|dc.description.abstract||The purpose of this work was to investigate and delve into the heat transfer in pool boiling of some single-component fluids and some binary and ternary mixtures on a vertically oriented cylindrical heater with different surface roughness. Mainly, to understand the effect of surface roughness on the heat transfer coefficients.
Heat transfer coefficients in nucleate pool boiling of two widely used 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 for different surface roughnesses. The heater surface was polished by diamond grinding and emery paper 1200 to an average surface roughness Ra of 0.02 and 0.13 μm for Dowtherm A and Dowtherm J, respectively. It was also 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. Thus, a new correlation for predicting the pool boiling heat transfer coefficient for Dowtherm A and Dowtherm J is proposed in the present work. The correlation is an adapted Gorenflo (VDI,1993) correlation, and is valid for pressures between 0.2 and 5 bar, heat fluxes up to 230 kW/m2 and surface roughness between 0.02 and 3.6 μm. The accuracy of the correlation is within ±15% compared to experimental data.
Five single-component hydrocarbons namely Methanol, Benzene, Toluene, 1-Pentanol and 1,2-Propanediol, were also investigated, and their pool boiling heat transfer coefficients were obtained on four different heater surface microstructures varying from Ra= 0.2 μm to Ra= 4.36 μm at ca. 1 bar and heat fluxes ranging from ca. 25 to 260 kW/m2. The experimental results on the smooth and rough heater surfaces were compared to predictions from correlations in the literature, and they were in good agreement with the correlation of Mostinski and Gorenflo, respectively. The experimental data for single-component fluids were used in the investigation of pool boiling heat transfer coefficients in some binary and ternary mixtures. The influence of surface roughness on the heat transfer coefficient was also correlated in terms of the average integral surface roughness Ra. The obtained surface roughness exponent n≈0.18 was in reasonable agreement with published values (n=0.133 to 0.2) in the literature.
Pool boiling heat transfer coefficients of some binary and ternary non-azeotropic mixtures were obtained experimentally using the same test facility as for Dowtherm fluids at nearatmospheric pressure with several surface roughnesses. The fluid mixtures used were Methanol/Toluene, Methanol/1-Pentanol, Methanol/Toluene/1-Pentanol, Methanol/1- Pentanol/1,2-Propanediol and Methanol/Benzene/1-Pentanol. In the ternary mixtures containing Benzene and 1,2-Propanediol, a constant mole fraction of 30% of these components was used. The heat fluxes were varied in the range of 20 to 235 kW/m2. The cylindrical heater surface was polished to an average surface roughness of 0.2 μm, and sandblasted yielding surface roughness of 2.98, 3.55 and 4.35 μm. The results were compared to predictions from literature correlations based on different modeling approaches. For binary mixtures, the heat transfer coefficient reduction was roughly proportional to the boiling range. For the ternary mixtures, the relationship between heat transfer coefficient reduction and boiling range or phase envelope was less pronounced. The differences between the different correlation approaches were significant. Overall, the correlations based on the boiling range as parameter generally performed qualitatively better than correlations using the vapor/liquid equilibrium composition difference.
Increasing the surface roughness resulted in an increasing heat transfer coefficient, and the increase was observed to be dependent on the heat flux and fluid composition. The effect of surface roughness on heat transfer coefficient was investigated. Correlations accounting for surface roughness on heat transfer coefficient for binary and ternary mixtures were developed. The correlations use a modified Thome and Shakir correlation with an additional term accounting for the influence of the surface roughness. The correlations were validated with the obtained experimental results for heat transfer coefficients for binary and ternary mixtures at near atmospheric pressure, heat fluxes up to 235 kW/m2, and surface roughness between 0.2 and 4.36 μm. The accuracy of the correlations for binary and ternary mixtures is within ±15% compared to experimental data.||nb_NO