Milivoje M. Kostic

Heat transfer to a viscoelastic fluid in laminar flow through a rectangular channel

Abstract The measured local and mean Nusselt numbers for a viscoelastic fluid in laminar flow through a rectangular channel are found to be much higher than those of a purely viscous fluid or a Newtonian fluid. The differences cannot be explained on the basis of a superimposed free convection effect. Rather, the increase is due primarily to secondary flows which are induced in the viscoelastic fluid as a result of the normal force differences acting at the boundaries which are unique to elastic fluids. The pressure drop behavior is unaffected by the presence of secondary flows and predictions based on a purely viscous power law model give good agreement with the measured values.

On turbulent drag and heat transfer reduction phenomena and laminar heat transfer enhancement in non-circular duct flow of certain non-Newtonian fluids

The fascinating friction drag and heat transfer reduction phenomena associated with turbulent flows of so-called ‘drag-reducing fluids’ are not well understood. It is believed that elastic fluid properties are strongly related to these phenomena. However, not all drag-reducing fluids are viscoelastic, nor are all viscoelastic fluids drag-reducing, suggesting that drag reduction and viscoelasticity are probably incidentally accompanying phenomena. Furthermore, the limited research to date has revealed considerable heat transfer enhancement (virtually without friction drag increase) in laminar non-circular duct flows with certain polymer solutions, and has shown that all utilized fluids were indeed viscoelastic1.It is argued here that turbulence suppression (i.e. flow laminarization), due to flow-induced anisotropic fluid structure and properties, is a determining factor for the reduction phenomena—not the fluid elasticity—while the latter may be a major cause for the laminar heat transfer augmentation. It is certain that many challenges in this interesting and useful area will keep researchers very busy well into the next century and beyond.

A novel computerized viscometer/rheometer

An innovative, Couette‐type viscometer/rheometer was developed, designed, and fabricated with the main objective being to measure viscosity and elastic properties of low‐viscous, non‐Newtonian, and visco‐elastic fluids, like dilute polymer solutions. The goal was to simplify and improve some existing drawbacks of commercial instruments employing several novel design solutions, particularly with regard to instrument precision and sensitivity. With a single pair of cylinders and a torsion bar stiffness of 0.116 Nm/rad, viscosities from 0.5 to 50 000 cP (centi‐Poise) in a shear‐rate range from 1 to 200 s−1, and oscillatory tests from 0.1 to 10 Hz, could be measured. The transducers’ electronic signals are handled by a software, developed in C language, and an IBM‐PC compatible computer with a data acquisition board. The innovative design solutions (use of a cruciform torsion bar, optoelectronics sensors, and a novel alignment procedure) have improved the critical instrument performance and reduced the number...

Revisiting The Second Law of Energy Degradation and Entropy Generation: From Sadi Carnot's Ingenious Reasoning to Holistic Generalization

Sadi Carnots ingenious reasoning of reversible cycles (1824) laid foundations for The Second Law before The First Law of energy conservation was even known (Joule 1843) and long before Thermodynamic concepts were established in 1850s. A century later, Bridgman (1941) ‘complained’ that “there are almost as many formulations of The Second Law as there have been discussions of it.” Even today, The Second Law remains so obscure, due to the lack of its comprehension, that it continues to attract new efforts at clarification, including this one.The Laws of Thermodynamics have much wider, including philosophical significance and implication, than their simple expressions based on the experimental observations—they are The Fundamental Laws of Nature: The Zeroth (equilibrium existentialism), The First (conservational transformationalism), The Second (irreversible directional transformationalism), and The Third (unattainability of emptiness). They are defining and unifying our comprehension of all existence and tr...

The Elusive Nature of Entropy and Its Physical Meaning

Entropy is the most used and often abused concept in science, but also in philosophy and society. Further confusions are produced by some attempts to generalize entropy with similar but not the same concepts in other disciplines. The physical meaning of phenomenological, thermodynamic entropy is reasoned and elaborated by generalizing Clausius definition with inclusion of generated heat, since it is irrelevant if entropy is changed due to reversible heat transfer or irreversible heat generation. Irreversible, caloric heat transfer is introduced as complementing reversible heat transfer. It is also reasoned and thus proven why entropy cannot be destroyed but is always generated (and thus over-all increased) locally and globally, at every space and time scales, without any exception. It is concluded that entropy is a thermal displacement (dynamic thermal-volume) of thermal energy due to absolute temperature as a thermal potential (dQ = TdS), and thus associated with thermal heat and absolute temperature, i.e., distribution of thermal energy within thermal micro-particles in space. Entropy is an integral measure of (random) thermal energy redistribution (due to heat transfer and/or irreversible heat generation) within a material system structure in space, per absolute temperature level: dS = dQSys/T = mCSysdT/T, thus logarithmic integral function, with J/K unit. It may be also expressed as a measure of “thermal disorder”, being related to logarithm of number of all thermal, dynamic microstates W (their position and momenta), S = kBlnW, or to the sum of their logarithmic probabilities S = −kB∑pilnpi, that correspond to, or are consistent with the given thermodynamic macro-state. The number of thermal microstates W, is correlated with macro-properties temperature T and volume V for ideal gases. A system form and/or functional order or disorder are not (thermal) energy order/disorder and the former is not related to Thermodynamic entropy. Expanding entropy to any type of disorder or information is a source of many misconceptions. Granted, there are certain benefits of simplified statistical descriptions to better comprehend the randomness of thermal motion and related physical quantities, but the limitations should be stated so the generalizations are not overstretched and the real physics overlooked, or worse discredited.

