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www.nature.com/npjmgrav ARTICLE OPEN Cryogenic spray quenching of simulated propellant tank wall using coating and ﬂow pulsing in microgravity 1✉ 1 1 1 2 J. N. Chung , Jun Dong , Hao Wang , S. R. Darr and J. W. Hartwig In-space cryogenic propulsion will play a vital role in NASA’s return to the Moon mission and future mission to Mars. The enabling of in-space cryogenic engines and cryogenic fuel depots for these future manned and robotic space exploration missions begins with the technology development of advanced cryogenic thermal-ﬂuid management systems for the propellant transfer lines and storage system. Before single-phase liquid can ﬂow to the engine or spacecraft receiver tank, the connecting transfer line and storage tank must ﬁrst be chilled down to cryogenic temperatures. The most direct and simplest method to quench the line and the tank is to use the cold propellant itself that results in the requirement of minimizing propellant consumption during chilldown. In view of the needs stated above, a highly efﬁcient thermal-ﬂuid management technology must be developed to consume the minimum amount of cryogen during chilldown of a transfer line and a storage tank. In this paper, we suggest the use of the cryogenic spray for storage tank chilldown. We have successfully demonstrated its feasibility and high efﬁciency in a simulated space microgravity condition. In order to maximize the storage tank chilldown efﬁciency for the least amount of cryogen consumption, the technology adopted included cryogenic spray cooling, Teﬂon thin-ﬁlm coating of the simulated tank surface, and spray ﬂow pulsing. The completed ﬂight experiments successfully demonstrated that spray cooling is the most efﬁcient cooling method for the tank chilldown in microgravity. In microgravity, Teﬂon coating alone can improve the efﬁciency up to 72% and the efﬁciency can be improved up to 59% by ﬂow pulsing alone. However, Teﬂon coating together with ﬂow pulsing was found to substantially enhance the chilldown efﬁciency in microgravity for up to 113%. npj Microgravity (2022)8:7 ; https://doi.org/10.1038/s41526-022-00192-w INTRODUCTION According to publications by the members of the Space Cryogenic Thermal Management Group at NASA Glenn Research In NASA’s return to the Moon mission and future mission to Mars, 6 7 Center, Doherty et al. and Myer et al. provided the main areas of a highly efﬁcient cryogenic thermal-ﬂuid management technology research and development for space cryogenic thermal manage- is among the indispensable requirements for successful lunar and ment. The tank-to-tank transfer of propellants in space is mars space missions. The planned propellant fuel depot deployed composed of transfer line chilldown, receiver tank chilldown, in the Lower-Earth-Orbit (LEO) for future deep-space missions, and no-vent ﬁll of the receiver tank. Among all three areas, and the human-carrying spacecraft ﬂying lunar and mars missions 1–4 receiver tank chilldown is considered the most important area as are designed to utilize liquid cryogenic fuels and oxydizers . For the amount of cryogen consumed is the largest. In this paper, we the human mars surface mission, one of the enabling technologies report an advance in microgravity tank wall chilldown heat is the efﬁcient transfer of propellant from the fuel depot to the transfer using a thin-ﬁlm coating and spray cooling. spacecraft propellant storage tank . The actual tank-to-tank The chilldown of the receiver tank wall by spray cooling using propellant transfer, however, has yet to take place, mainly due liquid cryogen is a liquid-to-vapor phase change quenching to the lack of cryogenic quenching data in reduced microgravity process that is characterized by the so-called “boiling curve” as for designing the transfer system. As the existing technology on shown in Fig. 1. This curve represents the tank wall surface heat cryogenic chilldown can only offer relatively very low efﬁciencies ﬂux,q , plotted against the wall surface degree of superheating, and it has not been developed under the microgravity conditions, T T , where T is the surface temperature and T is the W sat W sat a new technology with much higher efﬁciencies and veriﬁed saturation temperature corresponding to the boiling ﬂuid bulk under microgravity conditions is therefore needed for future pressure. A quenching process follows the route D→C→B→A. space missions. Therefore, during chilldown the heat transfer on the tank wall In order to maximize the storage tank chilldown efﬁciency, the surface always experiences ﬁlm boiling ﬁrst due to a very hot tank technology proposed includes cryogenic spray cooling, Teﬂon wall surface. Because the heat ﬂuxes in ﬁlm boiling are relatively thin-ﬁlm coating of the tank inner surface, and spray ﬂow pulsing. quite low, ﬁlm boiling regime always occupies the major portion The completed ﬂight experiments successfully demonstrated that of the total quenching time. Accordingly, the thermal energy cryogenic spray cooling is the most efﬁcient cooling method for efﬁciency in the traditional quenching process is extremely low. the tank wall chilldown in microgravity. Teﬂon coating together According to Shaeffer et al. , the average quenching efﬁciency is with ﬂow pulsing was found to substantially enhance the about 8% that provides a strong incentive to ﬁnd more efﬁcient chilldown efﬁciency in microgravity. The feasibilities of charge- methods for the space storage tank chilldown process. hold-vent for tank chilldown and no-vent-ﬁll for tank ﬁlling in Progressive advances in high power density electronics and microgravity were also successfully demonstrated. high-performance energy systems have precipitated the need for 1 2 Cryogenics Heat Transfer Laboratory Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611-6300, USA. NASA Glenn Research Center, Cleveland, OH 44135, USA. email: jnchung@uﬂ.edu Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; J.N. Chung et al. gravity conditions onboard parabolic ﬂight. The copper block was ﬁrst heated by seven cartridge heaters to a prescribed tempera- ture and then cooled down by spraying water or CFC-113 onto the nickel-plated surface which is only 19 mm in diameter. They observed that the heat transfer in the low heat ﬂux regime below the CHF was enhanced by the reduction in gravity for both ﬂuids. However, the effects of reduced gravity act differently on these two ﬂuids at CHF. The CHF for CFC-113 was decreased in a low gravity level whereas the CHF of water increased. Kato et al. also reported the vanishing of the gravity effect on the heat transfer at high spray volume mass ﬂuxes. As a follow-up study, Yoshida et al. conducted a more comprehensive study of the