A review of nanofluid preparation,stability, and thermo‐physical properties

This paper represents a comprehensive review on the preparation and stability of nanofluid, the convective heat transfer coefficient and different thermo‐physical properties such as thermal conductivity, specific heat capacity, viscosity, and so on. Here, for each thermo‐physical property, measurement methods, enhancement mechanisms, and criticisms of different studies are also presented. However, based on the available literature, it is concluded that a nanofluid has, in general, better thermo‐physical properties even at a very low particle concentration (typically 1% or less) than conventional heat transfer fluids. The only drawback is high viscosity which leads to a higher pressure drop. At a very low particle concentration, this drawback can be minimized. Three tables are provided for three thermo‐physical properties namely thermal conductivity, specific heat capacity, and viscosity, which can be used as a ready reference for calculating the nanofluid properties.     

Thermal analysis in coated cutting tools

This work studies the heat influence in cutting tools considering the variation of the coating thickness and the heat flux. K10 and diamond tools substrate with TiN and Al₂O₃ coatings were used. The numerical methodology utilizes the ANSYS^® CFX software. Boundary conditions and constant thermo physical properties of the solids involved in the numerical analysis are known. To validate the proposed methodology an experiment is used. The TiN and Al₂O₃ coatings did not show satisfying results during a continuous cutting process. It showed a slight reduction in the heat flux for the 10 (µm) TiN and Al₂O₃ coatings.     

Pool Boiling Heat Transfer Performance of a Dielectric Fluid With Low Global Warming Potential

Due to growing concerns over anthropogenic effects on the climate, there is increasing need to replace engineered fluids of high global warming potentials (GWPs), such as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), with more environmentally friendly alternatives in thermal management systems. This article presents experimental data and compares various correlations for predicting the pool boiling heat transfer coefficient of a new low-GWP fluid, FK-649. Using a pressurized boiling facility with a smooth aluminum heater, the critical heat flux (CHF) and heat transfer coefficient were measured for the pool boiling of FK-649 at various saturation conditions. The commonly used refrigerant tetrafluoroethane (R-134a) is tested in the same pressurized facility to act as a benchmark for the new fluid. While R-134a exhibited a higher heat transfer coefficient and CHF, this behavior is expected from the fluid properties. Two-phase heat transfer performance of FK-649 is expected to be similar to that of the fluorocarbon FC-72. Experimental data are compared to predictions using the Rohsenow, Borishanskii–Mostinski, Stephan and Abdelsalam, Cooper, and Leiner correlations. Where applicable, empirical constants are obtained by using a least-squares fit to experimental data. The Rohsenow correlation yielded the best result with a new surface–fluid constant C _s,_f of 0.0037 for FK-649.     

A fractal model for critical heat flux in pool boiling

In this paper, a fractal model for the high heat flux nucleate boiling region and for the critical heat flux (CHF) is proposed. The expression for the critical heat flux (CHF) is derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for CHF is found to be a function of wall superheat, the contact angle and physical properties of fluid. The relation between CHF and the number of active nucleation sites is obtained from the fractal distribution of active nucleation sites on boiling surfaces. The contact angle and the physical properties of fluid have the important effects on CHF. The predicted CHF from a boiling surface based on the proposed fractal model is compared with the existing experimental data. An excellent agreement between the proposed model predictions and experimental data is found.     

A Complete Experimental Investigation on The Rheological Behavior of Silver Oil Based Nanofluid

In this study, thermo‐physical properties including thermal conductivity, viscosity, density and specific heat capacity of an oil based nanofluid including silver as to be nanoparticles have been experimentally studied. The results indicate an enhancement in thermal conductivity which was depended on bulk temperature and volume fraction of utilized nanofluids. Viscosity data show a significant increment through volume fraction increasing. In addition, the specific heat capacity and density of nanofluids were studied experimentally and it was found that, all measured rheological properties of these nanofluids, were not in agreement to published correlations.     

Compound Material Thermal Parameters for a Layered Material Resembling an Automobile Firewall

Heat transmission through a layered compound material consisting of carbon steel backing and insulation (either aluminized silver or fiberglass) was examined to determine thermal properties. The heat flux impinged on the insulation material side of this simulated "firewall." A heat transfer model was developed that could, in principle, be used to predict the heat transfer through layered compound materials using techniques of thermal property parameter estimation. The parameter estimates are based on thermocouple measurements of surface temperatures during heating on both sides of the material. The experiment analyzed in this article involved a vertical plate exposed on the insulation side to a transient step-applied radiant heat flux. The transient temperature measurements were fitted to heat transfer models. Thermal diffusivity and Biot number were estimated using ordinary least-squares nonlinear regression.     

