Heat and mass transfer coefficients at the surface of elliptical cylinders placed in a turbulent air flow

Abstract

Heat and mass transfer coefficients were measured at the surface of elliptical cylinders of three different lengths (0·10 m, 0·15 m and 0·20 m) and five ratios of major to minor axis placed in a cross flow of air. The flow was characteristic of food pilot plants, i.e. an air velocity ranging from 0·5m s −1 to 2·0 m s −1 and a turbulence intensity, Tu, of about 12%. In agreement with theory, the transfer coefficients measured at this turbulence intensity were higher than those usually indicated in chemical engineering literature, which are based on experiments in ‘aeronautic’ wind tunnels where the turbulence intensity is close to 0%. The air flow properties (air velocity and turbulence intensity) have a greater effect than body shape characteristics (length and ratio). Velocity and turbulence intensity measurements in a chilling room and a dryer showed that the flow is intermittent and highly turbulent (Tu from 17 to 60%). Thus, the turbulence has to be taken into account in food engineering studies.