Skip Navigation
Flight Opportunities

Heat Pipe Limits in Reduced Gravity Environments

Completed Technology Project

Project Introduction

Heat Pipe Limits in Reduced Gravity Environments

Fission power provides game-changing solutions for powering advanced NASA missions. Radiators are needed to reject waste heat from Fission Power Systems (FPS). Titanium – water heat pipes are being considered for use in the radiators of a fission power system option for lunar exploration. Embedded in the radiators and deployed on the surface, heat pipes would operate as thermosyphons, a subset of heat pipes that have no wick in their condenser. Their design is determined in part by the flooding limit which is attributed to the interfacial shear force at the boundary between liquid and vapor, and occurs when concurrent vapor flow is so severe that liquid flow is prevented. Flooding is determined by the thickness of the fluid film on the walls and the interaction of fluid flow with concurrent vapor counter flow, both inversely proportional to gravity. The planned test objective of this project is to validate the gravity-dependent flooding limit model for thermosyphons. Titanium – water heat pipes are being considered for use in the radiators of a fission power system option for lunar exploration. Embedded in the radiators and deployed on the surface, heat pipes would operate as thermosyphons. Thermosyphons are a subset of heat pipes that have no wick in their condenser. Their design is determined in part by the flooding limit which is attributed to the interfacial shear force at the boundary between liquid and vapor, and occurs when concurrent vapor flow is so severe that liquid flow is prevented. Flooding is determined by the thickness of the fluid film on the walls and the interaction of fluid flow with concurrent
 vapor counter flow, both inversely proportional to gravity. Evaluating thermosyphon performance in 1 g may lead to over estimating thermosyphon performance in 0.16 g. A single thermosyphon on a reduced gravity flight may not experience flooding at a pre-determined condition and may respond sluggishly to changes in power while searching for the flooding regime. An array of identical thermosyphons operating over a range of power values, with some operating well below the flooding limit and some well above, would bracket the onset of flooding between known power extremes. Purposely small thermosyphons with minimal thermal mass would respond to changes in ambient gravity promptly. Operating the array through multiple 0.16 g parabolas on a given flight day would provide repetitive observations of those power values that trip into the flooding regime upon 0.16 g exposure. Lesser power extremes on a second flight day would provide greater resolution to the conditions which cause flooding. Additional 0.38 g parabolas on a third and fourth flight day would identify thermosyphon response to a simulated Martian gravity environment. Titanium is a proven heat pipe containment and water is a proven working fluid. An array of thermosyphons is proposed for exposure on the low-g aircraft. With each containing a fraction of a gram of water, the power to the array would be minimal placing a small demand on aircraft electrical resources. Heat rejection by forced air cooling would place a small demand on cabin air conditioning. A laptop computer would gather thermocouple data along the length of each thermosyphon indicating the presence or absence of flooding during flight. Shatto et al. (1996) give an analytic model for the fluid flowing down the wall of a thermosyphon as a function of equilibrium mass flow, based on the work of Katto and Watanabe (1992). The effect of the condensing vapor on the fluid film thickness is similar to a Nusselt analysis where the dependence on gravity appears as the reciprocal of the acceleration due to gravity. The interfacial shear force also appears as the reciprocal of the acceleration due to gravity and also depends on gravity in a complex way, brought into the analysis via the Bond number. The purpose of this work is to gather data on the low-g aircraft to validate the Shatto thermosyphon model at multiple gravity values.

More »

Primary U.S. Work Locations and Key Partners

Technology Transitions

Project Library

Share this Project

Organizational Responsibility

Project Management

Project Duration

Technology Maturity (TRL)

Target Destination

Light bulb

Suggest an Edit

Recommend changes and additions to this project record.
^