The heat transfer distributions for flow passing through a two-pass (one-turn) and a three-pass (two-turn) passages with 180-degree 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 non-uniform mass transfer coefficients measured around a sharp 180-degree turn exhibit the effects of flow separation, reattachment and impingement, in addition to secondary flows. Results of the three-pass passage indicate that heat transfer characteristics around the second turn is virtually the same as that 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% 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% higher than that of the two-pass one. This 15% 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 varies significantly with the Reynolds number.

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