effects of gravity on spray cooling. Two different heaters were used in this study. One is similar to the copper block described by Kato et al. except that the surface was plated with chromium and 50 mm in diameter. The other was Fig. 1 A typical boiling curve. This cure illustrates different boiling a glass prism plated with a thin transparent indium tin oxide (ITO) regimes and corresponding ﬂow patterns. ﬁlm as the heating element. This transparent glass heater was used in order to visualize the liquid deposition on the heater innovative thermal management technologies to ensure reliable surface for steady-state spray cooling while a copper block was performance and reduce the payload of thermal management used for transient spray cooling test. The working ﬂuids used were systems. Such systems include high current density propulsion water and FC-72. A series of ground-based tests and parabolic systems, high power electronics for energy conversion, high ﬂight tests were performed by varying the test parameters such as power optical sensors, as well as high power microelectronics working ﬂuid, heater surface orientation, heat ﬂux at heater packaged within environmental enclosures. In order to manage surface, the mass ﬂux of the coolant as well as heater types. They the progressively increasing heat ﬂux requirements for thermal reported that gravity level had little effect in the nucleate boiling management systems, a spray cooling system has been proposed regime. Moreover, they suggested a coupled effect of gravity and and under constant development for the past sixty years. Liang spray volume mass ﬂux on CHF. In the case of a low spray volume and Mudawar indicated that spray cooling possesses several ﬂux, neither the magnitude nor the direction of gravity affected advantages: high ﬂux heat dissipation, low and fairly uniform CHF. However, the CHF under reduced gravity is higher than that surface temperature, and ability to cool relatively large surface under the hypergravity by 10 percent. They also noted the areas using a single nozzle. signiﬁcant inﬂuence of gravity on the ﬁlm boiling regime when 9,10 In very recent papers, Liang and Mudawar provided a highly the Webber number was low. And they argued that the comprehensive, thorough, and complete review of the spray deterioration of the heat transfer during the ﬁlm boiling in the cooling research up to 2017. Almost all of the published papers case of low Webber number and reduced gravity condition is due were using water and refrigerants and we found only three papers to a lack of secondary impingement on the heater surface. on the study of terrestrial cryogenic spray cooling. Sehmbey As indicated by the literature review above, we only found a et al. performed a liquid nitrogen spray cooling experiment to handful of terrestrial cryogenic spray cooling research papers. gather heat transfer characteristics to facilitate the operation of However, there has been no attempt on microgravity and reduced power electronics at very low temperatures. Four different nozzles gravity cryogenic spray cooling using either room-temperature liquids at various pressures were used to study the variation in spray or cryogens. We believe that the current paper is the ﬁrst to report the cooling heat transfer at liquid nitrogen temperature. The effect of experimental data on cryogenic spray cooling in reduced gravity. nozzle and ﬂow rate on the critical heat ﬂux (CHF) and overall heat 16 According to transient conduction heat transfer theory , if two transfer characteristics were presented. Cooling heat ﬂuxes close materials A and B were put together in contact, then the 6 2 to 1.7 × 10 W/m were realized at temperatures below 100 K. The 00 instantaneous heat ﬂux q from material A to material B is given 4 5 2 A!B mass ﬂow rate range was from 6.1 × 10 to 3.2 × 10 kg/h m . They by Eq. (1) below, 6 2 demonstrated that a high heat ﬂux (over 1.0 × 10 W/m ) cooling k T T k T T technique, such as spray cooling, will have to be used to realize all A A;i s B B;i s q ¼ ¼ (1) A!B 1=2 1=2 the advantages of low-temperature operation. Following their ðÞ πα t ðÞ πα t A B experimental study, Sehmbey et al. further provided empirical Where T and T are constant temperatures of A and B just correlations for liquid nitrogen spray cooling. They offered a A;i B;i before contact, respectively. Also, thermal conductivities of A and general semiempirical correlation (based on macrolayer dryout B are k and k , respectively. α and α are thermal diffusivities of model) for spray cooling CHF for different liquids and spray A B A B A and B, respectively. T is the interface temperature, while t is the conditions. An empirical correlation for heat ﬂux was also s elapsed time after the contact. presented. They also pointed out the importance of surface 00 1=2 We can see that q is a function of t during the roughness for spray cooling with liquid nitrogen. It was discovered A!B transient . In essence, initially, the heat transfer rate between the that the rougher surfaces have signiﬁcantly higher heat transfer two materials is very high, but it also drops off quickly. As a result, rates and similar CHFs occurring at lower temperatures. for the pulsed ﬂow quenching process, at the moment when the Somasundara and Tay investigated the intermittent liquid pulsed ﬂow is switched on in a duty cycle, the disk surface gets in nitrogen spray cooling for applications, which require higher contact with a fresh cooling ﬂuid that induces a peak in the heat heater operating temperatures (−180 to 20 °C). This intermittent transfer rate that produces higher cooling rates than the spray cooling process can be adjusted using the mass ﬂow rate, pulsing frequency, and duty cycle (percentage of open time in one streaming ﬂow case. Based on Eq. (1), the duty cycle (DC) of the cycle) to match the required cooling rate on the target. The pulse ﬂow is the dominant factor on the cooling enhancement intermittent spray experiments were conducted for various ranges solely by the exponential decay of the thermal transport in time, of surface temperatures. the effect of different periods is only of the second-order effect. 