Small Vertical Tube Research Articles

Study on Convective Heat Transfer of Supercritical Nitrogen in a Vertical Tube for Liquid Air Energy Storage

The convective heat transfer behavior of supercritical nitrogen (S-N2) has played a significant role in optimizing the design of recently emerging cryogenic cold storage and recovery systems. However, studies on S-N2 heat transfer have been relatively scarce, not to mention that there is a legitimate urge for a robust numerical model to accurately predict and explain S-N2 heat transfer under various working conditions. In this paper, both experimental and numerical studies were conducted for convective heat transfer of S-N2 in a small vertical tube. The results demonstrated that the standard k-ε model performed better for predicting the key heat transfer characteristics of S-N2 than the SST k-ω model. The effects of heat flux and inlet pressure on the heat transfer characteristics under a large mass flux were evaluated. The variation mechanisms of local heat transfer performance were revealed by illustrating radial profiles of thermophysical properties and turbulent parameters of N2. It was found that the local performance variation along the flow direction was mainly determined by the radial profile of specific heat while the variation of the best local performance with the ratio of heat flux to mass flux was mainly determined by the radial profile of turbulent viscosity.

Direct numerical simulation of convective heat transfer of supercritical pressure in a vertical tube with buoyancy and thermal acceleration effects

Supercritical pressure fluids are widely used in heat transfer and energy systems. The benefit of high heat transfer performance and the successful avoidance of phase change from the use of supercritical pressure fluids are well-known, but the complex behaviours of such fluids owing to dramatic thermal property variations pose strong challenges to the design of heat transfer applications. In this paper, the turbulent flow and heat transfer of supercritical pressure $\textrm {CO}_2$ in a small vertical tube influenced by coupled effects of buoyancy and thermal acceleration are numerically investigated using direct numerical simulation. Both upward and downward flows with an inlet Reynolds number of 3540 and pressure of 7.75 MPa have been simulated and the results are compared with corresponding experimental data. The flow and heat transfer results reveal that under buoyancy and thermal acceleration, the turbulent flow and heat transfer exhibit four developing periods in which buoyancy and thermal acceleration alternately dominate. The results suggest a way to distinguish the dominant factor of heat transfer in different periods and a criterion for heat transfer degradation under the complex coupling of buoyancy and thermal acceleration. An analysis of the orthogonal decomposition and the generative mechanism of turbulent structures indicates that the flow acceleration induces a stretch-to-disrupt mechanism of coherent turbulent structures. The significant flow acceleration can destroy the three-dimensional flow structure and stretch the vortices resulting in dissipation.

Heat Transfer Coefficient, Pressure Gradient, and Flow Patterns of R1234yf Evaporating in Microchannel Tube

Abstract This paper presents the heat transfer coefficient, pressure gradient, and flow pattern of R1234yf in a microchannel tube. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at the exit of data points are presented in the same plot. The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg m−2s−1, heat flux from 0 to 6 kW m−2, vapor quality from 0 to 1, and inlet saturation temperature from 10 to 30 °C. Comparing the correlations to the HTC measurements at very low quality (about 0.1), Gorenflo, D., and Kenning, D. (2010, Pool Boiling, in: VDI Heat Atlas, 2nd ed, Springer, pp. 757–788) agree with the results. As vapor quality increases, pressure gradient increases. The adiabatic pressure gradient is a strong function of mass flux and saturation pressure (temperature). Flow patterns of R1234yf are also affected by mass flux and saturation pressure. The heat transfer coefficient is a strong function of mass flux and heat flux. The saturation temperature has a smaller effect on HTC in the condition range (10 – 30 °C). Under the test range, the accelerating pressure drop is insignificant compared to friction. Comparing to the results, Mishima, K., and Hibiki, T. (1996, “Some Characteristics of Air-Water Two-Phase Flow in Small Diameter Vertical Tubes,” Int. J. Multiph. Flow, 22(4), pp. 703–712) and Muller-Steinhagen, H., and Heck, K. (1986, “A Simple Friction Pressure Drop Correlation for Two-Phase Flow in Pipes,” Accessed March 1, 2018)., 20, pp. 297–308.) have small mean absolute error (MAE) to predict local pressure gradient. For the heat transfer coefficient, Sun, L., and Mishima, K. (2009, “An Evaluation of Prediction Methods for Saturated Flow Boiling Heat Transfer in Mini-Channels,” Int. J. Heat Mass Transf, 52(23–24), pp. 5323–5329) and Gungor, K. E., and Winterton, R. H. S. (1986, “A General Correlation for Flow Boiling in Tubes and Annuli,” Int. J. Heat Mass Transf, 29(3), pp. 351–358) have an MAE less than 30%.

