for the surface treatment to improve hydrophilic properties of plastic film and polyethylene. However, due to the effective treating area of a single pen-like atmospheric plasma torch is still very limited, the current status of the device is still far from the practical applications. In view of this, the development of the new design of the “pen-like atmospheric plasma torch array”, which is composed of multiple pen-like atmospheric plasma torches, is undergoing. It is expected the array design of the plasma-torch system could increase the effective treating area and satisfy the need of practical applications. The present paper carried out the numerical simulations of the thermofluid fields induced by the devices with dual and four pen-like plasma torches. The major interest is to exploit the different flow features of single-, dual- and four-plasma torch system. The results revealed in the present study provide fundamental understanding of the thermofluid fields induced by the multiple pen-like plasma torches, which are worthwhile for the design work of the array-type devices.

Application of In-Flight Melting Technology by RF Induction Thermal Plasmas to Glass Production

An innovative in-flight glass melting technology with induced thermal plasmas was developed for the purpose of energy conservation and environmental protection. Two-dimensional modeling was used to simulate the thermofluid fields in the plasma torch. The in-flight melting behavior of glass raw material was investigated by various analysis methods. Results showed that the plasma temperature was up to 10000 K with a maximum velocity over 30 m/s, which made it possible to melt the granulated glass raw material within milliseconds. The carbonates in the raw material decomposed completely and the compounds in the raw material attainted 100% vitrification during the in-flight time from the nozzle exit to substrate. The particle melting process is similar to the unreacted-core shrinking model.

Numerical investigation of cooling effect on platinum nanoparticle formation in inductively coupled thermal plasmas

A mathematical model is developed to simulate the comprehensive systems of platinum nanoparticle synthesis using an argon inductively coupled thermal plasma flow with forced cooling portions. Numerical investigation using the model is conducted to clarify and discuss the effects of several cooling methods on the formation mechanisms of nanoparticles in distinctive thermofluid fields with strong two dimensionality. The computational results show that cooling by a radial gas injection, and a counterflow, engenders the remarkable promotion of nanoparticles.

Numerical simulation of flow in a natural draft wet cooling tower – The effect of radial thermofluid fields

This paper presents a two-dimensional axisymmetric two-phase simulation of the heat and mass transfer inside a natural draft wet cooling tower with particular emphasis on determining the extent of the non-uniformities across the tower. The water droplets in the spray and rain zones are represented with droplet trajectories written in Lagrangian form. The heat and mass transfer in the fill is represented using source terms implemented with Poppe style transfer coefficients. These coefficients are defined through empirical correlations which capture the functional dependence of the system. The model has the capability to represent non-uniformities in fill layout and water distribution, which traditional one-dimensional models are unable to resolve. The results show a largely uniform velocity profile across the tower radius with the greatest non-uniformity occurring at the outer edge of the tower. The water outlet temperature was found to vary by 6 K (∼40%) between the tower centre and exterior under reference tower conditions. This is largely due to the rise in air temperature and humidity through the rain zone. The considerable non-uniformity of heat transfer across the tower is shown to be tied to the performance of the rain zone. The effect of tower inlet height on radial non-uniformity is surprisingly small but the model is very sensitive to changes in water flow rate. At small fill depths the water temperatures entering the rain zone tend to be higher, which in turn slightly increases the heat transfer rate here and the over-all non-uniformity in the tower.

Modeling of non-equilibrium argon–hydrogen induction plasmas under atmospheric pressure

Modeling of induction thermal plasmas has been performed to investigate chemically non-equilibrium effect for dissociation and ionization. Computations were carried out for argon–hydrogen plasmas under atmospheric pressure. The thermofluid and concentration fields were obtained by solving of two-dimensional modeling. This formulation was presented using higher-order approximation of the Chapman–Enskog method for the estimation of transport properties. A deviation from the equilibrium model indicates that argon–hydrogen induction plasmas should be treated as non-equilibrium for dissociation and ionization near the torch wall.

Numerical and experimental investigation on the thermofluid fields induced by a pen-like atmospheric nonthermal plasma torch

The present paper carried out the numerical simulation and experimental study of the thermofluid fields induced by a pen-like atmospheric nonthermal plasma torch, which is a very simple device to produce atmospheric plasma and has been successfully applied for surface treatment to improve the hydrophilic properties of plastic film and polyethylene. The device consists of a small cylindrical stainless steel pipe with 6.5 mm diameter and 170 mm length inserted into a ceramic tube as the inner electrode. A ring outer electrode made of stainless steel is fixed at near the edge of the ceramic. The 13.56 MHz RF power was applied between the two electrodes to produce a surface discharge. The power into the torch is 60 W. The driving gas is argon and the flow rate varies in the range of 3–12 L min −1. Fundamental research results describing the major features of the flow fields induced by the plasma torch are provided in the present study. The stress is laid on the investigation of the influence of gas flow rate, which is one of important operation conditions, on the flow fields excited by the plasma torch. The principal results show that as the power input is fixed the increase of the gas flow rate will reduce the plasma temperature. The plasma penetration length stretching toward the atmospheric surroundings is also found to be closely related with the gas flow rate. Experimental confirmation by measuring the rotational temperature using optical spectroscopic method has been carried out, and a reasonable agreement between calculation and experiment has been obtained. These results provide fundamental understanding of the thermofluid fields induced by a pen-like atmospheric plasma torch, and also build up a basic predictive capability by numerical simulations to perform the further detailed investigation for practical applications in future.

