Analysis of oil flow distribution in power transformer winding cooling ducts using temperature measurements

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In this paper a method of temperature front measurement is proposed to quantify flow distribution within the cooling ducts of a power transformer winding. The winding will be prefilled with transformer oil at a certain temperature, and oil at another temperature pumped into the system. Velocities can then be estimated by keeping track of the oil temperature change in the oil ducts using temperature sensors. Due to the effect of buoyancy, the velocity field will not be constant during the temperature front measurement but with the help of transient CFD modeling the variation can be taken into account. The method has been successfully tested on a mock-up model of a transformer oil-guided disc winding section, and a comparison has been performed with steady state measurements using Laser Doppler Anemometry as well as both steady and unsteady CFD modeling results. Application of the temperature front method in real transformers will also be discussed.dc201

Numerical prediction of the transport and pyrolysis in the interior and surrounding of dry and wet wood log

The numerical simulation of the pyrolysis process of a dry and wet birch wood log in a cylindrical heating chamber is preformed. The model includes the flow inside and outside the porous wood log and accounts for convective, conductive and radiative heat transfer. A two-step pyrolysis reaction scheme is used to model the conversion from wood to tar, gas and char. The results of the simulations compare well with the authors experimental data which are presented in terms of radial temperature distribution and mass reduction, for both dry and wet cases. Our transient simulations provide us with the detailed flow field inside and outside the wood log. It clearly shows not only the existence but also the structure of the pyrolysis gas plumes leaving the wood. These plumes have only been visualised experimentally by few authors [Brackmann C et al. Optical and mass spectroscopy study of the pyrolysis gas of wood particles. Appl Spectros 2003;57(2):216-22, [12]] without any quantitative measurements and the present investigation gives a realistic estimation that we presently use to evaluate its impact on the heat and mass transfer, and on the momentum balance and the particle dispersion in a near future work. The gas plumes have a maximum velocity magnitude ranging between 0.1 and 0.2 m s-1 and vanish when all the wood gas is produced. It is shown that increasing the convective flow around the wood log do not significantly modify the pyrolysis gas plume structure and seems to have small effect on the overall heating and the pyrolysis process which are mainly controlled by the thermal radiation from the hot surrounding walls.Wood pyrolysis Porous media Mass transfer Heat transfer Computational fluid dynamics

Computational fluid dynamics modeling of real scale slab reheating furnace

Understanding the complex transient flow and heat transfer characteristics inside the furnace is a precondition to optimize furnace operation while maintaining high standard steel products. In the industrial furnaces often hot spots have been observed inside the slabs due to the current alignment of the burners. The control of the slab heating uses surface temperature as boundary condition, and it is thus important to predict this as accurate as possible. Transient flow behavior and interactions between the burners make it challenging to find an optimal burner arrangement. However, Computational Fluid Dynamics (CFD) is one of the robust methods which can be used to simulate the flow and perceive the complexity in combustion and radiation associated with the furnace. The scope of this work is to simulate the slab reheating furnace to understand the flow pattern and provide the results as input to simplified zonal radiation method to use it in online control system in real plant. In this paper we present the CFD simulation results for a real scale slab reheating furnace in 3D and compare them with measurements. The simulation predicts the gas temperature similar to the measurement points. However, the slab surface and average temperature deviate from continuous curve generated from the interpolation based on measurements from few thermocouples. The flow pattern and temperature distribution suggests the interpolation method to improve using the results from the simulation.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Visualization of gas–liquid mass transfer and wake structure of rising bubbles using pH-sensitive PLIF

A planar laser-induced fluorescence (PLIF) technique for visualizing gas–liquid mass transfer and wake structure of rising gas bubbles is described. The method uses an aqueous solution of the pH-sensitive dye Naphthofluorescein and CO2 as a tracer gas. It features a high spatial resolution and frame rates of up to 500 Hz, providing the ability to capture cinematographic image sequences. By steering the laser beam with a set of two programmable scanning mirrors, sequences of three-dimensional LIF images can be recorded. The technique is applied to freely rising bubbles with diameters between 0.5 and 5 mm, which perform rectilinear, oscillatory or irregular motions. The resulting PLIF image sequences reveal the evolution of characteristic patterns in the near and far wake of the bubbles and prove the potential of the technique to provide new and detailed insights into the spatio-temporal dynamics of mass transfer of rising gas bubbles. The image sequences further allow the estimation of bubble size and rise velocity. The analysis of bubble rise velocities in the Naphthofluorescein solution indicates that surfactant-contaminated conditions are encountered