REGIONAL HEAT-TRANSFER IN 2-PASS AND 3-PASS PASSAGES WITH 180-DEG SHARP TURNS

The heat transfer distributions for flow passing through two-pass (one-turn) and three-pass (two-turn) passages with 180-deg sharp turns are studied by using the analogous naphthalene mass transfer technique. Both passages have square cross section and length-to-height ratio of 8. The passage surface, including top wall, side walls, and partition walls, is divided into 26 segments for the two-pass passage and 40 segments for the three-pass passage. Mass transfer results are presented for each segment along with regional and overall averages. The very nonuniform mass transfer coefficients measured around a sharp 180-deg turn exhibit the effects of flow separation, reattachment, and impingement, in addition to secondary flows. Results for the three-pass passage indicate that heat transfer characteristics around the second turn are virtually the same as those around the first turn. This may imply that, in a multiple-pass passage, heat transfer at the first turn has already reached the thermally developed (periodic) condition. Over the entire two-pass passage, the heat transfer enhancement induced by the single-turn is about 45 to 65 percent of the fully developed values in a straight channel. Such a heat transfer enhancement decreases with an increase in Reynolds number. In addition, overall heat transfer of the three-pass passage is approximately 15 percent higher than that of the two-pass one. This 15 percent increase appears to be Reynolds number independent. The pressure loss induced by the sharp turns is found to be very significant. Within the present testing range, the pressure loss coefficient for both passages is Reynolds number dependent.