Analysis of a Liquid Nitrogen Evaporative Spray Cooling Cell from Rini Technologies

Transcription

1 Analysis of a Liquid Nitrogen Evaporative Spray Cooling Cell from Rini Technologies by John F. Schill ARL-TR-5670 August 2011 Approved for public release; distribution unlimited.

2 NOTICES Disclaimers The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. Citation of manufacturer s or trade names does not constitute an official endorsement or approval of the use thereof. Destroy this report when it is no longer needed. Do not return it to the originator.

3 Army Research Laboratory Adelphi, MD ARL-TR-5670 August 2011 Analysis of a Liquid Nitrogen Evaporative Spray Cooling Cell from Rini Technologies John F. Schill Sensors and Electron Devices Directorate, ARL Approved for public release; distribution unlimited.

4 REPORT DOCUMENTATION PAGE Form Approved OMB No Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports ( ), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) August REPORT TYPE Final 4. TITLE AND SUBTITLE Analysis of a Liquid Nitrogen Evaporative Spray Cooling Cell from Rini Technologies 3. DATES COVERED (From - To) August to October a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) John F. Schill 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army Research Laboratory ATTN: RDRL-SEE-M 2800 Powder Mill Road Adelphi MD PERFORMING ORGANIZATION REPORT NUMBER ARL-TR SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR S ACRONYM(S) 11. SPONSOR/MONITOR'S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited. 13. SUPPLEMENTARY NOTES 14. ABSTRACT An evaporative spray cooling (ESC) cell was received from Rini Technologies of Oviedo, Fl. The cell was tested over the period of August October At this time, there is no closed form expression to describe the spray cooling technique used, and it seems each individual application must be analyzed by its individual characteristics. Much was learned during the testing process, and many subsequent modifications were made to our test system. Data are presented to show this evolution, and results that were achieved at Rini Technologies are presented for comparison. Many improvements were made to our ESC test chamber, and this study will be used as a base line for comparing various cooling techniques in the future. 15. SUBJECT TERMS Liquid nitrogen, evaporative spray cooling, heat transfer 16. SECURITY CLASSIFICATION OF: a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT UU 18. NUMBER OF PAGES 20 19a. NAME OF RESPONSIBLE PERSON John F. Schill 19b. TELEPHONE NUMBER (Include area code) (301) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 ii

5 Contents List of Figures Acknowledgments iv v 1. Introduction 1 2. Theory of Operation (8, 9) 2 3. Test Procedure 5 4. Results 6 5. Summary and Conclusion 7 6. References 9 List of Symbols, Abbreviations, and Acronyms 10 Distribution List 11 iii

6 List of Figures Figure 1. Here the stainless steel cell is shown in the center of the vacuum chamber on a fiveaxis positioning system....1 Figure 2. A schematic of the Rini cell....2 Figure 3. Isotherms of a vapor plotted on a p-v curve. Shows the transition from liquid to vapor at constant temperature and pressure....3 Figure 4. Standard boiling curve showing the various regimes of boiling....4 Figure 5. A summary of the data taken at ARL along with data received from Rini Technologies....7 iv

7 Acknowledgments I would like to thank David Van Ee, Ben Saarloss, and Jennifer Lindauer of Rini Technologies for their expert advice and assistance during this project. v

8 INTENTIONALLY LEFT BLANK. vi

9 1. Introduction With the advent of diode-pumped solid-state lasers for use in high energy laser (HEL) systems, various methods of removing waste heat have been explored. With the output power of HEL systems exceeding 100 kw and a power conversion (optical power out/electrical power in) that can be reasonably assumed to be 50% or less (1), these systems produce a large amount of waste heat that needs to be compensated for. Because the efficiency of both solid-state laser materials and the diodes used to pump these materials improves greatly when they are cooled to liquid nitrogen (LN2) temperature (77 K), it is very interesting to the HEL community to explore methods of cooling laser materials and the diodes used to pump them. In this report, I evaluate the performance of an LN2 evaporative spray cooling (ESC) cell received from Rini Technologies of Oviedo, FL. The cell is shown in figure 1 inside a vacuum chamber that can be pumped down to 1 mtorr, and a schematic of the cell is shown in figure 2. It is very difficult to analyze the performance of an ESC system because so many variables, such as droplet size, droplet velocity, and cold plate orientation, affect its performance (2 6). In addition, the cold plate material itself has a significant effect on the performance (3). Because of these difficulties, it is not possible to make simple closed form predictions of the performance of any ESC system. One must evaluate each system based on the shape of the nozzle, the pressure driving the spray, and the material the spray is impinging on. Also, there are factors in the operation of the cell that also affect the cooling ability, as are discussed further in section 2. One might wonder why one would use ESC given all these difficulties. The answer is that pool boiling with LN2 on copper can achieve a heat transfer of 1 W/m 2 K (3) and spray cooling can achieve an increase of nearly a factor of ~5 in this number (7). Figure 1. Here the stainless steel cell is shown in the center of the vacuum chamber on a five-axis positioning system. 1

