Natural convection heat transfer from a horizontal cylinder to mercury under magnetic field

This paper presents the experimental data of natural convection heat transfer from a horizontal cylindrical heater immersed in mercury pool under the two applied magnetic fields of which directions are perpendicular and parallel to the direction of gravity, respectively. The presence of both magnetic fields causes increase in heater surface temperature and liquid temperature surrounding the heater and also increase in thermal boundary layer thickness, at the fixed surface heat flux. From these results, it is expected that the magnetic field affects the inception of boiling. Variation of local heat-transfer coefficient along the periphery of the heater is also discussed.     

Three-Dimensional Study of Natural Convection in a Horizontal Channel With Discrete Heaters on One of Its Vertical Walls

Three-dimensional natural convection in a horizontal channel with an array of discrete flush-mounted heaters on one of its vertical walls is numerically studied. Effects of thermal conductivities of substrate and heaters and convection on outer sides of the channel walls on heat transfer are examined. The substrate affects heat transfer in a wider range of thermal conductivities than do the heaters. At lower heater thermal conductivities a higher heat portion is transferred by direct convection from the heaters to the adjacent coolant. However, higher substrate conductivity is associated with higher heat portion transferred through the substrate. The innermost heater column is found to become the hottest heater column due to the lower coolant accessibility. The heat transfer in the channel is strongly influenced by convection on the outer sides of the channel walls. Correlations are presented for dimensionless temperature maximum and average Nusselt number.     

The Effect of Alumina–Water Nanofluid on Natural Convection Heat Transfer Inside Vertical Circular Enclosures Heated from Above

Experimental investigation on natural convection heat transfer is carried out inside vertical circular enclosures filled with Al₂O₃–water nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%), and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The top surface of the enclosure is heated using a constant-heat-flux flexible foil heater while the bottom surface is subject to cooling using an ambient air stream. Various heat fluxes are used to generate heat transfer through the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. A general correlation for the average Nusselt number with the modified Rayleigh number is obtained using the volume fraction and the aspect ratio as parameters to cover both enclosures. The results show that the Nusselt number for the alumina–water nanofluid is less than that of the base fluid. This means that using the alumina–water nanofluids adversely affects the heat transfer coefficient compared to using pure water. It is also found that the degree of deterioration depends on the concentration ratio as well as the aspect ratio of the enclosure.     

Unsteady conjugate natural convection in a porous cavity boarded by two vertical finite thickness walls

The objective of this study is to investigate unsteady conjugate natural convection in a porous cavity sandwiched by finite conductive walls considering time-periodic boundary conditions and local thermal non-equilibrium. The top and bottom boundaries are assumed to be isolated and the continuity of temperature and heat transfer are considered in interface boundaries. The effect of varying a plethora of parameters such as Rayleigh number, Thermal conductivity ratio, wall thickness, and non-dimensional frequency on the streamlines, isotherms, and Nusselt number has been studied. It is shown that, apart from non-dimensional frequency and wall thickness, the amplitude of periodic fluid Nusselt number is an increasing function of all aforementioned parameters. Furthermore, aside from Rayleigh number and heat transfer coefficient, the behavior of the solid Nusselt number is the same as fluid Nusselt number. Eventually, the time-averaged Nusselt number and heat transfer through the vertical walls for different values of non-dimensional frequencies are calculated.     

Effect of Relative Humidity on the Prediction of Natural Convection Heat Transfer Coefficients

Results of an investigation into the sensitivity of natural convection heat transfer correlations with respect to relative humidity are presented. Given the relatively small values of natural convection heat transfer coefficients, small changes in the thermophysical properties can have a significant impact on the values predicted by theoretical/empirical correlations. In this study, the thermophysical properties are assumed to be those of a dry air and water vapor mixture. The mole fractions are determined as a function of relative humidity. Several widely used natural convection heat transfer correlations have been examined to determine the impact of varying the relative humidity on the predicted Nusselt number. The results show a general trend of an increasing Nusselt number with relative humidity. The results presented in this paper provide an engineering tool for obtaining accurate values of natural convection heat transfer coefficients for a moist air environment using only the thermophysical properties of dry air.     

