Flow and Phase Change Phenomena of Cryogens and Cryogenic Fuels

Cryogenics is the science and technology of devices that operate, and processes that occur below a temperature of 123 K (-150 °C, -238 °F). The field of cryogenics is vast and growing and include manufacturing, biomedical, bioengineering, and aerospace, among others. Boiling of cryogens is encountered in numerous applications, most notably in the fuel delivery and transportation systems of spacecraft. In comparison with combustion of fuels at temperatures where fuel components are gaseous, cryogenic fuels such as, liquid hydrogen (LH2) and liquid methane (LCH4), along with liquid oxygen (LO2) as the oxidizer, allow for significantly greater thrust during combustion. While the cooling properties of LH2 have been studied in the past to a considerable level, the boiling heat transfer characteristics of LCH4 and LO2 are not well understood. Accurately modeling of the boiling process for common fluids is difficult and often is accompanied by considerable uncertainty. Modeling the boiling of cryogens is even more complex. Models and correlations for various pool and flow boiling regimes are many; however, most of these models are based on experimental data that include little to no cryogens. In fact, many of these models are inconsistent across various data sets of the same fluid. The purpose of Part I of this thesis is to compile and analyze the available cryogenic experimental data, select the data that are reliable, and compile relevant models and correlations pertinent to LH2, LCH4, and LO2. For each cryogen, reliable correlations for the various boiling regimes were identified and recommended. Specifically, we focused on available experimental data for the nucleate and film boiling regimes in pool boiling, and the nucleate and post-critical heat flux (CHF) (i.e. film, dispersed flow boiling) regimes dealing for flow boiling. The transition out of nucleate boiling was also investigated by reviewing the available experimental data as well as CHF models and correlations for both pool and flow boiling. Part II of this thesis deals with periodic flow of gases at cryogenic temperatures through microporous structures. Such flows occur in the regenerators of Stirling-type cryocoolers. The regenerator is a key component of a pulse tube or Stirling cryocooler and is also a major contributor to irreversibility and losses in cryocoolers. Experimental pore-level characterization of such regenerators is difficult due to their microporous structure. By direct numerical simulation the hydrodynamic parameters were found for a microporous structure idealized as a metallic fill powder common to some recent cryocooler regenerators that operate at very low temperatures. The intrinsic permeability and the Forchheimer coefficients were found for parameter ranges of interest to pulse tube cryocoolers, and were correlated.Ph.D