Use of Inorganic Aqueous Solutions for Passivation of Heat Transfer Devices

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1 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 : , Swreilly@ucla.edu 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 1.1. Background In many devices, protecting the device from a potentially harmful environment is of paramount of importance in increasing the device lifetime. Environmental concerns can degrade, destroy, or form harmful materials that interfere with normal operation. For instance, in a heat pipe, use of water in conjunction with aluminum is an incompatible combination as water will oxidize the surface of aluminum, forming aluminum oxide and hydrogen. Hydrogen, is non-condensable and will eventually build up in the condenser and block the area available for condensation. Heat pipes are incredibly useful phase change heat transfer devices for use in many applications, such as electronics cooling and space based devices. A heat pipe functions by adding heat at one end and removing it from the other. Where the heat is added, liquid is evaporated, raising the pressure inducing flow of vapor to the condenser. Once the liquid has condensed it travels down the walls of the tube back to the evaporator. In the simplest version of a heat pipe, a thermo-siphon or Perkins tube, the walls of the tube are smooth and the device is oriented perpendicular to gravity with heat added at the bottom. The fluid is inserted and is put under a vacuum of a given strength in order to manipulate the saturation temperature. The saturation temperature is generally chosen to be application specific, so that operation occurs in a prescribed range. Once the device is sealed it is considered prepared for use. As mentioned previously, water is an incompatible working fluid for use with aluminum. In general, water is an attractive working fluid due to its high specific heat capacity and its vaporization point can be manipulated around room temperature with interior pressure. Furthermore, Aluminum can be attractive casing material due to its high thermal conductivity (approximately half that of copper) and low density compared to copper. In applications where weight is paramount, such as space, this can make tremendous impact on the design of a project. Aluminum heat pipes are therefore forced to use fluids with lower specific

2 heat capacities than water for these applications. This work focuses on a method by which a fluid, with many similar thermophyiscal properties to water can be used with an aluminum casing. Various chemical constituents are added to the liquid which form an inert interface between the 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. KMnO 4 CaCr 2 SrCr 2 MgCr 2 Ag 2 Cr 2 Figure 1: Chemical constituents of IAS The fluid itself was originally believed to be a solid-state, hyper-conductive surface treatment for copper tubing as it was originally proposed by Professor Qu, in China. These so called Qu Tubes or Super tubes generated significant interest in the United States as a result of several favorable performance claims. Performance of delivered devices was inconsistent, though, especially when tested assuming that the device was performing in a solid state mode. The significant presence of chromates might suggest that the fluid was originally inspired by chromate passivation schemes, popular for use with aluminum alloys. Rocco (2004) compared two different methods of chromate coatings on Al/Zn alloys which were designed to discourage corrosion and allow increased adherence of paint. The increased corrosion resistance might permit water to be in constant contact with an aluminum surface and actively resist oxidation. Previous work by Reilly (2010) showed that IAS performed better than water when used with porous copper evaporators. In the instance of the wicks, the IAS degraded performance over time which led to the discovery of deposits in the wick, seen in Figure. Figure 2: SEM of IAS Treated Copper Porous Media SEM photographs were taken showing many ligature shaped deposits in the interstitial spaces between particles and clusters within the wick. It was speculated at the time that these deposits might not only have an effect on the physical structure and hydrodynamic properties of the wick, but also might affect the surface interactions and reactions. This hypothesis was inspired in part due to testing on the IAS performed at the Naval Research Lab which showed that IAS formed 100x less hydrogen in a reaction test with aluminum as compared to water. Thermophsyical Property testing performed by Amouzegar showed that the properties of the IAS varied very little from water. Enthalpy of vaporization and surface tension were virtually identical to that of water, despite generally improved performance in comparison with that of water. However, significant reduction of contact angle was noted between liquid water and surfaces that had been treated by IAS. The origin of this performance gain is the focus of future work. However, it is worth noting, that even though many of the properties of the IAS do not differ very much from water, they are generally more favorable than evaporative heat transfer fluids currently used in aluminum pipes. If the passivation of aluminum by IAS can be verified, significant improvement in performance over current devices, such as reduction of weight and construction cost, etc., can be achieved. 2. EXPERIMENT

