Development of a Miniaturized High Intensity Cryogenic Flow Boiler

Transcription

1 Development of a Miniaturized High Intensity Cryogenic Flow Boiler James Morin, Allison Bender, Edward Hodgson and James Yanosy Hamilton Sundstrand Space Systems International, Inc. Copyright 2002 Hamilton Sundstrand Space Systems International, Inc. Scott Downing and Dam Nguyen Hamilton Sundstrand ABSTRACT An extremely compact heat exchanger is being developed which can boil cryogenic fluids with a liquid heat source at temperatures close to its freezing point. Freezing of the heat source fluid, e.g. water is precluded by the normal flow arrangement. Boiling and superheating of the cryogen occurs as the fluid approaches the heat source in a stack of bonded jet-array laminations. This heat exchanger technology is important in many applications where the storage of fluids at cryogenic temperatures offers substantial advantages in terms of system weight and volume. Often, as in several advanced portable life support system concepts, the advantages include the use of the cryogen as a heat sink in system thermal management. Realizing this benefit and safely conditioning the stored fluid for use requires effective heat transfer between the cryogen and a secondary heat transport fluid. Historically, this has resulted in a need for relatively large heat exchange devices and operational problems associated with the tendency of common heat transport media to freeze at temperatures well above the cryogenic fluid storage conditions. Hamilton Sundstrand has been developing an advanced high intensity heat exchanger design that simultaneously achieves dramatic reductions in the heat exchanger size and eliminates secondary fluid freezing problems. Preliminary testing has demonstrated the ability of this device to boil and thermally condition cryogenic nitrogen or oxygen with circulating water over a wide range of flow conditions without risk of freezing water in the device. This paper presents the design concept for a prototype and a brief summary of its development from an earlier sub-scale compact high intensity cooler for electronics (CHIC). We will also describe the design, analysis, and manufacturing work being performed on the prototype heat exchanger. INTRODUCTION Heat exchange is an essential part of every life support system and frequently a major contributor to system weight and volume. While these parameters are of great concern for all space life support systems, they are especially critical for portable systems like those used in spacesuits such as the NASA Extravehicular Mobility Unit (EMU). To support future exploration missions to planetary surfaces, these systems must be dramatically reduced in weight and volume. Advances in heat exchanger technology have been studied as one promising approach to achieving these reductions through the application of microchannel or high intensity heat exchanger designs. Recently, a significant development effort has been focused on one particularly challenging application, the use of metabolic waste heat to vaporize and thermally condition breathing gas derived from a storage system. This application requires heat transfer between fluids where one boils at a temperature far below the freezing point of the other. Past experience with such systems [1, 2] has shown this to be a knotty problem even without the added requirement for ultra-compact heat exchange. We have been attacking this problem by adapting an advanced heat exchange technology (the CHIC or Compact High

