Papers by Keyword: Heat Transfer Coefficient

Abstract: Condensation heat transfer has been evaluated experimentally on the tube side of three different circular tubes with inner diameter of 6.2, 7.5 and 9.2mm, respectively. Two-phase fluid flow conditions include mass fluxes from 200 to 320kg/m²s, qualities between 0.1 to 0.9, and heat flus range of 5 to 20kW/m² at a fixed saturation temperature of 48°C. Results showed that the average heat transfer coefficient increased with the increase of vapor quality, mass flux and heat flux, but decreased with inner diameter. The experiment results are compared with the existing heat transfer coefficient correlations, and a new correlation is developed with good prediction.

Abstract: Recently the physical model for the temperature field generated by thermophysical single-probe sensor in a shape of planar disc has been derived. The model accounts cylindrical sample having final radius and infinite length. The prototype of measuring electronics RTA was build that operates with auto balancing bridge. For the measurements the single-transition method is used. While the measurement error of the temperature response measured by thermocouple was 1% at the height of temperature response 1°C, the sensitivity of the measurement with the automatically balanced bridge is better and the signal to noise ratio is improved about 10 times. The measuring electronics was tested for measuring the temperature response using a single-sensor. This sensor generates the heat pulse and sense the temperature response in the same time. The next advantage is that the temperature response to the generated step-wise pulse is much smaller and it can be reduced to the level of 1-8° C compared to stationary method like guarded hot plate method. The effect of the temperature field generated by the probe is much smaller, which in the case of measurement of porous materials at the presence of moisture is beneficial, as the resulting redistribution of moisture under the created temperature gradient is negligible. There were derived basic models for the evaluation of this type of experiment. The probe should be in form of full cylinder or an annulus. The derived temperature function counts the outer and inner diameter of annulus. Putting the inner radius to zero we get the solution for full circle probe, so the model is universal. The model counts also the heat losses from the outer surface of the sample, when the time of measurement exceed the time when the penetration depth of generated heat pulse rich the outer radius of sample. The estimated output parameters from single measurement are the thermal conductivity, thermal diffusivity and specific heat. The heat transfer coefficient is additional parameter fitted as free parameter of the model.

Abstract: TIn this present study, the forced convection heat transfer performance of different fluids, namely, Al₂O₃-water, and CuO-water nanofluid has been studied experimentally in an automobile radiator. Three different concentration of nanofluid in the range of 0-1.0 vol.% have been prepared by addition of Al₂O₃ and CuO nanoparticles into water. The test fluid flow rate can be varied in the range of 2 l/min to 5 l/min to have a fully turbulent regime. Obtained results demonstrate that the fluid circulating rate can improve the heat transfer performance. The heat transfer performance of CuO-water nanofluid was found better than the other heat transfer fluids. Furthermore, the Nusselt number is found to increase with the increase in the nanoparticle concentration and nanofluid velocity.

Abstract: For a number of methods used in the cooling heat treatment technique, are required quenching media with cooling rate lower than that of water and larger than that of oil. This can be achieved in industrial practice by using the synthetic quenching type media like gels or emulsions. The synthetic quenching coolants offer advantages such as non-flammability, safety of use and low cost price. The cooling medium to be tested is emulsifiable oil dissolved in water at various concentrations and the testing temperature is 50°C. In the paper were measured cooling curves for specified synthetic media and calculated for the same media the cooling rate variation and heat transfer coefficient on intervals. The experimental data were compared with those obtained from traditional media: water and heat treatment oil TT 50.

Abstract: The boron steel quenching requirement on hot forming manufacturing processes allows the industry to create tailored parts to improve their mechanical functionality. During the cooling, the microstructure of the material changes depending on the imposed cooling rate. However, an accurate prediction of the cooling ratios is needed in order to correctly design the process. In this work the interfacial heat transfer coefficient (HTC) has been determined at different contact conditions, varying the initial die temperature. Experimental tests have been realized in a SCHMIDT micro servo-press, which is able to compensate the thermal contraction of the blank and tools to precisely keep constant the contact pressure. Temperature evolution of the tools and the blank has been monitored with nine thermocouples. For the determination of the heat transfer coefficient (HTC) an analytical-numerical method has been used leading to a fast and reliable calculation method able to determine the HTC value for each process time. This methodology allows relating the HTC to the blank temperature, difference on temperature on the interface to improve the tailor tempering of boron alloys simulation.

