Flow phenomena, heat and mass transfer in microchannel reactors

The studies of flow phenomena, heat and mass transfer in microchannel reactors are beneficial to estimate and evaluate the ability of microchannel reactors to be operated for a given process reaction such as Fischer-Tropsch synthesis. The flow phenomena, for example, the flow regimes and flow patterns in microchannel reactors for both single phase and multiphase flow are affected by the configuration of the flow channel. The reviews of the previous works about the analysis of related parameters that affect the flow phenomena are shown in this report. In order to predict the phenomena of Fischer-Tropsch synthesis in microchannel reactors, the 3-dimensional computational fluid dynamic simulation with commercial software package FLUENT was done to study the flow phenomena and heat transfer for gas phase Fischer-Tropsch products flow in rectangular microchannel with hydraulic diameter 500 ¿m and length 15 cm. Numerical solution with slip boundary condition was used in the simulation and the flowphenomena and heat transfer were determined.

On the Reversed Brayton Cycle with High Speed Machinery

This work was carried out in the laboratory of Fluid Dynamics, at Lappeenranta University of Technology during the years 1991-1996. The research was a part of larger high speed technology development research. First, there was the idea of making high speed machinery applications with the Brayton cycle. There was a clear need to deepen theknowledge of the cycle itself and to make a new approach in the field of the research. Also, the removal of water from the humid air seemed very interesting. The goal of this work was to study methods of designing high speed machinery to the reversed Brayton cycle, from theoretical principles to practical applications. The reversed Brayton cycle can be employed as an air dryer, a heat pump or a refrigerating machine. In this research the use of humid air as a working fluid has an environmental advantage, as well. A new calculation method for the Braytoncycle is developed. In this method especially the expansion process in the turbine is important because of the condensation of the water vapour in the humid air. This physical phenomena can have significant effects on the level of performance of the application. Also, the influence of calculating the process with actual, achievable process equipment efficiencies is essential for the development of the future machinery. The above theoretical calculations are confirmed with two different laboratory prototypes. The high speed machinery concept allows one to build an application with only one rotating shaft including all the major parts: the high speed motor, the compressor and the turbine wheel. The use of oil free bearings and high rotational speed outlines give several advantages compared to conventional machineries: light weight, compact structure, safe operation andhigher efficiency at a large operational region. There are always problems whentheory is applied to practice. The calibrations of pressure, temperature and humidity probes were made with care but still measurable errors were not negligible. Several different separators were examined and in all cases the content of the separated water was not exact. Due to the compact sizes and structures of the prototypes, the process measurement was slightly difficult. The experimental results agree well with the theoretical calculations. These experiments prove the operation of the process and lay a ground for the further development. The results of this work give very promising possibilities for the design of new, commercially competitive applications that use high speed machinery and the reversed Brayton cycle.

Bernoulli correction to viscous losses: Radial flow between two parallel discs

For a massless fluid (density = 0), the steady flow along a duct is governed exclusively by viscous losses. In this paper, we show that the velocity profile obtained in this limit can be used to calculate the pressure drop up to the first order in density. This method has been applied to the particular case of a duct, defined by two plane-parallel discs. For this case, the first-order approximation results in a simple analytical solution which has been favorably checked against numerical simulations. Finally, an experiment has been carried out with water flowing between the discs. The experimental results show good agreement with the approximate solution

Lifetime enhancement of scroll rings by spatiotemporal fluctuations

The dynamics of three-dimensional scroll rings with spatiotemporal random excitability is investigated numerically using the FitzHugh-Nagumo model. Depending on the correlation time and length scales of the fluctuations, the lifetime of the ring filament is enlarged and a resonance effect between the time scale of the scroll ring and the time correlation of the noise is observed. Numerical results are interpreted in terms of a simplified stochastic model derived from the kinematical equations for three-dimensional excitable waves.

Lifetime enhancement of scroll rings by spatiotemporal fluctuations

The dynamics of three-dimensional scroll rings with spatiotemporal random excitability is investigated numerically using the FitzHugh-Nagumo model. Depending on the correlation time and length scales of the fluctuations, the lifetime of the ring filament is enlarged and a resonance effect between the time scale of the scroll ring and the time correlation of the noise is observed. Numerical results are interpreted in terms of a simplified stochastic model derived from the kinematical equations for three-dimensional excitable waves.

