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CPOTE2020 logo
6th International Conference on
Contemporary Problems of Thermal Engineering
Online | 21-24 September 2020

Abstract CPOTE2020-1261-A

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The effect of surface temperature distribution on net force acting on reacting particles

Ewa KARCHNIWY, Silesian University of Technology, Poland
Sławomir SŁADEK, Silesian University of Technology, Poland
Adam KLIMANEK, Silesian University of Technology, Poland
Andrzej SZLĘK, Silesian University of Technology, Poland
Wojciech ADAMCZYK, Silesian University of Technology, Poland
Agnieszka KORUS, Silesian University of Technology, Poland

Nowadays, much of the research and design of solid fuels combustion is carried out numerically. It is therefore important to ensure that numerical models reflect the investigated processes with satisfactory accuracy. In numerical simulations of industrial-scale systems with reacting particles it is typically assumed that a particle can be characterized by single temperature. According to this assumption, surface reactions occur with the same rate over the entire particle surface. Consequently, the flow induced by production of chemical species at the surface (the so called Stefan flow) is also symmetric and there is no net force acting on the particle due to this transport phenomenon. In reality, however, particles are not spherical, their surfaces are not homogeneous and temperature at the surface is in general non-uniform. The force which is a result of this non-uniformity can influence the particle trajectory. This in turn might further affect combustion characteristics. In this work we examine the effect of the temperature non-uniformity and attempt to incorporate this effect into computational fluid dynamics model. First, we performed measurements of the temperature distributions on surfaces of several thousand reacting particles in O2/N2 and O2/CO2 atmospheres, ranging in size from 50 to 200 μm. Two fuel types were studied, i.e. lignite and hard coal in ambient temperature of 1273K. Two-color pyrometry method, with two synchronized monochromatic high-resolution cameras were used to measure the temperatures. Based on the measurements, probability distributions were found that describe how the temperature is likely to be distributed on a surface of the particles. Numerical model was then developed in which the external surface of the particle is discretized into a number of sub-surfaces. For each such surface the temperature is prescribed based on the corresponding probability distribution and force due to Stefan flow is computed. This allows to determine the net force acting on the particle due to temperature differences on its surface. This function is then incorporated into ANSYS Fluent code through the User Defined Function mechanism. Simulations of a simplified test combustion chamber are performed and the effect of the temperature non-uniformity on the net force acting on particles due to Stefan flow was assessed. Our analysis shows that the force is non-negligible, especially at high temperatures.

Keywords: Reacting particle temperature, Coal combustion, Tracking particles, Stefan flow, Two-color pyrometry
Acknowledgment: The research leading to these results has received funding from the Opus 13 Research Programme operated by the Polish National Science Centre in th frame of Project Contract No UMO-2017/25/B/ST8/00957