10 1) Top flange 7) Liquid nozzle layer 2) Exhaust tube 8) Spray nozzle layer 3) Phase separator 9) Interface layer 4) Vent tube 10) Spray cone 5) Inlet tube 11) Heat source 6) Vapor tube Figure 2. A schematic of the Rini cell. 2. Theory of Operation (8, 9) There are three main forms of boiling heat transfer: pool boiling, flow boiling, and ESC. These three are in fact different manifestations of the same property of a liquid, its latent heat of evaporation. In this process, heat is transferred into the liquid at constant temperature and pressure producing a vapor that is carried away by convection. Figure 3 shows this process in graphical form on a pressure versus volume curve. Curve P, Q, R, C represents the saturated liquid. The temperatures along this portion of the curve are the boiling points or the saturation 2

11 temperatures, T SAT, that are a function of pressure. Curve C, R, Q, P represents the saturated vapor as functions of pressure. At any given pressure, there is a temperature were the liquid becomes completely saturated with energy. This is the boiling point, P, Q, R, also designated the saturation temperature, here T 1 at pressure p p is one. As heat is added, the temperature and pressure remain constant until the entire mass of liquid is converted into vapor P. At 1 ATM or 0 psig, the standard boiling point, T SAT, for LN2 is 77 K. At all other pressures, it is different. For this reason, it is important to keep the pressure in the spray cone (10 in figure 2) as close as possible to 1 ATM. This can be achieved by keeping the outlet tube (2 in figure 2) large. Figure 3. Isotherms of a vapor plotted on a p-v curve. Shows the transition from liquid to vapor at constant temperature and pressure. In all three of these processes, when LN2 is brought into contact with a hot surface, violent boiling takes place, much like what happens when water is poured into a hot pan. From the perspective of a single drop, as the drop reaches the hot surface the bottom portion of the drop begins to evaporate. This vapor then produces a cushion between the hot surface and the liquid. Heat now must be transferred through the vapor via conduction before it can take part in the evaporation process. Conduction is a much less efficient heat transfer process than evaporation and convection. Figure 4 shows a typical boiling curve. The regime of cushioned or film boiling is in the area of E above the Leidenfrost point, defined as that temperature where the cushion begins to form. For our purpose, we want to operate in the area between B and C. Nucleate boiling is a process where many tiny bubbles are formed at the hot surface. These bubbles are then carried away from the surface by convection until they are free from the liquid. New liquid fills in behind at the surface where the bubbles were formed. C is the point of greatest heat 3

12 transfer or Critical Heat Flux (CHF). Note that the lower axis of the boiling curve is the temperature difference between the surface to be cooled, T S, and the saturation temperature of the liquid, in our case, LN2 T SAT = 77 K. After cooling the system so that the interface layer (9 on the side of 10 in figure 2) is as close to 77 K as possible, it is important to take as much time as needed at each increase of heat source to allow the system to come to a temperature below the Leidenfrost point. It is desired that the system operate at or below the temperature of CHF, the nucleate boiling regime. Figure 4. Standard boiling curve showing the various regimes of boiling. Boiling can be described in terms of vapor pressure. The boiling point occurs when the vapor pressure of the liquid is equal to the vapor pressure of the atmosphere around it. Since the vapor pressure of the liquid increases as the temperature of the liquid rises, boiling will not occur unless the temperature of the hot surface is hotter than T SAT. In addition, the vapor pressure of the liquid must also overcome the pressure of the pool of liquid above it. Bubble formation at deeper depths requires higher pressure, and therefore, higher temperatures. In the boiling process, the difference in temperature between the liquid and the vapor in the bubble is the driving force in the heat transfer process between the two phases. The pressure difference between the vapor and liquid is balanced by the surface tension at the interface of the bubble and liquid. In order for bubbles to form, there must be an excess temperature ΔT between 4