Heat Transfer Enhancement of Supercritical Nitrogen Flowing Downward in a Small Vertical Tube: Evaluation of System Parameter Effects

In this paper, the heat transfer enhancement (HTE) of supercritical nitrogen flowing downward in a vertical small tube (diameter 2 mm) is studied using the commercial software CFX of Ansys16.1, to provide theoretical guidance on the design of high-performance heat transfer systems. An effective numerical simulation method, which employs the SSG Reynolds stress model with enhanced wall treatment, is applied to study the heat transfer of supercritical nitrogen under typical working conditions. The objective is to evaluate the effect of the main parameters taking into account the buoyancy and flow acceleration effects. Simulation results are compared with results calculated from three well-known empirical correlations and the applicability of empirical correlation is discussed in detail. It is discovered that the Watts and Chou correlation accurately fits the simulation results of supercritical nitrogen and the Dittus-Boelter and Jackson correlations can only be used for high-pressure conditions. The HTE of supercritical nitrogen is closely related to the laminar sub-layer and buffer layer of a boundary layer. The buoyancy effect on the HTE should be considered at low mass flux conditions, and thermal acceleration can be completely ignored for the cases studied. The special HTE featured by the increment in heat transfer coefficient with increasing heat flux is discovered at low pressure, and simulation results proved that this HTE is caused by the combined actions of buoyancy as well as significant variations in specific heat and viscosity.

Heat and mass transfer modeling during laminar condensation of non-cryogenic downward fluids flow in a small vertical tube

The purpose of this paper is to investigate mass and heat transfer in the process of film condensation of vapor-air mixture for non-cryogenic fluids flow in a small vertical tube. A two-phase mathematical model is developed to model the mixture and liquid film. The governing equations for mixture and liquid-film have been resolved using a numerical method. Furthermore, this phenomenon analyzed is linked to a steady-state. Therefore, the development of numerical codes allows us to investigate the effect of implicated parameters on this phenomenon. Ethanol and methanol as non-cryogenic typical working fluids are realized for a good understanding of the heat and mass transfer mechanism during condensation. In this way, several effects of influencing parameters were examined. The predicted results showed a good agreement with experimental data.

Experimental Study of Condensation Heat Transfer in Helical Coil Heat Exchanger

Helical coils are used as one of the heat transfer enhancement techniques in many engineering applications such as in nuclear industry, chemical and petrochemical industries, HVAC systems, and many other engineering applications. In nuclear industry, helical coils are used as a method for exchanging heat in large sodium-cooled fast breeder reactor. In comparison with straight-tube heat exchangers, rate of heat transfer for helical coil heat exchanger is significantly high. This is due to secondary flow pattern in planes normal to the main flow direction of fluid in the helical coiled tube. The present work is an experimental investigation on heat transfer characteristics during steam condensing inside a small diameter vertical helical coil tube with the cooling water flowing in the shell in counter flow direction. The effect of mass flow rate and saturation temperature of steam on the heat transfer coefficient is investigated and plotted. The results confirm the significant improvement in the heat transfer coefficient for condensation of steam.

Steady and transient critical heat flux for subcooled water in a mini channel

The transient critical heat fluxes (CHFs) were measured for water flowing in a small vertical tube with exponentially increasing heat inputs. A stainless steel tube (SUS 304) with an inner diameter of 1.0mm and a length of 47.4mm was used as a test tube, which was mounted vertically in the experimental water loop. In the experiment, the upward flow velocity ranged from 9.4 to 13.4m/s and the inlet subcooling ranged from 85 to 145K. The heat generation rate was exponentially increased as a function of Q0exp(t/τ). The period of the heat generation rate was ranged from 82ms to 30s. Experimental results indicated that the CHFs increased with decreasing period of heat generation rates and increasing flow velocity. For shorter periods of the heat generation rate, the CHFs were significantly higher. The CHFs were affected by the period of the heat generation rate and flow velocities. The empirical correlations of the transient CHFs for the small tube were obtained based on the experimental data.

