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

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 (R_a) of 0.02 and 0.13 μm for Dowtherm A and J respectively, and roughened to an average surface roughness (R_a) of 2.42 μm by emery paper 40, and by sandblasting to an average (R_a) 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.