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For improving the quality of life of most of the 6 billion plus population in the world in general and that of the developing countries like India in particular (which constitute one-sixth of the world population), what is needed most under the prevailing scenario of global competition is “advanced technology”, the technology that can produce “More from Less”, in addition to a few essential inputs like capital, availability of skilled labour, productivity, work culture, etc. Unless we are able to bridge the prevailing technological gap – the gap between technologies a developed country has and the technologies the developing country needs – it would be a futile attempt to bring the quality of life of our vast population to an acceptable level. Looking back to the history of progressive advances in engineering science and material science and their effect on technology development in the society, we can distinctly observe the massive contribution of advances in heat and mass transfer operations in such technological advancement to produce more products from less material and has energy inputs. Even though heat and mass transfer operations are usually considered to be the main unit operations associated in chemical engineering science and chemical industries, these two unit operations have all along played a crucial role in nuclear engineering too which, as all of you are aware, is essentially a multi-disciplinary branch of engineering science dedicated essentially to generation of electricity from nuclear fuels. For any successful and complete nuclear fuel cycle, heat and mass transfer has a crucial role to play, right from winning the fuel from natural ores, utilizing the fuel along with other essential inputs at nuclear grade quality in a nuclear reactor for generation of power and finally recovery of precious fissile material from the spent fuel before the HL - nuclear wastes are vitrified for storing under surveillance prior to final disposal back to the nature in deep repository in an acceptable form. Accordingly, we the members of the DAE family in our mission to produce safe, reliable, economical and yet eco-friendly power from nuclear fuel on a sustainable basis, have been fortunate to experience a phenomenal advances in heat and mass transfer operation while establishing a complete nuclear fuel cycle which is totally indigenous – a task which only a few other countries in the world have succeeded to achieve so far. In fact, India is acknowledged as a developed nation as far as nuclear science and technology is concerned. Decades of our R&D efforts have resulted in our indigenous 12 PHWR nuclear reactors that are operating at world class level in terms of both safety and capacity factors and 4 more PHWR reactors in the pipeline out of which 2 x 500 MWe PHWR are in full swing of construction. This has not only generated fresh interests in nuclear energy in India but also resulted in better appreciation of the ground reality that for a country like India, nuclear energy is not just essential but it is an inevitable option if we are serious to improve the quality of life of our vast population. Advances made in the field of heat and mass transfer has played an important role behind the success story of nuclear energy and I have no hesitation to say that the field of nuclear science and technology has always been the most fascinating and challenging hunting ground for those interested to pursue the best in heat and mass transfer. In fact, some of the most outstanding developments in the area of heat transfer have taken place in Nuclear Research Laboratories. However, keeping in mind most of the participants in today’s Conference may not be directly linked with nuclear industry, it would be apt not to discuss further on this issue before we dwell upon the advances in heat and mass transfer in general outside the domain of nuclear energy.
Advances in Heat and Mass Transfer Fundamentals
As indicated earlier, fundamentals of doing business in the present world call for producing “More from Less”. Accordingly, development of energy efficient newer technology with more efficient heat and mass transfer equipment and their operation at PINCH mode have become the bottom line of industries. The heat exchanger pinch analysis, mass transfer network and water pinch analysis (for moving towards achieving zero liquid discharge philosophy) are extremely useful tools for all practicing engineers in the field of heat and mass transfer. The advent of advanced computing techniques and accurate measuring instruments have now enabled us to understand the fundamentals of this complex transport phenomena much better at micro level, resulting in better design capability, taking into consideration the advances made in the material science as well as process control, without resorting to many expensive tests that would have been required otherwise. Simultaneously, there has also been a tremendous quest for better understanding in some of the fundamental areas of heat and mass transfer with an objective of evolving the design procedure based on the newly established knowledge base for more advanced compact and energy efficient heat and mass transfer equipment. Such an understanding at fundamental level is, in fact, essential when we are dealing with heat and mass transfer problems at nano scale (compared to our existing understanding where boundary layers are at micron scale). BARC has done a pioneering work for the first time in the world in this context for estimating the self diffusivity as well as cross diffusivity for mixture of real fluids using Mode Coupling Theory (MCT) and Density Functional Theory
(DFT).
Advances in Mass Transfer Equipment in Conventional Industries.
As you are aware, mass transfer operation is most dominant in the recovery and purification of materials and liquid-liquid extraction and membrane separation processes are the two commonly adopted mass transfer operation in industries.
