No abstract

Spray cooling of hot steel surfaces is an inherent part of continuous casting and heat treatment. When we consider the temperature interval between room temperature and for instance 1000 °C, different boiling regimes can be observed. Spray cooling intensity rapidly changes with the surface temperature. Secondary cooling in continuous casting starts when the surface temperature is well above a thousand degrees Celsius and a film boiling regime can be observed. The cooled surface is protected from the direct impact of droplets by the vapour layer. As the surface temperature decreases, the vapour layer is less stable and for certain temperatures the vapour layer collapses, droplets reach the hot surface and heat flux suddenly jumps enormously. It is obvious that the described effect has a great effect on control of cooling. The surface temperature which indicates the sudden change in the cooling intensity is the Leidenfrost temperature. The Leidenfrost temperature in spray cooling can occur anywhere between 150 °C and over 1000 °C and depends on the character of the spray. This paper presents an experimental study and shows function for prediction of the Leidenfrost temperature based on spray parameters. Water impingement density was found to be the most important parameter. This parameter must be combined with information about droplet size and velocity to produce a good prediction of the Leidenfrost temperature.

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.

The cooling of vertically moving strips is used very often to obtain the required material properties. Water spray cooling has to be used when a high cooling intensity is needed. Our Heat Transfer and Fluid Flow Laboratory is equipped with a testing device which allows vertical movement of a heated experimental plate (sheet). Two different sizes of flat-jet nozzles were tested with different water pressures and angles of the water impact (inclination angles of the spraying bar). The water-pressure range was between 2 bar and 9.3 bar and the angle of the water impact changed from 20°to 40°. The dependence of the heat-transfer coefficient on the surface temperature was evaluated for each experiment. Interesting results were obtained from the comparison of these experimental results, showing that the heat-transfer coefficient and the Leidenfrost temperature increase with the increasing water pressure. Very interesting results were obtained during the tests with different inclination angles. The highest heat-transfer coefficient was obtained for the angle of 20°and the lowest value of the heat-transfer coefficient was obtained for the angle of 40°at the surface temperatures of around 200°C. Keywords: spray cooling, flat-jet nozzles, impact angle, water impingement density, Leidenfrost temperature Ohlajanje vertikalno premikajo~ih se trakov se pogosto uporablja za zagotovitev zahtevanih lastnosti materiala. Kadar je potrebna velika intenzivnost hlajenja, se uporablja ohlajanje z brizganjem vode. Laboratorij za prehajanje toplote in tok fluidov je opremljen s preizkusno napravo, ki omogo~a vertikalno premikanje eksperimentalne plo{~e (jeklo). Preizku{eni sta bili dve razli~ni dimenziji {ob pri razli~nih tlakih vode in razli~nih kotih pr{enja vode (nagibni kot palice za brizganje). Obmo~je tlaka vode je bilo med 2 bar in 9,3 bar, kot vodnega curka pa med 20°in 40°. Za vsak poskus je bila ocenjena odvisnost koeficienta prehajanja toplote od temperature povr{ine. Dobljeni so bili zanimivi rezultati iz primerjave eksperimentalnih podatkov, ki ka`ejo, da koeficient prehajanja toplote in Leidenfrostova temperatura nara{~ata z ve~anjem tlaka vode. Zanimive rezultate smo dobili tudi pri poskusih z razli~nimi vpadnimi koti. Najvi{ji koeficient prehajanja toplote je bil dose`en pri kotu 20°, najni`ja vrednost koeficienta prehajanja toplote pa je bila dose`ena pri kotu 40°p ri temperaturi povr{ine okrog 200°C. Klju~ne besede: hlajenje z brizganjem, {obe s plo{~atim curkom, vpadni kot curka, gostota udarca vode, Leidenfrostova temperatura

Steel production and processing are connected with high temperatures. Due to a reaction between hot surface of the steel and oxygen contained in surrounding atmosphere, oxides are formed on the surface of the steel. Created layer of oxides is called scales and has influence on cooling and quality of steel. Thickness and structure of scale layer are influenced by chemical composition of the steel, temperature and atmosphere during oxidation. Scale layer can be considerably porous which has a significant influence on thermal conductivity of this layer, because air pores have much lower thermal conductivity compared to scales. Steel samples were prepared and porosity of scale layer was studied. Further, the average thermal conductivity of porous scale layer was determined for different regimes of oxidation by FEM modelling. It was found that the average thermal conductivity of porous scale layer is influenced not only by porosity of scale layer, but also by distribution of air pores, which can has a significant effect.