2 Intensity Cooler) developed at Hamilton Sundstrand for cooling high power electronics. The goal of this effort has been to develop and demonstrate the ability to design and manufacture components, which can support many applications of cryogenic fluids with secondary heat transfer loops. To accomplish this, we have worked to produce a full size prototype capable of meeting the needs for breathing gas conditioning and peak heat load rejection for an advanced personal life support system considered by NASA as a candidate for future exploration missions. To date, we have evaluated alternatives and selected a design approach, conducted a proof of concept test that helped refine design analysis correlations, developed a design for the heat exchanger and its component parts, manufactured the parts and begun the optimization of the assembly diffusion bonding process. The design, data developed to date, and the development process are described in this paper. DESIGN CONCEPT EVOLUTION The project was born out of the desire for a smaller and lighter heat exchanger for advanced space suit systems and the recognition that a new design approach would be required to eliminate freeze up problems commonly encountered in existing personal life support systems using cryogenic gas storage. The concept evolved through an evaluation of the design requirements for the application, definition of desirable design characteristics to achieve those requirements, and collaboration between engineering personnel at different Hamilton Sundstrand locations. Design requirements appropriate to a current NASA advanced Extra-Vehicular Activity (EVA) life support system concept [3] were selected as an appropriate target point for this research. This concept requires that the heat exchanger provide the capability to vaporize and warm between 1.0 and 5.1 lbm/hr of cryogenic oxygen from a 100 psi (~-313 F) storage vessel using heat derived from a.40 gal/min water flow which enters the heat exchanger at between 55 and 84 F. HS implementation concepts required two additional cryogen conditioning passes each carrying up to 60% of the original cryogen flow with similar entry temperature and pressure. Delivered gas exiting the heat exchanger is required to be warmer than 32 F under all conditions demanding a heat exchanger effectiveness greater than 92% in the vapor superheat region in addition to boiling the cryogen. To achieve this in a small package and operate successfully in the system under all conditions requires that the heat exchanger: 1) provide high film coefficients for heat transfer in both the water and oxygen passes 2) maintain metal temperatures in the water channels above 32 F 3) avoid degraded cryogen vaporization performance due to film boiling. Concept studies and prior research in cryogen boiling systems showed that the last two of these needs were incompatible with direct heat exchange between the boiling cryogen and water heat source. Film coefficients for boiling below the Leidenfrost point are too high to permit wall temperatures above freezing. High heat transfer fluxes possible with nucleate boiling result in wall temperatures below the freezing point. As a result, both ice formation and film boiling typically result in this region making the heat exchanger less effective and vulnerable to freeze-up. Based on these considerations, a conceptual approach was developed which combined microchannel design and a flow configuration in which vapor in the superheat region provided an intermediate layer between the water heat exchange medium and the boiling cryogen. A concept sketch is shown in Figure 1. Not to Scale Cryogenic O2 Vapor From Coil Liquid O2 From Dewar Vapor Layers Isolate LCG Water From Boiling Liquid to Eliminate Freezing Figure 1. Original cryogenic microchannel flow boiler concept sketch. Discussion of this concept with engineers at another HS location led to the suggestion that the needs for this application could be more simply met by adapting a high intensity heat sink concept they had developed for electronics cooling. This concept, like a microchannel heat exchanger design, limits boundary layer thickness to achieve highly effective heat transfer without incurring excessive pressure drop.

3 Figure 2. Conceptual Drawing of Flow Arrangement for Prototype O 2 Boiler Most importantly, it also inherently provides a flow geometry in which the heat exchange fluids pass through a series of layers successively closer to (or further from) the direct interface between the cold and warm fluids, exactly the pattern required to ensure a freeze-resistant design for our application. This flow arrangement, where the fluid moves through a series of bonded orifice and spacer plates, was first introduced by Bland, et al.[4], to achieve very high heat transfer fluxes (~645 w/in 2 ). The general flow pattern is shown conceptually in Figure 2. The boiling oxygen streams cascade through multi-jet impingement arrays that are separated by spacer plates containing open windows for the jets. The impingement and rearranging flow pattern affords high transfer coefficients. The high wetted surface area density also results in an efficient and compact heat transfer structure. Jet Impi Preliminary analysis indicated that the application was feasible and that a full size unit capable of handling maximum flows of 200 lb/hr of water and 5 lb/hr of liquid oxygen would be approximately 2 inches x 4 inches x ½ inch thick, ideally suited for the application. A small test assembly combining backto-back CHICs (Compact High Intensity Cooler) heat exchangers, see Figure 3, was tested with liquid nitrogen and water as a proof of concept experiment. This feasibility testing demonstrated that the water would not freeze and supported the team s effort to proceed with the design, manufacture and testing of a development unit. FEASIBILITY TEST The CHIC concept has never been used in a cryogenic application. Furthermore, special concerns arise for the space suit breathing system application because the cryogen is being warmed using water. There are two primary concerns about applying the back-to-back CHIC concept to the space suit breathing system application. The primary risk is waterside freezing within the heat exchanger, which Figure 3. Close-up photograph of the heat exchanger with the insulation removed to dramatize the thermal differences. The lower white frosted half is flowing LN2 while the upper half is flowing room temperature water. The penny, attached to one of the feed tubes, provides scale as to the size of the unit that was tested. may cause blockage, degraded performance, and potential structural damage. Another concern is that the small flow passageways will result in an unacceptably high-pressure drop. Testing of the back-to-back CHIC and modeling were used to determine the feasibility of extending this type of impingement heat exchanger to the space suit breathing system application. The testing was conducted with liquid nitrogen and water at various flow rate combinations to evaluate heat transfer performance and pressure drop relationships. The results show that the overall heat exchanger performance varied significantly with both water and nitrogen flow rates. The testing proved that the multilayer impingement microchannel concept could be used to warm cryogenic nitrogen with room temperature water without freezing the water and

