Unaprijeđena metoda za određivanje toplinske difuzivnosti prehrambenih proizvoda

Iako su toplinska svojstva prehrambenih proizvoda od velike važnosti za prehrambenu tehnologiju, toplinska difuzivnost složenih materijala uglavnom nije poznata i potrebno ju je odrediti eksperimentalno. Za određivanje toplinske difuzivnosti prehrambenih proizvoda predložena je jednostavna eksperimentalna metoda temeljena na praćenju temperaturnog odziva valjkastog uzorka pri nagloj promjeni temperature stijenke materijala. U radu su opisana i uspoređena tri pristupa procjene parametra toplinske difuzivnosti na temelju eksperimentalnih podataka. Oni se zasnivaju na uporabi: 1) linearne ili 2) nelinearne regresije analitičkog modela te 3) numeričkog inverznog rješavanja modela procesa prijenosa topline. Numerički model zamjenjuje pretpostavku idealne skokovite promjene temperature stijenke, s izmjerenom naglom promjenom temperature stijenke materijala. Ustanovljeno je da pretpostavka skokovite (idealne) promjene temperature stijenke materijala može rezultirati značajnom sustavnom pogrješkom s nižom procjenom toplinske difuzivnosti. Za jednostavnu numeričku obradu rezultata predstavljen je računalni potprogram napisan u MATLAB-u s razvijenim grafi čkim sučeljem koji je pridružen javnoj domen

Tumačenje mikrovalnog magnetootpora iznad i ispod Tc u supravodljivim monokristalima YBa2Cu3O7−δ

Field dependence of the microwave surface resistance in YBa2Cu3O7−δ single crystal is analysed in terms of the mixed state and superconducting fluctuations. Careful fitting of the appropriate parts of the magnetoresistance curves yields the upper critical field, and mean field Tc. The microwave fluctuations conductivity is found to scale as predicted for a three dimensional (3D) superconductor.Analizirana je magnetska ovisnost mikrovalnog površinskog otpora monokristala visokotemperaturnog supravodiča YBa2Cu3O7−δ u području miješanog stanja i u području supravodljivih fluktuacija. Prilagođivanjem magnetootpornih ovisnosti u području miješanog stanja na teorijske izraze (teorija efektivne vodljivosti) dobiva se gornje kritično polje Bc2 i temperatura faznog prijelaza Tc u aproksimaciji srednjeg polja. Mikrovalna fluktuacijska vodljivost se svodi na istu mjeru (skalira) kao što je i predviđeno za 3D supravodič

Testing of Concrete Abrasion Resistance in Hydraulic Structures on the Lower Sava River

The paper deals with the issues of resistance of concrete linings to long-term abrasion loading caused by waterborne particles, particularly for the proposed hydro power plants on the Sava River in Slovenia. The main purpose of the research work was to define the possibility of forecasting the process of concrete lining wear on the Sava River dam structures based on the standard procedures of abrasion resistance testing. Abrasion resistance of concrete has been researched in accordance with the standard ASTM C 1138 and Böhme (DIN 52108) methods. The research work was based on a comparison between laboratory results and measurements of abrasion resistance of concrete under natural conditions by performing test plots in the stilling basin of the Vrhovo HPP. Concrete composites with different mechanical properties have been analysed within the research programme. The analysis showed a qualitative similarity of the level of concrete abrasion between laboratory simulations and measurements in the field, as well as suitability of the ASTM C 1138 laboratory method for the assessment of\ud abrasion resistance of concretes in the spillway of the HPP chain on the Lower Sava River

Tumačenje mikrovalnog magnetootpora iznad i ispod Tc u supravodljivim monokristalima YBa2Cu3O7−δ

