This is due to longer flow lengths of vortices for flat tubes than for round tubes. 8b (flat tubes with VGs) are higher than in fig. 7 and 8 show that vortex generators cause the appearance of extra peaks in Nu(x,y). Peak values and fin areas with enhanced heat transfer due to horseshoe vortices are larger for round tubes than for flat tubes. In front of the tubes in the second and last rows Nusselt number enhancement due to horseshoe vortices can be seen for round and flat tubes, see figs. Here this could not be avoided, because the fin area and tube cross-section were kept the same as for the round tube configuration. In a heat exchanger design the flat tube would be appropriately removed from the fin edge. Because this tube is placed at the edge of the fin, no horseshoe vortex appears as it appears in front of the first row tube of the fin-tube element with round tubes. 8 at the middle of the fin leading edge corresponds to the location of the first flat tube. As expected, large Nusselt numbers are obtained at the leading edge of the test fin. The location of the tubes are dark areas in figs. Local fin Nusselt number for arrangement with flat tubes, (a) without VGs, (b) with VGs. The experiments were performed using air ranging in Reynolds number from 9,400 to 28,200, where the Reynolds number here is defined as Re = U♲H/ν.įig. Furthermore vortex structures behind the generators were visualized by a flash light sheet method using smoke as a tracer supplied at upstream. Then the streamwise and spanwise local heat transfer coefficients hx, hz = qw/(t wx-t bx) were obtained. Bulk temperatures of the flow t bx were obtained by adding to the initial temperature increments of temperature equivalent to the total amount of heat generated between the starting point of heating and the measurement point. The other strips are used for measurement of streamwise local wall temperatures. One strip is set with the blades, and allowed to move laterally to obtain the details of the spanwise wall temperature profile. Seven strips downstream of the blades are provided with nine thermocouples each for the spanwise temperature distributions.
The wall surface temperatures t wx were measured by means of copper-constantan thermocouples with ϕ 70 μ m soldered on the back of the foil. The heat transfer surfaces, the top and bottom inside surface of the duct, were produced by attaching stainless-steel foil of 30 μm thickness divided into nineteen strips across the flow direction the surface heat flux qw was maintained uniform under direct current. Spanwise pressure distributions were measured using ϕ 0.4mm static pressure taps installed laterally at discrete intervals downstream of the generators, while pressure drop was determined from the hydraulic pressure gradient lines obtained from measurements of the pressure upstream and downstream of the blades. Experimental apparatus and arrangement of vortex generators
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