Chilldown Process Research Articles

Investigation of Cryogenic Chilldown in a Complex Channel Under Low Gravity Using a Sounding Rocket

To realize high-performance cryogenic propulsion systems, the chilldown sequence has to be improved. Because the chilldown is carried out under low gravity, the effect of gravity on the two-phase flow, especially at low flow rate, should be investigated. To understand the physics under low gravity, an experiment was conducted using a sounding rocket. Two identical test sections with different mass flow rates simulated part of a turbopump, each of which has a complex flowpath including slits and a dead end. Using liquid nitrogen, the flight experiment obtained data of temperatures, pressures, void fractions, and video frames of liquid motion. Then, the flight experiment data were compared to the ground data taken under normal gravity, revealing that the slits played an important role in the chilldown process and that the test sections were quickly chilled down under low gravity. The slits of the test sections formed liquid jets, and their behaviors were different from those in the ground experiment. In the flight experiment, the jets easily reached the dead end of the test sections and cooled down the whole walls due to the increase in inertia and wettability; however, such behaviors were hardly observed in the ground experiment. The difference between the ground and flight is significant at lower flow rate.

Transient modeling of cryogenic two-phase flow boiling during chill-down process

Accurate predictions of chill-down time and propellant consumption as well as a clear understanding of the performance and characteristics of cryogenic pipeline chill-down are significant for achieving the safe and high-efficiency cryogenic propellants fueling. On the basis of available nitrogen experimental data, detailed mechanism analysis and comprehensive verification are carried out for three characteristic points and four heat transfer regimes respectively, and a group of correlations for cryogenic pipeline chill-down are proposed. Besides, an accurate chill-down model involving intricate interaction of phase change, heat transfer and fluid dynamics is established with quasi-steady state method. To further reveal the unknown mechanism and phenomena of chill-down boiling, flow regime and heat transfer performance at typical moments and wall temperature drop characteristics at typical locations are studied respectively, which provide feasibility for more efficient refueling and arouse concern for potential safety hazard.

Numerical modeling of flow film boiling in cryogenic chilldown process using the AIAD framework

Flow film boiling plays a dominant role in cryogenic chilldown process, which involves complicated heat transfer and flow regime transition. Nevertheless, existing researches about flow film boiling with cryogenic fluids are relatively limited. In this study, a Computational Fluid Dynamics (CFD) model based on a wall heat flux partition algorithm is built. The AIAD framework implemented in the two-fluid model is employed to appropriately calculate the drag force on the liquid-vapor interfaces. The CFD model is validated by the satisfactory coincidence between the simulated heat fluxes and experimental data in literature. Accordingly, the two-phase interaction on the flow regime and heat transfer is further investigated. The results reveal that the vapor film beneath the bulk liquid becomes thinner due to the drag force on the liquid-vapor interface. In addition, FFT analysis on the pressure drop shows that dominant frequency of the interfacial waves in the tube mainly locates around 2.8 Hz. The normalized intensity indicates that fluctuation becomes more violent with the increase of superheat and inlet liquid flow rate. Finally, comparison between correlations and experimental data indicates that a correlation of heat transfer coefficient considering both film boiling effect and forced convective flow effect needs to be proposed.

Experimental research and numerical simulation on cryogenic line chill-down process

The empirical heat transfer correlations are suggested for the fast cool down process of the cryogenic transfer line from room temperature to cryogenic temperature. The correlations include the heat transfer coefficient (HTC) correlations for single-phase gas convection and film boiling regimes, minimum heat flux (MHF) temperature, critical heat flux (CHF) temperature and CHF. The correlations are obtained from the experimental measurements. The experiments are conducted on a 12.7 mm outer diameter (OD), 1.25 mm wall thickness and 7 m long stainless steel horizontal pipe with liquid nitrogen (LN2). The effect of the lengthwise position is verified by measuring the temperature profiles in near the inlet and the outlet of the transfer line. The newly suggested heat transfer correlations are applied to the one-dimensional homogeneous transient model to simulate the cryogenic line chill-down process, and the chill-down time and the cryogen consumption are well predicted in the mass flux range from 26.0 kg/m2 s to 73.6 kg/m2 s through the correlations.

