A novel approach to heat transfer enhancement using trapezoid shaped spiral strips to promote tumble and swirl in a slot shaped channel used in heat exchangers

摘要

Heat transfer results for a given slot shaped channel with a 3:1 aspect ratio are presented using various configurations of a trapezoid shaped spiral wound strips to enhance swirl and tumble motion in the channel. The Reynolds numbers investigated range from 10,000 to 50,000 and are based on the characteristics of the fluid at the channel inlet. The ratio of absolute temperatures between the wall and fluid are on the order of 0.8 to 0.9. A combination of thermochromic liquid crystal techniques and thermocouples were used to create a temperature vs. time map. Duhamel's superposition theorem was then used to determine the local heat transfer coefficients (h) and heat transfer enhancement factors (Nu/Nu_o). In one series of testing a straight center inlet with a radiused entry was used to reduce entry effects. In a second series of test a 90 degree inlet geometry was used to enhance turbulence at the entry. Three combinations of helical strips were tested using a single, double, and pentuple spiral design. The pitch of the helix remained constant in all tests at 0.75" (18 mm) as well as the height of the strip at 0.0625" (1.6 mm), yielding a p/e (pitch/rib height) ratio of 12. The resulting flow in the channel creates a tumble motion as the main channel fluid encounters the strips and a swirl motion as the fluid is directed through the spiraling helix. Many studies involving heat transfer using swirl enhancement have been presented in literature using round passages with wire spring inserts or twisted tapes, typically used in heat exchangers. In turbine aero foils, particularly in the mid-span region, rectangular channels with various configurations of trip strips are used to enhance heat transfer. The results of the tests presented in this paper show local heat transfer enhancement (Nu/NU_o) values greater than seven and subsequent average values for the entire channel greater than three at the higher Reynolds numbers along with relatively low normalized friction factors.