Papers by Keyword: Heat Transfer Coefficient

Abstract: The mould temperature distribution has a great influence on the semi-solid diecasting. In the present study the temperature distribution of a plane-shaped mould was investigated by using the method of numerical simulation and experiment. The results showed that the preheating mould temperature field was affected by three important simulation parameters, the heat transfer coefficient h_oil between the heat transfer oil and the mould, the heat transfer coefficient h_air between the mould and the air, and the heat transfer coefficient h_contact between the mould core and the mould frame. The simulation results showed that (1) with the increase of h_oil, the overall mould temperature imcreased; (2) with the increase of h_air, the overall mould temperature decreased, while the surface temperature gradient of mould frame grad T_-f and the temperature difference between the mould core and the mould frame ∆T increased; (3) With the increase of h_contact, ∆T decreased and the temperature of mould frame increased. When the heat oil temperature T_oil=290°C, the heat transfer coefficients were optimized as h_oil=500Wm^-2K^-1, h_air=7Wm^-2K^-1, and h_contact=1000Wm^-2K^-1 according to the experimental results. The average temperature difference between the simulation result and the experimental result was 3.45°C, and the average relative error was 1.73%.

Abstract: Finite Element Modeling (FEM) has been employed widely to analyze material flow behavior and identify potential defects in a hot forging process before try-out. Normally, the isothermal assumption should be used to simulate this process because the forming time was extremely shot around 0. 5 s – 1 s due to a high velocity of a press machine. However, in some cases when the contact pressure and contact area are extremely high, the heat could significantly dissipate to the forming dies. In case of Yoke flange simulation the isothermal condition could not be used to identify the defect as occurring in the real process. The forging defect (i.e. insufficient gap) was found at the apex of a workpiece in the rough or preform step. In this study, the non-isothermal assumption was used for investigating the defects. The forming process was divided in 3 steps; namely the transportation step when the billet was transferred from an induction furnace to the forging dies by conveyer, the rough forging and the finish forging steps. Temperatures, loads and gaps between workpiece and die at each step of the forming processes were measured and compared with the simulation results. For developing the reliable simulation model, the suitable heat transfer coefficients for each step would be determined. The heat transfer during the forming steps had an effect on the material flow and, the non-isothermal simulation model and could identify the insufficient gap in the rough step.

Abstract: This paper describes the details of a quantitative experimental and numerical study on the influence of solidification conditions, including the apparent interfacial heat transfer coefficient (IHTC) between the die and solidifying metal, on the resulting local microstructure. Multiple runs of the commercial casting simulation package, ProCASTTM, are used to model the mold filling and solidification events employing a range of IHTC values. The simulation results are used to estimate the centreline cooling curve at various locations through the casting. The centreline cooling curve, together with the die temperature and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate is used to calculate the resulting grain size and skin thickness via previously established relationships. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent IHTC, which, in this study, was approximately 12000 W/m²·K. Additional useful observations from the numerical study suggest that the IHTC has a significant influence on the skin thickness and grain size in both the skin and core regions of the casting, while the effect of die temperature is limited to influencing the skin grain size only.

Abstract: This paper presents a theoretical-experimental study for the prediction of the interfacial heat transfer coefficient during the horizontal directional solidification of an Al-3wt.%Cu alloy on water cooled stainless steel chill under transient heat flow conditions. Eight thermocouples were connected with the casting and the time-temperature data were recorded automatically. The thermocouples were placed at 5, 10, 15, 20, 30, 50, 70 and 90 mm from the metal-mold interface. A numerical technique which compares theoretical and experimental thermal profiles was used to measure the heat transfer coefficient values. This has permitted the evaluation of the variation of this thermal parameter along the solidification which is represented by a power equation that shows the time dependence during the process given by h_i = constant (t)^-n, which represents the best fit between the experimental and calculated curves. The obtained results also include the variation of both primary and secondary dendritic arm spacings of alloy analyzed as a function of heat transfer coefficient. These dendrite arm spacings were found to decrease as the values of this coefficient are increased. Finally, an experimental law of the Hall-Petch type is proposed relating the resulting microhardness to the heat transfer coefficient investigated.

