Tube bundles in crossflow

In the following, the heat transfer and pressure drop in a tube bundle in crossflow will be investigated. The individual tubes in the bundle are either in alignment with each other or in a staggered arrangement, according to Fig. 3.25. 

The heat transfer is greater in a staggered arrangement at the same Reynolds number. However this is paid for with a larger pressure drop. With reference to Fig. 3.25a to c, the separation between the tube centres perpendicular to the flow direction is known as transverse pitch sq, tube separation in the flow direction as 

The flow around and therefore the heat transfer around an individual tube within the bundle is influenced by the detachment of the boundary layer and the vortices from the previous tubes. The heat transfer on a tube in the first row is roughly the same as that on a single cylinder with a fluid in crossflow, provided the transverse pitch between the tubes is not too narrow. Further downstream the heat transfer coefficient increases because the previous tubes act as turbulence generators for those which follow. From the fourth or fifth row onwards the flow pattern hardly changes and the mean heat transfer coefficient of the tubes approach a constant end value. As a result of this the mean heat transfer coefficient over all the tubes reaches for an end value independent of the row number. It is roughly constant from about the tenth row onwards. This is illustrated in Fig. 3.26, in which the ratio F of the mean heat transfer coefficient Oim(zR) up to row zR with the end value am (zR — oo) = amoo is plotted against the row number zR. 

The Nusselt number Num from (3.211) is to be multiplied with a geometric factor /A. With this we obtain the Nusselt number 

3 Convective heat and mass transfer. Single phase flow 

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Fig. 3.25 Tube bundle in crossflow, a aligned tube arrangement b staggered tube arrangement with the smallest cross section perpendicular to the initial flow direction c staggered tube arrangement with the smallest cross section in the diagonal...

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