INVESTIGATION OF FLOW PATTERNS INSIDE NOZZLE AND SPRAY CHARACTERISTICS OF R134A FLASHING SPRAY

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00097 INVESTIGATION OF FLOW PATTERNS INSIDE NOZZLE AND SPRAY CHARACTERISTICS OF R134A FLASHING SPRAY Xinsheng Wang, Bin Chen * State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, 28 West Xian Ning RD, Beiling District, Xi an, , Shaanxi, China Presenting Author: wangxinsheng@stu.xjtu.edu.cn * Corresponding Author: chenbin@mail.xjtu.edu.cn ABSTRACT The flow patterns of liquid inside nozzles have great effect on spray characteristics in flashing spray. A transparent nozzle was manufactured to investigate the flow characteristic inside the nozzle during flashing spray and its effects on spray characteristics. The flow patterns of R134a inside the nozzle and spray characteristics outside the nozzle were recorded by a high-speed CCD camera under different injection pressure of 0.77MPa, 0.80MPa, 0.90MPa and 1.50MPa, and the ambient condition was atmospheric. The relationship between the internal flow inside the nozzle and external flashing spray were analyzed. It was found that the spray pattern is extremely unstable when random nucleation appears inside the nozzle, and the homogenous bubbly flow inside the nozzle contributes to form stable spray patterns. The nucleation and bubbly flow inside the nozzle would decrease the spray cone angle of R134a flashing spray. KEYWORDS: Flashing spray, Visualization, Internal flow, Spray pattern 1. INTRODUCTION The phenomena and mechanism of flashing spray atomization were widely studied since early 1960s by Brown and York [1]. According to the transition of spray pattern from fluffy plume to pencil-shape with increasing the superheat, Oza et.al [2, 3] proposed two regimes of flashing spray, i.e., the internal-flashing regime (breakup occurs within the nozzle) and the external-flashing regime (breakup occurs outside the nozzle) to emphasize the breakup mechanism of flashing spray in the high degree of superheat regime, which means nucleation appears inside the nozzle under high superheat degree and primary atomization develops before the fluid flows into the air. However, a later experiment about flashing spray with heated water passing through an orifice (diameter of 0.34mm and length of 1.37mm) carried by Reitz [4] demonstrated that an intact core still existed around the nozzle exit under high superheat degree and the unbroken jet expelled smaller droplets upon leaving the nozzle, indicating that the breakup didn t occur inside the nozzle, which cast doubt on the flashing atomization theory of Oza et al. Kurschat et al.[5]. Vieira et al. [6] also documented the existence of a metastable liquid core using convergent nozzles. Park and Lee [7] gave a visualization research on the water flashing spray using circular transparent nozzles. They found that bubbly, slug, or annular flow pattern appears inside the nozzle before 1

