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1.
Results of an experimental investigation of heat and mass transfer and wall shear stress at gas-liquid flow in a vertical tube are presented. Local wall shear stress and mass transfer coefficients were measured by an electrochemical method. Experiments were performed in the range of Reynolds number variation with respect to liquid Rci, = 8.5 × 103-5.4 × 104, gas Reg = 3 × 103-1.4 × 105, pressure 0.1-1 MPa. The relationship between heat and mass transfer and wall shear at gas-liquid flows is shown to exist. The results of measuring heat and mass transfer coefficients are generalized by formulas applied to calculate heat and mass transfer in single-phase turbulent flow. 相似文献
2.
High-speed visualization of adiabatic flow and heat transfer rate determination for constant wall heat flux conditions were performed to study the flow and heat transfer behavior of non-boiling, gas–liquid two-phase vertical upward flow in a 2.00 mm-diameter tube. Liquids of different volatilities, including water, ethylene glycol, and hexadecane, were employed to investigate the roles of convective heat transfer and evaporation for a wide range of flow conditions encompassing Taylor, slug-annular, and annular flow regimes. The heat transfer rate is found to depend strongly on the flow regime. Significant evaporative cooling was observed for the volatile system at high gas flow rates. A heat transfer enhancement up to fivefold over that for the liquid-only flow was observed in the annular flow regime. 相似文献
3.
Results of experimental investigation of a bubbly gas-liquid flow in an inclined flat channel are presented. Themeasurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and with different values of the volumetric gas flow rate ratio. The hydrodynamic structure wasmeasured bymeans of an electrochemical method using miniature shear stress probes. Values of average shear stress and heat transfer coefficient for different orientation of the channel were found. It is shown that in a bubbly gas-liquid flow the shear stress and heat transfer depend substantially on the channel inclination angle. 相似文献
4.
The results of numerical simulation of the structure of non-isothermal polydisperse bubbly turbulent flow and heat transfer behind a sudden tube expansion are presented. The study was carried out at a change in the initial diameter of the air bubbles within d m1 = 1–5 mm and their volumetric void fraction β = 0–10 %. Small bubbles are available in almost the entire cross section of the tube, while the large bubbles pass mainly through the flow core. An increase in the size of dispersed phase causes the growth of turbulence in the liquid phase due to flow turbulization, when there is a separated flow of liquid past the large bubbles. Adding the air bubbles causes a significant reduction in the length of the separation zone and heat transfer enhancement, and these effects increase with increasing bubble size and their gas volumetric flow rate ratio. 相似文献
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6.
Results of experimental investigation of the bubbly gas-liquid flow in a vertical annular channel are presented. The average and pulsation shear stresses and distributions of local void fraction were measured by the electrochemical method on both channel walls. It is shown that with a rise of gas flow rate ratio the value of wall shear stress increases significantly, and this effect becomes higher at a decrease in superficial liquid velocity. A presence of the gas phase effects significantly shear stress on the inner wall. Relative intensity of shear stress pulsations increases similarly on both channel walls. 相似文献
7.
The article presents results of an experimental study of the effect of gravitational orientation of the flow along its lower/upper solid boundaries on reduction of turbulent drag and void fraction profiles with injection of gas through a porous channel wall. The shear stress on the wall was measured in the Reynolds number range Rex = (0.23–1.1) × 107 by floating element transducers; the void fraction profile was determined using a fiber-optic sensor. The void fraction in the inner (near-wall) region of the boundary layer was shown to be a key parameter for turbulent drag reduction. The size of the inner region depends on the gas flow rate, the fluid velocity, the distance downstream of the gas generator, and the gravitational orientation of the wall. 相似文献
8.
毛细管内薄液膜轮廓和传热特性研究 总被引:2,自引:0,他引:2
本文认为毛细管的相变传热机理为液膜的导热和表面蒸发;表面蒸发受蒸汽温度、汽液界面的温度以及汽液压力差的共同控制。汽液流动机理为流动受脱离压力梯度、毛细力梯度支配。汽液相互作用机理为存在由于蒸发导致的动量转移切应力和由于汽液流速不同产生的摩擦切应力。提出的物理模型中较为全面地考虑了毛细管内传热、汽液流动及其相互作用。对毛细管半径和传热功率对薄液膜轮廓和传热特性影响程度的计算结果表明,随着毛细管半径的减小、传热功率的增大,蒸发界面区的长度会有所减小,这是针对微小空间得出的不同于常规情况的结论。 相似文献
9.