Modeling of non-equilibrium argon–oxygen induction plasmas under atmospheric pressure

Modeling of induction thermal plasmas has been performed to investigate chemically non-equilibrium effect for dissociation and ionization. Computations were carried out for argon–oxygen plasmas under atmospheric pressure. The thermofluid and concentration fields were obtained by solving two-dimensional modeling. This formulation was presented using higher-order approximation of the Chapman–Enskog method for the estimation of transport properties. A deviation from the equilibrium model indicates that argon–oxygen induction plasmas should be treated as non-equilibrium for dissociation and ionization. The present modeling would give the guidance for the rational design of new material processing using thermal plasmas.

Numerical Analysis of Metallic Nanoparticle Synthesis Using RF Inductively Coupled Plasma Flows

A thermal plasma flow is regarded as a multifunctional fluid with high energy density, high chemical reactivity, variable properties, and controllability by electromagnetic fields. Especially a radio frequency inductively coupled plasma (RF-ICP) flow has a large plasma volume, long chemical reaction time, and a high quenching rate. Besides, it is inherently clean because it is produced without internal electrodes. An RF-ICP flow is, therefore, considered to be very useful for nanoparticle synthesis. However, nanoparticle synthesis using an RF-ICP flow includes complicated phenomena with field interactions. In the present study, numerical analysis was conducted to investigate the synthesis of metallic nanoparticles using an advanced RF-ICP reactor. An advanced RF-ICP flow is generated by adding direct current (DC) discharge to a conventional RF-ICP flow in order to overcome the disadvantages of a conventional one. The objectives of the present work are to clarify the formation mechanism of metallic nanoparticles in advanced RF-ICP flow systems and to detect effective factors on required synthesis. A two-dimensional model as well as a one-dimensional model was introduced for nanoparticle growth to investigate effects of spatial distributions of thermofluid fields in RF-ICP flows on synthesized nanoparticles. In an advanced RF-ICP flow, a characteristic recirculation zone disappears due to a DC plasma jet. Larger numbers of nanoparticles with smaller size are produced by using an advanced RF-ICP flow. Thermofluid fields in RF-ICP flows can be controlled by applied coil frequency by means of skin effect. Larger numbers of nanoparticles with smaller size are produced near the central axis. Dispersion of particle size distributions can be suppressed by higher applied coil frequency through control of RF-ICP flows. Applied coil frequency can be a remarkably effective factor to control nanoparticle size distribution.

Comprehensive Numerical Simulation of an Indirect Internal Reforming Tubular SOFC

A comprehensive numerical simulation of a cell-based indirect internal reforming tubular Solid Oxide Fuel Cell has been conducted. Two-dimensional axisymmetric thermo-fluid fields and non-axisymmetric electric potential/current fields in the tubular cell were simultaneously solved. As a result, complex interactions between the thermo-fluid and electrical fields were clearly demonstrated. It was also revelated how the thermal field and power generation characteristics of the cell are affected by catalyst density distribution for the reforming in the cell. A linear distribution of the catalyst greatly reduced the maximum temperature and temperature gradients of the cell with little negative impact on the power generation performance of the cell.

Numerical analysis of oxygen induction thermal plasmas with chemically non-equilibrium assumption for dissociation and ionization

Modeling of induction thermal plasmas has been performed to investigate a chemically non-equilibrium effect for dissociation and ionization. Computations were carried out for oxygen plasmas under atmospheric pressure. The thermofluid and concentration fields were obtained by solving of two-dimensional modeling. This formulation was presented using higher-order approximation of the Chapman–Enskog method for the estimation of transport properties. A deviation from the equilibrium model indicates that oxygen induction plasmas should be treated as non-equilibrium for dissociation and ionization. The present modeling would give the guidance for the rational design of new material processing using thermal plasmas.

Computational simulation of a particle-laden RF inductively coupled plasma with seeded potassium

In the present study, the plasma properties, the thermofluid fields and the induction electromagnetic fields and further the in-flight particle behaviors in the radio frequency inductively coupled plasma (RF-ICP), which is electrically enhanced by seeding potassium vapor, are investigated by computational simulation. Since an electrically enhanced plasma is also influenced considerably by the applied coil frequency, the plasma structure and the in-flight particle behaviors in seeding potassium vapor are correspondingly influenced and have distinctive profiles. Due to the electrical enhancement of RF-ICP with potassium seeding, the frequency effect on particle heating becomes more considerable.