Heat transfer enhancement in chilldown process with electrospun nanofiber coating

Heat transfer enhancement in chilldown process with electrospun nanofiber coating

Cryogenics 101 (2019) 75–78 Contents lists available at ScienceDirect Cryogenics journal homepage: www.elsevier.com/locate/cryogenics Heat transfer...

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Cryogenics 101 (2019) 75–78

Contents lists available at ScienceDirect

Cryogenics journal homepage: www.elsevier.com/locate/cryogenics

Heat transfer enhancement in chilldown process with electrospun nanofiber coating

T



Katsuyoshi Fukibaa, , Satoru Tokawab, Hiroki Kawashimaa, Hiroki Adachic a

Department of Mechanical Engineering, Grad. School of Integrated Science and Technology, Shizuoka Univ., 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan Department of Applied Mechanics, Grad. School of Fundamental Science and Engineering, Waseda Univ., 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan c Department of Mechanical Engineering, Shizuoka Univ., 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Boiling heat transfer Heat transfer enhancement Cryogenic fluid Chilldown

In this study, a method to enhance boiling heat transfer in cryogenic fluids by using nanofiber coating was proposed and validated. For this method, a nanofiber coating of polyvinyl alcohol (PVA) was fabricated on the surface of a copper plate via electrospinning, which is a technique that uses a high-voltage electric field. A chilldown experiment conducted with the copper plate and liquid nitrogen revealed that the nanofiber coating had the potential to reduce the chilldown time drastically. It was observed that the heat flux through the nanofiber-coated plate increased immediately after the chilldown period began, and it exceeded that of the plate without coating for the entire test duration. Consequently, the chilldown (boiling) curve did not have a minimum heat flux point, which is generally seen in pool boiling process. The critical heat flux was twice as large as that of the conventional method with a bare copper plate. Overall, the chilldown curve of the nanofibercoated plate was distinct from a conventional boiling curve with a bare plate.

1. Introduction Today, cryogenic fluids have many kinds of applications, especially in transportation vehicles such as rockets [1] and automobiles [2]. Before it can be loaded with cryogenic fluids, the pipe system needs to undergo a process called “chilldown” to cool its temperature level from ambient to that of the cryogenic fluids. At the beginning of the chilldown, the temperature difference between the pipes and the cryogenic fluid is large. This condition leads to film boiling. The heat transfer rate during film boiling is extremely low due to the formation of a vapor film on the walls of the pipe. The film has poor conductivity and does not allow the wall to contact the liquid fluid, which results in low heat transfer efficiency. This phenomenon is called Leidenfrost effect. As a result of this effect, the duration of the chilldown process increases. Many methods to improve boiling heat transfer have been proposed in the past. Some of these methods propose increasing the surface area or number of nucleation sites [3,4], or introducing a layer of coating with low thermal conductivity [5–7], etc. The methods to increase the number of nucleation sites are usually not valid for enhancing heat transfer in film boiling. Therefore, these methods are not suited for the chilldown process with cryogenic fluids. In contrast, the method using a layer of coating with low thermal conductivity is effective for the process. This technique has ancient origins and our previous study



summarized the history of it [8]. Our previous study also revealed that a thin layer of coating with low thermal conductivity fabricated on the inner walls of a fluid-carrying pipe can reduce the chilldown time of pipe systems. The results indicated that the time taken for the chilldown process could be reduced to about 40% of that of the method that does not use such a layer. However, there are a few limitations to this approach in terms of efficiency, and we thought that a new method to bypass these limitations and conduct the chilldown process more efficiently was needed. The methods to increase the surface area or number of nucleation sites consist of various configurations. Some of these are valid for enhancing heat transfer in film boiling. We focused on one of the configurations proposed by Mazor et al. [9], in which frost was used to enhance heat transfer. The study demonstrated that frost accumulation on a metallic surface could accelerate the chilldown time by 10 times the actual rate. However, frost layers are fragile and melt when exposed to ambient temperature. Therefore, this technique is not suitable for engineering applications. In this study, we proposed the use of nanofibers for the same purpose. The term nanofiber denotes fibers with a diameter of the order of nanometers. Recently, there has been a tremendous growth in research on nanofibers and their commercial application [10]. Strong and efficient nanofibers can be fabricated easily by means of electrospinning,

Corresponding author. E-mail address: [email protected] (K. Fukiba).

https://doi.org/10.1016/j.cryogenics.2019.06.004 Received 5 December 2018; Received in revised form 4 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0011-2275/ © 2019 Elsevier Ltd. All rights reserved.

