Physical Effects of Variable Thermophysical Fluid Properties on Flow and Thermal Development in Micro-Channel

Micro-scale cooling is an efficient and effective cooling technique to achieve the goal of higher heat removal capabilities. The present research focuses to find the physical effects of fluid property variations on flow and thermal development in micro-channel. The effects of temperature-dependent density, viscosity, and thermal conductivity variations on single-phase laminar forced convection are numerically investigated. The problem is especially simulated for hydrodynamically and thermally developing water flow in micro-channel with no-slip, no-temperature jump, and constant wall heat flux boundary conditions. It is observed that the density variation induces radially inward flow due to continuity, which sharpens the axial velocity profile and decreases Nusselt number compared to constant property solution. The axial velocity profile significantly alters due to viscosity variation. This alteration varies along the micro-flow and it induces radially flow due to flow continuity. The reducing rate of Nusselt number for viscosity variation is substantially lower than constant property solution due to a significant flattening effect of the axial velocity profile, which augments the Nusselt number. Thermal-conductivity variation across the flow induces radial conduction, which enhances convection compared to constant property solution. Additionally, the effects of thermophysical fluid property variations on static gauge pressure drop are also investigated

Selection of Materials and Design of Multilayer Lightweight Passive Thermal Protection System

A methodology has been established aiming to design a lightweight thermal protection system (TPS), using advanced lightweight ablative materials developed at the NASA Ames Research Center. An explicit finite-difference scheme is presented for the analysis of one-dimensional transient heat transfer in a multilayer TPS. This problem is solved in two steps, in the first step, best candidate materials are selected for TPS. The selection of materials is based mainly on their thermal properties. In the second step, the geometrical dimensions are determined by using an explicit finite-difference scheme for different combinations of the selected materials, and these dimensions are optimized for the design of lightweight TPS. The best combination of material employs silicone impregnated reusable ceramic ablator (SIRCA), Saffil, and glass-wool for the first, second, and third layer, respectively

Numerical Investigation on Laminar Microconvective Liquid Flow With Entrance Effect and Graetz Problem due to Variation in Thermal Properties

This work deals with the analysis of forced convection in single-phase laminar flow of liquid through microsized circular geometry with a diameter of 100 x 10(-6) m. The problem with hydrodynamically and thermally developing flow in the entrance region with no-slip, no-temperature jump and constant wall heat flux boundary condition is numerically studied. Two-dimensional (with axisymmetry) simulation is carried out to understand the effect of fluid property variations on flow development and heat transfer. Pure continuum-based governing equations are solved to predict the significance of momentum and energy transport due to temperature-dependent viscosity and thermal conductivity variation, respectively. The radial inward flow is induced due to temperature-dependent density variation that sharpens the axial velocity profile. The investigation also analyzes the change in Nusselt number with locations in the channel for Graetz problem with uniform heating condition. The flattening of axial velocity profile, radial temperature profile, and inward radial flow influences the heat flow characteristics. The investigations in computational domain show that Nusselt number in thermal entrance region deviates from constant properties solution due to scaling effects

Scale-invariant entropy-based theory for dynamic ordering

Dynamically Ordered self-organized dissipative structure exists in various forms and at different scales. This investigation first introduces the concept of an isolated embedding system, which embeds an open system, e.g., dissipative structure and its mass and/or energy exchange with its surroundings. Thereafter, scale-invariant theoretical analysis is presented using thermodynamic principles for Order creation, existence, and destruction. The sustainability criterion for Order existence based on its structured mass and/or energy interactions with the surroundings is mathematically defined. This criterion forms the basis for the interrelationship of physical parameters during sustained existence of dynamic Order. It is shown that the sufficient condition for dynamic Order existence is approached if its sustainability criterion is met, i.e., its destruction path is blocked. This scale-invariant approach has the potential to unify the physical understanding of universal dynamic ordering based on entropy considerations. (C) 2014 AIP Publishing LLC

Theoretical investigation of scaling effects from macro-to-microscale convection due to variations in incompressible fluid properties

We re-examine the governing continuum-based conservation equations for single-phase laminar forced convection of liquids, incorporating temperature dependence of fluid viscosity and thermal conductivity. The effects of property variations become highly significant from macro-to-microscale convection. Further, the effects of property variations along the flow become more significant relative to property variations over the cross section; thereby necessitating consideration of additional convection mechanisms at the microscale, in addition to mechanisms known at the macroscale. The same Brinkman number that determines convection with viscous dissipation, also determines convection with variations in fluid properties; but now the significance in convection increases with decreasing Brinkman number. (C) 200

Effect of temperature-dependent viscosity variation on fully developed laminar microconvective flow

