An investigation of the use of step-graded metal felt wicks to improve heat pipe performance

This dissertation is an investigation of the modeling, design, and testing of metal felt wicks in an effort to improve the heat transfer limit of a flat plate constant conductance heat pipe through step-grading the wick. Twelve different screening samples of metal felts were characterized for porosity, effective radius, pore size distribution, and liquid permeability, and from this set, two sets of step-graded wicks with corresponding non-graded controls were manufactured. The screening set of metal felts had substrate wire diameters ranging from 6 to 50 μm. The porosity was measured to be in a range of 0.55 to 0.91. The effective radius was measured to be in a range of 6.1 to 281 μm. The mean flow pore radius was measured to be in a range of 2.3 to 110 μm. The permeability was measured to be in a range of 1.2 to 891 μm 2. The two step-graded wicks were measured to have an effective radius of 44 and 35 μm and a permeability of 792 and 170 μm2, respectively. The two control wicks were measured to have an effective radius of 500 and 211 μm and a permeability of 859 and 391 μm2, respectively.; An analytical model was developed to predict the performance of the wicks. This model, named FLATPIPE, predicted that the step-graded wicks would provide a significant, 2.6 and 10.7 times, improvement in the capillary heat transfer limit compared to non-graded control wicks.; The four wicks were tested in specially designed wick test stand that was instrumented to allow for accurate measurement of wick performance. Water was initially used as the working fluid, but concerns with the poor wettability of water on the stainless steel wicks led to the use of methanol. The effective thermal conductivity of the wicks was measured to be in a range of 0.86–3.4 W/m°C. The conductance of the wicks was measured to be in a range of 0.9–4.6 W/°C. The heat transfer limit of the non-graded base wicks was measured to be in the range of 4.2–26.7 W and was generally higher than the capillary limit predicted by the model FLATPIPE. The failure mode of non-graded base wicks was inconclusive between the capillary heat transfer limit and the boiling heat transfer limit. The heat transfer limit of the step-graded wicks was measured to be in the range of 2.2–11.5 W and was about an order of magnitude lower than the capillary heat transfer limit predicted by the model FLATPIPE. The failure mode of step-graded wicks was determined to be the boiling heat transfer limit.; It was determine that boiling was likely occurring in both the non-graded and step-graded wicks. The non-graded wicks had a path for vapor to escape and, hence, were boiling tolerant. The step-graded wicks trapped the vapor, resulting in premature failure when compared to the predicted capillary heat transfer limit.