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1.
In this article, the electrohydrodynamic (EHD) effects on nucleate boiling are studied by developing a numerical modelling of EHD effect on bubble deformation in pseudo-nucleate boiling conditions. The volume of fluid (VOF) method is employed to track the interface between the gas–liquid two phases; the user-defined code is written and added to the commercial software FLUENT to solve the electric field and the corresponding electric body force. On this basis, the model is applied to study the EHD effects on heat transfer and fluid flows. An initial air bubble surrounded by liquid CCl4 and attached to a horizontal superheated wall under the action of electric field is studied. The results of the EHD effect on bubble shape evolution are compared with those of available experiments showing good agreement. The mechanism of EHD enhancement of heat transfer and the EHD induced phenomena including bubble elongation and detachment are analyzed in detail.  相似文献   

2.
Electrohydrodyamic (EHD) heat transfer enhancement and flow control methods are becoming increasingly popular in engineering science and applications both in terrestrial and low gravity applications. The correct choice of the working fluid is essential for the design and performance of EHD hardware and can pose challenge because some working fluids with favorable EHD properties can be unstable or hazardous. In this paper key properties and criteria for the selection of working fluids for single-phase (liquid) as well as gas–liquid and vapor–liquid two-phase electrohydrodynamic experiments and applications are discussed. Key physical and electrical properties as well as environmental and safety issues are reviewed for the sample fluids PF-5052, FC-72, R141b, cyclohexane and pure water. Microgravity experiments impose additional demands on the selection of the working fluids. Some of these demands are addressed by contrasting bubble dimensions and shapes at detachment, estimated using a simple thermodynamic model, in terrestrial and microgravity conditions with and without electric fields. Data are obtained using a simplified analytical model and verified experimentally.  相似文献   

3.
Heat transfer enhancement from cylindrical heat sources as electronic components established at the bottom of duct with in-line arrangement and also from the bottom by electrohydrodynamic (EHD) actuator has been investigated experimentally. Air flow is drawn to the duct with various Reynolds numbers based on hydraulic diameter of inlet of the test section (Re = 0, 500, 1100, 2500 and 3870) that include natural convection (confined and unconfined cases) and forced convection (laminar and turbulent flows). Wire electrodes are arranged in transverse direction and perpendicular to the main flow with two various arrangements and high voltages are applied up to 30 kV in the wires. The results revealed that the second electrode arrangement (three wires over the ribs) is more effective due to more enhancement of heat transfer and less corona power consumption in comparison with the first one (four wires between the ribs). Also the electric field is obviously more effective for low Reynolds numbers.  相似文献   

4.
An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.  相似文献   

5.
The heat transfer and fluid flow behavior of water based Al2O3 nanofluids are numerically investigated inside a two-sided lid-driven differentially heated rectangular cavity. Physical properties which have major effects on the heat transfer of nanofluids such as viscosity and thermal conductivity are experimentally investigated and correlated and subsequently used as input data in the numerical simulation. Transport equations are numerically solved with finite volume approach using SIMPLEC algorithm. It was found that not only the thermal conductivity but also the viscosity of nanofluids has a key role in the heat transfer of nanofluids. The results show that at low Reynolds number, increasing the volume fraction of nanoparticles increases the viscosity and has a deteriorating effect on the heat transfer of nanofluids. At high Reynolds number, the increase in the viscosity is compensated by force convection and the increase in the volume fraction of nanoparticles which results in an increase in heat transfer is in coincidence with experimental results.  相似文献   

