Cryogenic Boiling and Two-Phase Flow during Pipe Chilldown in Earth and Reduced Gravity

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.      

Modeling of a horizontal circulation open loop in two-phase helium

In the process of the cryogenic cooling system design of the superconducting magnet of the R³B spectrometer, heat and mass transfer in a two-phase He I natural circulation loop with a horizontal heated section has been investigated experimentally. The experiments were conducted on a 2 m high experimental loop with a copper tube of 10 mm inner diameter uniformly heated over a length of 4 m. All data were obtained near atmospheric pressure. Evolution of the mass flow rates as a function of heat flux in steady state condition are presented and compared to a numerical model that have been developed to assist the design of such a cooling scheme. The model is based on a one-dimensional equations system, which includes mass, momentum and energy balances. It is based on the homogeneous model with a specific friction coefficient for the horizontal heated section. The model reproduces with an acceptable accuracy the experimental results and now serves as a tool for the design.     

Two-Phase Flow and Heat Transfer During Chilldown of a Simulated Flexible Metal Hose Using Liquid Nitrogen

For many industrial, medical and space technologies, cryogenic fluids play irreplaceable roles. When any cryogenic system is initially started, it must go through a transient chill down period prior to normal operation. Chilldown is the process of introducing the cryogenic liquid into the system, and allowing the system components to cool down to several hundred degrees below the ambient temperature. The chilldown process is an important initial stage before a system begins functioning. The objective of this paper is to investigate the chilldown process associated with a flexible hose that was simulated by a channel with saw-teeth inner wall surface structure in the current study. We have investigated the fundamental physics of the two-phase flow and quenching heat transfer during cryogenic chilldown inside the simulated flexible hose through flow visualization, data measurement and analysis. The flow pattern developed inside the channel was recorded by a high speed camera for flow pattern investigation. The experimental results indicate that the chilldown process that is composed of unsteady vapor-liquid two-phase flow and phase-change heat transfer is modified by the inner wall surface wavy structure. Based on the measurement of the channel wall temperature, the teeth structure and the associated cavities generally reduce the heat transfer efficiency compared to the straight hose. Furthermore, based on the measured data, a complete series of correlations on the heat transfer coefficient for each heat transfer regime was developed and reported.     

Demonstration of liquid nitrogen wicking using a multi-layer metallic wire cloth laminate

Cryogenic heat transport devices are the most basic and critical component for the thermal integration between the cryogenic component and its cooling source. In space environments, containment of heat transfer fluid inside a capillary structure is critical due to the absence of gravity. Cryogenic heat pipes using the capillary force for circulation may provide a solution for heat transfer in space applications due to its independence of gravity and transport distance. To achieve a high effective capillary performance, several options of wicking structures have been investigated. An efficient wicking flow of liquid nitrogen is demonstrated with a sintered, multi-layer, porous lamination of metal wire (pore size as low as 5 μm) in an open cryogenic chamber. The test data are presented in this paper. This technology has potential for use in development of improved cryogenic heat transfer devices and containment of cryogenic propellants under micro-gravity environment.     