13 the hot surface and the liquid, ΔT=T S -T SAT. The greater pressure inside the bubble is offset by the surface tension of the liquid vapor interface in the bubble. In pool boiling, the heat flux is inversely proportional to the square root of the surface tension of the liquid. In ESC, there is no additional pressure (heat) required to overcome the weight of the liquid or its surface tension. Evidence of this effect is the fact that the heat flux is inversely proportional to the volume of the droplets. The smaller the droplets are, the greater the heat flux becomes. In ESC, heat transfer is achieved by wetting the hot surface with a thin layer of liquid. As this liquid evaporates, the spray jet replenishes the liquid that has vaporized and risen from the surface. Another consideration is the temperature difference, ΔT, required to drive the heat through the interface from the heat source. This transfer occurs by conduction of heat through the interface described by equation 1:, (1) where k is the coefficient of thermal conductivity and A is the area of the interface that is participating in heat transfer. ΔT in this instance is the temperature difference between the heat source and the inner surface of the interface (9 on the side of 10 in figure 2). In our case, using copper tungsten as the interface material, this requires a ΔT of several K to move 100 W/cm 2 across the interface. Information gleaned from the Rini Web site (10), indicates that LN2 spray cooling is capable of removing 170 W/cm 2 at 110 K. The problem with this is there is not enough information to gauge how good this system is. Here we are not told if this temperature of 110 K is at the heat source or at the inner surface of the interface (9 on the side of 10 in figure 2). Because of this ambiguity, it is impossible to be certain of the meaning of the numbers given. I will return to this point in the conclusion. 3. Test Procedure A resistive heater of 5.1 Ω capable of providing 200 W of heat was soldered to the interface plate (11 of figure 2) with indium solder. For the initial tests, the interface plate is made of a commercially available 80%/10% copper/tungsten alloy chosen to improve the thermal expansion match between the interface and the solid-state laser material. Unlike copper, the alloy did not wet with solder well, so the interface was coated with titanium/platinum/gold (Ti/Pt/Au) with thickness of 500/600/1000 Å, respectively. This method was borrowed from Juan Ochoa at the Massachusetts Institute of Technology s (MIT) Lincoln Laboratory. A 180-L self pressurized LN2 dewar with a 50-psi relief valve was used to provide the driving force for the LN2 (5 of figure 2). Flow valves were placed at input and output to regulate the pressure at the spray jet (10 of figure 2). A heat exchanger with heater tapes was placed at the output to ensure that any LN2 5

14 that had not evaporated in the cell would be converted to vapor before being exhausted into the room. Initially, it was not understood that the pressure at the spray jet had to be kept as low as possible (per figure 3), so the output line (2 of figure 2) was made of ¼-in stainless tubing. Later this line was changed to ½-in stainless tubing and a pressure gauge was added to the output to insure the pressure at the spray jet would remain below 2 psig. Many different methods were tried to secure a thermocouple to the interface. In the final iteration, very small groves were machined into the interface and the thermocouples were soldered into the groves. This is the method used by Rini Technologies. As mentioned earlier, the test cell is placed in a vacuum chamber of m 3 that is pumped down to ~1 mtorr. After starting the LN2 flow, it would take about 15 min to cool the interface down to below 100 K. At around that point, data would be taken. Input power (heat), interface temperature, inlet and outlet flow rate, and outlet pressure were monitored. 4. Results A summary of the data collected over many tests of the spray cooling cell, along with data provided by Rini Technologies on their testing of the same cell, are shown in figure 5. The scatter plots represent the data taken at the U.S. Army Research Laboratory (ARL), and the line graphs were done at Rini Technologies. The green plot, ARL data 2, was the most recent data taken at ARL. For the Rini data, the differential pressures listed are mostly irrelevant because our LN2 supply only provided 50 psi, making the differential pressure between 5 and 2 in figure 2 nearly 50 psi at all times. ARL data 1 and 3 were taken with the copper tungsten interface at outlet pressures of 8 10 psig. Rini s outlet pressure was always kept below 2 psig. As depicted in figure 3, this is one of the reasons that heat transfer occurred at higher temperatures than at Rini. ARL data 2 was taken with a copper only interface and outlet pressures of 0 2 psig. These are most likely the reasons this data more closely match what should be expected. 6

