Heat transfer between a supernatant gas and a flowing vibrofluidized bed of solid particles

Abstract

The purpose of this study is to develop and demonstrate a novel process for heat recovery from hot exhaust gases. This process involves direct contact of a hot gas with a countercurrently flowing vibrofluidized bed of cold solid. Based on a simple heat-transfer model, an "apparent" heat-transfer coefficient between the air and solid was calculated. The temperature profile of the air as a function of heat-exchanger length was used to determine the "apparent" area for heat transfer in the model. Analysis, based on factorial-design experiments, showed that increasing the airflow rate and applied vibrational intensity, as well as decreasing the baffle height of the system served to increase the "apparent" heat-transfer coefficient. Increasing the solid flow rate produced higher heat-transfer coefficients only when the baffle was lowered past a certain "critical" height. Under optimum conditions investigated, a gas-to-bed heat-transfer coefficient of about 270 W/m²-K was obtained with a heat exchanger length of 0.71 m. "Cold-flow" experiments of the system were used to explain the heat-transfer trends. A condition analogous to "flooding" determined the operating range of the "flowing" vibrofluidized-bed heat exchanger. As a result of this work, significant progress has been made on the evolutionary development of a vibrofluidized-bed heat exchanger to be used for future heat recovery.