Heat Transfer and Fluid Flow Laboratory (HeatLab) is specialized in the research and application of spray cooling and hydraulic descaling processes. The research covers applications of spray cooling in processes such as continuous casting, hot and cold rolling and heat treatment.

Accurate thermal boundary conditions are essential for the correct modelling of thermal processes in the industry. Cooling, for instance, has a significant influence on a material microstructure, which is critical for final mechanical properties. Therefore, accurate control of a temperature field in the material body enables precise control of a final product's quality. Advanced cooling studies and simulations lead to achieving, for instance, better flatness of metal strips during heat treatment, saving water consumption during work rolls cooling, minimizing undesirable non-homogenous overcooling during continuous casting, etc.

Scales formed on the steel surface when exposed to high temperature have a significant negative influence on final product quality, such as surface roughness and quality, mechanical properties (scales influence heat transfer during the heat treatment process), product flatness, etc. Therefore, scales have to be removed to ensure the required steel standards.

HeatLab is equipped with unique experimental devices that enable the complex study of the water impact pressure distribution and the simulation of cooling processes as close as possible to real process. Laboratory equipment allows spraying on a moving samples of various shapes and dimensions (plate, tube, roll, rail, etc.), which are embedded by thermocouples. High-performance data logging (10 kHz for each channel), together with in-house advanced inverse calculations software, enables detailed determination of a heat transfer coefficient. Heatlab is able to measure thermophysical material properties and thermal resistance between two surfaces.

More information is available at: https://www.heatlab.cz/research/

Secondary cooling in continuous casting process

To design a secondary continuous casting cooling unit, the heat transfer coefficient at the slab surface, dependent on position and surface temperature, needs to be known for various conditions. There is no accurate analytical solution, the problem has to be solved experimentally. The process is simulated in the laboratory by the relative movement of the tested specimen and the spraying nozzles. The specimen for the test is made of an austenitic plate (typically 600x320x25 mm) with a set of thermocouples inset. The spraying nozzle (or set of nozzles) is moved under the heated experimental plate (1200°C) reversibly and it cools the surface only in forward movement. Datalogger records measured temperatures at a depth of 2 mm under the surface together with the nozzle position. The inverse calculation is used to determine a heat transfer coefficient dependence on the position and surface temperature. The measured heat transfer coefficient is used for the description of the cooling process and its optimization through cooling simulations.

Device for heat transfer coefficient measurement at very high temperatures

Cooling of work rolls

The cooling of work rolls is an important process in the hot rolling technology and cannot be taken as a marginal issue. The HeatLab designs cooling of rolls to reach their long working life with respect to energy savings and carbon footprint. For the roll cooling investigation, the surface temperature and heat transfer coefficient at the roll needs to be determined. The only reliable method of describing heat transfer under industrial conditions is to perform measurements either during the rolling campaign or in the lab. However, the measurements carried out in the lab using eligible experimental methods are much cheaper and less time-consuming. A specialized rotating test benches were developed to simulate cooling processes on rotating surfaces. The roll diameter is 650 mm (350 mm is also available) and the width is 600 mm. The roll is equipped with a test segment with 8 thermal sensors connected to the data logger. The data obtained by the thermocouples during the roll cooling experiment gives information about temperature history at defined positions 0.4 mm under the roll surface. Each data point is also connected to information about the angle of roll rotation and thus, the data collected in time order can be converted into position order. The measured temperatures are used to calculate the heat transfer coefficient and surface temperature history.

Heat transfer in the roll gap is simulated by small punch tests where the heat transfer coefficient between the slab and roll surface is determined for specified industrial conditions. Heat transfer coefficient boundary conditions are used in the developed software SimRoll. It enables the computation of temperatures on the surface and inside the working roll and thermal crown during rolling campaign. Another software, SimCool, enables the computation of temperatures on the surface and inside the slab, as well as spray and HTC patterns. Commercial software such as Ansys and Comsol Multiphysics are also widely used during design and optimization. Typical result of simulation is temperature distribution in the work roll and stress analysis which can extend the service life of the work rolls. Cooling optimization in the rolling process leads usually to decrease water consumption up to 50% depending on the condition of the rolling line.

Measurement of heat transfer coefficient during work roll cooling

Product cooling

The HeatLab is capable of designing cooling headers for interstand cooling at hot strip mills and for cooling systems in continuous annealing lines. The design is optimized using a laboratory-developed linear test benches for horizontal and vertical movements. They allow the experimental study of spray cooling for various nozzle types, pressures or header configurations for coolant temperatures in the range of 20-90°C. One linear stand is a 8 meters long rotatable frame with a trolley and designed for horizontal movement. The austenitic experimental plate (typical dimensions of 320 x 300 x 25 mm) is embedded with thermal sensors and fixed on the trolley. The plate is moved through a cooling zone with a maximal velocity of 10 ms-1. Thermocouples are connected with a datalogger that records temperatures (in the depth of 0.8 mm under the sprayed surface) and the corresponding plate position in the cooling zone. Another stand is for vertical movement which is typical in continuous annealing lines. Temperatures recorded during experiments are used in inverse calculation and heat transfer coefficient distribution on the position and surface temperature is determined. The boundary condition is used to create a numerical model of the temperature field in the cooled material. The numerical model predicts the controllability of the cooling device and assists in the preparation and adjustment of newly designed or redesigned cooling units in a real plant.

Heat transfer coefficient measurement

https://www.youtube.com/watch?v=IhXS0MWQo_8

Developments of high-pressure hydraulic descaling

Hydraulic descaling is a process in the steel manufacturing industry, particularly in continuous casting and hot rolling. The goal of descaling is removing oxide layers from a hot steel surface with minimal cooling intensity. Scales formed on the steel surface have a significant negative influence on final product quality, such as surface roughness and quality, mechanical properties (scales influence heat transfer during the heat treatment process), product flatness, etc. Therefore, scales have to be removed to ensure the required steel standards.

The descaling process involves mainly impinging the steel surface with high-pressure water jets (feeding water pressure of 10 – 40 MPa). The impact pressure and thermal shock from the clusters of water disturb and remove the scale layer. An alternative descaling method is shot blasting, where small steel spheres are shot on the surface.

Critical aspects of hydraulic descaling are impact pressure distribution and strip temperature distribution. HeatLab uses self-developed devices and software to study and optimize descaling processes. HeatLab is able to measure static impact pressure distribution for sets of nozzles. The linear stand device allows the study of heat transfer processes on hot-moving surfaces where a heat transfer coefficient dependence on the surface temperature is a typical result. This boundary condition is used for thermal analysis of the cooling process caused by impinging water and it can be minimized. The quality of the descaling process is verified using hot quality descaling experiments for given type of nozzles and spray patterns. A real material is heated and oxidized and then moved through the descaling section. The descaling quality is studied by measuring the residual scale layer and performing image analysis using optical microscopy and scanning electron microscope.

Important aspect of descaling is its energy requirement, when the descaling system consumes upto 70 % of the total energy of the rolling mill, depending on the condition of the rolling line. This means that improving this system leads to significant savings and a reduction in the CO2 footprint

Hydraulic descaling experiment

Hydraulic descaling – the influence of water jet overlap on cooling intensity