Experimental investigation of heat transfer from elliptic tubes heat exchanger immersed in a fluidized bed

Abstract

AbstractIn this work, heat transfer, wall-to-bed, characteristics from elliptic tubes heat exchangers, both inline and staggered tubes arrangements, were studied experimentally. Bundles of twenty tubes of the same total surface area of 0.015 m2 and a length of 200 mm are immersed in a bed of granulated charcoal of different grain sizes, 2, 4 and 6 mm. Air was used as a fluidizing fluid. One of these tubes which was made of copper, instrumented, electrically heated and positioned in the middle of tube bundle, while the others are wooden dummy to be influenced only by the presence of the neighbouring tubes. Measurements are reported in the range of fluidization number within range of 1 to1.4. The results showed that, the local Nusselt number around the elliptic heated tube is about uniform with its maximum value at the tube sides and minimum at the stagnation and rear points of the tube. In addition, the average Nusselt number increases with the increase of the fluidization number and with the decrease in particle diameter. Heat transfer characteristics were found to improve in staggered tube bundle if compared with the inline tube arrangements. The present results of pressure drop and average Nusselt numbers were validated with a numerical model of the same conditions for the case of particle diameter of 2 mm. Good qualitative and quantitative agreements have been achieved between experimental and numerical data. Finally, correlation equations for the average heat transfer in terms of Nu, fluidization number and tube to particle diameter ratio are deduced.