3 2.1. Experimental Method Aluminum tubes approximately 1 meter in length were constructed. These tubes were charged with various amounts of the IAS fluid. Based on the test results from the NRL report, it was desired to characterize the range of non-reactivity of the IAS with aluminum. The first tubes were welded shut after being charged with the IAS fluid but these tubes eventually failed, due to the formation of non-condensable gasses. It is speculated that the high temperatures associated with the arc welding process that was used to seal the tubes initiated a metallurgical change in the aluminum casing that allowed oxidation between the water in the IAS and the wall. Due to the failure of welding, low temperature solder was used as a sealing method H2 Alloy aluminum tubing was used in conjunction with 6061 alloy end caps to construct the tube. This work will document the tubes which were used with low temperature solder as these tubes have so far, showed no sign of formation of formation of non-condensable gasses. The tubes were laid in a bench top set up and the condenser was inclined approximately 3 degrees above horizontal. 12 thermocouples were arranged on the outer surface of the tube as seen below in Figure. Figure 3: Schematic of Test setup Two 250 W cartridge heaters were embedded in a copper block which encased the base of the tube. The cartridge heaters were connected to an unregulated, variable voltage controller or variac. The heater and cooling blocks were left uninsulated because losses were throught to be small. The tubes used a copper condenser block, in which were hollow passageways to allow water flow in and out for a heat exchanger setup. The tubes were first tested to determined the critical heat flux. This heat flux is determined by the point where the pressure drop required to maintain the evaporation rate defined by the enthalpy of vaporization exceeds the available capillary/graivatational pressure drop, causing a catastrophic dryout in the evaporator. This is evident in the data by a severe rise of the temperatures inside the evaporator. This was done in order to ensure that the critical value was not reached during lifetime testing. Once the tubes reached dryout, heat flux was cut to the tube and the evaporator temperature was allowed to fall to approximately 60 oc. At this point, the heat flux was again supplied, though below the point of dryout. As mentioned previously, the tubes were connected to an unregulated power supply, meaning that, the power supplied to the tubes varied as the load varied as supplied from a common 208V 20 amp wall socket. This effect is evident when examining the data as the results from the temperature readings tend to vary somewhat over the day. Data was recorded continuously but the tube was only actively monitored sporadically, in order to determine if an error occurred. A temperature measurment was recorded electronically, approximately every 10 minutes for lifetime testing. The data shown here is the result of lifetime testing conducted over 2 weeks, but the testing has continued without evidence of degradation over 7 weeks Experimental Results Figure shows the results of maximum flux testing while Figure, and Figure show the results of lifetime testing of one of the aluminum tubes. In Figure 4, you can see the dryout point cleary occuring at about 40 minutes as an input heat flux of approximately 280W was achieved. Because the critical heat flux was 280W, the experiment was conducted with an input power of 270W. Figure 4: Result of Critical heat flux testing on aluminum tubes The next graph, Figure, shows the results of

4 testing performed with a solder sealed tube cooled by closed loop circulating water. The individual temperatures show a variation of approximately 10% of the measured value about the mean. Over the course of the week of testing, the temperatures increased by about 10 o C everywhere but the condenser, as the device came to steady state. Calorimetry in the condensor indicated that the heat exchanger initially removed 140W but rose to about 200W by the time the experiment reached steady state. Figure 5: First week, lifetime testing The second graph, Figure, shows the results from the second week of testing. Again, the variation of the temperature measurements varies approximately 10% around the sample mean. Note that by now, the temperatures remained constant through out the week, indicating the tube had reached steady state. Figure 6: Second week, lifetime testing The results clearly show no conventional trend of the temperatures changing over the lifetime of the tube testing, once the tubes reached steady state. This is a strong indicator that no formation of non-condensable gas is building up in the condenser. Typically, in this type of testing, failure would be indicated by a drop in temperatures in the condenser, a sharp rise in the evaporator temperatures and a gradient forming in the adiabatic region of the tube. This behavior is explained by the loss of evaporative heat transfer and conduction of energy through the skin of the tube becoming the dominant energy transfer method. In both cases, the tubes were sealed with low temperature solder, which prevents heating beyond 150 C to avoid melting of solder, leading to device failure due to lost of vacum. 3. FUTURE WORK Tests are currently underway which first of all utilize a regulated power supply in order to smooth out many of the data points in future testing. Also, it is likely that natural convection from the heater block and condensor block is significant enough to warrant insulation. Based on the calorimetry results, approximately 1/3 of the input power was lost to the environment during testing. Further, use of higher temperature solders might permit the operating range of these devices to be extended. Testing is already under way with new tubes to test for longer periods of time than the 2 weeks reported in this work. It is hoped to acquire data over more than 3 months to have a clearer picture of the behavior of IAS driven aluminum tubes over long periods of time. Many long duration tests are needed to substantiate the reliability of these tubes before they can be deployed in various appications. As stated previously, it is believed that the vapor produced by evaporating IAS is predominantly water, but previous testing results showed the presence of traces of solutes. This has sparked interest in sampling the vapor produced in an active device en situ. A special aluminum tube is being constructed which will have a small siphon near the entrance to the condenser region. This special device will allow the researchers to extract some of the vapor for mass analysis in gas phase. This analysis will help determine whether the vapor transport has an active role in passivating the walls of the tube. Further, if non-condensable gasses do form as a result of sealing techniques or some other