SO3 formation under oxy-CFB combustion conditions

Due to the enrichment of SO2 and H2O, SO3 formation during oxy-fuel circulating fluidized bed (CFB) combustion may significantly increase over the air fired case and this requires special attention in terms of safety consideration. In an attempt to better elucidate the formation mechanism of SO3 under oxy-fuel CFB conditions, homogenous and heterogeneous experiments were performed using a small vertical tube reactor to model the SO3 formation condition in the back pass channels and then the mechanism deduced was further validated by tests and measurements using a pilot-scale 50kWth oxy-fuel CFB combustor with wet flue gas recycle. Results show that replacing N2 by CO2 does not change the SO3 formation levels while the addition of water enhances SO3 formation. The increased O2, SO2, H2O concentrations along with increasing temperature are favorable for enhancing SO3 formation over the range of tested parameters. Fe2O3, CuO and V2O5 are shown to be able to catalyze SO2 conversion to SO3 under oxy-fuel atmosphere; of these V2O5s catalyzing ability is the strongest. Fly ash can either catalyze the SO3 formation or absorb SO3, depending on the temperature and the alkalinity of the ash. The results from the pilot plant burning bituminous coal demonstrate that SO3 concentration in the flue gas is about 4.5 times higher during oxy-fuel combustion than that under air combustion.

Experimental investigation of convection heat transfer of n-decane at supercritical pressures in small vertical tubes

This paper presents experimental investigations of the convection heat transfer of n-decane at supercritical pressures in vertical tubes with inner diameters of 0.95 and 2.00mm for various inlet pressures, heat fluxes and flow directions. The effects of the heat fluxes, property variations, buoyancy and flow acceleration on the convection heat transfer were studied. The results show that for high inlet Reynolds numbers neither buoyancy nor flow acceleration affects heat transfer significantly, while for low inlet Reynolds numbers buoyancy strongly reduces heat transfer coefficient in upward flow cases. The threshold for Bo∗ was found to be about 2.0×10−7. Two local Nusselt number correlations for the convection heat transfer of supercritical pressures n-decane with and without buoyancy effect were developed based on the experimental data.

An experimental study on two-phase pressure drop in small diameter horizontal, downward inclined and vertical tubes

An experimental study of two-phase pressure drop in small diameter tubes orientated horizontally, vertically and at two other downward inclinations of ?= 300 and ? = 600 is described in this paper. Acrylic transparent tubes of internal diameters 4.0, 6.0, and 8.0 mm with lengths of 400 mm were used as the test section. Air-water mixture was used as the working fluid. Two-phase pressure drop was measured and compared with the existing correlations. These correlations are commonly used for calculation of pressure drop in macro and mini-microchannels. It is observed that the existing correlations are inadequate in predicting the two-phase pressure drop in small diameter tubes. Based on the experimental data, a new correlation has been proposed for predicting the two-phase pressure drop. This correlation is developed by modification of Chisholm parameter C by incorporating different parameters. It was found that the proposed correlation predicted two-phase pressure drop at satisfactory level.

Modelling the Mach bands illusion by means of a diffusion model.

First, we criticize the validity of the principle of lateral inhibition. Second, on the basis of illusory phenomena and stabilized retinal images, we point out that the retina does not code the absolute luminance; the retina forwards a relative luminance sketch towards higher levels of the visual system. However, at the level of conscious processing the perceptual counterpart of absolute luminance, brightness, is available. Therefore, it is reasonable to assume that a reconstruction process is carried out by the visual system, which recovers the inner representation that corresponds to the retinal light distribution from the coded relative luminance sketch. We provide an illustrative description of a computational model of this reconstruction process. The basis of the reconstruction is a mathematically provable theorem, according to which if image P is produced from image I by Laplacian filtering, and then P is used as the sources and sinks of a homogeneous linear diffusion process, then the equilibrium of the diffusion will be identical to the original image I. We have illustrated this by a one-dimensional heat diffusion example, and by a series of test tubes connected to each other, also in one dimension. Brightness illusions are considered as a side effect of this diffusion-based reconstruction process. If the diffusion process deviates from the principle of homogeneous linearity, then the result of the reconstruction will deviate from the original image I. We showed a concrete illustration of this with regards to the Mach bands illusion: here we violated the principle of homogeneous linearity by means of inserting a small vertical tube serving as a serial resistance between each test tube and the horizontal connecting tube. This violation resulted in a change of water level in the source and the sink test tubes corresponding to the Mach bands illusion.