14 17 Kato et al. studied the gravity effects on liquid spray cooling Chung et al. found that the pulsed ﬂow would raise the using a nickel-plated copper block in terrestrial and variable chilldown efﬁciency up to 67% over the continuous ﬂow case for npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; J.N. Chung et al. Film heaters Sensors UPS power supply Relays power supply DAQs 80L dewar FM transmitter Laptop Precooler Solenoid valve Test section Gas cylinder rig Vaporizers Flow meter Vacuum pump a b Fig. 2 Photographic images of the experimental system. a Front view, b Back view. the convective chilldown of a metal pipe. Chung et al. also reported that the chilldown efﬁciency increases with decreasing DC, but it is insensitive to the period. The basis of quenching heat transfer enhancement by the low- thermal conductivity coating is given in Chung et al. . As shown in Chung et al. for convective metal tube chilldown, both thermal diffusivities and thermal conductivities ratios between the stainless steel and the coating material are involved, but clearly, the thermal conductivity ratio dominates the transient process such that the low- thermal conductivity coating can facilitate more than an order of magnitude larger drop of the tube wall surface temperature for the 18,19 initial period after the quenching is initiated. Chung et al. used thin-layers of Teﬂon for enhancing heat transfer during chilldown of a metal pipe and they found that the coatings could increase the chilldown heat transfer efﬁciency up to 109 and 176% in terrestrial 19 18 gravity and microgravity , respectively. The primary objective of the current set of microgravity Fig. 3 The ﬂuid piping schematic and instrumentation diagram of experiments is to obtain transient quenching heat transfer the experimental system. The valves and important components of characteristics of a typical storage tank wall surface simulated by the ﬂuid network. Relief valve settings, the burst disk setting, and a metal circular disk. The disk transient temperature history or a pressure regulator settings are also included. BD burst disk, BV ball chilldown curve during spray quenching from room temperature valve, CV check valve, FM ﬂow meter, GN2 gaseous nitrogen, GV to LN2 saturation temperatures was measured. One of the two globe valve, LN2 liquid nitrogen, PG pressure gauge, PR pressure regulator, PT pressure transducer, RV relief valve, SV solenoid valve, disks was coated with a low-thermal conductivity thin Teﬂon layer TC thermocouple, Vap vaporizer, 3 V three-way valve. to evaluate the heat transfer enhancement. The effectiveness of the coating was evaluated by a comparison of chilldown efﬁciencies with a coating to those of a bare surface disk. Tests well as provides the LN2 for the prechilling of all the ﬂuid were carried out with a set of pulse ﬂow conditions that includes components upstream of the test section prior to the actual 40, 50, and 70% duty cycles with 1 s period over a wide range of chilldown test. Before the experiment, the 80 L dewar is topped off test section inlet pressure levels and corresponding mass ﬂow with industrial-grade liquid nitrogen from a standard Airgas 180- rates. The effectiveness of the pulse ﬂow was evaluated by a Liter dewar through the LN2 ﬁll port. The subcooler is essentially a comparison of chilldown curves with ﬂow pulsations to those with simple shell-tube heat exchanger and it serves two functions. The constant and continuous ﬂows. ﬁrst one is to subcool the liquid nitrogen before it enters the test section such that the thermodynamic state of the liquid entering the test section can be determined. During the subcooler METHODS operation, the inner ﬁnned tube of the subcooler is totally Experimental system submerged in the liquid nitrogen bath on the shell-side. Since the Figure 2 shows the photos of the parabolic ﬂight experimental pressure inside the tube is always higher than that on the shell- system. The test chamber of this apparatus is designed to side, the liquid nitrogen bath is always colder than that inside the comprise two nozzles to spray liquid nitrogen onto two separate tube. Thus, heat is removed from the liquid in the tube side. The stainless-steel circular disks simultaneously. All the system second function is to save the liquid nitrogen during the pre-test components except the high-pressure gas cylinder ﬁt into a (L × chilldown of the upstream components of the test section. The W × H) 1.4 m × 0.8 m × 1 m 8020 aluminum frame. This highly vapor generated on the shell-side is separated by gravity and is integrated thermal-ﬂuid system was installed on the ﬂoor of ZERO- vented outside the system. G Corporation’s Boeing 727-200 F aircraft to perform the The test section is basically a vacuum chamber where the parabolic ﬂight disk chilldown experiment in a simulated reduced cryogenic spray cooling of the disk takes place during the gravity environment. The reduced gravity is achieved through experiment and it is made mostly by off-the-shelf vacuum ﬂying the aircraft in a parabolic trajectory and each parabola −2 components. The exploded view of the test section is given in provides about 17–20 s reduced gravity (10 g) period. Fig. 4, which shows the conﬁguration of the two test disks and two The experimental apparatus consists of four essential ﬂuid units spray nozzles. Two spray nozzles are placed at the center between together with auxiliary components, ﬂuid piping and instrumenta- tions. The ﬂuid piping schematic and instrumentation diagram is two disks inside the 10-inch cubic vacuum chamber. It is noted shown in Fig. 3. The 80 L double-wall cryogenic dewar supplies the that the ﬂow direction is perpendicular to the disk and the ﬂow is LN2 to the test section for performing the disk chilldown test as parallel to the gravity. Two stainless-steel disks, cut from a Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2022) 7 J.N. Chung et al. 16-gauge 304 stainless-steel sheet with a thickness of 0.058 the plate. Eight MINCO ﬁlm heaters (Hap 6945 and 6946) are inches, were mounted at opposite sides of the chamber, for attached to the back of the disk to reheat the stainless-steel example, the front and back or top and bottom. Depending on the plate back to the initial temperature for the next test after each purpose of the test, either one set of nozzle and plate or two sets chilldown test. The ﬂow coming out of the test section goes into two separate of nozzles and plates are installed. For the ground test, only a single nozzle and one plate were installed inside the test chamber. vaporizers in parallel (labeled as Vap2 and Vap3 in Fig. 3). The The orientation of the heat transfer disk surface with respect to vaporizers are basically heat exchangers made from tube bundles, the gravity direction is varied by placing the stainless-steel disk at which evaporate any remaining liquid nitrogen coming out of the the bottom, side, or top of the cubic test chamber, and they are test section and also heat up the cold nitrogen vapor to above 0 °C before venting pure vapor out of the system. Each vaporizer is referred to as upward, vertical, and downward conﬁgurations. For the ﬂight tests, two different disk plates and two nozzles were made by packing eight grooved copper tubes that have star- installed as shown in Fig. 4. Each disk