ケヤキ樹冠の対流熱伝達率

It is important in grinding for the fluid to remove heat from the grinding contact zone to avoid thermal damage to the workpiece surface and/or subsurface layers. The cooling effect of grinding fluid can be quantified by a convection heat convective heat transfer coefficient acting in the grinding zone. This article presents values of the convection heat transfer coefficient based on published experimental results for measured grinding temperatures. This article also presents a new convective heat transfer model based on principles of applied fluid dynamics and heat transfer. Predicted values for the convection heat transfer coefficient calculated from the model are compared with results from experiments obtained under a range of grinding conditions and with experimental data obtained from published literature. The results demonstrate that the new convection heat transfer coefficient model improves the accuracy of prediction and helps explain the variation in the value of convection heat transfer coefficient under varying process conditions. The results also show that convection efficiency strongly depends on the grinding wheel speed, grinding arc length and fluid properties. This new model can be readily introduced into current thermal models of the process used to predict and control grinding temperatures.

Thermally activated building systems (TABS) are one of the most commonly utilised structural cooling methods in construction industries. This method of structural cooling has pipes encapsulated in building surfaces. This investigation analyses the internal convective heat transfer coefficient (CHTC) and radiative heat transfer coefficient (RHTC) for TABS in tropical conditions. The overall heat transfer coefficient is experimentally evaluated with HFP01 heat flux sensors and wet bulb globe thermometers. Calculated and actual CHTC were compared with the interrelationship available in the referred literature and with the constant value recommended by the standard. The results from the measurements indicate that the average CHTC and RHTC are 2.66 W/m2K and 5.7 W/m2K, and it conforms with EN ISO 6946 standards for tropical climates. The CHTC was also quantified with many correlations developed in the scientific literature to determine the best model for the TABS environment. From statistical error analysis, it was determined that Khalifa and Marshall's model was suited with the most minor error of 3 % compared to the other models.

Numerical evaluation of the convective and radiative heat transfer coefficients for preterm neonate body segments inside an incubator

The investigation of relation between radiative and convective heat transfer coefficients at the ceiling in a cooled ceiling room

New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room

Heat transfer study of external convective and radiative coefficients for building applications

Experimental and numerical studies on convective heat transfer from various urban canopy configurations

The convective heat transfer is an important component in the estimation of thermal balance of energy for arrays of buildings immersed in a turbulent boundary layer. This study proposes a novel cost-effective validation approach using targeted field measurements and numerical simulations as an alternative to the wind-tunnel and full-scale field measurement approaches that typically require significant human resources and instrumentation to develop convective heat transfer coefficient (CHTC) for buildings located in the urban environment. This study first introduces new CHTC correlations for regular arrays of cubic buildings. A field measurement is then conducted in an actual urban thermal environment characterized by the plan area density λp = 0.25. Afterwards, this urban thermal environment is numerically simulated using CHTC values for the specified λp and appropriate thermal boundary conditions. The results of the numerical simulations are compared with the measured air temperatures to indirectly validate the CHTCs for external surfaces of buildings. The results show that the difference between the simulated and measured air temperatures is small and typically within 5%. Furthermore, this study created a calibrated building energy simulation model to deploy newly developed CHTCs for building heating energy consumption calculations. Specifically, the energy simulations used both newly developed and commonly used CHTCs to analyze the influence of CHTCs on the heating energy simulation results. A comparison between the simulated heating and actual heating energy consumption shows that the developed CHTCs have a positive influence on the accuracy of the energy simulation results.

