共查询到20条相似文献,搜索用时 31 毫秒
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An experimental study of mixed-convection heat and mass transfer flow of a CuS04 + H2S04 + H2O solution in a horizontal channel is performed by using an electrochemical system. Unstable temperature and concentration gradients imposed by a heated cathodic bottom initiate an ascending (secondary) flow of light boundary-layer fluid into the freestream. To visualize the flow, the shadowgraph technique is used. Electrical measurements are used to obtain mass transfer rates and to infer a Sherwood number. The ranges of the parameters studied in the present work for the square channel are Pr = 5-7, Sc = 1,700-2,300, Re = 100-1,000, Grm= 1.27 X 106 and Grab = 4.06 × 106-2.4S × 107. 相似文献
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Boiling heat transfer in a refrigerant R 21 flow in a microchannel heat sink is studied. A stainless steel heat sink with
a length of 120 mm contains ten microchannels with a size of 640×2050 μm at cross-section with a wall roughness of 10 μm.
The local heat-transfer coefficient distribution along the heat sink length is obtained. The ranges of parameters are: mass
flow from 68 to 172 kg/m2s, heat fluxes from 16 to 152 kW/m2, and vapor quality from 0 to 1. The maximum values of the heat transfer coefficient are observed at the inlet of microchannels.
The heat transfer coefficients decrease substantially along the length of channels under high heat flux conditions and, on
the contrary, change insignificantly under low heat flux condition. A comparison with the well-known models of flow boiling
heat transfer is performed and the range of applicability is defined. 相似文献
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The flow boiling heat transfer of water in a microchannel heat sink with variable initial vapor quality at the inlet is investigated.
The stainless steel microchannel heat sink contains ten 640 × 2050 μm channels with a length of 120 mm; the wall roughness
is 10 μm. The data on the local heat-transfer coefficient distribution in heat sink length are obtained in the range of mass
fluxes from 30 to 90 kg/m2s, heat fluxes from 40 to 170 kW/m2, and vapor qualities from 0 to 1. The heat transfer instability associated with dry spots resulting from insufficient wetting
of channel walls introduces substantial contribution to the heat transfer mechanism and leads to decreasing heat transfer
in heat sink length downward the flow. The developed method for calculating the flow boiling heat transfer of water in a microchannel
heat sink allows more accurate prediction of heat transfer drop than available methods. 相似文献
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J. L. Consalvi Y. Pizzo A. Kaiss J. L. Torero B. Porterie 《Combustion Theory and Modelling》2013,17(3):451-475
The steady state burning rate of vertically oriented slabs of poly-methyl-methacrylate (PMMA) is numerically investigated. Model predictions are compared with measurements and results of the laminar boundary layer (LBL) theory. The numerical model provides a solution of the Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot production and radiation. The modelling of condensed phase processes is based on the one-dimensional heat transfer equation and pyrolysis is treated as a phase change using the latent heat approach. Results show that the pyrolysing region can be divided into three regions. In the laminar part of the flow (Gr x < 4.3 × 107), the predicted normalised burning rate, [mdot]″ p x/μ∞, is a power-law function of Gr x with an exponent close to that of the LBL theory, surface re-radiation being the primary source of discrepancies. From the LBL theory for free flow, it is demonstrated that the local burning rate is inversely proportional to the shear velocity gradient. This is globally confirmed by numerical model results. At Gr x = 4.3 × 107 the change in slope of the burning rate observed experimentally, which indicates the end of the laminar flow region, is reproduced numerically. From Gr x = 2.5 × 109 model results show that the surface mass flux of pyrolyzate increases with x, in agreement with experimental data in literature. 相似文献
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Results of numerical investigation of the boundary layer thickness on turbulent separation and heat transfer in a tube with
an abrupt expansion are shown. The Menter turbulence model of shear stress transfer implemented in Fluent package was used
for calculations. The range of Reynolds numbers was from 5·103 to 105. The air was used as the working fluid. A degree of tube expansion was (D
2/D
1)2 = 1.78. A significant effect of thickness of the separated boundary layer both on dynamic and thermal characteristics of
the flow is shown. In particular, it was found that with an increase in the boundary layer thickness the recirculation zone
increases, and the maximum heat transfer coefficient decreases.
