Chilldown Process Research Articles

Enhancing pipe chilldown with axial grooves filled with silicone sealant

This study proposes a method to accelerate the chilldown process in pipes using liquid nitrogen flow. Axial grooves were machined along the inner surface of the pipe and then filled with silicone sealant. This approach achieves a shorter chilldown time compared to previously proposed methods. Initial experiments involving pool boiling in liquid nitrogen were conducted to determine the optimal groove width and the ratio of groove-to-sealant area. The shortest chilldown time in pool boiling was achieved when the groove pitch was close to the capillary length. The effect of the copper-to-silicone sealant area ratio on the chilldown process was minimal. The shortest chilldown time was achieved with a surface that had a 2 mm pitch and a copper-to-silicone sealant area ratio of 0.25. The chilldown time was 1/4.4 of that of the bare surface. Pipes with different pitch and groove widths were tested in a flow chilldown experiment. Using the surface texture, the chilldown time of a stainless-steel pipe with an outer diameter of 1/2″ and a length of 120 mm from room temperature to the saturation temperature was reduced to a maximum of 1/3.6. The shortest cooling time is obtained at a groove pitch of 2 mm. In addition to the shorter cooling time, the amount of liquid nitrogen required for chilldown was also reduced.

Experimental and numerical analysis of the chill-down process of a large horizontal cryogenic storage tank

An experimental system is built to investigate the chill-down process of a 60 m3 horizontal cryogenic storage tank. Accordingly, a numerical model of the chill-down process is constructed to calculate the two-phase flow transition and thermal stress distribution. The chill-down processes with flow rates of 35 L/min and 43 L/min are investigated. The results show that the chill-down process of the wall can be divided into three stages, including slow cooling by cryogenic gaseous nitrogen, rapid cooling by boiling liquid nitrogen, and conduction cooling in contact with liquid nitrogen. Moreover, the maximum deformation in horizontal tanks shifts from the middle to the end of the bottom due to the uneven temperature variation during the chill-down process. The study highlights a positive correlation between the temperature gradient and the stress value. As the liquid accumulates, a large circumferential stress appears at the liquid level, resulting in the maximum stress section at the same height as the liquid level. The maximum stress except the constraint is about 248 MPa at a flow rate of 43 L/min. Overall, the analysis of the experimental and simulation results in this work provides useful information and guidance for the safe use of large horizontal cryogenic storage tanks.

Study on Optimization of Chilldown Process for A New Generation Launch Vehicle

Focused on chilldown procedure of liquid oxygen filling for second stage of a new generation launch vehicle, the method of chilldown through liquid oxygen pump has been put forward and compared with traditional method that of gas pressurization. The results showed that it could reduce total time, lower system state changes, and improve efficiency and reliability of liquid oxygen loading operations. Aimed at liquid oxygen flow of chilldown through the pump, the model of liquid oxygen pump operating has been established by considering variable frequency, regulating valve opening and liquid level of vehicle tank. It can provide scientific guidance and basis for flow control of chilldown through the pump and liquid oxygen tests.

Characteristics of Flow Development and Boiling Transitions in the Liquid Oxygen Chill-Down Process in a Straight Horizontal Exit-Contracted Pipe

Liquid oxygen chill-down in a straight horizontal pipe was studied experimentally. The effect of the entrance corner was excluded, and much denser wall temperature sensors along the pipe have been set compared to the present studies. In this way, the chill-down process, as well as the development of the flow pattern, has been drawn for every test. As a result, the mechanism of LO2 chill-down would be obtained for various pressure sections. For cases with stable pressure below 1.25 MPa, liquid rewetting in the pipe is controlled by the propagation of quenching fronts. For cases with a higher pressure, liquid rewetting in the second half of the pipe is controlled by the sudden liquid fill-in. Based on the transition points obtained, heat transfer coefficients on the Leidenfrost point and critical heat flux have been correlated for various pressure sections using new approaches. Conclusions show that the correlation equations are dependent on the chill-down mechanisms.