Improvement in pipe chilldown process using low thermal conductive layer

A method for reducing the time and total mass of cryogenic fluid required for a chilldown process in piping was experimentally investigated in this study. The inner wall of a pipe with an outer diameter of 1/4″ (=6.35mm) was coated with Polytetrafluoroethylene, which has a low thermal conductivity. Liquid nitrogen (LN2) was supplied to the pipe at a constant tank pressure of 120–170kPa. The fluctuations of the two-phase flow, which were composed of LN2 and gas phase nitrogen, were observed. A pipe without an insulating layer and three other pipes with insulating layers of thicknesses 23μm, 63μm, and 91μm, respectively, were used in the experiment. The results indicated that the temperature of the minimum heat flux point (MHF) was higher for the pipe with the insulating layer. This increased temperature caused earlier transition to nucleate boiling. Furthermore, the total mass of LN2 consumed in the chilldown process could be retrenched up to a maximum of 64%. The heat flux decreased after reaching the MHF point; however, heat flux after MHF point is not dominant to overall chilldown time. The effect of the layer to increase the temperature of MHF point is dominant to overall chilldown time, which results in the decrease in the chilldown time and the total mass of LN2 consumed in the chilldown process.

Experimental investigation on chill-down process of cryogenic flow line

This paper describes the cryogenic chill-down experiments that are conducted on a 12.7mm outer diameter, 1.25mm wall thickness and 7m long stainless steel horizontal pipe with liquid nitrogen (LN2). The pipe is vacuum insulated during the experiment to minimize the heat leak from room temperature and to enable one to numerically simulate the process easily. The temperature and the pressure profiles of the chill-down line are obtained at the location which is 5.5m in a distance from the pipe inlet. The mass flux range is approximately from 19kg/m2s to 49kg/m2s, which corresponds to the Reynolds numbers range from 1469 to 5240. The transient histories of temperature, pressure and mass flow rate during the line chill-down process are monitored, and the heat transfer coefficient and the heat flux are computed by an inverse problem solving method. The amplitude of the pressure oscillation and the oscillating period become larger and longer at higher pressure conditions. In the low mass flux conditions, the critical heat flux in horizontal pipes is not sensitive to mass flux, and is higher than that in vertical pipes. Kutateladze’s correlation with the constant coefficient, B=0.029, well matches the experimental data in the current work. In nucleate flow boiling regime, heat transfer coefficient, h, is proportional to (q″)n, and n is equal to 0.7.

Development and validation of an analytical charge–hold–vent model for cryogenic tank chilldown

To enable future in-space cryogenic fuel depots, efficient methods are required for the tank chilldown and transfer process to minimize the amount of consumed propellant. This paper presents a transient charge–hold–vent (CHV) analytical model which is compared against the single set of liquid hydrogen tank chilldown data. The model solves conservation of mass and energy, uses quasi-unsteady steps in time to simulate natural convection and jet impingement heat transfer, and tracks two-phase fluid thermodynamic properties as the chilldown process evolves. The additional cooling potential made available through a cyclic tank venting process is also calculated in the code. The flexible code can simulate multiple CHV cycles, be used to optimize tank geometry and nozzle location, and provide predictions on the amount of propellant mass used for each cycle. This model compares within 7% of the liquid hydrogen tank chilldown experimental data.

Pressure Surge During Cryogenic Feedline Chilldown Process

Cryogenic fluid entering a warm feedline absorbs heat and undergoes rapid flash evaporation leading to pressure surges, which can retard the flow inside the feedline. It may have serious repercussion in operation of the rocket engine during start up. Experimental and numerical studies are carried out to examine the effect of inlet pressure and initial feedline temperature on pressure surges. An analytical model using sinda/fluint software is developed to investigate this complex two-phase flow phenomenon including the various boiling regimes that exist during line chilling. The numerical study is carried out considering 1D flow through a cryogenic feedline of 2.47 m long and 0.01 m inner diameter with liquid nitrogen at 77.3 K as working fluid. Predictions are made for the inlet pressure in the range of 0.28–0.76 MPa and initial wall temperature of 200 K and 300 K. Subsequently, an experimental test rig is setup and the model is validated with the experimental data. The studies show that within the range of parameter considered, the magnitude of pressure surge increases exponentially with increase in inlet pressure and decreases with the prechilling of feedline.