Field dependence of the microwave surface resistance in YBa2Cu3O7−δ single crystal is analysed in terms of the mixed state and superconducting fluctuations. Careful fitting of the appropriate parts of the magnetoresistance curves yields the upper critical field, and mean field Tc. The microwave fluctuations conductivity is found to scale as predicted for a three dimensional (3D) superconductor.Analizirana je magnetska ovisnost mikrovalnog površinskog otpora monokristala visokotemperaturnog supravodiča YBa2Cu3O7−δ u području miješanog stanja i u području supravodljivih fluktuacija. Prilagođivanjem magnetootpornih ovisnosti u području miješanog stanja na teorijske izraze (teorija efektivne vodljivosti) dobiva se gornje kritično polje Bc2 i temperatura faznog prijelaza Tc u aproksimaciji srednjeg polja. Mikrovalna fluktuacijska vodljivost se svodi na istu mjeru (skalira) kao što je i predviđeno za 3D supravodič

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES

Calcium Sulfoaluminate Eco-Cement from Industrial Waste

In this paper, the potential benefits offered by calcium sulfoaluminate cement (CSA) production from industrial wastes or by-products already present in Republic of Croatia have been addressed. A variety of industrial wastes, namely phosphogypsum (PG), coal bottom ash (BA) and electric arc furnace slag (EAFS) were used as raw materials to provide additional environmental advantages in production of CSA. Mass fraction of Ye’elimite, the principal hydraulic mineral in the prepared CSA was determined by quantitative X-ray powder diffraction. Calculated phase composition based on 18 sub-systems of five-component assemblages, each containing five clinker phases, showed promising results. The hydration behavior of CSA clinkers was investigated by calorimetric and Vickers hardness tests. In conclusion, CSA production offers an alternative and feasible method of industrial waste minimization

Numerical phase-field model validation for dissolution of minerals

Modelling of a mineral dissolution front propagation is of interest in a wide range of scientific and engineering fields. The dissolution of minerals often involves complex physico-chemical processes at the solid–liquid interface (at nano-scale), which at the micro-to-meso-scale can be simplified to the problem of continuously moving boundaries. In this work, we studied the diffusion-controlled congruent dissolution of minerals from a meso-scale phase transition perspective. The dynamic evolution of the solid–liquid interface, during the dissolution process, is numerically simulated by employing the Finite Element Method (FEM) and using the phase–field (PF) approach, the latter implemented in the open-source Multiphysics Object Oriented Simulation Environment (MOOSE). The parameterization of the PF numerical approach is discussed in detail and validated against the experimental results for a congruent dissolution case of NaCl (taken from literature) as well as on analytical models for simple geometries. In addition, the effect of the shape of a dissolving mineral particle was analysed, thus demonstrating that the PF approach is suitable for simulating the mesoscopic morphological evolution of arbitrary geometries. Finally, the comparison of the PF method with experimental results demonstrated the importance of the dissolution rate mechanisms, which can be controlled by the interface reaction rate or by the diffusive transport mechanism.Fil: Yang, Sha. Universitat Technische Darmstadt; AlemaniaFil: Ukrainczyk, Neven. Universitat Technische Darmstadt; AlemaniaFil: Caggiano, Antonio. Universitat Technische Darmstadt; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; ArgentinaFil: Koenders, Eddie. Universitat Technische Darmstadt; Alemani

Simple Model for Alkali Leaching from Geopolymers: Effects of Raw Materials and Acetic Acid Concentration on Apparent Diffusion Coefficient

This paper investigates alkali leaching from geopolymers under various concentrations of acetic acid solutions. The effects of the raw metakaolin purity as well as fly ash-based geopolymer mortars and pastes are considered. A new methodology for (acetic) acid attack is proposed, adapting standard approaches, where the concentration of the leached alkali in the exposure solution is measured over time. The applicability of a simple diffusion-based mathematical model to determine the apparent diffusion coefficient (Dapp) for geopolymer pastes and mortars was validated. At the end of the paste tests, microstructural alterations of the specimens’ cross-sections were analyzed microscopically, revealing occurrence of degradation across the outermost surface parts and, especially under acid attack, the formation of long cracks that connect the surface with the intact inner zone. Drastically different Dapp are discussed in terms of the differences in the mix designs, principally resulting in different alkali-binding capacities of the geopolymers, while the acid promoted dissolution and increased porosity. As a result of this interpretation, it was concluded that Dapp is governed mainly by the chemistry of the alkali release from the gel, as it overruled the effects of porosity and cracks