Influence of thermodynamic models on the prediction of pool boiling heat transfer coefficient of dilute binary mixtures

In this paper, large number of experiments has been performed on saturated pool boiling heat transfer to three different dilute binary mixtures at various heat fluxes (up to 100 kW/m²) and five different concentrations (1–5 vol.% of heavier component). The test mixtures include water/glycerol, water/mono‐ethylene glycol (MEG), and water/di‐ethylene glycol (DEG). The effects of the main operating parameters such as heat flux, concentration, and bubble dynamics on the pool boiling heat transfer coefficient are also investigated. Furthermore, it is shown that physical properties of the mixtures have a considerable effect on the prediction of pool boiling heat transfer coefficients using the available correlations. In almost all of the existing correlations, some physical properties are strongly involved which can be estimated using different thermodynamic models. These models for the calculation of specific heat, density, heat of vaporization, and surface tension do not give exactly similar results and consequently, the heat transfer coefficient obtained from a specified predictive correlation can be tolerated according to the method used for the calculation of the physical properties. This point is usually ignored by investigators and they compare their experimental data with the correlations without reporting which thermodynamic models have to be used for the calculation of the physical properties. In this study, the prediction of Schlünder correlation has been compared with the present experimental data. Results show that the prediction ability of the Schlünder correlation is strongly dependent on the method used for the estimation of the required physical properties.     

Forced convective heat transfer to supercritical water flowing in tubes

Experimental investigations were made of heat transfer to supercritical water flowing in a horizontal tube and vertical tubes. A comprehensive set of data was obtained for pressures from 226 to 294 bar, bulk temperatures from 230 to 540°C, heat fluxes from 116 to 930 kW/m² and mass velocities from 310 to 1830 kg/m²s. Because the physical properties of supercritical fluids change rapidly with temperature in the pseudocritical region, the heat transfer coefficients show unusual behavior depending upon the heat flux. At low or modetate heat fluxes relatively to the flow rate, a satisfactory correlation was obtained, which predicts reasonably well the enhanced heat transfer coefficients near the pseudocritical point. The several characteristics of the deterioration in heat transfer which occurs at high heat fluxes were clarified, and the limit heat flux for the occurrence of the deterioration was determined in connection with the flow rate.     

Experimental investigation on thermo‐driving heat transfer in porous medium in a centrifugal force field

Based on the research in turbine blade super cooling techniques, experiments were carried out to investigate the thermo‐driving heat transfer phenomenon in a new cooling configuration filled with porous medium in a centrifugal force field. Moreover, the heat transfer effectiveness of the configuration was compared to one with no porous medium. The results showed that the thermo‐driving heat transfer effectiveness of the configuration filled with porous medium could be enhanced by increasing the rotating speed, heat flux, and cooling air speed. Meanwhile, the heat transfer effectiveness of the configuration was strengthened with the installation of porous medium. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(6): 329– 338, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20219     

Study of the transient thermal wave heat transfer in a channel immersed in a bath of superfluid helium

Thermal wave is a very interesting phenomenon in which heat is transported in the wave mode. It is different from the ordinary Fourier heat transfer discipline in which heat is transported in the diffusive mode. In the present study, transient thermal wave (second sound wave) heat transfer in He II (superfluid helium) is numerically studied. Quantized vortices in He II, a phenomenon related to the superfluid nature, which is an important factor affecting the behavior of the thermal wave has been taken into account. The present results show that the shape of the thermal wave does not deform seriously and the amount of the heat contained in and transported by the thermal wave does not decrease as the thermal wave transmits along the channel when it is free from the quantized vortices; while the shape of the thermal wave starts to deform at the moment of the emission of the thermal wave and the amount of the heat transported by the thermal wave decreases when it is subject to the quantized vortices. The deformation is in stronger magnitude in the case of the larger heat flux. The surplus amount of the heat which cannot be carried away by the thermal wave accumulates in the thermal boundary layer formed by the dense quantized vortices and then is transferred in a diffusion-like mode. It is found that Gorter–Mellink equation is not suitable to describe the transient heat transfer process in He II.     

