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Objective:
The objective of the proposed project is to investigate the effect of a nanofluid-mixture of lubricant dispersed with passive diamond nanoparticles on R134a pool boiling.
Problem:
Nanofluids are liquids that contain dispersed nano-size particles. Water, ethylene glycol, and lubricants have been successfully used as base fluids in making nanofluids. Carbon, metals, and metal oxides have served as nanoparticles. The exciting and promising characteristic of nanofluids for heat transfer applications is that some nanoparticles at small volume fractions (less than 0.4 %) result in the nanofluid having a thermal conductivity that is more than 40 % greater than that of the pure base fluid.
The proposed FY08 Federal investment in Nanotechnology via the National Nanotechnology Initiative (NNI) is over $1.45 billion bringing the total investment since 2001 to over $8.3 billion. NIST has received funding from the NNI for work in materials, optoelectronics, nanobioscience, and environmental nanotechnology. In addition, NIST continues to invest in nanotechnology projects outside and within its Center for Nanoscale Science and Technology (CNST) at the Advanced Measurement Laboratory. However, no NIST work (besides this study) is currently being conducted to characterize the heat transfer properties of nanofluids. In addition, there are only a few true nanofluids heat transfer publications in the open literature due in part to the difficulty in creating sufficiently stable dispersions of nanoparticles in liquids, especially water. Recent advancements in nanofluid technology create opportunities for many applications including improving the energy efficiency of refrigerant chillers. There is an opportunity for BFRL to serve the refrigeration and air-conditioning industry in a pre-competitive way with fundamental heat transfer data for both water-based and lubricant-based nanofluids. Considering that nanofluids are becoming more stable and the corresponding lack of heat transfer data, there is the potential for BFRL to lead in the development of nanofluids heat transfer research. The difficult part of the problem, which is also the long-term project goal, is applying the measured data to develop a fundamental model to predict refrigerant/nanolubricant pool-boiling heat transfer based on refrigerant, lubricant and nanoparticles properties.
Approach:
The long-range plan of the proposed project is to investigate the potential of lubricant based and water based nanofluids for improving the energy efficiency of refrigerant chillers and the fundamental mechanisms associated with heat transfer enhancement. Nanoparticles of various size, shape, and material will be investigated.
The possibilities for heat transfer enhancement with nanolubricants can best be explored fundamentally by studying the effects of nanoparticle size, shape, and material on performance. So far in our studies we have determined that nanoparticles thermal conductivity is not necessarily the primary determining factor for whether a heat transfer enhancement is achieved or not. By systematically investigating the key parameters that characterize nanolubricants, we can unlock the fundamental mechanisms that govern refrigerant/nanofluid boiling. The new understanding can be used to develop a refrigerant/nanolubricant pool-boiling model based on the salient characteristics of the nanoparticles. The model can be used to predict the boiling performance of refrigerant/nanolubricants mixtures that have not been measured and to maximize heat transfer to realize energy savings.
The FY06 and FY07 projects investigated the influence of CuO concentration on R134a/lubricant pool boiling. The pinnacle result was that a 4 % volume fraction CuO solution of the base lubricant of one mixture gave a heat transfer enhancement of between roughly 40 % and 275 %. These results depended upon the temperature difference between the heated surface and the bulk refrigerant and other factors. The FY08 project aim is to improve our understanding of the heat transfer mechanisms of nanofluids and to determine if much larger increases in heat transfer can be achieved with a significant increase in thermal conductivity by testing nanolubricants with diamond nano particles. The same base lubricant as was used in the FY06 and FY07 studies will be used in the FY08 study in order to isolate the effect of the nanoparticles properties on heat transfer. The effect of increased thermal conductivity on the nanofluid heat transfer is not expected to be consistent with macroscopic behavior. Consequently, the proposed research is a logical progression from the first in order to establish fundamental data for understanding the mechanisms that control nanofluid heat transfer.
The study will start with tests of R134a mixed with the pure base lubricant. Roughly eighty discrete operating points will be recorded to characterize the steady-state heat flux versus temperature (boiling curve) of three mixtures with different lubricant mass fractions (0.5 %, 1 %, and 2 %). These will be the reference measurements for the study. Then, diamond nanoparticles will be added to the base lubricant to form the nanolubricant containing 4 % of nanoparticles (by volume). In order to determine the effect of the nanolubricant, the boiling curve for three mixtures of R134a and the nanolubricant will be measured at the same three mass fractions as was done for the R134a and the pure base lubricant. Ratios of the heat flux for the R134a with the nanolubricants to the heat flux for the R134a with the pure base lubricant will be made as a function of heat flux for each of the three compositions.
The nanoparticle size will be measured with the use of our recently purchased non-invasive backscatter (NIBS) light scattering apparatus. In addition, the thermal conductivity will be measured using the transient line-heat-source method. Both measurement instruments were purchased with FY07 invested equipment funds. The Materials and Construction Research Division will help us to evaluate the degree of particle agglomeration in the mixture using their light scattering apparatus. In this way, we will have an independent verification of the dispersion properties so that, in the future, the boiling phenomenon can be modeled using the measured values for particle size and mixture volume fraction.
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