Accounting for adsorption and desorption in Lattice Boltzmann simulations

We report a Lattice-Boltzmann scheme that accounts for adsorption and desorption in the calculation of mesoscale dynamical properties of tracers in media of arbitrary complexity. Lattice Boltzmann simulations made it possible to solve numerically the coupled Navier-Stokes equations of fluid dynamics and Nernst-Planck equations of electrokinetics in complex, heterogeneous media. With the moment propagation scheme, it became possible to extract the effective diffusion and dispersion coefficients of tracers, or solutes, of any charge, e.g., in porous media. Nevertheless, the dynamical properties of tracers depend on the tracer-surface affinity, which is not purely electrostatic and also includes a species-specific contribution. In order to capture this important feature, we introduce specific adsorption and desorption processes in a lattice Boltzmann scheme through a modified moment propagation algorithm, in which tracers may adsorb and desorb from surfaces through kinetic reaction rates. The method is validated on exact results for pure diffusion and diffusion-advection in Poiseuille flows in a simple geometry. We finally illustrate the importance of taking such processes into account in the time-dependent diffusion coefficient in a more complex porous medium.

Chaos in resonant-tunneling superlattices

Spatiotemporal chaos is predicted to occur in n-doped semiconductor superlattices with sequential resonant tunneling as their main charge transport mechanism. Under dc voltage bias, undamped time-dependent oscillations of the current (due to the motion and recycling of electric field domain walls) have been observed in recent experiments. Chaos is the result of forcing this natural oscillation by means of an appropriate external microwave signal.

Simulació de vent en temps real per paisatges

La simulació de la realitat és un fenomen que va sorgir fa uns anys per tal de predir esdeveniments sense haver de malbaratar recursos. El problema inicial de la simulació va ser la necessitat de simplificar la realitat a causa de la manca de capacitat dels ordinadors de l’època. Amb aquest projecte volem ajudar, per exemple, a estudis científics sobre la difusió de la contaminació en grans nuclis a causa de l’efecte del vent, càlculs de trajectòries amb forces externes degudes al vent, o incorporar en el món de la multimèdia efectes realistes de vent. El principal objectiu d’aquest projecte és desenvolupar un sistema que permeti realitzar simulacions realistes de vent per un paisatge 2D, i estudiar com el vent és veu afectat per la geometria de l’escena. Un punt important, és que tot ha de ser en temps real. Per aconseguir-ho, utilitzarem tècniques basades en el mètode de Lattice-Boltzmann, el qual consisteix en una xarxa regular que representa el fluid en posicions discretes, i estudiar com flueix. Escollint els paràmetres correctes de la simulació, es pot demostrar que aquest mètode convergeix a les equacions continues de Navier-Stokes, les qual són les més importants per descriure el comportament macroscòpic d’un fluid. Per accelerar tots els càlculs, utilitzarem la capacitat i la potencia de les targes gràfiques, ajustarem l’algorisme per poder-lo utilitzar en paral•lel, tot tenint en compte les restriccions de les GPUs. També haurem de generar un sistema per poder llegir les escenes 2D sobre les que realitzarem la simulació. Finalment, haurem de “pintar” el vent per tal de poder visualitzar el resultat de la simulació

Supersaturation-controlled crystallization

Identification of product requirements and quality, together with the management of production are key issues in chemical engineering. Quality control of crystalline products is part of the quality of many industrially manufactured products like paper, paintings, medicines and fertilizers. In most crystallization cases, quality is described with the size, polymorph, shape and purity of the crystal. The chemical composition, hydrodynamics and driving force, together with the operating temperature are in a key position when the properties of a crystalline product are controlled with the crystallization process. This study concentrates on managing the identified properties of a crystalline product with the control of a driving force. The controlling of the driving force can be based on the change of solubility or the change of concentration. Solubility can be changed with temperature, pressure and an antisolvent. The concentration of crystallizing compound, the solute can be changed with the evaporation of the solvent and with the addition of a reagent. The present study focuses on reagent addition and temperature change as methods of changing the level of the driving force. Three control structures for direct control of supersaturation are built, one for cooling crystallization and two for reactive crystallization. Closed loop feedback control structures are based on the measurement of the solute concentration with attenuated total reflection - Fourier transform infrared spectrometer. The details of the reagent feed are analyzed with experimental studies and with results of computational fluid dynamic simulations of the inert particle pulse in the premixer and inert particle injection to the mixing tank. Nucleation in conditions of controlled reactive crystallization is analyzed with Nielsen’s equation of homogeneous nucleation. The resulting control systems, based on regulation of supersaturation, can be used to produce the desired polymorph of an organic product. The polymorph composition of product crystals is controlled repeatably with the decision of a set value of supersaturation level.