Multiobjective thermo‐economic and thermodynamics optimization of a plate–fin heat exchanger

In the present work, a multiobjective heat transfer search (MOHTS) algorithm is proposed and investigated for thermo‐economic and thermodynamic optimization of a plate–fin heat exchanger (PFHX). Heat exchanger effectiveness and total annual cost (TAC) are considered as thermo‐economic objective functions. Similarly, entropy generation rate and heat exchanger effectiveness are considered as thermodynamic objective functions. Six design variables including flow length of cold and hot streams, no flow length, fin height, fin pitch, and fin offset length are considered as decision variables. Effectiveness and accuracy of the proposed algorithm are evaluated by analyzing application examples of a PFHX. The results obtained using the proposed algorithm for thermo‐economic considerations are compared with the available results of NSGA‐II and TLBO in the literature. Results show that 3.56% to 10.29% reductions in TAC with 0.48% to 0.81% higher effectiveness are observed using the proposed approach compared to TLBO and NSGA‐II approaches. Additionally, the distribution of each design variable in its allowable range is also shown for thermo‐economic consideration to identify the level of conflict on objective functions. The sensitivity analyses of design variables on the objective functions value are also performed in detail.     

Drift flux model for large diameter pipe and new correlation for pool void fraction

A void fraction for a bubbling or boiling pool system is one of the important parameters in analyzing heat and mass transfer processes. Using the drift flux formulation, correlations for the pool void fraction have been developed in comparison with a large number of experimental data. It has been found that the drift velocity in a pool system depends upon vessel diameter, system pressure, gas flux and fluid physical properties. The results show that the relative velocity and void fraction can be quite different from those predicted by conventional correlations. In terms of the rise velocity, four different regimes are identified. These are bubbly, churn-turbulent, slug and cap bubble regimes. The present correlations are shown to agree with the experimental data over a wide range of parameters such as vessel diameter, system pressure, gas flux and physical properties.     

Static heat transfer to liquid helium in open pools and narrow channels

The transfer of heat to boiling liquid helium has been measured in open pools and narrow channels. In open pools a marked dependence of heat transfer on the orientation of the heated surface is observed. The maximum heat flux for nucleate boiling varies from 1 W/cm² with the heated surface horizontal facing upwards to about 0·1 W/cm² with the surface horizontal facing downwards. In a narrow vertical channel the maximum heat flux is reduced to about 0·15 W/cm² for a rectangular channel 10mm × 1 mm (50 cm length), and appears to decrease linearly with the channel dimension. The heat transfer is considerably increased in the narrow channel when the fluid is pressurized.     

Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux

Current cooling and heating distribution systems that use water as secondary fluid exhibit limited thermal capacity which can only be overcome by high flow rates and large (volume) capacity. A successful way to enhance the thermal capacity of secondary fluid systems is by incorporating microencapsulated phase change material (MPCM) slurry. However, a full understanding of the physical properties and heat transfer characteristics of MPCM slurry in the 2–8 °C range (35.5–46.5 °F) still is lacking. In the paper, latent heat of fusion, melting and freezing points, and temperature- and concentration-dependent viscosity data, are presented. Results indicate that selection of nucleating agent type and concentration is required to prevent the supercooling phenomenon. Pressure drop and convective heat transfer data were measured using a heat transfer loop operated at different flow rates and heat flux values. Results indicate that the phase change process and slurry mass fraction affect the heat transfer process. The paper also examines the impact of using enhanced surface tubing in combination with MPCM slurry under constant heat flux and turbulent conditions.     

Effect of thermo physical properties of heat exchanger material on the performance of latent heat storage system using an enthalpy method

In this paper, a simple two‐dimensional theoretical model based on enthalpy formulation of a latent heat storage system has been developed to study the effects of thermo physical properties of heat exchanger container materials on the thermal performance of the storage system. Numerical results show that thermal conductivity, specific heat and density of the heat exchanger container materials increases, the melting time of the PCM decreases. Numerical results also show that high value of thermal conductivity of the heat exchanger container materials did not make significant contribution on the melt fraction. It is also found that initial temperature of the PCM does not have very important effects on the melting time, while the boundary wall temperature play an important role during melting. Copyright © 2005 John Wiley & Sons, Ltd.     