Liquid Membrane Extraction
Compared to liquid-liquid extraction, Liquid Membrane Extraction (LME) is a relatively new separation method. It is also known as Liquid-Pertraction where both extraction and stripping processes are carried out in one step through what we call “diffusion limited transport process” across the thickness of a membrane made of the solvent used. Unlike “equilibrium limited transport” of normal liquid-liquid extraction (where the driving source is progressively reduced as mass transfer operation proceeds, resulting in the necessity of a multi stage operation), the driving force in liquid membrane extraction is very high and can be kept constant by maintaining the chemical potential in the stripping site to nearly zero by suitable complex formation. Hence, Liquid Membrane Extraction (LME) can complete the extraction task in nearly a single stage. Further, the organic solvent requirement in LME is extremely small because it acts as a short time mediator within the liquid membrane where it complexes with the solute of the feed which is subsequently broken down to release the solvent free making the same solvent available for formation of further complexes, while the desired solute diffuses out of liquid membrane to the stripping solution side. This unique feature makes LME in general and Emulsion Liquid Membrane (ELM) in particular (because of its very high surface area per unit volume >
1000m2/m3) the most effective technique for recovery of values from dilute or large volume solution, e.g., recovery of heavy metals from dilute solutions/waste streams including removal of radioactive materials from nuclear wastes.
Various commercial simulators for different types of extraction tasks involving multi-component and multi-stage extraction (which consist of the required mathematical models with suitable numerical algorithms) are excellent tools to establish the most optimum condition for a given system. However, these tools have deficiencies in terms of their inadequacies to handle cases of mass transfer operation that involve high heat generation and also for basic understanding of several issues like inter-drop and drop-bulk coalescence, effect of interfacial contamination, etc.
Solvent Extraction using Super Critical Fluid
Now a days, no discussion on mass transfer is complete without making a special mention of the newcomer to the family of solvents extraction, namely, Super Critical Fluid (SCF) extraction where a pure substance above the thermo dynamic critical temperature and pressure is used as solvent [and hence called Super Critical Fluid (SCF)]. The most widely used SCF today is CO2 because of its favourable Tc and Pc at .31°C ad 73 bar respectively for extraction of various natural products such as fragrance from spices and essential oils. In fact, there are commercial plants operating with
CO2 SCF for decaffeination of green coffee seeds and extraction of HOP for liquor industries.
The SCF has many interesting properties such as liquid like density and solvation power in one hand but gas like diffusivity and viscosity on the other. Further, near zero surface tension of SCF leads to possibility of creating additional surface area for mass transfer without any extra energy expenditure, as opposed to what is normally needed for creation of dispersed phase in conventional solvent extraction. Finally, SCF leaves behind no solvent residue either in product or to the environment. Realising the importance of these facts, several developed countries have established dedicated research establishments which are known as Green Technology Institutes.
Unlike the conventional solvent extraction, where we analyse liquid-liquid equilibrium, the SCF extraction deals with single phase in SCF region. In the case of SCF, Re increases by several folds and Sc almost disappear because the viscosity of SCF is near zero. Hence, use of conventional mass transfer correlations that express Sherwood number (Sh) as a function of Reynolds number (Re) and Schmidt number (Sc) may not work here. Instead, we need to look into the possibility of proposing entirely a new approach to estimate mass transfer co-efficient in SCF media. We need to understand the SCF phase equilibrium for which improved equations of states need to be postulated and verified.
Simultanous Heat and Mass Transfer
From the topic of mass transfer, when we look towards the technologies that utilize simultaneously heat and mass transfer operation, we invariably think of Fluidized Bed Technology (FBT) and its second generation version of Circulating Fluidized Bed Technology (CFBT), both of which have been and will continue to have major role in wide spectrum of chemical, power and nuclear industries.
Virtues like uniform axial and radical temperature profile that ensures better control of chemical reactions without hot spots have resulted in adoption of FBT in Indian nuclear fuel cycle for a number of applications involving highly exothermic reactions.
Further, the adaptability of low grade coals along with low pollution emission, high combustion efficiency and above all the much improved quality of the ash generated have attracted the FBT in the form of Circulating Fluidized Bed Technology (CFBT) as the most promising technology for power generation. CFBTs can be used in combined cycle power plant where pressurized gasification with gas turbine and atmospheric combustion with steam turbine can be adopted as a substitute for the usually accepted clean power generation based on natural gas which is becoming increasingly unviable due to increasing price of natural gas.
