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.

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

We present a new Nusselt number correlation for spray cooling at large Reynolds numbers and high surface temperatures for water sprays impinging perpendicularly onto a flat plate. A large set of experimental data on spray cooling of hot surfaces with water has been analyzed, including the water temperature effects. For large-scale cooling, such as in industrial processes, large number of injection parameters such as number, type, pressure, and angle of the spray injection has led to a multitude of correlations that are difficult for general and practical applications. However, by synthesizing a set of experimental data where all of the above parameters have been varied, we find that the Nusselt number and therefore the heat transfer coefficient can be cast accurately as a function of the Reynolds number. Water is widely used as the coolant during spray cooling, and has a specific phase change characteristic. At large Reynolds number (Re > 100,000) and surface temperature (Ts > 600˚C) ranges, which are of interest in large-scale spray cooling, the effect of water temperature is quite significant as it affects the film boiling close to the surface. This effect also has been parameterized using experimental data.

Heat treatment is increasingly used in the heavy industry. The main advantage of this method is the achievement of the required material and mechanical properties. Heat treatment allows for a manufacturing process, which can improve product performance by increasing the steel strength, hardness and other desirable characteristics. The microstructure, grain size and chemical composition of steel affect its overall mechanical behavior. Heat treatment is an efficient way to manipulate the properties of a steel product by controlling the cooling rate. It can be expressed using the heat-transfer coefficient (HTC). The controllability of the cooling process is very important. Mist and water nozzles may provide good controllability of the HTC. An experimental stand was designed and built. The stand consists of a movable trolley with a test sample, which moves under a spray at a given velocity. Sensors record the temperature history of the tested material. This experimental stand enables simulations of a variety of cooling regimes and evaluations of the final structures of tested samples. The same experimental stand is also used for designing cooling sections in order to determine the required heat-treatment procedures and the final structures. This paper describes a cooling-section design procedure for obtaining the required structure and mechanical properties of rails. Keywords: heat transfer, heat treatment, cooling, heat-transfer coefficient, spray cooling Uporaba toplotne obdelave se v te`ki industriji pove~uje. Glavna prednost te metode je, da se dose`e zahtevane mehanske lastnosti materiala. Toplotna obdelava omogo~a postopke izdelave, ki lahko izbolj{ajo lastnosti proizvodov s tem, da pove~ajo trdnost jekla, trdoto in druge za`eljene zna~ilnosti. Mikrostruktura, velikost zrn in kemijska sestava jekla vplivajo na mehanske lastnosti. Toplotna obdelava je u~inkovita pot za vplivanje na lastnosti jeklenega proizvoda s kontroliranjem hitrosti ohlajanja. Lahko se jo izrazi z uporabo koeficienta prenosa toplote. Mo`nost kontrole postopka ohlajanja je zelo pomembna. Obvladanje procesa ohlajanja je zelo pomembno. Vodna para in vodne {obe omogo~ajo dobro kontrolo koeficienta prenosa toplote (angl. HTC). Na~rtovano in postavljeno je bilo eksperimentalno stojalo. Stojalo sestoji iz vozi~ka z vzorcem, ki se pomika pod {obe z dano hitrostjo. Senzorji bele`ijo temperaturno zgodovino vzorca. Eksperimentalno stojalo omogo~a simulacijo razli~nih re`imov ohlajanja in oceno kon~ne mikrostrukture preizku{enega vzorca. Isto stojalo je uporabno tudi kot orodje pri na~rtovanju hladilnih odsekov za dolo~anje postopka toplotne obdelave in kon~ne mikrostrukture.^lanek opisuje postopek na~rtovanja odseka za izvajanje hlajenja, za zagotavljanje`eljene mikrostrukture in mehanskih lastnosti`elezni{kih tirnic.