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This publication provides a dataset of 54 experiments, carried out with supercritical CO 2 (sCO 2) in two heated horizontal pipes with inner diameters of 4 and 8 mm at a pressure of approximately 7.75 MPa. The mass fluxes were set to 400 and 800 kg/m2s and the heat flux was varied from 30 to 130 kW/m2, resulting in a heat to mass flux ratio of 38–225 J/kg. The influences of the inner pipe diameter, the Reynolds number and the heat to mass flux ratio on the thermal stratification were studied. Temperature differences up to 90 K between the bottom and the top of the pipe have been detected. By varying the bulk fluid inlet temperature, the thermal inflow lengths have been quantified for a fully developed temperature stratification with up to 180 inner pipe diameters. The Petukhov and Richardson buoyancy criteria show that the experimental results are strongly influenced by buoyancy. For the Jackson buoyancy criterion, a new threshold of higher than 160 is proposed. The validation of four empirical Nusselt correlations shows that the Bishop correlation has the lowest total average deviation of 52%, especially for the bottom site of both pipes with an average deviation of 29%. • 54 Experiments with heated horizontal sCO 2 flows in pipes were carried out. • Thermal stratifications were investigated with temperature differences up to 90 K. • Thermal inflow lengths were quantified with up to 180 d i for fully developed flows. • Three different criteria for buoyancy and one for flow acceleration were tested. • Four different heat transfer correlations were validated against experimental data. [ABSTRACT FROM AUTHOR]

In this publication, the cooling heat transfer coefficient (h t c) is investigated in a 3 mm diameter tube with vertical flow orientation. Commonly used calculation methods of the heat transfer coefficient are presented. Although developed for heating of sCO 2 , the mixed convection criterion is used to evaluate the heat transfer deterioration. The effects of the CO 2 mass flux of 141 − 354 k g / m ² s and bulk fluid temperatures of 20 − 50 ° C with a constant pressure of 80 b a r on the heat transfer were examined. The upwards flow shows a steady decrease in the h t c with the reduction of the mass flux. However, the downwards flow shows significant effects of buoyancy. At low mass flux the distinct peak in the h t c at the pseudocritical temperature (T p c) disappears. The deteriorated heat transfer in the downwards flow showed significant lower wall temperatures compared to the upwards flow. The separate evaluation of the liquid-like and gas-like region indicates that the deterioration may vary based on the fluid properties. The results were regressed within +-15% in the forced convection region and +-40% in the mixed convection region. The comparison with the literature data of vertical cooling in 6 mm showed different behaviour which indicates an influence of the tube diameter. • Heat transfer of sCO2 depends strongly on the flow direction. • The flow direction influences the heat transfer much stronger at low mass flux. • Heat transfer deterioration leads to a strong decrease in the wall temperature. [ABSTRACT FROM AUTHOR]

Abstract: In this paper an experimental investigation of main influencing parameters on the dynamics of a metal hydride reaction bed is presented. The metal hydride used in this work is LmNi4.91Sn0.15 and the applied reaction bed is based on a capillary tube bundle heat exchanger with a large heat transfer surface. Based on experimental investigations of the hydrogen distribution in the capillary tube bundle reaction bed a limitation of the reaction bed dynamics due to insufficient hydrogen transport can be excluded. However, it is shown that the desorption dynamics of the reaction bed can be limited by the intrinsic kinetics of the used AB5 alloy. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a detailed experimental analysis of a metal hydride based cooling system is presented. For the high temperature side an AB5 type alloy (LmNi4.91Sn0.15) was chosen, whereas an AB2 type alloy (Ti0.99Zr0.01V0.43Fe0.09Cr0.05Mn1.5) is used for the low temperature side. Due to very good heat and also mass transfer characteristics (among others, large heat transfer surface area) of the utilized capillary tube bundle reaction bed, very short half-cycle times in the order of 100 s have been reached. Consequently, the specific cooling power of the system is up to 780 W per kg desorbing metal hydride – depending on the temperature boundary conditions. The system was experimentally analyzed for different cooling and ambient temperatures, whereas the heating temperature was fixed to 130 °C. [Copyright &y& Elsevier]

