Effect of non-uniform electric fields on convective heat transfer in a colloidal fluid
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The effect of radial electric fields on the convective heat transfer coefficient of a colloidal fluid (haematite in distilled water) has been studied as a function of various quantities such as time, electric field, pH value, colloidal concentration and orientation. The electroconvective heat transfer coefficient exhibits a 'timing' effect as well as an 'aging' effect. Whereas an AC field always enhances the heat transfer, a DC field produces an enhancement that is almost vertical in the vicinity of the origin. An increase in particle concentration increases the heat transfer coefficient, while a sharp rise in heat transfer coefficient is observed when the surface charge of the collidal particle is increased. Furthermore, the convective heat transfer coefficient increases when the inclination of the cylinder is changed from the horizontal to the vertical position.Keywords:
Churchill–Bernstein equation
Particle (ecology)
Heat transfer enhancement
Churchill–Bernstein equation
Sitting
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This paper describes the experiments to determine the convective heat transfer coefficients on a synthetic heat transfer fluid flowing in a Shell-and-Tube heat exchanger. The analysis of results is carried out by application of the Wilson plot Technique, on the basis of which, the convective heat transfer coefficients were experimentally obtained for the fluid flowing inside the tube. The convective heat transfer coefficient of oil derived through Wilson plot is then compared with the convective heat transfer coefficients obtained using the classical thermal resistance equation. An empirical correlation between the convective heat transfer coefficient of oil with respect to its mean velocity of flow in the tube and the bulk oil temperature has been proposed. A correction factor of 2.3 and exploration of the exponent value of 0.2 pertaining to the velocity of oil was obtained. The values of convective heat transfer coefficients obtained after applying the correction factor are consistent with the values reported in the literature for oil-water heat transfers. The variation of the heat transfer coefficients at different temperatures is attributed to factors like vapor blanketing effect, surface temperature measurement difficulty as well as dependence of convection phenomenon on surface geometry and physical conditions of the fluids. Experimental results obtained for a temperature range of 50-200°C are extrapolated upto 400°C, the actual upper operational fluid temperatures used in concentrated solar parabolic trough power plant. The test method proposed in this paper can be useful for the development of oil as heat transfer fluids, where already established or commercialized oil is compared with the oil under development, in the same test setup and under similar test conditions.
Churchill–Bernstein equation
Film temperature
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Abstract Most engineering applications have boundary layers; the convective transport of mass, momentum and heat normally occurs through a thin boundary layer close to the wall. It is essential to predict the boundary layer heat transfer phenomenon on the surface of various engineering machines through calculations. The experimental, analogy and numerical methods are the three main methods used to obtain convective heat transfer coefficient. The Reynolds analogy provides a useful method to estimate the heat transfer rate with known surface friction. In the Reynolds analogy, the heat transfer coefficient is independent of the temperature ratio between the wall and the fluid. Other methods also ignore the effect of the temperature ratio. This paper summarizes the methods of predicting heat transfer coefficients in engineering applications. The effects of the temperature ratio between the wall and the fluid on the heat transfer coefficient predictions are studied by summarizing the researches. Through the summary, it can be found that the heat transfer coefficients do show a dependence on the temperature ratio. And these effects are more obvious in turbulent flow and pointing out that the inaccuracy in the determination of the heat transfer coefficient and proposing that the conjugate heat transfer analysis is the future direction of development.
Film temperature
Churchill–Bernstein equation
Mass transfer coefficient
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Convective heat transfer was measured for a heated axisymmetric air jet impinging on a flat surface. It was found that the local heat transfer coefficient does not depend explicitly upon the temperature mismatch between the jet fluid and the ambient fluid if the convection coefficient is defined in terms of the difference between the local recovery temperature and target surface temperature. In fact, profiles of local heat transfer coefficients defined in this manner were found to be identical to those measured for isothermal impinging jets.
Churchill–Bernstein equation
Film temperature
Entrainment (biomusicology)
Isothermal process
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For obtaining the convective heat transfer coefficient between gas and particles at high temperature, heat transfer experiments and mathematical model simulation were carried out both for a single sphere and for a counter-current moving bed. If the radiative heat transfer should be evaluated quantitatively, the convective heat transfer coefficient based on the previously empirical equations reported by Ranz and other researchers were available for the evaluation of convective heat transfer around a single sphere. From the experiments and the mathematical model simulation with heat transfer of the moving bed, the following empirical equation was obtained as the equation for determining the convective heat transfer coefficient in the moving bed.Nu=2.0+0.39Reρ1/2Pr1/3
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The effect of radial electric fields on the convective heat transfer coefficient of a colloidal fluid (haematite in distilled water) has been studied as a function of various quantities such as time, electric field, pH value, colloidal concentration and orientation. The electroconvective heat transfer coefficient exhibits a 'timing' effect as well as an 'aging' effect. Whereas an AC field always enhances the heat transfer, a DC field produces an enhancement that is almost vertical in the vicinity of the origin. An increase in particle concentration increases the heat transfer coefficient, while a sharp rise in heat transfer coefficient is observed when the surface charge of the collidal particle is increased. Furthermore, the convective heat transfer coefficient increases when the inclination of the cylinder is changed from the horizontal to the vertical position.