New computer software for defining the three-dimensional geometry for a centrifu- gal compressor impeller

In this thesis, a computer software for defining the geometry for a centrifugal compressor impeller is designed and implemented. The project is done under the supervision of Laboratory of Fluid Dynamics in Lappeenranta University of Technology. This thesis is similar to the thesis written by Tomi Putus (2009) in which a centrifugal compressor impeller flow channel is researched and commonly used design practices are reviewed. Putus wrote a computer software which can be used to define impeller’s three-dimensional geometry based on the basic geometrical dimensions given by a preliminary design. The software designed in this thesis is almost similar but it uses a different programming language (C++) and a different way to define the shape of the impeller meridional projection.

Laskentasovelluksen kehittäminen modulaariselle vesiputkikattilalle

Modulaarisella vesiputkikattilalla tarkoitetaan täysin konepajalla valmistettavissa olevaa kattilaa, joka voidaan kuljettaa yhtenä tai muutamana suurena moduulina työmaalle. Tässä diplomityössä käsiteltiin modulaarisen vesiputkikattilan laskentasovelluksen ke-hittämistä KPA Unicon Oy:lle. Työn tavoitteena oli tarkastella modulaarisen vesiputkikattilan lämpöteknistä mitoitusta ja suunnittelua sekä kehittää laskentasovellus, jonka avulla voidaan arvioida kattilan mittoja ja painoa. Laskentasovellus laadittiin Microsoftin Excel-alustalle, josta se on myöhemmin mahdollista siirtää muille ohjelma-alustoille. Laskentasovelluksessa hyödynnetään lämmönsiirron ja virtaustekniikan laskentayhtälöitä sekä -menetelmiä. Sovellukseen valitut laskentayhtälöt sekä -menetelmät ovat yleisesti hyväksyttyjä ja käytännössä testattuja. Diplomityön tuloksena valmistui laskentasovellus, joka kykenee modulaarisen vesiputkikattilan lämpötekniseen mitoittamiseen. Sovelluksen avulla voidaan mitoittaa kattilan tulipesä, tulistimet, höyrystinpinnat sekä ekonomaiseri. Laskentasovellusta on tarkoitus hyödyntää yrityksen tarjousvaiheen projekteissa sekä mahdollisesti kattiloiden esisuunnittelussa. Laskentasovelluksen laatimista varten ei ollut mahdollista hyödyntää toiminnassa olevien kattiloiden prosessitietoja, koska ensimmäiset tämän tyyppiset kattilat ovat asenteilla. Sen sijaan sovelluksen antamia laskentatuloksia verrattiin toisen mitoitusohjelman antamiin tuloksiin, joiden perusteella laskentasovelluksen voidaan olettaa antavan oikeita tuloksia.

Kiertoleijupetikattilateknologia ja sen matemaattinen mallinnus energiantuotantoprosessissa

Työn teoreettisessa osuudessa tehdään katsaus kiertoleijupetiteknologian eri osa-alueisiin: leijupedin virtausdynamiikkaan, hiukkaserottimeen ja kiintoaineen palautusmekanismiin. Myös teknologian historiaa ja muita käyttötarkoituksia energiantuotannon ohella käydään läpi. Termodynamiikkaa sekä lämmönsiirron ja voimalaitosprosessien teoriaa käsitellään mallinnuksessa tarvittavilta osin. Mallinnusosiossa käydään läpi kiertoleijupetihöyrykattilan matemaattisen mallin tekoprosessia. Malli perustuu yleisesti saatavilla oleviin yhtälöihin ja korrelaatioihin. Mallintaminen koostuu höyrykattilan jakamisesta lämpöpintoihin ja niiden mitoittamisesta. Mallissa esitetään myös näkemys siitä, miten lämpö siirtyy savukaasuun ja miten petimateriaalin kierto tapahtuu tulipesässä.

