Two-component two-phase critical flow

U radu je predstavljen model dvokomponentnog dvofaznog kritičnog strujanja dvofazne mešavine. Model je validiran poređenjem računskih rezultata sa vrednostima izmerenim na PUMA eksperimentalnoj instalaciji. Model je zasnovan na jednodimenzionalnom modifikovanom homogenom izentropskom dvofaznom strujanju tečnosti i gasa. Homogeni model je modifikovan tako što je klizanje između faza uzeto u obzir pri određivanju zapreminskog udela gasne faze u dvofaznoj mešavini, što određuje i gustinu dvofazne mešavine. Klizanje između faza je sračunato na osnovu Chisholm korelacije koja je pogodna za niže vrednosti masenog protočnog udela gasne faze kao i na osnovu Zivi korelacije koja određuje maksimalnu vrednost klizanja između faza. Na mestu kritičnog isticanja brzina dvofazne mešavine je jednaka brzini zvuka i sračunata je na osnovu takozvanog zamrznutog modela dvofaznog strujanja koji ne uzima u obzir fazni prelaz. Ostvareni rezultati su prikazani i upoređeni zajedno sa dobro poznatim Fauske modelom dostupnim u literaturi. Pokazano je da Fauske model daje više vrednosti kritičnog masenog fluksa u odnosu na izmerene vrednosti, dok model prikazan u radu daje prihvatljiva slaganja sa izmerenim podacima.A model of two-component two-phase critical flow is presented. The modelling approach is based on one-dimensional homogeneous gas-liquid two-phase isentropic flow of mixture. The homogeneous model is modified by taking into account the void fraction and two-phase mixture density dependence on velocity slip. The velocity slip is calculated using Chisholm correlation that depends on the gas phase quality and Zivi correlation for the prediction of the maximum velocity slip values. At the location of the critical flow the two-phase mixture velocity equals sonic velocity and it is calculated with the so-called ''frozen sonic velocity' model. The model is validated against data measured in air water flow at the PUMA experimental facility. Obtained results are presented together with the predictions by the well-known Fauske model. It is shown that Fauske model overpredicts measured critical mass fluxes, while the present model shows acceptable agreement with the measured data

Two-component two-phase critical flow

U radu je predstavljen model dvokomponentnog dvofaznog kritičnog strujanja dvofazne mešavine. Model je validiran poređenjem računskih rezultata sa vrednostima izmerenim na PUMA eksperimentalnoj instalaciji. Model je zasnovan na jednodimenzionalnom modifikovanom homogenom izentropskom dvofaznom strujanju tečnosti i gasa. Homogeni model je modifikovan tako što je klizanje između faza uzeto u obzir pri određivanju zapreminskog udela gasne faze u dvofaznoj mešavini, što određuje i gustinu dvofazne mešavine. Klizanje između faza je sračunato na osnovu Chisholm korelacije koja je pogodna za niže vrednosti masenog protočnog udela gasne faze kao i na osnovu Zivi korelacije koja određuje maksimalnu vrednost klizanja između faza. Na mestu kritičnog isticanja brzina dvofazne mešavine je jednaka brzini zvuka i sračunata je na osnovu takozvanog zamrznutog modela dvofaznog strujanja koji ne uzima u obzir fazni prelaz. Ostvareni rezultati su prikazani i upoređeni zajedno sa dobro poznatim Fauske modelom dostupnim u literaturi. Pokazano je da Fauske model daje više vrednosti kritičnog masenog fluksa u odnosu na izmerene vrednosti, dok model prikazan u radu daje prihvatljiva slaganja sa izmerenim podacima.A model of two-component two-phase critical flow is presented. The modelling approach is based on one-dimensional homogeneous gas-liquid two-phase isentropic flow of mixture. The homogeneous model is modified by taking into account the void fraction and two-phase mixture density dependence on velocity slip. The velocity slip is calculated using Chisholm correlation that depends on the gas phase quality and Zivi correlation for the prediction of the maximum velocity slip values. At the location of the critical flow the two-phase mixture velocity equals sonic velocity and it is calculated with the so-called ''frozen sonic velocity' model. The model is validated against data measured in air water flow at the PUMA experimental facility. Obtained results are presented together with the predictions by the well-known Fauske model. It is shown that Fauske model overpredicts measured critical mass fluxes, while the present model shows acceptable agreement with the measured data

Skorohod-Olevsky viscous sintering model sensitivity to temperature distribution during the sintering process

