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Use of Inorganic Aqueous Solutions for Passivation of Heat Transfer Devices

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Use of Inorganic Aqueous Solutions for Passivation of Heat Transfer Devices 10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011 Use of Inorganic Aqueous Solutions for Passivation of Heat Transfer Devices Sean Reilly a, Ladan Amouzegar a, H. Tom Tao b and Ivan Catton a a University of California at Los Angeles, Los Angeles, CA, 90095, USA b Posnett International Co. Ltd., Walnut, CA 91789, USA Tel :310.825.8185 , E-mail: [email protected] ABSTRACT Heat pipes are an effective means by which to transfer heat while maintaining a low temperature difference between the source and sink. Typically a heat pipe is constructed of a conductive material, such as copper, and filled with a working fluid at a given pressure. The favorable heat transfer performance is accomplished because heat pipes transfer energy through both sensible and latent heat. The high heat of vaporization of water and favorable operating range for electronics make it an appealing working fluid for use in heat pipes. However, one drawback of using water based heat pipes is a limitation on the type of casing material that may be used. In particular, water is incompatible for use with aluminum because water will form non-condensable gas when in contact with aluminum. This is unfortunate because aluminum is particularly lightweight and has high thermal conductivity (approximately half that of copper). In this work, an inorganic aqueous solution (IAS), with similar thermophysical properties to water, has been tested as a working fluid for use in heat pipes. Further, a technique by which this fluid can be used effectively with aluminum is presented. It is believed that constituents present in solution react with the surface to passivate it and the presence of ions in the vapor indicate continuous deposition of material throughout the device. It will be shown that the fluid demonstrates similar performance to water heat pipes with no indication of non-condensable gas formation after continuous operation for more than 7 weeks. Lifetime and performance testing will be shown in this work and future applications of this development suggested. The development of aluminum based heat pipes using fluid which has similar latent heat properties to water presents a significant increase over state of the art aluminum devices. Keywords: Inorganic Aqueous Solution (IAS), Non-Condensable Gas 1. INTRODUCTION simplest version of a heat pipe, a thermo-siphon or Perkins tube, the walls of the tube are smooth 1.1. Background and the device is oriented perpendicular to gravity In many devices, protecting the device from a with heat added at the bottom. The fluid is potentially harmful environment is of paramount inserted and is put under a vacuum of a given of importance in increasing the device lifetime. strength in order to manipulate the saturation Environmental concerns can degrade, destroy, or temperature. The saturation temperature is form harmful materials that interfere with normal generally chosen to be application specific, so that operation. For instance, in a heat pipe, use of operation occurs in a prescribed range. Once the water in conjunction with aluminum is an device is sealed it is considered prepared for use. incompatible combination as water will oxidize As mentioned previously, water is an the surface of aluminum, forming aluminum oxide incompatible working fluid for use with and hydrogen. Hydrogen, is non-condensable aluminum. In general, water is an attractive and will eventually build up in the condenser and working fluid due to its high specific heat block the area available for condensation. capacity and its vaporization point can be Heat pipes are incredibly useful phase change manipulated around room temperature with heat transfer devices for use in many applications, interior pressure. Furthermore, Aluminum can such as electronics cooling and space based be attractive casing material due to its high devices. A heat pipe functions by adding heat at thermal conductivity (approximately half that of one end and removing it from the other. Where copper) and low density compared to copper. In the heat is added, liquid is evaporated, raising the applications where weight is paramount, such as pressure inducing flow of vapor to the condenser. space, this can make tremendous impact on the Once the liquid has condensed it travels down the design of a project. Aluminum heat pipes are walls of the tube back to the evaporator. In the therefore forced to use fluids with lower specific - 153 - heat capacities than water for these applications. Previous work by Reilly (2010) showed that This work focuses on a method by which a fluid, IAS performed better than water when used with with many similar thermophyiscal properties to porous copper evaporators. In the instance of the water can be used with an aluminum casing. wicks, the IAS degraded performance over time Various chemical constituents are added to the which led to the discovery of deposits in the