Natural Convection Experiments Research Articles

Investigation of natural convection heat transfer of self-heating packed beds

This study investigated heat transfer characteristics of natural convective flow in the packed bed geometry experimentally and numerically. The natural convection experiments for self-heating condition were performed using the copper sulfate electroplating system based on the analogy between heat and mass transfers. Also, the same conditions were simulated with Lattice-Boltzmann Method (LBM) and the results were compared. The spheres diameters were d = 4, 6 and 10 mm and the corresponding Rayleigh numbers were 5.43 × 106, 1.83 × 107 and 8.48 × 107 respectively. The packed bed height to sphere diameter ratios, H/d were 5, 10 and 20. The Nusselt numbers (Nud), velocity and shear stress distributions of both analyses were in good agreement. Both results showed the same order (10−2 m/s) of irregular velocity distributions resulting from the random arrangement of the spheres. In spite of the laminar flow condition, wake and vortex generated by the packed bed geometry increased the turbulent kinetic energies to one- or two-order higher values than those at the entrance. The temperature profiles of flow inside packed bed were approximately uniform for d = 4 mm cases but there were temperature gradients in case of d = 10 mm. This revealed that the use of smaller fuel sphere can prevent from unwanted hot spots inside the packed bed. The Nud values from LBM results and newly developed empirical correlation for natural convective flow in packed bed were consistent.

Numerical analysis of natural convection behavior in density stratification induced by external cooling of a containment vessel

It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the initial and boundary conditions on the analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments. A density stratification layer was initially formed, and natural convection was induced by the external cooling of the vessel. Mixing by natural convection eroded the density stratification. We applied the RANS model with dynamic turbulent Schmidt number model. We used the measured temperature and gas concentration as the boundary and initial conditions. The erosion velocity did not change much with the initial gas distribution. The temperature boundary condition of small internal structures significantly influenced the fluid temperature distribution, and we evaluated this influence of the small internal structure quantitatively.

Influence of position on the natural and forced convective heat transfer of a single heating sphere in a packed bed

Natural and forced convection experiments on a single heating sphere in a packed bed were performed by varying the axial and radial positions of the heating sphere. Based on the analogy between heat and mass transfer, mass transfer experiments were conducted using a copper electroplating system. The sphere diameter was varied from 0.004 to 0.010 m, which correspond to the Rad of 5.43 × 106–8.48 × 107. The Sc was 2,014. In the forced convection experiments, the range of superficial velocity was 0.01 to 0.58 m/s, which correspond the Red of 43–5,076. For natural convective flow, the influence of sphere position on the heat transfer of single heating sphere in the packed bed was not observed. However, for forced convective flow, the local heat transfer inside the bed was affected by the variation of the sphere position. The heat transfer increased at the downstream layer of bed due to the intensified turbulence level by the porous structure. The heat transfer near wall region was also enhanced due to the high local porosity. Based on the experimental results, we developed the correlation of local heat transfer for forced convection by adding the multiplier to the existing correlation.

Using Graphene-Based Grease as a Heat Conduction Material for Hectowatt-Level LEDs: A Natural Convection Experiment

In this study, a self-adjusting concentration of graphene thermal grease was developed to reduce the contact surface thermal resistance of 50 W light-emitting diodes (LEDs). The purpose was to identify an important type of heat conduction material with a high thermal conductivity coefficient, which can be applied to the contact surface of various high-heat sources or concentrated heat sources to achieve seamless heat transfer with an extremely low thermal resistance state. The contact heat conduction material conductivity reached the highest K value of 13.4 W/m·K with a 15 wt.% self-adjusting concentration of graphene grease. This material could continuously achieve a completely uniform and rapid thermal diffusion of heat energy. Therefore, we performed an analysis of chip-on-board light-emitting diodes (LEDs) with a highly concentrated heat source, which showed excellent heat dissipation under natural convection heat transfer. As such, this study achieved the natural convection mechanism and a heat sink volume thermal performance capacity of 473,750 mm3 for LEDs under 50 W, but those over 50 W require an enhanced forced convection solution and a heat sink volume thermal performance capacity between 473,750 mm3 and 947,500 mm3. If the heat source dissipation reaches 100 W, the volume capacity must be at least 947,500 mm3 for lighting equipment applications. In the experimental study, we also verified and analyzed the research data, including an analysis of the measured data, grease component wt.%, heat sink material selection, increase in heat sink volume, heat transfer path, and contact surface, a discrimination analysis of infrared thermal images, and an analysis of flow visualization, which were conducted to ensure quantitative and qualitative improvement, provide a mechanism for judging the technical performance, and provide research results to enable discussion.