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which is a technique that uses a high-voltage electric field. In this process, thin fibers are wound over a metallic surface if a polymeric liquid solution is scattered into the electric field. This paper reports the results of an experiment where a thin nanofiber layer of polyvinyl alcohol (PVA) was fabricated on a copper plate. The ability of the nanofiber to enhance pool boiling heat transfer was evaluated using liquid nitrogen as the cryogenic liquid. Our literature survey revealed that the first study to utilize nanofibers for heat transfer was conducted by Srikar et al. [11]. In this study, they employed nanofibers for intensification of drop or spray impact cooling. Then Shiha-Ray et al. [12] improved the method with electroplating. Shinha-Ray et al. also studied the effect of gravity on drop impact cooling enhanced with nanofiber [13,14]. Subsequently, Jun et al. [15], Sahu et al. [16,17], and Sinha-Ray et al. [18] utilized nanofibers for heat transfer enhancement of pool boiling. They used water, ethanol, heptanol and Novec 7300 as the liquid for pool boiling, and the boiling regime was limited to nucleate boiling. The most important boiling regime in chilldown process of cryogenic fluid is film boiling because the temperature difference between the fluid and wall is large. In this regime, nanofiber layers affect the boiling mechanism and vary the characteristics drastically. This study focuses on the improvement of chilldown process with liquid nitrogen by using nanofiber layers.

these images are different from the copper plate used in the pool boiling experiment mentioned in Section 2.2. These samples are nanofiber layers on another copper plate of t1 × 20 × 20 mm in dimension, which were fabricated under the same electrospinning conditions as the copper plate used in the pool boiling experiment. Since PVA nanofibers are non-conductive, the nanofiber layers were coated with Pt spattering before the SEM imaging. The images indicate that the diameter of the fibers are of the order of a nanometer. In Fig. 2(b), the copper plate surface can be viewed through the gaps between the fibers. From the image, it can be inferred that the number of nanofiber strands in the vertical direction of the image is 10, which means that the thickness of the nanofiber layer is only a few micrometers. 2.2. Setup for pool boiling experiments In the pool boiling experiments, a t6 × 50 × 50 mm copper plate coated with a nanofiber layer was immersed in liquid nitrogen. The pool boiling heat transfer was investigated. Fig. 3 shows the experimental setup. The copper plate was insulated with polypropylene and its temperature was measured using a T-type thermocouple at 3 mm from the plate surface. We assumed that the temperature of the copper plate was uniform. The heat flux was then calculated with the temperature variation of the copper plate. It is important to note that the Biot number in this experiment at a temperature range under 120 K is larger than 0.1. In such a condition, temperature distribution cannot be neglected. Therefore, the heat flux in this temperature range should be considered carefully. Fig. 3(b) shows the surface appearance of the nanofiber layer. The layer looks like white paint on the surface. The fibers are too small to see the fabric structures by the naked eye.

