Attention is focused on physical situations in which separate zones of laminar and turbulent flow coexist within a given device. The specific motivating application for this study is a complex fluid-flow manifold used in the thermal management of electronic equipment. The adopted approach to the solution of this fluid-flow problem was to employ a turbulent-flow model for the entire solution space but to use several turbulence models as inputs. The goal of the study was to quantify the behavior of popular turbulence models in regions where the flow is laminar according to the accepted Reynolds number criteria. Three models were considered: 1) standard k-, 2) k-, and 3) SST. Evaluation of the models was based on the values of the ratio t/ in nominally laminar regions, where t and are, respectively, the turbulent and molecular viscosities. Values of this ratio well below one in a nominally laminar region would indicate that a turbulence model reduces, in effect, to a laminar model. It was found that the standard k- model failed to provide values of t/1 for channel Reynolds numbers as low as 370. On the other hand, both the k- and SST models yielded values of t/1 for Re=370. For these models, acceptably small values of the ratio were encountered for Reynolds numbers up to 370. At still higher Reynolds numbers, these turbulence models did not reduce to laminar models. Results for the relationship between the overall pressure drop and the system flowrate are also presented for use in practical applications.