Variation of Leidenfrost temperature in liquid nitrogen pipe chilldown process and its application in chilldown performance analysis

• A reliable model on Leidenfrost temperature of chilldown is suggested. • Re does not affect Leidenfrost temperature in low-conductivity coating tube chilldown process. • A throughout nucleate boiling-dominated chilldown could be caused by micro-fin tube. • Micro-fin pipe can reduce chilldown time and rapidly takes away thermal energy from metal wall. For a typical cryogenic pipe chilldown event, film boiling plays a dominate role in the view of temperature decrease range and time cost proportion. In the chilldown process, Leidenfrost temperature ( T L ) denotes the termination of film boiling and could be utilized to represent the chilldown progress to a certain extent. Therefore, T L is significant for the cryogenic chilldown but the associated researches are still insufficient. In the present study, an experimental system, adopting liquid nitrogen (LN 2 ) as the working fluid, is built to perform cryogenic chilldown tests under different conditions, and the variation of T L is concerned to assist the chilldown performance analysis. Current predictive models on T L are evaluated and the influences of existing chilldown acceleration approaches are discussed in virtue of T L variation. The results demonstrate that the chilldown test data sourced from different literature appear remarkable discrepancy with one another, and the model proposed by Darr and Hartwig could be suggested to estimate T L . Moreover, significant rise in T L could be observed when a chilldown acceleration approach is adopted, which demonstrates that T L variation could be used to assess the acceleration approach. If a low-conductivity coating layer pipe is adopted, increasing Re does not affect T L significantly. Comparatively, when using a micro-fin pipe in the chilldown event, Re influence on T L is still apparent. Even for the micro-fin case with Δ=50mm, the differences of different boiling regimes are hard to be recognized, and a throughout high heat flux could dominate the wall temperature decrease process. Therefore, balancing consideration of micro-fin structure and inlet Re should be involved in the design of a micro-fin pipe. In addition, although both of the low-conductivity coating layer technique and the micro-fin tube could realize the purpose of chilldown acceleration, the mechanisms and the characteristics are very different. The micro-fin pipe has the advantages of reducing chilldown time as well as rapidly taking away the thermal energy from the metal wall region. The low-conductivity coating layer appears the advantages of facilitating the liquid-wall heat transfer occurrence and reducing the flow resistance, but the existence of the coating layer simultaneously restrains the thermal removal from the metal wall region.

Experimental study on liquid nitrogen pipe chilldown performance under pulse flow conditions and adopting micro-fin pipe

• Pulse flow approach saves liquid mass in LH 2 chilldown but has no significant effect in LN 2 case. • Pulse LN 2 flow causes time cost increase but no definite effect in liquid mass. • Micro-fin pipe simultaneously reaches purposes of time saving and liquid saving. • Extending valve closing period could not produce significant liquid saving in LN 2 condition. High-efficient cold energy utilization of cryogenic liquid is beneficial to propellant mass saving in transfer pipe chilldown operation, and a pulse liquid flow approach has been proposed to reach this purpose. However, the liquid inlet pattern influence on the overall performance of chilldown process is still unclear. In the present study, a cryogenic chilldown test platform, using liquid nitrogen (LN 2 ) as the working fluid and covering the inlet Reynolds number (Re) range of 5000–45000, is constructed to investigate the pulse flow effect on cryogenic chilldown behaviors. Different pipe sections, including two straight pipes and three micro-fin pipes, are tested under different inlet conditions, and the data on chilldown time and liquid consumption are significantly analyzed. The test results show that a pulse flow inlet produces periodic fluctuation of heat flux during the chilldown progress, and a longer time is required to reach the chilldown purpose when compared to a continuous liquid inflow situation. To attain chilldown time saving, both of the approaches by increasing inlet Re and adopting micro-fin pipe could yield satisfactory results. Compared to the straight pipe case with wall thickness of 2 mm, the micro-fin pipe with fin interval of 50 mm (Δ = 50 mm) saves time cost by 40–45 %. However, the approach of pulse liquid flow produces a significant increase in chilldown time. In the view of liquid consumption, the pulse LN 2 flow approach could not produce a definite effect in reducing liquid mass, which is different from the liquid hydrogen (LH 2 ) chilldown situation. By contrast, the micro-fin pipe yields an excellent effect in saving liquid mass, with the maximum saving of 65 % when compared to a conventional continuous liquid case. Therefore, the micro-fin pipe could simultaneously reach the purposes of chilldown time reduction and liquid consumption saving. The chilldown thermal efficiency is also analyzed in the present studies, and the results demonstrate that there is no apparent increase in thermal efficiency by adopting a pulse flow approach, but the micro-fin pipe gives rise to a significant increase in thermal efficiency. When adopting a pulse flow operation with a longer valve off period, a longer chilldown time is needed but the liquid saving effect is negligible.