Abstract: Innovative product characteristics can be realized by hot roll bonding of two or more layers of different materials. To optimize the roll bonding process, an approach to align the strength differences in both materials by a temperature difference between the layers has been proposed. Therefore, the temperature distribution has to be investigated by finite element (FE) simulations. In these simulations the heat transfer coefficient (HTC) between the two aluminum layers is of great importance. With this coefficient the temperature transfer between the two layers can be determined in order to estimate the temperature field and the material strength difference in the layers.In hot roll bonding there are two ranges for the HTC depending on whether bond formation takes place or not. This effect can be used to determine at which pressures bond formation starts. To evaluate the HTC for this application and to determine its value ranges, a simple setup has been developed. This setup allows conducting experiments under defined temperature and pressure conditions. The resulting force-time measurements were used as input values for inverse FE-simulations, with the goal to gather the HTC by inverse modelling the temperature distributions of the specimens. First results show that the range of the pressure dependent HTCs leads up to 21 kW/(m²K) in the unbonded range. In the range where bonding occurred between the specimens, values over 150 kW/(m²K) were estimated. The data for the HTC was implemented in roll bonding simulations as an interaction property. A comparison between the simulated temperature curve and a measured temperature curve during roll bonding showed a good agreement between the temperature values.

Abstract: This research presents an experimental investigation on the heat transfer performance and pressure drop characteristics of a heat sink with miniature square pin fin structure using nanofluids as coolant. ZnO-water nanofluids with particle concentrations of 0.2, 0.4 and 0.6 vol.% are used as working fluid and then compared with the data for water-cooled heat sink. Heat sink made from aluminum material with dimension around 28 x 33 x 25 mm (width x length x thickness). The heat transfer area and hydraulic diameter of the each flow channel is designed at 1,565 mm² and 1.2 mm respectively. Uniform heat flux at the bottom of heat sink is achieved using an electric heater. The experimental data illustrate that the thermal performance of heat sink using nanofluids as coolant is average 14% higher than that of the water-cooled heat sink. For pressure drop, the data show that the pressure drop of nanofluids is a few percent larger than that of the water-cooled heat sink.

Abstract: This article looks at the effects of considering two-dimensional temperature distributions in analyzing different fin configurations (radial rectangular fins, planar rectangular fins) in contrast to the one-dimensional assumption commonly used in most design methodologies. The investigation of the temperature distributions along the length of the extended surfaces was performed both analytically and by using Computational Fluid Dynamics (CFD) software. The results obtained were then compared and the observed deviations reported. From these investigations, it was discovered that the one-dimensional approach does not always give good results for the heat fluxes and temperature distributions for plain and radial rectangular fins. This calls into question the validity of the one-dimensional assumption utilized in the design methodologies for heat exchange equipment incorporating plain and radial rectangular fins. Keywords: Fins, heat flux, heat transfer coefficient, Temperature distribution, One-dimensional analysis.

Abstract: The aim of this work is to optimize process design parameters of the hot rolling steel strip preposition ultra-fast cooling (UFC) system, and improve the stability and uniformity of micro heat transfer during the high strength cooling process of hot rolling strip. According to the technology and equipment feature of the pre-UFC system for a factory, the convective heat transfer process of single nozzle and strip in UFC system was studied numerically by the fluid-structure interaction finite element method (FEM). The influence of different parameters on the slot impinging jet heat transfer coefficient was obtained, such as jet angle, the jet velocity, the slot nozzle width and water temperature. The results show that on the ultra-fast cooling process of the strip in the initial temperature of 850 °C and thickness of 8 mm, the global average heat transfer coefficient can be increased with the increase of jet velocity, and decrease of the cooling water temperature. The jet angle and the slot nozzle width have minimal effect on it for the whole heat transfer zone. The local average heat transfer coefficient first increased and then decreased with the increase of jet angle and slot nozzle width at the jet impingement location.

Abstract: The estimation of thermal boundary conditions occurring during heat treatment processes is an essential requirement for characterization of heat transfer phenomena. In this work, the performance of five optimization techniques is studied. These models are the Conjugate Gradient Method, the Levenberg-Marquardt Method, the Simplex method, the NSGA II algorithm and a hybrid approach based on the NSGA II and Levenberg-Marquardt Method sequence. The models are used to estimate the heat transfer coefficient in 2D axis symmetrical case during transient heat transfer. The performance of the optimization methods is demonstrated using numerical experiments.