Natural and forced air convection operation in a direct solar dryer assisted by photovoltaic module for drying of green onion

The aim of this research was to develop a direct solar dryer assisted by photovoltaic module and evaluate its performance under natural and forced convection experiments. The photovoltaic module provides electrical power to eight coolers that allow the renewal of air inside the equipment. In this configuration, the dryer is capable of operating independently of electrical energy distribution grid. Moisture and colorimetric parameters were monitored during drying of green onion. Drying kinetics showed a constant rate period followed by a falling rate period for both operating conditions. The convection experiment showed a higher rate and external mechanisms may have controlled the process. Straight lines were adjusted to constants rate periods with R2 of 0.999. Page and Overhults models best described the falling rate periods, with R2 between 0.932 and 0.999. The effective diffusivity values were 5.15 × 10−9 m2/s and 1.15 × 10−8 m2/s for drying with natural and forced convection runs. The average efficiencies of the solar dryer and the specific energy consumption were 34.2%, 18.3 kWh/kg for the drying process with natural convection, and 38.3%, 16.4 kWh/kg for the drying process with forced convection. Little color variation was observed between fresh and dried green onions in the two operating conditions, which is consistent with the need to maintain the green color of the material.

Simultaneous estimation of fluid temperature and convective heat transfer coefficient by sequential function specification method

In pressurized water reactor (PWR) with multiple loops, abnormal working conditions occur when coolant pumps do not work in some loops. In these closed-loop pipelines, thermal stratification is formed under the effect of natural convection cooling and this poses a threat to the safe operation of the reactor. In this paper, the sequential function specification method (SFSM) is implemented to simultaneously predict the spatially and temporally varying internal fluid temperature and convective heat transfer coefficient of two-dimensional pipe under thermal stratification. In the direct problem, the finite volume method (FVM) is adopted to solve the governing equations subjected to initial and boundary conditions. In the inverse problem, the least-square method is employed to obtain the heat flux of the inner wall, and then Newton's law of cooling and the natural-convection correlation are adopted to obtain the fluid temperature and convective heat transfer coefficient. Numerical experiments of natural convection cooling are carried out under different outer wall boundary conditions to demonstrate the effectiveness of the proposed inversion method. The results of temperature estimation are consistent with the results of natural cooling numerical experiment and the relative error is below 4%, whereas the convective heat transfer coefficient is slightly worse due to the difference in the extraction positions. The effects of the number of involved future time steps and the measurement noise on the accuracy of the inverse analysis are also investigated in detail. Our results show that the fluid temperature and convective heat transfer coefficient of natural cooling may be effectively estimated using the proposed inverse method.

Energy and exergy analysis of forced and natural convection indirect solar dryers: Estimation of exergy inflow, outflow, losses, exergy efficiencies and sustainability indicators from drying experiments