Flow Patterns of Oil/Water Two-Phase Upflows in a Small Vertical Tube

The flow patterns of oil/water two-phase upward flows in a small vertical tube with an internal diameter of 0.010 m were investigated by high speed video system. Using stainless steel tube as test section, transparent plastic tube as observing section and deionized water and kerosene (density of 796kg/m3) as working fluids, 5 flow patterns, i.e., annular, churn, slug, bubble and dispersed droplets, were observed under the experimental conditions. The transition boundaries of these flow patterns were compared with the literature and theoretical models of Taitel et al [1] for gas-liquid upward flows. There are some differences of the transition boundaries between the present study and the literature of either gas-liquid or liquid-liquid systems. The theoretical models of Taitel et al can well predict the transition boundaries from annular to churn and from churn to bubble.

Magnetic Field Structures in a Facular Region Observed by THEMIS and Hinode

The main objective of this paper is to build and compare vector magnetic maps obtained by two spectral polarimeters, i.e. THEMIS/MTR and Hinode SOT/SP, using two inversion codes (UNNOFIT and MELANIE) based on the Milne-Eddington solar atmosphere model. To this end, we used observations of a facular region within active region NOAA 10996 on 23 May 2008, and found consistent results concerning the field strength, azimuth and inclination distributions. Because SOT/SP is free from the seeing effect and has better spatial resolution, we were able to resolve small magnetic polarities with sizes of 1" to 2", and we could detect strong horizontal magnetic fields, which converge or diverge in negative or positive facular polarities. These findings support models which suggest the existence of small vertical flux tube bundles in faculae. A new method is proposed to get the relative formation heights of the multi-lines observed by MTR assuming the validity of a flux tube model for the faculae. We found that the Fe 1 6302.5 \AA line forms at a greater atmospheric height than the Fe 1 5250.2 \AA line.

Experimental and numerical study of convection heat transfer of CO 2 at super-critical pressures during cooling in small vertical tube

Convection heat transfer of CO 2 at super-critical pressures during cooling in a vertical small tube with inner diameter of 2.00 mm was investigated experimentally and numerically. The local heat transfer coefficients were determined through a combination of experimental measurements and numerical simulations. This study investigated the effects of pressure, cooling water mass flow rate, CO 2 mass flow rate, CO 2 inlet temperature, flow direction, properties variation and buoyancy on convection heat transfer in small tube. The results show that the local heat transfer coefficients vary significantly along the tube when the CO 2 bulk temperatures are in the near-critical region. The increase of specific heat and turbulence kinetic energy due to the density variation leads to the increase of the local heat transfer coefficients for upward flow. The buoyancy effect induced by density variation leads to a different variation trend of the local heat transfer coefficients along the tube for upward and downward flows. The numerical simulations were conducted using several k–ε turbulence models including the RNG k–ε model with a two-layer near wall treatment and three low-Reynolds number eddy viscosity turbulence models. The simulations using the low-Reynolds number k–ε model due to Yang–Shih has been found to be able to reproduce the general features exhibited in the experiments, although with a relatively large overestimation of measured wall temperatures. A better understanding of the mechanism of properties variation and buoyancy effects on convection heat transfer of CO 2 at super-critical pressures in a vertical small tube during cooling has been developed based on the information generated by the simulation on the detailed flow and turbulence fields.

Pressure drop of two-phase plug flow in round mini-channels: Influence of surface wettability

In the present experimental study, the pressure drop of two-phase plug flows in round mini-channels was investigated for three different tube materials, i.e., glass, polyurethane and Teflon, respectively, with their inner diameters ranging from 1.62 to 2.16 mm. Air and water were used as the test fluids. In the wet-plug flow regime (wet wall condition at the gas portions), the pressure drop was reasonably predicted by the homogeneous flow model or by the correlations of Mishima and Hibiki [K. Mishima, T. Hibiki, Some characteristics of air-water two-phase flow in small diameter vertical tubes, Int. J. Multiphase Flow 22 (1996) 703–712] and Chisholm [D. Chisholm, A theoretical basis for the Lockhart-Martinelli correlation for two-phase flow, Int. J. Heat Mass Transfer 10 (1967) 1767–1778]. On the other hand, in the dry-plug flow regime (dry wall condition at the gas portions), the role of the moving contact lines turned out to be significant. To take into account the effect of the moving contact lines, a modified Lockhart–Martinelli type correlation was proposed, which fitted the measured pressure-drop data within the mean deviation of 6%.

