An inlet header of a microchannel heat exchanger is provided with a first insert disposed within the inlet header and extending substantially the length thereof, and having a plurality of openings for the flow of refrigerant into the internal confines of the inlet header and then to the channels. A second insert, disposed within the first insert, extends substantially the length of the first insert and is of increasing cross sectional area toward its downstream end such that annular cavity is formed between the first and second insert. The annular cavity of decreasing cross sectional area provides for the maintenance of a substantially constant mass flux of the refrigerant along the length of the annulus so as to thereby maintain an annular flow regime of the liquid and thereby promote uniform flow distribution to the channels.