General correlations for pressure drop and heat transfer for single-phase turbulent flows in ribbed tubes

Heat exchangers are an integral part of any power plant, sometimes representing as much as one-third of the total investment. Recently, new techniques have been considered to increase the effectiveness of heat exchangers. Particular attention has been directed toward augmentation or enhancement of convective heat transfer coefficients through use of so-called second generation technology methods such as roughened or extended surfaces and turbulator inserts;The present study examines the use of internally ribbed tubes instead of smooth tubes for shell-and-tube heat exchangers. The study was divided into three phases: (1) development of fundamental correlations for single-phase turbulent heat transfer and pressure drop that are applicable to a wide range of internally ribbed tubes; (2) experimental testing of the correlations under varied conditions of heating, using water as the medium, and cooling, using air as the working fluid; and (3) optimization of the design parameters of these tubes for increasing the heat duty under constant surface area and pumping power and for reducing the surface area under constant heat duty and pumping power;During the first phase, the friction factor and the heat transfer coefficient were separately correlated applying statistical methods to an extensive data base gathered from the literature. These correlations are superior to correlations previously proposed;In the second phase, the correlations were further ratified for water flowing in ribbed tubes under heating conditions and for hot air flowing in a coiled tube under cooling conditions. In all, four tubes and five coils were tested. In addition, simple flow visualization experiments were conducted to gain insight into the mechanisms involved in ribbed tube flows;In the last phase, the correlations were utilized to determine the optimum design variables on the basis of the objective of either maximizing heat transfer or minimizing heat exchanger area for an Ocean Thermal Energy Commission power plant application;An increase in performance of over 60% was observed when the heat exchanges alone were considered for optimization. When parasitic losses for the system were included, the performance increase was around 30%

General correlations for pressure drop and heat transfer for single-phase turbulent flows in ribbed tubes

Heat exchangers are an integral part of any power plant, sometimes representing as much as one-third of the total investment. Recently, new techniques have been considered to increase the effectiveness of heat exchangers. Particular attention has been directed toward augmentation or enhancement of convective heat transfer coefficients through use of so-called "second generation technology" methods such as roughened or extended surfaces and turbulator inserts;The present study examines the use of internally ribbed tubes instead of smooth tubes for shell-and-tube heat exchangers. The study was divided into three phases: (1) development of fundamental correlations for single-phase turbulent heat transfer and pressure drop that are applicable to a wide range of internally ribbed tubes; (2) experimental testing of the correlations under varied conditions of heating, using water as the medium, and cooling, using air as the working fluid; and (3) optimization of the design parameters of these tubes for increasing the heat duty under constant surface area and pumping power and for reducing the surface area under constant heat duty and pumping power;During the first phase, the friction factor and the heat transfer coefficient were separately correlated applying statistical methods to an extensive data base gathered from the literature. These correlations are superior to correlations previously proposed;In the second phase, the correlations were further ratified for water flowing in ribbed tubes under heating conditions and for hot air flowing in a coiled tube under cooling conditions. In all, four tubes and five coils were tested. In addition, simple flow visualization experiments were conducted to gain insight into the mechanisms involved in ribbed tube flows;In the last phase, the correlations were utilized to determine the optimum design variables on the basis of the objective of either maximizing heat transfer or minimizing heat exchanger area for an Ocean Thermal Energy Commission power plant application;An increase in performance of over 60% was observed when the heat exchanges alone were considered for optimization. When parasitic losses for the system were included, the performance increase was around 30%.</p

Flow Through Coated Coronary Stented Arteries: A Review

Click on the DOI link to access the article (may not be free).This paper reviews the recent literature on the stent applications and their recent advancements due to some controversies. The paper consists of two parts flow modeling of stents and a comparison between normal and stenosed artery. It also includes a comparison of stent geometries to determine hemodynamically favorable stents. In spite of many advantages bare metal stents are found to be unfavorable. However the study of stents based on flow and design recommend stent coatings. In comparison with bare metal stents, metal coated stents proved to be more efficient in terms of restenosis. Most of the metal coatings have less fluoroscopic visibility and low tensile strengths and hence drug eluting stents were studied. Although drug eluting stents were proved to be efficient in controlling restenosis, they have issues in patients with co-morbidities. For this reason, polymer and carbon coatings were studied in detail which showed better results than drug eluting stents. Biodegradable stents on the other hand inhibit smooth muscle cell proliferation and a decrease in neointimal proliferation. At present research on polymer, carbon and biodegradable stents are actively pursued for stent therapy applications