Experimental Study on Heat Transfer Characteristics of Static Flash ...

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ScienceDirect Energy Procedia 105 (2017) 4776 – 4781

The 8th International Conference on Applied Energy – ICAE2016

Experimental Study on Heat Transfer Characteristics of Static Flash Evaporation Process Qingzhong Yang, Dan Zhang, Daotong Chong, Yu Wang, Junjie Yan* State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an, Shaan

Abstract An experimental study on flash evaporation process in static flash evaporation of aqueous NaCl solution was presented. Experiments were carried out with initial concentration of waterfilm varied from 0 to 0.15, initial height from 0.1 to 0.4 m, and superheat from 1.8 to 49.5 K. Instant superheat was the driving force during flash evaporation. A negative correlation was found between temperature drop rat e and instant superheat. Instant heat transfer coefficient increased with the increase of instant superheat. A peak value of instant superheat existed during static flash. The evolution of instant superheat was affected by initial superheat, or initial hei ght, or initial concentration of waterfilm. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2017 2016The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). under responsibility of of ICAE Selection and/or peer-review Peer-review under responsibility of the scientific committee the 8th International Conference on Applied Energy. Keywords:Static flash evaporation; Instant superheat; Aqueous NaCl solution; temperature drop rate

1. Introducti on Flash evaporation defines the quickly evaporation leading to a significant temperature drop when a given waterfilm is exposed to sudden pressure drop below its saturation pressure. Due to the significant performance on mass transfer and separation, flash evaporation has been widely studied by scholars and been applied in desalination [1], geo-thermal p lant [2], pharmaceutical industry [3], and so on. The waterfilm in static flash evaporation has no macroscopic horizontal velocity before flash evapora tion occurs. The evolution of instant superheat strongly affected the flash evolution. Instant superheat was the link between boiling and steam-carrying effect. This paper focused on the evolution of instant superheat during flash evaporation process. Experimental study was carried out with init ial concentration changing fro m 0.05 to 0.15, init ial height fro m 0.1 m to 0.4 m and in itial superheat between 1.8 K and 49.5 K. Instant superheat dynamically changed during static flash evaporation. Experimental re sults suggested a

* Corresponding author. Tel.: +86 29 82665741; fax: +86 29 82665741. E-mail address: [email protected].

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.940

Qingzhong Yang et al. / Energy Procedia 105 (2017) 4776 – 4781

significant negative correlat ion between temperature drop rate and instant superheat. The dependence of instant superheat on initial conditions was investigated in this paper. Nomenclature T

temperatue, 0 C

H

height of waterfilm, m

τ

time, s

ΔT

superheat , K

fm

concentration of waterfilem

2. Experiential system and uncertainty analysis 2.1. Experiential system As showed in Fig 1, The experimental system is composed of high pressure part and low pressure part. Heater and flash chamber are the main components in the high pressure part. The sampling duration in this paper continued for 20 s. Flash evaporation already phased into gradual evaporation stage when data sampling continued for 20 s. NEF only slowly decayed with t ime. Considering that data sampled in the first 20 s included the main process characteristics in static flash evaporation, the data was sampled for 20 s.

Fig. 1 experimental system

2.2. Uncertainty analysis

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The experimental range of main parameters in this study is listed in Table 1.The uncertainty analysis results for all directly and indirect ly measured values are calculated accord ing to the method proposed by Moffat [4], and are listed in Table 2. T able 1. Experimental range of main parameters Parameter

Experimental range

H0 /m ΔT/K fm0

0.1-0.4 1.8-49.5 0-0.15

T able 2. Uncertainty analysis results Parameter

Absolutely uncertainty

Minimal measured value

Maximal uncertainty

T/ć H/m τ/s p/MPa ΔT/K fm

0.2 5.0×10 -4 0.025 1.1×10 -3 0.2 5.0×10 -4

40.6 0.1 20 8.27×10 -3 1.8 0.05

4.9×10 -3 5.0×10 -3 1.25×10 -3 0.133 0.111 0.01

3. Result and analysis 3.1. Instant superheat Superheat caused by pressure drop in flash chamber is the driving force of static flash. In the dynamic process of static flash, the instant superheat (Eq 1) also changes with ti me. As shown in Fig 2, the instant superheat rises sharply to satisfy the superheat of nucleation while the electro magnetic is opened (point 2 ). After the breakup of bubbles, ᇞTtr decreases sharply. Then ᇞTtr increases to the maximu m value ƻ 4 ). Finally, ᇞTtr drops to an equilibriu m value. Even the maximu m value of ᇞTtr cannot reach the (point ƻ value of ᇞT (in itial superheat of static flash evaporation) in the process of static flash evaporation. ᇞT (Eq 2) represents the maximu m temperature drop of static flash evaporation in theory. ᇞTtr is the real driving force in every moment of static flash evaporation. (1) 'Ttr W =Tw W -Ts p f W 'T =Tw,0 -Ts p f ,e

