The flow and heat transfer in spirally fluted tubes were investigated numerically.The velocity vector fields in spirally fluted tubes with different groove depths under different Reynolds numbers were compared.The influences of the thread groove on the two types of flows and heat transfers were investigated.It is found that the spiral flow is enforced with the increasing groove depth “e”,the disturbance caused by eddy is gradually enhanced and the heat transfer in the pipe also increases.However,when e rises up to a certain value,the enhancement of spiral flow concentrates mainly in the center region of pipes which has little effect on heat transfer.In the area near the wall where the flow type has a great influence on heat transfer,the change of spiral flow is small and therefore it has little effect on heat transfer enhancement.The center of the eddy becomes a ‘still zone’ which weakens the heat transfer.In addition,when e is kept increasing,the still zone enlarges.