A helically grooved copper heat pipe with ethanol as the working fluid has been fabricated and tested on a centrifuge table. The heat pipe was bent to match the radius of curvature of the table so that uniform transverse (perpendicular to the axis of the heat pipe) body force fields could be applied along the entire length of the pipe. By varying the heat input (Qin = 25 to 250 W) and centrifuge table velocity (radial acceleration |a⃗r| = 0 to 10g), information on dry out phenomena, circumferential temperature uniformity, heat lost to the environment, thermal resistance, and the capillary limit to heat transport was obtained. Due to the geometry of the helical grooves, the capillary limit increased by a factor of five when the radial acceleration increased from |a⃗r| = 0 to 6.0g. This important result was verified by a mathematical model of the heat pipe system, wherein the capillary limit to heat transport of each groove was calculated in terms of centrifuge table angular velocity, the geometry of the heat pipe and the grooves (including helix pitch), and temperature-dependent working fluid properties. In addition, a qualitative study was executed with a copper-ethanol heat pipe with straight axial grooves. This experimental study showed that the performance of the heat pipe with straight grooves was not improved when the radial acceleration was increased from |a⃗r| = 0 to 10.0g.
The program CAPLIM predicts the capillary limit of an axially-grooved heat pipe as a function of working temperature and heat pipe geometry. Calculations for water, ethanol, and ammonia are built into the program, and a user-defined function is provided to allow for the addition of other working fluids. The thermal-fluid properties of the working fluids are approximated by polynomial curve fits as functions of temperature. The Visual Basic programming language incorporated into Microsoft EXCEL (Version 5.0a) is used to perform the calculations and plot the capillary limit as a function of temperature. A sample calculation is provided to ensure proper operation of the program. A listing of the program code is available which includes a variable dictionary and an explanation of the program flow.This program can be used as a design tool to quickly evaluate potential heat pipe/working fluid combinations.
NOMENCi ATUREvapor core didmeter, m drag coefficient friction coefficient, ( N/m2)/W-m acceleration due to gravity, m/s2 latent heat of vaporization, J/kg thermal conductivity, W /(m-K) permeability, m2 length, m number of grooves pressure, N/m2 heat transfer rate, W heat transfer rate at the capillary limit, W heat transport factor a t the capillary limit, W-m radius, m principal radii of curvature, m Reynolds number temperature, O K
A helically-grooved copper heat pipe with ethanol as the working fluid has been fabricated and tested on a centrifuge table. The heat pipe was bent to match the radius of curvature of the table so that uniform transverse (perpendicular to the axis of the heat pipe) body force fields could be applied along the entire length of the pipe. The steady-state performance of the curved heat pipe under transverse body force fields was determined by varying the heat input (Qin = 25 to 250 W) and centrifuge table velocity (radial acceleration |a→r| = 0 to 10-g). The thermal resistance decreased with increasing heat input until dryout was reached. As dryout commenced, the thermal resistance increased. Due to the geometry of the helical grooves, the capillary limit increased by a factor of five when the radial acceleration increased from |a→r| = 0 to 6.0-g. This important result was verified by a mathematical model of the heat pipe system, wherein the capillary limit of each groove was calculated in terms of centrifuge table angular velocity, the geometry of the heat pipe and the grooves (including helix pitch), and temperature-dependent working fluid properties.
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