UNSTEADY-FLOW AND HEAT-TRANSFER FOR CYLINDER PAIRS IN A CHANNEL

The incompressible, two-dimensional, unsteady flow past a pair of cyli nders of square cross-section, placed in tandem normal to the flow in a channel, has been investigated by dye visualization and direct numer ical simulation. The objective has been to evaluate the effect of cyli nder separation distance, lambda, on the flow behavior and heat transf er, for cylinder diameter ratios, D/d, of 1 and 2 over a range of Reyn olds numbers 200 less-than-or-equal-to Re less-than-or-equal-to 1600, based on the larger (downstream) cylinder diameter. A comparison betwe en the experimental and numerical results for cylinders of equal cross -section dimensions shows very good agreement. The results for D/d = 1 reveal three distinct flow patterns as a function of lambda and Re wh ich, apparently, have not been previously reported: (1) For 0.25 less- than-or-equal-to lambda less-than-or-equal-to 4 with Re less-than-or-e qual-to 200, the inter-cylinder flow consists of a pair of steady coun ter-rotating eddies which do not exchange fluid with the surrounding f low and eddy shedding is observed only for the downstream cylinder. (2 ) For 0.25 less-than-or-equal-to lambda less-than-or-equal-to 1.0 with 400 less-than-or-equal-to Re less-than-or-equal-to 1600, vertical flo w oscillations arise in the inter-cylinder space, and the periodic ing estion of backward-jetting fluid from the top and bottom walls of the downstream cylinder into the inter-cylinder space is observed. For fix ed lambda the unsteadiness increases with Re but only the downstream c ylinder sheds large eddies. (3) At a critical inter-cylinder spacing r elated to Re according to lambda(c) is similar to Re-2/3, the shedding of large eddies also occurs at the upstream cylinder and this results in a highly mixed inter-cylinder flow. For a cylinder diameter ratio of D/d = 2, with the smaller cylinder located upstream of the larger h eated cylinder, it is shown that for Re fixed an optimal location exis ts for the upstream cylinder such that the heat transfer from the down stream cylinder is maximized. The optimal location corresponds to a sp acing smaller than the critical inter-cylinder spacing.