RECENT PROGRESS ON EXPERIMENTAL RESEARCH OF CRYOGENIC TRANSPORT LINE CHILLDOWN PROCESS

Chilldown or quenching is a complicated process that initiates the cryogenic fluid line transport, and it involves unsteady two-phase heat and mass transfer. To advance our understanding of this process, we have reviewed recent experimental investigations. The chilldown process can be generally divided into three regimes: film boiling, transition boiling and nucleate boiling, and each regime is associated with a different flow pattern and heat transfer mechanism. Under low flow rate conditions, it is concluded that the two-phase flow regime is dispersed flow in the film boiling regime. The dispersed liquid phase is in the form of long filaments as the transport line is chilled down, and the vapor phase is generally superheated. Microgravity experiment for line chilldown was conducted basically in the film boiling region. Bottom wall heat flux was found to decrease under microgravity condition. Under the experimental condition, the gravity effect does not show a strong dependence on wall temperature and inlet flow rate.

Comparison of cryogenic flow boiling in liquid nitrogen and liquid hydrogen chilldown experiments

This paper presents a comparison between experimental results from recent liquid hydrogen (LH2) transfer line chilldown experiments at high Reynolds (Re) numbers versus liquid nitrogen (LN2) experiments conducted at low Re numbers. Parasitic heat leak, inner wall temperatures, inner wall heat fluxes, and heat transfer coefficients are computed to compare between the two systems. Analysis of temperature traces and flow visualization indicates that the chilldown process evolves much more rapidly at higher Re numbers due to a quick transition from vapor flow to annular liquid flow and near immediate liquid contact along the pipe walls. The lower kinematic viscosity and surface tension of LH2, along with reduced parasitic heat leak and higher Re numbers relative to the LN2 experiments, causes chilldown to proceed almost immediately into the nucleate boiling regime, in comparison to low Re flows where >75% of the chilldown is spent in vapor film boiling.

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.

An experimental study on liquid nitrogen pipe chilldown and heat transfer with pulse flows

In order to provide the optimized chilldown strategy for a cryogenic transport line, a study on pulse flow chilldown was conducted. A comparison between continuous flow and pulse flow patterns on heat transfer characteristics was presented to reveal the effect of flow oscillation. Liquid nitrogen was used as the working fluid and upward flows with Reynolds numbers ranging from 2500 to 7000 in a round tube were examined. To gain a better understanding of the effect of flow pulsation on the chilldown process, different square wave pulse flow strategies were examined. Analysis of the ability of different flow patterns to chill down transport lines indicated that there is no single best strategy for all boiling regimes and Reynolds numbers. In terms of the efficiency, the continuous flow is optimal for most boiling regimes at low Reynolds numbers. On the other hand, pulse flow is ideal in the film boiling regime at higher Reynolds numbers. In terms of the total efficiency, continuous flows with high Reynolds numbers are generally more efficient at transferring heat than other patterns or those of lower Reynolds numbers. If a short chilldown duration is a priority, the continuous flow consistently offers the quickest chilldown times. A general guideline for chilldown strategy is suggested as a conclusion.

An experimental study on flow patterns and heat transfer characteristics during cryogenic chilldown in a vertical pipe

In the present paper, the experimental results of a cryogenic chilldown process are reported. The physical phenomena involve unsteady two-phase vapor–liquid flow and intense boiling heat transfer of the cryogenic fluid that is coupled with the transient heat conduction inside pipe walls. The objective for the present study is to compare the chilldown rates and flow patterns between the upward flow and downward flow in a vertical pipe. Liquid nitrogen is employed as the working fluid and the test section is a vertical straight segment of a Pyrex glass pipe with an inner diameter of 8mm. The effects of mass flow rate on the flow patterns, heat transfer characteristics and interface movement were determined through experiments performed under several different mass flow rates. Through flow visualization, measurement and analysis on the flow patterns and temperature variations, a physical explanation of the vertical chilldown is given. By observing the process and analyzing the results, it is concluded that pipe chilldown in a vertical flow is similar to that in microgravity to some extent.

