Straight ultra-thin vapor chambers tested under different modes for different vapor duct thicknesses

This study aims to demonstrate, through careful temperature measurements and infrared images, that tests for a vapor chamber (VC) under a VC mode or a heat pipe (HP) mode, reach very different thermal performances due to different severities for water accumulation at the condenser end. Both modes have been frequently adopted in the literature with the difference overlooked. Experiments are conducted for 100 × 20 mm² straight ultra-thin vapor chambers (UTVCs) with a 0.17 mm or a 0.12 mm vapor duct thickness and a total thickness of 0.45 mm or 0.4 mm, respectively. Three different charges, an over-charge, a charge with the charge ratio slightly larger than 1.0, and an under-charge, are prepared for each vapor duct thickness. Severe water accumulation over the condenser is observed under the HP mode, yielding drastic temperature drops therein. Under the HP mode the maximum heat transfer rate (Q_max) drops to about only 3–4 W with vapor duct thickness (h) = 0.17 mm and fails at actual vapor input ∼3 W with h = 0.12 mm. Water accumulation cannot be totally removed in the HP mode when the UTVC is placed vertically, although slight improvement on the thermal performance can be achieved with the assistance of gravity. Under a HP mode, the minimum vapor chamber resistance (R_min) ranges between 0.7–1.0 K/W, while under a VC mode R_min is 0.14–0.29 K/W. Q_max appears slightly reduced for the thinner vapor duct thickness due to the larger vapor resistance.