(259f) Visualization of Bubble Dynamics in a Natural Circulation Boiling Loop

Jithender Naik L, Jyoti Bhati, Swapan Paruya[1]

Chemical Engineering Department, NIT Durgapur, West Bengal, India-713209

 ABSTRACT

The study of bubble dynamics during boiling of liquid is mainly intended to estimate bubble life cycle (bubble inception to collapse), bubble size, bubble nucleation site density, bubble release frequency, bubble velocity and their parametric variations. Bubble dynamics also includes lift-off, merger and departure. The motivating factors for studying bubble dynamics are: partitioning of energy into vapor and liquid phases and the influence of bubble dynamics on the wall heat transfer¹. On the other hand, the studies on bubble dynamics using natural circulation boiling loop (NCBL) facility is extremely important due to the fact that the bubble dynamics has a significant impact on flow instabilities in the loop². NCBL is generally used for controlled heat removal in boiling water reactors (BWRs), simplified boiling water reactors (SBWRs), nuclear steam generators, thermal drum-type boilers and all the thermosiphon reboiler units of chemical, refining and petrochemical plants, etc. Several types of bubble have been observed in NCBL recently. Several researchers^3-5 have performed boiling experiments with water in vertical annulus. They studied on various bubble parameters i.e., bubble lift-off diameter, bubble release frequency bubble nucleation site density and etc. The vapor bubble dynamics while travelling through subcooled liquid need to be studied in detail. The study on bubble sliding length and vapor bubble velocity has not been carried out in much detail. In the present study, the bubble diameters and bubble velocities have been studied, when the bubbles are travelling through subcooled liquid. The bubble behavior was captured using a high-speed digital video camera for a convective subcooled boiling of water in a vertical annulus. Although it is a water circulation loop, the experiments have been done at very low liquid velocity and mass flux (0.02m/s and 23.64kg/m²s respectively) to clearly understand bubble growth rate phenomena in the loop. Due to very low mass flux and liquid velocity, it is considered as pool boiling. The bubble diameter and velocity were quantified by analyzing the captured images. These bubble parameters have wide applications e. g. bubble size to determine the heat transfer coefficient and interfacial area of vapor bubble. The bubble velocity is used to calculate the interfacial heat transfer coefficient in the cases when vapor bubble is in subcooled liquid and in superheated liquid.

In the present study, the authors performed the visualization of the growing water-vapor bubbles during subcooled boiling in a heated vertical tube to measure diameter and velocity under varying surface temperature. The experiments were carried out at surface temperature varying106.66 to 117.68^oC, the superheat from 2.86 to 13.61K and the inlet subcooling from 31.08 to 40.89K at a constant system pressure of 1.170 bar. The test section is 1m long. ID and OD of the tube are 20.96mm and 26.71mm respectively. Six segmental heater-coils are wound around 1m long aluminum tube for uniform and safe heating. This aluminum tube is placed outside the process tube through which the water flows. The visualization was performed at the outlet of the heater section. The dynamics of vapor bubbles are captured with high speed camera at 500fps. Micro - Nikkor 55mm lens is used to capture the bubble dynamics in the boiling loop. The size of each pixel of a high speed camera is 8µm × 8µm (0.008mm × 0.008mm). We obtained different bubble sizes and velocity in different surface temperature. From the results, we saw that the mean bubble velocity varied from 0.25 to 0.45 m/s and the mean bubble diameter from 1.89 mm to 4.1 mm. We used MATLAB program for the image analysis to find the centroids and Feret diameter of the vapor bubble. We also compared our experimental maximum bubble diameter and mean bubble velocity with well-known theories^{6-7 .}and noted a fairly good agreement between theories and experiment. In particular, the results are very close to the theoretical results in relatively low superheats.

References:

1. Dhir VK, Abarajith HS, Li D. Bubble dynamics and heat transfer during pool and flow boiling. Heat Transfer Eng. 2007; 28:608–624.
2. Karmakar A, Goswami N, Paruya S. Subcooled boiling oscillations in natural circulation boiling loop at low pressure. AIChE J. 2014; 60 (1): 375–386.
3. Prodanovic V, Fraser D, Salcudean M. Bubble behaviour in subcooled flow boiling of water at low pressures and low flow rates. Int J of Multiphase Flow. 2002; 28:1-19.
4. Okawa T, Ishida T, Kataoka I, Mori M, Bubble rise characteristics after the departure from a nucleation site in vertical upflow boiling of subcooled water. Nucl Eng Des. 2005; 235:1149–1161.
5. Situ R, Hibiki T, Ishii M, Mori M. Bubble lift-off size in forced convective subcooled boiling flow. Int J Heat Mass Transf. 2005; 48:5536–5548.
6. Plesset MS, and Zwick SA. Growth of Vapour Bubbles in Superheated Liquids. J Appl Phys, 1954; 25:493–500.
7. Cole R. A Photographic Study of Pool Boiling in the Region of the Critical Heat Flux. AICHE J. 1960; 6:533-538.