Natural convection heat transfer of the low prandtl number fluid with solidified layer formation

Experimental and analytical studies have been performed on natural convection heat transfer in the low Prandtl number fluid of the molten metal with a rapid solidified layer formation. The experimental results on the relationship between the Nusselt number and Rayleigh number were compared with the existing correlations. The temperature distribution and the heat transfer rate have been evaluated using the numerical model. The experimental study has shown that a solidified layer formation leads to a decrease of natural convection heat transfer rate in the metal pool, due to the solidified layer's action as a conducting thermal barrier. The experimental results are more similar to Globe and Dropkin's correlation than any others. Even though the Rayleigh number rapidly decreases due to an increase of the solidified layer thickness, the Nusselt number does not rapidly decrease because of the aspect ratio effect. The numerical results on the temperature profile and the heat transfer rate in the metal pool region match well with the experimental data.     

Transient natural convection heat transfer of liquid D-mannitol on a horizontal cylinder

Transient and steady state natural convection heat transfer for D-mannitol on a horizontal cylinder was investigated experimentally at various liquid temperatures and heat input conditions. To clarify the natural convection phenomena of D-mannitol, transient and steady heat transfer coefficients were measured under various liquid temperatures of D-mannitol and periods of heat generation rates from a horizontal platinum cylinder. The platinum cylinder with a diameter of 1 mm and a length of 43.5 mm was used as the test heater in this experiment. Experimental results indicated that the steady heat transfer coefficient of D-mannitol was affected by the liquid temperature. As the liquid temperature increased, it was understood that the effect of liquid temperature weakened. When the period of the heat generation rate was changed, the heat transfer process was divided into natural convection heat transfer and conductive heat transfer. It was considered that the conductive heat transfer was more dominant as the period of the heat generation rate decreased. The empirical correlations of steady and transient heat transfer coefficients for D-mannitol were obtained.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

Pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating

We study the pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating. Nanofluids are the mixture of de-ionized water and Al₂O₃ particles with 0.1%, 0.2%, 0.5% and 1.0% weight concentrations. The microheater is a platinum surface by 50 × 20 μm. Three types of bubble dynamics were identified. The first type of bubble dynamics is for the boiling in pure water, referring to a sharp microheater temperature increase once a new pulse cycle begins, followed by a continuous temperature increase during the pulse duration stage. Large bubble is observed on the microheater surface and it does not disappear during the pulse off stage. The second type of bubble dynamics is for the nanofluids with 0.1% and 0.2% weight concentrations. The microheater surface temperature has a sharp increase at the start of a new pulse cycle, followed by a slight decrease during the pulse duration stage. Miniature bubble has oscillation movement along the microheater length direction, and it disappears during the pulse off stage. The third type of bubble dynamics occurs at the nanofluid weight concentration of 0.5% and 1.0%. The bubble behavior is similar to that in pure water, but the microheater temperatures are much lower than that in pure water. A structural disjoining pressure causes the smaller contact area between the dry vapor and the heater surface, decreasing the surface tension effect and resulting in the easy departure of miniature bubbles for the 0.1% and 0.2% nanofluid weight concentrations. For the 0.5% weight concentration of nanofluids, coalescence of nanoparticles to form larger particles is responsible for the large bubble formation on the heater surface. The microlayer evaporation heat transfer and the heat transfer mechanisms during the bubble departure process account for the higher heat transfer coefficients for the 0.1% and 0.2% nanofluid weight concentrations. The shortened dry area between the bubble and the heater surface, and the additional thin nanofluid liquid film evaporation heat transfer, account for the higher heat transfer coefficient for the 0.5% nanofluid weight concentration, compared with the pure water runs.     

相变微胶囊悬浮液强制对流换热实验研究

提出了一种可以同时作为储能介质与传热流体的新型相变微胶囊悬浮液(MPCS),设计和搭建试验台,分别在层流和湍流条件下在等热流密度的光滑圆管中对MPCS进行了强制对流换热实验,研究了悬浮液浓度、流量、泵送功率和加热速率对其流动及传热特性的影响。结果表明:对于质量分数为5%的MPCS,当微胶囊中相变材料分别处于固体、固体-液体和液体状态时,对应的努塞尔数平均增大了23.9%、20.5%和9.1%;与纯水相比,MPCS作为在热力系统应用的传热流体可以实现强化传热,但是需要在传热实验中控制好相变过程才能使MPCS的传热性能优于水。     

