Transient heat transfer to liquid helium from bare copper surfaces in a vertical orientation — I: Film boiling regime

An experiment on transient heat transfer is described in which partial quench and recovery in a composite superconductor is simulated. Results show that transient heat transfer in the film boiling regime can be expressed as the sum of the steady state term, q_s(τ), and a term that characterizes heating or cooling associated with a thin layer of helium vapour created at the test surface. Specifically the transient heat transfer rate is given as a function of τ by:

$\text{q}{}_{\mathrm{t}}\text{(θ) = q}{}_{\mathrm{s}}\text{(θ) + a(θ)}\text{d}\text{(θ)}\text{d}\text{t}$

.This is in a form directly usable for the computation of partial quench and recovery processes in a composite superconductor.The proportionality function a(τ) represents an effective heat capacity of the vapour layer. Like q_s(τ) it is strongly dependent on surface conditions. Because of this term, transient heat transfer in the film boiling regime can be much higher in the heating cycle and much lower in the cooling cycle than steady state heat transfer: it is hysteretic with temperature cycling.The function a(τ) also quantitatively determines the vapour layer thickness as a function of temperature. The effect of channel gap on transient heat transfer is qualitatively interpreted in terms of the layer thickness relative to the channel size. A criterion is given for a proper choice of channel sizes in superconducting magnet windings.     

Heat transfer and hydrodynamics of channelled cryogenic liquids in fields of centrifugal forces

This Paper reports experimental results on the hydrodynamics and heat transfer during a free-convective and forced motion of cryogenic liquids within a channel in the field of centrifugal forces. Investigations were carried out on the hydrodynamics and heat transfer of a two-phase flow of nitrogen and helium in the heated axial part of the ⊓-shaped duct in the 50–300 range of relative accelerations. It was found that during the free-convective motion the volume flow of liquid nitrogen, with heat fluxes varying from 3 × 10³ to 1 × 10⁴Wm⁻², increased more than 30 times. The volume flow is accompanied by large oscillations and increases with growing relative accelerations. The heat transfer coefficients also are shown to grow with the relative acceleration, which is due to an increase in the hydrostatic pressure at the radial inlet of the duct. Experimental results are presented concerning the heat transfer intensity during forced motion of two-phase helium along a heated axial channel of a rotating □-shaped duct at flow rates <- 7.5 × 10⁻⁵kgs⁻¹. At the relative acceleration of ≈ 100 the heat transfer and critical heat flux are observed to increase with the flow rate. At flow rates <- 10⁻⁴ kg s⁻¹the heat transfer to helium is the same as during pool boiling.     

Heat transfer to helium-I during different boiling regimes at high centrifugal accelerations

Heat transfer during nucleate and film boiling of helium and also the boiling crises up to relative accelerations of (1 ÷ 2) × 10³ have been investigated. The heat transfer surface was a flat copper heater. The heat transfer during nucleate boiling proved to be independent of the relative accelerations. For film boiling, it was found that $\text{α ～ η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$. The dependence of the peak nucleate boiling heat flux and the minimum film boiling flux on the relative acceleration is non-monotonic.     

Investigation on the heat transfer characteristics during flow boiling of liquefied natural gas in a vertical micro-fin tube

This paper presents an experimental investigation on the heat transfer characteristics of liquefied natural gas flow boiling in a vertical micro-fin tube. The effect of heat flux, mass flux and inlet pressure on the flow boiling heat transfer coefficients was analyzed. The Kim, Koyama, and two kinds of Wellsandt correlations with different F_tp coefficients were used to predict the flow boiling heat transfer coefficients. The predicted results showed that the Koyama correlation was the most accurate over the range of experimental conditions.     

A magnet system design for reduced gravity environment

Simulating a reduced gravity environment experienced in spaceships in a laboratory setting for studying different technical aspects (fluid transfer or propellant behavior, for example) is a primordial step prior to extraterrestrial explorations. We first present some results on boiling heat transfer in helium under reduced gravity using a commercial magnet and point out the limitations in volume and magnetic force homogeneity to perform reduced gravity experiments with such a non ad hoc magnet. Then, we present a new magnetic design to create a reduced gravity environment in large volume suitable for boiling test experiments in oxygen. Based on a modified design we present the magnetic configurations that allow compensating gravity for different elements such as hydrogen, water or helium but in smaller volumes. We detail the different aspects of winding techniques to achieve the requirements on magnetic force.     

