Characteristics of dynamic behavior and local heat transfer around single row tubes immersed in floating particles

Abstract

To improve the performance of a heat exchanger on a fluidized bed, low density particles (polystyrene hollow sphere) were employed. Pressure loss caused by the insertion of particles was of special concern. The tests were conducted using a single row of horizontal cylindrical tubes ( D = 10 and 20 mm), and data were obtained by varying the location of the tubes, static bed heights, tube pitches, and air velocities. The mean heat transfer coefficient was evaluated in relation to the pressure loss. Aiming at a phenomenologic understanding of the heat transport processes, a fast response capacitance sensor was developed to investigate the movement of particles near the surface of tubes. The local instantaneous heat transfer coefficient was measured using a miniature heat flow sensor (response time = 0.02 s). The following results were obtained: 1. The time averaged local heat transfer coefficients were found to be significantly different for different angular positions and to be affected by both particle size and L 0, as well as by the airflow rate. 2. Very significant improvements in the heat transfer coefficients were obtained in the backside region of the tubes compared with those in the front region when u > 2 u mg , where u mf is the superficial air velocity of at minimum fluidization. 3. The capacitance trace versus time can be directly related to the variations in local solid's density near the tube surface. 4. The capacitance signals indicated that the bed-surface contact characteristics differ distinctively at various angular positions, and that they are closely connected to air velocity. 5. Signals from the heat flow sensor correspond with those of the capacitance sensor. Mechanisms of heat transfer were discussed based on the results of the relation between the local heat transfer coefficient and both the time average and the r.m.s. average of density of floating particles.