The flow and heat transfer performances of thermalhydrolyzed sewage sludge in twisted oval tubes were numerically investigated and the mechanism of enhanced heat transfer was analyzed in this paper. The velocity and temperature distributions at different aspect ratios and twist ratios were obtained, and the effect of Reynolds number on the flow and heat transfer was clarified. The results reveal that the heat transfer efficiency could be greatly increased by the generation of vortex and secondary flow but with the growth of pressure drop. Furthermore, a larger twist ratio leads to a smaller thermal performance factor, and as the aspect ratio increases to 1.96, the thermal performance factor reaches the maximum value and then descends. By compared with the results of smooth round tubes, the twisted oval tubes show much improved heat transfer efficiency at a low Reynolds number(<5 000).