Interaction of salicylic acid and its derivates with calcite crystals of different morphologies

Istraživanja interakcija kalcijeva karbonata s aktivnim farmaceutskim tvarima od velike su važnosti za njegovu potencijalnu primjenu kao mineralnog nosača lijekova. U ovom radu istraženi su mehanizmi i dosezi interakcija malih organskih molekula, korištenih kao modelne aktivne farmaceutske tvari, s dobro definiranim kristalnim sjemenom kalcita različite strukture površine. Razrađen je postupak priprave čistog, monodisperznog i dobro definiranog kalcita u taložnom sustavu Ca(OH)2(s)-H2O(l)-CO2(g). Kao modelne molekule korištene su stearinska kiselina i salicilna kiselina te njeni funkcionalni derivati. Istražena je adsorpcija stearata na kristalima kalcita uz korištenje spektroskopskih i termogravimetrijskih mjerenja, kao i mjerenja hidrofobnosti površine. Određena je minimalna koncentracija stearata potrebna za formiranje monosloja za različite morfologije kalcita. Istražena je kinetika rasta kristalnog sjemena uz dodatak salicilne kiseline, 5-amino salicilne kiseline, N-saliciloil-L-asparaginske kiseline i N-saliciloil-L-glutaminske kiseline. Utvrđeno je da se rast kalcita zbiva na spiralnoj dislokaciji te da skalenoedarski kalcit raste sporije od romboedarskoga. Pronađeno je da se funkcionalni derivati salicilne kiseline preferirano adsorbiraju na specifična mjesta na površini i inhibiraju kristalni rast, pri čemu N-saliciloil-L-asparaginska kiselina ima najizraženiji utjecaj. Na temelju analize kinetičkih parametara zaključeno je o mehanizmu interakcija. Rezultati detaljnih kinetičkih mjerenja su dodatno poduprti rezultatima adsorpcijskih, spektroskopskih i akustoforetskih mjerenja. Određen je maksimalni doseg adsorpcije salicilne kiseline i njenih funkcionalnih derivata pri čemu su kemijska i konformacijska svojstva pripravljenih derivata korelirana sa strukturom površine čvrste faze.Investigation of basic interactions between calcium carbonates and active pharmaceutical compounds is of great importance because of the possibility to use the carbonates as a mineral carrier in drug delivery systems. In this study the mode and extent of interactions of small organic molecules, chosen as pharmaceutically relevent model compounds, with calcite crystals of different morphologies are described. A method for the preparation of pure, monodispersed and well-defined calcite crystals in the Ca(OH)2(s)-H2O(l)-CO2(g) precipitation system was developed. Stearic acid, salicylic acid and its functional derivatives were used as model active pharmaceutical substances. Adsorption of stearate was studied by FT-IR spectroscopy, thermogravimetric analysis and hydrophobicity measurements. Minimal amount of stearate necessary to build up the monolayer and coresponding cross sectional area of one stearate molecule were estimated for different calcite morphologies. The crystal growth kinetics of well defined rhombohedral and scalenohedral calcite seed crystals in the systems containing salicylic acid, 5-aminosalicylic acid, N-salicyloil-L-aspartic acid or N-salicyloil-L-glutamic acid, were investigated. The analysis of the crystal growth kinetic parameters indicated that the crystal growth of calcite is controlled by integration of growth units into the spiral step. Growth rate of scalenohedral calcite is found to be lower than for rhombohedral calcite. The addition of functional salicylate derivates into the precipitation systems caused the inhibition of their growth as well as change of growth mechanism. Based on the kinetic parameters, conclusions on the mechanisms of interactions were gained. The results indicated that salicylate molecules preferentially adsorb along the steps on the growing calcite surfaces. The results of crystal growth kinetic experiments were additionally supported with the results of the appropriate adsorption, spectroscopic and acustophoretic measurements. The extent of adsorption of salicylates was determined. Chemical and conformational properties of used functional derivatives were correlated with different surface structures of calcite crystals