901 publications from this institution
Experimental investigations of the flow and the associated heat transfer were conducted in two-dimensional microchannels in order to test possible size effects on the laws of hydrodynamics and heat transfer and to infer optimal conditions of use from the measurements. The test section was designed to modify easily the channel height e between 1 mm and 0.1 mm. Measurements of the overall friction factor and local Nusselt numbers show that the classical laws of hydrodynamics and heat transfer are verified for e > 0.4 mm. For lower values of e, a significant decrease of the Nusselt number is observed, whereas the Poiseuille number continues to have the conventional value of laminar developed flow. The transition to turbulence is not affected by the channel size. For fixed pressure drop across the channel, a maximum of heat transfer rate density is found for a particular value of e. The corresponding dimensionless optimal spacing and heat transfer rate density are in very good agreement with the predictions of Bejan and Sciubba (1992). This paper is the first time that the optimal spacing between parallel plates is determined experimentally.
This paper reports in closed form the similarity heatfunctions H for laminar boundary layer flow on a flat wall. Plots of the constant-H lines (‘heatlines’) show that the path of convection from a hot free stream to a cold wall is unlike the path of convection from a hot wall to a cold fluid. The true path of convection in laminar boundary layer flow is visualized in charts drawn for both heat transfer modes (cold wall, hot wall), several Prandtl numbers (0.02, 0.72, 7) and isothermal walls and constant-flux walls. The paper stresses the heat transfer features that are brought into view for the first time by the heatline patterns. As a supplementary contribution, the paper reports the exact similarity solution for the wall with uniform flux in the Pr→ 0 limit, and proposes a closed-form local Nusselt number correlation that covers the entire Pr range.
