Local Heat Transfer Phenomena Research Articles

Influence of nozzle geometry on wall static pressure coefficient of the submerged turbulent jet impinging on a smooth flat surface

The experimental work is carried out to study the influence of nozzle geometry on the flow characteristics of the submerged turbulent jet impinging on a smooth flat surface. Three nozzle geometry configurations, viz. parabolic, conical and exponential, are considered. Depending upon the nozzle exit diameter, the Reynolds number of 12,000–23,500 and jet-to-plate distance of 0.5–6 is considered. At a higher jet-to-plate distance (z/d = 6), a secondary peak was observed at a radial distance around r/d = 0.4 from the stagnation point. It is due to the transition of fluid flow from laminar to turbulent on the impingement surface. At all Re and z/d, parabolic nozzle geometry resulted in maximum value as compared with other nozzle geometry configurations. This may be attributed to higher pressure losses in the conical nozzle and exponential nozzle than the parabola nozzle because of streamlined flow and this information can be used for local heat transfer phenomena for optimum conditions.

A discrete element approach to model packed bed thermal storage

A discrete 1-D Eulerian and 3-D Lagrangian model was developed to analyze heat transfer in packed beds composed of spherical particles to study a in more detail the solid phase phenomenon. The proposed thermal model simulates the particles using the discrete element method, generating a particle distribution inside the packed bed. It considers direct and indirect conduction, convection, and radiation and evaluates the local heat transfer phenomena. One of its main advantages is its flexibility to deliver results at the particle level, which provides detailed information than continuous models. A comparison with experimental data from the literature indicates good agreement, with mean absolute errors lower than 8 K. Finally, through a sensitivity analysis, it was demonstrated that the selection of an appropriate Nusselt number is essential because it is related to the convective heat transfer, which corresponds to 85.8% of the total heat transfer during the charging process. For the standby process, particle fluid conduction is the predominant heat transfer mechanism, accounting for 27.5% of the total heat transfer.

Development and Assessment of an Isotropic Four-Equation Model for Heat Transfer of Low Prandtl Number Fluids

In the simple gradient diffusion hypothesis, the turbulent Prandtl number (Prt) with a constant of 0.85 is difficult to accurately predict for liquid metals having low Prandtl numbers (Pr), while a four-equation model can improve this solution by introducing the turbulence time-scale into the calculation of turbulent thermal diffusivity. However, the four-equation model’s transport form and numerical stability are so complex that suitable commercial code is lacking. Therefore, an isotropic four-equation model with simple Dirichlet wall boundary conditions is built in the present work. Based on the open-source computational fluid dynamics program OpenFOAM, the fully developed velocity, temperature, Reynolds stress, and heat flux of low Pr fluids (Pr = 0.01–0.05) in the parallel plane are obtained by numerical simulation. The results show that the time-average statistics predicted using the present four-equation model are in good agreement with the direct numerical simulation data. Then, the isotropic four-equation model is used to analyze the flow and heat of liquid metal (Pr = 0.01) in a quadrilateral infinite rod bundle. The numerical results are compared with the various and available experimental relationships. The Nusselt numbers calculated using the isotropic four-equation model are betweenness the available correlations, while the turbulent Prandtl number model using a constant of 0.85 over predicts heat transfer. More detailed local heat transfer phenomena and distribution of low Pr fluids are obtained using the present isotropic four-equation model.

Influence of thermal boundary conditions on local convective heat transfer in coiled tubes

Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature.

Experimental validation of numerical model for evaluation of local heat transfer coefficient in coiled tubes

The efficiency of the enhanced heat transfer equipment was widely investigated in literature by adopting both experimental and numerical approaches. Obviously, the numerical tools allow improving the knowledge of the fundamental heat transfer mechanisms in curved geometries, since they enable to evaluate punctually the velocity and temperature distributions. On the other hand, it has to be pointed out that the numerical results can be considered reliable only if they are validated throughout the comparison with either analytical results or experimental data. The validation of numerical models is usually carried out by considering average heat transfer performances, therefore this approach does not guarantee that these models enable to evaluate the local heat transfer phenomena with the same accuracy. In the present paper a promising numerical tool for evaluating local convective heat transfer in coiled-tubes, within the laminar regime, is presented. The Navier-Stokes and energy equations are solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes, within the framework of the SIMPLE algorithm. The robustness of the numerical model is assessed by comparing the numerical outcomes to the local experimental data available in literature.

MEMS sensor for high resolution measurement of boiling heat transfer

We have developed a micro heat flux sensor for measuring boiling heat transfer at high-resolution. The heat flux sensor consists of two thin film resistance temperature detectors (RTDs) stacked in the substrate thickness direction. The heat flux can be calculated through the one-dimensional transient heat conduction calculation using measured two temperatures. Two kinds of experiments as preliminary test were conducted. First one is the local heat flux measurement during liquid droplet contact on the sensor substrate. The sensor successfully detected a huge heat transfer of over 3MW/m2 with a quick temperature change of ~105K/s just after droplet contact. And another is measurement of the local heat flux beneath a single pool boiling bubble. The heat flux over 1 MW/m2 induced by microlayer evaporation was measured under a bubble. The developed sensor was confirmed to be useful for the measurement of fast and local heat transfer phenomena.

