Pool Boiling Heat Transfer of Dowtherm-A and Dowtherm-J on Smooth and Roughened Vertical Heater Surfaces

Abstract

Heat transfer coefficients in nucleate pool boiling of the organic heat transfer fluids Dowtherm A and Dowtherm J were obtained experimentally using a vertical electrically heated cylindrical carbon steel surface. The experiments were carried out at system pressures between 0.2–4.9 bar and heat fluxes in the range 20 to 230 kW/m2 and using different surface roughnesses. The heater surface was polished with emery paper 1200 to an average surface roughness (Ra) of 0.02 and 0.13 μm for Dowtherm A and J respectively, and roughened to an average surface roughness (Ra) of 2.42 μm by emery paper 40, and by sandblasting to an average (Ra) of 1.3 and 3.6 μm. The experimental results on the roughened surfaces showed that the heat transfer coefficient increased with increasing pressure, increasing surface roughness and increasing heat flux. A new correlation for predicting the pool boiling heat transfer coefficient for Dowtherm A and Dowtherm J is proposed. The correlation is an adapted Gorenflo [1], [2] correlation, and is valid for pressures between 0.2 and 5 bar, heat fluxes up to 230 kW/m2 and surface roughnesses between 0.02 and 3.6 μm. The accuracy of the correlation is within ±15% compared to experimental data.