Experimental studies on a two stage pulse tube cryocooler reaching 2.5K

A two stage Pulse Tube Cryocooler (PTC) is designed and fabricated which reaches a no-load temperature of 2.5K in the second stage and similar to 60 K in the first stage respectively. The system provides a cooling power of similar to 250 mW at 5K in the second stage. Stainless steel meshes (size 200) and lead (Pb) granules are used as the first stage regenerator materials and combination of Pb with Er3Ni / HoCu2 are used as the second stage regenerator materials. The system operates at 1.6 Hz using a 6 kW water cooled helium compressor. Studies conducted by varying the dimensions of Pulse Tubes and regenerators show that the dimensions of the Pulse Tubes are more critical to the performance of the Cryocooler than those of the regenerators. Experimental studies show that the optimum volume ratios of Er3Ni to Pb and HoCu2 to Pb in the second stage regenerator should be 3:2 and 2:3 respectively for the best performance. Further, systems with HoCu2 performed better than those with Er3Ni. The theoretical analysis of the system has been carried out using a simple isothermal model. The experimentally measured cooling powers are in good agreement with the theoretical predictions.

Probing the existing magnetic phases in Pr0.5Ca0.5MnO3 (PCMO) nanowires and nanoparticles: magnetization and magneto-transport investigations

We show from conventional magnetization measurements that the charge order (CO) is completely suppressed in 10 nm Pr0.5Ca0.5MnO3 (PCMO 10) nanoparticles. Novel magnetization measurements, designed by a special high field measurement protocol, show that the dominant ground state magnetic phase is ferromagnetic-metallic (FM-M), which is an equilibrium phase, which coexists with the residual charge ordered anti-ferromagnetic phase (CO AFM) (an arrested phase) and exhibits the characteristic features of a `magnetic glassy state' at low temperatures. It is observed that there is a drastic reduction in the field required to induce the AFM to FM transition (similar to 5-6 T) compared to their bulk counterpart(similar to 27 T); this phase transition is of first order in nature, broad, irreversible and the coexisting phases are tunable with the cooling field. Temperature-dependent magneto-transport data indicate the occurrence of a size-induced insulator-metal transition (TM-I) and anomalous resistive hysteresis (R-H) loops, pointing out the presence of a mixture of the FM-M phase and AFM-I phase.

LOX separation studies using cryogenic vortex tube

In-flight collection of air, pre-cooling, liquefaction and separation of liquid oxygen (LOX) are key technologies for futuristic launch vehicles, Vortex tube technology is one of the few potential technologies for this application. Extensive studies have been carried out on straight and conical vortex tubes for developing vortex tube technology for high purity LOX separation. Studies show that 12mm. diameter conical vortex tube with L/D of 10 could achieve LOX purity of similar to 96% with separation efficiency of similar to 14% indicating that it is not possible to obtain both high LOX purity and high separation efficiency simultaneously in a single vortex tube. However, it is possible to achieve both high LOX purity and separation efficiency by staging of vortex tubes. LOX purity of 96% and separation efficiency of similar to 73.5% has been achieved for second stage vortex tube supplied with pre-cooled air having 60% oxygen purity. LOX purity has been further increased to 97% by applying controlled heating power over liquid oxygen flowing discharge surface of the vortex tube.

Numerical studies on channel formation and growth during solidification: Effect of process parameters

In the present work, solidification of a hyper-eutectic ammonium chloride solution in a bottom-cooled cavity (i.e. with stable thermal gradient) is numerically studied. A Rayleigh number based criterion is developed, which determines the conditions favorable for freckles formation. This criterion, when expressed in terms of physical properties and process parameters, yields the condition for plume formation as a function of concentration, liquid fraction, permeability, growth rate of a mushy layer and thermophysical properties. Subsequently, numerical simulations are performed for cases with initial and boundary conditions favoring freckle formation. The effects of parameters, such as cooling rate and initial concentration, on the formation and growth of freckles are investigated. It was found that a high cooling rate produced larger and more defined channels which are retained for a longer durations. Similarly, a lower initial concentration of solute resulted in fewer but more pronounced channels. The number and size of channels are also found to be related to the mushy zone thickness. The trends predicted with regard to the variation of number of channels with time under different process conditions are in accordance with the experimental observations reported in the literature.

Interrupted creep behaviour of Mg alloys developed for powertrain applications

A conventional magnesium alloy, AZ91D, and two creep resistant magnesium alloys, developed for powertrain applications, MRI 153M and MRI 230D, are prepared by high pressure die casting. These alloys are tested for their creep behaviour in the continuous manner, as is the Current practice, and in the interrupted manner, which represents the real life Situation more closely. It is observed that the interrupted creep tests give rise to a primary creep appearing at the beginning of each cycle resulting in a higher average strain rate than that encountered in the continuous creep tests. Further, the shorter the cycle time, higher is the average strain rate in the interrupted creep tests. A higher average strain rate will give rise to a higher strain over the same period. This is attributed to the recovery taking place during the cooling and heating between two cycles. The effect of additional precipitation during interrupted creep tests depends on the nature of the precipitates. The additional precipitation of beta phase during the cooling and heating between two cycles increases the steady state strain rate in the AZ91D and MRI 153M alloys. whereas the additional precipitation of C36 phase during the cooling and heating between two cycles decreases the steady state strain rate in the MRI 230D alloy. (C) 2009 Elsevier B.V. All rights reserved.

