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Further progress in the miniaturization of electronic circuits, engines and apparatus is bound up with the development of small heating and cooling devices. Recent investigations show that the calculation models for micro evaporator channels markedly differ in many respects from those used for the design of normally dimensioned channels. The present contribution is concerned with the characteristics of micro heat exchangers.

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1,500 lm, and rectangular channels with height values of 300-700 lm and a width of 10 mm. In both facilities, the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus, evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 kg/m 2 s and 350 kg/m 2 s and heat fluxes from 20 kW/m 2 to 350 kW/m 2 at an inlet pressure of 0.3 MPa are examined. Infrared (IR) thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms, and the determination of the local heat transfer coefficients (HTC). Measurements are carried out in initial, saturated, and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with currently available correlations and with a physically founded model developed for convective boiling.

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1500 μm, and rectangular channels with height values of 300–700 μm and a width of 10mm. In both facilities the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 and 350 kg/(m2s) and heat fluxes from 20 to 350 kW/m2 at an inlet pressure of 0.3 MPa are examined. Infrared thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms and the determination of the local heat transfer coefficients. Measurements are carried out in initial, saturated and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with available correlations and with a physically founded model developed for convective boiling.

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