(2)

Fig 2 flash evolution and the instant superheat


3.2. Instant heat transfer characteristics As shown in fig 3, the temperature of waterfilm d rops during the static flash evaporation. dT is the tangent at any point along curve Tw. It represents the instant temperature drop rate and the slope of any tangent is negative. The larger the absolute value of dT is, the faster the temperature of waterfilm drops. Fig 4 described the instant heat transfer coefficient and instant superheat in the same static flash evaporation. A peak value of instant heat transfer coefficient exists during the static flash evaporation. The tendency of instant heat transfer coefficient changes correspondingly in agreement wit h instant superheat. Instant heat transfer coefficient increases with increasing instant superheat. Instant heat transfer coefficient and instant superheat reaches their maximu m values at the same time.

Fig 3 Temperature drop rate and the instant superheat

Fig 4 evolutions of instant heat transfer coefficient and instant superheat

3.3. Instant superheat evolution under different intial conditions Fig.5 shows the evolution of ᇞTtr with time under different initial superheat. ime duration τf containing the maximu m ᇞTtr is the major phases of static flash evaporation. Tw and p f both drops rapidly during τf. τ f increases while the in itial superheat increases. With the increase of ᇞT, the maximu m value of ᇞTtr increases.

Fig. 5 instant superheat under different initially superheats

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The evolution of ᇞTtr under different init ial waterfilm concentrations is showed in Fig 6. The maximu m value of ᇞTtr increases as the initial waterfilm concentration rises. Intensity of static flash evaporation increases at this moment, wh ich is consistent with the visual results. Although the ᇞTtr of fm0 =0.15 is larger than the ᇞTtr of fm0 =0.05 at τe, intensity of flashing is not higher. It is worth noting that δ 1 causing a rise in flashing intensity is bigger than δ 2 Ǆ

Fig.6 instant superheat under different initial waterfilm concentrations

Fig 7 describes the evolution of ᇞTtr under different initial water heights. The maximu m value of ᇞTtr decreases with the increase of in itial waterfilm height. Besides, the time of the maximu m ᇞTtr is earlier in the case of lower in itial waterfilm height. The hydrostatic pressure at the bottom of the waterfilm rises. Ts (τ) corresponding to the pressures at the bottom of the waterfilm increases. Tw (τ) for the nucleation increase. Although the ᇞTtr of H0 =0.4 m is larger than the ᇞTtr of H0 =0.2 m at τe, intensity of flashing is not higher.

Fig 7 instant superheat under different initial waterfilm heights


4. Conclusion Experimental study on static flash evaporation process of aqueous NaCl solution was carried out. The initial concentration of waterfilm varied fro m 0 to 0.15, initial height fro m 0.1 m to 0.4 m, init ial superheat from 1.8 to 49.5 K respectively. The dynamic process of static flash evaporation was analyzed. Instant superheat was the driving force during flash evaporation. A significant negative correlat ion was found between temperature drop rate and instant superheat. Instant heat transfer coefficient increases with the rise of instant superheat. A peak value of instant superheat existed during static flash. But, the maximu m value of instant superheat was less than the initial superheat in static flash. The maximu m instant superheat increased with the rise of initial concentration, or the init ial superheat. Increasing initial water film height reduced the maximu m initial superheat during static flash.. 5. Copyright Authors keep full copyright over papers published in Energy Procedia Acknowledgements This work was supported by the National Natural Science Foundation of China (51306148/ 51 436006) and the National Basic Research Program of China (973 Program, Grant Number 2015CB251504). References [1] S.R. Hosseini, M. Amidpour , A. Behbahaninia, Thermoeconomic analysis with reliability consideration of a combined power and multi stage flash desalination plant, Desalination 278 (2011) 424-433. [2]T .A.H. Ratlamwala, I. Dincer, Comparative efficiency assessment of novel multi-flash integrated geothermal systems for power and hydrogen production, Applied T hermal Engineering 48 (2012) 359-366. [3]Kumar V, Prud'homme RK. Nanoparticle stability: processing pathways for solvent removal. Chemical Engineering Science, 2009, 64 (6): 1358-1361. [4]R.J. Moffat, Contributions to the theory of single-sample uncertainty analysis. Journal of Fluid Engineering, 104 (2) (1982) 250-260.

Biography Junjie Yan is a professor of State Key Laboratory of Mult iphase Flow in Po wer Engineering, Xi’an Jiaotong University, Xi’an China. He received his Ph.D. fro m Xi’an Jiaotong University in 1998. His research interests include emulation and optimizat ion of thermal system, steam-water two phase flow, cogeneration of cooling, heating and power.

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