Qincheng Bi

Co-current air-water two-phase flow patterns in vertical triangular microchannels

Characteristics of co-current upward air–water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866 mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters (dh=2.886 and 1.443 mm). However, for the smallest channel (dh=0.866 mm), dispersed bubbly flow pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of dh=0.866 mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally, flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.

Pressure drop characteristics of gas-liquid two-phase flow in vertical miniature triangular channels

Experimental data are presented for the gas velocity, the void fraction, and the pressure drop of upward co-current air–water two-phase flow through vertical miniature triangular channels having hydraulic diameters of 0.866, 1.443 and 2.886 mm, with superficial air velocity ranging from jg=0.1 to 100 m/s and superficial water velocity ranging from jl=0.08 to 6 m/s. A correlation is developed for predicting the pressure drops of single-phase laminar and turbulent flow through the miniature triangular channels based on that proposed by Churchill (S.W. Churchill, Friction-factor equation spans all fluid flow regimes, Chem. Eng. 84(24) (1977) 91–92) for circular tubes. This work shows that the pressure drop of two-phase flow in the miniature triangular channels can be well predicted by the Lockhart–Martinelli correlation (R.W. Lockhart, R.C. Martinelli, Proposed correlation of data for isothermal two-phase two-component flow in pipes, Chem. Eng. Prog. 45 (1949) 39–48) if the newly proposed friction factor correlation for single-phase flow is adopted.

Taylor bubbles in miniaturized circular and noncircular channels

The study of two-phase ̄ow and boiling heat transfer in a con®ned space or micro-sized geometry has received considerable attention in the past few years because of its applications in the cooling of a wide variety of devices, such as high-density multi-chip modules in supercomputers, high-heat̄ux modular electronics, high-powered X-ray and other diagnostic devices, high̄ux heat exchangers in aerospace systems, cryogenic cooling systems in satellites, and so on. One of the important aspects associated with two-phase ̄ows in microchannels is to study the bubble behaviors. In this work, the motion of elongated bubbles formed in miniature channels with stagnant liquids is investigated. Drift̄ux model is widely used for the analysis of gas±liquid two-phase ̄ows, primarily because it is applicable to various ̄ow patterns and a wide range of void fractions. According to the theory (Wallis, 1969), the velocity of the gas phase in a tube is given by

Numerical Simulation of a Falling Ferrofluid Droplet in a Uniform Magnetic Field by the VOSET Method

This article presents a two-dimensional numerical simulation of the dynamics of a ferrofluid droplet falling in a non-magnetic fluid under a uniform magnetic field. A new interface tracking method called VOSET (a coupled volume-of-fluid and level set method) is used for capturing the evolution of interface. The Continuum Surface Force (CSF) model is used to model the surface tension force. A linearly magnetizable ferrofluid is assumed, and the magnetic force caused by the jump of magnetic permeability across the interface is incorporated into the Navier-Stokes equation as a body force. The effects of the magnetic Bond number, susceptibility, Weber number, Reynolds number, and magnetic field direction on the motion and deformation of the droplet are investigated. The results show that the increase in magnetic Bond number or susceptibility leads to the larger deformation of the droplet. For a high Weber number, even the formation of a teardrop-like droplet and an oblate elliptical-cap droplet occurs in the vertical and horizontal magnetic fields, respectively.

Experimental Performance Comparison of Shell-Side Heat Transfer for Shell-and-Tube Heat Exchangers with Different Helical Baffles

In this study, experiments were carried out to study the effects of baffle overlap proportion on the shell-side flow resistance and heat transfer performance of the shell-and-tube heat exchangers with helical baffles (STHXsHB). Three STHXsHB with an overlap proportion of 10% and helix angles of 20°, 30°, and 40° were tested. Comparisons were made of the experimental data of the STHXsHB with the same helix angles but 50% overlap proportion. The theory of entransy dissipation was employed to evaluate the irreversible loss in STHXsHB with different helix angles and overlap proportions. The results indicated that both the baffle overlap proportion and the helix angle have a great effect on the shell-side flow resistance and heat transfer. For a given helix angle, the comprehensive performance of STHXsHB with small overlap proportion is always better than that with large overlap proportion at the same mass flow rate or Reynolds number on the shell side. However, for the same heat transfer area, working conditions, and helix angle, the STHXsHB with large baffle overlap proportion has less irreversibility in the heat exchange process, according to the theory of entransy dissipation. In addition, experimental results demonstrated that the configuration of the relatively large helix angle and baffle overlap proportion is the preferred alternative in STHXsHB.