is sandwiched by two bored shaped inner insertions into a 2.5″ schedule 40 stainless-steel pipe. vacuum ﬂanges. Two PTFE gaskets were compressed against the The tube bundles are heated up to 200 °C before each test by a stainless-steel disk such that the cubic test chamber and the back high-temperature heating tap that is wrapped around the outer of the disk can be sealed. The outermost ﬂange on each test disk surface of the stainless-steel pipe. The Labview program monitors assembly is connected to the vacuum pump such that the back of and controls the on and off of the heating tap by the combination the stainless-steel disk is insulated from the surroundings by of a K-type thermocouple, NI 9211 thermocouple input module, NI drawing a vacuum to minimize the parasitic heat input from the 9472 digital output module, and a mechanical relay. If the outside environment. For measuring the disk transient tempera- experiment is performed onboard the aircraft, the gas coming out ture history during the chilldown, 25 thermocouples (TCs) were of the vaporizers is vented outside the aircraft cabin through soldered to the vacuum side (back) of each stainless-steel disc. As rubber hoses that connect to the vent ports on the cabin wall. shown in Fig. 5, a total of 24 TCs were distributed on the 6 Similarly, another vaporizer, Vap1 ensures the proper venting of concentric circles (6 rings, R1, R2, …, R6) in addition to one (TC25) gaseous nitrogen coming out of the subcooler. placed at the center. Only one TC (TC5) is placed on the outermost The data acquisition system including the Labview program point near the outer boundary. TC5 is 3.7″ away from the center of and National Instrument Compact DAQ hardware collects all sensor data and displays the real time on a laptop at a sampling rate of 16 Hz. NI 9214 TC modules read all the T-type TCs (Omega). NI 9205, an analog input module, reads all the voltage signals from pressure transducers (Omega PX 409V5A) and the 4–20 mA current signals (through a 249-ohm resistor) from the Coriolis liquid ﬂow meter (Micro motion CMF025). The Labview program controls the opening and closing of the solenoidal valve, SV1, through a combination of NI USB 6009 and a Solid- State relay. In the case of a continuous spray, the relay energizes the solenoid valve after receiving a continuous voltage signal. However, in the case of an intermittent spray, the relay energizes and de-energizes SV1 according to a rectangular waveform voltage signal from the VI. In the current experiment, we used two types of disks. In Fig. 4 A CAD drawing of the test section. The test chamber is 10- addition to the bare surface stainless-steel disk, we also added a inch cubic and it houses two spray nozzles. coated disk that is a stainless-steel disk coated with a low-thermal Fig. 5 Schematic of thermal couple placement. Locations of 25 thermal couples are shown on six concentric circles. npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA J.N. Chung et al. conductivity thin-ﬁlm Teﬂon layer on one side of the disk surface. Table 1. Measured quantities and their uncertainties. Speciﬁcally, the coating material was made of Fluorinated Ethylene Propylene (FEP) by DuPont and classiﬁed by DuPont as Symbol Quantity Measurement method Uncertainty Teﬂon 959G-203 that is a black color paint and has a thermal T Plate temperature T-type thermocouple 1 K or 1.5% conductivity of 0.195 W/mK (DuPont publication ). The coating below 273 K was put on using the dip and drain process. The thickness of the P Test section Kulite CTL-190M140BARA 7 kPa coating is estimated at around 20–30 microns. The thickness of pressure the Teﬂon coating was estimated by previous experiences δ SS plate thickness Calipers 0.03 mm obtained from an identical coating method. In our previous pipe R Radial position of Ruler 1.6 mm (1/ chilldown experiment , the thickness of the coating on the tube x TCs on Ring x 16″) inner surface was measured by the high-resolution X-ray ṁ LN mass ﬂow rate Coriolis ﬂow meter 0.3% computer tomography (CT) scan. Since we used the same method 2 M Mass of the ﬂuid scale 0.1 g to coat the disk plate as that used in the tube and expected the components thickness of the coating is similar to that of the tube coating. (tube, tee, nozzle) Experimental procedure To perform the chilldown test, mainly four steps are followed, and Table 2. Experimental conditions for the six ﬂight cases. these are initial starting, precooling, testing, and reheating. The initial starting is the step where all the electrical devices are turned Case P (psig) Duty cycle (%) Period (second) g-level in on. This includes running the preprogrammed Labview script and 1 80 100 NA microgravity turning on the vacuum pump (Turbo Lab 80). Once the Labview script is running, it will automatically set the output voltage of the 2 80 40 1 microgravity DC power supply for the pressure transducers and turn on the 3 80 70 1 microgravity heating cables of the vaporizers. This step takes about 30 min 4 60 100 NA microgravity mainly due to the time required for heating up the vaporizers to 5 90 100 NA microgravity 200 °C. Meanwhile, the globe valves, GV1, GV2, GV3 are open for 6 90 50 1 microgravity the next step. The second step involves the precooling and prechilling of all the piping and ﬂuid components upstream of the test section to make sure that only the liquid phase of working Experimental uncertainty ﬂuid enters the test section and it proceeds ﬁrst by rotating the Table 1 lists all the uncertainties or the independently measured three-way valve, 3V1, from GV1 to GV2, and setting the desired quantities. The uncertainty for the chilldown thermal efﬁciency is testing pressure on the pressure regulator, PR. Once the solenoidal discussed above in the Results section. valve SV2 is open by clicking the virtual button on the Laptop screen, then the liquid nitrogen starts to ﬂow from the 80 L dewar Reporting summary into the shell-side of the subcooler. Before the liquid nitrogen can Further information on research design is available in the Nature ﬁll up the shell-side of the subcooler, the ﬂow path upstream of Research Reporting Summary linked to this article. the test section has to be chilled down. This step prevents the boil-off of liquid nitrogen before it ﬂows into the test section. Once the inner tube of the subcooler is full (can be determined by RESULTS the proﬁle reading of the TC located inside the subcooler), the Microgravity experiments completed system is ready for the experiment. The chilldown test is started The University of Florida ﬂight team led by Professor Jacob Chung simply by clicking the virtual start bottom on the laptop screen, then the solenoid valve, SV1, will be opened according to the performed parabolic