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Development of Cryocooler Based High Performance Cryosorption Pump
No full textThe aim of this work is to develop high performance cryosorption (or cryoadsorption) pumps specifically for fusion applications. An actual cryopump for the above application will use the supercritical liquid helium flow through the channels embedded in the large scale cryopanels. In this case, the liquid helium requirement (both as normal and as supercritical fluids) will be large, depending on the size of the cryosorption pump. However, in a research laboratory wherein such large quantities of liquid helium are not available, an alternate arrangement of cooling the cryopanels has to be considered.
One of the possible options can then be as follows. A scaled-down version of the cryopanel can be used and cooled by a two stage cryo-refrigerator system with adequate cooling power. This system is known as cryocooler based cryosorption pump. Due to the availability of a two stage GM cryocooler with a refrigeration power of ~ 1.5 W at 4.2 K in our laboratory, which can be used for the above purpose, the main objective of this work is the “development of a cryocooler based high performance cryosorption pump”.
The cryopanel which is mounted on the second stage cold head of the cryocooler is not necessarily a single panel, but is usually a set of panels (stacked one over the other) and consists of mainly three components and they are: (a) the metallic panel made of copper and cooled by the cryocooler (b) the adhesive to bind the adsorbent onto the metallic panel and (c) the adsorbent (in the present case, activated carbon (AC)) which is used to adsorb the gas molecules. By this arrangement, the adsorbent gets cooled to the lowest possible temperature to enable cryopumping.
To develop the cryocooler based high performance cryosorption pump, we need to select: (a) the best adsorbent (with large adsorption surface area) and adhere it on a cryopanel to evaluate its performance as a cryopump and (b) the best adhesive with high thermal conductivity, high bonding strength and ability to withstand several thermal cycles.
The surface area of an adsorbent in the range of temperatures range from 4.5 K to 77 K can be arrived at by a micropore analyser ( Model: ASIQ, Quantachrome, USA) integrated with a two stage GM cryocooler (Janis: SRDK415D), with helium as adsorbate gas at 4.5 K and nitrogen as adsorbate gas at 77 K. Based on the above studies, we can choose the best activated carbon with high surface area.
Next, the best adhesive which is used for binding the adsorbent onto the panel is chosen on the basis of its high thermal conductivity. The thermal conductivity of the adhesive has been measured using two dedicated thermal conductivity measurement systems namely: (a) liquid helium based Janis SuperVariTemp (SVT) cryostat and (b) two-stage GM cryocooler based experimental setup developed in our laboratory in the temperature range from 4.5 K to 300 K.
In order to make comparative studies of cryosorption of different activated carbons, a standard cryopanel, such as the one used in the commercial cryopump (Make: Varian, Model Ebara SP8) has been chosen in our studies. (Henceforth, this will be designated as “Commercial cryopanel”). In other words, the physical dimensions of all the cryopanels fabricated with indigenous activated carbons are exactly the same as that of the commercial cryopanel. By this, the experimental results of pumping speeds of different indigenous AC cryopanels can be benchmarked against the commercial cryopanel and the best performing activated carbon cryopanel can be arrived at.
In the following, we discuss the works carried out for the development of the cryocooler based high performance cryosorption pump.
a) A specially prepared indigenously developed Knitted Carbon Cloth (KCC/IIS01) is found to have a larger surface area for adsorption compared to the other adsorbents. This adsorbent has a surface area of ~ 3000 m2/g for helium adsorption at 4.5 K, which is significantly higher than those of granular charcoals which are in the range of ~ 1600 m2/g for similar experimental conditions. This AC cloth has been used for the development of our cryosorption pump.
b) A special epoxy based adhesive (SEBA/IIS01) with higher thermal conductivity, (measured using the experimental setups mentioned earlier) in the temperature range from 4.5 K to 7 K (which is generally the operating temperature range of a cryosorption pump for efficient pumping of helium gas) compared to the commercially available epoxy adhesives such as STYCAST 2850 FT and G10 Cryocomp has been developed indigenously and used.
c) Using the above Knitted Carbon Cloth KCC/IIS01 and the epoxy adhesive SEBA/IIS01, cryopanel has been prepared and studied for its performance. The pumping speeds of the developed cryopanel are improved on an average by factors of 1.55 and 1.54 when compared with those of commercial panel for gases such as hydrogen (H2) and helium (He) respectively in the specific pressure range.
