Rough surfaces with enhanced heat transfer for electronics cooling by direct metal laser sintering

Experimental evidences are reported on the potential of direct metal laser sintering (DMLS) in manufacturing flat and finned heat sinks with a remarkably enhanced convective heat transfer coefficient, taking advantage of artificial roughness in fully turbulent regime. To the best of our knowledge, this is the first study where artificial roughness by DMLS is investigated in terms of such thermal performances. On rough flat surfaces, we experience a peak of 73 % for the convective heat transfer enhancement (63 % on average) compared to smooth surfaces. On rough (single) finned surfaces, the best performance is found to be 40 % (35 % on average) compared to smooth finned surface. These results refer to setups with Reynolds numbers (based on heated edge) within 3,500 < Re_L < 16,500 (corresponding to 35,000 < Re_D < 165,000 in terms of Reynolds number based on hydraulic diameter). Experimental data are obtained by a purposely developed sensor with maximum and mean estimated tolerance intervals of +/- 7.0 % and +/- 5.4 %, respectively. Following the idea by Gioia et al. [Phys. Rev. Lett. 96 (2006) 044502], we propose that heat transfer close to the wall is dominated by eddies with size depending on the roughness dimensions and the viscous (Kolmogorov) length scale. An excellent agreement between the experimental data and the proposed analytical model is finally demonstrated

Design of an Experimental Facility for Building Airflow and Heat Transfer Measurements

An experimental facility was constructed and instrumented to conduct heat transfer experiments and flow visualization. The experimental facility is a 1/5-scale model of a full-scale two story building with aspect ratio (H/L) of 0.625. Natural convection experiments were conducted with hot and cold end walls with the rest of the enclosure insulated. Air was the working medium inside the experimental facility. The Grashof number of the flow was ~ 108. Smoke flow visualization and PIV were conducted for the flow field inside the experimental facility. The heat transfer results obtained agreed fine with the results reported in the literature. The smoke visualization and PIV techniques showed the existence of thin boundary layer next to the isothermal walls and stationary core inside the enclosure. The seeding particles used for PIV were olive oil droplets. The flow was two dimensional as proved by the temperature measurement; smoke visualization, and PIV results. PIV showed the existence of reversing loops next to the top and bottom of the enclosure. The PIV technique worked well with natural convection flow. Olive oil droplets of 1 - 3 micor meter, successfully seeded the flow field, and had a sufficiently long settling time to allow reconstruction of the buoyantly driven flow field prior to image capturing.Mechanical & Aerospace Engineerin

Higher-Order Spectral Analysis to Identify Quadratic Nonlinearities in Fluid-Structure Interaction

Hydrodynamic forces on a structure are the manifestation of fluid-structure interaction. Since this interaction is nonlinear, these forces consist of various frequencies: fundamental, harmonics, excitation, sum, and difference of these frequencies. To analyze this phenomenon, we perform numerical simulations of the flow past stationary and oscillating cylinders at low Reynolds numbers. We compute the pressure, integrate it over the surface, and obtain the lift and drag coefficients for the two configurations: stationary and transversely oscillating cylinders. Higher-order spectral analysis is performed to investigate the nonlinear interaction between the forces. We confirmed and investigated the quadratic coupling between the lift and drag coefficients and their phase relationship. We identify additional frequencies and their corresponding energy present in the flow field that appear as the manifestation of quadratic nonlinear interaction

The future of the Arab Regional Regime according to the Regional and International Changes: 1990-2012