2. Experimental setup 2.1. Fabrication of nanofibers Fig. 1 shows an experimental setup to fabricate nanofibers on a copper plate. A high-voltage DC power source generates a strong electric field between a syringe and a copper plate. A syringe pump ejects a polymeric liquid solution into the electric field at a constant flow rate. The solution is then drawn to the copper plate and scattered. The solvent evaporates while scattering, and the nanofiber is accumulated on the plate. In this study, a PVA solution with volume concentration of 8% was used. The voltage, distance between the tip of the needle and the copper plate, and flow rate of the solution were 10 kV, 100 mm and 6 mL/h, respectively. The dimensions of the copper plate are t6 × 50 × 50 mm. Since the nanofibers accumulate around the central axis of the needle, the amount of nanofiber corresponds to the distance from the central axis. To maintain uniformity of the nanofibers on the surface of the copperplate, the location of the plate was varied at every 30-s interval. The overall time taken to fabricate the nanofibers was 180 s. Electrospinning can be used to fabricate a nanofiber layer of arbitrary thickness by varying the fabrication time. A method to measure the thickness of the nanofiber layer accurately has not been established yet. A previous attempt to measure the same using an electric thickness gauge did not produce successful results. Fig. 2 shows the scanning electron microscope (SEM) images of the PVA nanofiber layer on the copper plate as fabricated by electrospinning. The samples shown in

3. Results Fig. 4 shows the temperature history of the nanofiber-coated copper plate measured using a thermocouple. Furthermore, the figure shows the history of a bare copper plate without nanofibers and a copper plate with foamed metal on the surface. These data were referred from our previous studies [19] in which the same experimental setup was used. The foamed metal was fabricated from copper. The thickness and porosity of the foamed metal are 1 mm and 91%, respectively. The foamed metal was fixed to the surface via soldering. In this figure, the time at which the temperature becomes 273.15 K is defined as 0 s. The end of the chilldown time was defined as the time at which the temperature variation over time, dT/dt, becomes 0.1 K/s. The duration to the end of the chilldown was 138.8 s for the bare surface without nanofibers and 39.3 s for the surface with nanofibers, which is 28% of the duration of the former. This duration for chilldown is almost the same as that of the foamed metal. Here, we define the heat transfer intensification coefficient, ε , following the definition by Mazor et al. [9,20].

ε=

tbare tcoated

(1)

The heat transfer intensification coefficient for the nanofiber surface is 3.53. Fig. 5 shows the chilldown (boiling) curve calculated with the temperature history shown in Fig. 4. In this figure, the chilldown experiment progresses from high temperature (right) to low temperature (left). It should be noted that the temperature in this figure is that at the middle of the plate, which is different from the temperature on the nanofiber surface. In the case of the bare copper plate without nanofibers, film boiling occurs first because the temperature difference between the plate and liquid is large at the beginning of the test. The heat flux starts to decrease as time proceeds. Then the heat flux reaches the minimum value at the Leidenfrost point of about 100 K, and the transition to the nucleate boiling starts. The heat flux reaches the maximum value in the process from transition boiling to nucleate boiling. Finally,

Fig. 1. Experimental apparatus for electrospinning. 76

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a) × 500

b) × 2000

Fig. 2. SEM images of the PVA nanofiber layer on the copper plate.

a) Schematic diagram

b) Exterior appearance of the plate with fiber

Fig. 3. Experimental setup for observing pool boiling heat transfer.

Fig. 4. Temperature history for a bare copper plate, a plate with foamed metal and a plate with the nanofiber layer.

Fig. 5. Chilldown curves for a bare copper plate, a plate with foamed metal and a plate with the nanofiber layer.

the temperature of the plate reaches the saturation temperature and the heat transfer stops. In contrast, the heat flux for the chilldown curve in the case of metal with nanofiber layer starts to increase at the beginning of the test. This results in the curve losing the Leidenfrost point. The

temperature difference between the plate and the liquid gradually decreases with the progress of the chilldown period. Therefore, the heat flux should decrease if the effect of the nanofiber layer is only to