4 without structurally damaging the unit. This testing was the first time that the impingement microchannel heat exchanger had been used in a boiling and/or cryogenic application. The testing set-up is shown in Figure 4. The water side of the heat exchanger was supplied from a charged tank to insure a constant temperature source. The filter protects the small heat exchanger passageways, and a ball float rotometer was used to determine the mass flow rate. On the liquid nitrogen side the use of a 100-psi dewar along with the LN 2 coil-chiller ensured sub-cooled liquid at the heat exchanger inlet. The water bath was used to raise the nitrogen temperature to assure all gas phase flow at a constant temperature for accurate readings. The heat exchanger and LN 2 supply lines were heavily insulated. 100 PSI LN2 DEWAR 15u FILTER GN2 LN2 LINE COIL CHILLER 15u FILTER WATER FLOW CONTROL VALVE 60 L WATER TANK P T P T T P LN2 FLOW CONTROL VALVE T MICRO-CHANNEL HEAT EXCHANGER P T T 70 deg F HEAT EXCHANGER WATER BATH FLOW METER TO DRAIN Figure 4. Schematic of Back-to-Back CHIC Test Setup The test results were extremely favorable. The interface temperature between the water and N 2 CHICs (measured by a small inch diameter thermocouple) varied between 34 F and 73 F as shown in Figure 5. This provides sufficient margin against freezing over a wide range of liquid nitrogen and water flow rate combinations. The results also show that the desired pressure drop requirements for a space suit breathing system are achievable. A preliminary requirement established for the development unit is 10 psid at 200-lbm/hr water flow and 3 psid at 5-lbm/hr liquid nitrogen flow. The recorded pressure drops for the sub-scale test CHIC were slightly higher than desired for each fluid, but the heat exchanger design can be adjusted to compensate for the desired pressure drop A data point from the testing at Windsor Locks was selected for use in HS Rockford s CFD ICEPAK FLOW METER HOOD VENT model of the sub-scale back-to-back CHIC. This point was chosen because the magnitude of its nitrogen flow and available heat transfer area are similar to the magnitude of the nitrogen flow for the proposed development unit and its larger available heat transfer area. These results were also encouraging. Temperature ( F) Various LN 2 and Water Flow Conditions Figure 5. Measured Interface Temperature Shows Tolerance To Freezing The calculated pressure drops were well below the specified preliminary requirements for both fluids. The heat exchanger effectiveness was very high as both fluids left at approximately the same temperature. Finally, the predicted interface temperature was 48 F, which would preclude any freezing problems during operation at these fluid flow rates. A Visual Basic/spreadsheet model of the back-toback configuration CHIC was developed to predict the CHIC performance in this cryogenic application. Data from the feasibility testing was used to calibrate the model and thereby more confidently design the prototype unit. The model assumes a one dimensional temperature distribution in the axial conduction bars. The predicted heat transfer and pressure drop is computed based on the assumed plate temperature and local fluid conditions. This assumed heat transfer per plate also defines the thermal gradient in the solid conduction bars between impingement windows. A converged heat transfer solution is achieved by iteratively improving the temperature gradient and the heat duty per plate. The model was also used to determine the unit s ability to prevent waterside freezing and for preliminary sizing for the space suit breathing system. A case was considered with oxygen and water flow rates comparable to those used for the spacesuit backpack system, 6.1 lbm/hr and 180 lbm/hr, respectively. The preliminary analytical 13 Data Point