Fill-In and Boiling Transition Characteristics during the Liquid Oxygen Chill-Down Process in a Vertical Exit-Contracted Pipe

Liquid oxygen chill-down in a vertical exit-contracted pipe was investigated experimentally. The wall temperatures were recorded in detail to describe the filling and chill-down process of the experimental section. Two quenching fronts, the exit one and the inlet one, were detected, and their propagations were found. Results show that the chill-down process is controlled mainly by the formation and propagation of quenching front, which are determined by the pressure level. With the increase of pressure, the roles of both propagation of quenching front and inlet quenching front undergo decreasing. On the vertical section, the effect of circumferential position was discussed in detail and the dominant point was identified, which determines the boiling transition time of the dominated points on the current cross-section. Based on the experimental data, two correlations were suggested for dominant point and dominated points, respectively, to predict heat flux on Leidenfrost, heat transfer coefficient on Leidenfrost, and critical heat flux. One equation was approved to predict heat transfer coefficient on critical heat flux point for both sorts of points. All of these correlations could produce reliable predictions.

Nitrogen flow boiling and chilldown experiments in microgravity using pulse flow and low-thermally conductive coatings

The enabling of in-space cryogenic engines and cryogenic fuel depots for future manned and robotic space exploration missions begins with technology development of advanced cryogenic fluid management systems upstream in the propellant feed system. Before single-phase liquid can flow to the engine or customer spacecraft receiver tank, the connecting transfer line must first be chilled down to cryogenic temperatures. The most direct and simplest method to quench the line is to use the cold propellant itself. When a cryogenic fluid is introduced into a warm transfer system, two-phase flow quenching ensues. While boiling is well known to be a highly efficient mode of heat transfer, previous work has shown this efficiency is lowered in reduced gravity. Due to the projected cost of launching and storing cryogens in space, it is desired to perform this chilldown process using the least amount of propellant possible, especially given the desire for reusable systems and thus multiple transfers. This paper presents an assessment of two revolutionary new performance enhancements that reduce the amount of propellant consumed during chilldown while in a microgravity environment. Twenty-eight cryogenic transfer line chilldown experiments were performed onboard four parabolic flights to examine the independent as well as combined effect of using low thermally conductive coatings and pulse flow on the chilldown process. Across a range of Reynolds numbers, results show the combination significantly enhances performance in microgravity, with a reduction in consumed mass up to 75% relative to continuous flow for a bare transfer line.

An Experimental Study on Cryogenic Spray Quenching of a Circular Metal Disk II—Chilldown Efficiency and Effects of Coating and Flow Pulsing

Abstract This is the second part of a two-paper series that presents the results of liquid nitrogen spray quenching of a stainless steel (SS) disk plate. The first paper focuses on the heat transfer characteristics of the liquid nitrogen spray quenching, whereas the second paper emphasizes the overall chilldown performance and efficiency. Serving as the baseline information for evaluating the effects of disk surface coating and pulse flow, the results of continuous-flow spray chilldown of a bare surface SS disk are summarized first. We found that for continuous-flow spray chilldown of a bare surface disk, the chilldown efficiency is mainly a function of the average spray mass flow rate with the trend of decreasing efficiency with increasing average mass flow rate. Additional experiments were performed to evaluate the enhancement of cryogenic spray quenching by three techniques: (1) using intermittent pulse sprays on SS bare surface, (2) coating the SS surface with a layer of low thermal conductivity Teflon film, and (3) spraying liquid nitrogen intermittently on the coated SS surface. In general, the results indicate that all three methods effectively produced higher spray efficiencies. However, it was also found that the Teflon coating was more effective than the flow pulsing. The coating produced a quick transition from film boiling to transition boiling. This quick transition shortens the poor heat transfer film boiling period and thus facilitates the switch to much higher heat transfer transition boiling and nucleate boiling periods earlier to expedite the chilldown process and enhance the heat transfer.

