Impact of bed motion on the wall-to-bed heat transfer for powders in a rotary kiln and effect of built-ins

Kurzfassung

This study analyses experimentally the heat transfer from the wall to the particle bed in a rotating drum with and without addition of built-ins for improving the mixing of the powder. Two types of materials were employed, sand as a granular material (dp,mean = 200 μm) and cement raw meal as a powdery material (dp,mean = 15 μm). A rotating cylinder heated through the outer shell was operated in batch mode, with rotational speeds of 1, 2 and 3 rpm and fillings of 5 and 10 %. The sand achieved a rolling motion, which makes the measured heat transfer coefficient predictable with theoretical models. At the contrary, cement raw meal showed very little mixing in a cylinder without built-ins and the measured heat transfer coefficient was much lower than the model predictions. Using longitudinal strips along the wall caused the powder to collapse regularly, increasing its heat transfer by a factor of 2–3, up to the range of predictions from theoretical models. Thus, a motion comparable to the rolling motion can be established with such built-ins which only cause limited dust formation. Furthermore, the limitations of the measurement method with rotating thermocouples are pointed out which mostly stem from the thermocouple response time and convectional losses, which are increased once built-ins are employed.