The induction of a propagating normal zone (quench) in a superconductor by local energy release

Experiments were performed to measure the minimum energy, E_c of an instantaneous point source, needed to induce a propagating normal transition (quench) in a superconductor. The energy pulse was supplied to the superconductor by a miniaturized heater element which had a thermal time constant of ～ 20 μs. E_c depends on the current distribution in the conductor and is lower for a nonhomogeneous distribution. A computer calculation solving the nonhomogeneous heat equation agrees within a factor of two with the results for adiabatic conditions (no radial heat transfer) without adjustable parameters. The results obtained with the conductor placed in liquid helium could be fitted into the calculation with parameters close to measured transient heat transfer data.     

The study of entropy generation during flow boiling in a micro-fin tube

Increasing in the heat transfer rate in flow boiling is a common and key issue for engineers. Generally, the heat transfer coefficient augmentation methods are divided into two main categories (active and passive methods). In passive methods the increase in heat transfer rate causes the increase in pressure drop. In order to evaluate the contribution of heat transfer and pressure drop mechanisms, the entropy generation analysis is used. In this paper, the entropy generation in micro-fin tube is investigated under flow boiling condition. The effect of different geometrical parameters and flow conditions is discussed on pressure drop contribution and heat transfer one in entropy generation, irreversibility distribution ratio (IDR) and Bejan number (Be). The frictional pressure drop and heat transfer coefficient in the micro-fin tube and the helically coiled one are compared as two enhancements passive heat transfer methods with the smooth straight tube in the literatures. Therefore, by introducing entropy generation number (N_s), the favorable geometry between the micro-fin tube and the helically coiled one with respect to the smooth straight tube is recognizable at equivalent boundary conditions.     

Investigation of entropy generation in a helically coiled tube in flow boiling condition under a constant heat flux

There are many methods to augment the heat transfer rate in flow boiling in industrial applications. The helically coiled tubes are one of the best geometries to enhance the heat transfer rate. The entropy generation analysis is an appropriate tool to evaluate the contribution of heat transfer and pressure drop mechanisms. In the present paper, the entropy generation in the helically coiled tube under flow boiling is studied. The optimum tube and coil diameters under specified conditions are found. The effect of different flow conditions such as mass velocity, inlet vapor quality, saturation temperature, and heat flux on contributions of pressure drop and heat transfer in entropy generation is discussed. The Bejan number (Be) and irreversibility distribution ratio (IDR) at different saturation temperatures versus mass velocity are plotted. The comparison between entropy generation and contributions of pressure drop and heat transfer for the helically coiled tube and the straight one is presented. The entropy generation number (N_s) for different flow conditions is plotted. The entropy generation analysis shows that there is a favorable region to use the helically coiled tube with respect to the straight one.     

The stability of Nb3Sn-composite conductors cooled by normal and superfluid helium

The stability of Nb₃Sn superconductors, which were additionally stabilized by soldering copper to the prereacted cable, was investigated. Cooling channels in a sample coil provided access to LHe in the 4.2 K and the 1.85 K temperature range. The measurements show that a heat flow rate of 0.6 W cm^?2 at 4.2 K in the Nb₃Sn composite conductor guarantees a stable operation even at local heat inputs of about 0.5 J onto the surface of the conductor. In superfluid helium, recovery up to the take-off values of current in magnetic fields between 8 T and 13 T was found. A theoretical analysis was performed to explain the experimental results with respect to the cooling channel geometry, magnetic fields, and the local energy inputs to initiate a normal conducting region.     

Forced convection heat transfer to liquid helium I in the nucleate boiling region

Heat transfer and critical heat fluxes to helium boiling in a 2 mm id copper tube (100 mm long) were measured in the pressure range 1.1–1.5 atm and at mass velocities 18–96 kg m^?2s^?1. Corresponding Reynolds numbers are (1.2–6.2) × 10⁴. Experimentally obtained heat transfer coefficients show satisfactory agreement with those calculated according to the Kutateladze equation but with less pronounced pressure dependence. It was found that in the boiling region developed quality did not influence the heat transfer coefficient. An expression was obtained, which describes with ±10% error, the dependence of critical heat flux on mass flow rate in the pressure range 1.1–1.5 atm and mass quality 0.33–0.6.     

