Saturated Liquid Nitrogen Research Articles

Absorption of liquid nitrogen into porous materials used in the cryogenic cold chain

A dry-shipper is a dewar used to transport frozen biomedical samples at cryogenic temperature. The inside of the dewar is lined with a porous material that absorbs and prevents the spillage of liquid nitrogen during transportation. In these porous materials vapor might be trapped during filling of the dry-shipper leading to a lower transport and storage time. The conditions under which the vapor is formed and the relationship with the porous material properties is not well understood. We studied the impact of the pore size distribution on the vapor retention in the porous materials by comparing liquid nitrogen absorption in aluminosilicate material with relatively large pores (1-100 μm) and calciumsilicate with small pores (∼0.45 μm). Both samples were immersed into saturated liquid nitrogen and a comparison of the absorbed liquid volume fraction with the porosity showed the calciumsilicate sample was completely filled with liquid, whereas the aluminosilicate contained a vapor fraction of about twenty percent. As a further investigation, we studied the absorption characteristics in subcooled liquid nitrogen. In this case, both materials absorbed liquid equivalent to their respective void fraction indicating no vapor pockets in the material. From these results, we propose a design property window for potential new porous materials for use in the dry-shippers.

Degradation Characteristics of REBCO Tapes According to Electrical Breakdown

Second-generation (2G) high-temperature supercon-ducting (HTS) coated conductors are widely used to develop high-voltage superconducting apparatuses, such as HTS fault current limiters and HTS cables. In this study, experiments on electrical breakdown were performed in saturated liquid nitrogen (LN2) at a temperature of 77K to verify the relation between the applied voltage and electrically degraded characteristics of REBa2Cu3Ox (REBCO, RE: rear earth) tapes, such as the critical current (Ic) and index n. We used three types of REBCO tapes to determine the effect of the stabilizer material on the electrical degradation characteristics of silver, copper, and brass. A sphere electrode sys-tem was used to conduct electrical breakdown experiments under AC and lightning impulse voltages. Accordingly, we determined that the electrical degradation characteristics of REBCO tapes de-pend on the stabilizer material according to the applied voltage.

Two-phase pressure drop study for cryosurgical probes using one-dimensional homogeneous model

Cryosurgery is an effective medical treatment approach for treating solid cancerous malignancy, as it is a minimally invasive treatment therapy. The main aim of cryosurgery is to use extreme cold to destroy abnormal tissues while minimizing damage to healthy surrounding tissue. Recent technological advancements in cryogenics have attracted the attention of many researchers to study the low-temperature cryosurgical devices, generally termed cryoprobes. Saturated Liquid Nitrogen (LN2) from pressurized dewar is fed to the cryoprobe through a transfer hose. The fluid evaporates as it flows downstream because of various heat loads and experiences pressure drop. In this paper, a two-phase pressure drop solver is presented following a simplified one-dimensional analysis to estimate pressure losses occurring throughout the system. A qualitative discussion on the contribution of various pressure drop components is provided. It is observed that the major pressure loss occurs along the length of the cryoprobe tubes. The effect of the cryoprobe tube dimensions on the pressure drop is also studied. It is concluded that the miniaturized probe design demands higher system inlet pressure.

인가전압의 크기에 따른 2세대 고온 초전도선재의 열화특성에 관한 연구

2G High temperature superconducting (HTS) GdBCO conductors are usually considered as proper superconductors to develop a superconducting fault current limiter and a superconducting cable which are operated in a high voltage condition. 2G HTS GdBCO conductors manufactured by SuNAM Co., Ltd. can be classified by a metal stabilizer: silver, copper, brass, and stainless steel. In previous study, the electrical performance of 2G HTS GdBCO conductors with copper stabilizer according to electrical breakdown voltage is reported. In this study, dielectric experiments are performed in saturated liquid nitrogen (LN₂) at 77K temperature to analyze the characteristics of electrical degradation such as critical current and index number of 2G HTS GdBCO conductors with copper and brass stabilizer according to amplitude of applied voltage. All dielectric experiments are performed with AC and lightning impulse voltages by using a sphere-to-plane electrode system. It is found that the electrical performance of 2G HTS GdBCO conductors is degraded according to the amplitude of applied voltage.

