Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

Measurement of liquid tin heat transfer coefficients within the temperature range of 506–1170 K

The thermal conductivity and thermal diffusivity coefficients of liquid tin within the temperature range of 506–1170 K were measured by the laser flash technique. The measurement errors for the heat transfer coefficients were equal to ±(2.5–3.5)%. Approximation equations and the reference data tables were obtained for the temperature dependency of the properties. The measurement results were compared with the available literature data. The Lorentz number temperature dependence was calculated up to 1000 K.     

Visualization of the solid–liquid equilibria for non-flammable mixed refrigerants

Non-flammable mixed refrigerant (NF-MR) Joule Thomson (J–T) refrigerators have desirable characteristics and wide cooling temperature range compared to those of pure J–T refrigerators. However, the operating challenge due to freezing is a critical issue to realize this type of refrigerator. In this paper, the solid–liquid phase equilibria (i.e. freezing point) of the NF-MR which is composed of Argon (Ar), R14 (CF₄), and R218 (C₃F₈), has been experimentally investigated by a visualized apparatus. The accuracy of the apparatus is experimentally verified with pure refrigerants and selected binary mixed refrigerants. Freezing points of the ternary NF-MRs have been measured with the molar compositions from 0.1 to 0.8 for each component. Each test result is simultaneously acquired by a camcorder for visual inspection and temperature measurement during a warming process. Experimental results reveal that the specific MR, with R14 molar composition higher than 0.4, can achieve remarkably low freezing temperature even below 77 K. These unusual freezing point depression characteristics of the MR can be a useful information for designing a cryogenic MR J–T refrigerator to reach temperatures less than 77 K.     

Investigation of transient conduction–radiation heat transfer in a square cavity using combination of LBM and FVM

In this paper, the effect of surface radiation in a square cavity containing an absorbing, emitting and scattering medium with four heated boundaries is investigated, numerically. Lattice Boltzmann method (LBM) is used to solve the energy equation of a transient conduction–radiation heat transfer problem and the radiative heat transfer equation is solved using finite-volume method (FVM). In this work, two different heat flux boundary conditions are considered for the east wall: a uniform and a sinusoidally varying heat flux profile. The results show that as the value of conduction–radiation decreases, the dimensionless temperature in the medium increases. Also, it is clarified that, for an arbitrary value of the conduction–radiation parameter, the temperature decreases with decreasing scattering albedo. It is observed that when the boundaries reflect more, a higher temperature is achieved in the medium and on boundaries.     

Modelled heat transfer coefficients for Al–7 wt-%Si alloy castings unidirectionally solidified horizontally and vertically downwards

Abstract

A model is described for the calculation of the interfacial heat transfer coefficient during the unidirectional solidification of Al–7 wt-%Si alloy castings against a water cooled copper chill. The model includes the deformation of the initial solidified skin of the casting into a convex shape within the first seconds of solidification. Thereafter, heat transfer from the casting to the chill takes place through a central contact area and an outer annulus where local separation has occurred. Modelled heat transfer coefficients for solidification horizontally and vertically downwards are compared with experimentally determined values and show broad agreement. Some limitations of the model which prevent better agreement with the experimental values are discussed.     

Effects of momentum transfer on particle dispersions of dense gas–particle two-phase turbulent flows

A modified momentum transfer coefficient of dense gas–particle two-phase turbulent flows is developed and its effect on particle dispersion characteristics in high particle concentration turbulent downer flows has been numerically simulated incorporating into a second-order moment (USM) two-phase turbulent model and the kinetic theory of granular flow (KTGF) to consider particle–particle collisions. The particle fractions, the time-averaged axial particle velocity, the particle velocities fluctuation, and their correlations between gas and particle phases based on the anisotropic behaviors and the particle collision frequency are obtained and compared using traditional momentum transfer coefficients proposed by Wen (1966), Difelice (1985), Lu (2003) and Beetstra (2007). Predicted results of presented model are in good agreement with experimental measurement by Wang et al. (1992). The particle fluctuation velocity and its fluctuation velocity correlations along axial–axial and radial–radial directions have stronger anisotropic behaviors. Furthermore, the presented model is in a better accordance with Lu’s model in light of particle axial velocity fluctuation, particle temperature, particle kinetic energy and correlations of particle–gas axial–axial velocity fluctuation. Also, they are larger than those of other models. Beetstra’s model is not suitable for this downer simulation due to the relative lower particle volume fraction, particle collision and particle kinetic energy.     

