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The ever-increasing power density of electronic devices fosters development of cooling systems. Among them, two-phase heat transfer devices in general and Loop Heat Pipes (LHP) in particular offer advantages by being completely passive while being able to cope with high heat fluxes. However, LHP manufacturing and especially integration is complicated by the loop architecture, which is not found in conventional Heat Pipes (HP). In an attempt to fit LHP layout into HP-like housing, a novel Heat Loop Pipe (HLP) device was designed, manufactured and tested in application to cooling a portable high-power (80 W) LED luminaire with a focusing reflector. The HLP utilizes co-axial layout of vapor and liquid lines and an additional wick in the condenser. The butane-charged, 350 mm-long HLP showed unexpectedly high resistance in the adverse orientation (evaporator above the condenser), which was an order of magnitude higher than in reflux orientation (0.8 K/W vs. 0.03 K/W). A possible explanation of this behavior in terms of vapor superheating in the evaporator channels is given. Analysis of the test results with help of p-T diagram justifies this assumption. Finding the root cause for such behavior requires further study.
Ušakovs et al. (Sat,) studied this question.