This paper addresses the fundamental problem of maximizing the thermal contact between an entire heat-generating volume and a pulsating stream of coolant that bathes the volume. The coolant flows through an array of round and equidistant tubes. Two laminar flow configurations are considered: stop-and-go flow, where the reservoir of coolant is on one side of the volume, and back-and-forth flow, where the volume is sandwiched between two reservoirs of coolant. The total heat transfer rate between the volume and the coolant is determined numerically for many geometric configurations in the pressure drop number range 102⩽B⩽106, and Pr⩾1. The optimal tube radius and the maximum volumetric heat transfer rate are determined numerically. The numerical optimization results are later predicted based on scale analysis by matching the longitudinal and transversal time scales of the temperature field in each tube, for each pulsation stroke. The predicted scales lead to power-law formulas that correlate the results and summarize the optimal geometry. The optimal tube size is nearly the same in stop-and-go flow and back-and-forth flow, and is independent of the pulsation frequency.
A picture is worth a thousand words. This article is about a picture known as “heatlines” since 1983, and “synergy” since 1998. Both concepts, heatlines and synergy, are about visualizing the physics of convection, which is the combination (superposition) of heat conduction lines and enthalpy flow lines over a material in motion. Heatlines and synergy are reviewed here comparatively. This comparison reveals that synergy is a remake of heatlines, and that synergy has no physical connection with heat transfer enhancement. At bottom, it has become a lot easier to take an existing idea change some key words and drawings and publish the old idea as new.
This paper shows that the hot spot temperature of an electronic module with finned air heat sink can be reduced by allowing the fin thickness and height to increase in the flow direction x. The hot spot temperature decreases by about 15% if the thickness of a plate fin with constant height increases as x0.42 .The decrease is approximately 30% if the height of a constant-thickness plate fin increases as.x, i.e. if the fin shape is almost like a triangle when viewed from the side. In addition to lowering the hot spot temperature, the fin thickness and height variations recommended by this study lead to considerably more uniform temperature distributions on the module surface on which the fins are installed.
Technical Briefs Natural Convection Experiments in a Triangular Enclosure D. Poulikakos, D. Poulikakos Department of Mechanical Engineering, University of Illinois at Chicago, Chicago, Ill. 60680 Search for other works by this author on: This Site PubMed Google Scholar A. Bejan A. Bejan Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information D. Poulikakos Department of Mechanical Engineering, University of Illinois at Chicago, Chicago, Ill. 60680 A. Bejan Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 J. Heat Transfer. Aug 1983, 105(3): 652-655 (4 pages) https://doi.org/10.1115/1.3245635 Published Online: August 1, 1983 Article history Received: September 20, 1982 Online: October 20, 2009
The uniform distribution of stresses in flying-wing aircraft improves the aeroelastic flight envelope. In this paper, we document the effect of wing cross-section configuration on the stress distribution and flutter characteristics of a flying-wing aircraft. We determined the flow of stresses through the wing structure, and changed the structure to avoid stress strangulations. The emerging structure is more stable. We used the computer programs Gmsh, Variational Asymptotic Beam Sectional Analysis, and Nonlinear Aeroelastic Trim and Stability of High Altitude Long Endurance Aircraft. The wing structure was evolved by holding fixed the flight condition, mass per unit length, and material type. The results indicate that particular configurations of wing cross sections favor a uniform stress distribution, and therefore aeroelastic stability. The configuration with higher flutter speed is associated with the smoother flow of stresses through the wing structure.
No abstract is provided for this article.
In this paper we propose to optimize the geometric configuration of a component by maximizing the global thermodynamic performance of the much larger system that contains the component. This “integrative” approach departs from current thermodynamic optimization practice in which the configuration of a component (e.g., heat exchanger) is optimized by itself, in isolation. In the present example the larger system is an aircraft and the component is its environmental control system (ECS). We show that the configuration of the ECS impacts the performance (exergy destruction, fuel consumption) of the aircraft in two ways, not one: through its own irreversibility, and its weight-related contribution to the power required to sustain the flight. By minimizing the thermodynamic losses at the aircraft level, we deduce all the geometric details of the cross-flow heat exchanger that dominates the weight and structure of the ECS. The optimized geometry is robust with respect to changes in some of the operating parameters that have to be specified. The integrative method illustrated in this paper is generally applicable to the optimization of architecture in other systems where all the functions are driven by the exergy of the fuel installed onboard.
