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

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

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

Coherence lengths and anisotropy in MgB2 superconductor

Field and temperature microwave measurements have been carried out on MgB2 thin film grown on Al2O3 substrate. The analysis reveals the mean field coherence length xi_{MF} in the mixed state and a temperature independent anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the superconducting transition, the scaling of the fluctuation conductivity yields the Ginzburg-Landau coherence length with a different anisotropy ratio gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR

Relating Ettringite formation and rheological changes during the Initial cement hydration : a comparative study applying XRD analysis, rheological measurements and modeling

In order to gain a deeper understanding of the rheological development of hydrating ordinary Portland cement (OPC) pastes at initial state, and to better understand their underlying processes, quantitative X-ray diffraction (XRD) analysis and rheological measurements were conducted and their results combined. The time-dependent relation between phase development and flow behavior of cement paste was investigated at two different temperatures (20 and 30 °C), over a period of two hours. Regarding the phase development during hydration, ettringite precipitation was identified as the dominant reaction in the first two hours. For both temperatures, the increasing ettringite content turned out to correlate very well with the loss of workability of the reacting cement paste. An exponential relationship between ettringite growth and flow behavior was observed that could be explained by applying the Krieger-Dougherty equation, which describes the influence of solid fraction on the viscosity of a suspension.DFG, 386869775, Rheologie wirksame Additive in Portlandzement-basierten Formulierungen - Von nano/mikro-skaligen Strukturen zu makroskopischen EigenschaftenDFG, 387065993, Formfüllungsvermögen von Frischbetonen - ein zeit- und hydratationsabhängiger Ansat

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

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