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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 , 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 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 incompatible working fluid for use with and . 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

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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 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

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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 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. testing has continued without evidence of The tubes were laid in a bench top set up and the degradation over 7 weeks. condenser was inclined approximately 3 degrees above horizontal. 12 thermocouples were 2.2. Experimental Results arranged on the outer surface of the tube as seen Figure shows the results of maximum flux below in Figure . 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

Figure 3: Schematic of Test setup 270W. 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 Figure 4: Result of Critical heat flux testing on catastrophic dryout in the evaporator. This is aluminum tubes evident in the data by a severe rise of the The next graph, Figure , shows the results of

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testing performed with a solder sealed tube cooled The results clearly show no conventional trend by closed loop circulating water. The individual of the temperatures changing over the lifetime of temperatures show a variation of approximately the tube testing, once the tubes reached steady 10% of the measured value about the mean. state. This is a strong indicator that no formation Over the course of the week of testing, the of non-condensable gas is building up in the temperatures increased by about 10 oC condenser. Typically, in this type of testing, everywhere but the condenser, as the device came failure would be indicated by a drop in to steady state. Calorimetry in the condensor temperatures in the condenser, a sharp rise in the indicated that the heat exchanger initially evaporator temperatures and a gradient forming in removed 140W but rose to about 200W by the the adiabatic region of the tube. This behavior is time the experiment reached steady state. 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 Figure 5: First week, lifetime testing during testing. Further, use of higher temperature solders might permit the operating The second graph, Figure , shows the results range of these devices to be extended. from the second week of testing. Again, the variation of the temperature measurements varies Testing is already under way with new tubes to approximately 10% around the sample mean. test for longer periods of time than the 2 weeks Note that by now, the temperatures remained reported in this work. It is hoped to acquire data constant through out the week, indicating the tube over more than 3 months to have a clearer picture had reached steady state. 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 Figure 6: Second week, lifetime testing

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unforeseen event, this gas can be extracted for ACKNOWLEDGMENT sampling as well to aid in determining its source. We would like to acknowledge the support for Furthermore, if trace amounts of non-condensable gasses are detected, this information could be this work under DARPA BAA08-18 MACE. In extrapolated to try and determine an estimate of particular, we would like to acknowledge our program manager, Dr. Avram Bar-Cohen. The the lifetime of the device as determined by views, opinions, and/or findings contained in this formation of non-condensable gasses. article/presentation are those of the Finally, other types of aluminum alloy are author/presenter and should not be interpreted as being tested to determine compatibility with the representing the official views or policies, either IAS fluid. The tests discussed in this work were expressed or implied, of the Defense Advanced conducted solely with 3003-H2 aluminum. Research Projects Agency or the Department of Other aerospace alloys of aluminum such as 5061, Defense. 6061 and 6030 are being produced in order to characterize their compatibility with IAS. In REFERENCES parallel with compatibility testing, these newer tubes will also be tested with an eye towards [1] Rocco, A.M. et al,” Evaluation of chromate actual performance, rather than just life time passivation and chromate conversion testing. Testing will be conducted at various coating on 55% Al-Zn coated ”, input heat fluxes and condenser conditions. The Surface and Coatings Technology , vol. device will remain in an active state during all 179, , pp. 135-144, 2004. testing to help determine failure modes of IAS [2] Reilly, S., Catton, I. ” Utilization of driven devices. Testing with these new alloys Advanced Working Fluids in Heat Pipes”, and conditions might permit a wider range of Proc. of the ASME/JSME Joint heat devices to be used with IAS. Transfer Conference, AJTEC2011-44360, 2011. 4. CONCLUSION [3] Mills, A. Heat Transfer. Homewood, IL: A novel Inorganic Aqueous Solution (IAS) is Irwin,1992. presented for use as a working fluid in a heat pipe [4] Rao, P. 2009. “Thermal Characterization and other phase change devices. Lifetime testing Tests of the Qu Tube Heat Pipe.” Masters was conducted with 3003-H2 alloy Aluminum Thesis, University of Alabama, Huntsville tubes which were charged with the IAS fluid in air. This testing was motivated by previous [5] Wasekar, V. M., and R. M. Manglik. 2000. investigations conducted at UCLA concerning the “Pool Boiling Heat Transfer in Aqueous evaporation heat transfer applications of the IAS Solutions of an Anionic Surfactant”. fluid as well multiple studies regarding the Journal of Heat Transfer 122, no. 4: 708. passivation of surfaces with Inorganic chemicals. [6] Das, S. 2003. “Pool boiling The results of the current life time testing show a characteristics of nano-fluids”. lack of failure from non-condensable gas International Journal of Heat and Mass formation with a working fluid made of primarily Transfer 46, no. 5 (February): 851-862 water for more than 7 weeks. The current work has motivated future work to investigate IAS [7] Kendig, M., Buchheit, R., “ Corrosion compatibility with other aluminum alloys as well Inhibition of Aluminum and Aluminum as various performance environments. Alloys by Soluable Chromates,Chromate Characterization of the active passivation of the Coatings, and Chromate-Free Coatings”. device through investigation of the constituents Corrosion, Vol. 59, No. 5, 2003 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.

NOMENCLATURE Inorganic Aquoeous Solution – IAS Non-Condensable Gas - NCG

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