Investigation of the heat conductivity of niobium in the temperature range 0.05–23 K

We report thermal conductivity measurements on a single-crystal niobium specimen of resistivity ratio 33,000 over the temperature range 0.05–23 K in the superconducting state and above 9.1 K in the normal state. The axis of the niobium rod was [110] oriented. The surface roughness was varied by sandblasting of the sample. The values of the thermal conductivity in the range from the lowest temperatures up to the maximal value covered a range of six orders of magnitude (=2×10^–5 W cm^–1 K^–1 at 50 mK to =22 W cm^–1 K^–1 at 9 K). Above 2 K the results for the untreated and the sandblasted sample are in accord, whereas below 2 K the influence of the sample surface is discernible. The various conduction and scattering mechanisms are discussed.     

The effect of internal particle heat conduction on heat transfer analysis of turbulent gas–particle flow in a dilute state

In many cases the conduction mechanism inside a particle can not be ignored (large particles, low thermal conductivity and high porosity) during turbulent gas–particle flows. However, the accurate solution might be difficult to apply. Therefore, we first develop here the ability to conduct accurate solution and then we define the criterion for which the internal conductivity might be ignored. A combination between commercial C.F.D. code and user defined programs was developed to predict numerically the gas–particle velocity and temperature profiles. The selected criterion (defined at the outlet of the pipe’s cross-section), referred to the relation between the computational desirable average temperature difference without ignoring internal heat conductivity and the average particles temperature by ignoring internal heat conductivity, determines whether to consider the heat conduction mechanism in numerical simulations or to ignore it. It was found that the average particles temperature for T _p =  f(r) is lower than the case when T _p =  constant. Also, it was found that the non-dimensional temperature difference criterion is a continuous function of [Bi ×  (d _p/D)] for a specific geometry, various pipe and particle diameters, various particles’ thermal conductivities, constant heat flux and Re number. The numerical code enables to extend the classical criterion for Bi number of solids to various gas–particle systems and different operational conditions.     

Optimisation of Ni–Ti shape memory alloy response time by transient heat transfer analysis

Nickel–Titanium (Ni–Ti) shape memory alloys (SMAs) are commonly applied in commercial actuator design due to the high associated fatigue and tensile strengths, low cost and high activation temperature. Consequently, Ni–Ti SMAs provide an opportunity for the development of novel electromechanical actuators. However, the cooling response time is typically of significantly larger duration than the associated heating response time. The applicability of SMA actuators would be significantly greater if the cooling response time was reduced to allow a symmetric, high speed activation profile. This work provides insight into the opportunities associated with enhancing thermal heat transfer efficiency to achieve this objective. An explicit model of the temperature of Ni–Ti SMA wire is developed to estimate the temperature–time profile during resistive heating. A finite-difference equation is developed to predict the associated temperature during cooling. These models are used to confirm that for a typical scenario, the cooling stage dominates the total response time, and that lagging with a highly conductive media can be used to dramatically reduce the cooling response time. The finite-difference equation is validated against steady state data, and extended to provide insight into the effects of SMA lagging, including the effects of periodic excitation on cooling rate and the minimum observed SMA temperature during a heating cycle. The outcomes of this work are generally applicable to any axisymmetric transient heat transfer optimisation problem.     

Account for heat transfer between elements of a plane parallel strip–foundation friction unit

An analytical solution of the boundary-value heat conduction problem for a tribosystem consisting of a semiinfinite foundation and a plane-parallel strip sliding at a constant velocity along the foundation surface is obtained. The thermal contact between the strip and the foundation is partial. The asymptotics of the solution for low and high values of time have been found. For the materials of the friction pair “metal ceramics strip–pig iron foundation” the influence of the coefficient of thermal conductivity of the contact on temperature distribution was studied.     