Numerical modeling of cryogenic chilldown process in terrestrial gravity and microgravity

In this paper, a numerical model is developed to predict the cryogenic chilldown process of a vertical tube for both terrestrial and microgravity conditions. The flow field is covered by four distinct regions, which are single-phase vapor region, dispersed flow region, inverted annular flow region, and single-phase liquid region. Heat transfer mechanisms are dictated by the flow patters. A two-fluid model is employed to analyze the dispersed flow region and the inverted annular film boiling region. Gravity effect on the chilldown process is also investigated. Model results indicate that film boiling heat transfer decreases with decreasing gravity level for the bottom flooding condition. The model results show a good agreement with the experimental data.

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.

Cryogenic chilldown process under low flow rates

In this paper, we investigated the chilldown process of a horizontal tube by a liquid nitrogen flow with low mass fluxes. The flow patterns and heat transfer characteristics are studied experimentally. Visualization results illustrate the successive states of liquid–wall interaction with different mass fluxes. For a horizontal tube, the liquid filament–wall interaction is an important contributor to the heat transfer on the bottom wall while the upper wall is cooled by forced convection with superheated vapor. A phenomenological model is developed, in which the heat transfer on the bottom wall is composed of film boiling and forced convection while for the upper wall, forced convection is the only mechanism of heat transfer. A good agreement is achieved between the model prediction and the experimental result.

One-dimensional thermohydraulic code THESEUS and its application to chilldown process simulation in two-phase hydrogen flows

THESEUS is a thermohydraulic code for the calculation of steady state and transient processes of two-phase cryogenic flows. The physical model is based on four conservation equations with separate liquid and gas phase mass conservation equations. The thermohydraulic non-equilibrium is calculated by means of evaporation and condensation models. The mechanical non-equilibrium is modelled by a full-range drift-flux model. Also heat conduction in solid structures and heat exchange for the full spectrum of heat transfer regimes can be simulated. Test analyses of two-channel chilldown experiments and comparisons with the measured data have been performed.

Reliability study of a digital radio : Wilson Acosta. IEEE Proc. Reliab., 199 (1982)

A method for reducing the time and total mass of cryogenic fluid required for a chilldown process in piping was experimentally investigated in this study. The inner wall of a pipe with an outer diameter of 1/4″ (=6.35 mm) was coated with Polytetrafluoroethylene, which has a low thermal conductivity. Liquid nitrogen (LN2) was supplied to the pipe at a constant tank pressure of 120–170 kPa. The fluctuations of the two-phase flow, which were composed of LN2 and gas phase nitrogen, were observed. A pipe without an insulating layer and three other pipes with insulating layers of thicknesses 23 μm, 63 μm, and 91 μm, respectively, were used in the experiment. The results indicated that the temperature of the minimum heat flux point (MHF) was higher for the pipe with the insulating layer. This increased temperature caused earlier transition to nucleate boiling. Furthermore, the total mass of LN2 consumed in the chilldown process could be retrenched up to a maximum of 64%. The heat flux decreased after reaching the MHF point; however, heat flux after MHF point is not dominant to overall chilldown time. The effect of the layer to increase the temperature of MHF point is dominant to overall chilldown time, which results in the decrease in the chilldown time and the total mass of LN2 consumed in the chilldown process.

Engineers for VLSI—a scarce commodity : Ivan R. Saddler. IEEE Trans. Components Hybrids mfg Technol.CHMT-5 (1), 131 (1982)

A method for reducing the time and total mass of cryogenic fluid required for a chilldown process in piping was experimentally investigated in this study. The inner wall of a pipe with an outer diameter of 1/4″ (=6.35 mm) was coated with Polytetrafluoroethylene, which has a low thermal conductivity. Liquid nitrogen (LN2) was supplied to the pipe at a constant tank pressure of 120–170 kPa. The fluctuations of the two-phase flow, which were composed of LN2 and gas phase nitrogen, were observed. A pipe without an insulating layer and three other pipes with insulating layers of thicknesses 23 μm, 63 μm, and 91 μm, respectively, were used in the experiment. The results indicated that the temperature of the minimum heat flux point (MHF) was higher for the pipe with the insulating layer. This increased temperature caused earlier transition to nucleate boiling. Furthermore, the total mass of LN2 consumed in the chilldown process could be retrenched up to a maximum of 64%. The heat flux decreased after reaching the MHF point; however, heat flux after MHF point is not dominant to overall chilldown time. The effect of the layer to increase the temperature of MHF point is dominant to overall chilldown time, which results in the decrease in the chilldown time and the total mass of LN2 consumed in the chilldown process.