Use of an Inorganic Aqueous Solution to Prevent Non-Condensable Gas Formation in Aluminum Heat Pipes

Abstract

Heat pipes are used in many applications as an effective means for transferring heat from a source to a sink. The basic heat pipe typically consists of a solid metal casing within which a working fluid is sealed inside at a given pressure. The latent heat transfer via the heat pipe’s working fluid allows it to carry a larger amount of heat energy than would normally be possible with an identically dimensioned solid metal rod. Water is often used as a working fluid due to its high heat of vaporization and suitable operating range for electronics cooling. For many applications, especially space, aluminum is desired as a casing material for its high thermal conductivity, low weight, and low cost. However, water is incompatible for use with aluminum heat pipes because it forms a non-condensable gas (NCG), hydrogen, when they contact. In this work, an inorganic aqueous solution (IAS), which has thermophysical properties similar to water, has been used as the working fluid with an aluminum alloy 5052-H2 casing. The prepared thermosiphon underwent long-term lifetime testing and the results indicate no tube failure or significant NCG formation for the duration of the 9 week study. Furthermore, the data indicate that the IAS fluid not only inhibited NCG production but also led to a reduction in heat pipe thermal resistance over time. It is believed that the chemicals in IAS react with the aluminum surface to create a compact oxide layer and electrochemical reaction which prevents hydrogen generation. A secondary, hydrophilic surface coating is also generated by the fluid on top of the first oxide (passivation) layer. This hydrophilic layer is believed to be responsible for the heat transfer enhancement which was observed during testing and the reduction in ΔT (defined as Tevap−Tcond) over time. Aluminum heat pipes used currently in practice utilize ammonia, or other non-water based working fluids, which have inferior latent heats of vaporization compared to water or an aqueous-based fluid such as IAS. The use of aluminum heat pipe casings in combination with a water-based fluid such as IAS has the potential to provide a significant increase in heat transport capability per device unit mass over traditional ammonia charged aluminum heat pipes.