Churchill–Bernstein equation
Particle (ecology)
Heat transfer enhancement
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For heat and mass transfer in thermal processes, convective heat transfer is the main mechanism of heat transfer from solid to fluid media in the presence of bulk motion. When fluid passes a solid surface due to an external driving force, this is called forced convection. In contrast, when fluid moves due to density differences caused by a temperature variation in the fluid, this is called free or natural convection. According to Newton’s law of cooling, the rate of heat flow through a fluid is determined by the convective heat transfer coefficient. The convective heat transfer coefficient is usually derived as a constant parameter from empirical correlations for given flow conditions. The convective heat transfer coefficient is a temperature-dependent parameter under variant operating conditions. Also, it strongly depends on the speed of air, which changes with respect to time. The determination of the convective heat transfer coefficient for air under actual convection processes is proposed by using thermoelectric modules. Based on the Seebeck effect and energy balance, voltage signals are mathematically related to the convective heat transfer coefficient in real-time. The effectiveness of the proposed thermoelectric modules in determining the convective heat transfer coefficient in a real-time implementation is presented in this research. The methodology can be generalized by further developing applications for the thermoelectric module.
The convective heat transfer coefficient strongly depends on the speed of air. The speed of air or wind speed can be determined when the convective heat transfer coefficient is known. The principle of the Seebeck effect is applied for quantifying the convective heat transfer coefficients of airflow over two thermoelectric modules. The modules are embedded on the vertical wall and the horizontal wall within a small air passage. According to dimensional analysis, the convective heat transfer coefficients depend strongly on wind speed. Wind speeds are systematically determined from the Seebeck voltages of the thermoelectric modules. A prototype of a thermoelectric anemometer is implemented, to measure the wind speeds within a wind tunnel and under open field conditions. The results confirm that there is good agreement between the wind speed measurements from the proposed thermoelectric anemometer and the conventional measurements of wind speed, carried out in a wind tunnel and in open field conditions.
Churchill–Bernstein equation
Film temperature
Thermoelectric cooling
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The convective heat transfer coefficient of nozzle internal flow in a liquid oxygen-kerosene rocket engine is simulated with Roe-FDS computation format and a standard k-e turbulence model.The effects of mesh Reynolds number and constant wall temperature on the convective heat transfer of nozzle internal flow are analyzed.The simulated result is in good agreement with the result of engineering estimation when the order of magnitude of first near-wall mesh altitude is 10-3 mm.In a given scope,the convective heat transfer coefficient increases with the increase of flow velocity,and the influence of constant wall temperature on the convective heat transfer coefficient is indistinctive.
Churchill–Bernstein equation
Discharge coefficient
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The objective of this paper is to estimate the natural convective heat transfer coefficients for the human body at sedentary posture and its segments, based on measured dry heat dissipation rate with heat flow meters. The following results were obtained : 1) The natural convective heat transfer coefficients for trunk, forearm, thigh and shin had distribution over the skin. The mean values of local coefficients at each segment were calculated and factors were determined to estimate the mean value of natural convective heat transfer coefficients from that at the measured point. 2) The estimated local convective heat transfer coefficient for the peripheral segments were agreed with the previous experimental data and the estimation formula based on non-dimen- sional heat transfer coefficient formula concerning simple solids which were simulated to the human segments. The convective heat transfer coefficients for a head and a trunk were fairly agreed with the previous studies. 3) Convective heat transfer coefficient for the whole body were calculated as the weighted mean of the local convective heat transfer coefficients for each segments. The value stood at 3.7 to 3.8W/m2
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Convective heat transfer coefficient was measured at different volume fraction of graphite-water nano-fluids with Re in scope of 3 000—6 500 by using self-designed nano-fluid flow and convective heat transfer performance of the experimental device.The experimental results showed that graphite nano-particles improved the convection heat transfer coefficient of water;the volume fraction of graphite nano-particles in water and convective heat transfer coefficient is nearly linear relationship;Nu number increases linearly with Re increasing;the random motion of nano-particles at flow state and the thermal scattering have a significant effect on increase of convective heat transfer coefficient.
Churchill–Bernstein equation
Volume fraction
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