Investigation of the transient characteristics of a micro heat pipe

An analytical investigation was conducted to determine the potential advantages of incorporating very small (100 jim) heat pipes directly into semiconductor devices. As a result of this investigation, a transient numerical model capable of predicting the thermal behavior of these micro heat pipes during startup or variation in the evaporator thermal load was developed. This numerical model was used to identify, evaluate, and better understand the phenomena that govern the transient behavior of micro heat pipes as a function of the physical shape, the properties of the working fluid, and the principal dimensions. The modeling results were compared with the steady-state results from an earlier experimental investigation and were shown to accurately predict the steady state dry out limit for two different test pipes. Using the verified numerical model, the parameters that affect the axial heat transport capacity were evaluated. The results of this evaluation indicate that in micro heat pipes reverse liquid flow occurs in the liquid arteries during startup and/or rapid transients. In addition, the wetting angle was found to be one of the most important factors contributing to the transport capacity. Nomenclature A = cross-sectional area Cp = specific heat d = distance j = evaporation M = molecular weight m = mass flow rate P = pressure Q = heat transfer q = heat transfer rate R = universal gas constant r = radius of curvature T = temperature t = time V = velocity W = wetted perimeter x — length or distance y = distance