Performance studies on mechanical + adsorption hybrid compression refrigeration cycles with HFC 134a

This paper presents the results of an investigation on the efficacy of hybrid compression process for refrigerant HFC 134a in cooling applications. The conventional mechanical compression is supplemented by thermal compression using a string of adsorption compressors. Activated carbon is the adsorbent for the thermal compression segment. The alternatives of bottoming either mechanical or thermal compression stages are investigated. It is shown that almost 40% energy saving is realizable by carrying out a part of the compression in a thermal compressor compared to the case when the entire compression is carried out in a single-stage mechanical compressor. The hybrid compression is feasible even when low grade heat is available. Some performance indictors are defined and evaluated for various configurations.

Surface spin glass behavior in sol–gel derived La0.7Ca0.3MnO3 nanotubes

We report the fabrication of La0.7Ca0.3MnO3 nanotubes (LCMONTs) with a diameter of about 200 nm, by a modified sol-gel method utilizing nanochannel alumina templates. High resolution transmission electron microscopy confirmed that the obtained LCMONTs are made up of nanoparticles (8-12 nm), which are randomly aligned in the wall of the nanotubes. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves as well as a cusplike peak in the ZFC curve gives strong support for surface spin glass behavior.

Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX)

Before the onset of the south Asian summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30–32°C. Climatological mean mixed layer depth in spring (March–May) is 10–20 m, and net surface heat flux (Q net ) is 80–100 W m−2 into the ocean. Previous work suggests that observed spring SST warming is small mainly because of (1) penetrative flux of solar radiation through the base of the mixed layer (Q pen ) and (2) advective cooling by upper ocean currents. We estimate the role of these two processes in SST evolution from a two-week Arabian Sea Monsoon Experiment process experiment in April–May 2005 in the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling, although the ocean gains heat at the surface: Q net is about 105 W m−2 in the warming phase and 25 W m−2 in the cooling phase; penetrative loss Q pen is 80 W m−2 and 70 W m−2. In the warming phase, SST rises mainly because of heat absorbed within the mixed layer, i.e., Q net minus Q pen ; Q pen reduces the rate of SST warming by a factor of 3. In the second phase, SST cools rapidly because (1) Q pen is larger than Q net and (2) advective cooling is ∼85 W m−2. A calculation using time-averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal timescale. Buoy and satellite data suggest that a typical premonsoon intraseasonal cooling event occurs under clear skies when the ocean is gaining heat through the surface. In this respect, premonsoon SST cooling in the north Indian Ocean is different from that due to the Madden-Julian oscillation or monsoon intraseasonal oscillation.

Effect of process parameters on transport phenomena during solidification in the presence of electromagnetic stirring

In the present work, a numerical study is performed to predict the effect of process parameters on transport phenomena during solidification of aluminium alloy A356 in the presence of electromagnetic stirring. A set of single-phase governing equations of mass, momentum, energy and species conservation is used to represent the solidification process and the associated fluid flow, heat and mass transfer. In the model, the electromagnetic forces are incorporated using an analytical solution of Maxwell equation in the momentum conservation equations and the slurry rheology during solidification is represented using an experimentally determined variable viscosity function. Finally, the set of governing equations is solved for various process conditions using a pressure based finite volume technique, along with an enthalpy based phase change algorithm. In present work, the effect of stirring intensity and cooling rate are considered. It is found that increasing stirring intensity results in increase of slurry velocity and corresponding increase in the fraction of solid in the slurry. In addition, the increasing stirring intensity results uniform distribution of species and fraction of solid in the slurry. It is also found from the simulation that the distribution of solid fraction and species is dependent on cooling rate conditions. At low cooling rate, the fragmentation of dendrites from the solid/liquid interface is more.

Optimization of friction stir welding parameters for dissimilar aluminum alloys

Assembly consisting of cast and wrought aluminum alloys has wide spread application in defense and aero space industries. For the efficacious use of the transition joints, the weld should have adequate strength and formability. In the present investigation, A356 and 6061 aluminum alloys were friction stir welded under tool rotational speed of 1000-1400 rpm and traversing speed of 80-240 mm/min, keeping other parameters same. The variable process window is responsible for the change in total heat input and cooling rate during welding. Structural characterization of the bonded assemblies exhibits recovery-recrystallization in the stirring zone and breaking of coarse eutectic network of Al-Si. Dispersion of fine Si rich particles, refinement of 6061 grain size, low residual stress level and high defect density within weld nugget contribute towards the improvement in bond strength. Lower will be the tool rotational and traversing speed, more dominant will be the above phenomena. Therefore, the joint fabricated using lowest tool traversing and rotational speed, exhibits substantial improvement in bond strength (similar to 98% of that of 6061 alloy), which is also maximum with respect to others. (C) 2010 Elsevier Ltd. All rights reserved.