The study focuses on the future of the Arab regional regime during the period 1990- 2012 in the light of regional and international changes. The statement of the study is finding out the future of the Arab regional regime according to the regional and international changes that affected the structure of this regime, and tries to look up the future of this regime and its effectiveness up on the continuum of the Arab psychological formation unity in the unity of hope, ambition and aims. The study aims at clarifying the rule of Arab regional regime organizations in facing the internal and external challenges. The study also aims at clarifying opinions of the Arab regional regime to several regional and international matters such as the Second and Third Gulf Wars, negotiation between Israel and Arabs, regional and international projects as the Middle East project, the middle project, Euro-Middle Project, the Israeli attack on Lebanon and Gaza, and explaining effects of Arab revolutions on the future of Arab regional regime. The study used the historical method and the descriptive method. The study concluded that interfacial interactions to the Arab regional regime exposed to abatement and strong retreat, and they were mostly conflict interactions that increased division between Arabs and made Arab solidarity lose its essence especially when the Arab collective work came to the lowest level after the crisis of the Gulf Second. Also, the study concluded that the absence of economic and political reformations in Arab countries had led to occurrence of opposite popular motions which typified in some demonstrations and revolutions against the ruling regimes in this regime, and that caused several regimes to collapse as what happened in Tunis, Egypt Libya and what is going on now in Syria. These revolutions forced other Arab governments in other countries to make internal reformations fearing of new revolution against them, as what is happening now in Saudi Arabia and other countries. The study also concluded that those revolutions have succeeded in impairing principle of excessive real among Arab people. So, the desperate and subdued Arabic citizen has been no longer existent, and he became a rejecter to subservience. These revolutions have also activated and the role of the Arab States League and made it strong after it was non-effective and tottering organization. The happened change in the Arab States League became clear through its opinion in the Libyan and Syrian revolutions, and its supporting to these revolutions. The study recommended making a plenty of real, internal, economic and political reformations beginning from every Arab country to include all the units of the Arab regime. In addition, these needed political reformations require improving and reforming relationships between the ruler and people. That means all community and people parties should participate in the political process through transparent elections. This thing will help preventing western pretexts to interfere in Arab issues claiming of protecting human rights and minorities’ rights, who live bad conditions in some Arab countries, and reforming the bad conditions in Arab countries. The study also recommended developing and reforming organizations of the Arab regime, treating and solving problems of these organizations through being conformed to their resolutions and improving their covenants, activating the role of the general secretary of the Arab States League and giving him more influence and expanding the popular participation in this league through civil community organizations in order to pave the way for setting up popular organizations on the national level. The study also recommended through what preceeded, we conclude that the Arab Countries were –and still- and will be a spot of greedy for the other countries due to the advantages they have as the economical and natural wealth, the strategic situation for the other countries. Throught looking again to the period between (1990-2012), we can find the Arab regime’s attempts to fight these greedies are not satisfied enough specially the economic and security issues. The study also recommends in setting up new links of cooperation between the Arab countries and the other great countries in the world such as countries of the European Union, Japan, China and Turkey, which may impair the control of the only power in the region “United States’’ on the Arab region

Investigation of Coriolis Forces Effect of Flow Structure and Heat Transfer Distribution in a Rotating Dimpled Channel

Abstract Large-eddy simulations are used to investigate Coriolis forces effect on flow structure and heat transfer in a rotating dimpled channel. Two geometries with two dimple depths are considered, δ=0.2 and 0.3 of channel height, for a wide range of rotation number, Ro b =0.0-0.70, based on mean bulk velocity and channel height. It is found that the turbulent flow is destabilized near the trailing side and stabilized near the leading side, with secondary flow structures generated in the channel under the effect of Coriolis forces. Higher heat transfer levels are obtained at the trailing surface of the channel, especially in regions of flow reattachment and boundary layer regeneration at the dimple surface. Coriolis forces showed a stronger effect on the flow structure for the shallow dimple geometry (δ=0.2) compared to the deeper dimple where the growth and shrinkage of the flow recirculation zone in the dimple cavity with rotation were more pronounced than the deep dimple geometry (δ=0.3). Under the action of rotation, heat transfer augmentation increased by 57% for δ=0.2 and by 70% for δ=0.3 on the trailing side and dropped by 50% for δ=0.2 and by 45% for δ=0.3 on the leading side from that of the stationary case

A Study of Heat Transfer Augmentation for Recuperative Heat Exchangers: Comparison Between Three Dimple Geometries

This study presents an investigation of the heat transfer augmentation for the purpose of obtaining high effectiveness recuperative heat exchangers for regeneration. The focus of the present work is in the fully developed portion of a 2:1 aspect ratio rectangular channel characterized by dimples applied to one wall at channel Reynolds numbers of 10, 000, 18, 000, 27, 000, and 36, 000. The dimples are applied in a staggered-row, racetrack configuration. In this study, a segmented copper test section was embedded with insulated dimples in order to minimize (to a negligible level) the heat transfer within the dimpled feature. The insulated material used to create a dimpled geometry isolates the heat transfer within the dimple cavity from the heat transfer augmentation on the surrounding smooth walls promoted by the flow disturbances induced by the dimple. Results for three different geometries are presented, a small dimple feature, a large dimple, and a double dimple. The results of this study indicate that there is significant heat transfer augmentation even on the non-featured portion of the channel wall. Overall heat transfer augmentations for the small dimples are between 13-27%, large dimples between 33-54%, and double dimples between 22-39%, with highest heat transfer augmentation at the lowest Reynolds number for all three dimple geometries tested. Copyright © 2011 by ASME