Analysis of solid vapour heat transfer in helium vapour columns at low temperatures

An analysis of the available data for heat transfer in low temperature helium vapour columns with vertical temperature gradients is presented. Previous studies have shown that a vertical short plate immersed in the core region of the vapour column yielded heat transfer coefficients higher by a factor up to four times the values predicted from the conventional correlation: Nu = f (Gr, Pr). The present paper applies an analytical solution which has been developed to describe natural convection from a vertical surface to a thermally stratified fluid at ambient temperatures. This solution contains a stratification parameter which is related to the dimensionless temperature gradient in the bulk fluid. It is shown that this analytical solution can be used to describe the observed enhanced heat transfer in low temperature helium vapour columns over a wide range of stratification parameters.     

Experimental Study of the Relation Between Heat Transfer and Flow Behavior in a Single Microtube

The flow boiling heat transfer in microchannels have become important issue because it is extremely high-performance heat exchanger for electronic devices. For a detailed study on flow boiling heat transfer in a microtube, we have used a transparent heated microtube, which is coated with a thin gold film on its inner wall. The gold film is used as a resistance thermometer to directly evaluate the inner wall temperature averaged over the entire temperature measurement length. At the same time, the transparency of the film enables the observation of fluid behavior. Flow boiling experiments have been carried out using the microtube under the following conditions; mass velocity of 105 kg/m² s, tube diameter of 1 mm, heat flux in the range of 10 ~ 380 kW/m² s, and the test fluid used is ionized water. Under low heat flux conditions, the fluctuations in the inner wall temperature and mass velocity are closely related; the frequency of these fluctuations is the same. However, the fluctuations in the inner wall temperature and heat transfer coefficient are found to be independent of the fluctuation in the mass velocity under high heat flux conditions.     

Temperature oscillations in thin metallic tapes with transport current at liquid nitrogen temperature

Temperature oscillations were observed in thin metallic tapes cooled by liquid nitrogen at current densities above 10⁸ A/m². A boiling hysteresis in the transition region between conductive heat transfer and nucleate boiling caused a temperature decrease and a change in the resistance of the tape. The differential resistance was negative and the current voltage curve had a non-linear character. The amplitude of the resistance change and character of the oscillations depended on the type of metal used, current ramp rate, geometry of the tape, and on its position with respect to the liquid surface. The effect was most pronounced in Ni tapes; the resistance change during the temperature drop achieved a value of about 30%. Dumped and un-dumped oscillations were observed after the temperature drop.     

Design and development of a helium injection system to improve external leakage detection during liquid nitrogen immersion tests

The testing of assemblies for use in cryogenic systems commonly includes evaluation at or near operating (therefore cryogenic) temperature. Typical assemblies include valves and pumps for use in liquid oxygen-liquid hydrogen rocket engines. One frequently specified method of cryogenic external leakage testing requires the assembly, pressurized with gaseous helium (GHe), be immersed in a bath of liquid nitrogen (LN₂) and allowed to thermally stabilize. Component interfaces are then visually inspected for leakage (bubbles). Unfortunately the liquid nitrogen will be boiling under normal, bench-top, test conditions. This boiling tends to mask even significant leakage.One little known and perhaps under-utilized property of helium is the seemingly counter-intuitive thermodynamic property that when ambient temperature helium is bubbled through boiling LN₂ at a temperature of −195.8 °C, the temperature of the liquid nitrogen will reduce.This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN₂ bath below boiling point through the controlled injection of ambient temperature gaseous helium and then to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN₂. Helium saturation testing is performed and injection system sizing is discussed.     

Non-stationary helium heat transfer

This Paper presents the results of an experimental investigation of heat transfer to helium in stationary conditions, in conditions of increase and decrease of heat power at various rates and with pulse heating. Heat transfer both from a free surface and in a coaxial gap of 0.5 mm was studied. The experiments were carried out within the range of liquid saturation temperatures from 1.94 to 4.23 K. The values of the heat loads in a pulse were ≈1.5 – 15 times the steady-state value of the critical heat flux for helium nucleate boiling. It has been shown that transient thermal processes may have higher heat transfer coefficients compared with steady-state conditions. The most pronounced effect is observed under pulse heating conditions.     

