Evaluation of helical wire inserts for CHF enhancement

The U.S. is evaluating helical wire inserts (HWI) for heat transfer enhancement and critical heat flux (CHF) performance in divertor cooling channels. This concept is attractive because it: (1) is cost-effective to integrate into parallel, gun-drilled channels using a simple brazing operation with no special tooling; (2) can accommodate selective placement where enhancement is most needed in long, curved channels, and (3) has been shown in past testing to reach CHF enhancement values comparable to swirl tape inserts. Two bare copper (CuCrZr, Elbrodur G) mock-ups were fabricated to determine the preferred combination of wire size and helix pitch for the 10-mm-diameter cooling channels selected by the U.S. One mock-up used 1-mm-dia. wire wound on a 5 and 10-mm pitch. The other used 1.5-mm-dia, wire wound on a 10 and 20-mm pitch. Each pitch extended over half the 200-mm length of the copper bar. The inserts were made from Inconel 600 wire and brazed to the channel wall at 980°C using a Cu-Mn-Ni alloy. Gas-fan cooling was used after brazing to prevent undesired metallurgical reactions in the CuCrZr alloy and was followed by thermal aging to recover properties. CHF tests were performed in the 30-kW Electron-Beam Test System at Sandia using 100°C and 130°C subcooled, 4 MPa water flowing at 7 and 12 m/s. Coolant flow was reversed during testing so that each of the two wire pitches could be placed at the entrance end. Because the heated length is short near CHF levels, thermocouple locations were selected so local effects of the wire insert wrap beneath the heated surface could be evaluated