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Acknowledgements

increase the surface area. Previous studies [5–8] showed that a thin low-thermal-conductivity layer like polytetrafluoroethylene (PTFE) increases the minimum heat flux temperature, which results in the reduction of the chilldown time. One of characteristics of the chilldown curve for this method is the increase in the critical heat flux temperature. However, the critical heat flux temperature in Fig. 5 does not increase compared with that of the bare copper plate. Mazor et al. [20] pointed out that their heat transfer enhancement method with frost could be explained by a one-dimensional model. The model describes the process of boiling on the array of frost cylinders where different stages of boiling occur at the same time along the cylinders. This type of hybrid boiling seems to occur in the current experiment with the nanofiber plate and result in the chilldown curve in Fig. 5. The chilldown curve of the copper plate with nanofiber layer is similar to that with foamed metal used in our previous study [19]. The heat transfer enhancement method using electrospun nanofibers is simple and convenient. Electrospinning can also be used to fabricate fibers inside tubes. This is advantageous for applications such as industrial cryogenic transfer piping system. Another advantage is that electrospinning can be used to fabricate fibers of arbitrary thickness by varying the duration of high voltage application. Despite this, electrospinning has a few limitations as well. First, the dispersion of this method is large. Electrospinning is affected by various factors, such as solution, voltage, ambient temperature, and humidity [10]. It is difficult to repeatedly fabricate a nanofiber layer with the same fiber diameter and thickness. Second, although the nanofiber layer is sturdier than frost, which was used in the study by Mazor et al. [9], its structure is fragile. At the end of this experiment, we left the copper plate with nanofiber coating in our laboratory. The plate was cold, so the condensation droplets appeared on it. The nanofiber dissolved into the droplets and disappeared. Generally, the speed of PVA dissolution into water at ambient temperature is not so high. We are thinking that surface area increase as nanofibers causes increase in the speed of PVA dissolution. One of countermeasures for this issue is changing the materials. Non water soluble polymer like polyacrylonitrile probably solves this problem. Another countermeasure is a method with coatings. For this, the fabrication of thin Pt coating on electrospun PVA surface via sputtering was attempted in this study. The thickness of the Pt coating was about 100 nm, which was estimated using the duration of the sputtering. A water droplet was poured with a pipette on the plate with nanofibers coated with Pt. However, the previous observation of dissolving was not seen in this case. This kind of coating could vary the surface property and have an effect on boiling heat transfer. Further research is to be undertaken to clarify this aspect.

