characteristics of flow field in t he header of a plate2fin heat exchanger were inves2 tigated wit h particle image velocimet ry ( PIV) . It is found t hat the fluid flow maldist ribution is very serious for conventional header while t he improved header configuration wit h p unched baffle can effectively imp rove the performance. When the p unched baffle is proper in lengt h

the small holes are in staggered arrangement and t he p unched ratio gradually increases f rom t he cent ral axis to the boundary along t he baffle lengt h

t he performance of flow dist ribution in t he plate2fin heat exchanger is effectively improved by t he optimum design of header configuration. The flow mald2 ist ribution parameter S decreases f rom 11210 to 01209 and t he ratio of the maximum velocity to<br />t he minimum decreases f rom 231163 to 11756.