wick, liquid which form an inert interface between the seen in Figure . liquid and the casing allowing for continuous use with no evidence of formation of non-condensable gasses. An explanation of the fluid will be presented followed by the experimental method, results, sealing techniques and future work 1.2. What is IAS? The Inorganic Aqueous Solution (IAS) mentioned in this work is a complex mix of approximately 9 chemical constituents, including water, which can be seen in Figure , which shows the results of chromatography performed on the fluid. Figure 2: SEM of IAS Treated Copper Porous Media KMnO 4 SEM photographs were taken showing many ligature shaped deposits in the interstitial spaces CaCr 2O7 between particles and clusters within the wick. It was speculated at the time that these deposits SrCr O might not only have an effect on the physical 2 7 structure and hydrodynamic properties of the wick, but also might affect the surface interactions and MgCr O 2 7 reactions. This hypothesis was inspired in part due to testing on the IAS performed at the Naval Ag Cr O 2 2 7 Research Lab which showed that IAS formed 100x less hydrogen in a reaction test with Figure 1: Chemical constituents of IAS aluminum as compared to water. The fluid itself was originally believed to be a Thermophsyical Property testing performed by solid-state, hyper-conductive surface treatment Amouzegar showed that the properties of the IAS for copper tubing as it was originally proposed varied very little from water. Enthalpy of by Professor Qu, in China. These so called vaporization and surface tension were virtually “Qu Tubes” or “Super tubes” generated identical to that of water, despite generally significant interest in the United States as a improved performance in comparison with that of result of several favorable performance claims. water. However, significant reduction of contact Performance of delivered devices was angle was noted between liquid water and inconsistent, though, especially when tested surfaces that had been treated by IAS. The assuming that the device was performing in a origin of this performance gain is the focus of solid state mode. future work. However, it is worth noting, that The significant presence of chromates might even though many of the properties of the IAS do suggest that the fluid was originally inspired by not differ very much from water, they are chromate passivation schemes, popular for use generally more favorable than evaporative heat with aluminum alloys. Rocco (2004) compared transfer fluids currently used in aluminum pipes. two different methods of chromate coatings on If the passivation of aluminum by IAS can be Al/Zn alloys which were designed to discourage verified, significant improvement in performance corrosion and allow increased adherence of paint. over current devices, such as reduction of weight The increased corrosion resistance might permit and construction cost, etc., can be achieved. water to be in constant contact with an aluminum surface and actively resist oxidation. 2. EXPERIMENT - 154 - 2.1. Experimental Method temperatures inside the evaporator. This was done in order to ensure that the critical value was Aluminum tubes approximately 1 meter in not reached during lifetime testing. length were constructed. These tubes were charged with various amounts of the IAS fluid. Once the tubes reached dryout, heat flux was Based on the test results from the NRL report, it cut to the tube and the evaporator temperature was was desired to characterize the range of allowed to fall to approximately 60 oC. At this non-reactivity of the IAS with aluminum. The point, the heat flux was again supplied, though first tubes were welded shut after being charged below the point of dryout. with the IAS fluid but these tubes eventually failed, As mentioned previously, the tubes were due to the formation of non-condensable gasses. connected to an unregulated power supply, It is speculated that the high temperatures meaning that, the power supplied to the tubes associated with the arc welding process that was varied as the load varied as supplied from a used to seal the tubes initiated a metallurgical common 208V 20 amp wall socket. This effect change in the aluminum casing that allowed is evident when examining the data as the results oxidation between the water in the IAS and the from the temperature readings tend to vary wall. somewhat over the day. Due to the failure of welding, low temperature Data was recorded continuously but the tube solder was used as a sealing method. 3003-H2 was only actively monitored sporadically, in order Alloy aluminum tubing was used in conjunction to determine if an error occurred. A temperature with 6061 alloy end caps to construct the tube. measurment was recorded electronically, This work will document the tubes which were approximately every 10 minutes for lifetime used with low temperature solder as these tubes testing. The data shown here is the result of have so far, showed no sign of formation of lifetime testing conducted over 2 weeks, but the formation of non-condensable gasses.
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