Numerical Investigations of Pool Boiling over a Vertical Tube at Sub-atmospheric Pressures or with In-tube Condensation

負壓或有管內冷凝條件下豎直管外池式沸騰的數值研究

Student thesis: Doctoral Thesis

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Award date14 Oct 2019

Abstract

Regarding the pool boiling over a vertical tube or tube bundles, the two-phase flow and heat transfer behaviors usually vary along the tube height especially under the sub-atmospheric pressures or with coupled heat transfer of the in-tube condensation. When pool boiling occurs at sub-atmospheric pressures, which is widely employed in adsorption refrigeration and heat pump systems, the static pressure induced by the liquid height can be of the same order of magnitude as the local saturation pressure which results in the decreasing of saturation temperature and subcooling with the pool height, so that the heat transfer behavior could be very different from that under the atmospheric pressure. When the boiling heat is provided by the in-tube condensation, which is commonly applied in passive heat removal heat exchangers of nuclear power plants, the pool boiling phenomenon differs along the tube height due to the flow of the in-tube hot fluid and the heat transfer conjugation. Therefore, the information of the local heat transfer characteristics is important for the optimal design of the heat exchangers working under such conditions. However, the current experimental techniques are still inadequate to capture the details of the local two-phase flow behavior and heat transfer characteristics, such as local void fraction distribution, local heat transfer coefficient distribution and bubble dynamics. In addition, the experimental study is usually very expensive. The recent advances in multiphase CFD techniques provide effective and economical tools for the investigation of the internal mechanics of the pool boiling heat transfer. Furthermore, the pool boiling at sub-atmospheric pressures has not gained much attention yet, and the studies on the effects of in-tube condensation on external pool boiling heat transfer are also very limited, especially numerical investigations.

In the present study, a 2D axisymmetric numerical model is established for the pool boiling heat transfer and two-phase flow outside a vertical tube at sub-atmospheric pressures or with in-tube condensation. The volume of fluid interface tracking method is employed based on the Euler-Euler multiphase frame. The user-defined subroutines applicable to the VOF model are established for the simulations of the phase change processes. The applicability of phase change models has been analyzed. The numerical models have been validated by experimental data in the literatures, and good agreement has been achieved with tuned mass transfer coefficients. The correlations of mass transfer coefficients with heat flux in the Lee model at different pressures are presented.

The boiling heat transfer and two-phase flow outside a vertical tube at sub-atmospheric pressures is studied numerically for the first time in the present study. The simulations are performed under the operating pressure of 1-10 kPa, heat flux from 22.4 to 100 kW/m2 and liquid subcooling of 0-20 K. The effects of tube length and liquid height on the boiling heat transfer have been studied. Two-phase flow pattern and bubble behavior are analyzed and compared for different sub-atmospheric pressures. Under the condition of lower pressures, because of the decrease of liquid subcooling with the liquid height, boiling occurs at the top of the tube wall firstly, and then gradually develops downwards. The bubbles keep growing while sliding upwards along the wall and coalescing with the adjacent bubbles but will not detach from the wall until leaving the top part of the tube. The bubble sliding velocity increases with the bubble size and tube height. From the computational results, the wall superheat increases with the decreasing operating pressure and the liquid subcooling for the same heat fluxes. The heat flux required for the onset of nucleate boiling is higher for the boiling outside a vertical tube than that for the boiling on a horizontal plate. Unlike the sub-atmospheric pool boiling on a horizontal surface, the heat transfer over a vertical tube is not sensitive to the change of the liquid height. Furthermore, the averaged wall temperature is higher on the longer tubes due to the significant bubble coalescence.

The local two-phase flow during the conjugated heat transfer of in-tube condensation and external pool boiling is studied numerically by an interface-tracking method for the first time in the present study, too. The modeling is conducted with in-tube mean steam flow velocity of 1.9 – 3.2 m/s and steam superheat of 5 K at 1atm at the top inlet. The simulations illustrate that bubbly flow prevails in the bottom area all through the nucleate boiling stage with the bubble sliding, coalescence, and split-up, while cap bubbly flow and cap turbulent flow dominate the middle region and the upper region. The wake effect plays significant effects on the heat transfer in the middle and upper region through inducing turbulence to the bulk fluid. The thickness of the condensate film in the tube side increases along the flow direction. The film is close to smooth laminar state in the upper region and becomes wavy laminar as flowing downwards to the bottom region. The pool side HTC fluctuates and exhibits a slight decrease along the flow direction. The computational result is generally in coincidence with the experimental results. The boiling heat transfer coefficient firstly decreases then increases with the increasing steam velocity.

The present study shows that the two-phase flow pattern evolves along the tube height during the pool boiling over a vertical tube. At sub-atmospheric pressures, increasing operating pressure and liquid subcooling can enhance the pool boiling heat transfer coefficient, while higher liquid height and longer tube length will deteriorate the heat transfer coefficient. When in-tube condensation is involved, the boiling heat transfer coefficient fluctuates and exhibits a slight decrease along the flow time. Also, the boiling heat transfer is affected by the in-tube steam flow rate.

    Research areas

  • Pool boiling, Sub-atmospheric pressure, CFD, Vertical tube, Condensation, Heat transfer coefficient, Phase change model