Wrapped multilayer insulation design and testing

New vehicles need improved cryogenic propellant storage and transfer capabilities for long duration missions. Multilayer insulation (MLI) for cryogenic propellant feedlines is much less effective than MLI tank insulation, with heat leak into spiral wrapped MLI on pipes 3–10 times higher than conventional tank MLI. Better insulation for cryogenic feed lines is an important enabling technology that could help NASA reach cryogenic propellant storage and transfer requirements. Improved insulation for Ground Support Equipment could reduce cryogen losses during launch vehicle loading. Wrapped-MLI (WMLI) is a high performance multilayer insulation using innovative discrete spacer technology specifically designed for cryogenic transfer lines and Vacuum Jacketed Pipe (VJP) to reduce heat flux.The poor performance of traditional MLI wrapped on feed lines is due in part to compression of the MLI layers, with increased interlayer contact and heat conduction. WMLI uses discrete spacers that maintain precise layer spacing, with a unique design to reduce heat leak. A Triple Orthogonal Disk spacer was engineered to minimize contact area/length ratio and reduce solid heat conduction for use in concentric MLI configurations.A new insulation, WMLI, was developed and tested. Novel polymer spacers were designed, analyzed and fabricated; different installation techniques were examined; and rapid prototype nested shell components to speed installation on real world piping were designed and tested. Prototypes were installed on tubing set test fixtures and heat flux measured via calorimetry. WMLI offered superior performance to traditional MLI installed on cryogenic pipe, with 2.2 W/m² heat flux compared to 26.6 W/m² for traditional spiral wrapped MLI (5 layers, 77–295 K). WMLI as inner insulation in VJP can offer heat leaks as low as 0.09 W/m, compared to industry standard products with 0.31 W/m. WMLI could enable improved spacecraft cryogenic feedlines and industrial hot/cold transfer lines.     

Impact of cooling condition and filling ratio on heat transfer limit of cryogenic thermosyphon

In this study, the heat transfer limits of two cryogenic thermosyphons with different cooling conditions and filling ratios are experimentally studied and discussed. The cryogenic thermosyphons are fabricated with the same inner structures and their heat transfer performances are tested. The heat transfer limit of the cryogenic thermosyphon can reach 180.0 W through improving the cooling condition at moderate filling ratios. Meanwhile, it is found that the dry-out limit occurs not only at low filling ratios, but also at high filling ratios in the case of poor cooling condition. The mechanism behind the dry-out limit at high filling ratios is analyzed and the critical heat flux is predicted by a model that describes the heat and mass balance of the working fluid. A fluctuating period is observed in the vicinity of the boiling limit, and the critical heat flux corresponding to the boiling limit is predicted by an empirical correlation.     

EDU liquid acquisition device outflow tests in liquid hydrogen: Experiments and analytical modeling

This paper presents experiments and modeling of the most recent set of liquid acquisition device (LAD) vertical outflow tests conducted in liquid hydrogen. The Engineering Development Unit (EDU) was a relatively large tank (4.25 m³) used to mimic a storage tank for a cryogenic storage and transfer flight demonstration test. Six 1-g propellant tank outflow tests were conducted with a standard 325 × 2300 rectangular cross-section curved LAD channel conformal to the tank walls over a range of tank pressure (158–221 kPa), ullage temperature (22–39 K), and mass flow rate (0.0103–0.0187 kg/s) per arm. An analytical LAD channel solver, an exact solution to the Navier-Stokes equations, is used to model propellant outflow for the LAD channel. Results shows that the breakdown height of the LAD is dominated by liquid and ullage gas temperatures, with a secondary effect of flow rate. The best performance is always obtained by exposing the channel to cold pressurant gas and low flow rates, consistent with the cryogenic bubble point model. The model tracks the trends in the data and shows that the contribution of flow-through-screen pressure drop is minimized for bottom outflow in 1-g, versus the standard inverted outflow.     

Eulerian–Lagrangian method for three-dimensional simulation of fluidized bed coal gasification

A comprehensive three-dimensional numerical model has been developed to simulate the coal gasification in a fluidized bed gasifier. The methodology is based on the multiphase particle-in-cell (MP-PIC) model, which uses an Eulerian method for fluid phase and a discrete particle method for particle phase. Dense particulate flow, mass and heat transfer, homogeneous and heterogeneous chemistry between phases and within the fluid mixture are considered. The dynamics of the particle phase is calculated by solving a transport equation for the particle distribution function (PDF) f. Particle collisions and chemical reactions are solved on a grid cell with particle properties mapped from discrete particles to the grid. Solid mass consumed or produced in reactions changes the size of particles. Simulations were carried out in a coal gasifier with a height of 2.0 m and a diameter of 0.22 m at atmosphere. The calculated product gas compositions compare well with the experimental data. The formation of flow patterns, profiles of particle species and gas compositions, distributions of reaction rates and consumption of carbon mass were investigated under different operating conditions.     

