Responses of an axially rotated tube versus twisted tube design as two solutions to enhance the thermal performance under a non-uniform heat flux condition: A numerical study

Abstract

• Heat transfer in rotating and twisted tubes are studied and compared numerically. • A maximum increase of 137% in Nusselt number occurs at N=10.5 for the 3-meter pipe. • Pipe rotation eliminates the liquid stratification phenomenon in the pipe. • A maximum increase of 2.5% in outlet temperature occurs at twist ratio S=0.1. • A maximum loss in the pressure coefficient reaches 32.8% at twist ratio S=0.1. Enhancement of heat transfer in tubes exposed to non-uniform heat flux applications is a key factor in increasing such applications' efficiency. In the present study, heat transfer in an axially rotating pipe with concentrated heat flux at the lower section of the pipe is numerically investigated and compared to the twisting technique in such applications. The effects of pipe’s length (1, 2, and 3 meters) and rotation rate (N=0-10.5), which is the ratio between the rotational and axial Reynold’s numbers, are studied numerically using ANSYS Fluent while the effect of the twist ratio (S=0.1-1) is studied in the twisted tube. The numerical analysis reveals that the pipe axial rotation has a negative impact on the average outlet temperature where a maximum decrease of 7.7% is reached in the 3-meter pipe at a rotation rate of N=10.5 with respect to the stationary pipe while the effect of the twisting technique was significant as the outlet temperature has a maximum increase of 9.3% at S=0.1. On the other hand, the pipe rotation increases the temperature homogeneity between the liquid layers thus reducing the liquid stratification phenomenon in the pipe. The average reduction percentage in the temperature difference between the pipe wall and the fluid center is 69.7% at N=10.5, however; this reduction reached a maximum value of only 11% at twist ratio S=0.1. This is evidenced by the increase in the Nusselt number where the numerical results show a maximum increase of 137% in the Nusselt number at N=10.5 for the 3-meter pipe whereas this increase reached 9.8% at a twist ratio of S=0.1 compared to the plain tube. Therefore, pipe axial rotation increases the temperature homogeneity by reducing the temperature gap between the pipe wall and the fluid center by reducing the pipe wall temperature thus making the pipe capable of absorbing greater heat amounts at larger lengths while the twisting technique increases the outlet temperature taking into consideration that this is a passive technique that does not consume power.