Evaporative heat transfer of liquid nitrogen for energy systems applications

The rapid rise in global CO2 emissions due to increasing demand for fossil fuels has led to the pursuit of alternative energy sources with reduced emissions for producing power. Liquid nitrogen or other cryogenic liquids have the potential to replace or augment current energy sources in cooling and power applications. This can be done by the rapid evaporation and expansion processes that occur when liquid nitrogen is injected into hotter fluids in mechanical expander systems. Earlier works in this area did not lead to any commercialised technologies. In addition, previous works did not quantify the heat transfer rates achievable when liquid nitrogen is directly immersed into a heated liquid. The potential of liquid nitrogen as a source of cold stored thermal energy has motivated this study. The expansion work achievable when liquid nitrogen is injected into a mechanical expander containing a heated liquid has been studied thermodynamically and the required heat transfer rates estimated. The evaporation process of single liquid nitrogen droplets when submerged into n-propanol, methanol, n-hexane, and n-pentane maintained at different temperatures have been investigated experimentally and numerically. The evaporation process is quantified by tracking the growth rate of the resulting nitrogen vapour bubble that has an interface with the bulk liquid. The experimental data suggest that the bubble volume growth is proportional to the time and the bubble growth rate is mainly determined by the initial droplet size. A comparison between the four different bulk liquids indicates that the evaporation rate in n-pentane is the highest, possibly due to its low surface tension. A scaling law based on the pure diffusion-controlled evaporation of droplet in an open air environment has been successfully implemented to scale the experimental data. The more detailed bubble growth rates have been modelled by a heuristic one-dimensional, quasi-steady, conduction-based model. An effective thermal conductivity has been introduced to account for the complex dynamics of the droplet inside the bubble and the subsequent convective processes in the surrounding vapour. The evaporation process of non-cryogenic droplets such as n-pentane and n-hexane when submerged into water, aqueous glycerol and glycerol maintained at different temperatures has been investigated experimentally and numerically. Three different evaporation regimes have been found for these droplets based on the superheat provided by the bulk liquid with regions of high heat transfer determined. Such regions of high heat transfer can be obtained when a spray of liquid nitrogen droplets is injected into a superheated immiscible liquid inside a mechanical expander resulting in greater utilisation of the cold stored thermal energy in liquid nitrogen thereby making such systems practical.