Gas to gas heat transfer in micro heat exchangers

Abstract

By the method of mechanical manufacture of microstructures jointly developed by the Karlsruhe Nuclear Research Center (KfK) and Messerschmitt-Bölkow-Blohm (MBB) very compact cross-flow micro heat exchangers have been fabricated whose active volumes are 1 × 1 × 1 cm 3 and whose typical channel cross-sections are 75 × 90 μm 2. Measurements with nitrogen, argon and helium as heat transfer fluids were performed on two copper and stainless steel finned plate heat exchangers. With the stainless steel heat exchanger overall heat transfer coefficients up to 1700 W m −2 K −1 have been attained in the range investigated using helium as the fluid. For low flow rates greatly reduced overall heat transfer coefficients have been measured compared with the values to be anticipated for laminar flow. In some ranges higher heat transfer rates have been achieved with the stainless steel heat exchanger compared with the system made from copper. This leads to the conclusion that the heat transfer behavior for low gas flow rates is largely determined by heat conduction in the longitudinal direction within the wall and fin materials. With a homogeneous model the influence of longitudinal heat conduction in the stationary walls can be explained and is in fairly good agreement with the experimental data.