Experimental investigation of forced flow boiling of nitrogen in a horizontal corrugated stainless steel tube

Experimental investigation is performed on the heat transfer characteristics of forced flow boiling of saturated liquid nitrogen (LN₂) in a horizontal corrugated stainless steel tube with a 17.6 mm maximum inner diameter. The local heat transfer coefficients (HTCs) are measured at two mass flow rates with a wide range of wall heat fluxes. The effects of the heat flux, mass flow flux and vapor quality on the two-phase heat transfer characteristics are discussed. The results reveal that the local HTCs increase with the heat flux and mass flow flux. The measured local HTCs present a strong dependence on the heat flux. The circumferential averages of the HTCs for the present corrugated tube are compared with the empirical correlations proposed for the smooth tubes, and the results show that the heat transfer is enhanced due to the area augmentation.     

Investigation of two-phase heat transfer coefficients of argon–freon cryogenic mixed refrigerants

Mixed refrigerant Joule Thomson refrigerators are widely used in various kinds of cryogenic systems these days. Although heat transfer coefficient estimation for a multi-phase and multi-component fluid in the cryogenic temperature range is necessarily required in the heat exchanger design of mixed refrigerant Joule Thomson refrigerators, it has been rarely discussed so far. In this paper, condensation and evaporation heat transfer coefficients of argon–freon mixed refrigerant are measured in a microchannel heat exchanger. A Printed Circuit Heat Exchanger (PCHE) with 340 μm hydraulic diameter has been developed as a compact microchannel heat exchanger and utilized in the experiment. Several two-phase heat transfer coefficient correlations are examined to discuss the experimental measurement results. The result of this paper shows that cryogenic two-phase mixed refrigerant heat transfer coefficients can be estimated by conventional two-phase heat transfer coefficient correlations.     

Pressure drop reduction and heat transfer deterioration of slush nitrogen in triangular and circular pipe flows

Slush fluids such as slush hydrogen and slush nitrogen are characterized by superior properties as functional thermal fluids due to their density and heat of fusion. In addition to allowing efficient hydrogen transport and storage, slush hydrogen can serve as a refrigerant for high-temperature superconducting (HTS) equipment using MgB₂, with the potential for synergistic effects. In this study, pressure drop reduction and heat transfer deterioration experiments were performed on slush nitrogen flowing in a horizontal triangular pipe with sides of 20 mm under the conditions of three different cross-sectional orientations. Experimental conditions consisted of flow velocity (0.3–4.2 m/s), solid fraction (0–25 wt.%), and heat flux (0, 10, and 20 kW/m²). Pressure drop reduction became apparent at flow velocities exceeding about 1.3–1.8 m/s, representing a maximum amount of reduction of 16–19% in comparison with liquid nitrogen, regardless of heating. Heat transfer deterioration was seen at flow velocities of over 1.2–1.8 m/s, for a maximum amount of deterioration of 13–16%. The authors of the current study compared the results for pressure drop reduction and heat transfer deterioration in triangular pipe with those obtained previously for circular and square pipes, clarifying differences in flow and heat transfer properties. Also, a correlation equation was obtained between the slush Reynolds number and the pipe friction factor, which is important in the estimation of pressure drop in unheated triangular pipe. Furthermore, a second correlation equation was derived between the modified slush Reynolds number and the pipe friction factor, enabling the integrated prediction of pressure drop in both unheated triangular and circular pipes.     