Results of experimental investigation of a bubbly gas-liquid flow in horizontal and weakly inclined (from −20° to +20°) flat
channel are presented. These measurements were carried out within the 0.2–1 m/s range of superficial velocities and volumetric
gas flow rate ratio of up to 0.2. The hydrodynamic structure was measured by the electrochemical method with application of
wall shear stress and conductivity microprobes. During the experiments signals of shear stress on the upper channel wall and
local gas flow rate ratio were recorded completely. After numerical treatment of recorded signals the profiles of local gas
flow rate ratio were obtained, average shear stress and its relative mean square pulsations on the upper channel wall were
determined. It is shown that under the studied regimes the bubbles are grouped into clusters, and the bubbly flow is presented
by alternation of bubbly clusters and single-phase liquid with separate bubbles and without them. Average wall shear stress
and absolute shear stress pulsations in the range of bubbly clusters and beyond them were determined. Histograms of probability
density distribution were obtained for the wall shear stress on the upper wall. It is shown that average shear stress and
absolute pulsations in clusters are significantly higher than those in the flow zone free from bubbles.
The work was financially supported by the Russian Foundation for Basic Research (No. 07-08-00405a). 相似文献
10.
Over a range of 70 < Re a < 9,600, 7 < Pr a < 130, 0 < β < 0.12, and 0.7 < n < 1, circumferential wall temperatures for air–water and air–aqueous polymer (viscoelastic) solution flows over a horizontal cylinder were measured experimentally. The 2.5-cm-diameter and 7.5-cm-long cylinder was heated by passing direct electric current through it. The peripherally averaged heat transfer coefficient for relatively dilute viscoelastic–air solutions, at any fixed flow rate of liquid phase, increases with β. Such increase is more pronounced at lower flow rates of liquid phase. For relatively more elastic solutions, the two-phase heat transfer decreases with increasing β. Such reduction is more pronounced at higher flow rates of liquid phase. A new correlation is proposed for predicting the Nusselt number for air–viscoelastic fluid flows over a heated cylinder in cross flow. 相似文献
11.
The effect of blowing through a localized slot on the wall turbulence dynamics and heat transfer process is analyzed by direct
numerical simulations in a fully developed turbulent channel flow. The severity parameter is mild and there is no flow separation
induced by the blowing. The shear stress transport and temperature energy budget is discussed in detail. The wall shear and
flux decreases immediately downstream the slot in a similar manner but the Reynolds analogy does not hold over the slot. The
physical process is governed by the production and pressure redistribution over the slot in a complex manner. The turbulent
transport and especially the advection play an essential role in the heat transfer mechanism. 相似文献
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13.
Nanofluids, because of their enhanced heat transfer capability as compared to normal water/glycol/oil based fluids, offer the engineer opportunities for development in areas where high heat transfer, low temperature tolerance and small component size are required. In this present paper, the hydrodynamic and thermal fields of a water–γAl2O3 nanofluid in a radial laminar flow cooling system are considered. Results indicate that considerable heat transfer enhancement is possible, even achieving a twofold increase in the case of a 10% nanoparticle volume fraction nanofluid. On the other hand, an increase in wall shear stress is also noticed with an increase in particle volume concentration. 相似文献
14.
The natural gas hydrate plugging problems in the mixed pipeline are becoming more and more serious. The hydrate plugging has gradually become an important problem to ensure the safety of pipeline operation. The deposition and heat transfer characteristics of natural gas hydrate particles in the spiral flow pipeline have been studied. The DPM model (discrete phase model) was used to simulate the motion of solid particles, which was used to simulate the complex spiral flow characteristics of hydrate in the pipeline with a long twisted band. The deposition and heat transfer characteristics of gas hydrate particles in the spiral flow pipeline were studied. The velocity distribution, pressure drop distribution, heat transfer characteristics, and particle settling characteristics in the pipeline were investigated. The numerical results showed that compared with the straight flow without a long twisted band, two obvious eddies are formed in the flow field with a long twisted band, and the velocities are maximum at the center of the vortices. Along the direction of the pipeline, the two vortices move toward the pipe wall from near the twisted band, which can effectively carry the hydrate particles deposited on the wall. With the same Reynolds number, the twisted rate was greater, the spiral strength was weaker, the tangential velocity was smaller, and the pressure drop was smaller. Therefore, the pressure loss can be reduced as much as possible with effect of the spiral flow. In a straight light flow, the Nusselt number is in a parabolic shape with the opening downwards. At the center of the pipe, the Nusselt number gradually decreased toward the pipe wall at the maximum, and at the near wall, the attenuation gradient of the Nu number was large. For spiral flow, the curve presented by the Nusselt number was a trough at the center of the pipe and a peak at 1/2 of the pipe diameter. With the reduction of twist rate, the Nusselt number becomes larger. Therefore, the spiral flow can make the temperature distribution more even and prevent the large temperature difference, resulting in the mass formation of hydrate particles in the pipeline wall. Spiral flow has a good carrying effect. Under the same condition, the spiral flow carried hydrate particles at a distance about 3–4 times farther than that of the straight flow. 相似文献
15.