Experimental study on liquid oxygen chill-down in a horizontal exit-contracted pipe

Liquid oxygen chill-down in a horizontal exit-contracted pipe was investigated experimentally. The outer wall temperatures were recorded in the detailed manner. Results show the temperatures in the center length of the pipe decrease first, which shows much different manners from those in the transport pipe. This indicates that liquid accumulates in the center length to form quenching head, which propagates to the both sides of the pipe during the liquid oxygen chill-down process in the exit-contracted pipe. Based on the present set of data, the correlations on temperature of Leidenfrost point was corrected, and critical heat flux was well correlated by new expression from bubble separation theories. Results show that, for high pressure or high flow rate cases, Leidenfrost point is controlled by the increase of instable wave on the liquid propagation area. However, for other cases, Leidenfrost point is controlled by heat transfer.

Experimental investigation on cryogenic chilldown performance under high-Reynolds number condition and using interior micro-fin structure

Future aerospace vehicle requires a fast propellant fueling and a rapid launching. In the present study, a high-pressure chilldown test platform is established, and two different tube structures, including a straight tube and an interior micro-fin tube, are tested under different inlet Reynolds number (Re) conditions. The test results show that for a typical chilldown case through the straight tube, different boiling mechanisms successively dominate the fluid-wall heat transfer, and inverse annular flow film boiling occupies the maximum proportion of wall temperature decrease. Increasing inlet flow rate could accelerate the chilldown rate, and the time cost in Re=45,000 case is only about 15% of that in Re=5,000 case. For the proposed interior micro-fin tube, the test results verify its excellent performance in improving chilldown rate. Under Re=5,000 condition, the time cost for micro-fin tube case is only about 70 s, which is far less than the time cost by a straight tube case. This astonishing performance is mainly because the micro-fin structure induces a radial velocity component in the inverse annular vapor film. The radial velocity strike acts on the liquid-vapor interface, yielding an interface fluctuation or even a liquid column breakup. Therefore, the liquid phase could contact the tube warm wall earlier, and then a sustained high heat flux gives rise to significant temperature decrease effect throughout the whole chilldown process.

Numerical investigation on chill-down and thermal stress characteristics of a LH2 tank during ground filling

To investigate the thermal and structural characteristics of a flight-scale LH2 tank during ground fillings, a CFD model and a structural analysis model are established to simulated the chill-down process and the induced thermal stress behavior of the tank, respectively. Results show that, at the early stage of filling, a severe temperature gradient appears at the liquid level, leading to a remarkable local concentration of thermal stress, while the maximal thermal deformation is at the outlet region. After the local wall is chilled down sufficiently, the temperature jump at the interface vanishes as well as the local thermal stress, while the maximal thermal deformation is located at the middle height of tank. The thermal stress is most serious at the beginning stage of filling and the maximum appears at the tank bottom. Moreover, the non-uniformity of the temperature distribution and the average thermal stress level within the tank wall both increase with the filling rate. At a filling rate of 7.5 kg·s−1, the maximal thermal stress and thermal deformation of the target tank are more than 70 MPa and 30 mm.

Investigation on heat transfer in line chill-down process with various cryogenic fluids

