In MTR reactors the fuel elements are usually of flat plate-type and the cooling water flows through gaps of 2 to 4 mm. Therefore, correlations developed for parallel flat plates are usually used for their study. Nevertheless, in the hydrodynamic regime of transition, the flow is not stable and, under high heat fluxes, there are no reliable correlations that can be applied in this geometry. The aim of the present work is to determine experimentally the heat transfer coefficient for water in this geometry in the rank of 3000< Re <10000 and for relatively high heat fluxes (between 24 W/cm2 and 32 W/cm2). The experimental setup consists of a 200 liters water tank, pressurized to 1.7 atm, which discharges its content through the vertical test section. The flow is downwards and it is controlled by a valve located downstream. The flow rate is measured with a orifice plate connected to a DPcell and the pressure with a JUMO pressure digital sensor. The test section is a rectangular channel constructed with two 62 cm long, 6 cm wide and 6mm thickness aluminum plates, separated with Teflon strips which set the gap for the flow passage. Gaps or plate separations of 2.7 mm and 3.8 mm were studied. The aluminum plates were electrically heated from the rear using a Bruker BMN 70/700 direct current power supply. The tests were made at 18 kW and 24 kW. The wall temperatures were measured with K-type thermocouples placed in different axial positions. The temperature measurements, the DPcell signal and the pressure signal were acquired with a digital card connected to a PC. Two different inlet flow temperatures were considered 10°C and 38°C. With this arrangement, measurements of wall temperatures evolutions were obtained and the local coefficient of convection h(z) was calculated. Comparisons with turbulent correlations for flat plates indicate that the measured temperatures of wall are greater than expected. It was also observed that there are zones where subcooled boiling is reached. There are some particular aspects that could affect the reliability of correlations for flows between flat plates. The first phenomenon is the change of viscosity close to the wall due to the very high heat fluxes. There may also appear buoyancy effects, though we believe they have a minor importance. And finally the long thermal and hydrodynamic development distances, which may delay the appearance of turbulence in the channel.

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