절연파괴에 따른 2세대 고온 초전도선재의 열화특성에 관한 연구

2G high temperature superconducting (HTS) wires have been widely adopted to develop high voltage superconducting apparatuses such as a superconducting fault current limiter (SFCL), a superconducting cable, a superconducting transformer, and so on. A study on the dielectric degradation characteristics of 2G HTS wires due to electrical breakdown should be performed to enhance the stability and to accelerate the commercialization of high voltage superconducting apparatuses. In this study, the degradation characteristics of 2G HTS wires, SCN-04 made by SuNAM Co., Ltd. are analyzed by dielectric experiments with AC and lightning impulse voltage in saturated liquid nitrogen (LN2) of 77K. The 2G HTS wire used in this study has silver stabilizing layers on both sides of GdBCO layer. Also, the substrate is made with stainless steel and whole 2G HTS wire is surrounded by copper. It is verified that the degree of degradation of the 2G HTS wire is affected by the magnitude of the applied voltage and the type of voltage. Also, cracked GdBCO layer due to electrical breakdown is shown by scanning electron microscope (SEM) image. The degradation characteristics such as critical current and index n of 2G HTS wire due to electrical breakdown can be explained by mechanical cracked structure of a GdBCO layer.

작은 원통형 풀에서 액체질소의 막비등 열전달에 관한 실험적 연구

In this study, experiments on the pool boiling of saturated liquid nitrogen in a small cylindrical pool were conducted, and the film boiling heat transfer for different pool depths was observed. Test surface made of stainless steel was set horizontally upward. The test surface was heated up to approximately 250℃ by an infrared heater, after that it was cooled down by liquid nitrogen. Temperature history was measured using three thermocouples, and based on the measured temperature history, the surface temperature and heat flux were calculated by solving numerically the inverse heat conduction problem. The experimental results showed that the heat flux decreased slightly with an increased depth of the cylindrical pool in the film boiling region. This was thought to be due to the increase in the relative area occupied by the bubbles on the heat transfer surface. However, the effect of the pool depth on film boiling heat flux is not significant and the film boiling heat flux in the small cylindrical pool can be predicted from the correlations of the previous study.

Design and operation of the in-vessel cryopump for the new upper divertor in ASDEX Upgrade

To study alternative divertor configurations in the ASDEX Upgrade (AUG) tokamak a new upper divertor, Div-IIo, with two in-vessel coils and a cryopump (CP) will be installed in 2022. For efficient density control in the new divertor configuration during plasma discharges, the CP has to guarantee an effective pumping speed for deuterium (D2) of at least Seff = 15 m3/s. The CP will be installed behind the inner divertor with a toroidal pumping gap of width 30 mm between the inner and outer divertor plates. It consists of 7 modules with a major radius of R = 1.2 m and a total active length of 5.4 m.To investigate reactor relevant D2-He gas mixtures during upper single null discharges, the liquid helium cryo-panels are coated with activated charcoal as cryosorbent.For quick regeneration of the charcoal from the accumulated gases, heating wires on the cryo-panels are installed.To reduce the pumping speed during high density discharges, an in-vessel cryo valve between two modules is installed. The pumping speed can be reduced by 35%.The cryo-panels operate with saturated liquid helium (LHe) at a temperature of 4.4 K. The thermal radiation shield consisting of reflector and chevron-baffle operate with saturated liquid nitrogen (LN2) at 80 K. To supply the new upper CP with LHe and LN2 the existing LHe-LN2-supply of the lower CP will be used. Therefore, an additional distributor box and a multi-channel transfer line to the upper CP have been installed in 2019.

Моделирование и экспериментальное исследование процесса вакуумной откачки паров жидкого азота

We performed the required theoretical computations to simulate the process of pumping saturated liquid nitrogen vapours, accounting for the assumptions adopted. The paper provides a schematic of the test bench and describes the sequence of experiment stages. We checked whether the storage Dewar and volumetric vacuum pump were selected correctly so as to ensure the required evacuation level above the liquid nitrogen surface. We built our test bench and conducted an experiment involving thermostatting in the 63 to 77 K temperature range, and achieved a phase transition in the nitrogen, it turning from liquid to solid. We computed the mass of the nitrogen evaporated required to reach its triple point temperature. We calculated the effective evacuation rate in the system, taking into account the conductivity of separate components. We determined the minimum vapour evacuation time required. We plotted system temperature and pressure as functions of time. The paper describes possible modernisation of the experimental installation, that is, introducing additional crystallisation centres and monitoring the liquid nitrogen mass using electronic scales

Cooldown of insulated metals in saturated and subcooled liquid nitrogen

The cooling rate of metallic objects quenched in liquid nitrogen can be enhanced by coating its surface with a material that has a low thermal effusivity. An early transition from film to nucleate boiling regime caused due to the formation of cold spots at the liquid-coating interface is reported as the reason for this enhanced cooling rate. However, untill now, optimization of the coating thickness to minimize the overall cooling time has only been an empirical proposition. Inspired by experimental data a phenomenological model is proposed. Using this model, an approximate insulation coating thickness that will approach the fastest cool down of an insulated metal quenched in liquid nitrogen can be predicted. This model is verified with experimental data of several copper cylinders coated with different thickness of epoxy quenched in saturated as well as subcooled liquid nitrogen. The optimum coating thickness reduces significantly with the degree of liquid sub-cooling.