Abstract: The formation of bubbles on an orifice plate involves a moving contact line, especially in case of poor wetting conditions. The dynamics of the moving contact line and contact angle have a significant impact on the bubble departure size. Therefore, for the numerical simulation, an appropriate contact line boundary condition is essential for a correct prediction of the bubble formation. Numerical tests have been performed on two kinds of contact line models, one is a contact line velocity dependent model (Model-A, a commonly used model) and the other is a stick-slip model (Model-B). The calculation results using Model-A depend greatly on the prescribed maximum contact line velocity. With Model-B a parameter-independent prediction can be obtained provided that the mesh is sufficiently fine. The dynamic advancing and receding contact angles, which are two required inputs to both models, have a significant influence on the predicted bubble departure diameter, if the contact line moves beyond the inner rim of the orifice. The effect of wettability on the bubble departure size is realized via the variation of the maximum contact diameter. When the contact line sticks to a small region near the inner rim of the orifice, such as the bubble formation on a thin-walled nozzle, the effects of the contact angle and contact line models are negligible. [Copyright &y& Elsevier]

• Heat transfer of sCO 2 depends strongly on the fluid temperature and pressure. • Cooling water temperature leads to a trade-off in heat transfer performance. • CO 2 mass flux influences the heat transfer much stronger near the pseudocritical temperature. • The wall fluid properties are most appropriate for sCO 2 cooling Nusselt correlation. [Display omitted] The heat transfer of carbon dioxide near the critical point cooled in a horizontal tube with 2 mm inner diameter was investigated experimentally. The local heat transfer coefficients (htc) were determined by temperature measurements inside the tube wall along the 1.2 m cooled length. The effects of the CO 2 mass fluxes in the range of 400–1300 kg/m2s, CO 2 inlet pressures between 77 and 85 bar, CO 2 bulk fluid temperatures between 10 and 85 °C and cooling water temperatures between 10 and 40 °C on the heat transfer were examined. The experimental results are compared with existing correlations for sCO 2 heat transfer in small diameter tubes. A modified Dittus-Boelter equation was developed to predict the heat transfer coefficients with an accuracy of 20%. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique. [Copyright &y& Elsevier]

Abstract: Experimental investigations on flow boiling phenomena in a vertical narrow rectangular microchannel with the hydraulic diameter were carried out. The experiments were performed under fluid-inlet subcooled conditions with de-ionised and degassed water for different mass fluxes. Investigations on pressure drop and heat transfer during single- and two-phase flow have been carried out. Moreover, flow visualisation of the two-phase flow patterns along the channel was performed using a digital high-speed video camera. The present work outlines local heat transfer coefficients for three mass fluxes (200, 700 and 1500 kg m−2 s−1) and heat fluxes (30–110, 35–150 and 65–200 kW m−2) during two-phase flow. The visual observations were used to obtain a better insight about the heat transfer mechanisms. The goal of this work is to find the circumstance of the best heat transfer circumstance in microchannels. [Copyright &y& Elsevier]

Abstract: Experiments on flow boiling of water are carried out for a rectangular vertical channel with dimension 0.86×2.0mm2 (width×depth, hydraulic diameter d h =1.2mm). The Confinement number Co ∗ for the investigated range of operating parameters is about 2.2. So, applying the criterion Co ∗ >0.5 for microchannels, the investigated channel has to be classified as “micro”. Investigations of heat transfer, pressure drop and flow pattern were carried out for different mass and heat fluxes and their relationships were analysed. [Copyright &y& Elsevier]

Abstract: A series of tests have been carried out with a miniature loop heat pipe (mLHP), which has been developed for consumer electronics cooling, for horizontal and four vertical orientations under different sink temperatures. The mLHP has a cylindrical evaporator of 5 mm outer diameter and 29 mm length. The steady-state operating characteristics are similar for different orientations except for the orientation where the evaporator is above the compensation chamber. At an evaporator temperature of 75 °C, an evaporator heat load up to 70 W can be reached with thermal resistance of about 0.2 °C/W. The transient behavior of the mLHP is studied in detail. In general, the mLHP can be started up with very low power input (5 W). Big temperature oscillations in the liquid line were found in many cases, however, the temperature oscillations in the evaporator are minimum. The orientations greatly influence the operating characteristics of the mLHP. At least for the horizontal orientation, the overall performance of the tested mLHP is satisfying. [Copyright &y& Elsevier]

