Study on flow and heat performance of thermal-hydrolyzed sludge in a double-pipe heat exchanger with a series of inner corrugated tubes

Abstract

In this paper, a double-pipe heat exchanger with a series of corrugated tubes as inner tubes is studied for the adequate cooling of thermal-hydrolyzed sludge. A numerical model, built on the Spalart-Allmaras model, is validated to have the ability for characterizing the flow and heat transfer performance of sludge. The influencing factors, including the flow parameters of the sludge and cooling water, and the geometric configurations of the corrugated tube are parametrically investigated. The ratio of Nusselt number (Nu/Nu0), the coefficient of flow resistance (f/f0) of the sludge, the synthesis factor of performance (η) and the ratio of the heat transfer increase over the pump power increase (ζ) are used as the performance assessment indexes. The results show that increasing Resludge can negatively affect η and η tends to be stable when Rewater is higher than 1.2 × 104. Decreasing S/D and increasing H/D result in a higher ratio of flow resistance f/f0 and a higher ratio of Nu/Nu0. The corrugation height (H) shows a larger effect than the corrugation length (S). In practical applications, the optimal flow rates of the sludge for the studied corrugated tubes are recommended to be within the range of 1.2–1.5 m/s.