2 discharge, and they thought the internal flow pattern dominated the external spray behavior outside the nozzle. Gunther and Wirth [8] found that when the temperature of water increase to 130 degrees Celsius, nucleation appears inside the capillary nozzle and the spray pattern changes from jet to a fine spray. Recently, Zhang et al. [9] performed an visualization on flashing spray of methanol using transparent slit nozzle. They found that bubble formation inside the nozzle and the spray pattern were dominated by superheat degree and suggested that the area fraction of superheated jet near the nozzle has a positive relationship with the bubble number density inside the nozzle, which means the breakup of superheated jet is directly related to the intensity of bubbly formation rate inside the nozzle. Although there are differences of opinions on whether the primary atomization can be realized by the evaporation inside the nozzle, it has been accepted that the internal flow inside the nozzle has a significant effect on the external spray characteristics [10]. In the hope of providing further understanding about the relationship between the internal flow and external spray characteristics, a transparent straight nozzle was designed, and the internal flows as well as external spray images were recorded by a high speed CCD camera. 2. EXPERIMENTAL SETUP AND METHODOLOGY A schematic diagram of the experimental apparatus to visualize the flashing spray was illustrated in Fig.1 (a). Non-toxic commercial cryogen R134a (Dupont, USA) stored in a storage tank was released to a pressure stabilizer. The pressure stabilizer is equipped with a pressure transducer and a pressure relief valve, and a high pressure nitrogen vessel is connected to the pressure stabilizer to provide constant injection pressure. Cryogen R134a flowed into a nozzle fixed on a three-dimensional translational electric positioner (with resolution of 8μm, WN105TA300M by Beijing Winner Optics Instruments Co., China) though hose tube, and a solenoid valve (ZC51-8B-6.3, Dun Ming, China) was used to control the spray. All signals were acquired by a DAQ board (NI: M-6251, USA). A high speed camera (Fastcam SA-Z, Photron, Japan) was employed to capture the internal flow and external spray pattern, which was illuminated by backlight methodology. For shadowgraphy measurement, a LED lamp was located opposite to the CCD camera, with the spray in between. Snapshots of the external spray and flow inside the nozzle were taken successively at the speed of 75000fps and fps respectively. The structure of transparent nozzle used in the research is shown in Fig.1 (b) with inner diameter (D) of 1.45mm and length (L) of 60mm. The ambient condition was atmospheric with the temperature of 29 C and the saturation pressure of R134a is 0.75MPa. (a) (b) Fig. 1 (a) Schematic of experiment system (b) structure of transparent nozzle 2

3 3.1 Visualization of internal flow 3. RESULTS AND DISCUSSION Fig.2 visualized the internal flow of R134a inside the nozzle under different inlet pressure. Bubble nucleation and two-phase flow inside the nozzle were captured by the high speed CCD camera clearly. With back-illumination of the LED lamp, the white area in the nozzle represents the liquid phase region, whereas the black area is the gas-liquid two-phase flow region due to the reflection and refraction of light caused by the gas liquid interface. When the injection pressure was 0.77MPa, evaporation appeared before the nozzle since the injection pressure was close to the saturation pressure, and the static pressure before the nozzle would be reduced to a value less than saturation pressure during the spray. Large bubbles formed before the nozzle would break up rapidly when they flowed into the nozzle because of the enhanced turbulence, which contributed the development of bubbly flow on the whole cross section inside the nozzle as displayed in Fig.2 (a), and the breakup process inside the nozzle was captured by high speed CCD with microscopic lens, as presented in Fig.2 (b). Extreme instability of the internal flow appeared when the injection pressure increased to 0.80MPa, as shown in Fig.2 (c). The position where nucleation appeared inside the nozzle was random, and bubbly flow was easily formed once the stable nucleus appeared inside the nozzle. Fig. 2 (a) Internal flow pattern under injection pressure of 0.77MPa (b) bubbles break-up around nozzle entrance under injection pressure of 0.77MPa (c) variation of internal flow under injection pressure of 0.80MPa (d) several typical internal flow patterns under injection pressure of 0.90MPa (e) internal flow visualizations under injection pressure of 1.50MPa 3

