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This work experimentally characterizes the critical heat flux (CHF) and minimum film boiling heat flux (MFBHF) in additively manufactured cooling channels for regeneratively-cooled rocket engines during high pressure saturated internal forced convective boiling of liquid nitrogen (LN2). Three different channels with hydraulic diameters of 1.8 mm, 2.3 mm and 2.5 mm were fabricated by the National Aeronautics and Space Administration (NASA) Marshall Space Flight Center (MSFC). The channels were fabricated using Powder Bed Fusion (PBF) advanced 3D printing of the rocket engine material, GR-Cop42, a copper-chrome-niobium alloy. The fabricated channels were tested using a custom-built cryogenic High Heat Flux Test Facility capable of operating up to 4 MPa of pressure and 10 MW/m2 of heat flux. The channels were asymmetrically heated from the bottom to simulate the performance of the cooling channels of a rocket engine. The high-pressure flow boiling tests were performed at 1.38 MPa with respective saturation temperature of 109 K using LN2 as the working fluid in horizontal orientation of the channels. The volumetric flowrate of LN2 is held approximately constant at 47 cm3/s for all channels. The experiments were performed beyond the CHF to ensure film boiling inside the channels, and then gradually decreased the given power until MFBHF was reached. A CHF of 543 kW/m2 and a MFBHF heat flux of 486 kW/m2 were achieved for the 1.8 mm hydraulic diameter channel. Furthermore, the experimentally measured CHF values were compared with the correlations available in literature. More than 84% increase in CHF has been experimentally measured for the additively manufactured rough cooling channels as compared to the CHF prediction based on literature correlation for smooth channels.

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This paper presents an experimental characterization of liquid nitrogen (LN2) flow boiling in additively manufactured minichannels. There is a pressing need of concerted efforts from the space exploration and thermal transport communities to design high-performance rocket engine cooling channels. A close observation of the literature gaps warrants a systematic cryogenic flow boiling characterization of asymmetrically heated small (<3 mm) non-circular channels fabricated with advanced manufacturing technologies at mass flux > 3000 kg/m2s and pressure > 1 MPa. As such, this work presents the LN2 flow boiling results for three asymmetrically heated additively manufactured GR-Cop42 channels of 1.8 mm, 2.3 mm, and 2.5 mm hydraulic diameters. Twenty different tests have been performed at mass flux~3805–14,295 kg/m2s, pressures~1.38 and 1.59 MPa, and subcooling~0 and 5 K. A maximum departure from nucleate boiling (DNB)-type critical heat flux (CHF) of 768 kW/m2 has been achieved for the 1.8 mm channel. The experimental results show that CHF increases with increasing LN2 flow rate (337–459 kW/m2 at 25–57 cm3/s for 2.3 mm channel) and decreasing channel size (307–768 kW/m2 for 2.5–1.8 mm channel). Finally, an experimental DNB correlation has been developed with 10.68% mean absolute error.

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