Abstract

Revolving heat pipes have been proposed to be used for electric motor cooling and rotating heat exchangers (Thoren, 1984; Gi and Maezawa, 1990). Revolving heat pipes are defined herein as the case in which the axis of rotation is parallel to (but offset from) the central axis of the heat pipe. A literature survey has shown that most researchers utilized smooth--walled non--tapered revolving heat pipes because of their simplicity of manufacture (Thoren, 1984; Gi and Maezawa, 1990; Niekawa et al., 1981; Pokorny et al., 1984; Curtila and Chataing, 1984). However, smooth--walled heat pipes have not proven to be greatly effective because the working fluid is held against the outboard wall of the pipe, leaving a significant portion of the pipe wall dry. A remedy is to have large volumes of working fluid in the heat pipe, which increases the weight of the heat pipe and the potential for imbalance of the rotating system. In addition, it has been found that slight adverse tilt angles between the axis of rotation and horizontal, where the evaporator section is above the condenser section, can severely affect the performance of revolving non--tapered heat pipes. Another possibility is to install the heat pipe with a slight tilt (offset of the evaporator section from the axis of rotation is greater than that of the condenser section), such that the working fluid is forced back to the evaporator section due to the favorable body forces. However, this is not always possible due to the design of the machine to be cooled. Yet another alternative is to machine a slight taper on the interior of the pipe wall, but this severely limits the overall heat pipe length due to difficulties encountered in such a machining process. Therefore, a simple mechanism to reliably return the working fluid back to the evaporator section is needed. Recently, an experimental and analytical study was performed in which the capillary limit of a curved helically--grooved revolving heat pipe was found to increase with rotational speed (Thomas et al., 1998). This was due to the geometric nature of the helically--grooved wick structure. The helically--grooved copper tubing was found to be commercially available at a relatively low cost. The results of the aforementioned study (Thomas et al., 1998) has prompted the present analysis to determine the possibility of using helically--grooved straight heat pipes for the thermal management of rotating equipment. A mathematical model was formulated to determine the operating limits of revolving helically--grooved heat pipes. The capillary limit calculation was complicated by the variation of the body force field along the length of the groove and around the circumference of the heat pipe. This required an analysis of the total body force imposed on the liquid along the length of the helical grooves. The boiling and entrainment limits were calculated using methods described by Faghri (1995).