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As fuel injection systems advance towards higher injection pressures and the combustor environment increases in both temperature and pressure in the pursuit of improved emissions and efficiency, advanced combustion strategies are required. Injecting fuel as a supercritical fluid has the potential to improve fuel/air mixing and eliminate steps in the spray vaporization process. Experiments are carried out on a heated fuel injector in an open-air test cell using Mie scattering, Schlieren imaging, and long-distance microscopy to investigate changes in spray characteristics with varying temperature and pressure. Spray angle, spray penetration length, and vapor-liquid ratio data are collected and evaluated. Near-nozzle imaging shows distinct changes in spray morphology during the initial microseconds of spray formation. Sprays injected under conditions further into the supercritical regime exhibit increased spray angle and vapor-to-liquid ratio. Spray penetration is found to decrease with increasing temperature. A jump in vapor-to-liquid ratio is observed in the vicinity of 568 K, indicating a transition in spray behaviour trending towards more rapid fuel/air mixing across the transcritical region. Changes in the micro-scale structure of the spray during the initial microseconds of spray formation exhibit this same narrow transition region. A significantly greater fraction of the spray plume is observed to be in a vapor or vapor-like state at a given time after injection initiation as the injection conditions are advanced into the supercritical state. These findings indicate that injection fuel as a supercritical fluid has the potential to improve the mixing of a fuel/air charge, and thus, improve combustion quality.
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Predicted roundtrip efficiency for compressed air energy storage using spray-based heat transfer
Compressed air energy storage (CAES) is a low-cost, long-duration storage option under research development. Several studies suggest that near-isothermal compression may be achieved by injecting water droplets into the air during the process to increase the overall efficiency. However, little is known about the thermal-fluid mechanisms and the controlling nondimensional parameters of the expansion process, which has previously been assumed to mirror the compression process. Furthermore, the isothermal round-trip efficiency and the impact of spray-based CAES have not been investigated. This study uses a validated 1-D model for compression and expansion with spray injection to complete a parametric analysis to analyze the thermal-fluid time-dependent physics and resulting roundtrip isothermal efficiency of a CAES system. Comparing the results for compression and expansion simulations, compression is found to have a higher isothermal efficiency than expansion for the same set up. The polytropic index for both compression and expansion tends to decrease and approach the ideal isothermal limit as nondimensional mass loading increases and as nondimensional Crowe number (ratio of thermal response time to domain time) decreases. As such, the highest efficiency designs are those with slow compression speeds and high spray flow rates to achieve high mass loading and those with small droplets to achieve low Crowe numbers—as long as spray work is neglected. If spray work is included, the optimum spray conditions shift to those with lager drop sizes. For example, roundtrip isothermal efficiency peaks around 95 % at a mass loading of 14 and at Crowe numbers <0.1 with a pressure ratio of ten. The results indicate that near-isothermal CAES compression and expansion is possible but that spray work should be included for significant mass loadings (e.g. greater than unity). Further investigation is recommended to consider effects of multi-dimensionality, turbulence, wall-interactions, and droplet dynamics.
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- Award ID(s):
- 2324460
- PAR ID:
- 10600391
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Energy Storage
- Volume:
- 72
- Issue:
- PC
- ISSN:
- 2352-152X
- Page Range / eLocation ID:
- 108461
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.est.2023.108461