나선형 튜브 열교환 방식의 포화 상태 액체질소의 비등열전달 특성에 대한 실험적 연구

나선형 튜브 열교환 방식의 포화 상태 액체질소의 비등열전달 특성에 대한 실험적 연구

Experimental study on the heat transfer characteristics of saturated liquid nitrogen flow boiling in small-diameter horizontal tubes

Experiments were performed to investigate the heat transfer characteristics of the saturated liquid nitrogen flow boiling in a horizontal tube with an internal diameter of 1.98 mm. The heat transfer coefficients were obtained within a wide range of operation conditions, i.e. inlet pressure from 230 to 420 kPa, mass flux from 200 to 690 kg·m−2·s−1, heat flux from 1.5 to 67.1 kW·m−2, and vapor quality from 0 to 0.81. In the low vapor quality region, heat transfer coefficients showed strong dependence on heat flux and pressure but varied little with mass flux or vapor quality, indicating the dominant role of the nucleate boiling over the convective evaporation. The influence of convective evaporation became noticeable in the intermediate-to-high vapor quality region. A modified correlation was proposed by including both the influences of the nucleate boiling and the convective evaporation, which could predict the experimental data with MAD of 16.4%.

Effect of surface condition to temperature distribution in living tissue during cryopreservation

The temperature distribution of the simulated living tissue is measured for the improvement of the cooling rate during cryopreservation when the surface condition of the test sample is changed by covering the stainless steel mesh. Agar is used as a simulated living tissue and is filled inside the test sample. The variation of the transient temperature with mesh by the directly immersion in the liquid nitrogen is measured. The temperatures on the sample surface and the inside of the sample are measured by use of type T thermocouples. It is confirmed that on the sample surface there is the slightly temperature increase than that in the saturated liquid nitrogen at the atmospheric pressure. It is found by the comparison of the degree of superheat with or without the mesh that the surface temperature of the test sample with the mesh is lower than that without the mesh. On the other hand, the time series variations of the temperature located in the center of the sample does not change with or without the mesh. It is considered that the center of the sample used is too deep from the surface to respond to the boiling state on the sample surface.

Inferential framework for two-fluid model of cryogenic chilldown

We report a development of probabilistic framework for parameter inference of cryogenic two-phase flow based on fast two-fluid solver. We introduce a concise set of cryogenic correlations and discuss its parameterization. We present results of application of proposed approach to the analysis of cryogenic chilldown in horizontal transfer line. We demonstrate simultaneous optimization of large number of model parameters obtained using global optimization algorithms. It is shown that the proposed approach allows accurate predictions of experimental data obtained both with saturated and sub-cooled liquid nitrogen flow. We discuss extension of predictive capabilities of the model to practical full scale systems.

Investigation on CHF of saturated liquid nitrogen flow boiling in a horizontal small channel

The critical heat flux (CHF) characteristics of saturated liquid nitrogen flow boiling in a horizontal small channel were experimentally investigated over a wide range of heat flux, mass flow flux and inlet pressure. The results show that the departure from nucleate boiling (DNB) type CHF and the dry-out type CHF dominate the heat transfer separately depending on the vapor quality. The mass flow flux and the axial position along the channel affect the DNB type CHF, which occurs at the end of the channel with a vapor quality of 0.4. The higher mass flow flux and inlet pressure increase the dry-out type CHF as well as the corresponding vapor quality. These two types of CHF are both significantly affected by the gravity effect. By comparing the dry-out CHF with six existing correlations, it is found that almost all the correlations overpredict the CHF. This is because the flow boiling instability is relatively evident in small channels, which leads to the periodical vibration of the mass flow flux and consequently the earlier dryout in the lower mass flow flux region. In comparison, Qu and Mudawar's and Zhang's correlations match better with the dry-out type CHF under the present operating conditions.

