Experimental evaluation of pairs of inline tubes of different size as components for heat exchanger tube bundles

A passive heat exchanger design for process intensification, reduced flow fluctuations, pressure-drop and fouling is evaluated experimentally. A primal geometry of two unequally sized cylinders placed in tandem arrangement under cross-flow is initially examined in order to establish the effect on the wake fluctuations, compared to the single cylinder case. Three pairs of cylinders were considered with a diameter ratio of d/D = 0.5 and at different longitudinal pitches, SL/D = 1.5, 2.0 and 2.5, at the subcritical Reynolds number of ReD = 3900. The streamwise velocity fluctuations in the wake of the cylinders were found to be lower for the cylinder pairs as compared to the case of the single cylinder. The preferred primal arrangement with unequally sized cylinders was extended to form a heat exchanger tube bundle, where the effect of the transverse distance between the tubes (ST/D) was also studied. Three tube bundle configurations were examined with d/D = 0.5, SL/D = 1.5 and ST/D = 3.6 and ST/D = 2.5, and a common arrangement with d/D = 1.0, SL/D = 2.5 and ST/D = 3.6. Measurements were performed with regard to the heat transfer on the external surface of the tubes and the pressure drop across each configuration. When compared to the conventional tube bundle arrangement, the arrangement with d/D = 0.5, SL/D = 1.5 and ST/D = 3.6 led to specific (per unit volume) heat transfer enhancement by 40% with indications of a negligible penalty in pressure drop.