15 Figure 5. A summary of the data taken at ARL along with data received from Rini Technologies. 5. Summary and Conclusion The result that was predicted was not achieved at ARL. If I choose a single data point from the Rini data on the 47-psi curve (140 W, 1.8 cm 2 interface, and ΔT=15 K), this corresponds to a heat flux of 5.2 W/cm 2 K, which is very close to that reported on their Web site (10). Our best result ARL data 2 (75 W, 1.8 cm 2, and ΔT=11 K) is a heat flux of 3.8 W/cm 2 K. Lack of experience with the technique was surely the reason for this difference. ARL data 1 indicates that we can achieve the overall heat transfer but not without high temperature differences. This is a result of working in the area above the Leidenfrost point, an unstable area of less than maximum heat flux. The improvements that should be made to the system should include the following: a better heat exchanger to insure that the control valve in the outlet system does not freeze; a larger outlet, perhaps ¾-in stainless tube, to insure the lowest, 1 ATM, pressure in the spray cone; and development of a pressure regulation system in order to keep the inlet pressure constant. Rini personnel suggest that using a relief valve to provide pressure is unstable. Rini Technologies has noted that failure to keep the inlet pressure constant caused the temperature at 7

16 the interface to fluctuate. They also noted that for this particular cell operating at heat fluxes above 100 W caused the interface temperature to fluctuate, so they suggested that we stay at 100 W. As a closing note, it is difficult to say that this technique will be viable for HEL system cooling. A recent test made on high power diode bar stacks showed that flow nucleate boiling could achieve a heat transfer of (1400 W, 4 cm 2 interface, ΔT=25 K) 14 W/cm 2 K. A factor of almost three times better than ESC. 8

17 6. References 1. Leisher, P., et al. Mitigation of Voltage Defect for High-efficiency InP Diode Lasers Operating at Cryogenic Temperatures. IEEE Photonics Technology Letters 2002, 22, (24), Aguilar, G., et al. Theoretical and Experimental Analysis of Droplet Diameter, Temperature, and Evaporation Rate Evolution in Cryogenic Sprays. Int. Jour. Of Heat and Mass Transfer 2001, 44, Thome, John R. Engineering Data Book III; Wolverine Tube, Inc., 9-9, Visaria, M.; Mudawar, I. Application of Two-phase Spray Cooling for Thermal Management of Electronic Devices. IEEE Transactions on components and Packaging Technologies 2009, 32 (4), Chen R. H., et al. Droplet and Bubble Dynamics in Saturated FC-72 Spray Cooling in a Smooth Surface. Journal of Heat Transfer 2008, 130, Chen, R. H.; Chow, L. C.; Navedo, J. E. Effects of Spray Characteristics on Heat Flux in Subcooled Water Spray Cooling. Int. Jour. Of Heat and Mass Transfer 2002, 45, Rini, D.; Chen, R.; Chow, L. Bubble Behavior and Nucleate Boiling Heat Transfer in saturated FC-72 Spray Cooling. Journal of Heat Transfer 2002, 124, Eastop, T.; McConkey, A. Applied Thermodynamics for Engineering Technologists. Pearson, Essex, 27 32, Cengel, Y. Heat Transfer: A Practical Approach; McGraw Hill, Hightstown, , Rini Technologies Web site. (accessed July 2011). 9

18 List of Symbols, Abbreviations, and Acronyms ARL Au CHF ESC HEL LN2 MIT Pt Ti U.S. Army Research Laboratory gold critical heat flux evaporative spray cooling high energy laser liquid nitrogen Massachusetts Institute of Technology platinum titanium 10

19 NO. OF COPIES ORGANIZATION 1 DEFENSE TECHNICAL (PDF INFORMATION CTR only) ATTN DTIC OCA 8725 JOHN J KINGMAN RD STE 0944 FORT BELVOIR VA DIRECTOR US ARMY RESEARCH LAB ATTN IMNE ALC HRR 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL CIO LL 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL CIO MT 2800 POWDER MILL RD ADELPHI MD NO. OF COPIES ORGANIZATION 1 DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE M M DUBINSKIY 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE P GILLESPI 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE G WOOD 2800 POWDER MILL RD ADELPHI MD TOTAL: 14(1 ELEC, 13 HCS) 4 DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE O J SCHILL 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE O N FELL 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE O J WHITE 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SEE M L STOUT 2800 POWDER MILL RD ADELPHI MD

20 INTENTIONALLY LEFT BLANK. 12