Experimental study on heat transfer characteristics of LN2 saturated flow boiling in a horizontal corrugated tube

The two-phase heat transfer of saturated liquid nitrogen (LN2) flow boiling in a horizontal corrugated tube was experimentally studied. The experiments were conducted at different inlet pressures of 0.180–0.461MPa and mass fluxes of 163.1–257.1kgm−2s−1 under a wide range of heat flux of 16,000–40,000Wm−2. The flow pattern was identified to be unstratified flow according to the circumferential distributions of heat transfer temperature differences. Two heat transfer regimes were observed in the present experiment: the nucleate boiling dominated heat transfer and the convective evaporation dominated heat transfer. Due to the thinning of the liquid film at the tube inner surface caused by helical fin, nucleation suppression and convection augmentation were observed and the variation of the circumferential average HTC with the flow parameters became complex depending on the relative importance of the two heat transfer mechanisms. The effects of heat flux, vapor quality, inlet pressure and mass flux on the circumferential average heat transfer coefficient (HTC) are discussed. The results show that the nucleate boiling heat transfer is enhanced with the increase of heat flux, while the convective evaporation is enhanced with the increase of mass flux and vapor quality, and the decrease of pressure. In order to improve the prediction of HTCs, the Klimenko correlation for cryogenic fluids are included in an additive model. Based on the experimental data of circumferential average heat transfer coefficients, the parameterization of the additive model including the convective and nucleate boiling components was obtained for the LN2 saturated flow boiling in the corrugated tube. Compared to the Klimenko correlation for the flow boiling of cryogenic fluids with a fully wetted perimeter in smooth tubes, the proposed parameterization of the additive model improves the prediction of HTCs in the convective evaporation dominated regime.

Heat transfer during transition to nucleate boiling

A wide variety of empirical studies that consider transition to nucleate boiling regime has been published so far. Most of them tend to quantify the process-specific heat transfer by means of statistical approximation of the experimental data. The derived relations are therefore valid in a limited range of heat flux and pressure variation. The present work describes a general way to estimate heat transfer coefficient upon the transition to nucleate boiling regime. An expression for the heat transfer coefficient is theoretically derived when the effects of the heating surface roughness are taken into account. This expression is validated against series of experiments on saturated water and liquid nitrogen at the atmospheric pressure. Our theory agrees well with experimental results.

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 17.6mm 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.

Experimental results of flooding experiments in an inclined tube with liquid nitrogen and its vapor

Counter-current two-phase flow behaviors of saturated liquid nitrogen and its vapor at the onset of flooding are experimentally investigated. The experiments are carried out in a vacuum-insulated 20mm i.d. transparent tube with the inclination angles of 30°, 45° and 60° corresponding to the horizontal. The common slug flow phenomenon happened with water–air is not observed with liquid nitrogen–vapor, instead, the big interfacial wave is found to be crushed to tiny droplets. The phenomenal difference is primarily attributed to the larger viscosity of water than liquid nitrogen. Correspondingly, the sharp rise of pressure drop with water–air is largely due to the blockage of gas flow by the formed slug, while it is primarily due to the tiny droplet entrainment for the liquid nitrogen–vapor pairs. The effects of inclination angles on the incipient flooding velocity are specially emphasized and investigated. A new correlation base on Ohnesorge number and modified Froude number are presented, and the results coincide with the experimental data of both room-temperature and cryogenic fluids with the uncertainty of 20%.

Experimental investigation on ejecting low-temperature cooling superconducting magnets

With the development of the high-temperature superconducting (HTS) materials and refrigeration technologies, using ejecting refrigeration to cool the superconducting materials becomes the direction of HTS applications. In this paper, an experimental study has been carried out on the basis of the theory of analyzing the ejecting low-temperature cooling superconducting magnet. The relationship between area ratios and refrigeration performance at different system pressures was derived. In addition, the working fluid flow and suction chamber pressure of the ejector with different area ratios at various inlet pressures have been examined to obtain the performance of ejectors under different working conditions. The result shows that the temperature of liquid nitrogen can be reduced to 70K by controlling the inlet water pressure when the pressurized water at 20°C is used to eject the saturated liquid nitrogen, which can provide the stable operational conditions for the HTS magnets cooling.

Boiling incipience in liquid nitrogen induced by a step change in heat flux

In the present paper are reported boiling experiments performed on a 25μm brass wire immersed in saturated liquid nitrogen at 1bar. A boiling curve performed under quasi-stationary heating conditions is presented. The boiling performance of the system induced by a step change in heat flux is next investigated. It is shown that a premature transition to film boiling is systematically observed when the applied heat flux is higher than 80% of the steady critical heat flux. This transition is however likely to happen as soon as the applied heat flux exceeds the steady minimum heat flux value. Boiling incipience results from the activation of pre-existing vapour embryos entrapped in the cavities of the wall. A gradual activation of the nucleation sites is observed for low values of the applied heat flux. Values higher than qFDNB″ (minimum heat flux for fully developed nucleate boiling measured under stationary heating conditions) lead to explosive boiling incipience in which bubble dynamics and interfacial motions are intense enough to produce an acoustic emission.