Abstract
This paper presents results of an experimental investigation on pressure drop and heat transfer for a wide range of Reynolds and Prandtl numbers ranging from 8 < Pr < 60 and 40 < Re < 3500, for flat tubes without and with passive inserts. For three different kinds of passive insert designs, the impact on heat and momentum transfer due to coaction of the total set of passive inserts with different shape and amount was investigated. Experimental results were analyzed regarding two main aspects: Heat transfer mechanisms and pressure drop induced by friction and form drag forces due to the presence of different shapes. After heat and momentum transfer mechanisms for each passive insert design were analyzed, heat transfer and pressure drop enhancement were compared to each other, leading to an efficiency discussion. Different concepts for efficiency evaluation, which are cited in literature, were applied to the presented experimental data. Pros and cons of the different concepts are discussed. Finally, we propose an equation for evaluation of total performance, which fully respects the energetic and exergetic aspects of heat transfer and pressure drop enhancement.
Highlights
Numerous investigations on heat and momentum transfer with passive inserts in tubes and pipes have been published in the past
This paper presents results of an experimental investigation on pressure drop and heat transfer for a wide range of Reynolds and Prandtl numbers ranging from 8 < Pr < 60 and 40 < Re < 3500, for flat tubes without and with passive inserts
The questions to be answered were, which flow modifications or phenomena are induced by the three different passive insert types and in what way do they affect heat transfer and pressure drop? The bulk and boundary layers will be influences in different ways with the present passive inserts, which is why on the one hand we considered different portions of heat transfer realized by minimizing boundary layer thickness and inducing turbulence in core flow and on the other hand pressure drop induced by friction and drag forces
Summary
Numerous investigations on heat and momentum transfer with passive inserts in tubes and pipes have been published in the past. From so called “dimples”, which are primarily designed to stop the formation or realize a reformation of the boundary layers [5], to passive inserts such as pins, which will induce flow detachment and turbulence downstream [6,7,8,9]. Such “pins” are columns with arbitrary cross sectional shape such as circle, oval, drop, or diamond, with an insert height to channel height ratio of one, meaning that the cylindrical passive insert stretches from bottom to top of the channel. Other types of passive inserts, which are mainly used for minimizing the thermal boundary layer, are characterized by a lower insert height to channel height ratio
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