ﬂight experiments in one ﬂight on 12 November 2018. Since we only had one parabolic ﬂight to perform preset waveform signals either to continuously or intermittently ﬂow nitrogen into the test section for spraying on the target disks. theexperiments,wewereonly abletouse one test section conﬁguration. As shown in Fig. 4, the test section is composed of Once all the temperature readings from the TCs drop to the liquid two disks facing each other and they were sprayed on by two nitrogen temperature and maintain at a steady-state, the disk separate spray nozzles, respectively, sharing the same liquid chilldown experiment is complete. The solenoid valve, SV1, is then nitrogen feed line such that we could obtain two sets of chilldown closed by clicking the virtual stop bottom. Next, the reheating step starts to prepare the stainless-steel disc plates for the next test. To data with identical spray ﬂow condition simultaneously in one experimental run. As discussed in the Method section, one target heat up the plates after chilldown, the ﬁlm heaters are turned on by clicking the heating virtual bottom on the screen. Once the was a stainless-steel disk coated with a low-thermal conductivity Teﬂon 959G-203 thin-ﬁlm layer , and the other was the bare plates are heated back to room temperature, the ﬁlm heaters are turned off. This marks the beginning of the next cycle of testing surface stainless steel disk without any coating to serve as the baseline case for evaluating the coating effects. Table 2 lists the six which starts with the precooling step. In addition to the experimental procedure discussed above, cases we performed during the one parabolic ﬂight experiment. Due to the limited runs allowed, only three pulse ﬂow cases were next, we need to mention the simulated microgravity environ- ment on the parabolic ﬂight. The variable gravity condition inside performed. Also, the only one-second period was chosen due to the airplane was created when the airplane is ﬂying a parabolic limited microgravity times and the relative insensitivity of period to trajectory . The microgravity period is always sandwiched heat transfer. It is noted that the gravitational acceleration for Mars 2 2 between two 1.8-g periods where g is the earth’s gravity of is 3.711 m/s that is 37.8% of earth’s gravity of 9.81 m/s .In our 9.81 m/s . The microgravity period nominally lasts between 18- notation, the Martian gravity is 0.378 g where g is earth gravity. As 25 s. For our research ﬂights, in order to maintain the acceleration mentioned above, the gravity under simulated microgravity levels within ±0.01 g, the microgravity period is around 18–20 s. environment in the parabolic ﬂight is about ±0.01 g. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2022) 7 J.N. Chung et al. closest to the green group, and the purple group is the farthest. It is very clear that the closer to the center the faster the rate of chilldown. The center (TC25) is always the fastest chilldown point. As a result, all the chilldown curves are aligned in a predictable order which is that the chilldown curve from a TC that is located farther from the center is always positioned to the right side of the curve from a TC located closer to the center. Boiling curves For every chilldown curve, there exists a corresponding unique boing curve. A boiling curve illustrates the heat transfer characteristics during chilldown by providing the inner wall transient surface heat ﬂux as a function of inner wall surface temperature. Figure 7a2, b2 provide two sets of chilldown curve versus boiling curve plots with data obtained by the center TC25 and TC9 on Ring 3, respectively, for Case 1 using the bare surface disk. It is noted that the tube inner surface temperature and heat ﬂux were obtained using the measured outer wall surface temperature through an inverse conduction method developed by Burgraff . Readers are referred to current 17,19 authors’ previous papers for details. The chilldown curve and corresponding boiling curve are shown in Fig. 7.Speciﬁcally, Fig. 7a, b show the corresponding boiling curves as explained in Fig. 1 where the heat transfer regimes, Leidenfrost point (LFP) and CHF point as a function of the disk wall surface temperature. According to Fig. 7a, the LFP is found just before the sharp slopechangeofthe chilldown curve, and the almost vertical line belongs to the transition regime, Fig. 6 Typical chilldown curves. a A typical set of full 25 chilldown CHF, and nucleate boiling regime. regimes. Comparing between Fig. curves from a bare surface stainless-steel disk spray chilldown 7a, b, the heat transfer rates at the center were much higher than experiment, b a simpliﬁed set of chilldown curves from Fig. 3a. those at the TC9 location due to higher spray mass ﬂuxes at the disk center that resulted in much quicker chilldown time at the center. Typical chilldown curves Disk plate chilldown thermal efﬁciencies As shown in Fig. 5, for measuring the disk transient temperature variation history at different locations during the chilldown for In order to measure the spray chilldow performance, we adopted a determining the quenching efﬁciency, 25 thermocouples (TCs) spray thermal efﬁciency concept that measures how much of the were soldered to the vacuum side (back) of the test disk. A total of cooling capacity of the supplied spray cryogenic liquid is actually 23 TCs are distributed on six concentric circles (six rings, R1, R2, utilized in chilling down the disk. The chilldown efﬁciency as deﬁned …., R6) in addition to one (TC25) placed at the center and one below represents the percent of available total quenching capacity of (TC5) placed on the outside near the outer boundary. TC5 is 3.7″ the liquid cryogen supplied that is actually utilized in cooling the disk away from the center of the disk plate (target of spray cooling) from room temperature to liquid saturation A chilldown curve for a local point on the disk is deﬁned as the temperature of LN2 corresponding to spray chamber pressure. The transient temperature history at this point recorded by a thermal spray chilldown thermal efﬁciency for a disk plate, η , is therefore CD couple (TC) during the chilldown process. So, these curves are the deﬁned as, plots of the temperatures measured by the TCs soldered on the back Removed η ¼ ´ 100% (2) of the disk that registered the disk back local surface temperature CD Available histories during chilldown. Figure 6a shows a typical set of full 25 chilldown curves from a spray quenching experiment. The chilldown In the above, Q is the total thermal energy removed from Removed curves showninFig. 