An asymptotic analysis of a simple model for the structure and dynamics of the Ramdas layer

This paper presents a complete asymptotic analysis of a simple model for the evolution of the nocturnal temperature distribution on bare soil in calm clear conditions. The model is based on a simplified flux emissivity scheme that provides a nondiffusive local approximation for estimating longwave radiative cooling near ground. An examination of the various parameters involved shows that the ratio of the characteristic radiative to the diffusive timescale in the problem is of order 10(-3), and can therefore be treated as a small parameter (mu). Certain other plausible approximations and linearization lead to a new equation whose asymptotic solution as mu --> 0 can be written in closed form. Four regimes, consishttp://eprints.iisc.ernet.in/cgi/users/home?screen=EPrint::Edit&eprintid=27192&stage=core#tting of a transient at nominal sunset, a radiative-diffusive boundary ('Ramdas') layer on ground, a boundary layer transient and a radiative outer solution, are identified. The asymptotic solution reproduces all the qualitative features of more exact numerical simulations, including the occurrence of a lifted temperature minimum and its evolution during night, ranging from continuing growth to relatively sudden collapse of the Ramdas layer.

A simple miniature furnace for routine collection of single crystal x-ray data up to 1000°C on the Hilger and Watts linear diffractometer

A miniature furnace suitable for routine collection of x-ray data up to 1000°C from single crystals on the Hilger and Watts linear diffractometer, without restricting the normally allowed region of reciprocal space on the diffractometer, is described. The crystal is heated primarily by radiation from a surrounding current-heated, stationary platinum coil wound on a silica bracket. The coil is split at its middle to provide a 4 mm gap for crystal mounting and x-irradiation. The crystal, mounted on a standard goniometer head, can be rotated and centred freely, as in the room temperature case. There is no need for any radiation shields or water-cooling arrangement. Investigations up to 1500°C are possible with slight modifications of the furnace.

Preparation of lower oxide of sulphur

RECENT work on the lower oxide of sulphur1,2 has established that disulphur monoxide (S2O) or its polymeric form is produced when sulphur is burnt in oxygen under reduced pressure. It has now been shown that it is possible to make use of an oxide of a heavy metal as a source of limited supply of oxygen to prepare the disulphur monoxide. For example, when a mixture of finely powdered cupric oxide and sulphur (1 : 5 by weight) is heated under vacuum in a glass tube gaseous products are evolved. which, on cooling in a trap surrounded by liquid air, will give an orange-red condensate (S2O)x. This condensate also gives off sulphur dioxide in stages as the temperature is raised, finally leaving a residue of elemental sulphur. Copper sulphide and excess of sulphur are left behind in the reaction tube.

A low temperature Weissenberg camera

Details of the design and operation of a Weissenberg camera suitable for x-ray investigations between -180°c and 200°c are presented. The camera employs a novel arrangement of spur and bevel gears to couple the goniometer spindle to the worm rod which controls the range of oscillation. The entire drive system and the goniometer assembly are mounted on a support which permits the insertion or removal of a cylindrical cassette from the gear-box side without disturbing the cooling assembly and the layer-line screen. The cassette can also be inserted from the opposite side. The specimen can be cooled either directly by a stream of liquid air or by the cold gas from its evaporation. Condensation of moisture at low temperatures is prevented by heating the layer-line tubes internally.

Energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals

Thermotropic liquid crystals are known to display rich phase behavior on temperature variation. Although the nematic phase is orientationally ordered but translationally disordered, a smectic phase is characterized by the appearance of a partial translational order in addition to a further increase in orientational order. In an attempt to understand the interplay between orientational and translational order in the mesophases that thermotropic liquid crystals typically exhibit upon cooling from the high-temperature isotropic phase, we investigate the potential energy landscapes of a family of model liquid crystalline systems. The configurations of the system corresponding to the local potential energy minima, known as the inherent structures, are determined from computer simulations across the mesophases. We find that the depth of the potential energy minima explored by the system along an isochor grows through the nematic phase as temperature drops in contrast to its insensitivity to temperature in the isotropic and smectic phases. The onset of the growth of the orientational order in the parent phase is found to induce a translational order, resulting in a smectic-like layer in the underlying inherent structures; the inherent structures, surprisingly, never seem to sustain orientational order alone if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. The Arrhenius temperature dependence of the orientational relaxation time breaks down near the isotropic-nematic transition. We find that this breakdown occurs at a temperature below which the system explores increasingly deeper potential energy minima.