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Flow pattes in upstream and downstream straight tubes of sudden-changedareas in ahorizontal straight pipe were experimentally examined. Both sudden-expansioncross-section (SECS)and sudden-contraction cross-section (SCCS) were investigated. The flow patte mapsupstream anddownstream were delineated and compared with those in straight tubes with uniformcross-sections.The effects of the SECS and SCCS on flow pattes were discussed and analyzed.Furthermore, flowpatte transition mechanisms resulting in occurrences of different flow patteswere simplydiscussed and some transition criteria for the flow patte transitions were deduced byusing the non-dimensionlized analysis method.