Master of Science in Mechanical Engineering (MSME)
Micro heat pipes have been used to cool computer chips, but their heat transfer coefficients are low compared with that of conventional heat pipes. To understand this deficiency, an idealized micro heat pipe is proposed that has simplified interfacial geometry but retains the essential physics. It is a long and narrow cavity of rectangular cross section with the bottom made of a wetting material and the top a non-wetting material. A wetting liquid fills the bottom half of the cavity, and its vapor fills the rest. This configuration ensures that the liquid-vapor interface is pinned at the contact line. Since the pipe is long, the evaporation motion at a cross-sectional plane can be taken as two-dimensional. This two-dimensional motion is governed by a Nusselt number Nu and a Marangoni number M, and is solved in the limits of Nu and M tending to infinity. It is found that evaporation occurs mainly near the contact line in a small region of size Nu-1W, where W is the half width of the pipe. The nondimensional evaporation rate is of order Nu-1 ln Nu. This is used to analyze unidirectional fluid flow and heat transfer along the pipe. Analytic solutions are found for the axial temperature and vapor pressure. The solutions capture for the first time the three distinct regions (evaporation, adiabatic, and condensation) commonly observed in heat pipes. It also explains why the regions do not appear in micro heat pipes. Effective thermal conductivity is studied and improvements are suggested.
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Zhang, Jin, "A model of heat and mass transfer in an idealized micro heat pipe" (2002). LSU Master's Theses. 2677.