Heat transfer and ignition characteristics of millimeter scale particles in turbulent flow field

The combustion of fuel in strong turbulent field widely exists in actual industrial devices. It is very important to study the effect of turbulence fluctuation on the heat transfer and ignition of fuel for accurately understanding combustion process. At present, there have been some very detailed studies on the evaporation and ignition of gaseous fuels and droplets in turbulent flow field, while for the study of heating and ignition of solid particles, most experimental methods, such as drop tube furnace, single particle furnace, and flat flame burner, etc., are laminar flow field. Research methods with turbulent flow filed mainly include high speed jet, one dimensional furnace and swirl burner, but there are some problems such as poor optical visibility and difficulty in controlling turbulent intensity. To this end, by establishing a four fan counter turbulence test rig that can operate at high tem peratures, a near homogeneous and isotropic turbulent flow field with adjustable turbulence intensity was estab lished, and the effect of turbulence intensity on the heating and ignition of single particles in millimeter scale was studied. By measuring the transient velocity distribution of the flow field and the heating curve of particle at different fan speeds and temperatures, the heat transfer characteristics of the particles at different temperatures and turbulence intensities were obtained. Based on the experiment results of the heating of a copper particle with a diameter of 4.4 mm, a particle heat transfer model considering turbulence fluctuation was proposed, and the model was validated using the experimental data of a copper particle with a diameter of 2 mm. The research results show that the flow field of the measurement area is isotropic and the fluctuation velocity is much larger than the time average velocity. The fluctuation velocity increases linearly with the fan speed. With the increase of fluctuation velocity, the ignition of coal particles is advanced and the heating rate of a copper particle is accelerated, indicating that the effect of turbulence on the enhancement of heat transfer cannot be ignored. The general Ranz Marshall correlation will obviously underestimate the heating history of the particles under strong turbulence conditions, thus causing the calculated particle ignition delay time to be too large. By introducing an additional turbulent effect term into the Ranz Marshall correlation and fitting its coefficients according to the experimental results in a strong turbulent flow field, the enhancement effect of turbulence on the heat transfer of large particles can be accurately characterized.