In this study, energy and exergy analysis (EEA) was performed during drying of okra (Abelmoschus esculentus) in an indirect solar dryer (ISD) under forced and natural convection and results were compared for better assessment of the performance of ISD and optimize the drying process by considering the quantity of energy. A divergent duct integrated with central processing unit (CPU) fans was attached to solar air collector (SAC) in forced convection ISD and this fitting was removed during natural convection experiments. The average SAC and drying efficiencies were 74.98% and 24.95% under forced convection and the same were 61.49% and 20.13% under natural convection, respectively. The exergy inflow, outflow and losses were determined for the collector and drying chamber for both setups. The exergy outflow of SAC was in a range of 1.04–46.85 W and 1.13–50.94 W, in forced and natural convection mode, respectively. The exergy loss for the drying chamber varied from 0.062 to 21.99 W and 0.394–24.99 W, under forced and natural convection, respectively. The values of average exergy efficiencies for SAC and drying chamber in forced convection ISD were 2.03 and 59.32%, while in natural convection, these were 2.44 and 55.45%, respectively. In forced convection ISD, the improvement potential was in the range of 0.0095–10.51 W while the sustainability index and waste exergy ratio varied from 1.19 to 17.05 and 0.06 to 0.83, respectively. EEA analysis revealed that the performance of forced convection was better than the natural convection ISD. Uncertainty of all parameters was estimated and updated in all plots in terms of error bars.

Comparisons of 2-D and 3-D IVR experiments for oxide layer in the three-layer configuration

We performed 3-D (3-dimensional) IVR (In-Vessel Retention) natural convection experiments simulating the oxide layer in the three-layer configuration, varying the aspect ratio (H/R). Mass transfer experiment was conducted based on the analogy to achieve high RaH′s of 1.99 × 1012–6.90 × 1013 with compact facilities. Comparisons with 2-D (2-dimensional) experiments revealed different local heat transfer characteristics on upper and lower boundaries of the oxide layer depending on the H/R. For the 3-D shallow oxide layer, the multi-cell flow patterns appeared and the number of cells was considerably increased with the H/R decreases, which differs with the 2-D experiments that the number of cells was independent on H/R. Thus, the enhancement of the downward heat transfer and the mitigation of the focusing effect were more noticeable in the 3-D experiments.

Natural convection heat transfer characteristics of TiO2–H2O nanofluids in a cavity filled with metal foam

In this paper, TiO2–H2O nanofluids are used instead of traditional mediums such as water and oil as the heat transfer medium. For the purpose of research on the natural convection heat transfer characteristics of TiO2–H2O nanofluids in a cavity filled with metal foam, a natural convection experiment system filled with different pore density (PPI) copper foam was established. The experimental results showed that the filling of the metal foam greatly improves the heat transfer effect in the cavity. When the PPI of the metal foam copper is 5 and 25, the Nusselt numbers are similar and larger than that of PPI = 15. The heat transfer effect of water in a cavity with the PPI = 5, PPI = 15 and PPI = 25 metal foam can be enhanced by 58.8%, 31.4% and 63.3% compared with that without metal foam, respectively. The heat transfer effect increases with the concentration of nanofluids. TiO2–H2O nanofluids with 0.1%, 0.3% and 0.5% mass fraction can enhance heat exchange by 6.3%, 22.9% and 32.1% compared with water, respectively. In addition, as the heating power increases, the heat transfer is improved while the heat transfer enhancement efficiency deteriorates.

Controllable natural convection in a rectangular enclosure filled with Fe3O4@CNT nanofluids

Superparamagnetic nanofluids based on magnetic composite nanoparticles were obtained, and natural convection experiments were conducted to enhance heat transfer performance by controlling the mass ratio of materials and the magnetic field. A 2:1 mass ratio for Fe3O4 and CNT was selected to conduct natural convection experiments as it provides better cost and efficiency of heat transfer compared to those for mass ratios of 1:1 and 4:1. The heat transfer and flow of natural convection were controlled by the heating power and the strength of magnetic field. The efficiency of natural convection with Fe3O4@CNT nanofluids increased from 14.1% to 29.1%, when increasing the heating power of base fluids from 3.528 W to 14.364 W without magnetic field, it further increased from 6.7% to 18.4% with the external magnetic field increased from 5 to 40 mT. The influence of magnetic field on heat transfer was also studied by simulation of non-isothermal flow, which was consistent with the experimental results. This research provided an approach to control the convective heat transfer coefficients by changing the mass ratio of materials, heating power, and external magnetic field.