This is a review of the main features of current activity in engineering thermodynamics research. It starts with the fundamentals of why processes and devices are inefficient: irreversibility, entropy generation (as distinct from the property entropy), exergy destruction, and the Gouy - Stodola theorem. The main trend today is the use of these principles to optimize global performance and configuration. The emphasis is on design, that is, the generation of the configuration of the energy system. This is accomplished by balancing (distributing, spreading) the irreversibilities of the energy system. This trend is fueled not only by the need for innovation (design without bias), but also by the availability of increasingly more powerful computational tools. The examples illustrated in this paper are counterflow heat exchangers, tree-shaped flow structures, flight, and sizes of organs for complex flow systems. At the fundamental level, the generation of the optimal configuration for maximal thermodynamic performance is an opportunity for engineers to shed light on the basics of self-optimization in nature. This latest trend at the engineering-science interface is constructal theory and design.
No abstract is provided for this article.
In this paper we rely on the constructal law of maximization of flow access in order to construct a theory of geometry generation (selection, evolution) during molten droplet impact. We show that immediately after impact the liquid spreads inviscidly as a ring with a radial velocity that scales with the initial impact velocity. If the initial droplet is small and slow enough, the ‘splat’ comes to rest (dies) viscously, as a disc. If the droplet is large and fast enough, the ring splashes and is continued outward by needles that grow radially until they are arrested by viscous effects. We optimized the number of needles such that the total splash time is minimum. The theoretical dimensionless group that governs the selection of geometry (G) is the ratio of two lengths, the final radius of the disc that dies viscously, divided by the radius of the still inviscid ring that just wrinkles. Splats form when G ⩽O(1) and splashes are favored when G ⩾O(1). Experimental measurements reported in the literature confirm several of the features of the constructal development of splat vs. splash flow architecture.
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.The Journal of Mechanics publishes original research in all fields of theoretical and applied mechanics.The Journal especially welcomes papers that are related to recent technological advances, such as micro/nanomechanics, medical and biological systems, and microscale heat transfer.The contributions, which may be analytical, experimental or numerical, should be of significance to the progress of mechanics.
This paper addresses the fundamentals of the phenomenon of steady heat transfer by rolling contact between two bodies at different temperatures. The contact region is modeled according to the classical Hertz theory, by which the bodies undergo elastic deformation and the contact area has the shape of an ellipse. In the first part of the study it is shown that when the two bodies make contact continuously over the elliptical area, the overall heat transfer rate is proportional to the square root of the Peclet number based on the ellipse semiaxis parallel to the tangential (rolling) velocity. In the same case the heat transfer rate increases as the square root of the normal force (F) between the two bodies. The second part of the study treats the case when the rolling contact is made through a large number of asperities (contact sites) distributed over the elliptical contact area. The heat transfer rate is again proportional to the square root of the Peclet number. When the asperities are distributed randomly, the heat transfer rate increases as F5/6. In the case of regularly distributed asperities that undergo elastic deformation, the heat transfer rate is proportional to F13/18. The high Peclet number domain covered by this study is discussed in the closing section of the paper.
This is an experimental and theoretical study of the process of time-dependent solidification in an enclosed liquid cooled from the side. The focus is on the effect of high Rayleigh number convection, and the convection-conduction interaction across the freezing front. Experiments at Ra ≈ 1011 and Ste ≈ 0.1 show the existence of a weakly turbulent flow in the early stages of the solidification process. The parametric domain of this study is expanded through the use of a theoretical model of the combined conduction and convection process with time-dependent stratification in the liquid pool. The accuracy of this model is demonstrated by comparing its predictions with the experimental measurements. The effect that the time, the Stefan number and the conductivity and diffusivity ratios have on the solidification rate is documented graphically.
In this paper we investigate a new design concept for generating multi-scale structures in natural convection with the objective of maximizing the heat transfer density, or the heat transfer rate per unit of volume. The flow volume is filled with vertical equidistant heated blades of decreasing lengths. The spacings between the blades are optimized for maximal heat transfer density. Smaller blades are installed in the center plane between two adjacent longer blades, in the entrance region where the boundary layers are thin and the fluid is unheated. Based on the same principle, new generations of even smaller blades are added stepwise to the multi-scale structure. Constructal theory is applied to each new generation of blades, and this method leads to the optimal spacings between blades and the optimal lengths scales. As the number of length scales increase, the flow rate and the volume-averaged heat transfer density increase. It is also shown that there is a smallest (cutoff) length scale below which the boundary layers are no longer distinct, and where the sequence of generating optimal length scales ends. The maximized heat transfer density increases as the optimized complexity of the flow structure increases.
In this paper, we present a natural language processing system that can be used in hospital surveillance applications with the purpose of identifying patients with pneumonia. For this purpose, we built a sequence of supervised classifiers, where the dataset corresponding to each classifier consists of a restricted set of time-ordered narrative reports. In this way the pneumonia surveillance application will be able to invoke the most suitable classifier for each patient based on the period of time that has elapsed since the patient was admitted into the hospital. Our system achieves significantly better results when compared with a baseline previously proposed for pneumonia identification.