Experimental investigation on the performance of a neon cryogenic oscillating heat pipe

An experimental investigation is conducted to study the performance of a cryogenic oscillating heat pipe (OHP) using neon as the working fluid. The stainless steel OHP with an inner diameter of 0.9 mm has 4 turns, and the lengths of the evaporator, condenser section and adiabatic section are 35 mm, 35 mm and 95 mm, respectively. The temperature of the evaporator and condenser and the pressure of the OHP are measured. The results show that the cooling down process of the OHP from room temperature to the working temperature can be significantly accelerated by charging with neon. During the pseudo steady-state operation process, the temperature of evaporator and the pressure of the OHP increase with increasing heat input. When the dry out appears, the temperature of evaporator rises quickly, and the pressure of the OHP drops sharply. In addition, the effective thermal conductivity of the OHP at the different heat inputs and the different filling ratios is calculated. It increases with increasing heat input, and there exists an optimum filling ratio which makes the maximum effective thermal conductivity. For this OHP, the optimum filling ratio is 24.5%, at which the effective thermal conductivity is 6100–22,180 W/m K.     

Experimental investigation of heat transfer enhancement in helical coil heat exchangers using water based CuO nanofluid

The present work deals with the study of heat transfer enhancement using water based CuO nanofluids in the helical coil heat exchanger. Nanofluids were prepared using two-step method by using wet chemical method. Nanofluids with various volume percentage between 0 and 0.5 of CuO nanoparticles and their flow rate between 30 and 80 LPH (Reynolds number ranging from 812 to 1895, Laminar flow regime) were considered in the present study. The setup consists of a test section (helical coil), cooler, reservoir, pump, flow meter, thermocouples and flow controlling system. The temperature measurements were carried out with the help of thermocouples. The investigation was carried out to study the effect of particle loading and flow rate on heat transfer coefficient and Nusselt number. It has been found that the increase in the loading of CuO nanoparticles in base fluid shows a significant enhancement in the heat transfer coefficient of nanofluid. In the present study, at 0.1 vol% concentration of CuO nanoparticles in nanofluid, enhancement in heat transfer coefficient was 37.3% as compared to base fluid while at 0.5 vol%, it is as high as 77.7%. Also with the increase in the flow rate of the CuO nanofluid, significant increase in heat transfer coefficient was observed.     

Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Validation of an EcosimPro® model for the assessment of two heat load smoothing strategies in the HELIOS experiment

HELIOS experiment, installed at CEA Grenoble, is a scaled down helium loop for investigating high pulsed loads on superconducting magnet cooling circuits of Tokamak. Heat loads have to be smoothed down in order to ensure refrigerator stability. A real time simulation is of interest for reproducing the thermohydraulic phenomenon observed experimentally. The modelling work has been carried out with EcosimPro simulation software combined with a specific cryogenic library. Existing components were modified with additional features, particularly for taking into account 1D fluid transport. The model comprises a closed loop with forced flow supercritical helium at 4.4 K and 5 bar connected through heat exchangers to a saturated helium bath at 1.1 bar. The new heat load mitigation strategies presented in this paper are based on two kinds of regulations in order to smooth the helium mass flow retuning to the refrigerator. First strategy uses the bath as a thermal buffer by acting on bath outlet control valve. Second one uses the variation of the circulator speed to induce a delay in the arrival of the heat loads into the helium bath. The model with the two controls is validated against comparisons with experimental data.     

Cryogenic capillary screen heat entrapment

Cryogenic liquid acquisition devices (LADs) for space-based propulsion interface directly with the feed system, which can be a significant heat leak source. Further, the accumulation of thermal energy within LAD channels can lead to the loss of subcooled propellant conditions and result in feed system cavitation during propellant outflow. Therefore, the fundamental question addressed by this program was: “To what degree is natural convection in a cryogenic liquid constrained by the capillary screen meshes envisioned for LADs?” Testing was first conducted with water as the test fluid, followed by liquid nitrogen (LN₂) tests. In either case, the basic experimental approach was to heat the bottom of a cylindrical column of test fluid to establish stratification patterns measured by temperature sensors located above and below a horizontal screen barrier position. Experimentation was performed without barriers, with screens, and with a solid barrier. The two screen meshes tested were those typically used by LAD designers, 200 × 1400 and 325 × 2300, both with Twill Dutch Weave. Upon consideration of both the water and LN₂ data, it was concluded that heat transfer across the screen meshes was dependent upon barrier thermal conductivity and that the capillary screen meshes were impervious to natural convection currents.     