Estimation of local heat transfer coefficient from natural convection experiments using liquid crystal thermography and Bayesian method

In this work, an inverse methodology is developed for estimating the local heat transfer coefficients on a vertical plate embedded with the three discrete heat sources, under steady state natural convection, with the temperatures measured at the adiabatic surface without disturbing the fluid flow, using simple conduction/surrogate model and Bayesian inference. Liquid crystal thermography (LCT), an optical measurement method based on the colour-temperature relationship of thermochromic liquid crystal sheet (TLC) is used to determine the temperature field of the adiabatic surface. Bayesian framework with Metropolis Hastings-Markov chain Monte Carlo (MH-MCMC) sampling method is considered for exploring the posterior distribution to estimate the parameters in terms of point estimates like mean, Maximum a posteriori (MAP) and standard deviation. A parity plot between simulated (using retrieved parameters) and measured TLC temperatures shows good agreement.

3D large eddy simulation (LES) calculations and experiments of natural convection in a laterally-heated cylindrical enclosure for crystal growth

The ammonothermal crystal growth technique is one of the most effective methods used for growing Gallium Nitride (GaN) crystals in terms of the final product quality and manufacturing efficiency. Popular applications of GaN crystals include light emitting diodes (LEDs) and high-frequency electronic devices. A laterally-heated cylindrical enclosure with a top to bottom temperature gradient is considered in this study to investigate the fluid mechanics and heat transfer in an ammonothermal crystal growth reactor. Three-dimensional (3D) large eddy simulations (LES) results of natural convection in a laterally heated cylindrical reactor, using the commercial computational fluid dynamics (CFD) software, ANSYS FLUENT, are presented in this paper for a Rayleigh number (Ra) of 8.8×106. The Ra is defined based on a length scale that is equal to the ratio of volume to the lateral area of the cylinder (R/2). In addition, an experiment of a geometrically- and dynamically-similar geometry is developed, and particle image velocimetry (PIV)-based flow visualizations are carried out for the purpose of validating the numerical model. Comparisons between experiments and numerical simulations showed that flow patterns were qualitatively similar, and Fourier transforms of velocity magnitudes at selected points in the domain matched reasonably well. An added interesting observation in the simulation was the existence of temperature inversion, which has potential implications on the choice of mineralizer (acidic/basic).

Experimental mixed convection heat transfer from an isothermal horizontal square cylinder

Laminar natural and mixed convection heat transfer from an isothermal horizontal square cylinder was experimentally studied. The study includes the design and construction of a test rig to fulfill the research requirements. The natural convection experiments were established for three different side lengths of the square cylinder (25, 30 and 40mm) which in atmospheric air give Grashof numbers, Gr of 9.895×104, 1.499×105 and 4.388×105. For mixed convection, using the three cylinders, Reynolds number, Re ranged from 48 to 670, and the attack angle, Ψ from 0° to 180° with steps of 30°. The experimental results gave the effects of Grashof, Reynolds and Richardson numbers on the average Nusselt number for air (Pr=0.7). The experimental results were correlated and compared with previous work.

Sedimentation in nanofluids during a natural convection experiment

This study presents an experimental investigation of the thermophysical behavior of γ-Al2O3–deionized (DI) H2O nanofluid under natural convection in the classical Rayleigh–Benard configuration, which consists of a cubic cell with conductive bottom and top plates, insulated sidewalls and optical access. The presence of nanoparticles either in stationary liquids or in flows affects the physical properties of the host fluids as well as the mechanisms and rate of heat and mass transfer. In the present work, measurements of heat transfer performance and thermophysical properties of Al2O3–H2O nanofluids, with nanoparticle concentration within the range of 0.01–0.12vol.%, are compared to those for pure DI water that serves as a benchmark. The natural convective chamber induces thermal instability in the vertical direction in the test medium by heating the medium from below and cooling it from above. Fixed heat flux at the bottom hot plate and constant temperature at the top cold plate are the imposed boundary conditions. The Al2O3–H2O nanofluid is tested under different boundary conditions and various nanoparticle concentrations until steady state conditions are reached. It is found that while the Rayleigh number, Ra, increases with increasing nanoparticle concentration, the convective heat transfer coefficient and Nusselt number, Nu, decrease. This finding implies that the addition of Al2O3 nanoparticles deteriorates the heat transfer performance due to natural convection of the base fluid, mainly due to poor nanofluid stability. Also, as the nanoparticle concentration increases the temperature at the heating plate increases, suggesting fouling at the bottom surface; a stationary thin layer structure of nanoparticles and liquid seems to be formed close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. This layer structure imposes additional thermal insulation in the system and thus appears to be responsible in a big extend for the reported heat transfer degradation. Also, for relatively high nanoparticle concentrations of 0.06 and 0.12vol.%, as the heating flux increases the rate of heat transfer deterioration increases. Specifically in the case of maximum nanoparticle concentration, 0.12vol.%, when the turbulence intensity increases, by increasing the applied heat flux, the Nusselt number remains constant in comparison with lower nanoparticle concentrations. This behavior can be attributed mainly to the physical properties of the Al2O3 nanopowder used in this study and the resulting interactions between the heating plate and the nanoparticles.

Anisotropic turbulent model based on V2f model for natural and mixed convection in enclosed spaces

In previous work (Zhang et al. 2007) several Reynolds-averaged Navier–Stokes turbulent models were evaluated by the classic natural convection experiment. It was demonstrated that the V2f turbulent model shows the best overall performance compared to other models in terms of accuracy of temperature, velocity, and turbulent kinetic energy. However, all compared turbulent models cannot simulate the pseudo-laminar phenomenon in the center zone of the classic natural convection experiment. Moreover, the V2f model needs to solve four equations, and the transport equation for the wall normal stress () and the elliptic equation for the relaxation function (f) make the model numerically unstable. Based on the idea of the V2f model, an anisotropic model (BV2fAM) was developed. In the BV2fAM model, as the turbulent kinetic intensity normal to streamlines was adopted and computed by local turbulent kinetic energy and local flow characteristic variable instead of the transport equation. The validity of the developed BV2fAM model was confirmed by two benchmark cases (classic natural convection in a tall cavity and mixed convection in a square cavity). It can be seen that the developed BV2fAM model can yield better simulated results of temperature, velocity, and turbulent kinetic energy than other compared models. Moreover, it can simulate the pseudo-laminar phenomenon in the center zone of the classic natural convection experiment.

Thermophysical and natural convection characteristics of ethylene glycol and water mixture based ZnO nanofluids

Stabilized nanofluids have been prepared by dispersing dry ZnO nanoparticles in different base fluids including 75:25, 85:15 and 95:5 (by volume) of ethylene glycol (EG) and deionized water (DW) mixtures. Thermal conductivity and viscosity experiments have been conducted at temperatures between 15°C and 55°C with mass fraction of nanoparticle of 5.25%. What’s more, natural convection characteristics of EG/DW based ZnO nanofluids were also investigated. The thermal conductivity of nanofluid with more DW is bigger because the network of direct and tight connection in spatial structures between atoms expedites heat transfer between nanoparticles, and it increases with increasing the temperature as expected. Meanwhile, the viscosity of nanofluid increases with increasing the concentration of EG. When the temperature increases, the intermolecular distance becomes larger, which leads to the decrease of the viscosity. Natural convection experiment shows that the presence of nanoparticles deteriorates the natural convective heat transfer under present condition.

