Dynamics of Vapor Bubble in a Variable Pressure Field

This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced.In this problem,the Rayleigh–Plesset equation is treated considering gas content,surface tension,and viscosity,displaying possible multiple expansion–compression cycles.Second,a similar investigation is conducted for the case when the bubble originates near the rounded leading edge of a thin and slightly curved foil at a small angle of attack.Mathematically the flow field around the foil is constructed using the method of matched asymptotic expansions.The outer flow past the hydrofoil is described by linear(small perturbations)theory,which furnishes closed-form solutions for any analytical foil.By stretching local coordinates inversely proportionally to the radius of curvature of the rounded leading edge,the inner flow problem is derived as that past a semi-infinite osculating parabola for any analytical foil with a rounded leading edge.Assuming that the pressure outside the bubble at any moment of time is equal to that at the corresponding point of the streamline,the dynamics problem of a vapor bubble is reduced to solving the Rayleigh-Plesset equation for the spherical bubble evolution in a time-dependent pressure field.For the case of bubble collapse in an adverse pressure field,the spectral parameters of the induced acoustic pressure impulses are determined similarly to equivalent triangular ones.The present analysis can be extended to 3D flows around wings and screw propellers.In this case,the outer expansion of the solution corresponds to a linear lifting surface theory,and the local inner flow remains quasi-2D in the planes normal to the planform contour of the leading edge of the wing(or screw propeller blade).Note that a typical bubble contraction time,ending up with its collapse,is very small compared to typical time of any variation in the flow.Therefore,the approach can also be applied to unsteady flow problems.

Method for passivation of propellant residues in orbital stage tank of launch vehicle

The problem of removing unused liquid propellant residues from the tanks of spent spacecraft and orbital stages of Launch Vehicles(LV)leads to their explosion and the formation of space debris in orbits.To provide a solution to this problem,a method for removing liquid propellant residues from the LV tanks after the mission completion is proposed.The method is based on the gasification of liquid propellant residues in the tanks under acoustic-vacuum exposure and the discharge of the gasification products into the surrounding outer space.Experimental investigations were carried out on a Ground-based Experimental Installation(GEI)to determine the coefficient of heat transfer from the surface of an acoustic radiator to a liquid.The obtained coefficient was then used to calculate the energy costs for the gasification of kerosene.Numerical estimates are given on the example of the tank with kerosene residues from a spent second stage of the LV“Soyuz-2.1 v”.The optimal discharge rate at which kerosene does not freeze is 0.14 m^(3)/s.Moreover,the acoustic exposure leads to an increase in the mass of evaporated kerosene over a given time by96.1%,and the energy costs are 1756.7 kJ(approximately 50% of the remaining electrical energy).

Heat Transfer Enhancement Due to Marangoni Flow Around Moving Bubbles During Nucleate Boiling

Nucleate boiling is a very efficient method for generating high heat transfer rates from solid surfaces; however, the fundamental physical mechanisms governing nucleate boiling heat transfer are not well understood. The heat transfer mechanisms around stationary and moving bubbles on very thin microwires were analyzed numerically to evaluate the effect of the bubble motion on the heat transfer from the wire surface. The numerical analysis accurately models the experimentally observed bubble movement and fluid velocities. The analytical model includes the effects of the Marangoni flow around the bubble and the evaporation and condensation within the bubble. The analysis shows that the heat transfer was significantly enhanced by the Marangoni flow around the outside of the bubble which transfers at least twice as much en- ergy from the wire as the heat transfer directly from the wire to the bubble. The enhanced heat transfer due to the Marangoni flow was evident for both stationary and moving bubbles. The moving bubbles also created a wake that further enhanced the heat transfer from the wire. Since the Marangoni number for water is greater than for ethanol for the same conditions, the Marangoni flow and, hence, the bubble velocities are predicted to be greater in water than in ethanol.