Dynamics of Evaporation and Cooling of a Water Droplet During the Early Stage of Depressurization

This paper reports an experimental and numerical study of evaporation and cooling of a water droplet during the early stage of depressurization in a test vessel. During the experiment, a distilled water droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature, and the droplet surface temperature was captured by an infrared thermograph. Experimental data indicated a large temperature difference within the droplet during the early stage of depressurization. A thermodynamic analysis of the experimental data found that the pressure reduction was not fast enough to induce liquid superheating and thus equilibrium evaporation was expected. A mathematical model was then constructed to simulate the droplet evaporation process. The model solves one-dimensional heat conduction equation for the temperature distribution inside the water droplet, with the convective heat transfer inside the droplet simplified through an effective conductivity factor. A simplified treatment was introduced to quantify the convective evaporation due to air movement and droplet swing induced by sudden opening of the electro-magnetic valve and the following air exiting. The model-predictions agree well with the measured temperature data, demonstrating the soundness of the present model.Copyright

Convective heat transfer characteristics of supercritical hydrocarbon fuel in small non-circular cross-section channels

Convective heat transfer characteristics of supercritical pressure hydrocarbon fuel were experimentally investigated in electrically heated small non-circular channels with hydraulic diameter ranging from 2.00 to 3.00 mm. In this research, the system pressure and fuel mass velocity were maintained at supercritical (3.0 MPa) and 840.0 kg/(m 2 ∙s), while fuel inlet temperature varied from 200.0 ~ 450.0 °C. Heat transfer characteristics of various cross-section channels were obtained in view of the temperature profiles on the channel outside surface. Experimental results were compared among different non-circular channels. The effects of inlet fuel temperature, heat flux, aspect ratio or length to diameter ratio on the convective heat transfer characteristics were discussed. Due to the varieties of length to diameter ratio, the unique heat transfer characteristics of non-circular cross-section channels seemed to make them suitable for the application of hypersonic vehicle. However, this study did not consider the pyrolytic reaction in the high temperature range. The coking characteristics of small non-circular channels under high temperature may trade off the benefits acquired in the heat transfer characteristics.

Effects of Baffle Overlap Proportion on Shell-Side Performance of Shell and Tube Heat Exchanger With Helical Baffles

Different overlap configurations of discontinuous helical baffles affect the flow pattern of the shell-side fluid directly, and thus there is a significant impact on the flow and heat transfer characteristics of the shell-side fluid. In the present paper, experiments were carried out to study the impact of baffle overlap proportion on the shell-side flow and heat transfer performance of the shell-and-tube heat exchanger with helical baffles (STHEHB). Two different shell-side friction factors, the friction factor per helical pitch (fs,1B) and the friction factor per tube length (fs,1m), were defined based on different reference lengths. The results showed that, since the baffle overlap proportion leads to different helical pitch as well as flow fields in shell side, opposite conclusions are obtained by choosing different reference length. Based on the same Reynolds number, the shell-side Nusselt number of the STHEHB with 10% baffle overlap is higher than that with 50% baffle overlap. The reason is that the larger baffle overlap proportion produces more serious leak flows and weakens the heat transfer in shell side. The comparison of heat transfer coefficient per unit pressure drop versus shell-side flow rate showed that the STHEHB with smaller baffle overlap proportion has better comprehensive heat transfer performance, but the difference between the two decreases gradually with the increase of the flow rate.Copyright

Visualization and flow boiling heat transfer of hydrocarbons in a horizontal tube

Visualizations of a specific hydrocarbon fuel in a horizontal tube with 2.0 mm inside diameter were investigated. The experiments were conducted at mass velocity of 213.4, 426.5 and 640.2 kg/ (m2⋅s), diabatic lengths of 140, 240 and 420 mm under the pressure from 2.0-2.7 MPa. In the sub-pressure conditions, bubbly, intermittent, stratified-wave, churn and annular flow patterns were observed. The frictional pressure drops were also measured to distinguish the patterns. The development of flow patterns and frictional pressure drop were positively related to the mass velocity and the heat flux. However, the diabatic length of the tube takes an important part in the process. The residence time of the fluid does not only affect the transition of the patterns but influence the composition of the fuel manifested by the fuel color and carbon deposit. The special observational phenomenon was obtained for the supercritical pressure fluid. The flow in the tube became fuzzier and pressure drop changed sharply near th...

Forced Convection Heat Transfer Using High Temperature and Pressure Water in an Upward-Inclined Tube

Within the range of pressure from 9 to 28 MPa, mass flux from 600 to 1500 kg/m2 s, heat flux at inside wall from 200 to 600 kW/m2 , and wall temperature up to 650 °C, experiments were conducted to research the forced convection heat transfer of water in an inclined upward tube with an inclination angle of 20 deg and an inner diameter of 26 mm. According to the experimental data, the effects of pressure and heat flux on heat transfer of water were analyzed in detail. In the subcritical pressure region, it was found that heat transfer characteristics of water are not uniform along the circumference of the inclined tube. Temperature of the top is always higher than that of the bottom, which can be attributed to the buoyancy effect in the inclined tube. In the supercritical pressure region, natural convection makes the low-density hot fluid gather at the top of the inclined tube; hence, heat transfer condition is deteriorated and wall temperature is increased. Furthermore, the criterions of Petukhov and Jackson were selected to judge the buoyancy effect in the inclined upward tube. The result seems acceptable but these criterions should be further improved to get a better applicability for an inclined tube.