SPRAY CHARACTERIZATION, CRITICAL HEAT FLUX AND FLOW BOILING PHYSICS OF CRYOGENS UNDER TERRESTRIAL AND MICROGRAVITY CONDITIONS

With the growing interest in space exploration, cryogenic technologies involving two-phase flow and heat transfer are in high demand to successfully procure advanced space applications such as fuel depots and nuclear thermal propulsion (NTP) systems for deep space missions. However, the unique and extreme thermal properties of cryogenic fluids introduce distinct flow boiling fluid physics and energy transport phenomena, which differ significantly from those observed with conventional fluids. Understanding the unique two-phase physics in cryogenic flow boiling remains an ongoing challenge. Furthermore, the lack of readily available microgravity cryogenic steady-state heat transfer data hinders the assessment of gravitational effects on cryogenic flow boiling. This study aims to elucidate the fundamental two-phase flow and heat transfer physics of cryogenic flow boiling system by conducting (i) experimental, (ii) theoretical/empirical, and (iii) computational investigations of cryogenic flow boiling using liquid nitrogen (LN2) under various gravity environments, encompassing terrestrial, partial, and microgravity conditions.The first part of study presents detailed measurements of droplet diameter and velocity for liquid nitrogen (LN₂) sprays discharged into ambient air. Using a high-resolution Phase Doppler Particle Analyzer (PDPA), both mean values and statistical distributions of droplet size and velocity were obtained. Five full-cone spray nozzles were tested across a range of injection pressures. PDPA data were used to develop new correlations for Sauter Mean Diameter (d₃₂), Arithmetic Mean Diameter (d₁₀), and Mean Droplet Velocity (uf). Results show that increasing injection pressure leads to higher uf but lower d₃₂ values. Evaluation of existing d₃₂ correlations primarily developed for water sprays revealed significant predictive errors when applied to LN₂ data. To address this, new dimensionless correlations were derived for d₃₂, d₁₀, and uf, incorporating Reynolds and Weber numbers. These correlations demonstrated strong predictive performance when validated against the experimental data. The spray cone angle was found to be relatively insensitive to injection pressure but consistently smaller than the manufacturer's specified angle for water sprays. This reduction in cone angle is attributed not only to differences in thermo physical properties such as viscosity, surface tension, and latent heat of vaporization but also to ambient heat transfer causing evaporation at the spray's periphery, where droplet density is lowest. The newly developed LN₂ correlations provide valuable predictive tools essential for the design and optimization of cryogenic spray systems, particularly in future space applications. This section further explores the effect of non-flashing conditions on spray distribution.The second part of this study investigates 1-ge horizontal flow boiling of liquid nitrogen with a near-saturated inlet along a circular tube of dimensions 8.5-mm inner diameter and 680-mm heated length. Experiments were conducted using a payload developed for eventual parabolic flight experiments. The operating parameters varied are mass velocity of 406.76–1572.77 kg/m2s, inlet quality of -0.05 to -0.01, and inlet pressure of 336.29–493.07 kPa. High speed video recordings are presented to explain two-phase flow structure and regime transitions which are visualized through a transparent tube in a visualization section situated downstream of the heated tube. Recognized flow patterns are bubbly, plug, slug, stratified annular, and annular. Heat transfer results are presented and discussed in terms of flow boiling curve trends. The gravitational effects on two-phase fluid physics and heat transfer by conducting the first-ever experimental measurement of cryogenic flow boiling performance using a steady-state heated method in a reduced gravity environment. Parabolic flight experiments were performed to acquire both heat transfer measurements and high- speed video of interfacial behaviors, under varying gravity levels (microgravity, hyper gravity, lunar gravity, and martian gravity). The experiments involved flow boiling of liquid nitrogen (LN2) with a near-saturated inlet along a circular heated tube of dimensions 8.5-mm inner diameter and 680-mm heated length. The operating parameters varied are mass velocity of 398.3 – 1342.8 kg/m2s, inlet quality of -0.08 to -0.01, and inlet pressure of 413.68 – 689.48 kPa. Captured microgravity flow patterns range from bubbly to annular, all having vapor structures that are larger than those under higher gravity levels. Under microgravity, absence of buoyancy yields symmetrical vapor structures without flow stratification, laying a physical foundation for the distinct two-phase heat transfer trends during LN2 flow boiling in microgravity.For the last part of the study, this paper presents an investigation into critical heat flux (CHF) and boiling during liquid nitrogen (LN2) spray cooling of the heated aluminum surface in terrestrial and microgravity environments. For the investigations, the cryogenic pressure chamber was configured to house aluminum nitride (AlN) heaters that provided heat flux and temperature was measured of the heated surface using type E thermocouples that were calibrated. Visualization of the spray and boiling phenomena, including the identification of the mechanisms that determine critical heat flux, was also conducted. For the identification of the boiling behavior, the full-cone sprays generated by the Spraying Systems TG0.7 and TG2 nozzles were utilized in the experiment to generate an array of pressure differentials (∆P) between the injectors and the chamber. For terrestrial gravity, the critical heat flux increases monotonically then starts reducing as more pressure differential is applied, which shows that after some pressure values, the inertia effect of the liquid flux is supplemented by the formation of a hard film on the surface reduces the effect. CHF value is similar in the microgravity tests performed during the parabolic flight tests (comprising 60 parabolas over two flight days) when compared with the results at similar ΔP in terrestrial tests, which indicates that the contribution of buoyancy forces is negligible in comparison with the effects of spray momentum in maintaining the wetting and the removal of the vapor created in the surface. CHF was detected in the periphery of the surface with the lowest volume flux, with the increase in the heat flux leading to the transition from the steady-state film evaporation to the peripheral dry out in a transient state before the attainment of CHF. It has been observed that LN₂ spray CHF essentially depends on the hydrodynamics based on the spray nozzle and the thermodynamic conditions at the inlet.