Studies on the processing of nickel base porous wicks for capillary pumped loop for thermal management of spacecrafts

Nickel base sintered porous wicks have potential application as capillary structure in two-phase heat transfer loops of a heat dissipation system, like capillary pumped loop (CPL) and loop heat pipe (LHP). A porous wick is located inside the evaporator of the CPL system and it transports working fluid in the loop by capillary action.In the present work, experimental trials were carried out to achieve porous wick having high porosity with interconnected pores of average size less than 5 μm, higher aspect ratio (L/D >10) and permeability better than 10 m-Darcy (10^?14 m²). A carbonyl nickel powder (2–7 μm) was used as raw material. Loose carbonyl nickel powders (2–7 μm) were sintered in graphite mould under hydrogen atmosphere at different temperatures in order to optimize porosity, pore size and permeability of the sintered wick. For mechanical and physical properties characterization, samples were cut from sintered rod using EDM wire cut to avoid pore closure. Profile making on the sintered rod is also done by wire EDM. Microstructural characterization as well as the effect of W-EDM on the surface pores was done using Scanning Electron Microscopy (SEM). Surface profile making through W-EDM had shown encouraging result. After optimization of process parameters cylindrical wick (L/D ratio: 10) with porosity of 64%, average pore size of 5 μm and a permeability of 70 m-Darcy could be realized.The present paper explain the details of processing of cylindrical shaped porous wicks through sintering technique and effect of EDM on surface pores characteristic.     

Cryogenic helium gas circulation system for advanced characterization of superconducting cables and other devices

A versatile cryogenic test bed, based on circulating cryogenic helium gas, has been designed, fabricated, and installed at the Florida State University Center for Advanced Power Systems (FSU-CAPS). The test bed is being used to understand the benefits of integrating the cryogenic systems of multiple superconducting power devices. The helium circulation system operates with four sets of cryocooler and heat exchanger combinations. The maximum operating pressure of the system is 2.1 MPa. The efficacy of helium circulation systems in cooling superconducting power devices is evaluated using a 30-m-long simulated superconducting cable in a flexible cryostat. Experiments were conducted at various mass flow rates and a variety of heat load profiles. A 1-D thermal model was developed to understand the effect of the gas flow parameters on the thermal gradients along the cable. Experimental results are in close agreement with the results from the thermal model.     

Magnetic field effects on force convection flow of a nanofluid in a channel partially filled with porous media using Lattice Boltzmann Method

In this paper the Lattice Boltzmann Method (LBM) is utilized to investigate the effects of uniform vertical magnetic field on the flow pattern and fluid–solid coupling heat transfer in a channel which is partially filled with porous medium. Al₂O₃–water nanofluid as a work fluid with temperature sensitive properties is forced to flow into the channel while the top and bottom walls of the channel is heated and kept at a constant temperature. In the present study, with respect to previous works and experimental data, a new correlation is presented for density of Al₂O₃–water nanofluid as a function of temperature. The result also shows that the step approximation which is used for the complex boundaries of porous medium is reliable. Finally, the effect of various volume fractions of nanoparticles (ϕ = 0%, 3%, 5% and 7%) and different magnitude of magnetic field (Ha = 0, 5, 10 and 15) on the rate of heat transfer are thoroughly explored. In accordance with the results, by raising the nanoparticle volume fraction, average temperature and velocity at the outlet of the channel increase and the average Nusselt number rises dramatically. In addition, the increase the Hartmann number leads to the slow growth in the average Nusselt number, although the outlet average temperature and velocity shows a little drop.