Formation of silicon hollow spheres via electromagnetic levitation method under static magnetic field in hydrogen–argon mixed gas

A formation mechanism of a silicon hollow sphere via electromagnetic levitation method under high static magnetic field in hydrogen atmosphere is discussed in this paper. Since the convection in the levitated silicon melt which is caused by the electromagnetic field is restrained by a high static magnetic field, it is possible to examine a solidification of silicon from an equilibrium silicon melt with hydrogen except an effect of convection. The solidified silicon sphere has an extrusion which is formed during the solidification due to reduced stress in the melt. When a nucleation of pore occurs before the formation of the extrusion, a silicon hollow sphere is formed. Since the chance to form the nucleation of pore increases with increasing hydrogen concentration in the melt, a silicon hollow sphere is formed when the solidification is performed in higher hydrogen partial pressure. In the present case, a silicon hollow sphere is formed by the solidification in 50%H₂–50%Ar atmosphere. However, non-porous silicon spheres are formed by the solidification in 25%H₂–75%Ar or 100%Ar atmosphere.     

Investigation on TIG welding of SiCp-reinforced aluminum–matrix composite using mixed shielding gas and Al–Si filler

Using He–Ar mixed gas as shielding gas, the tungsten inert gas (TIG) welding of SiCp/6061 Al composites was investigated without and with Al–Si filler. Welded joint with filler were submitted to tensile tests. The microstructure and fracture morphology of the joint were examined. The results show that adding 50 vol.% helium in shielding gas improves the arc stability, and seams with high-quality appearance are obtained when the Al–Si filler is added. In addition, the interface reaction between SiC and matrix is greatly suppressed when using Al–Si filler. The microstructure of the welded joint displays non-uniformity with many SiC particles distributing in the weld center. The average tensile strength of weld joints with Al–Si filler is 70% above that of the matrix composites under annealed condition.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Improved pressure–volume–temperature method for estimation of cryogenic liquid volume

One of the most important issues in a liquid propellant rocket is to measure the amount of remaining liquid propellant under low gravity environment during space mission. This paper presents the results of experiment and analysis of a pressure–volume–temperature (PVT) method which is a gauging method for low gravity environment. The experiment is conducted using 7.4 l tank for liquid nitrogen with various liquid-fill levels. To maximize the accuracy of a PVT method with minimum hardware, the technique of a helium injection with low mass flow rate is applied to maintain stable temperature profile in the ullage volume. The PVT analysis considering both pressurant and cryogen as a binary mixture is suggested. At high liquid-fill levels of 72–80%, the accuracy from the conventional PVT analysis is within 4.6%. At low fill levels of 27–30%, the gauging error is within 3.4% by mixture analysis of a PVT method with specific low mass flow rate of a helium injection. It is concluded that the proper mass flow rate of a helium injection and PVT analyses are crucial to enhance the accuracy of the PVT method with regard to various liquid-fill levels.     

Virial coefficients of neon,argon, and krypton at temperatures up to 3000°k

The second and third virial coefficients are calculated for a (12–7, ) pair model potential. With their help the fourth virial coefficient is determined from the experimental p, , and T data. The limits of applicability of the equation of state obtained is indicated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 6, pp. 974–977, June, 1987.     

Influence of cryogenic thermal treatment on mechanical properties of an Al–Cu–Mg alloy

In this study, a cryogenic thermal treatment is developed and its effects on mechanical properties and precipitates are investigated. Water-quenched samples were immersed in liquid nitrogen and reheated in hot oil at 180°C or boiling water for 5?min. Finally, the samples were artificially aged at 190°C for 12?h. The results indicated a notable increase of about 75?MPa in the ultimate tensile strength in comparison to T6 heat-treated alloy. TEM observations revealed that the S(S′) precipitates were fine and uniformly distributed in the microstructure due to reheating in hot oil and subsequent aging treatment.     

What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?

Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al₂O₃ – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al₂O₃/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide.