The entrainment of liquid droplets in the vapor stream is one of the five fundamental limitations occurring in heat pipes. Although recent analytical investigations have led to a better understanding of the phenomena that govern this limit, these investigations have resulted in wide variations in the predicted level at which entrainment should occur. In contrast to these numerous analytical investigations, very few experimental investigations have been conducted. In these, entrainment has only been observed in heat pipes where the capillary structure was flooded. No experimental evidence has been found to indicate entrainment of droplets in properly primed heat pipes, suggesting that entrainment may never occur in heat pipes, due to the presence of the capillary structure which retards the growth of surface waves. It is proposed here to conduct an experimental investigation of the entrainment in capillary pumped structures, i.e., heat pipes. This investigation would build upon the information gained from previous work conducted in the Conduction Heat Transfer Laboratory at Texas A&M University and would employ a laser diffraction particle sizing technique and laser Doppler anemometry to measure the size and velocity distributions of entrained droplets. The objective of this investigation would be to determine the onset of liquid entrainment in capillary pumped structures as a function of the flooding level and axial heat flux. Several different types of heat pipe wicking structures would be evaluated to determine the quantity of fluid entrained, along with the size and velocity distributions of the entrained liquid droplets. The result of this investigation would be a better understanding of liquid droplet entrainment in capillary pumped structures such as heat pipes and an experimental determination of the amount of flooding necessary for entrainment to occur as a function of the capillary pore size.
