An experimental study of a confined and submerged impinging jet heat transfer using Al2O3-water nanofluid

An experimental investigation was performed to study the heat transfer performance of a 36 nm-Al₂O₃-particle–water nanofluid in a confined and submerged impinging jet on a flat, horizontal and circular heated surface. The tests were realized for the following ranges of the governing parameters: the nozzle diameter is 3 mm and the distance nozzle-to-heated-surface was set to 2, 5 and 10 mm; the flow Reynolds number varies from 3800 to 88 000, the Prandtl number from 5 to 10, and the particle volume fraction is ranging from 0 to 6%. Experimental data, obtained for both laminar and turbulent flow regimes, have clearly shown that, depending upon the combination of nozzle-to-heated surface distance and particle volume fraction, the use of a nanofluid can provide a heat transfer enhancement in some cases; conversely, for other combinations, an adverse effect on the convective heat transfer coefficient may occur. Within the experimental parameters used, it has been observed that highest surface heat transfer coefficients can be achieved using an intermediate nozzle-to-surface distance of 5 mm and a 2.8% particle volume fraction nanofluid. Nanofluids with high particle volume fractions, say 6% or higher, have been found not appropriate for the heat transfer enhancement purpose under the confined impinging jet configuration. On the other hand, for a very small and a large distance of nozzle-to-heated-surface, it has been observed that the nanofluid use does not provide a perceptible heat transfer enhancement and has, for some particular cases, produced a clear decrease of the convective heat transfer coefficient while compared to that obtained using distilled water.     

Innovations in the ENHS reactor design and fuel cycle

A number of approaches were explored for improving characteristics of the encapsulated nuclear heat source (ENHS) reactor and its fuel cycle, including: increasing the ENHS module power, power density and the specific power, making the core design insensitive to the actinides composition variation with number of fuel recycling and reducing the positive void coefficient of reactivity. Design innovations examined for power increase include intermediate heat exchanger (IHX) design optimization, riser diameter optimization, introducing a flow partition inside the riser, increasing the cooling time of the LWR discharged TRU, increasing the minor actinides' concentration in the loaded fuel and split-enrichment for power flattening. Another design innovation described utilizes a unique synergism between the use of MA and the design of reduced power ENHS cores.

Also described is a radically different ENHS reactor concept that has a solid core from which heat pipes transport the fission power to a coolant circulating around the reflector. Promising features of this design concept include enhanced decay heat removal capability; no positive void reactivity coefficient; no direct contact between the fuel clad and the coolant; a core that is more robust for transportation; higher coolant temperature potentially offering higher energy conversion efficiency and hydrogen production capability.     

Orienting Fibers by Extrusion in Reinforced Reactive Powder Concrete

This paper deals with the problem of orienting short metal fibers (12 mm length, aspect ratio 60) in a reactive powder concrete during an extrusion process to manufacture reinforced tubes. By choosing an appropriate geometry for the die it is possible to orient the fibers in an orthoradial direction, which is the most efficient if one wishes to reinforce the tube for an internal pressure loading. This result is obtained with both a theoretical and an experimental approach. In a theoretical framework the authors carried out numerical modeling with a constitutive behavior deduced from the anisotropic suspensions theory. This calculus enabled the authors to design the shape of the die. Tubes have then been extruded and the fibers' orientation has been measured with optical microscopy.     

Surface roughness variation of thin wall milling, related to modal interactions

High-speed milling operations of thin walls are often limited by the so-called regenerative effect that causes poor surface finish. The aim of this paper is to examine the link between chatter instability and surface roughness evolution for thin wall milling. Firstly, the linear stability lobes theory for the thin wall milling optimisation was used. Then, in order to consider the modal interactions, an explicit numerical model was developed. The resulting nonlinear system of delay differential equations is solved by numerical integration. The model takes into account the coupling mode, the modal shape, the fact that the tool may leave the cut and the ploughing effect. Dedicated experiments are carried out in order to confirm this modelling. This paper presents surface roughness and chatter frequency measurements. The stability lobes are validated by thin wall milling. Finally, the modal behaviour and the mode coupling give a new interpretation of the complex surface finish deterioration often observed during thin wall milling.     

Synthesis and characterization of new cationic surfactants derived from lactic acid

A series of new cationic surfactants was prepared by condensation of lactic acid with N,N-dimethyl propylamine, followed successively by acetylation with acid chloride or ester and by quaternization with dimethylsulfate. Each surfactant and its precursors' structural assignments were based on infrared spectra and ¹H and ¹³C nuclear magnetic resonance spectra. Critical micelle concentrations, contact angles, kinetics of drop spreading, and foam capacities were determined.     

Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms

This work investigates emulsion templating to synthesize hexadecane oil/geopolymer composites. In a system with hexadecane as the internal (dispersed) phase and an alkali activated continuous phase without added surfactant, adding aluminosilicate clay particles does not increase resistance against creaming or coalescence, while adding a surfactant (L35 or CTAB) stabilizes the solid-liquid interface. Infrared studies and rheological studies of the associated geopolymerization determined that the presence of the organic phase or surfactant has no significant effect on the geopolymerization kinetics, as determined by the change in time of the Si-O-T IR stretching frequency and the rheological moduli involved during the process. The stabilization of the organic template is reminiscent of Pickering emulsion even though we employ a much greater amount of inorganic material for geopolymer formation. Although the addition of surfactant has a significant effect on the behavior of the paste, the percolation of the network remains unmodified, highlighting the fact that the phenomenon is not dependent on viscosity. Finally, rheological measurements were used to obtain the mass fractal dimension of the as-made gel network, which is able to differentiate the interfacial effect between surfactant molecules with a slightly denser interphase when a cationic surfactant is used.     

Destabilizing interactions between the partners of a bifunctional fusion protein

Hybrid MalE–GVP is a bifunctional protein in vitro sinceit binds maltose as protein MalE of Escherichia coli and sinceit is dimeric and specifically binds single-stranded DNA asprotein GVP of phage M13. The oxidation rate of a unique cysteineresidue was used to compare the stabilities of GVP in its freeand hybrid forms, under conditions where MalE was either foldedor unfolded by a denaturing agent. The results showed that boththe covalent link and tertiary non-covalent interactions betweenMalE and GVP destabilized GVP in MalE–GVP. To test whetherGVP had identical structures in its free and hybrid forms, mutationswere used as local conformational probes. The effects of thesemutations on the capabilities of MalE–GVP to dimerizeand to bind single-stranded DNA were assayed in vitro. Theywere compatible with the effects of the same mutations on theglobal activity of free GVP in vivo and with the effects thatcould be predicted from the known data on free GVP, in particularits crystal structure. Thus, one partner of a hybrid proteincan be destabilized by the other partner while maintaining itsstructural and functional characteristics.     

SUPPRESSION OF PERIOD DOUBLING CHATTER IN HIGH-SPEED MILLING BY SPINDLE SPEED VARIATION

Spindle speed variation is a well known technique to suppress regenerative machine tool vibrations, but it is usually considered to be effective only for low spindle speeds. In the current paper, spindle speed variation is applied to the high speed milling process, at the spindle speeds where the constant speed cutting results in period doubling chatter. The stability analysis of triangular and sinusoidal shape variations is made numerically with the semi-discretization method. It is shown that the milling process can be stabilized by increasing the amplitude of the spindle speed variation, while the frequency of the variation has no significant effect on the dynamic behaviour. The results are validated by experiments. Based on the analysis of the machined workpieces, it is shown that the surface roughness can also be decreased by the spindle speed variation technique.     

Engraftment of Prevascularized,Tissue Engineered Constructs in a Novel Rabbit Segmental Bone Defect Model

The gold standard treatment of large segmental bone defects is autologous bone transfer, which suffers from low availability and additional morbidity. Tissue engineered bone able to engraft orthotopically and a suitable animal model for pre-clinical testing are direly needed. This study aimed to evaluate engraftment of tissue-engineered bone with different prevascularization strategies in a novel segmental defect model in the rabbit humerus. Decellularized bone matrix (Tutobone) seeded with bone marrow mesenchymal stromal cells was used directly orthotopically or combined with a vessel and inserted immediately (1-step) or only after six weeks of subcutaneous “incubation” (2-step). After 12 weeks, histological and radiological assessment was performed. Variable callus formation was observed. No bone formation or remodeling of the graft through TRAP positive osteoclasts could be detected. Instead, a variable amount of necrotic tissue formed. Although necrotic area correlated significantly with amount of vessels and the 2-step strategy had significantly more vessels than the 1-step strategy, no significant reduction of necrotic area was found. In conclusion, the animal model developed here represents a highly challenging situation, for which a suitable engineered bone graft with better prevascularization, better resorbability and higher osteogenicity has yet to be developed.     

Fast Measurements of the Gas‐Liquid Diffusion Coefficient in the Gaussian Wake of a Spherical Bubble

A fast method is proposed for determining the oxygen gas‐liquid diffusion coefficient from measurements of the fluorescence quenching behind a bubble. The approach consists of capturing pictures of the concentration field at micro‐scale in the laminar bubble wake. The Gaussian concentration profiles measured in the wake are demonstrated to be systematically equivalent to an instantaneous plane diffusion case. The approach permits to accurately evaluate the gas‐liquid diffusivity in a very short time of around one second.