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Advanced capillary structures in grooved heat pipe evaporators − theoretical and experimental investigations

C. Brandt
Chair for Technical Thermodynamics, Darmstadt University of Technology, Petersenstr. 30, 64287 Darmstadt, Germany

Peter Stephan
Center of Smart Interfaces, Technische Thermodynamik, Technische Universitat Darmstadt, Petersenstrasse 17, D-64287 Darmstadt, Germany

Abstract

High performance heat pipes must show the following characteristics: low pressure drop along the liquid flow, high maximum capillary pressure, and high radial heat transfer coefficients in the evaporator and the condenser. The radial heat transfer in the evaporator depends very much on the local heat and mass transfer in a tiny micro region, where the liquid-vapor interface attaches to the wall. Taking these micro region effects into account, the authors suggested an advanced capillary structure to enhance the heat transfer coefficient in the evaporator. The structure consists of so-called 'Re-Entrant' grooves and additional micro grooves. The Re-Entrant grooves combine a high maximum capillary pressure, which is the driving force for the fluid flow in the heat pipe, with a low liquid pressure drop along the grooves. With the additional micro grooves very high heat transfer coefficients can be realized.
The heat and mass transfer through the advanced capillary structure was modeled in a first attempt by superposition of two two-dimensional models. For the experimental investigation a setup with a grooved evaporator plate has been developed. It allows the simulation of the heat and mass transfer processes in the adiabatic zone and the evaporator of an axially grooved heat pipe. A series of experiments has been carried out with water as working fluid and copper as wall material. The widths of the micro grooves are 300 µm and 500 µm, respectively. The vapor temperature varies between 25 °C and 70 °C. The grooved plates are inclined up to a tilt of 6 °. It is shown that for the advanced capillary structure the overall evaporative heat transfer coefficient increases considerably compared with the standard Re-Entrant groove geometry. For the standard Re-Entrant groove geometry and the advanced capillary structure with a micro groove width of 500 µm the experimental results and the numerical predictions agree well within the experimental uncertainty. For the advanced capillary structure with a micro groove width of 300 µm quantitative discrepancies exist.

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