U ovom radu proučavan je uticaj neuniformnosti temperaturnog polja po uzorku na rezultate simulacije procesa sinterovanja. Kao konstitutivni model ponašanja materijala upotrebljen je modifikovan fenomenološki Skorohod-Olevski viskozni model sinterovanja. Poređeni su rezultati simulacije dobijeni korišćenjem uniformne raspodele temperature, sa rezultatima koji koriste temperaturno polje dobijeno simulacijom zagrevanj auzorka. Poređenja se odnose na uzorke različiti hveličina, kako bi se ispitao različiti nivo neuniformnosti. Dobijeni rezultati u ovoj studiji pokazuju da postoje situacije u kojima je potrebno uzeti u obzir neuniformnost raspodele temperature po uzorku, odnosno da pojednostavljenje modela korišćenjem uniformne raspodele dovodi do neprihvatljivih grešaka.This paper investigates the influence of temperature field non-uniformity on sintering simulation results using the Skorohod-Olevsky viscous sintering model. As a difference to previous studies, here a thermal transient analysis is performed to provide a detailed temperature field over the component within sintering time. Results obtained using uniform temperature distribution are compared to those obtained using a nonuniform distribution derived from a transient thermal analysis. Results are compared for different geometry sizes, that lead to different temperature non-uniformity levels. The study has shown that the temperature nonuniformity cannot always be neglected and should be considered as a possible source of modeling error

Skorohod-Olevsky viscous sintering model sensitivity to temperature distribution during the sintering process

U ovom radu proučavan je uticaj neuniformnosti temperaturnog polja po uzorku na rezultate simulacije procesa sinterovanja. Kao konstitutivni model ponašanja materijala upotrebljen je modifikovan fenomenološki Skorohod-Olevski viskozni model sinterovanja. Poređeni su rezultati simulacije dobijeni korišćenjem uniformne raspodele temperature, sa rezultatima koji koriste temperaturno polje dobijeno simulacijom zagrevanj auzorka. Poređenja se odnose na uzorke različiti hveličina, kako bi se ispitao različiti nivo neuniformnosti. Dobijeni rezultati u ovoj studiji pokazuju da postoje situacije u kojima je potrebno uzeti u obzir neuniformnost raspodele temperature po uzorku, odnosno da pojednostavljenje modela korišćenjem uniformne raspodele dovodi do neprihvatljivih grešaka.This paper investigates the influence of temperature field non-uniformity on sintering simulation results using the Skorohod-Olevsky viscous sintering model. As a difference to previous studies, here a thermal transient analysis is performed to provide a detailed temperature field over the component within sintering time. Results obtained using uniform temperature distribution are compared to those obtained using a nonuniform distribution derived from a transient thermal analysis. Results are compared for different geometry sizes, that lead to different temperature non-uniformity levels. The study has shown that the temperature nonuniformity cannot always be neglected and should be considered as a possible source of modeling error

Analiza prikaza digitalnih modela terena primjenom različitih metoda interpolacije

In the paper is described the process of creating digital terrain models (DTM) using different interpolation methods. The analyses show the accuracy of the DTM obtained from topographic maps at different scales and using different interpolation methods. The quality and accuracy of DTM depends on the complexity of the terrain, data sources, and methods of height interpolation. The basic idea is the creation of the DTM for the selected area and the comparison of the results by applying appropriate interpolation methods. The aim of the research is to analyse the quality of the DTM model and to consider suitability of certain interpolation methods, based on the obtained results, i.e. their advantages and disadvantages. The experiment was done in the software environment ERDAS IMAGINE 2014.Rad opisuje postupak izrade digitalnih modela terena (DMT) primjenom različitih metoda interpolacije. Pritom se analiziraju prikaz i točnost DMT-a, dobiveni s topografskih karata različitih mjerila te primjenom različitih metoda interpolacije. Kvaliteta i točnost DMT-a ovise o složenosti reljefa, izvoru podataka, ali i o metodi interpolacije visina. Osnovna ideja u radu je kreiranje DMT-a za izabrano područje te primjenom odgovarajućih metoda interpolacije usporediti dobivene rezultate. Cilj istraživanja je analizirati kvalitetu DMT-a te na temelju dobivenih rezultata razmotriti pogodnost pojedinih metoda interpolacije, odnosno njihove prednosti i mane. Istraživanje je realizirano u softverskom okruženju ERDAS IMAGINE 2014

Fluid dynamic forces in the main steam pipeline of thermal power plant upon stop valves closure

A steam turbine trip is followed by a prompt closure of stop valves in front of the turbine and consequently to a pressure rise in the main steam pipeline. This steam hammer transient leads to the generation of intensive fluid dynamic forces that act along the pipeline axis and induce additional dynamic loads on the main steam pipeline. It is a common practice to assume a simultaneous closure of all stop valves in the safety analysis of the main steam pipeline. In the present paper computer simulations and analyses of the fluid dynamic forces are performed for several scenarios that take into account the possibility of delayed closure of the stop valve in front of the turbine. The influence of the failure of the steam by-pass line opening is considered too. The results show that the delay of the stop valve closure increases the maximum intensity of fluid dynamic force in the pipeline segment in front of the stop valve and decreases the intensity of fluid dynamic forces in segments along the pipeline. The failure of the by-pass line to open leads to prolonged steam pressure and fluid dynamic forces oscillation in pipeline segments. The simulations were performed with the in-house computer code based on the method of characteristics for the solving of the hyperbolic system of PDE that represent the mass, momentum and energy balance equations of the 1-D, compressible and transient fluid-flow. The obtained results are a support to safety analyses of thermal power plants under transient conditions

Fluid dynamic forces in the main steam pipeline of thermal power plant upon stop valves closure

