Through a combined effort between researchers in Norway and France, we examined the characteristics of two candidate nanoparticles, coffee and carbon black. Using different concentrations, temperatures, and dispersion methods, both thermal properties and stability were measured. These were essential to the development of the PT boiling measurement methodologies and the analysis of the boiling tests, contributing to the two publications and five conference presentations that emerged from this project.
Two new PT boiling measurement techniques were created. The first used acoustics, where frequencies are inversely proportional to bubble size and amplitudes are directly proportional to the number of bubbles. Initially, we desired to explore the smallest possible bubbles at several 100 kilohertz frequencies. However, we discovered that, at the highest frequencies, there was substantial attenuation (reduction in amplitude) and the acoustic signals were not distinguishable. Therefore, in furtherance of the technique development, we completed a study looking at how the nanofluids were attenuating the acoustic signals at higher frequencies. We then developed an experimental rig to capture clear, distinguishable acoustic signals in the auditory range (up to 20 kHz).
The second measurement technique used x-rays. Specifically, a CT-scanner into which the rig developed for acoustic studies was inserted. Here, we examined different imaging agent and nanoparticle concentrations and found the ideal combination (with the addition of post-processing) to image the bubbles during PT boiling. This worked down to a minimum diameter of 0.8 mm.
Understanding the limitations of the new techniques, we performed boiling tests at different concentrations of carbon black. This produced novel acoustic spectra that we could compare to understand how the nanoparticles were affecting boiling and the mechanism by which steam generation was enhanced.
