![]() ![]() ![]() ![]() They considered several cases of gaps of different sizes. ![]() who proposed a CFD model of a finned heat exchanger with a variable contact surface area between the tube and the fin. Recently, the topic was addressed by Singh et al. Less attention has been paid to cases where there is a lack of contact with the plate-fin along a part of the tube perimeter (no contact between the fin collar and tube). Taler and Oclon and Taler and Cebula changed iteratively the value of contact resistance in their CFD model in such a way that the thermal and flow quantities obtained from the numerical solution agreed with the measured ones. Aside from the experimental research aimed at estimating the thermal contact resistance, its value is also obtained through semiempirical methods, partly based on numerical analysis. Therefore, some of the researchers have undertaken studies taking into account the realistic fin-tube thermal contact and its impact on the thermal efficiency of heat exchangers. Non-ideal tube fin connection can cause significant thermal contact resistance which deteriorates heat transfer. Because of the additional amount of energy needed for the soldering process and some technical difficulties (ensuring high purity of soldered surfaces), mechanical methods of expansion are most frequently chosen. The quality of the joint can be improved by brazing the tube to the fin (the soldering process and its impact on heat transfer are discussed in detail in ). In this case, the lack of contact is always present at approximately 1/2 perimeter of the tube, and it is a feature essential for the assembly of the heat exchanger. For the egg-crate-type heat exchangers, which are used as evaporators in domestic refrigerators, slits between the tubes are introduced to allow one to pull the whole tube coil through the plate-fin packet. Formation of the gap can be also a result of the nonideal joint formation process or assembly requirements. These mechanical methods of expansion cause additional thermal resistance at the tubeāfin interface, due to the gaps between the contact surfaces of the fin collars and tubes. The contact is usually achieved by a tube expansion (plastic deformation of the tube by moving the expansion die or by applying pressure at its inner surface). The majority of these works assume an ideal tube fin contact that in practice has to be created during the manufacturing process. Nowadays, when detailed numerical simulations of fluid flow with heat transfer are within reach, many of the researchers carry out Computational Fluid Dynamics (CFD) studies to optimize the geometry of plate-fin and tube heat exchangers. Proven technology, reliability, and relatively low cost of manufacture make plate-fin and tube heat exchangers very popular, not only as components of refrigeration units but also as gas coolers/heaters, recuperative heaters, dryers or heat pipe, thermosyphon radiators. Plate-fin and tube heat exchangers are most widely used as evaporators and condensers in refrigeration systems (commercial refrigeration) and in other applications where heat transfer between liquid or evaporating/condensing fluid and gaseous working fluid is required. The introduction of a thin slit in the front of the tube leads to convective heat transfer enhancement, which should be further investigated. Therefore, reversing the flow direction can lead to up to a 15% heat transfer increase, if mainly the rear gaps are present. The rear gap position is the worst in the smallest gap range. Fin discontinuity in the front of the tube causes the smallest reduction of the heat transfer rate in comparison to the ideal tube-fin contact, especially for thin slits. Blocking of heat flow by the gap leads to the reduction of heat transfer rate. Velocity and temperature fields for cases with air gap were compared with ideal thermal contact cases. Different locations and sizes of an air gap spanning 1/2 circumference of the tube were considered for the range of airflow velocities. Several numerical simulations were carried out for a symmetric section of plate fin-tube heat exchanger to study the influence of air gap on heat transfer under periodic flow conditions. Various errors made in the manufacturing process can result in the formation of an air gap between the tube and fin. Plate fin-tube heat exchangers are widely used in air conditioning and refrigeration systems and other industry fields. ![]()
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