A numerical model for a bubble in nucleate pool boiling

Patil, Rajendra
Major Professor
Joseph Prusa
Committee Member
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Mechanical Engineering
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Mechanical Engineering

Growth of a hemispherical bubble in nucleate pool boiling is analyzed in two stages using a numerical technique. In the first stage, growth of a hemispherical bubble in viscous medium on an isothermally heated surface is investigated neglecting the microlayer contribution. The results are compared with an analytical solution for homogeneous growth in a superheated medium, to other numerical results for heterogeneous growth in an inviscid medium, and to previous experimental work. The flow field and temperature distribution around the bubble are found to exhibit similarity for large times when an appropriate non-dimensionalization is used. The Jakob number (Ja), and subcooling number (Sb) are the only similarity parameters controlling growth in this asymptotic regime;In the second stage, growth of a hemispherical bubble is investigated including the analyses of a microlayer, the non-condensible gas inside the bubble and the temperature distribution in the heated surface. The temperature distribution in the fluid and in the solid, and the microlayer profile is found to be self similar for large time. The microlayer profile is almost triangular except at the tail point, where it is very steep. The famous "bulk convection" theory is found to be questionable. In accordance with experimental observations recorded in the literature, the growth of a bubble with a microlayer is found to be equal to that in homogeneous superheated medium for equal Jakob numbers. Pressure in the bubble is almost equal to the pressure of the environment except in the extremely short initial period. The bubble growth relation in saturated boiling isR = const Ja[superscript]3/4[over] Pr[superscript]1/4 √tfor 10 ≤ Ja ≤ 640, 0.26 ≤ Pr≤ 2.33, where Pr is the Prandtl number and the value of the constant is approximately 5. The contribution to bubble growth from the microlayer has been found to be 90% for Ja~eq 10, Pr~eq 2.5, 99% for Ja~eq 80, Pr ~eq 1, and as high as 99.8% for Ja~eq 320, Pr~eq 0.2. The flow field inside the bubble is investigated with and without the microlayer. The gas inside the bubble is proved to be at an uniform state in both the cases.