R.A.M. Al-Hayes

Bio: R.A.M. Al-Hayes is an academic researcher from University of Birmingham. The author has contributed to research in topics: Bubble & Penetration (firestop). The author has an hindex of 2, co-authored 2 publications receiving 150 citations.

Dynamics of bubble growth and detachment from a needle

Abstract: Several aspects of the growth and departure of bubbles from a submerged needle are considered. A simple model shows the existence of two different growth regimes according to whether the gas flow rate into the bubble is smaller or greater than a critical value. These conclusions are refined by means of a boundary-integral potential-flow calculation that gives results in remarkable agreement with experiment. It is shown that bubbles growing in a liquid flowing parallel to the needle may detach with a considerably smaller radius than in a quiescent liquid. The study also demonstrates the critical role played by the gas flow resistance in the needle. A considerable control on the rate and size of bubble production can be achieved by a careful consideration of this parameter. The effect is particularly noticeable in the case of small bubbles, which are the most difficult ones to produce in practice.

Two-phase flow, boiling and condensation in conventional and miniature systems

Abstract: Providing a comprehensive introduction to the fundamentals and applications of flow and heat transfer in conventional and miniature systems, this fully enhanced and updated edition covers all the topics essential for graduate courses on two-phase flow, boiling, and condensation. Beginning with a concise review of single-phase flow fundamentals and interfacial phenomena, detailed and clear discussion is provided on a range of topics, including two-phase hydrodynamics and flow regimes, mathematical modeling of gas-liquid two-phase flows, pool and flow boiling, flow and boiling in mini and microchannels, external and internal-flow condensation with and without noncondensables, condensation in small flow passages, and two-phase choked flow. Numerous solved examples and end-of-chapter problems that include many common design problems likely to be encountered by students, make this an essential text for graduate students. With up-to-date detail on the most recent research trends and practical applications, it is also an ideal reference for professionals and researchers in mechanical, nuclear, and chemical engineering.

Two-Phase Microchannel Heat Sinks: Theory, Applications, and Limitations

Abstract: Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase microchannel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase microchannel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macrochannels. Conversely, other fluids may exhibit considerable “confinement” even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nanofluids and carbon nanotube coatings, with proven merits to single-phase macrosystems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of microchannel flow with those of jet impingement. [DOI: 10.1115/1.4005300]

The effect of elasticity on drop creation in T-shaped microchannels

Abstract: Polymeric microdrops of low viscosity, elastic fluids have been generated in T-shaped microfluidic devices using a cross-flow shear-induced drop generation process. Dilute (c/c* similar to 0.5) aqueous solutions of polyethylene oxide (PEO) of various molecular weights (3 x 10(5) -2 x 10(6) g/mol) were used as the drop phase fluids whilst silicone oils (5 mPa s <= eta <= 50 mPa s) were used as the continuous phase fluids. The effects of viscosity ratio and fluid elasticity on the mechanism of drop detachment and break-up, final drop size and frequency of drop formation were studied in this non-Newtonian-Newtonian multiphase system. The generation of thinning filaments between subsequent drops of these low viscosity fluids signifies the presence of elastic stresses in this low Reynold's number flow. Two distinct regions of filament thinning dynamics, a 'pre-stretch' region and an exponential self-thinning region, were observed for the highest molecular weight of PEO studied. The 'experimental' relaxation times extracted from the exponential self-thinning region were of the same order of magnitude as the calculated Zimm relaxation time but were shown to increase as the cross-flow shear was increased. This increase is associated with substantial pre-stretching of the polymer molecules within the forming neck prior to the onset of thread self-thinning. The presence of elasticity within these low viscosity fluids resulted in the production of secondary drops of varying sizes upon final breakup. The substantial threads between the primary drop and the nozzle display traditional bead-on-a-string morphologies, with the final drop size and polydispersity being a distinct function of cross-flow rate, dispersed flow rate and polymer molecular weight. (c) 2006 Elsevier B.V. All rights reserved.