The effects of temperature-dependent viscosity variation on fully developed flow through a microchannel are numerically investigated in this work. The effects of A(T) variation are able to couple the velocity and temperature fields. Therefore, the velocity and temperature profiles vary qualitatively along the micro-flow. Due to mu(T) variation, the concept of flow undevelopment (the reverse process of flow development) is observed in the flow regime. Four different flow regions have been observed in the flow regime and possible path is, developed undeveloping undeveloped developing. In the vicinity of the inlet, the hydrodynamic undevelopment of flow (the reverse process of hydrodynamic development of flow) is represented by the distortion in axial velocity profile which is confirmed from partial derivative/partial derivative(z) over bar(partial derivative(u) over bar/partial derivative(r) over bar)(W) > 0. The thermal undevelopment of flow (the reverse process of thermal development of flow).is expressed by the rapid enhancement in the convective heat transfer coefficient (h) along the flow due to significant flattening of the axial velocity profile. This study also investigates the Chilton-Colburn analogy for variable viscosity fluid at different mean velocities. The direct proportionality between the Chilton-Colburn j-factor for heat transfer, j(H)(=St.Pr-2/3) and the skin friction coefficient (C-f) is studied which shows the validity of the Chilton-Colbum analogy for variable viscosity fluid. It is observed that the Chilton-Colburn analogy is largely valid for lower values of mean velocity, but progressively fails as the mean velocity of the fluid increases. Additionally, the role of a modified non-dimensional parameter "IIS mu T" in flow friction is also investigated in this research. (C) 2015 Elsevier Masson SAS. All rights reserved

Modified thermodynamic principles unifying order existence and evolution

The 'negentropy' proposed first by Schroedinger is re-examined, and its conceptual and mathematical definition are proposed; which is shown to integrate Schroedinger's intention of its introduction, and the subsequent diverse notions in literature. This definition of negentropy is corroborated by its unique ability to state direct thermodynamic principles for order existence: 'Negentropy Principle', and order evolution: 'Principle of Maximum Negentropy Production'. These principles are the respective counterparts of the 'Entropy Principle', and the 'Law of Maximum Entropy Production'. The Principle of Maximum Negentropy Production combines the basic philosophies of the evolution theories postulated by Darwin and de Vries. The prime advantage of these two additional thermodynamic principles, is the resulting unified explanation of order creation, existence, evolution, and destruction; using thermodynamic principles only

Optimization of micro-heat sink based on theory of entropy generation in laminar forced convection

The entropy generation ((gen)) due to fluid friction and convective heat transfer is studied en-route the microscale. The (gen) for water-flow through a circular tube for the constant wall heat flux boundary condition is estimated. The number of tubes (N) en-route the microscale is increased by correspondingly decreasing each tube diameter, for fixed total mass flow rate and the total heat flow rate is also kept constant. There exists an optimum tube diameter (D-N,D-opt) and a corresponding optimum natural number N (N-opt) at which, the sum-total (gen) is minimum. Criterion for D-N,D-opt is obtained in terms of Reynolds number and a modified Brinkman number, which shows that D-N,D-opt depends only on the total heat flow rate. Unlike other reported studies, the fluid temperature in the denominator of the entropy generation terms is considered as local and variable. The difference in (gen) based on reported studies and this investigation increases significantly especially towards the microscale

NUMERICAL STUDY OF MICROCONVECTIVE WATER-FLOW CHARACTERISTICS WITH VARIATIONS IN PROPERTIES

A numerical program is developed to solve two-dimensional continnum-based governing differential equations for liquid flow in axisymmetric circular microchannel geometry. The effects of variable thermal properties in single-phase laminar forced convection with constant wall heat flux boundary conditions are studied. The numerical analysis of fully developed flow behavior investigates the effect of rho(T), mu(T), and k(T) on convection and friction characteristics in isolation and in combination. For the case of heated water, mu(T) variation and k(T) variation increases the Nusselt number due to the following effects: (1) nonnegligible radial convection causes flattening of the axial velocity profile, (2) reduction in wall temperature and axial bulk mean fluid temperature causes significant axial conduction along the flow. The effect of.(T) and mu(T) variation on friction is indirect as follows: the viscosity gradient and shear stress at the wall reduce along the flow; therefore, the Poiseuille number deviates from constant properties solution (Po = 64). The investigations also showed that pressure drop significantly differs at the microscale compared to the macroscale

Infrared Signature of Aircraft Engine with Choked Converging Nozzle

Incorporation of an infrared suppressor is generally accompanied by a compromise in engine performance, which indirectly reduces the effectiveness of infrared signature suppression. This investigation illustrates the percentage increase in the infrared signature level in the 1.9-2.9 mu m and 3-5 mu m bands resulting from an increase in engine backpressure in a jet engine due to a reduction in the exit area of a choked converging nozzle. The effectiveness of optically blocking the hot engine parts by reducing the choked nozzle-exit area is estimated. Thermodynamic offdesign point analysis of the jet engine is done using GasTurb software to evaluate the percentage reduction in thrust and the net change in infrared signature level for the reduced choked converging nozzle-exit area relative to that for the design point