| CPC F28D 7/16 (2013.01) [F28F 9/02 (2013.01); B33Y 10/00 (2014.12); B33Y 80/00 (2014.12)] | 24 Claims |

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1. A heat exchanger core comprising:
a plurality of internal passages extending in parallel to each other; and
a header passage communicating with the plurality of internal passages at ends of the plurality of internal passages in an extension direction of the internal passages,
wherein an inner wall of the header passage has greater surface roughness than passage walls of the plurality of internal passages,
wherein an inner wall of the header passage includes:
a first region located on one side in the extension direction of the plurality of internal passages; and
a second region located on another side in the extension direction and having an inclined surface whose angle with respect to the extension direction is not greater than 60 degrees;
wherein a partition wall between the header passage and an internal passage through which another fluid flow is along an extension direction of the plurality of internal passages.
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