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The interest for the heat transfer applications has been huge since its development until its current stage. The primarily areas where this technique has been developed is the refrigeration and automotive industries where the enhanced surfaces are widely used on their heat exchangers. Nowadays there is an aggressive competition in the process industry to incorporate this technology in heat exchangers. Though every heat exchanger is a potential candidate for the application of enhanced heat transfer, the possibilities for its application must be tested in order to achieve satisfactory and consistent results. During the earliest attempts to enhance heat transfer, plain (or smooth) surfaced were used. This surface requires a special surface geometry able to provide higher values per unit surface area in comparison with a plain surface. The ratio of of an enhanced heat transfer surface to the plain surface is called Enhancement Ratio " ". Thus, The heat transfer rate for a two-fluid counterflow heat exchanger is given by In order to better illustrate the benefits of enhancement, the total length 'L' of the tube is multiplied and divided in the equation Where is the overall thermal resistance per unit tube length. And it is given by The subscripts 1 and 2, describe the two different fluids. The surface efficiency is represented by employing extended surfaces. One aspect to take into consideration is that the latter equation does not include any fouling resistances due to its simplicity, which can be important. In order to enhance the performance of the heat exchanger, the term, must be increased. For achieving a reduced thermal resistance, the enhanced surface geometry may be used to increase one or both terms in relation to the plain surfaces, leading to a reduced thermal resistance per unit tube length, . This reduced term may be used to achieve one of the following three objectives: 1. Size reduction. maintaining the heat exchange rate constant, the length of the heat exchanger may be reduced, providing a heat exchanger of smaller proportions. 2. Increased . * Reduced : maintaining both and the length constant, can be reduced increasing thermodynamic process efficiency leading to reduced operation costs. * Increased heat exchange: Increasing and keeping a constant length will lead to an increased for fixed fluid inlet temperature. 3. Reduced pumping power for fixed heat duty. This will require smaller velocities of operation than the plain surface and a normally not desired, increased frontal area. Depending on the objectives for the design, any of the three different performance improvements can be used on an enhanced surface, and using any of the three mentioned performance improvements it is fully possible to accomplish it.〔 〕 ==Internal flow== There are several available options for enhancing heat transfer. The enhancement can be achieved by increasing the surface area for convection or/and increasing the convection coefficient. For example, the surface roughness can be used to increase in order to enhance turbulence. This can be achieved through machining or other kinds of insertions like coil-spring wire. The insert provides a helical roughness in contact with the surface. The convection coefficient may also be increased by an insert of a twisted tape that consists in a periodical twist through 360 degrees. Tangential inserts optimize the velocity of the flow near the tube wall, while providing a bigger heat transfer area. While, increased area and convection coefficient can be achieved by applying spiral fin or ribs inserts. Other aspects such pressure drop must be taken into consideration in order to meet the fan or pump power constraints. The Journal of Enhanced Heat Transfer provides plenty of information regarding the most recent developments in this field.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heat transfer enhancement」の詳細全文を読む スポンサード リンク
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