Heat transfer to supercritical helium,carbon dioxide,and water: Analysis of thermodynamic and transport properties and experimental data

Experimental data published on heat transfer to supercritical helium as well as peculiarities in the change in transport properties of helium, water, and carbon dioxide are analysed. Relationships such as (P_r = f[T/T_m]) in terms of dimensionless π isobars over the critical state temperatures for the liquids under consideration were found to coincide reasonably well. It is shown that the relationship Nu_b = 0.023 Re_b^0.8Pr_min^0.8 agrees fairly well with experimental data relating to supercritical helium for small temperature differences with ?_b/?_w < 3. For higher values of this relationship an analysis is given for possible regimes with deteriorated heat transfer in the case of forced convection of supercritical helium. The interest shown in the problems of heat transfer to supercritical helium is connected with a search for the solutions to the problem of applications of superconductivity to systems and apparatus on an industrial scale. Forced convection supercritical helium systems could be a promising technique for providing low temperature at a stable level.     

Shell side direct expansion evaporation of ammonia on a plain tube bundle with exit superheat effect

There are no data available on the direct expansion evaporation of refrigerant on the outside of a tube bundle. With the current ozone depletion and global warming issues it is critical to develop systems with low charge refrigerants especially with natural refrigerants such as ammonia which has zero ozone depletion potential (ODP) and zero global warming potential (GWP). This study presents results of an extensive experimental work on direct expansion of ammonia on a triangular pitch plain tube bundle with saturation temperature range −1.7 to −20 °C, heat flux range 5 to 45 kW m⁻² and exit superheat range 2 to 10 °C. The test matrix falls well within the practical operating conditions of industrial refrigeration systems. A typical increase in heat transfer coefficient was observed with saturation temperature and heat flux. The effect of exit super heat on the overall performance of the bundle was also reported. A correlation for outside boiling on a tube bundle in direct expansion mode was developed and compared with existing single tube pool, flooded bundle boiling and spray evaporation studies.     

Using the HELIOS facility for assessment of bundle-jacket thermal coupling in a CICC

In a Cable In Conduit Conductor (CICC) cooled by forced circulation of supercritical helium, the heat exchange in the bundle region can play a significant role for conductor safe operation, while remaining a quite uncertain parameter. Heat exchange between bundle and jacket depends on the relative contributions of convective heat transfer due to the helium flow inside the bundle and of thermal resistance due to the wrappings between the cable and the conduit.In order to qualify this thermal coupling at realistic operating conditions, a dedicated experiment on a 1.2 m sample of ITER Toroidal Field (TF) dummy conductor was designed and performed in the HELIOS test facility at CEA Grenoble. Several methods were envisaged, and the choice was made to assess bundle-jacket heat transfer coefficient by measuring the temperature of a solid copper cylinder inserted over the conductor jacket and submitted to heat deposition on its outer surface.The mock-up was manufactured and tested in spring 2015. Bundle-jacket heat transfer coefficient was found in the range 300–500 W m⁻² K⁻¹. Results analysis suggests that the order of magnitude of convective heat transfer coefficient inside bundle is closer to Colburn–Reynolds analogy than to Dittus–Boelter correlation, and that bundle-jacket thermal coupling is mainly limited by thermal resistance due to wrappings. A model based on an equivalent layer of stagnant helium between wraps and jacket was proposed and showed a good consistency with the experiment, with relevant values for the helium layer thickness.     

Review of correlations of flow boiling heat transfer coefficients for carbon dioxide

Carbon dioxide (CO₂) has quite different flow boiling heat transfer characteristics from conventional refrigerants due to its much higher reduced pressures that make its thermodynamic and transport properties very different. There were some studies evaluating the correlations of flow boiling heat transfer coefficient for CO₂. However, either the number of correlations covered or the number of data used was limited, resulting in inconsistent conclusions. This work presents a comparative review of existing correlations for flow boiling heat transfer coefficient of CO₂. There are 34 correlations analyzed and evaluated using 2956 experimental data points of CO₂ flow boiling heat transfer from 10 independent laboratories. The Fang (2013) correlation performs best with a mean absolute deviation of 15.5%. The evaluation analysis sets a channel transition criterion for flow boiling heat transfer of CO₂. Several topics worthy of attention for future studies are identified.