High mass quality, near critical pressure, flow boiling of CO2
Abstract
The proposed study seeks to reveal the nature of flow boiling heat transfer of carbon dioxide at a pressure close to the critical conditions at high mass qualities in microchannels. Specifically, reduced pressures (PR) ranging from 0.5 to slightly below 1 (i.e., pressures from ~3.7 MPa to ~7.37 MPa), mass fluxes from 1,000 kg/s to 10,000 kg/s, heat fluxes up to 500 W/cm2, and mass qualities up to 1 will be studied. (Note that these pressures correspond to saturation temperatures of ~2 C to 31 C). Several channel hydraulic diameters will be examined, ranging from 100 m to 400 m. New laser and optical diagnostic techniques will be developed to provide unprecedented measurements that will greatly assist in revealing the mechanisms controlling the heat transfer process under various operating conditions. Specifically, laser and optical diagnostic techniques will provide measurements of spatiotemporal liquid and vapor temperatures, liquid film thickness, and flow field. This will greatly complement our established microfabrication and microscale experimentation capabilities at high pressures. Mechanistic models and correlations for heat transfer coefficient (HTC), critical heat flux (CHF) conditions, and flow patterns will be developed based on these enhanced measurements. Results will be compared with established correlations developed mostly for flow boiling at low reduced pressures (i.e., PR of 0.1 and less). While flow instabilities are expected to diminish at high pressures, they will be monitored and, if applicable, characterized. Flow boiling in micro channels has been a topic of much interest over the last two decades because of its high heat transfer coefficient, a relatively uniform fluid temperature, low mass flux due to latent heat of vaporization, and ability to reside close to the electronic circuit and eliminate several thermal resistances. Because of concerns about flow instabilities, the critical heat flux (CHF) condition, the low mass qualities dictated by the CHF, and insufficient modeling fidelity, it is still not considered a viable cooling method for high power electronics. However, recent results, partially from our research group, suggest that flow boiling of carbon dioxide at high reduced pressures can overcome these hindrances and provide a reliable, robust, and effective cooling at the micro scale. Because of a lack of awareness and challenging experimentation requirements, flow boiling under these conditions is poorly understood. It isexpected that the nature of the heat transfer process is modified. For instance, nucleate boiling heat transfer is predicted to continue and dominant at much higher mass qualities than at lower pressures. Likewise, because the liquid-to-vapor density ratio is much closer to unity, the CHF conditions and mechanism are expected to vary considerably from their low-pressure counterparts. The proposed effort will include mostly experimental studies with limited numerical modeling. We will leverage our extensive knowledge and experience in convective heat transfer at the micro scale, emphasizing flow boiling, and more recently, high-pressure flow boiling, to provide the heat transfer scientific community new knowledge about this important cooling method. The state-of-the-art experimental facility will be used to develop new optical diagnostic capabilities to measure detailed heat transfer and fluid flow processes occurring inside the microchannels for the first time. Also, we will provide practical design tools (e.g., correlationsand models) to practitioners to enable the design of this new cooling technology.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 05, 2021
- Source ID
- N000142112653
Entities
People
- Yoav Peles
Organizations
- Office of Naval Research
- United States Navy
- University of Central Florida Board of Trustees