6a were obtained from the bare surface stainless- the disk plate by the cooling ﬂuid during chilldown and is deﬁned as, steel disk in 1-g condition. Figure 6bisasimpliﬁed version of Fig. 6a where only the medium chilldown curve for each ring is plotted in Q ¼ðÞ M cðÞ T T þðÞ M c ðT Removed TARGET p; SS initial final NONTARGET p; SS initial addition to the center TC25 and outer boundary TC5. A medium T Þ (3) NONTARGET; AVERAGE chilldown curve is the one with a chilldown time in the middle among those of the four curves from the same ring. At any instant, a where M is the mass of the chilldown target area that totally TARGET local point on the back surface is the warmest and the corresponding intercepts the spray cone and is completely cooled down by the point on the front surface is the coolest, therefore, chilldown at a spray ﬂuid to the saturated liquid nitrogen temperature. It is noted point is considered complete only when the back temperature at that that the target area is also a circular disk with a radius of r target point has reached the saturated liquid temperature corresponding to which is part of the whole disk with a radius of r from the center. the local pressure. Chilldown time is therefore deﬁned based on the c is the stainless steel speciﬁc heat for the disk plate material. P; SS disk back surface temperatures. As seen in Fig. 6a, based on the T and T are the initial temperature when the chilldown is Initial Final individual TC’s distance from the center, we have divided the started and the ﬁnal temperature of the target when the chilldown curves into four groups identiﬁed by red, green, yellow, and chilldown is completed, respectively. The end of chilldown temperature is the liquid saturation temperature corresponding purple double-headed arrows. The red group includes the center TC to the local pressure. M is the non-target area that does and all four TCs on R1, the green group includes all eight TCs on R2 NONTARGET not receive any liquid spray and is therefore equal to the total disk and R3, the yellow group includes all eight TCs on R4 and R5, and the plate mass, M minus that of the target area, purple group includes three TCs on R6. The color is also used to DISK PLATE identify the TCs as shown in Fig. 5. For example, TCs 2, 10, 14, 22, and M ¼ M M . T is the esti- NONTARGET DISK PLATE TARGET NONTARGET;AVE 25 belong to the red group. As a result, the red group is closest to the mated mass averaged temperature of the non-target area at the center, the green group is the next closest, the yellow group is next end of chilldown based on the conservation of thermal energy. npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA J.N. Chung et al. Fig. 7 Chilldown curve and boiling curves for Case 1, 80 psig inlet pressure, and continuous ﬂow with stainless-steel bare surface disk in micro-G. (a1) chilldown curve from center TC, (a2) boiling curve from center TC, (b1) chilldown curve from TC9 on Ring 3, and (b2) boiling curve from TC9 on Ring 3. T is calculated using the ﬂowing equation, Where q00 ðÞ t is time-dependent heat ﬂux due to heat NONTARGET;AVE Boundary of Target conduction at the interface between the target and non-target ðÞ M c T T ¼ Q (4) NONTARGET p; SS initial NONTARGET; AVERAGE NONTARGET areas. The non-target area is only cooled by conduction heat ﬂow t through the interface between the target and non-target areas as end Q ¼ q00 ðtÞ2πr t dt (5) the non-target surface area does not receive spray droplets for NONTARGET Boundary of Target Target Plate cooling. r is the radius of the target area and t is the Target plate Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2022) 7 J.N. Chung et al. Fig. 8 Simpliﬁed chilldown curves for 80 psig inlet pressure, continuous ﬂow (Case 1). a Teﬂon coated disk in micro-G, b stainless-steel bare surface disk in micro-G, c Teﬂon coated disk in terrestrial gravity, d stainless-steel disk in terrestrial gravity. Fig. 9 Simpliﬁed chilldown curves for micro-G, 80 psig inlet pressure. a 40% DC, 1 s period (Case 2) for Teﬂon coated disk, b 40% DC, 1 s period (Case 2) for stainless-steel bare surface disk, c 70% DC, 1 s period (Case 3) for Teﬂon coated disk, d 70% DC, 1 s period (Case 3) for stainless-steel bare surface disk. thickness of the disk plate. q00 ðÞ t can be estimated by chilldown curve at the outer boundary of the target area and the Boundary of Target using the commercial software, Ansys Fluent, with the two other at the outer boundary of the non-target area were used as measured chilldown curves as the boundary conditions. One the boundary conditions. npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA J.N. Chung et al. Fig. 10 Simpliﬁed chilldown curves for micro-G. a 60 psig inlet pressure, continuous ﬂow (Case 4) for Teﬂon coated disk, b 60 psig inlet pressure, continuous ﬂow (Case 4) for stainless-steel bare surface disk, c 90 psig inlet pressure, continuous ﬂow (Case 5) for Teﬂon coated disk, d 90 psig inlet pressure, continuous ﬂow (Case 5) for stainless-steel bare surface disk, e 90 psig inlet pressure, 50% DC, 1 s period (Case 6) for Teﬂon coated disk, f 90 psig inlet pressure, 50% DC, 1 s period (Case 6) for stainless-steel bare surface disk. Q is the total quenching capability supplied during the 2 where it is partially chilled down, and T = average Available AVE, TARGET sub2 chilldown process. It is deﬁned as, temperature of target subregion 2 at the end of chilldown, t . END Q ¼ M h (6) Available coolant fg Uncertainty of chilldown efﬁciency Where M is the total mass of coolant used and it can be Coolant To ﬁnd the uncertainty in chilldown thermal efﬁciency, η , Eq. (2) CD estimated as, to (8) were cast in terms of seven independent quantities. The End uncertainty of the chilldown efﬁciency was determined by _ (7) M ¼ mtðÞdt Coolant applying the individual uncertainties of the seven quantities (listed in Table 3) using the root-mean-square method. The mtðÞ is the recorded time-dependent coolant mass ﬂow rate and relative uncertainties for the coated disk and bare surface disk t is the end of chilldown time that corresponds to the time when End thermal efﬁciencies in microgravity range between 7.40 to 7.58% the entire target area has reached T .Therefore, M is the total Final Coolant and between 8.71 to 9.44%, respectively. mass of cryogenic coolant consumed inthe entire chilldownprocess. Table 4 lists the estimated chilldown efﬁciencies using Eqs. (1)–(7) h is the latent heat of vaporization per unit mass that means fg for all six cases. The mass ﬂow rates under microgravity for all six Q is the available total quenching capacity. Available cases are also listed. For each case, there are four entries for For cases where the target area is not totally chilled down microgravity gravity versus terrestrial gravity. Additionally, for each during the microgravity period, use the following for Q . <moved gravity