To enhance the thermal conductivity of SEBA/IIS01, fine powders of metallic fillers (such as aluminium, silver etc.) can be added to the pure epoxy adhesive. However it is also essential that the epoxy-aluminium composite adhesive should withstand the thermal cycling from 4.5 K to 300 K during its functioning as a cryosorption pump.
d) Now the thermal conductivities of epoxy-aluminium composites in the temperature range from 4.5 K to 300 K has been measured using the dedicated experimental setups for measurements of thermal conductivity (developed in-house). The measurements of thermal conductivity using the above experimental setups are based on one-dimensional Fourier heat conduction with longitudinal steady state heat flow method. Detailed experimental studies on thermal conductivity of epoxy and epoxy-aluminium composites have been carried out by the above experimental setups.
e) Further the thermal conductivities of the epoxy-aluminium composites have been theoretically predicted by analytical heat conduction models. Here, the epoxy forms the base matrix and aluminium powder forms the filler. Appropriate models have been developed both for the low and high volume fractions of the filler in the epoxy base matrix. The thermal conductivity values predicted by the models match quite well with the experimentally measured values of the epoxy-aluminium composite samples. Also the developed models are able to predict the thermal conductivity values of the published data.
f) By the addition of metallic (aluminium) filler particles to the epoxy adhesive, the thermal conductivity of the epoxy adhesive is increased. However, the downside of adding aluminium fine powder is the reduction of the bonding strength of the epoxy onto the cryopanel. An experimental setup has been developed to measure the strength of epoxy- aluminium composite adhesive. Based on the experimental studies, an optimum composition of the aluminium powder filler in the epoxy adhesive has been estimated as ~ 35 % by volume fraction. This epoxy-aluminium composite adhesive is designated henceforth as “EAL35”.
g) A new cryopanel has been fabricated wherein the activated carbon cloth KCC/IIS01 is bonded using EAL35. The pumping speeds of the newly developed cryopanel are improved on an average by factors of 3.63 and 3.60 when compared with those of commercial panel for gases such as hydrogen and helium respectively in the pressure range from 5E-6 to 4E-5 mbar.
Our studies have led to the development of a cryocooler based cryosorption pump with higher pumping speeds for gases such as H2 and He compared to the commercial cryopumps. Hence the present studies will be quite useful to the development of the appropriate cryosorption pumps for the Tokamak related applications
Design of a Vortex Tube based Refrigeration System
Get PDFVortex tube (VT) is a mechanical device with no moving parts. The fundamental principle of Vortex Tube is that it can split an incoming fluid flow of a constant pressure and constant temperature gas stream into two separate low pressure streams, one having higher enthalpy and the other having lower enthalpy than the inlet flow. So this device essentially works as a temperature separator. On separation from the device, a warmer flow exits through a terminal which is called the “hot end” and a low temperature stream comes out from another terminal known as the “cold end”. Just with a few bar pressure of compressed air at room temperature can produce a hot stream temperature of about 150°C and a cold stream temperature of about - 40°C. This temperature separation scheme allows us to get cooling and heating effect simultaneously using the same device which makes the Vortex tube one of the popular mechanical equipment and is used in many fields of engineering. The cooling or heating effect produced by this device is largely dependent on geometric parameters of the device itself. Since no exact theoretical correlation is there between the geometric parameters and the cooling (or heating) effect produced, VT design is solely based on empirical relations. There are quite a few geometric parameters which affect the cooling effect of this device and all the empirical correlation are needed to design the optimum VT for maximum cooling/heating effect. These relations can be derived in two ways, either by numerical methods or by experimental investigations. The first part of the thesis important geometric parameter of the VT namely the ratio of the “cold end” diameter (to the “tube diameter” , which has been numerically optimized in this work to achieve maximum temperature separation.
