The flow between pairs of flat plates was studied experimentally to gain insight into the operation of compact heat exchangers with interrupted surfaces. The plates were tested at low Reynolds number in both water and air streams. The investigation focused on the region of transition from steady to unsteady laminar flow between plates. A critical velocity was determined at which periodic oscillations were first observed. This velocity depends strongly on the thickness of the plates, t, plate length, L, and plate separation distance and weakly on flow disturbance level. Data for a range of geometries, 4 ≤ L/t ≤ 159, are correlated using plate wake width as a single plate length scale. The downstream plate was found to have a pronounced upstream influence on the critical velocity. In a low-disturbance-level stream the critical velocity may be lower than that required to produce detectable oscillations at the same point in the upstream plate wake in absence of the second plate. This feedback effect may be responsible for the relative insensitivity of the results to the turbulence level in the free stream.

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