Experimental study of an upward sub-cooled forced convection in a rectangular channel

The upward sub-cooled forced convection in a rectangular channel is investigated experimentally. The aim of the present work is the studying of the local heat transfer phenomena. Concerning the experimentation: the n-pentane is used as a working fluid, the independent variables are: the velocity in the range from 0.04 to 0.086 m/s and heat flux density with values between 1.8 and 7.36 W/cm2. The results show that the local Nusselt number distribution is not uniform along the channel; however, uniformity is observed in the mean Nusselt number for Reynolds under 1600. On the other hand, a new correlation to predict the local fluid temperature is established as a function of local wall temperature. The wall’s heat is dissipated under the common effect of the sub-cooled regime; therefore, the local heat transfer coefficient is increased. The study of the thermal equilibrium showed that for Reynolds less than 1500; almost all of the heat flux generated by the heater cartridges is absorbed by the fluid.

20313 高時空間分解能レーザ干渉法による沸騰熱伝達機構の研究

This study has been investigated characteristics of phase change heat transfer at the liquid-vapor interface around a boiling bubble using laser interferometry. Initially, to evaluate measurement accuracy of the used interferometric temperature measurement technique, a temperature field of a liquid phase around a heated fine wire was measured and the temperature data was compared to the result of a transient heat conduction simulation. It was indicated that laser interferometry was available for temperature measurement in fast and local heat transfer phenomena such as liquid phase heat transfer in boiling. In the boiling experiment, temperature field around a spherical bubble growing in the depressurized and superheated water was measured. Thin evaporative thermal boundary layer which had 50 μm thickness at 5 ms from the nucleation was successfully captured as deformation of interference fringes in the vicinity of the liquid-vapor interface.

Effect of Periodicity of Non-Uniform Sinusoidal Side Heating on Natural Convection in an Anisotropic Porous-Enclosure

A comprehensive numerical study on the natural convection in a hydrodynamically anisotropic as well as isotropic porous enclosure is presented, flow is induced by non uniform sinusoidal heating of the right wall of the enclosure. The principal directions of the permeability tensor has been taken oblique to the gravity vector. The spectral Element method has been adopted to solve numerically the governing differential equations by using the vorticity-stream-function approach. The results are presented in terms of stream function, temperature profile and Nusselt number. The result show that the maximum heat transfer takes place at y = 1.5 when N is odd.. Also, increasing media permeability, by changing K* = 1 to K* = 0.2, increases heat transfer rate at below and above right corner of the enclosure. Furthermore, for the all values of N, profiles of local Nusselt number (Nuy) in isotropic as well as anisotropic media are similar, but for even values of N differ slightly at N = 2.. In particular the present analysis shows that, different periodicity (N) of temperature boundary condition has the significant effect on the flow pattern and consequently on the local heat transfer phenomena.

열전달촉진체를 사용한 원관에서의 국소열전달 특성에 관한 연구

For the successful design of heat exchangers, it is very important to understand local heat transfer phenomena on the circular tube of heat exchangers. In the present study, experiments are performed for single circular tube and tube banks with and without heat transfer promoters. The naphthalene sublimation technique is employed to measure the local mass transfer coefficients, and the measured local mass transfer data are converted to the local heat transfer data using heat and mass transfer analogy. The distribution pattern of local Nusselt numbers on single circular tube with heat transfer promoters is similar to that without the heat transfer promoter, but average Nusselt numbers are greatly increased. In case of tube banks without the heat transfer promoter, the Nusselt numbers are much lower in the first row than those of other rows, but the local heat transfer coefficients on all rows are equalized when the heat transfer promoter is installed.

Heat transfer enhancement for fin-tube heat exchanger using vortex generators

Vortex generators are fabricated on the fin surface of a fin-tube heat exchanger to augment the convective heat transfer. In addition to horseshoe vortices formed naturally around the tube of the fin-tube heat exchanger, longitudinal vortices are artificially created on the fin surface by vortex generators. The purpose of this study is to investigate the local heat transfer phenomena in the fin-tube heat exchangers with and without vortex generators, and to evaluate the effect of vortices on the heat transfer enhancement. Naphthalene sublimation technique is employed to measure local mass transfer coefficients, then analogy equation between heat and mass transfer is used to calculate heat transfer coefficients. Experiments are performed for the model of fin-circular tube heat exchangers with and without vortex generators, and of fin-flat tube heat exchangers with and without vortex generators. Average heat transfer coefficients of fin-flat tube heat exchanger without vortex generator are much lower than those of fin-circular tube heat exchanger. On the other hand, fin-flat tube heat exchanger with vortex generators has much higher heat transfer value than conventional fin-circular tube heat exchanger. At the same time, pressure losses for four types of heat exchanger is measured and compared.