This work was supported by JSPS KAKENHI Grant Number JP17H03479. References [1] Galeev AG, Firsov VP, Antyukhov IV, Galeev AV. Research of heat transfer processes during pre-launch chilldown of PS consumption lines of upper-stage LV. Int J Hydrogen Energy 2017;42:24448–57. https://doi.org/10.1016/j.ijhydene.2017.07. 038. [2] Wolf J. Liquid-hydrogen technology for vehicles. MBS Bull 2002;27:684–7. https:// doi.org/10.1557/mrs2002.222. [3] Kim DE, Yu DI, Jerng DW, Kim MH, Ahn HS. Review of boiling heat transfer enhancement on micro/nanostructured surfaces. Exp Therm Fluid Sci 2015;66:173–96. https://doi.org/10.1016/j.expthermflusci.2015.03.023. [4] Das AK, Das PK, Saha P. Performance of different structured surfaces in nucleate pool boiling. Appl Therm Eng 2009;29:3643–53. https://doi.org/10.1016/j. applthermaleng.2009.06.020. [5] Cowley CW, Timson WJ, Sawdye JA. A Method for improving heat transfer to a cryogenic fluid. Boston: Springer US; 1962. p. 385–90. https://doi.org/10.1007/ 978-1-4757-0531-7_47. [6] Maddox JP, Frederking THK. Cooldown of insulated metal tubes to cryogenic temperatures. Boston: Springer US; 1966. p. 536–46. https://doi.org/10.1007/9781-4757-0522-5_57. [7] Lavalle GG, Carrica P, Garea V, Jaime M. A boiling heat transfer para-dox. Am J Phys 1992;60:593–7. https://doi.org/10.1119/1.17111. [8] Takeda D, Fukiba K, Kobayashi H. Improvement in pipe chilldown process using low thermal conductive layer. Int J Heat Mass Transf 2017;111:115–22. https://doi.org/ 10.1016/j.ijheatmasstransfer.2017.03.114. [9] Mazor G, Korin E, Nemirovsky D, Ladizhensky I. Frost formation as a temporary enhancer for quench pool boiling. Appl Therm Eng 2013;52:345–52. https://doi. org/10.1016/j.applthermaleng.2012.11.048. [10] Bhardwaj N, Kundu SC. Electrospining: A fascinating fiber fabrication technique. Biotechnol Adv 2010;28:325–47. https://doi.org/10.1016/j.biotechadv.2010.01. 004. [11] Srikar R, Gambaryan-Roisman T, Steffes C, Stephan P, Tropea C, Yarin AL. Nanofiber coating of surfaces for intensification of drop or spray impact cooling. Int J Heat Mass Transf 2009;52:5814–26. https://doi.org/10.1016/j. ijheatmasstransfer.2009.07.021. [12] Shinha-Ray S, Zhang Y, Yarin AL. Thorny Devil Nanotextured Fibers: The way to cooling rates on the order of 1 kW/cm2. Langmuir 2011;27:215–26. https://doi. org/10.1021/la104024t. [13] Shinha-Ray S, Yarin AL. Drop impact cooling enhancement on nano-textured surfaces. Part I: Theory and results of the ground (1 g) experiments. Int J Heat Mass Transf 2014;70:1095–106. https://doi.org/10.1016/j.ijheatmasstransfer.2013.11. 007. [14] Shinha-Ray S, Yarin AL. Drop impact cooling enhancement on nano-textured surfaces. Part II: Results of the parabolic flight experiments [zero gravity (0g) and supergravity (1.8g)]. Int J Heat Mass Transf 2014;70:1107–14. https://doi.org/10. 1016/j.ijheatmasstransfer.2013.11.008. [15] Jun S, Sinha-Ray S, Yarin AL. Pool boiling on nano-textured surfaces. Int J Heat Mass Transf 2013;62:99–111. https://doi.org/10.1016/j.ijheatmasstransfer.2013. 02.046. [16] Sahu RP, Sinha-Ray Sumit, Sinha-Ray Suman, Yarin AL. Pool boiling on nano-textured surfaces comprised of electrically-assisted supersonically solution-blown, copper-plated nanofibers: Experiments and theory. Int J Heat Mass Transf 2015;87:521–35. https://doi.org/10.1016/j.ijheatmasstransfer.2015.04.009. [17] Sahu RP, Sinha-Ray Sumit, Sinha-Ray Suman, Yarin AL. Pool boiling of Novec 7300 and self-rewetting fluids on electrically-assisted supersonically solution-blown, copper-plated nanofibers. Int J Heat Mass Transf 2016;95:83–93. https://doi.org/ 10.1016/j.ijheatmasstransfer.2015.11.094. [18] Sinha-Ray Sumit, Zhang W, Sahu RP, Sinha-Ray Suman, Yarin AL. Pool boiling of Novec 7300 and DI water on nano-textured heater covered with supersonicallyblown or electrospun polymer nanofibers. Int J Heat Mass Transf 2017;106:482–90. https://doi.org/10.1016/j.ijheatmasstransfer.2016.08.101. [19] Ono T, Tsutsumi N, Fukiba K, Kawashima H. Effect of metal surface coatings on chilldown with cryogenic fluid. Space transportation symposium, STCP-2017-028 2018. [in Japanese]. [20] Mazor GI, Ladizhensky, Nemirovsky D, Alfi A, Rabin A, Yehud E, Korin E. Accelerated cryogenic cooling caused by the temporary frost layer enhancer. 022901–022901 J Heat Transfer 2016;139. https://doi.org/10.1115/1.4034899.

4. Conclusions A method to enhance boiling heat transfer of cryogenic fluid with a nanofiber layer was proposed in this study. The nanofiber layer made of polyvinyl alcohol (PVA) was coated on the copper plate by using the electrospinning technique. Chilldown experiments using liquid nitrogen as the cryogenic liquid showed that the nanofiber layer reduced the chilldown time drastically. The chilldown curve for the plate with the nanofiber layer was distinct from that of the plate without the layer. Declaration of Competing Interest The authors declared that there is no conflict of interest.

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