This paper focuses on the transient process of the pipe cooling by the internal flow of cryogenic fluid, which is a so-called cryogenic line chill-down process. Due to the increasing use of cryogenic fluid in various industries, the heat transfer characteristics during the line chill-down process are followed with great interest. One of the biggest concern is that there are no universal heat transfer correlations for simulating the chill-down process. This research aims to extend the quenching database with liquid oxygen and predict the cryogenic line chill-down process with a one-dimensional homogeneous model. The line chill-down experiments are performed by liquid oxygen with the mass flux range from 15.9 kg/m2-s to 75.9 kg/m2-s. A 7 m long stainless steel 316 horizontal tube is used as the test section. The empirical correlations for heat transfer coefficient of single-phase vapor convection and film boiling are suggested based on the chill-down experimental data of liquid oxygen, liquid argon, and liquid nitrogen. The line chill-down process is simulated by applying the empirical correlations in a one-dimensional homogeneous model. The feasibility of the dynamic simulation model is verified in the mass flux up to 868 kg/m2-s by simulating the chill-down experimental data in the literature. Moreover, the characteristics of the critical heat flux, critical heat flux temperature, and minimum heat flux temperature are also investigated by comparing various experimental data and the empirical correlations. This study provides not only the open experimental data with liquid oxygen, which is rarely found in the literature but also the numerical simulation method, which predicts the line chill-down time within ± 10% error.

Effects of wall superheat and mass flux on flow film boiling in cryogenic chilldown process

Chilldown process is usually the initial operation for storing and transporting cryogenic fluids. During this process, film boiling is of great importance in terms of the large time span. In this research, a numerical model was built to study the flow regime and heat transfer characteristics of the film boiling with cryogenic fluids. An algebraic interface area density framework was coupled in the model to calculate the drag force at the liquid-gas interface. Accordingly, the effects of wall superheat and mass flow rate on heat flux and film thickness were investigated. Furthermore, the variance analysis method was employed to evaluate the weight of the effects of superheat and inlet mass flow and their coupling effect on the heat transfer coefficient. The results reveal that the wall superheat effect prevails over the mass flux effect on the heat transfer coefficient. Moreover, the two factors have a relatively weak coupling impact on the heat transfer coefficient. Finally, the features of pressure drop in the tube were further analyzed.

Two-phase convection heat transfer correlations for liquid hydrogen pipe chilldown

Recently, heat transfer correlations based on liquid nitrogen (LN2) and liquid hydrogen (LH2) pipe quenching data were developed to improve the predictive accuracy of lumped node codes like SINDA/FLUINT and the Generalized Fluid System Simulation Program (GFSSP). After implementing these correlations into both programs, updated model runs showed strong improvement in LN2 pipe chilldown modeling but only modest improvement in LH2 chilldown modeling. Due to large differences in thermal and fluid properties between the two fluids, results indicated a need to develop a separate set of LH2-only correlations to improve the accuracy of the simulations. This paper presents a new set of two-phase convection heat transfer correlations based on LH2 pipe quenching data. A correlation to predict the bulk vapor temperature was developed after analysis showed that large departures from thermal equilibrium between the liquid and vapor phases occurred during film boiling of LH2. Implemented in a numerical model, the new correlations achieve a mean absolute error of 19.5 K in the predicted wall temperature when compared to recent LH2 pipe chilldown data—an improvement of 40% over recent GFSSP predictions. This set of correlations can be implemented in simulations of the transient LH2 chilldown process. Such simulations are useful for predicting the chilldown time and boil-off mass of LH2 for applications such as the transfer of LH2 from a ground storage tank to the rocket vehicle propellant tank, or through a rocket engine feedline during engine startup.

An Experimental Study on the Heat and Mass Transfer of Liquid Nitrogen in a Loose Medium

Liquid nitrogen is a vital medium during the extinguishing and chilldown process of coal spontaneous combustion in coal mine goafs. In this paper, the heat and mass transfer of liquid nitrogen in a loose medium was investigated. A laboratory system including a temperature sensing system was designed and built to explore the effects of different nitrogen injection positions and angles on the chilldown effect. The results indicate that after liquid nitrogen injection, the temperatures in the liquid nitrogen flow area and the bottom of the model can be quickly reduced to −196 °C, which was the best chilldown effect zone. With the vaporization of the liquid nitrogen, the cryogenic nitrogen gradually diffused. At 20,000 s, the bottom temperature was about –63 °C, the middle was –30 °C, and the upper was 0 °C. When the model angle was 0°, the effective chilldown zone was the largest. As the angle increased, the effective chilldown volume and holding time decreased significantly. The model angle had a greater impact on the chilldown. For the position of nitrogen injection, the inlet was set slightly farther from the gas outlet’s position, leading to a larger coverage for the liquid nitrogen and cryogenic nitrogen chilldown. This study can provide a theoretical basis for the flow and chilldown of nitrogen injection in goafs.