Traditional heat transfer fluids, because of their low thermal conductivity, are the primary reason for flat-plate solar collectors' low thermal performance. Enhancing collector performance is feasible by replacing these fluids with nanofluids possessing better thermophysical properties. Despite substantial research on improving thermal efficiency using various nanofluids, the potential of magnetic nanofluids or ferrofluids which are responsive to external magnetic fields remains largely unexplored. Furthermore, there is a notable gap in understanding the hydraulic and economic aspects of ferrofluid-based flat-plate solar collectors (FPSCs). This research presents a comprehensive numerical investigation of the thermal and hydraulic performance of a FPSC utilising Fe3O4-water ferrofluid, both with and without the influence of an external magnetic field. It also compares the performance of a ferrofluid-based FPSC to that of a traditional water-based FPSC. Considering various parameters such as inlet temperature (293–333 K), mass flow rate (0.0027–0.05 kg/s), nanoparticle volume fraction (0.1–2%), magnetic flux density (0.02–0.1 T), solar radiation (100–1000 W/m2), ambient temperature (278–313 K), and wind speed (1–10 m/s), a combined approach using MATLAB and computational fluid dynamics (CFD) in ANSYS® Fluent was applied to investigate the heat losses, convection heat transfer, energy and exergy efficiencies, entropy generation, friction factor, pressure drop, and needed pumping power of the FPSC. An economic analysis was also conducted to evaluate the costs and benefits of the ferrofluid-based FPSC in the absence and presence of an external magnetic field. Without a magnetic field, employing a 2% volume fraction of Fe3O4 led to significant improvements in convection heat transfer coefficient, energy, and exergy efficiencies. However, this improvement was accompanied by increased friction factor and pressure drop. Through the performance index criterion, the optimum volume fraction of Fe3O4 was found to be 0.5%, resulting in a 12.90% increase in convection heat transfer coefficient, and a maximum improvement of 2.84% and 4.60% in energy and exergy efficiency, respectively, compared to a water-based collector. This came with a 3.60% increase in friction factor and a 1.98% increase in the pressure drop, raising the pumping power requirement by only 0.39%. Introducing a non-uniform transverse magnetic field further enhanced performance, with a 2.50%, 1.28%, and 1.74% improvement in convection heat transfer coefficient, energy, and exergy efficiency, respectively, using the optimum 0.5% volume fraction of Fe3O4. Compared to a water-based collector, employing 0.5% Fe3O4 led to a maximum 0.21% reduction in entropy generation, both in the absence and presence of a magnetic field. The application of the magnetic field to the ferrofluid-based FPSC improved the thermal performance mainly at low flow rates, low temperatures, and low volume fractions (of less than or equal to 0.5% Fe3O4). High volume fractions above 0.5% Fe3O4 in the presence of magnetic field at a very low flow rate of 0.0027 kg/s deteriorated the collector’s thermal performance. Additionally, the positive effects of the magnetic field diminished with increased mass flow rate and fluid inlet temperature. Economically, while Fe3O4-water ferrofluid can enhance FPSC’s performance, the total costs associated with ferrofluid and a magnetic field outweigh the benefits, rendering it economically unviable, particularly at volume fractions exceeding 0.5%. This economic consideration highlights the challenges of implementing ferrofluids in FPSCs on a large scale; emphasising the need for further research to optimise the cost-effectiveness ferrofluid-based FPSCs for practical applications.

Experimental evaluation of the convection heat transfer coefficient of large particles moving freely in a fluidized bed reactor

Local and overall convective heat transfer coefficients for human body with air ventilation clothing: Parametric study and correlations

Hyper-reduced-order model for estimating convection heat transfer coefficients of turbine rotors

Convection heat transfer coefficient between a spherical particle and a liquid was measured for the range 50<Rep<5000 and the applicabilities of empirical equations proposed previously to the estimation of convection heat transfer coefficients of solid-liquid system were investigated experimentally. An aluminum sphere, 32 mm in diameter, which was kept at about 60°C initially was dipped into a flow of water at about 25°C and the temperature change of the sphere was measured. The heat transfer coefficient was obtained from the temperature drop curve of the solid. The relationship between the Nusselt number, Nu, and the Reynolds number, Rep, was obtained from the results. Convection heat transfer coefficients at several flow systems were measured and the applicability of the equation corrected for the liquid velocity in the bed was examined. For packed bed system, it was found that the local velocity around a sphere became comparatively large, and convection heat transfer coefficients for packed beds tended to become large.

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.

Performance comparison of annular and flat-plate thermoelectric generators for cylindrical hot source