However, both FBT and CFBT technologies need lot of further understanding at fundamental level for their full exploitation (beyond the existing double boundary layer or pocket models for heat transfer and three phase model for mass transfer).
Advances in Heat and Mass Transfer in Nuclear Industry
As I said earlier, development of various types of present generation nuclear reactors (which includes the Fast Breeder Reactors) has been the most fascinating and most challenging tasks that have attracted the professionals in heat and mass transfer world over.
No doubt, the experience gained and the understanding obtained through continuous R&D efforts world over, nuclear reactors have achieved high levels of safety. However, the accident in PWR reactor at Three Mile Island in March, 1979 and in the RBMK reactor at Chernobyl in April, 1986, has posed a bigger challenge to the thermal hydraulic research world wide for still better understanding of thermal hydraulics and thermodynamic phenomena inside the reactor vessels in the event of such an accident condition. The world over Advanced Reactors are being designed by incorporating extremely reliable passive safety features which would ensure reactor safety even under very severe accident conditions.
Further, there is bigger challenge is to bring down the cost of the nuclear power through advanced design based on reduced factor of safety which calls for better understanding of the nuclear phenomenon coupled to hydrodynamic phenomenon. This requires the researchers in heat and mass transfer to adequately model and simulate the worst design basis accident by computer codes and validate the same through experiments.
Since it would not be apt to dwell on any of these issues in detail in an inaugural speech, I would like to mention just a few of the key areas that need to be addressed by the heat and mass transfer professionals in nuclear engineering in collaboration with our University system and other R&D institutions.
Transient and Accident Analysis of Nuclear Reactors
Analysis of nuclear reactors under transient and accident conditions are perhaps the most challenging task that have drawn the attention of thermal hydraulic specialists world over for development of computer codes as well as their experimental validation.
Apart from the various transients, a number of accident scenarios are also postulated and the consequences of such accidents are analysed and appropriate design provisions are to be made to mitigate their consequences. These accidents can be classified into two categories, viz. Design Basis Accidents (DBAs) and Beyond Design Basis Accidents (BDBAs)/Severe Accidents. The first category of accidents, viz., DBAs are those which have low but still a somewhat significant probability of occurrence (may be one in one million) and design provisions are usually made to mitigate their consequences.
An example of DBA is the Loss of Coolant Accident (LOCA) which involves failure of the pressure boundary of the primary coolant system. This will result in flashing out of the high pressure, high enthalpy coolant out of the system, thus causing degradation of cooling. The Emergency Core Cooling System (ECCS) is provided as an engineered safety feature to mitigate the consequences of LOCA.
Beyond Design Basis Accidents, on the other hand, have an extremely low probability of occurrence (< 10-8 to 10-9). The BDBA is the consequence of multiple failures where a postulated DBA is accompanied by simultaneous failure of the Engineered Safety Systems that have been provided to mitigate the consequences of the DBA. Because of the severity of the BDBAs, the consequences may spill over into the public domain. Only emergency preparedness measures are generally provided to tackle such situations. However, the recent trend is to provide some design provision to mitigate the consequences of certain BDBAs. This calls for precise understanding of the high temperature heat and mass transfers in fuel, clad and coolant interactions, molten material relocation, water explosions, etc. In this context, it is worth noting that in the case of Indian PHWRs, a multiple failure accident involving LOCA with simultaneous failure of ECCS will not result in as severe a situation as in the case of PWRs. This is because of the cold moderator surrounding the calandria tubes, which acts as a heat sink and helps in keeping the fuel temperature well below the melting point. However, with the continued R&D and experience gained, a number of new phenomena and scenarios, not identified earlier, have been established and methodologies to address these BDBA issues are being developed.
In the context of containment behaviour following an accident such as LOCA, apart from thermal hydraulics (which decides the pressure and temperature in the containment following the accident), phenomena such as behaviour of aerosol generated from the particular products released from fuel, behaviour of hydrogen generated from steam-clad reaction, etc., are also important issues.
Passive removal of Hydrogen from the containment atmosphere using pt/pd based catalytic recombiners is widely accepted method for Hydrogen mitigation. Detailed 3-dimensional analyses to understand transport behaviour in the containment need the solutions of sets of highly complex mass, momentum and energy conservation equations along with the appropriate representation of buoyancy turbulence and heat transfer associated with steam condensation (described earlier). Further, modelling of the behaviour of recombiners is a difficult task involving reaction kinetics, heat transfer and mass transfer due to buoyancy and convection in a complex steam-air-Hydrogen system. Deficiencies of theoretical models and computer codes on such analyses are to be taken care of by detailed experimental validation.