5 modeling predicted that the interface temperature of the unit would be well above freezing, approximately 70 F. The model indicated that a full size multi-jet, multi-layer micro-channel heat exchanger for a spacesuit backpack system would be approximately 2 x 4 x ½ thick. In conclusion, both preliminary analytic models and testing conducted on the back-to-back CHIC produced results which indicate that a similar CHIC design would be able to meet all the performance needs required for a space suit breathing system. Therefore, the next phase for the manufacture and testing of a development unit has been undertaken. PROTOTYPE DESIGN AND ANALYSIS Using the performance data from the LN 2 H 2 O feasibility heat exchanger a LO 2 H 2 O prototype unit for an advanced EVA space application was designed. Metabolically heated water is used to heat three separate oxygen streams. The inlet condition of the oxygen streams differs as a function of stream passage as well as by the operational point. The inlet condition of the oxygen can range from saturated liquid to superheated vapor and includes low quality (~10%) two-phase flows. The outlet condition ranges from 40 to 52 F. A summary of selected design points is shown in Table 1. The heat exchanger design is internally divided into four identical and parallel heat transfer segments or regions. The #2 O 2 and #3 O 2 streams flow in parallel and each occupy one heat transfer segment. The #1 O 2 stream is ported into two of the regions due to the larger heat duty and restrictive normalized pressure drop (psi/cfm). Key features in the thermal-hydraulic design were selected using a plate-to-plate stepwise heat transfer model. The normal flow arrangement results in an effectiveness curve for the heat exchanger that is nearly flat over a wide range of inlet conditions. At off-design conditions nearly all the heat transfer occurs in the early (entry) layers. With increasing heat duty the boiling zone will move axially closer to the interface. Design Conditions O 2 - #1 O 2 - #2 O 2 - #3 Water Flow, lbm/hr Inlet Temperature, F -260 F -250 F -244 F 56 to 80 Inlet Pressure, psia 134 psia 129 psia 125 psia 50 psia Inlet Condition Superheated Saturated Liquid 10% quality Vapor Liquid Exit Temperature, F 45 F 44 F 39 F ~53 F Maximum P, psid 4 psid 3 psid 3.0 psid 3.0 psid Heat Duty, Btu/hr to 1400 Table 1. Representative Design Cases for the LO 2 H 2 O Prototype Heat Exchanger

6 As evidenced in the proof of concept test results, the manifolding of the inlet and outlet streams is extremely important. If an outlet stream flows in a counter-flow direction next to an active heat exchanging passage, considerable regenerative heat transfer will occur. For example, if the vaporized cryogen from the last impingement layer is ported axially away from that interface in a passage adjacent to the heat transfer stack, it will be cooled (with possible condensation) by the inlet cold (boiling) flow. This same phenomenon, although less detrimental due to the much smaller temperature gradients, will occur on the waterside. For this reason, the outlet O 2 streams have been designed to cross the centerline interface plate and exhaust through passages on the waterside. This porting arrangement will also serve as an O 2 superheater. This porting arrangement is shown conceptually in Figure 6. A photograph of the prototype laminations and a bonded stack (without headers attached) is shown in Figures 7a and 7b. Pass #3 Input Pass #2 Input Liquid LO 2 From Dewar H2O in Pass #3 Outlet Warm GO 2 to use Pass #2 Outlet H 2 O out Pass #1 Outlet Figure 6. Porting Arrangement to Minimize Regenerative Heat Transfer 7a 7b Figure 7. Photographs of Development Hardware in Process Figure 7a shows the individual laminations that make up the heat exchanger and Figure 7b shows the configuration of a stacked and bonded assembly without manifolds.

7 PROTOTYPE MANUFACTURING STATUS Several sets of laminations were manufactured, and as seen in figure 8, a micro channel heat exchanger was successfully diffusion bonded and is presently waiting to undergo evaluation testing. CONCLUSION The work accomplished so far has shown that a compact high intensity flow boiler offers great promise as an effective means of conditioning cryogenically stored breathing gases. Heat transfer densities achieved are substantially higher than in more conventional compact heat exchangers. Specifically: Effective heat exchange and resistance to freezing is verified by both test and analysis results. Design analyses verify expected unit size advantages over more conventional heat exchangers. Test prototype manufacture progress to date has shown no unexpected problems. REFERENCES 1. Yanosy, J.L., (2000). Cryogenics and ECLSS- Past, Present, and Future Challenges. SAE Kosmo, J.J., (1998). Remote Field Site Space Suit Mobility Study Test Report. JSC Figure 8. Photograph showing a completed unit with manifolds and inlet/outlet tubing. 3. Mays, D.C., French, J., Nair, S.S., Miles, J.B., & Lin, C.H. (2000). Design of a Transient Thermal Model of the Cryogenic PLSS. SAE Bland, T.J., Niggemann, R.E., & Parekh, M.B. (1983), A Compact High Intensity Cooler (CHIC). SAE