Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05.0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction, factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper. (C) 2012 Elsevier Ltd. All rights reserved.