Abstract: Visualization experiments were carried out for boiling of propane, isobutane and their mixtures on a smooth and four reentrant enhanced tubes. The results shown by the video pictures indicate a strong mixture effect on nucleation and evaporation processes on the enhanced surfaces. The fluid physical properties also influence the bubble behavior and consequently affect the heat transfer performance. The effects of reentrant enhanced surface geometries are complicated and closely related to mixture effects, fluid properties and working conditions. Based on the analysis of the available visualization results, the different heat transfer performance for different enhanced surfaces and especially the difference between pure fluids and mixtures, as shown in an accompanying paper [Chen et al., Int. J. Heat Mass Transfer, in press], are partially explained. [Copyright &y& Elsevier]

Abstract: Pool boiling heat transfer experiments were carried out on a conventional smooth tube and four enhanced tubes with reentrant surfaces using propane, isobutane and their mixtures as working fluids for six saturation temperatures. The heat transfer performance is very different for different surface–fluid combinations. Compared to the smooth tube, the mixture boiling heat transfer degradation is more significant for the enhanced tubes. The current data are compared with available literature data for the same fluids and also with data for R12 and R134a. Experimental results of boiling hysteresis and for twin-tube bundles are also provided. Further explanations for the different heat transfer performances is provided by means of visualization in an accompanying paper [Y. Chen, M. Groll, R. Mertz, R. Kulenovic, Visualization and mechanisms of pool boiling of propane, isobutane and their mixtures on enhanced tubes with reentrant channels, submitted to Int. J. Heat Mass Transfer (H/S 04016)]. [Copyright &y& Elsevier]

Visualization experiments were carried out for nucleate pool boiling of propane and iso-butane on a horizontal smooth tube and two kinds of structured enhanced tubes. Some bubble dynamic parameters were measured. Comparisons were made between the smooth tube and the enhanced tubes for bubble growth rate, departure diameter, frequency, active site density, rise velocity and latent heat transfer. The effects of physical properties on the bubble dynamics are discussed. The present experimental results are compared with the model of Haider and Webb, relatively good agreement is found for enhanced surfaces. The augmentation mechanisms of heat transfer from structured enhanced tubes are also discussed. [Copyright &y& Elsevier]

The formation of bubbles on an orifice plate involves a moving contact line, especially in case of poor wetting conditions. The dynamics of the moving contact line and contact angle have a significant impact on the bubble departure size. This work presents visualization and numerical simulations on gas bubble formation on submerged orifices of different wettabilities and diameters. The transient bubble shape (image), departure diameter, contact line diameter, apparent contact angle, neck diameter, and neck height are obtained under various conditions. Under certain wetting and flow conditions, the relationship between moving contact line and apparent contact angle can be relatively well defined. However, this relationship cannot be generalized for other conditions. Two kinds of contact line models have been re-evaluated against experiments, one is a contact line velocity dependent model (Model-A) and the other is a stick-slip model (Model-B). Numerical simulations were performed using Model-B and the results show good agreement with the experiments. The bubble formation data provided are useful for supporting and verifying the numerical simulations. [ABSTRACT FROM AUTHOR]

Abstract: In this manuscript, the estimation of thermochemical properties such as enthalpy of formation (ΔH) and entropy of formation (ΔS) during hydriding and dehydriding reactions of three titanium (Ti), five misch metal (Mm) and eight lanthanum (La) based metal hydride alloys are presented. van't Hoff plots of the selected metal hydride alloys are constructed using the Pressure–Concentration–Temperature (PCT) characteristics measured at different operating temperatures. Reaction enthalpies during hydriding and dehydriding processes are expressed as a function of hydrogen storage capacity by a polynomial equation of order three. The average enthalpy of formation during dehydriding process of the investigated titanium and lanthanum based alloys are about 3–18% higher than their respective hydriding enthalpies. While, dehydriding enthalpies of LmNi4.49Al0.205Mn0.205Co0.1 and MmNi4.85Al0.15 are about 26.5% and 40% higher than their respective hydriding enthalpies. The variation in reaction enthalpies during the initial and final stages of hydriding process of titanium based alloys is in the range of 7–9%, while the respective ranges for misch metal and lanthanum based alloys are about 3–7.5% and 2.1–8.5%, respectively. [Copyright &y& Elsevier]