小液滴撞击壁面现象在喷雾冷却等领域都有广泛应用.为研究小液滴(微米)撞击热壁面(非沸腾区)传热过程,建立了二维液滴撞壁瞬态模型,并采用相场方法对小液滴换热过程中对流热通量和导热热通量的大小进行了对比.研究结果表明:液滴撞击壁面初期形成“冷斑”,有利于小液滴与壁面的传热;小液滴撞击壁面过程中热通量峰值存在于三相接触点附近,数量级在105—106 W/m2;小液滴撞击壁面过程中受壁面浸润性和液滴尺寸对传导热通量的影响较为显著,而速度和液滴尺寸对对流热通量的影响较为显著;大多数情况下,小液滴撞击壁面传导热通量数量级在103—105 W/m2,对流热通量数量级在104—106 W/m2,对流热通量大于传导热通量,在整个换热过程中占据主导地位. 相似文献
16.
The paper presents the first experimental results concerning the wall shear stress in an upward monodispersed microbubble
flow in vertical tube. A bubble generator using a microfluid focusing technique was designed to produce monodispersed submillimeter
bubbles. The experimental results allow to think that there is an optimal size of the bubbles and the optimal gas fraction
in the bubble sublayer that provide the maximal mass transfer coefficient beween the flow and the tube wall. 相似文献
17.
This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses
on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating
surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase.
Low concentrations of nanofluids (0.001–0.1 vol.%) are formulated by the two-step method and the average alumina particle
size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where
the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent
pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed.
The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations
and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone
can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling
system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating
surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially
promising for microchannel applications. 相似文献
18.
Xiaoqian Chen Yong Chen Yiyong Huang Yuzhu Bai Dengpeng Hu Shaoming Fei 《Acoustical Physics》2016,62(2):143-150
Axisymmetric acoustic wave propagating in a shear pipeline flow confined by a rigid wall is studied in the two-part paper. The effects of viscous friction and thermal conduction on the acoustic wave propagating in the liquid and perfect gas are respectively analyzed under different configurations of acoustic frequency and shear mean flow. In Part 2 of this paper, comprehensive analysis of the effects of shear mean flow and acoustic frequency on the features (relative phase velocity and attenuation coefficient) of the acoustic wave are numerically addressed in cases of water and perfect gas respectively. Comparisons between the non-isentropic and isentropic models are provided in details. Meanwhile, discussions of the thermoviscous effects on the acoustic wave between water and perfect gas are given. 相似文献
19.
This paper deals with experimental results on flowboiling heat transfer of liquid moving in an annular channel with unilateral
central heating under the conditions of a significant effect of capillary forces on the flow modes and heat transfer. Experiments
were carried out on boiling freon R318C in an annular channel with a gap of 0.95 mm and transparent outer wall. The inner
wall was heated by the electric current. The local heat transfer coefficients and flow modes are presented. The critical film
thickness corresponding to suppression of nucleate boiling was determined. 相似文献
20.
A model is presented to simulate the cooling processes during tumor cryosurgery with different kinds of flows through the cryoprobe. The heat flux between the cryoprobe wall and the tumor, the heat transfer coefficient under different inflow conditions are obtained numerically. The impact of the inlet mass flow rate, gas volume fraction on these parameters is investigated. It is found that the heat transfer coefficient decreased significantly when inflow changed from two-phase annular flow to droplet flow, and to gas flow. The inlet gas volume fraction and flow velocity only significantly affect the freezing ability of the probe when the inflow is gas or in droplet phase. Simulation of the tumor temperature profiles under different flow conditions show that the heat transfer coefficient is a crucial parameter in temperature prediction during cryosurgery. Results indicate that when the cryoprobe wall is assumed at a constant temperature conventionally, the cooling effect could be overestimated. It would be more reasonable to use the constant wall heat transfer coefficient to simulate the cooling progress under a specific flow. 相似文献