Bruce Webbon

Bubble formation at a submerged orifice in reduced gravity

Abstract We consider gas injection through a circular plate orifice into an ideally wetting liquid which results in the successive detachment of bubbles, each of which is regarded as a separate entity. At normal gravity and at relatively low gas flow rates, the growing bubble is modelled as a spherical segment that touches the orifice perimeter during the whole time of its evolution. If the gas flow rate exceeds a certain threshold value, a second stage of the detachment takes place that follows the first spherical segment stage. In this second stage, a nearly cylindrical stem forms at the orifice that lengthens as the bubble rises above the plate, and this stems feeds an almost spherical gas envelope stituated at the stem upper end. At high gas flow rates, bubble shape resembles that of a mushroom, and its upper envelope continues to grow until the gas supplied through the stem is completely cut off. This second stage always develops when gravity is sufficiently low, irrespective of the gas flow rate. There are two major factors that determine the moment of bubble detachment: the buoyancy force and a force due to the momentum flowing into the bubble with the injected gas. The buoyancy force dominates the process at normal gravity whereas the inflowing momentum force plays the key role under negligible gravity conditions. As gravity fluctuates, the interplay of these forces drastically influences bubble growth and detachment. At sufficiently low gravity, the bubble formation frequency is proportional to gas flow rate whereas the bubble detachment volume is independent of gas flow rate. At normal and moderately reduced gravity conditions, when the gas flow rate grows, bubble formation frequency slightly decreases and bubble detachment volume increases almost linearly. Effects of other parameters, such as the orifice radius, gas and liquid densities and surface tension coefficient are discussed.

Dynamics of vapour bubbles in nucleate boiling

Abstract This paper considers the behaviour of a vapour bubble formed at a nucleation site on a heated horizontal wall. This bubble is modeled as a spherical segment which is separated from the wall by a microlayer of intervening liquid. The liquid is presumed to be at rest at great distances from the bubble. In order to avoid unwarranted assumptions about forces acting on the bubble which are specific to all known models of bubble growth and detachment, we derive equations that govern bubble behaviour in a rigorous way from the variational equation that describes mechanical energy conservation for the whole system, which includes both the bubble and the liquid. The variational equation leads to a set of two mutually independent strongly nonlinear equations which govern bubble expansion and the motion of its centre of mass. Because these equations contain an extra unknown variable (the bubble vapour pressure), a supplementary equation that defines bubble vapour temperature must be formulated with allowance made for heat transfer to the bubble both from the bulk of the surrounding liquid and through the microlayer. The most important conclusion of this paper consists in the fact that surface tension effects result in an effective force that tends to transform the bubble into a sphere, thereby facilitating bubble detachment. This conclusion absolutely nullifies the generally, however erroneously, held belief that this effective force presses the bubble to the wall. By way of example, we consider the evolution of bubbles whose growth is thermally controlled. Our analysis provides quite a natural explanation for a number of repeatedly observed phenomena, such as the influence of gravity and surface tension on bubble growth rate and the dependence of bubble detachment size on thermophysical parameters.

The Isolated Bubble Regime in Pool Nucleate Boiling

Abstract We consider an isolated bubble boiling regime in which vapour bubbles are intermittently produced at a prearranged set of nucleation sites on an upward facing overheated wall plane. In this boiling regime, the bubbles depart from the wall and move as separate entities. Heat transfer properties specific to this regime cannot be described without bubble detachment size, and we apply our previously developed dynamic theory of vapour bubble growth and detachment to determine this size. Bubble growth is presumed to be thermally controlled. Two limiting cases of bubble evolution are considered: the one in which buoyancy prevails in promoting bubble detachment and the one in which surface tension prevails. We prove termination of the isolated regime of pool nucleate boiling to result from one of the four possible causes, depending on relevant parameters values. The first cause consists in the fact that the upward flow of rising bubbles hampers the downward liquid flow, and under certain conditions, prevents the liquid from coming to the wall in an amount that would be sufficient to compensate for vapour removal from the wall. The second cause is due to the lateral coalescence of growing bubbles that are attached to their corresponding nucleation sites, with ensuing generation of larger bubbles and extended vapour patches near the wall. The other two causes involve longitudinal coalescence either: (1) immediately in the wall vicinity, accompanied by the establishment of the multiple bubble boiling regime or (2) in the bulk, with the formation of vapour columns. The longitudinal coalescence in the bulk is shown to be the most important cause. The critical wall temperature and the heat flux density associated with isolated bubble regime termination are found to be functions of the physical and operating parameters and are discussed in detail.

An innovative exercise method to simulate orbital EVA work - Applications to PLSS automatic controls

An exercise method has been proposed which may satisfy the current need for a laboratory simulation representative of muscular, cardiovascular, respiratory, and thermoregulatory responses to work during orbital extravehicular activity (EVA). The simulation incorporates arm crank ergometry with a unique body support mechanism that allows all body position stabilization forces to be reacted at the feet. By instituting this exercise method in laboratory experimentation, an advanced portable life support system (PLSS) thermoregulatory control system can be designed to more accurately reflect the specific work requirements of orbital EVA.

A direct-interface, fusible heat sink for astronaut cooling

Astronaut cooling during extravehicular activity is a critical design issue in developing a portable life support system that meets the requirements of a space station mission. Some of the requirements are that the cooling device can be easily regenerable and nonventing during operation. In response to this, a direct-interface, fusible heat sink prototype with freezable quick-disconnects was developed. A proof-of-concept prototype was constructed and tested that consists of an elastic container filled with normal tap water and having two quick-disconnects embedded in a wall. These quick-disconnects are designed so that they may be frozen with the ice and yet still be joined to the cooling system, allowing an immediate flow path. The inherent difficulties in a direct-interface heat sink have been overcome, i.e., (1) establishing an initial flow path; (2) avoiding low-flow freeze-up; and (3) achieving adequate heat-transfer rates at the end of the melting process. The requirements, design, fabrication, and testing are discussed.

Development of a Thermal Control Coating for Space Suits

Past space suits and the current Shuttle suit, which are constructed primarily from fabric, use the Integrated Thermal and Micrometeoroid Garment, which insulates the astronaut from his environment. The new generation of hard suits affords designers the opportunity to incorporate thermal control into the suit structure. Environmental influence on the suit temperature and heat flux can then be minimized with a high reflectance coating. Candidate coatings have been identified and ranked on the basis of thermophysical properties; wear, corrosion and atomic oxygen degradation resistance; and coating process and cost. Laboratory determination of properties, thermal cycling and wear resistance tests are underway to identify the optimum coating. A computer model is being developed to evaluate various environmental configurations. Preliminary results are presented here.