The work was financially supported by the Russian Foundation for Basic Research (project codes 07-08-00025 and 06-08-00300). 相似文献
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An experimental study was performed focusing on heat transfer and friction coefficient associated with turbulent oscillating tube flow. For this goal an oscillating mechanism was designed. Experiments were conducted for the low oscillating frequency in the range of 0.008–1.988 Hz and dimensionless amplitude was chosen as X0 = 0.3, 0.6, and 0.9. Reynolds number was changed from 0.5 × 104 to 2.5 × 104. The bulk temperature of the fluid at the exit of the oscillating section was fond to be increasing with oscillating frequency and amplitude. For the oscillating cases, heat transfer enhancement is obtained 52% for f = 1.988 s?1, 40% for f = 1.320 s?1, and 28% for f = 0.008 s?1, in comparison with the smooth pipe at the highest Reynolds number. The results also showed that Nusselt number and friction coefficient also increased with increasing frequency and amplitude. 相似文献
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本文用固相反应烧结制备出Li2Mo2O6多晶材料。经X射线分析、红外光谱和电子顺磁共振谱(EPR)的研究,确定了它的结构是Li2Mo2O4和MoO2两个晶相组成的烧结体。钼离子以四价状态存在于MoO2晶相结构中。采用交流阻抗谱分析了晶界与温度变化的相关性。测得了样品的ln(σ总T)-1/T 曲线是由两段直线和一段曲线所组成;总电导率化能σ27℃=1.36×10-3(Ω·cm)-1,σ115℃=1.49×10-3(Ω·cm)-1,σ300℃=9.71×10-3(Ω·cm)-1,σ370℃=2.42×10-3(Ω·cm)-1;电导活化能E1=0.043eV,E2=0.235eV,E平均=0.76eV。采用维格纳极化电池法测得电子电导率σe,σe27℃=2.240×10-5(Ω·cm)-1,σe300℃=4.476×10-3(Ω·cm)-1。实验证明,室温下材料为固体电解质,300℃附近为良好的离子与电子混合导体。
关键词: 相似文献
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This paper presents the experimental analysis of aluminum BPHX, with dimensions of 215 × 80 × 61 mm, having transversal offset strip fins with two pitches of 5 and 6.8 mm using liquid to liquid to measure the heat transfer and pressure drop performance in the Reynolds range of transitional to turbulent regime [103, 104]. Firstly, the heat exchangers were tested using water on both sides. A heat transfer and friction coefficients empirical correlations were determined, and the resulted functions were compared with two other models presented in the literature. Secondly, the heat exchangers were measured using water and engine oil as hot fluid. 相似文献
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V. V. Zelenov E. V. Aparina S. A. Kashtanov D. V. Shestakov 《Russian Journal of Physical Chemistry B, Focus on Physics》2009,3(1):122-131
The uptake coefficients γ of chlorine nitrate on MgCl2 · 6H2O crystallites and a MgCl2 · 6H2O-NaCl mixture deposited from an aqueous solution are measured using a flow reactor with a movable salt-substrate-coated insert equipped in combination with a mass spectrometer at 295–428 K, [ClNO3] = (0.2?12) × 1012 cm?3, and [H2O] = 1.0 × 1012 ? 4.3 × 1015 cm?3. Immediately after the exposure of the salt substrate to a ClNO3 flow, γ(t) decreases exponentially with time, γ(t) = γ0 × exp(?t/τ) + γ s , to a steady-state level, γ s , which depends on the temperature and the ClNO3 and H2O concentrations. The main gas-phase product is Cl2, HOCl appears only when water vapor is admitted into the reactor. The coefficient of steady-state uptake on wetted MgCl2 · 6H2O at 295 K can be described by the approximation γ = a + b [H2O] with a = 3.5 × 10?3 cm3 and b = 3.2 × 10?18 cm3. The mechanism of the uptake of ClNO3 on MgCl2 · 6H2O is discussed. The experimental data are treated within the framework of a steady-state uptake model to estimate the heat of adsorption of ClNO3 on MgCl2 · 6H2O (Q ad = 62 kJ/mol) and the activation energy of the bimolecular heterogeneous reaction ClNO3 + Z s = 2Cl2 + Mg(NO3)2 · 6H2O (E a = 21.8 kJ/mol; Z s denotes a ClNO3-MgCl2 · 6H2O surface complex). When the MgCl2 · 6H2O: NaCl is varied from 0 to ~3 wt %, the steady-state uptake coefficient changes from the value corresponding to uptake on pure NaCl to that characteristic of uptake on pure MgCl2 · 6H2O. 相似文献