Pool Boiling Heat Transfer to Phosphoric Acid Solutions

Phosphoric acid is a weak electrolyte with complex physical properties. This complexity combined with its industrial importance has necessitated intensive studies into its heat transfer behavior. In this investigation, pool boiling heat transfer coefficients of phosphoric acid solutions have been measured over a wide range of acid concentrations. The effects of various operating parameters such as heat flux, temperature, and acid concentration have been investigated. Also, the bubble departure diameter and the number of active nucleation sites for phosphoric acid solutions are compared with those for pure water under identical conditions. A model was developed for pool boiling heat transfer of phosphoric acid solutions, which can easily be adapted for other weak electrolyte solutions. In this model, the correct boiling temperature at the vapor/liquid interface is determined rather than applying an arbitrary correction to the boiling heat transfer coefficient. The proposed model is confirmed by comparison between calculated and experimental data.     

A fractal analysis of dropwise condensation heat transfer

In this paper, a fractal model for dropwise condensation heat transfer is developed based on the fractal characteristics of drop size distributions on condensing surfaces. Expressions for the fractal dimension and area fraction of drop sizes are derived, which are shown to be a function of temperature difference between condensing surface and saturated vapor. The condensation heat transfer is found to be a function of the fractal dimension for drop sizes, maximum and minimum drop radii, the temperature difference, and physical properties of fluid. The predicted total heat flux from a condensing surface based on the present fractal model is compared with existing experimental data. Good agreement between the model predictions and experimental data is found, which verifies the validity of the present model.     

氯化铵水溶液定向凝固实验的传热分析

选用类合金NH4Cl-H2O二元溶液进行垂直定向凝固实验研究,再现过共晶合金结晶过程,测量记录凝固过程中的温度场和固、液相界面位置;重点分析了两相区的传热特性,包括局部热流和释放潜热,并尝试用实验数据与数值计算相结合的方法确定两相区局部固相分数与温度的关系曲线。研究表明:在结晶过程中,各点温度呈线性下降,局部热流在进入两相状态后达到峰值;各相区内温度梯度恒定,但相界面附近温度梯度变化显著。两相区凝固过程中,先期潜热释放总量大,总凝固分数大,两相区厚度迅速增长;随后总凝固分数随相界面迅速上移而急剧下降,经历准稳态过程后再缓慢上升。溶液沿凝固方向分层,NH4Cl质量分数逐渐增大,相应结晶温度逐渐升高。     

Heat and mass transfer of combusting monodisperse droplets in a linear stream

Heat and mass transfer phenomena in fuel sprays is a key issue in the field of the design of the combustion chambers where the fuel is injected on a liquid form. The development and validation of new physical models related to heat transfer and evaporation in sprays requires reliable experimental data. This paper reports on an experimental study of the energy budget, i.e. internal flux, evaporation flux and convective heat flux for monodisperse combusting droplets in linear stream. The evaporation flux is characterized by the measurement of the droplet size reduction by the phase Doppler technique, and the droplet mean temperature, required for the internal and convective heat flux evaluation, is determined by two-color, laser-induced fluorescence. The Nusselt and Sherwood numbers are evaluated from the heat and mass fluxes estimation, as a function of the inter-droplet distance. The results are compared to physical models available in the literature, for moving, evaporating and isolated droplets. A correction factor of the isolated droplet model, taking into account drop-drop interaction on the Sherwood and Nusselt numbers, is proposed.     

Salient features of Dufour and Soret effect in radiative MHD flow of viscous fluid by a rotating cone with entropy generation

BackgroundHere physical characteristics of convective magnetohydrodynamic flow of viscous liquid subject to a rotating cone are discussed. Dissipation, Joule heating, thermal flux and heat generation/absorption are scrutinized in energy expression. Physical aspects of diffusion-thermo and thermo diffusion effect are deliberated. Thermo-diffusion is the mechanisms of transportation in which particles are transferred in a multi-factor mixture determined by temperature gradient. Furthermore irreversibility analysis is considered.MethodNonlinear partial differential system are reduced to ordinary one with the help of similarity variables. Here we implemented ND solve technique to get numerical results for given nonlinear system.ResultsCharacteristics of influential variables for entropy optimization, velocity, concentration, Bejan number and temperature are scrutinized. Numerical outcomes of gradient of velocity and Nusselt and Sherwood numbers are examined through tabulated form. Velocity components are declined against higher velocity slip parameters. For larger estimation of heat generation and radiation parameters the temperature is upsurges. Entropy generation and Bejan number are boost up via rising values of diffusion and radiation parameters. For larger estimation of Brinkman number both Bejan number and entropy rate have opposite effect. Comparative studies of the current and previous results are discussed in tabularized form and have a good agreement.