Ентропијска анализа рада и одређивање оптималних димензија и радних услова супротносмерних предајника топлоте

Predajnici toplote su jedan od najšire primenjivanih uređaja u procesnoj tehnici i termotehnici. Oni se tradicionalno projektuju tako da svojom konstrukcijom obezbede zahtevani toplotni protok sa grejnog na grejani fluid, pri čemu se potpuno zanemaruju gubici radnih sposobnosti ovih fluida. Radi toga, u ovom radu je prikazana detaljna analiza rada suprotosmernih predajnika toplote sa stanovišta Drugog principa termodinamike. Radi lakše matematičke iterperetacije, uzeto je da oba radna fluida imaju termofizička svojstva idealnih gasova i da su predajnici toplote dobošastog tipa. Pokazano je, da se smanjenje gubitaka radne sposobnosti, osim izjednačavanjem toplotnih kapaciteta fluidnih tokova, može ostvariti i pravilnim izborom geometrijskih parametara predajnika toplote

Ентропијска анализа рада и одређивање оптималних димензија и радних услова супротносмерних предајника топлоте

Predajnici toplote su jedan od najšire primenjivanih uređaja u procesnoj tehnici i termotehnici. Oni se tradicionalno projektuju tako da svojom konstrukcijom obezbede zahtevani toplotni protok sa grejnog na grejani fluid, pri čemu se potpuno zanemaruju gubici radnih sposobnosti ovih fluida. Radi toga, u ovom radu je prikazana detaljna analiza rada suprotosmernih predajnika toplote sa stanovišta Drugog principa termodinamike. Radi lakše matematičke iterperetacije, uzeto je da oba radna fluida imaju termofizička svojstva idealnih gasova i da su predajnici toplote dobošastog tipa. Pokazano je, da se smanjenje gubitaka radne sposobnosti, osim izjednačavanjem toplotnih kapaciteta fluidnih tokova, može ostvariti i pravilnim izborom geometrijskih parametara predajnika toplote

IMPACT OF GRAZING ON SOIL ORGANIC MATTER AND PHYSICAL PROPERTIES OF A FLUVISOL IN NORTWEST SERBIA

The effects of long-term (>20 yr) grazing on the selected physical properties of a non carbonated silty-clay Fluvisols were studied in the region of the Kolubara Valley, Northwest Serbia. Two adjacent land-use types (native deciduous forest and natural pasture soils converted from forests for more than 20 years) were chosen for the study. Disturbed and undisturbed soil samples were collected from three sites at each of the two different land-use types from the depths of 0–15, 15–30 and 30–45 cm. In relation to the soil under native forest, soil organic matter content, total porosity and air-filled porosity were significantly reduced after long-term of grazing. The bulk density (0.99–1.48 g cm–3) and the saturated hydraulic conductivity (6.9.10–2–3.2.10–4 cm s–1) were significantly lower in forest compared to the adjacent pasture (ex-forest) soil (1.49–1.55 g cm–3 and 3.4.10–4–5.5.10–4 cm s–1, respectively). In addition, forest had significantly lower dry mean weight diameter (7.0–9.2 mm) and greater wet mean weight diameter (2.0–2.6 mm) for 0–45 cm depth compared with the pasture (8.8–9.4 mm and 1.8–2.3 mm, respectively). The decrease of soil organic matter content and reduction in aggregate stability under long-term grazing rendered the soil more susceptible to compaction. In conclusion, the results of this study indicate that removal of permanent vegetation in the conversion process from forest areas to pasture land may lead to loss of soil productivity and serious soil degradation. Obviously, there is a need for greater attention to developing sustainable land use practices in management of these ecosystems to prevent further degradation of pasture soils in the region

Variation of Soil Structure in the Foot and Toe Slopes of Mt. Vukan, East-central Serbia

This paper presents the variation of soil structure along the foot and toe slopes of Mt. Vukan, East-Central Serbia. The analysis of aggregate size distribution and structure indices were conducted by means of soil units, characteristic soil horizons and elevation differences along the study area. Soils of Great Field located at different elevations were found to have significant variation in ASD and soil structure indices. Topsoil horizon of Eutric Cambisols have higher MWD after dry sieving, but at the same time it has the highest variation in MWD after wet sieving, indicating low water stability, which is opposite to the coefficient of aggregability. We share an opinion that change in MWD better depicts soils structure stability to water. The results of correlation analysis indicated that clay content is correlated more to structure indices compared with SOM content. SOM is significantly correlated with ASD and soil structure indices only in Calcomelansols, whereas the significant correlation of clay content and soil structure is more evident in Eutric Cambisols and Non-calcaric Chernozems, compared with other soil units. Soil structure variation along the lowest chain of Catena might be strong, and that it has to be analyzed from the point of view of soil unit and their corresponding soil horizons