5 unforeseen event, this gas can be extracted for sampling as well to aid in determining its source. Furthermore, if trace amounts of non-condensable gasses are detected, this information could be extrapolated to try and determine an estimate of the lifetime of the device as determined by formation of non-condensable gasses. Finally, other types of aluminum alloy are being tested to determine compatibility with the IAS fluid. The tests discussed in this work were conducted solely with 3003-H2 aluminum. Other aerospace alloys of aluminum such as 5061, 6061 and 6030 are being produced in order to characterize their compatibility with IAS. In parallel with compatibility testing, these newer tubes will also be tested with an eye towards actual performance, rather than just life time testing. Testing will be conducted at various input heat fluxes and condenser conditions. The device will remain in an active state during all testing to help determine failure modes of IAS driven devices. Testing with these new alloys and conditions might permit a wider range of devices to be used with IAS. 4. CONCLUSION A novel Inorganic Aqueous Solution (IAS) is presented for use as a working fluid in a heat pipe and other phase change devices. Lifetime testing was conducted with 3003-H2 alloy Aluminum tubes which were charged with the IAS fluid in air. This testing was motivated by previous investigations conducted at UCLA concerning the evaporation heat transfer applications of the IAS fluid as well multiple studies regarding the passivation of surfaces with Inorganic chemicals. The results of the current life time testing show a lack of failure from non-condensable gas formation with a working fluid made of primarily water for more than 7 weeks. The current work has motivated future work to investigate IAS compatibility with other aluminum alloys as well as various performance environments. Characterization of the active passivation of the device through investigation of the constituents present in the vapor produced in the evaporator is proposed. The results of this work represent an interesting alternative to the current state of art of lightweight heatpipes which typically use fluids with much lower specific heat capacities. ACKNOWLEDGMENT We would like to acknowledge the support for this work under DARPA BAA08-18 MACE. In particular, we would like to acknowledge our program manager, Dr. Avram Bar-Cohen. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. REFERENCES [1] Rocco, A.M. et al, Evaluation of chromate passivation and chromate conversion coating on 55% Al-Zn coated Steel, Surface and Coatings Technology, vol. 179,, pp , [2] Reilly, S., Catton, I. Utilization of Advanced Working Fluids in Heat Pipes, Proc. of the ASME/JSME Joint heat Transfer Conference, AJTEC , [3] Mills, A. Heat Transfer. Homewood, IL: Irwin,1992. [4] Rao, P Thermal Characterization Tests of the Qu Tube Heat Pipe. Masters Thesis, University of Alabama, Huntsville [5] Wasekar, V. M., and R. M. Manglik Pool Boiling Heat Transfer in Aqueous Solutions of an Anionic Surfactant. Journal of Heat Transfer 122, no. 4: 708. [6] Das, S Pool boiling characteristics of nano-fluids. International Journal of Heat and Mass Transfer 46, no. 5 (February): [7] Kendig, M., Buchheit, R., Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluable Chromates,Chromate Coatings, and Chromate-Free Coatings. Corrosion, Vol. 59, No. 5, 2003 NOMENCLATURE Inorganic Aquoeous Solution IAS Non-Condensable Gas - NCG