Effect of ambient pressure on air side heat transfer and flow characteristics of plain finned tube heat exchanger

•Ambient pressure effect on air side heat transfer and flow characteristics was theoretically and experimentally analyzed.•Ambient pressure have impacts on development and separation of boundary layer as air flow around the tube.•Row number and fin pitch have smaller effect on air side heat transfer when ambient pressure drops to 40 kPa.•Air flow between fins is more laminar at lower ambient pressure.•Air side heat transfer coefficient corresponding to the same Re decreases as ambient pressure reduces.