In our efforts to design a VT based refrigeration system, optimization of the VT itself is not enough. A suitable heat exchanger (HX) which can extract the cold enthalpy from the VT also needs to be designed and cascaded with the VT to get the complete refrigeration system. The second part of the thesis is solely dedicated to the design of a suitable HX that can be used alongside a VT to produce refrigeration. The HXs design can be approached from two directions, dimensional aspect and material aspect. Rather than focusing on the dimensional aspect in this work we have concentrated of the material aspect of HX design. It is fairly obvious that the thermal conductivity (TC) of the HX material will play a crucial role on the cooling effect of the refrigeration system. Conventional metals with high TC can be used to design HXs but the downsides of using pure metals such as Copper, Iron are that they are heavy, quite expensive and highly reactive to corrosive fluids. Because of this, high TC ceramic material such as Aluminium Nitride (AlN) is quite often used to fabricate HXs and they are used for spot cooling in electronic systems. AlN has TC of 160 W/m-K which is high but not as high as of Copper or Iron. TC of AlN can be increased by mixing the right volume fraction of metal powder (such as pure Aluminium) with it to a great extent. So in a nutshell, instead of using pure AlN, if we use the particle reinforced binary composite [AlN + Al (powder)] to design a HX, we would achieve the benefits of having high TC as well as properties such as anti-corrosiveness, cost effectiveness and weight reduction.
In the above context, prediction of TC of particle reinforced composite materials containing a base material of low TC and a filler material of high TC is of utmost importance. Till now a very few analytical heat transfer models are available in the literature that can accurately predict the TC value of such composites especially when high volume fraction of filler particles is added to the base material or if more than one type of filler particles are added. So in this thesis, three analytical heat transfer models have been developed that can predict the TC of binary as well as tertiary particle reinforced composites.
The third and the final segment of the thesis deals with the performance study of a refrigeration system comprised of the optimized VT cascaded with a suitable HX made out of a particle reinforced composite material. The numerical results show how the HX effectiveness improves as the volume fraction of the filler particles in the composite increases.
The key results of the works described in the thesis are as follows:
• Through extensive numerical simulations it is shown that for = 0.5, the temperature separation in a VT is maximum.
• The heat transfer models developed to predict the thermal conductivity of binary composites, shows the trend of how thermal conductivity varies with increasing volume fraction of filler. It has been shown that initially the thermal conductivity increases linearly with a small slope, then after a critical volume fraction an abrupt increment of slope is observed due to the formation of continuous heat conduction paths within the composite. Further increase in volume fraction shows linear increment of thermal conductivity with lesser slope as before.
• The heat transfer model developed to predict the thermal conductivity of tertiary
composites is suitable for low volume fraction (< 20 %). The model shows the addition of one component into the base matrix affects the distribution of the other
component which is observed through the covariance.
• The last part of the thesis shows that compared to a pure AlN heat exchanger, a heat exchanger made of AlN + 30 % volume fraction of pure Aluminium powder, has increased heat exchanger effectiveness by more than 50 %.
Thesis outline is as follows:
• Chapter 1 is a brief introduction to Vortex Tube.
• Chapter 2 deals with the necessary literature review related to Vortex Tube as well as presently available heat transfer models that are equipped to handle composite materials to predict their TC.
• Chapter 3 elaborates numerical modeling and optimization of a critical parameter
( to achieve maximum temperature separation in a VT.
• Chapter 4 presents a stochastic heat transfer model to estimate the TC of Binary particle reinforced composites containing low volume fraction of filler particles.
• Chapter 5 describes the development of a computational heat transfer model to predict the TC of Particle Reinforced Binary Composite materials containing high volume fraction of filler element.
• Chapter 6 deals with a stochastic heat transfer model to calculate TC of Particle Reinforced Tertiary Composite materials containing low volume fractions of filler elements.
• Chapter 7 consolidates all the necessary concepts and data from previous chapters to design the final cascaded VT based refrigeration system and presents a performance study.
• The last chapter summarizes the entire work along with scope for future work
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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