Challenges in Heat and Mass Transfer in FBR
After years of successful operation of FBTR, India has entered into its second phase of reactor technology based on which a 500 MW(e) PFBR has been designed and its construction is to start soon.
Even though basic issues related to steady state and transient velocity field and temperature field distributed in complex geometry are by and large same as in thermal reactors, presence of both hot and cold sodium pools co-existing inside reactor causes peculiar thermal hydraulic problems, viz., sodium stratification and thermal striping. The sodium stratification is the sharp interface between the hot and cold layers of sodium (of about 100 mm thickness with temperature difference of about 100k) and thermal striping is the random temperature fluctuation with a frequency in the range 0.01 to 10 Hz in the zone of turbulence wherein the hot and cold sodium do not mix properly.
Both computer codes and experimental facilities have been developed in DAE which have created comprehensive capability in solving this thermal hydraulic problems. This expertise can be taken advantage of, for solving fluide flow and heat transfer problems in other industries also.
Challenges in Future Generation Reactor System
Now, let me mention a few of the challenges in the heat and mass transfer involved in the next generation reactor where more and more extremely reliable passive safety features are being introduced by innovative design so that the safe shut down condition of the nuclear reactor is ensured under any abnormal/accidental scenario without the intervention of any active device or operator intervention. Our 235 MWe vertical core type AHWR which will generate about 65-70% of power from thorium based fuel belongs to this category. Since most of the next generation reactors employ natural circulation for heat removal from the core, complete understanding of thermo hydrodynamic phenomena for buoyancy driven flows is very much essential. In addition to multi dimensional nature of the flow, the velocity field under the prevailing condition in such reactors may lead to phase separation. Two phase natural circulation can be affected by flow instabilities due to low driving forces and strong dependency of friction forces on void fraction. Critical heat flux under natural circulation is another important issue. For proper understanding of such multi-phase multi- dimensional flow phenomena in the next generation reactors, we may require computational tools extending beyond the modern generation tools in numerical methods and code structures. The available empirical co-relations are based on limited experimental database. This is a major challenge, therefore, for the Universities to develop advanced methods, instruments and techniques to measure the void fraction and its distribution inside the rod assembly, identify flow pattern transition in different flow geometries, measurement of interface heat, mass and momentum transfer. There is a strong need to develop these fundamental inputs in the context of development of future advanced reactors.
I am sure these challenges could ignite the minds of some of the young or even not so young professionals of heat and mass transfer community present this morning to accept these challenges and pursue their desire of achieving excellence in heat and mass transfer in their professional career by joining our nuclear energy programme.
We have the legacy of staying at the forefront of heat and mass transfer in nuclear energy inherited from our predecessors and this has to be preserved for our posterity with the help of our university system and other R&D institutions. Apart from the research funding through its BRNS wing, the Department also provides funds directly through other means such as Memorandum of Understanding to carry out research projects of interest in specific areas. In Kolkata itself, I am aware that currently, three BRNS projects are in progress at Jadhavpur University. One example of direct funding is the agreement with the Indian Institute of Technology, Mumbai, for carrying out R&D on specific problems related to the Thermal Hydraulics of Advanced Heavy Water Reactor.
I am aware that the Indian Society for Heat and Mass Transfer (ISHMT) during its existence over more than three decades, has been playing a very important role in bringing together professionals in the fields of Heat and Mass Transfer providing thereby a useful forum for exchange of latest informations on various aspects of Heat and Mass Transfer. Another significant thing to note is that since its first National Conference at IIT, Madras in 1971, this has become an International Conference since 1994 with participation of American Society of Mechanical Engineers (ASME). I vividly recall the tremendous enthusiasm within which delegates from India and abroad participated extensively in the first joint conference that was arranged by BARC in January, 1994. Since then, all the Units of DAE have been actively participating in this Conference, apart from providing financial assistance. I understand that in this Conference also about twenty delegates from BARC are participating.
I am sure, the collective wisdom of the professionals participating in this Conference will provide a meaningful guidance to the budding professionals in their pursuit for excellence in heat and mass transfer which is so essential for the growth of our national economy.
With these few observations, let me thank you all for your patient hearing and wish you all an academically exciting and professionally rewarding deliberations on various important issues of heat and mass transfer during your stay in Kolkata over the next two days. It is indeed a matter of great pleasure for me to inaugurate this Conference.
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