Abstract

Formation of vapor and gas bubbles and voids is normal and expected in flow processes involving extremely viscous fl uids in normal gravity. This paper is a follow -up of our previous paper AIAA 2009-1147. In that paper we reported on the effects of bubbles in extremely viscous fluids are epoxy-like filler materials before the epoxy fluids cure to their permanent form to create a mechanical bond between two substrates. When these fluids with a free liquid interface are exposed to vacuum, a rapid bubble expansion process may ensue. Bubble expansion might compromise the mechanical bond strength. In this paper, we use computational fluid dynamics to understand the bubble growth dynamics in constricted containers. The present analysis can be used to explain previous experiment data which deviated from Rayleigh-Plesset model. Nomenclature l = density of the liquid μ = liquid viscosity R = instantaneous radius of the bubble  = gas/liquid surface tension P = pressure of the gas inside the bubble P∞ = pressure in the liquid Ro = radius of the bubble at steady state Pi = initial pressure Ri = initial radius of the gas bubble I. Introduction Formation of vapor and gas bubbles and voids is normal and expected in flow processes involving fluids in normal gravity. In space and microgravity, the presence of bubbles in fluids may cause problems like bubble expansion especially when the fluid involved is intended to be an adhesive joining two solid materials upon bonding. On earth bubbles are widely used in industries where gas-liquid columns are important especially in chemical, petrochemical and biochemical industries. Important applications include oxidation processes, fermentation and waste water management. At times, bubbles are required for increasing the mixing processes as well as increasing the interfacial area between the phases leading to improved heat and mass transfer characteristics without the use of moving parts. In our previous studies, we determined analytically the bubble volume fraction that can be produced from different potential sources. We used experimentally methods to determine the dynamics of bubble expansion. In the present study, we have used numerical experiments to analyze bubble expansion under vacuum. The present study describes methods used to predict the behavior of gas and/or vapor bubbles in a fluid when the ambient pressure suddenly changes to vacuum. The study may be used to understand bubble distribution and void formation during the dispensing and precuring stages of epoxy-like adhesives in microgravity under vacuum. The following potential sources were identified for the origin of the gases, that encompass process variables as well as inherent phenomena in the curing process: (i) incomplete out-gassing, (ii) re-gasification due to leakage and off-gassing, (iii) leakage of pressurized gases used in the dispensing of the fluid, (iv) introduction of gases from other components required for adhesive bond strengthening, (v) volatiles evolved from cure reaction products (for example ethanol). Whereas voids and

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