Experimental investigation on two-phase flow instabilities in long-distance transportation of liquid oxygen

Two-phase instabilities are inevitably encountered in the cryogenic transport process, which are significant for the high-efficient utilization of cryogenic fluids as well as the safe operation of cryogenic devices, especially for long distance transportation systems. An experimental study is performed on a 200-meter-long transfer system with liquid oxygen to investigate the details with respect to the cryogenic two-phase instabilities. The fluid pressure, temperature as well as flow rate along the conveying pipelines are measured, and the pipeline chill-down, tank chill-down, filling and parking steps are conducted sequentially. The results demonstrate the important role of gas forced convection for long pipelines, and reveal the flow and thermal performance of chill-down process. The system damage caused by chill-down pressure surge might be possibly avoided through properly controlling the inlet pressure of the pipeline and simultaneously chill-down efficiency can be improved. Besides, the geyser phenomenon with 8 min duration and 0.18 MPa pressure amplitude was observed in a 10-meter-high vertical pipeline during the parking process, and the consumption rate of cryogenic fluids was greatly increased.

Heat transfer enhancement in chilldown process with electrospun nanofiber coating

In this study, a method to enhance boiling heat transfer in cryogenic fluids by using nanofiber coating was proposed and validated. For this method, a nanofiber coating of polyvinyl alcohol (PVA) was fabricated on the surface of a copper plate via electrospinning, which is a technique that uses a high-voltage electric field. A chilldown experiment conducted with the copper plate and liquid nitrogen revealed that the nanofiber coating had the potential to reduce the chilldown time drastically. It was observed that the heat flux through the nanofiber-coated plate increased immediately after the chilldown period began, and it exceeded that of the plate without coating for the entire test duration. Consequently, the chilldown (boiling) curve did not have a minimum heat flux point, which is generally seen in pool boiling process. The critical heat flux was twice as large as that of the conventional method with a bare copper plate. Overall, the chilldown curve of the nanofiber-coated plate was distinct from a conventional boiling curve with a bare plate.

Experimental investigation of line chill-down process with liquid oxygen

Cryogenic fluids, such as liquid oxygen (LOX) and liquid hydrogen are used as the propellants of space vehicles. Before initiating the normal operation state, all elements in the cryogenic system need to be chilled down by consumption of the incoming the cryogenic liquid. As a result, the heat transfer characteristics during the chill-down process are extremely concerned by engineers. This paper describes the experimental research on the line chill-down process with LOX. The experiments are conducted under various mass flux conditions on a stainless steel horizontal pipe with the dimensions of 12.7 mm outer diameter, 1.25 mm wall thickness, and 7 m length. The heat transfer characteristics in various heat transfer regimes are experimentally examined and analysed for comparison with those in chill-down processes with liquid nitrogen. Additionally, the accuracy of the previously suggested transient heat transfer correlations is evaluated being based upon the current experimental data.

Experimental investigation on line chill-down process by liquid argon

The cryogenic line chill-down process is an essential part of the cryogenic system application. As the heat transfer characteristics during the transient cooling down are highly affected by the fluid properties, universal heat transfer correlations, which are valid for the chill-down process with various kinds of cryogenic fluid, are required in many industries. This paper investigates cryogenic line chill-down process by using a 7 m long stainless steel horizontal pipe. Liquid argon is selected as the working fluid to examine the fluid property effects. The histories of the wall temperature, pressure, and mass flow rate are measured during the chill-down process. The heat transfer characteristics of liquid argon are analyzed and compared with those of chill-down process with liquid nitrogen. At the same time, the empirical correlations for the critical heat flux, critical heat flux temperature, and minimum heat flux temperature, which can well estimate the parameters for both of liquid nitrogen and liquid argon cases, are suggested. As a result, the critical heat flux is estimated with the mean absolute error (MAE) of 14.8%. The critical heat flux temperature and minimum heat flux temperature are predicted with the MAE of 3.4% and 1.9%, respectively. The empirical correlations are essential to construct a numerical model for simulating the cryogenic line chill-down process.