Inhaltszusammenfassung

For heat recovery purposes, shell and tube heat exchangers are often used to transfer heat from a hot exhaust gas to a colder liquid. The gas flow passes through the inner side of the tube bundle. Since the thermal resistance of this gaseous side dominates the overall thermal resistance, the heat flux can be increased by using corrugated tubes instead of smooth tubes. An economic corrugation of stainless steel tubes can be achieved by deforming the wall material with well-proportioned depre...For heat recovery purposes, shell and tube heat exchangers are often used to transfer heat from a hot exhaust gas to a colder liquid. The gas flow passes through the inner side of the tube bundle. Since the thermal resistance of this gaseous side dominates the overall thermal resistance, the heat flux can be increased by using corrugated tubes instead of smooth tubes. An economic corrugation of stainless steel tubes can be achieved by deforming the wall material with well-proportioned depressions in magnitude of 1/20 of the tube diameter. Thus, the acceleration and deceleration of the bulk flow will be negligible and the flow modification will only occur close to the wall, where the heat transfer coefficient will be set by favourable velocity and temperature gradients. Recurring zones will be created with a thinner viscous sublayer, than before as well as with higher temperature gradients perpendicular to the wall. These zones of reduced viscous sub-layer thickness alternate with zones of increased sublayer thickness and stagnant eddies. For better overall heat transfer, the effect of rising temperature gradients must dominate the isolating influence of areas with higher sublayer thickness and stagnant eddies. The altera-tion of the flow character will also change the pressure drop on the gaseous side, which must be considered simultaneously as it is always a further important feature of exhaust gas heat exchangers. It is evident that a close understanding of the velocity field in the corrugations is needed to improve the decisive parameters of the contour, such as depth and distance of the grooves. For a detailed understanding of the flow field in corrugated tubes, local measurements by a Laser Doppler Anemometer were done. The measurements were focused on the close-to-the-wall region, to examine the viscous sublayer and the stagnant eddies. For practical reasons, the measuring technique LDV was applied to isothermal flow in a cor-rugated tube made by acrylic glass. It is assumed, that the near wall effects of non-isothermal flow in real applications will have a minor effect on the flow shape in the wall grooves. The localization of upflow and downflow areas will be shown. » weiterlesen» einklappen

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Ingenieurwissenschaften