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Abstract
The current article experimentally documents the effects of the nanoparticles thermal conductivity and aspect ratio on the two-phase flow heat transfer coefficient and pressure drop of nanolubricants in mixture with refrigerant R410A. Two nanoparticles types were dispersed in the polyolester lubricant: spherical Alumina (γ-Al2O3) nanoparticles with 40 nm nominal particles diameter and ZnO nanoparticles with 20 to 40 nm nominal particles diameter and with an elongated shape. While the two nanolubricants had different nanoparticle aspect ratio, they shared similar thermal conductivity. In refrigerant R410A and polyolester mixtures, they led to measurably different heat transfer coefficient for two-phase flow boiling inside a horizontal 9.5 mm micro-fin evaporator tube. Depending on mass flux, concentration, and heat flux, nanolubricants provided either an enhancement or a degradation, supporting the hypothesis that the nanolubricants thermal conductivity was not the main property responsible for the heat transfer coefficient intensification during flow boiling. The current article presents an analysis of the nanolubricants heat transfer models that adopted homogeneous liquid phase modeling approaches during the two-phase flow process. A sensitivity analysis of the thermodynamic properties highlighted that these type of models were not able to satisfactorily predict the heat transfer coefficient and pressure drop of refrigerant-nanolubricants mixtures.
Nomenclature
| A (or HTC) | = | = heat transfer coefficient (kW/m2-K) (or Btu/hr-ft2-R) |
| HTF | = | = heat transfer factor (%) |
| k | = | = thermal conductivity (W/m-K) (or Btu/hr-ft2-R) |
| m | = | = mass (kg) (or lbm) |
| = | = mass flow rate (kg/s) (or lbm/hr) | |
| = | = mass flux (kg/m2-s) (or lbm/ft2-hr) | |
| NP | = | = nanoparticle type |
| NPconc | = | = mass, or weight, concentration (wt. %) |
| OMF | = | = oil mass fraction (%) |
| P | = | = pressure, (kPa) (or psi) |
| = | = pressure drop factor (%) | |
| ΔP | = | = test section pressure drop (kPa) (or psi) |
| = | = heat flux (kW/m2) (or Btu/hr-ft2) | |
| T | = | = temperature (°C) (°F) |
| x | = | = thermodynamic refrigerant quality |
| ω | = | = local oil mass fraction |
| Subscripts | ||
| 0 | = | = baseline |
| np | = | = nanoparticle |
| oil | = | = oil, lubricant |
| POE | = | = polyolester oil |
| ref | = | = refrigerant |
| s,i | = | = micro-fin tube inner surface |
| s,o | = | = micro-fin tube outer surface |
| ν | = | = kinematic viscosity (m2/s) (or ft2/s) |
Acknowledgments
The authors would like to acknowledge and thank Dr. Harry W. Sarkas and Nanophase Technologies Corporation, for providing the nanolubricant samples of the present work and technical consultation and Thiam Wong and Gennaro Criscuolo (respectively, Master student at Oklahoma State University and visiting undergraduate student from Politecnico di Milano, Italy) for helping collecting data for the current article.
Funding
The authors would like to thank ASHRAE for supporting this work through the ASHRAE Innovative Research Grant Program. Also the NSF IGERT program at Auburn University, for supporting this work.