4 The stochastic nature of nucleation is related to the local fluctuations inside the nozzle and results in the transition from a meta-stable to a stable phase[10]. It can be seen from Fig.2 (d) that when the inlet pressure reached 0.90MPa, the internal flow tended to be stable, and nucleation only appeared around the nozzle exit. However, sometimes nucleation tended to appeared on a particular side of the nozzle, which perhaps caused by the non-uniform roughness of the nozzle surfaces. The phenomenon of the nucleation on one side of the nozzle disappeared when the injection pressure increased to 1.50MPa, as presented in Fig.2 (e). 3.2 EXTERNAL SPRAY PATTERNS CORRESPONDING TO THE FLOW INSIDE NOZZLES Spray patterns corresponding to the flow inside the nozzle were shown in Fig.3. It is found that the spray pattern is closely related to the internal flow, which means the stable internal flow patterns would make the external spray morphology stable, and the spray pattern will be volatile when the instability appears inside the nozzle. Fig.3 (a) and Fig.3 (b) give the typical spray pattern under the injection of 0.77MPa and 1.50MPa respectively. The spray patterns are symmetrical along the nozzle axis and the fluctuation of spray morphology is week during the spray. It is worth noting that the expansion of spray radius only occurs a short distance around the nozzle when the injection pressure is 0.77MPa, while it would last in the whole field of vision under the injection pressure of 1.50MPa. According to the internal flow under the injection pressure of 0.80MPa presented in Fig.2 (c), the snapshots of the spray patterns in series are shown in Fig.3 (e). As a result of the inconstant nucleation inside the nozzle, the spray morphology is extremely unstable. It is interesting to note that the spray cone angle increases suddenly when a liquid slug flows out the nozzle, and the process starts from the image No.06 of Fig.3 (e). The nucleation and evaporation inside the nozzle would accelerate the flow of cryogen inside the nozzle, which would make the droplets faster along the axis, and the superheat degree of liquid around the nozzle exit would decrease owing to the evaporation inside the nozzle. Fig. 3 (a) Spray pattern under injection pressure of 0.77MPa (b) spray pattern under injection pressure of 1.50MPa (c)spray pattern with unilateral nucleation under injection pressure of 0.90MPa (d)spray pattern with annular nucleation under injection pressure of 0.90MPa (e) variation of spray patterns under the injection pressure of 0.80MPa 4

5 Moreover, the two phase flow of refrigerant leads to a higher flow resistance and lower sonic speed, which would reduce the mass flow of cryogen during the spray and all above may be the factors that the spray cone is large when less bubbles flow out the nozzle. Fig.3 (c) and Fig.3 (d) are two typical spray patterns under the injection pressure of 0.90MPa. It can be seen that the spray cone angle in the side of the nozzle without evaporation is significantly larger that than the bubbly flow side, which is coincident with the previous analysis. 4. CONCLUSION The internal flow and external spray morphology of R134a flashing spray using straight transparent tube nozzle under varies injection pressure were recorded by a high speed CCD camera, and the influence of internal flow on the external spray pattern was analyzed. The investigation shows that the spray pattern is directly affected by the flow pattern inside the nozzle, and the spray pattern is extremely unstable when random nucleation appears inside the nozzle, while homogenous bubbly flow inside the nozzle contributes to form stable spray patterns. The nucleation and evaporation inside the nozzle would increase the flow velocity, decline the mass flow of cryogen and decrease the superheat degree of cryogen around the nozzle exit. Hence the spray cone angle would be declined when the evaporation appears inside the nozzle during the R134a flashing spray. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China ( ). REFERENCES [1] R. Brown, J.L. York, Sprays formed by flashing liquid jets, AIChE Journal, 8(2) (1962) [2] R.D. Oza, J.F. Sinnamon, An experimental and analytical study of flash-boiling fuel injection, , SAE Technical Paper, [3] R. Oza, On the mechanism of flashing injection of initially subcooled fuels, Journal of fluids engineering, 106(1) (1984) [4] R.D. Reitz, A Photographic Study of Flash-Boiling Atomization, Aerosol Science and Technology, 12(3) (1990) [5] K. T, C. H, M. Ge, Complete adiabatic evaporation of highly superheated liquid jets, Journal of Fluid Mechanics, 236(3) (1992) [6] M.M. Vieira, J.R. Simoes-Moreira, Low-pressure flashing mechanisms in iso-octane liquid jets, Journal of Fluid Mechanics, 572 (2007) [7] B.S. Park, S.Y. Lee, An Experimental Investigation Of the Flash Atomization Mechanism, Atomization Spray, 4(2) (1994) [8] A. Gunther, K.E. Wirth, Evaporation phenomena in superheated atomization and its impact on the generated spray, International Journal of Heat and Mass Transfer, 64 (2013) [9] Y. Zhang, S. Li, B. Zheng, J. Wu, B. Xu, Quantitative observation on breakup of superheated liquid jet using transparent slit nozzle, Experimental Thermal and Fluid Science, 63 (2015) [10] E. Sher, T. Bar-Kohany, A. Rashkovan, Flash-boiling atomization, Progress in Energy & Combustion Science, 34(4) (2008)