Heat transfer phenomena and performance investigations for 3D fin-and-tube pulsating heat pipe heat exchanger under vertical and horizontal orientations

Abstract

The present heat exchanger is modified from an inexpensive commercial heat exchanger composed of three connected tube layers with an internal tube diameter of 5.6 mm. Under the vertical orientation, methanol with weaker surface tension can surprisingly achieve pulsation-like motion in the large ID with Bo numbers ranging between 3.37 and 3.73, well beyond the widely accepted criterion of Bo = 1.8 or 2.0. Under the horizontal orientation, the multi-layered structure greatly helps the pulsation in that liquid slug trains tend to drain down to the lower layer, thereby not only trigger but sustain continuous pulsation motion. Also, the low-surface-tension working fluid, like methanol, also activated a dynamic flowing behavior over the PHPHX unit. In terms of the thermal performance, the effectiveness increases by up to 50 % compared to the system with independent tube layers, even though both have the same total number of tubes. The system can accommodate up to three sets of PHPHX units, and the maximum effectiveness appears with the methanol for the filling ratio of 35 % under the horizontal orientation. With methanol overcoming local dry-out by installing multiple sets of PHPHX unit, the smooth movement of liquid slug trains, due to its lighter weight and lower viscosity, achieve more heat delivery even with a 2.2 times lower latent heat and a 3.51–3.64 times lower thermal conductivity than water.