Heat Exchange In Laminar Flow Research Articles

Heat transfer enhancement in laminar flow heat exchangers due to flapping flags

Heat transfer enhancement due to flapping flags in a heated duct flow is studied using three-dimensional (3D) fully coupled fluid–structure–thermal simulations. Following prior work, which was limited to two-dimensional models, we examine the mechanisms and the heat transfer performance for a more realistic, 3D model of a flag in a rectangular duct heat exchanger. We then examine the role of the flag aspect-ratio and spanwise confinement, which are key design parameters for this device. We find that the narrow flags do not exhibit sufficiently energetic flapping to generate any meaningful heat transfer enhancement. We also find that the wide flags significantly increase heat flux and an increase in the width of the flag can further increase the thermal enhancement factor.

CONJUGATED HEAT TRANSFER IN COCURRENT FLOW MULTI-STREAM HEAT EXCHANGERS

Considering the cross-sectional velocity profile to be piecewise-constant in each stream of a multi-stream heat exchanger for cocurrent thermally developing flow, this study analytically solves the related conjugated Graetz problem by using an integral transform method. Further, it obtains an analytical solution in an explicit form to the fluid temperatures that vary two-dimensionally. A numerical example is provided for the case of a triple-tube heat exchanger. The numerical results demonstrate the effects of the thermal conductivity ratios of the fluids and the Péclet number ratios on the temperature distribution in the streams. In addition, in order to show the importance of considering streamwise variations in the overall heat transfer coefficients when designing laminar flow heat exchangers, the amount of exchanged heat calculated by the presented analytical solution is compared with that predicted on the basis of bulk temperatures and constant overall heat transfer coefficients.

The Microtube Strip Heat Exchanger

Abstract The advantages of designing heat exchangers in the laminar-flow regime are discussed from a theoretical standpoint. It is argued that laminar-flow designs have the advantages of reducing thermodynamic and hydrodynamic irreversibilities, and hence increasing system efficiency. More concretely, laminar-flow heat exchangers are free from the turbulence-induced vibration common in conventional heat exchangers, and can thus offer longer life and greater reliability. The problems of manufacturing heat exchangers suited to laminar flow are discussed. A method of manufacture is outlined that allows compact, modular design. Experience with this method of manufacture is described. Experimental results with a prototype heat exchanger bank are presented; these results show good agreement with theory at moderate levels of effectiveness (75-85%), but fall below predicted values at higher levels. It is argued that this discrepancy results from flow maldistribution. The problems of fouling and flow maldistribution are briefly discussed, and some possible applications are mentioned.

Viscosity induced non-uniform flow in laminar flow heat exchangers

Laminar flow heat exchangers which cool oil in non-interconnected parallel passages can experience non-uniform flows and a reduction in the effective heat exchanger coefficient in a range of Reynolds number which varies with tube length and diameter, tube wall temperature and fluid inlet temperature. The method of predicting the reduction in effective heat transfer coefficient and the range of Reynolds number over which these instabilities exist is presented for a particular oil, Mobil aviation oil 120. Included, also, is the prediction of the effect of radial viscosity variation on the constant property magnitudes of friction and heat transfer coefficient.

Heat exchange in laminar flow through planar and quasiplanar channels of variable cross section

Formulas are derived for both the local and averaged over the surface Nusselt number. Experimental results are presented and the limits of applicability of the calculated formulas are indicated.