6.
Forced convection heat transfer of non-Newtonian nanofluids in a circular tube with constant wall temperature under turbulent flow conditions was investigated experimentally. Three types of nanofluids were prepared by dispersing homogeneously γ-Al2O3, TiO2 and CuO nanoparticles into the base fluid. An aqueous solution of carboxymethyl cellulose (CMC) was used as the base fluid. Nanofluids as well as the base fluid show shear-thinning (pseudoplastic) rheological behavior. Results indicate that the convective heat transfer coefficient of nanofluids is higher than that of the base fluid. The enhancement of the convective heat transfer coefficient increases with an increase in the Peclet number and the nanoparticle concentration. The increase in the convective heat transfer coefficient of nanofluids is greater than the increase that would be observed considering strictly the increase in the effective thermal conductivity of nanofluids. Experimental data were compared to heat transfer coefficients predicted using available correlations for purely viscous non-Newtonian fluids. Results show poor agreement between experimental and predicted values. New correlation was proposed to predict successfully Nusselt numbers of non-Newtonian nanofluids as a function of Reynolds and Prandtl numbers.  相似文献   

7.
Precise fluid temperature control in microfluidic channels is a requirement for many lab-on-a-chip and microreactor devices, especially in biotechnology where most processes are highly temperature sensitive. We demonstrate the concept of a microthermoelectric cooler integrated into a microfluidic channel in order to give rapid and localised fluid cooling. The key aspect of this concept is the use of a second imbedded microfluidic channel that is used as a miniature heat sink. An analytical thermal model has been derived that couples thermoelectric effects with fluid heat-transfer rates from both the hot and cold connections. Using this model, the effect on cooling performance of varying the thermal resistance between the hot and cold connections and the fluid has been quantified, as well as the effect of substrate thermal conductivity. If the substrate thermal conductivity is too high, heat leakage renders the thermoelectric cooler ineffective. The optimum electrical current for cooling has been shown to be a function of the thermal resistance of the heat sink. For thermoelectric coolers there is competition between temperature reduction and cooling power. Using this fact, based on the final fluid temperature required, we have calculated the maximum flow rate that will achieve this. Finally, a prototype integrated microthermoelectric cooler has been fabricated and tested.  相似文献   

8.
Liquid–solid two-phase flow with heat transfer is simulated, and the effect of temperature gradient within a solid particle on the particle behaviour and heat transfer is studied. The interaction between fluid and particles is considered with our original immersed solid approach on a rectangular grid system. The local heat flux at the fluid–solid interface is described with an anisotropic heat conductivity matrix, and the governing equation of temperature is time-updated with an implicit treatment for the diffusion term. The method is applied to a 2-D natural convection flow of a relatively low Rayleigh number including multiple particles. Heat transfer and particle behaviours are studied for different solid heat conductivities (ratio to the fluid conductivity ranging between 10−3 and 103) and solid volume fractions. Under a condition of relatively low heat conductivity ratio, the particles show a simple circulating flow. By increasing the heat conductivity ratio, a transition of the particulate flow is observed to oscillation mode around the domain centre due to the buoyancy force as a restitution force. The oscillation period is found to vary with the heat conductivity ratio, and it is related to the time scales for the heat transfer via fluid and solid.  相似文献   

9.
Uniform flow regime and constant effective thermal conductivity inside packed beds are commonly accepted in the evaluation of the fluid dynamics and heat transfer in such systems.However,several authors have confirmed the presence of an oscillatory velocity profile caused by the effective contribution of porosity profile in the fluid dynamic behavior of packed beds,which directly influences the heat transfer inside the beds.This paper describes the application of a pseudo-homogeneous mathematical ...  相似文献   

10.
The influence of variation in physical variables on the steady magnetohydrodynamic (MHD) Couette flow with heat transfer is studied. An external uniform magnetic field is applied perpendicular to the parallel plates and the fluid is acted upon by a constant pressure gradient. The viscosity and the thermal as well as electric conductivities are assumed to be temperature dependent. The two plates are kept at two constant but different temperatures, and the viscous and Joule dissipations are considered in the energy equation. A numerical solution for the governing nonlinear coupled equations of motion and the energy equation is obtained. The effect of the temperature-dependent viscosity, thermal conductivity, and electrical conductivity on both the velocity and temperature distributions is examined. H.A. Attia - On leave from: Dept. of Eng. Mathematics and physics, El-Fayoum University, El-Fayoum, Egypt  相似文献   

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