Heat exchanger

A heat exchanger comprising:

• a plurality of flat tubes;
• a first header-collecting pipe having inserted therein an end portion of each of the flat tubes;
• a second header-collecting pipe having inserted therein the other end portion of each of the flat tubes;
• and a plurality of fins joined to the flat tubes, the heat exchanger provided in a refrigerant circuit configured to perform a refrigerating cycle, wherein
• the second header-collecting pipe forms flow spaces which communicate with the plurality of flat tubes and in which a refrigerant being in a gas-liquid two-phase state flows upwardly when the heat exchanger functions as an evaporator,
• an effective cross-sectional area of the flow spaces is an area obtained by subtracting a projected area which corresponds to a portion of each flat tube located in a corresponding one of the flow spaces and which is projected onto a plane perpendicular to an axial direction of the second header-collecting pipe, from an area of a cross section of the corresponding one of the flow spaces which is perpendicular to the axial direction of the second header-collecting pipe, and
• the effective cross-sectional area of the flow spaces is set based on a mass flow rate at which the refrigerant flows into the flow spaces when the heat exchanger functions as the evaporator.