Heat transfer fin

1. A heat transfer fin having a flat plate shape and arranged in a heat exchanger through which combustion exhaust gas flows, the heat transfer fin comprising:

a plurality of heat-transfer-tube insertion holes aligned in a single stage;

a downstream cut portion provided downstream of a gas flow passage of the combustion exhaust gas between an adjacent pair of the plurality of heat-transfer-tube insertion holes, the downstream cut portion being formed so as to be recessed toward an upstream side of the gas flow passage of the combustion exhaust gas;

a downstream flange formed on a peripheral edge of the downstream cut portion so as to protrude from a first surface of the heat transfer fin; and

a plurality of protruding pieces formed between the downstream flange and a first heat-transfer-tube insertion hole of the adjacent pair of the plurality of heat-transfer-tube insertion holes located adjacent to the downstream flange so as to protrude alternately from the first surface and a second surface of the heat transfer fin,

wherein each of the plurality of protruding pieces is a cut-and-raised portion protruding only in a downstream direction of the gas flow passage of the combustion exhaust gas.

2. The heat transfer fin according to claim 1 , wherein the plurality of protruding pieces is a plurality of first protruding pieces, the heat transfer fin further comprising a plurality of second protruding pieces formed between the adjacent pair of the plurality of heat-transfer-tube insertion holes so as to protrude alternately from the first surface and the second surface of the heat transfer fin.

3. The heat transfer fin according to claim 2 ,

wherein the plurality of first protruding pieces are configured to be substantially orthogonal to the gas flow passage of the combustion exhaust gas flowing from the upstream side to a downstream side between the first heat-transfer-tube insertion hole of the adjacent pair of the plurality of heat-transfer-tube insertion holes and the downstream flange, and

wherein the plurality of second protruding pieces are configured to be substantially orthogonal to the gas flow passage of the combustion exhaust gas flowing from the upstream side to the downstream side between the adjacent pair of the plurality of heat-transfer-tube insertion holes.

4. The heat transfer fin according to claim 2 ,

wherein each of the plurality of first protruding pieces is a first cut-and-raised portion,

wherein each of the plurality of second protruding pieces is a second cut-and-raised portion protruding only in a downstream direction of the gas flow passage of the combustion exhaust gas, and

wherein the downstream flange has a protrusion height greater than protrusion heights of the first cut-and-raised portions and the second cut-and-raised portions protruding from the first surface of the heat transfer fin.

5. The heat transfer fin according to claim 1 , further comprising an upstream protrusion formed between the first heat-transfer-tube insertion hole of the adjacent pair of the plurality of heat-transfer-tube insertion holes and an upstream edge of the heat transfer fin so as to protrude from the first surface of the heat transfer fin.

6. The heat transfer fin according to claim 1 , further comprising an upstream cut portion upstream of the gas flow passage of the combustion exhaust gas between the adjacent pair of the plurality of heat-transfer-tube insertion holes, the upstream cut portion being formed so as to be recessed toward a downstream side of the gas flow passage of the combustion exhaust gas.