Heat transfer effects of chimney height, diameter, and Prandtl number

This study investigates the heat transfer enhancement of a chimney system, both experimentally and numerically, by varying the height and diameter of the chimney, and the Prandtl number of the working fluid. Mass transfer experiments are carried out using a sulfuric acid and copper sulfate electroplating system based on analogy concepts. Numerical simulations are executed using FLUENT 6.3. Natural convection experiments and numerical calculations performed without a chimney showed good agreement with the Le Fevre correlation for natural convection on a vertical plate. As the chimney height is increased, the heat transfer rates are enhanced for all Prandtl numbers, but the enhancement rates decrease as the Prandtl number increases. An optimal chimney diameter is found that maximizes the heat transfer. An increase in heat input or heated length results in an additional enhancement of the heat transfer, increasing the buoyancy force. Numerical results provide visualizations of the temperature and velocity fields in the chimneys, showing their interactions and flow regimes.

Natural convection experiments on the outer surface of an inclined helical coil

Natural convection heat transfer on the outside of an inclined helical coil was investigated experimentally. For a single turn helical coil laid on the horizontal plane, the heat transfer rate was very close to that of a horizontal cylinder. The heat transfer rate decreased when the coil was tilted. For inclined helical coils with higher numbers of turns, the heat transfer rates were influenced by plumes from the lower turns, exhibiting preheating, velocity, and chimney effects. For very small pitches, the preheating effect was dominant. For moderate pitches, the velocity and chimney effects became dominant. For larger pitches, the plume effect disappeared. The angle of inclination affects the physical arrangement of the helical coil and influences the heat transfer. The study was performed using very high Pr of 2014. An empirical correlation was proposed based on the experiments.

Natural convection experiments on the upward and downward faces of inclined plates using an electroplating system

Natural convection experiments were carried out on inclined plates. A copper sulfate electroplating system was adopted to simulate heat transfer based on analogy. The lengths of the copper plates were 0.1 and 0.35 m, corresponding to Grashof numbers of 8.06 × 107 and 3.45 × 109. The inclination of the plates varied from upward-facing (UF) horizontal to downward-facing (DF) horizontal. Test results for the DF plate agreed well with Rich’s theory, that the Nusselt number can be calculated by replacing g with gcosθ in the heat transfer correlation for a vertical plate and those for the UF plate reveal that the development of the boundary layer and the flow separation occurs repeatedly for the whole plate length. The copper-plating patterns for the UF plates visualized the location of flow separation. The empirical correlation predicting the critical distance and the heat transfer correlations was proposed.

Bayesian estimation of heat flux and thermal diffusivity using liquid crystal thermography

This paper presents a methodology for the estimation of heat flux (qs) strength and thermal diffusivity (α) of a Teflon block by using liquid crystal thermography (LCT) experiments and Bayesian inference. Steady and transient state, laminar natural convection experiments have been conducted at various heat fluxes on a Teflon block of size 60 × 60 × 60, (L × W × H, all in mm). At the center of the Teflon block, a 10 mm diameter heater combined with a hollow hemispherical (ID = 10 mm and OD = 20 mm) aluminum ball is placed where heat generation takes place. The temperature distribution at the front face of the Teflon block has been captured for transient heating, steady and transient cooling experiments, using calibrated R40C5W thermochromic liquid crystals (TLCs). Similarly, the simulated temperature distribution during transient heating, steady and transient cooling cases, has been determined by solving the three dimensional conduction model of the Teflon block using FLUENT to arrive at the average heat transfer coefficient, havg on all faces by match up with limited experiments with various heat fluxes at the center face of Teflon are boundary conditions. With the matched up havg, further simulations are carried out and the input (qs and α) and target (temperature) data sets from the numerical solution are trained using an artificial neural network (ANN) and this is used as a forward model for estimating parameters qs and α. For guess sample of parameter(s), the simulated and experimental temperatures are compared using a Bayesian frame work. The Metropolis-Hasting algorithm has been used to sample the parameter space. Two point estimates namely mean and the maximum a posterior (MAP) along with the associated standard deviation (SD) are determined. At steady state, qs and during transient cooling, α has been estimated sequentially. During transient heating, qs and α have been estimated simultaneously and the mean of the estimates obtained previously from sequential estimation are incorporated into the estimation process as priors for the simultaneous estimation. The effect of the priors has been quantified.