A Numerical Study of Compact Fin Array Geometries to Improve Additively Manufactured Heat Exchanger Performance

Metal-based additive manufacturing (AM) processes have proven that traditional design domains can be expanded for incorporation of new geometries to potentially improve the performance capabilities of existing thermal devices. Although improved heat-transfer performance has been demonstrated in AM compact heat exchangers, the improvement comes at the expense of increased pressure losses due to both the increased surface roughness of as-built AM surfaces and the increased physical dimensions of AM parts. In this study, the thermal and hydraulic performance of micron-scale fin arrays for compact offset strip fin heat exchangers are examined. By implementing small gaps between subsequent strip fins, it is anticipated that the additional flow area provided will reduce the inertial pressure loss as the working fluid traverses between subsequent columns of fins. To assess the efficacy of this design consideration, numerical simulations were performed to predict and analyze resultant pressure loss and heat-transfer performance. Simulations were preformed utilizing ANSYS Fluent, wherein symmetric samples of each fin array are modeled for offset distances ranging from
$\mathbf{0}-\mathbf{300}\ \boldsymbol{\mu}\mathbf{m}$
and fin lengths ranging from
$\mathbf{3175}-\mathbf{2875}\ \boldsymbol{\mu}\mathbf{m}$
. The model assumes a constant heat-flux boundary condition and utilizes a working fluid mixture of 50:50 Ethylene Glycol and Water (EGW) under laminar flow conditions
$(\mathbf{16} < \mathbf{Re} < \mathbf{52})$
. To allow for an equivalent comparison of each array design, thermal and hydraulic performance are characterized based on a non-dimensional ratio of the
$\mathbf{Colburn}-\boldsymbol{j}$
factor
$(\boldsymbol{j})$
and Fanning friction factor
$(\boldsymbol{f}_{\boldsymbol{app}})$
. Overall, the results demonstrated that the addition of small gaps can lead to a measurable reduction in pressure loss with minimal impact on thermal performance. However, results also demonstrated that the advantage of offset distance diminishes as offset distance increases, such that the increase in
$\boldsymbol{j}/\boldsymbol{f}_{\boldsymbol{app}}$
was 15% from
$\mathbf{0}-\mathbf{100}\boldsymbol{\mu} \mathbf{m}$
but only a 5% additional gain in
$\boldsymbol{j}/\boldsymbol{f}_{\boldsymbol{app}}$
was seen when increasing distance from
$\mathbf{200}\boldsymbol{\mu}\mathbf{m}$
to
$\mathbf{300}\boldsymbol{\mu} \mathbf{m}$
. This trend is likely attributed to a minimal decrease in local fluid velocity gradients providing only a minor reduction to inertial pressure loss. This study has demonstrated that the addition of small gaps between columns of fins is an effective technique to reduce pressure loss in potential AM compact heat-exchanger designs while minimally effecting heat-transfer performance. Insights from these numerical studies will be leveraged to guide future experimental studies to design, print, and test AM heat-exchangers for electronics cooling applications.