DESIGN OF A STEADY-STATE, PARALLEL-PLATE THERMAL CONDUCTIVITY APPARATUS FOR NANOFLUIDS AND COMPARATIVE MEASUREMENTS WITH TRANSIENT HWTC APPARATUS

A steady-state, parallel-plate thermal conductivity (PPTC) apparatus has been developed and used for comparative measurements of complex POLY-nanofluids, in order to compare results with the corresponding measurements using the transient, hotwire thermal conductivity (HWTC) apparatus. The related measurements in the literature, mostly with HWTC method, have been inconsistent and with measured thermal conductivities far beyond prediction using the well-known mixture theory. The objective was to check out if existing and well-established HWTC method might have some unknown issues while measuring TC of complex nano-mixture suspensions, like electro-magnetic phenomena, undetectable hot-wire vibrations, and others. These initial and limited measurements have shown considerable difference between the two methods, where the TC enhancements measured with PPTC apparatus were about three times smaller than with HWTC apparatus, the former data being much closer to the mixture theory prediction. However, the influence of measurement method is not conclusive since it has been observed that the complex nano-mixture suspensions were very unstable during the lengthy steady-state measurements as compared to rather quick transient HWTC method. The nanofluid suspension instability might be the main reason for very inconsistent results in the literature. It is necessary to expend investigation with more stable nano-mixture suspensions.Copyright

CRITICAL ISSUES AND APPLICATION POTENTIALS IN NANOFLUIDS RESEARCH

Development of many industrial and new technologies is limited by existing thermal management, and need for high-performance cooling. Nanofluids, stable colloidal mixtures of nanoparticles (including nanofibers and functional nanocomposites) in common fluids, have a potential to meet these and many other challenges. Colloidal nano-mixtures with functionally-stable and active-like nanostructures that may self-adjust to the process conditions, require systematic surface-chemistry study and enhancements (coatings with functional layers, surfactants, etc), in addition to investigation of thermo-physical characteristics and phenomena. A comprehensive, systematic and interdisciplinary experimental research program is necessary to study, understand and resolve critical issues in nanofluids research to date. The research must focus on both synthesis and a careful exploration of thermo-physical characteristics. Development of new-hybrid, drag-reducing nanofluids may lead to enhanced flow and heat transfer characteristics. The nanoparticles in these fluids yield increased heat-transfer while the long-chain polymers are expected to enhance flow properties, including active and functional interactions with nanoparticles, thus providing potential for many applications yet to be developed and optimized.Copyright

Analysis of Enthalpy Approximation for Compressed Liquid Water

It is custom to approximate solid and liquid thermodynamic properties as being a function of temperature only, since they are virtually incompressible, and Pdv boundary work may be neglected. Furthermore, in classical literature, for isothermal compression processes, a general “improvement” and correction for liquid enthalpy approximation is given by adding the “pressure correction,” vdP, to the corresponding saturation value. It is shown that such correction given for isothermal processes is generally valid for isentropic processes only. Analysis of water real properties, over the saturation temperature range and a wide pressure range up to 100 MPa, shows that the recommended corrections are only beneficial for higher pressures at smaller temperatures (below 200°C), insignificant for smaller pressures at most of the temperatures, about the same but opposite sign (thus unnecessary) for intermediate temperatures and pressures, and more erroneous (thus counterproductive and misleading) for higher temperatures and pressures, than the corresponding saturation values without any correction. The misconception in the literature is a result of the erroneous assumption, that due to incompressibility for liquids in general, the internal energy is less dependent on pressure than enthalpy. DOI: 10.1115/1.2175090

Entropy Generation Results of Convenience But without Purposeful Analysis and Due Comprehension—Guidelines for Authors

There is a growing trend in recently-submitted manuscripts and publications to present calculated results of entropy generation, also known as entropy production, as field quantities in a system or device control volume, based on prior calculation of velocity and temperature fields, frequently using CFD numerical methods. [...]

Work, Power, and Energy

Northern Illinois UniversityDeKalb, Illinois, United States1. Basic Concepts2. Forms, Classifications, and Conservation of Energy3. Work of Conservative and Nonconservative Forces:Work–Energy Principle4. Energy, Work, and Power of Rotating Systems5. Energy Sources, Conversion to Work, and Efficiency6. Energy Reserves and Outlook