condition, there are comparisons between the bare surface stainless-steel disk and the Teﬂon coated disk. For Cases 5 and 6, Q ¼ðÞ M cðÞ T T þðÞ M c T T Removed TARGET sub 1 p; SS initial final TARGET sub 2 p; SS initial AVE; TARGET sub 2 chilldown efﬁciencies for microgravity conditions were estimated þðÞ M c T T NONTARGET p; SS initial NONTARGET; AVERAGE based on incomplete mass ﬂow rates and marked by an *. The actual (8) complete mass ﬂow rates were not available for microgravity where,M = mass of target subregion 1 where it is conditions due to some intermittent lower ﬂow rates that were TARGET sub1 completely chilled down, M = mass of target subregion below the measurement threshold of the Coriolis ﬂow meter due to TARGET sub2 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2022) 7 J.N. Chung et al. Table 3. Individual uncertainty of the independently measured quantities. Symbol Quantity Measurement method Uncertainty −3 D Diameter of the target area Rulers 1.6 × 10 m ρ Density of test section NIST website 1% c Speciﬁc heat capacity of the test section NIST website 5% ΔT Temperature difference between the initial and the end of the test T-type thermocouple 1.5% τ Thickness of test section Calipers 3 × 10 m h Latent heat of nitrogen NIST website 5% fg m_ Mass ﬂow rate of liquid nitrogen Coriolis ﬂow meter 0.3% Table 4. Chilldown efﬁciencies and mass ﬂow rates for the six ﬂight cases. Reduced-G Ground-G Case Teﬂon coated Bare SS304 Mass ﬂow rate, kg/s Teﬂon coated SS304 1 (micro-g) 80 psig, continuous ﬂow 18.54% 13.38% 0.0253 23.19% 19.54% 2 (micro-g) 80 psig 40% DC, 1 s Period 28.50% 21.26% 0.0350 28.49% 21.74% 3 (micro-g) 80 psig 70% DC, 1 s Period 23.48% 13.65% 0.0239 21.17% 17.78% 4 (micro-g) 60 psig, continuous ﬂow 28.40% 17.83% 0.0100 24.50% 18.58% 5 (micro-g) 90 psig, continuous ﬂow 16.87%* 12.96%* 0.0264* 17.26% 14.32% 6 (micro-g) 90 psig 50% DC, 1 s Period 23.88%* 18.63%* 0.0255* 20.52% 16.76% low liquid capacity in the supply tank during later stages of the stainless-steel disk. It is noted that a complete chilldown up to parabolic ﬂight experiment. Therefore, these two cases are somewhat Ring 6 within the 17–20 s of reduced gravity time was only between continuous ﬂow and intermittent ﬂow conditions. When the feasible for the coated disks as the bare surface disk took much liquid capacity is lower than 50% in the tank, the pressure-driven ﬂow longer to reach the same conditions. For example, speciﬁcally mechanism was not able to completely ﬁll the entrance of the for Case 3 (Fig. 9d), Case 4 (Fig. 10b), and Case 6 (Fig. 10f), even transfer pipe completely with liquid during microgravity that resulted the center was not completely chilled down in 20 s. All the chilldown curves show similar characteristics of spray quench- in some mass ﬂow rates that are lower than the ﬂow meter measurement threshold. ing cooling where the local rate of cooling heat transfer is With the six cases investigated, the efﬁciencies range between inversely proportional to the distance between the local point 19–29% and 13–21% for coated disk and bare surface disk in and the center of the disk. In general, for all cases, the coating microgravity, respectively. However, in terrestrial gravity, the substantially expedited the chilldown process. For example, as efﬁciency ranges are 17–29% and 14–22% for coated disk and shown in Fig. 9afor Case 2 uptoRing4,the coated disk was bare surface disk, respectively. In general, the spray chilldown completely chilldown in 12.8 s, while the bare surface disk took (cooling) efﬁciencies using cryogenic ﬂuids are signiﬁcantly higher 25 s. More speciﬁcally, we will discuss quantitatively the than those using water or FC-72 that normally carry efﬁciencies enhancement of chilldown by the low-thermal conductivity 9,10 only from 5% to up to about 10% . coating below. Microgravity chilldown curves Effects of gravity on disk chilldown heat transfer Figure 8 presents the spray chilldown curves for Case 1 that has Based on the spray chilldown data obtained in microgravity and an inlet pressure of 80 psig and a continuous ﬂow. Figure 8a, b terrestrial gravity, the effects of gravity on spray quenching are are microgravity runs for coated and bare surface tubes, assessed in terms of the chilldown efﬁciency listed in Table 4.In respectively. To provide a comparison, Fig. 8c, d are terrestrial general, for continuous ﬂows, the gravity was found to enhance counterparts for coated and bare surface tubes, respectively. the chilldown efﬁciency for both coated and bare surface disks. The common feature for all four curves is that all curves show The chilldown efﬁciencies in microgravity are 2 to 32% lower than the typical quenching characteristics of a chilldown curve as those counterparts in terrestrial gravity for Cases 1, 4, and 5. The discussed above. For both 1-g and microgravity, the coated only exception is the coated disk in Case 4 where the efﬁciency is disk was chilled down much faster than the bare surface disk. 16% higher in microgravity. However, the trend is somewhat In microgravity, only the coated disk completed the chilldown mixed for pulse ﬂows. For Case 2, the gravity almost made no within the 17-s reduced gravity time. For example, as shown in difference. However, for Case 3, the efﬁciency is higher for coated Fig. 8a, bfor Case 1uptoRing4,the coated disk was tubes and lower for bare surface disks in microgravity versus in completely chilldown in 12.8 s, while the bare surface disk took terrestrial gravity. While for Case 6, the efﬁciencies are higher for 25 s. It is worth noting that under both 1-g and microgravity, both coated and bare surface disks in microgravity. for the coated tube, the chilldown curves of the center (TC25) To explain the above, we offer the following. For the continuous and Ring 1 (TC2) do not exhibit the typical sequence of starting ﬂows, the higher efﬁciencies in terrestrial gravity are basically due with ﬁlm boiling. In other words, these curves do not have a to two heat transfer enhancement mechanisms. Natural convec- distinct LFP. tion that is induced by gravity is the ﬁrst mechanism when the Figures 9 and 10 show the microgravity chilldown curves for ﬁlm boiling is taking place on the disk surface, while the draining the rest of ﬁve cases. For each case, there are two plots, one for and thinning of the liquid ﬁlm during the transition and nucleate the Teﬂon coated disk and the other for the bare surface boiling would be the second mechanism that enhances the heat npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA J.N. Chung et al. Table 5. Improvement of chilldown efﬁciency by coating. Reduced-G Ground-G Case Percent improvement Percent improvement 1 (micro-g) 80 psig, continuous ﬂow 38.6% 18.7% 2 (micro-g) 80 psig 40% DC, 1 s Period 34.1% 31.1% 3 (micro-g) 80 psig 70% DC, 1 s Period 72.0% 19.1% 4 (micro-g) 60 psig, continuous ﬂow 59.3% 31.9% 5 (micro-g) 90 psig, continuous ﬂow 30.2% 20.5% 6 (micro-g) 90 psig 50% DC, 1 s Period 28.2% 22.4% transfer. However, for the pulse ﬂows we may not have stable Effects of inlet pressure on chilldown heat transfer vapor ﬁlms or liquid ﬁlms for heat transfer enhancement. Cases 1, 4, and 5 provide information for assessing the inlet pressure effects. First, we need to stress that the mass ﬂow rate is actually proportional to the inlet pressure as the ﬂow is Effects of disk coating on chilldown heat transfer driven by the pressure difference between the inlet pressure In previous sections, we have already touched on some aspects of and the pressure at the outlet. Since the outlet pressure is the low-thermal conductivity coating effects. Here, we would relatively constant that makes the mass ﬂow rate to be totally summarize those effects. Table 5 lists the percent increase in dependent on the inlet pressure. As a result, it is the case that efﬁciency for the coated disk over the bare surface disk in both the higher the inlet pressure, the higher the mass ﬂow rate. As microgravity and terrestrial gravity. Consistently without any explained above, the chilldown efﬁciency is inversely propor- exception, the coating enhanced the heat transfer during spray tional to the mass ﬂow rate that leads to the outcome that Case chilldown as shown in Table 5 for all six cases in both microgravity 4 should possess the highest efﬁciency, Case 1 is in the middle, and terrestrial gravity. However, for each case, the improvement in andCase5is theleast efﬁcient for each of the four categories microgravity is always higher than that in terrestrial gravity that is (coated disk in microgravity, bare surface disk in microgravity, basically due to the fact the efﬁciencies for bare surface disks are coated disk in 1-g, and bare surface disk in 1-g). It turned out that only the bare surface disk in 1-g did not follow the all lower in microgravity than those in terrestrial gravity as explained above. Therefore, there is more room for improvement. predicted trend, where Case 1 has the highest efﬁciency instead of Case 4 and Case 5 is still the lowest. The results of coating effects thus conﬁrm the theoretical basis for low-thermal conductivity coating given above. DISCUSSION Effects of ﬂow pulsing on chilldown heat transfer The most important ﬁnding in the current research is that the bulk of the spray quenching enhancement and the corre- Let us ﬁrst compare the efﬁciencies for Cases 1, 2, and 3 for the ﬂow sponding spray cooling thermal efﬁciency improvement is pulsing effects as they all have the same inlet pressures of 80 psig. As largely due to the low-thermal conductivity thin-ﬁlm coating, can be seen from Table 4 that for Case 2 pulse ﬂow with 40% DC and especially in microgravity. As mentioned above, the poor heat 1s period, all four efﬁciencies are higher than the corresponding ones transfer ﬁlm boiling regime occupies the major portion of the in Case 1 of continuous ﬂow. Speciﬁcally, in microgravity, the chilldown time for the bare surface disk without coating that improvements in efﬁciencies due to pulsing are 54 and 59% for also translates into low quenching thermal efﬁciency. The low- coated and bare surface tubes, respectively. But for Case 3 with 70% thermal conductivity coating can facilitate a quick drop of the DC and 1 s period, the only pulse ﬂow improvement over the disk surface temperature that expedites the approach to the continuous ﬂow case is 27% for the coated disk in microgravity and LFP on the disk surface and the switch over from the ﬁlm the other three are either about the same (bare surface in boiling regime to the transition boiling regime, thus drastically microgravity) or less efﬁcient (coated disk and bare surface disk in shortens the ﬁlm boiling time and increases the rates of heat terrestrial gravity). Next, between Cases 5 and 6, all four efﬁciencies transfer. The conduction theory also predicts that the thicker from the pulse ﬂow Case 6 are higher than those corresponding ones the coating, the quicker the surface reaches the LFP. However, with the continuous ﬂow Case 5. Again, in microgravity, the once the quenching process enters the high heat transfer improvements in efﬁciencies due to pulsing are 42 and 44% for regimes of transition boiling and nucleates boiling, the coating coated and bare surface tubes, respectively. We may conclude that becomes an insulator that results in lower heat transfer rates as pulse ﬂows can improve the chilldown efﬁciency, but only with lower compared to those of bare surface disks. As a result, after duty cycles. reaching the LFP it is required that the thickness of the coating material be as thin as possible to expedite the disk cooling process. Based on the two scenarios, there should be an Combined effects of disk coating and ﬂow pulsing on optimal coating thickness such that it is not too thick to chilldown heat transfer in microgravity drastically reduce the conduction of heat from the disk to the First, let us compare the coated disk of Case 2 and the bare surface cooling ﬂuid, but also still thick enough to quickly drop the disk of Case 1 in microgravity. We found that the improvement surface temperature to the LFP. Since we only had one ﬂight, due to both coatings, and 40% DC and 1 s period pulse ﬂow is we were not able to test different coating thicknesses. 113% for a common inlet pressure of 80 psig. The other comparison is between the coated disk of Case 6 and the bare surface disk of Case 5 in microgravity. The improvement due to DATA AVAILABILITY both coating, and 50% DC and 1 s period pulse ﬂow is 84% for a The authors declare that the data supporting the ﬁndings of this study are available common inlet pressure of 90 psig. within the paper. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2022) 7 J.N. Chung et al. Received: 28 July 2021; Accepted: 21 January 2022; ACKNOWLEDGEMENTS Published online: 01 April 2022 A great amount of recognition is due to Peter Torsone, Noah Yudwitz, and Ben Frohn who helped carry out the parabolic ﬂight experiments both as ﬂyers and as ground support personnel. We are particularly grateful for the support and consultation REFERENCES provided by the Zero Gravity Corporation. This work was supported by the Flight Opportunities Program at National Aeronautics and Space Administration (NASA) 1. Mars Architecture Steering Group. 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Transf. 86, 373–380 (1964). npj Microgravity (2022) 7 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA
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