Abstract: This requires the use of additional reinforcement in order to prevent excessive or permanent deformation of PVC windows. In the paper particular attention was devoted to space located in a corrosive environment exposed to chemical agents. For this purpose, proposed to change the previously used steel profiles reinforcements made of Ti6Al4V titanium alloy corrosion-resistant in the air, at sea and many types of industrial atmosphere. Analysis of the thermal insulation properties of PVC windows with additional reinforcement of profile Ti6Al4V titanium alloy was performed. PVC window set in a layer of thermal insulation was analyzed. Research was conducted using Finite Element Analysis. Numerical models and thermal calculations were made in the program ADINA, assuming appropriate material parameters. The constant internal temperature of 20 ̊ and an outer-20 ̊ was assumed. The course of temperature distribution in baffle in time 24 hours and graphs of characteristic points was obtained. The time of in which followed the steady flow of heat, as well as the course of isotherm of characteristic temperature in the baffle was determined. On the basis of numerical analysis obtained vector distribution of heat flux q [W/m²] and was determined heat transfer coefficients U [W/m²K] for the whole window with titanium reinforcement . All results were compared with the model of PVC windows reinforced with steel profile.

Abstract: In contrast to the very rich literature on modeling and the determination of the thermal conductivity of nanofluids the forced convection data are limited. This work presents preliminary results of the experimental investigation of the forced convection heat transfer of water-Al₂O₃ nanofluids inside stainless steel tube with 8 mm internal diameter and 2000 mm length. Nanoparticles were tested at the concentrations of 0.01%, 0.1%, 1% and 5% by weight and the Reynolds number range encompasses laminar as well as turbulent flows.

Abstract: The paper attempted to assess the impact of a variable over time heat transfer coefficient on the courses of temperature obtained in the selected areas of the analysed element. The object of the tests was a steam valve. A constant value of this coefficient during such component design is assumed in standards. However, in view of the fact that in devices used in power engineering the state of aggregation of the medium and its flow rate frequently change, the value of the coefficient should be treated as variable. Its value depends on the temperature and pressure. It has been shown that changes in time of the heat transfer coefficient have a strong influence on the temperature distributions in the component.

Abstract: Mechanically fastened flexible roof waterproofing membranes - MEFAWAME is regarded as an effective way how to stabilize the roofsheathing in the new industrial buildings, shopping centers, but also widely used technology of roof reconstruction. This technology is fast, reliable, and relatively independent of the weather.

Abstract: A numerical model for determining the characteristics of flow and heat has been presented by modifying the eddy diffusivity equation of Sarma et al. The experimental data of thermo-physical properties determined using spherical particles in a wide range of concentration, particle size, materials and operating temperatures are available in the literature. The numerical analysis employed equations, which were developed using the experimental data of thermo-physical properties, friction factor and Nusselt number. Based on the agreement of the numerical results with the experimental data, the influence of concentration and temperature on the turbulent characteristics is presented. It is observed that SiO₂ nanofluid attained higher velocity and lower eddy diffusivity compared to Cu nanofluid at a concentration. The temperature gradient increases with concentration and decreases with temperature.

Abstract: Hydrocarbon refrigerants have been widely used to replace HFCs. As hydrocarbon, R-290 has no ODP (Ozone Depletion Potential) and negligible GWP (Global Warming Potential). This paper presents flow boiling heat transfer in small tube with R-290 and R-22. The test tube has inner diameter of 7.6 mm and length of 1.07 m. In order to determine the heat transfer coefficient, experiments were carried out for heat fluxes ranging from 10 to 25 kW/m², mass fluxes ranging from 204 to 628 kg/m²s, and saturation temperatures ranging from 1.87 to 11.9^o C. The study analyzed the heat transfer through the local heat transfer coefficient along the flow under the variation of these different parameters. In comparison with R-22, R-290 provides higher heat transfer coefficients. In the prediction of the heat transfer coefficients of R-22 and R-290, the correlation of Shah (1982) and Choi et.al. (2009) best fitted the present experimental result, respectively.