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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
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Transient heat transfer of liquid methane under forced convection in a 1.8 mm × 1.8 mm asymmetrically heated square channel was investigated. This study is aimed at understanding the heat transfer behavior of cryogenic propellant in cooling channels of a regeneratively cooled rocket engine at the start-up condition. To simulate high heat load conditions representative of regeneratively cooled rocket engines, a high heat flux test facility with cryogenic liquid handing capabilities was developed at the Center for Space Exploration Technology Research. The time history of inlet and outlet fluid temperatures and test section channel wall temperatures were measured at high heat flux conditions (from 1.19 to 3.80 MW/m2) and a Reynolds number (Re) range of 1.88 × 105 to 3.45 × 105. The measured wall temperature data point toward possible film boiling within the test section during certain tests, particularly with higher heat fluxes and lower Reynolds number conditions that resulted in higher wall temperatures. The transient average Nusselt numbers (NuL) of the channel obtained from the experimental measurements are lower than those calculated from the Sieder–Tate correlation (NuO); however, the ratio (NuL/NuO) increases with the increase in Reynolds number. The ratio is around 0.25 at the lower end of Re and then increases to 0.7 at the maximum Re studied in the present investigation. 相似文献
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This article directly investigates the effect of a cooling medium's coolant temperature on the condensation of the refrigerant R-134a. The study presents an experimental investigation into condensation heat transfer, vapor quality, and pressure drop of R-134a flowing through a commercial annular helicoidal pipe under the severe climatic conditions of a Kuwait summer. The quality of the refrigerant is calculated using the temperature and pressure obtained from the experiment. Measurements were performed for refrigerant mass fluxes ranging from 50 to 650 kg/m2s, with a cooling water flow Reynolds number range of 950 to 15,000 at a fixed gas saturation temperature of 42°C and cooling wall temperatures of 5°C, 10°C, and 20°C. The data shows that with an increase of refrigerant mass flux, the overall condensation heat transfer coefficients of R-134a increased, and the pressure drops also increased. However, with the increase of mass flux of cooling water, the refrigerant-side heat transfer coefficients decreased. Using low mass flux in a helicoidal tube improves the heat transfer coefficient. Furthermore, selecting low wall temperature for the cooling medium gives a higher refrigerant-side heat transfer coefficient. 相似文献
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The objective of this paper was to study the moisture and salt diffusivity during ultrasonic assisted-osmotic dehydration of cucumbers. Experimental measurements of moisture and salt concentration versus time were carried out and an inverse numerical method was performed by coupling a CFD package (OpenFOAM) with a parameter estimation software (DAKOTA) to determine mass transfer coefficients. A good agreement between experimental and numerical results was observed. Mass transfer coefficients were from 3.5 × 10−9 to 7 × 10−9 m/s for water and from 4.8 × 10−9 m/s to 7.4 × 10−9 m/s for salt at different conditions (diffusion coefficients of around 3.5 × 10−12–11.5 × 10−12 m2/s for water and 5 × 10−12 m/s–12 × 10−12 m2/s for salt). Ultrasound irradiation could increase the mass transfer coefficient. The values obtained by this method were closer to the actual data. The inverse simulation method can be an accurate technique to study the mass transfer phenomena during food processing. 相似文献