Critical heat flux of countercurrent boiling in an inclined small tube with closed bottom

The study was focused on the effect of the inclination angle on the critical heat flux of countercurrent boiling in an inclined uniformly heated tube with open top and closed bottom ends at zero inlet flow. The experimental results show that the CHF data of the small vertical tubes agree reasonably well with the predicting correlation proposed by Tien. The CHF data of the small inclined tubes decrease with reducing the inclination angle. The experimental data of the inclined tubes agrees reasonably well with the modified correlation, which is resulted from the conventional correlation for vertical tubes.

Boiling characteristics in small vertical tubes with closed bottom for nanofluids and nanoparticle-suspensions

An experimental study was carried out to understand the nucleate boiling characteristics and the critical heat flux (CHF) of water, the water based nanofluids and the water based nanoparticle-suspensions in vertical small heated tubes with a closed bottom. Here, the nanofluids consisted of the base liquid, the CuO nanoparticles and the surfactant. The nanoparticle-suspensions consisted of the base liquid and CuO nanoparticles. The surfactant was sodium dodecyl benzene sulfate. The study focused on the influence of the nanoparticles and surfactant on the nucleate boiling characteristics and the CHF. The experimental results indicated that the nanoparticle concentrations of the nanofluids and nanoparticle-suspensions in the tubes do not change during the boiling processes; the nanoparticles in the evaporated liquid are totally carried away by the steam. The boiling heat transfer rates of nanofluids are poorer than that of the base liquid. However, the boiling heat transfer rates of nanoparticle-suspensions are better than that of the base liquid. Comparing with the base liquid, the CHF of the nanofluids and the nanoparticle-suspensions is higher. The CHF is only related to nanoparticle mass concentration when the tube length and the tube diameter are fixed. The experiment confirm that there is a thin nanoparticle coating layer on the heated surface after the nanofluids boiling test but there is no coating layer on the heated surface after the nanoparticle-suspensions boiling test. This coating layer is the main reason that deteriorates the boiling heat transfer rates of nanofluids. An empirical correlation was proposed for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom.

A new method of entrainment fraction measurement in annular gas–liquid flow in a small diameter vertical tube

A new method was introduced to measure liquid entrainment fraction in gas–liquid two-phase upward annular flow in a vertical tube (i.d.=9.525 mm). In this method, a new liquid–gas separator was designed and the chemically-based titration method was used to effectively measure the entrainment fraction in real time. Experiments were conducted at low system pressure (∼1 atm), and relatively low gas and liquid superficial velocities ( V sg = 25.8 –45.5 m/s, and V sl = 0.15 –0.30 m/s). Data analysis shows that the results are repeatable and occupy the range commonly seen in annular flow. The entrainment fraction was found to be under 7% for all the experimental set points. The repeatability of the test results and comparisons with previous entrainment data indicate that the new technique can perform as well as the film removal technique.

EXPERIMENTAL INVESTIGATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A SMALL VERTICAL TUBE

An experimental study was performed to investigate the heat transfer characteristics of the non-boiling annular two-phase flow of nitrogen gas and subcooled water through a small vertical heated tube with uniform wall heat flux. The experimental results were compared with the numerical calculations based on the two-phase flow boundary layer model. The experimental data show that the annular two-phase flow of nitrogen gas and subcooled water in a small vertical tube can provide quite high heat transfer ability, while wall temperatures lie in a low range even if at quite high wall heat fluxes. In particular, the highest values of the wall temperatures are insensitive to the change of flowing and heating conditions. This characteristic is quite suitable for highly effective cooling of polymer electrolyte membrane fuel cells.

The effect of admixed material on the minimum explosible concentration of oil shale

The minimum explosible concentration (MEC) in the air atmosphere at the boundary between an explosion and no explosion in a dust cloud, has been investigated for several particle sizes of oil shale and for mixtures of oil shale and inert powder of different particle size. Limestone, stone dust and coarse particle size of oil shale were used as inert materials. Measurements were made in a standard small vertical tube apparatus. The results obtained indicated that the minimum explosible concentration is dependent on the particle size, i.e., values of MEC decrease with a decrease in the size of the particles. Below 70 μm, values of MEC become almost constant. Admixture of limestone as low as 5% to oil shale is sufficient to reduce the MEC values significantly.