A steam turbine trip is followed by a prompt closure of stop valves in front of the turbine and consequently to a pressure rise in the main steam pipeline. This steam hammer transient leads to the generation of intensive fluid dynamic forces that act along the pipeline axis and induce additional dynamic loads on the main steam pipeline. It is a common practice to assume a simultaneous closure of all stop valves in the safety analysis of the main steam pipeline. In the present paper computer simulations and analyses of the fluid dynamic forces are performed for several scenarios that take into account the possibility of delayed closure of the stop valve in front of the turbine. The influence of the failure of the steam by-pass line opening is considered too. The results show that the delay of the stop valve closure increases the maximum intensity of fluid dynamic force in the pipeline segment in front of the stop valve and decreases the intensity of fluid dynamic forces in segments along the pipeline. The failure of the by-pass line to open leads to prolonged steam pressure and fluid dynamic forces oscillation in pipeline segments. The simulations were performed with the in-house computer code based on the method of characteristics for the solving of the hyperbolic system of PDE that represent the mass, momentum and energy balance equations of the 1-D, compressible and transient fluid-flow. The obtained results are a support to safety analyses of thermal power plants under transient conditions

Boiling heat transfer modelling a review and future prospectus

This paper reviews the current status of boiling heat transfer modelling, discusses the need for its improvement due to unresolved intriguing experimental findings and emergence of novel technical applications and outlines the directions for an advanced modelling approach. The state-of-the-art of computational boiling heat transfer studies is given for: macro-scale boiling models applied in two-fluid liquid-vapour interpenetrating media approach, micro-, meso-scale boiling computations by interface capturing methods, and nano-scale boiling simulations by molecular dynamics tools. Advantages, limitations and shortcomings of each approach, which originate from its grounding formulations, are discussed and illustrated on results obtained by the boiling model developed in our research group. Based on these issues, we stress the importance of adaptation of a multi-scale approach for development of an advanced boiling predictive methodology. A general road-map is outlined for achieving this challenging goal, which should include: improvement of existing methods for computation of boiling on different scales and development of conceptually new algorithms for linking of individual scale methods. As dramatically different time steps of integration for different boiling scales hinder the application of full multi-scale methodology on boiling problems of practical significance, we emphasise the importance of development of another algorithm for the determination of sub-domains within a macro-scale boiling region, which are relevant for conductance of small-scale simulations

Analiza prikaza digitalnih modela terena primjenom različitih metoda interpolacije

In the paper is described the process of creating digital terrain models (DTM) using different interpolation methods. The analyses show the accuracy of the DTM obtained from topographic maps at different scales and using different interpolation methods. The quality and accuracy of DTM depends on the complexity of the terrain, data sources, and methods of height interpolation. The basic idea is the creation of the DTM for the selected area and the comparison of the results by applying appropriate interpolation methods. The aim of the research is to analyse the quality of the DTM model and to consider suitability of certain interpolation methods, based on the obtained results, i.e. their advantages and disadvantages. The experiment was done in the software environment ERDAS IMAGINE 2014.Rad opisuje postupak izrade digitalnih modela terena (DMT) primjenom različitih metoda interpolacije. Pritom se analiziraju prikaz i točnost DMT-a, dobiveni s topografskih karata različitih mjerila te primjenom različitih metoda interpolacije. Kvaliteta i točnost DMT-a ovise o složenosti reljefa, izvoru podataka, ali i o metodi interpolacije visina. Osnovna ideja u radu je kreiranje DMT-a za izabrano područje te primjenom odgovarajućih metoda interpolacije usporediti dobivene rezultate. Cilj istraživanja je analizirati kvalitetu DMT-a te na temelju dobivenih rezultata razmotriti pogodnost pojedinih metoda interpolacije, odnosno njihove prednosti i mane. Istraživanje je realizirano u softverskom okruženju ERDAS IMAGINE 2014

Boiling heat transfer modelling a review and future prospectus

This paper reviews the current status of boiling heat transfer modelling, discusses the need for its improvement due to unresolved intriguing experimental findings and emergence of novel technical applications and outlines the directions for an advanced modelling approach. The state-of-the-art of computational boiling heat transfer studies is given for: macro-scale boiling models applied in two-fluid liquid-vapour interpenetrating media approach, micro-, meso-scale boiling computations by interface capturing methods, and nano-scale boiling simulations by molecular dynamics tools. Advantages, limitations and shortcomings of each approach, which originate from its grounding formulations, are discussed and illustrated on results obtained by the boiling model developed in our research group. Based on these issues, we stress the importance of adaptation of a multi-scale approach for development of an advanced boiling predictive methodology. A general road-map is outlined for achieving this challenging goal, which should include: improvement of existing methods for computation of boiling on different scales and development of conceptually new algorithms for linking of individual scale methods. As dramatically different time steps of integration for different boiling scales hinder the application of full multi-scale methodology on boiling problems of practical significance, we emphasise the importance of development of another algorithm for the determination of sub-domains within a macro-scale boiling region, which are relevant for conductance of small-scale simulations