A high efficiency microturbine concept

There is a growing trend towards decentralized electricity and heat production throughout the world. Reciprocating engines and gas turbines have an essential role in the global decentralized energy markets and any improvement in their electrical efficiency has a significant impact from the environmental and economic viewpoints. This paper introduces an inter-cooled and recuperated two-shaft microturbine at 500 kW electric output range. The microturbine is optimized for a realistic combination of the turbine inlet temperature, the recuperation rate and the pressure ratio. The new microturbine design aims to achieve significantly increased performance within the range of microturbines and even competing with the efficiencies achieved in large industrial gas turbines. The simulated electrical efficiency is 45%. Improving the efficiency of combined heat and power (CHP) systems will significantly decrease the emissions and operating costs of decentralized heat and electricity production. Cost-effective, compact and environmentally friendly micro-and small-scale CHP turbine systems with high electrical efficiency will have an opportunity to successfully compete against reciprocating engines, which today are used in heat and power generation all over the world and manufactured in large production series. This paper presents a small-scale gas turbine process, capable of competing with reciprocating engine in terms of electrical efficiency.

A reaction engineering approach to homogeneously catalyzed glycerol hydrochlorination

The production of biodiesel through transesterification has created a surplus of glycerol on the international market. In few years, glycerol has become an inexpensive and abundant raw material, subject to numerous plausible valorisation strategies. Glycerol hydrochlorination stands out as an economically attractive alternative to the production of biobased epichlorohydrin, an important raw material for the manufacturing of epoxy resins and plasticizers. Glycerol hydrochlorination using gaseous hydrogen chloride (HCl) was studied from a reaction engineering viewpoint. Firstly, a more general and rigorous kinetic model was derived based on a consistent reaction mechanism proposed in the literature. The model was validated with experimental data reported in the literature as well as with new data of our own. Semi-batch experiments were conducted in which the influence of the stirring speed, HCl partial pressure, catalyst concentration and temperature were thoroughly analysed and discussed. Acetic acid was used as a homogeneous catalyst for the experiments. For the first time, it was demonstrated that the liquid-phase volume undergoes a significant increase due to the accumulation of HCl in the liquid phase. Novel and relevant features concerning hydrochlorination kinetics, HCl solubility and mass transfer were investigated. An extended reaction mechanism was proposed and a new kinetic model was derived. The model was tested with the experimental data by means of regression analysis, in which kinetic and mass transfer parameters were successfully estimated. A dimensionless number, called Catalyst Modulus, was proposed as a tool for corroborating the kinetic model. Reactive flash distillation experiments were conducted to check the commonly accepted hypothesis that removal of water should enhance the glycerol hydrochlorination kinetics. The performance of the reactive flash distillation experiments were compared to the semi-batch data previously obtained. An unforeseen effect was observed once the water was let to be stripped out from the liquid phase, exposing a strong correlation between the HCl liquid uptake and the presence of water in the system. Water has revealed to play an important role also in the HCl dissociation: as water was removed, the dissociation of HCl was diminished, which had a retarding effect on the reaction kinetics. In order to obtain a further insight on the influence of water on the hydrochlorination reaction, extra semi-batch experiments were conducted in which initial amounts of water and the desired product were added. This study revealed the possibility to use the desired product as an ideal “solvent” for the glycerol hydrochlorination process. A co-current bubble column was used to investigate the glycerol hydrochlorination process under continuous operation. The influence of liquid flow rate, gas flow rate, temperature and catalyst concentration on the glycerol conversion and product distribution was studied. The fluid dynamics of the system showed a remarkable behaviour, which was carefully investigated and described. Highspeed camera images and residence time distribution experiments were conducted to collect relevant information about the flow conditions inside the tube. A model based on the axial dispersion concept was proposed and confronted with the experimental data. The kinetic and solubility parameters estimated from the semi-batch experiments were successfully used in the description of mass transfer and fluid dynamics of the bubble column reactor. In light of the results brought by the present work, the glycerol hydrochlorination reaction mechanism has been finally clarified. It has been demonstrated that the reactive distillation technology may cause drawbacks to the glycerol hydrochlorination reaction rate under certain conditions. Furthermore, continuous reactor technology showed a high selectivity towards monochlorohydrins, whilst semibatch technology was demonstrated to be more efficient towards the production of dichlorohydrins. Based on the novel and revealing discoveries brought by the present work, many insightful suggestions are made towards the improvement of the production of αγ-dichlorohydrin on an industrial scale.