Feasibility Study of Heat Transfer Enhancement by Ultrasonic Vibration Under Subcooled Pool Condition

In this study, experimental tests were carried out to investigate the feasibility of heat transfer enhancement by ultrasonic vibration under a subcooled pool condition. A commercial stainless-steel heater was utilized along with a water tank, and three ultrasonic transducers were attached underneath the tank bottom to generate ultrasonic vibration with a frequency of 40 kHz and total power of 150 W. For demonstrating the effectiveness, the tests were performed with and without ultrasonic vibration under a constant water temperature of 30°C and pressure of 1 atm. The heights of heater were set at 15, 22, and 33 mm from the tank bottom, and the heat flux was operated from 7800 to 70800 W/m², which covered the regimes of single phase natural convection and subcooled nucleate boiling. Instantaneous signals of temperature and heat flux were recorded during the experiment, and the heat transfer coefficients were determined for each condition. The results show that the heat transfer coefficient can increase up to 3880 W/m²K and the enhancement ratio can reach up to 284% by ultrasonic vibration. Trends of heat transfer increments and enhancement ratios against heat flux and height were presented. This study successfully demonstrated the feasibility of heat transfer enhancement by ultrasonic vibrations.     

Free Solution Convection at Non-Isothermal Evaporation of Aqueous Salt Solution on a Micro-Structured Wall

Evaporation and heat transfer of layers of aqueous salt solutions have been studied. The behavior of salt solutions is compared for a smooth and micro-structured wall with a rectangular profile. The evaporation rate of the salt solution on the structured wall is 20–30% higher than on the smooth one at high salt concentration. Previously, it was thought that the heat transfer for solutions can be calculated for thin layers and films without taking into account the natural convection in liquid. In this paper, the liquid free convection is shown to play a key role. A simple model linking the solutal and the thermal Marangoni numbers and the Peclet number with free convection of the liquid on a hot structured wall is considered. For correct simulation of the non-isothermal heat and mass transfer, it is necessary to take into account local characteristics of thermal and velocity fields inside a layer of the salt solution, as well as to determine the average characteristic scales of circulation into the liquid. To simplify the analysis it is possible to effectively consider four types of characteristic convective scales, the role of which depends on the thickness and diameter of the solution layer, as well as on the wall temperature. The strong influence of free convection in a thin layer of the solution is extremely important for accurate modeling of a wide range of modern technologies. Intensification of heat transfer and evaporation due to the use of a structured wall can be applied in heat exchangers, to improve efficiency in desalination of water, in energy technologies (e.g., in heat absorption pumps), as well as in chemical technologies.     

Nanofluid multi-phase convective heat transfer in closed domain: Simulation with lattice Boltzmann method

The lattice Boltzmann method is applied to simulate the thermal field and flow field of nanofluid natural convection in a square cavity. The heat transfer characteristics of nanofluid are compared with that of water to explore nanofluid heat transfer mechanism. The flow field shows different characters at different Rayleigh number and the average Nusselt number is obtained changing with Rayleigh number.     

Numerical investigation of the thermal mechanism of the solid-liquid phase changing process

由于相变换热储能技术可以协调能量在时间和空间尺度的分配,成为了目前研究的热点问题。本工作用焓值法分别对充填低温无机盐相变材料的二维和三维管壳式相变储能换热器模型的储/放热特性进行了模拟研究,采用Boussinesq近似研究了液相区密度变化引起的自然对流的影响。研究表明换热器的入口温度对相变换热效率影响显著;在储热过程中自然对流发挥了重要作用,换热效率与液相区的运动状态直接相关,而放热过程中的热交换主要依靠热传导完成;三维模拟的结果表明换热管出口温度与管壁的平均努赛尔数高度相关,且换热管水平放置的换热效率略低于竖直放置。     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Numerical Investigation of Unsteady Natural Convection in an Inclined Square Enclosure With Heat-Generating Porous Medium

The unsteady laminar natural convection in an inclined square enclosure with heat-generating porous medium whose heat varies by a cosine function is investigated by a thermal equilibrium model and the Brinkman–Darcy–Forchheimer model numerically, with the four cooled walls of closure as isothermal. The numerical code based on the finite-volume method has been validated by reference data before it was adopted. Influence of dimensionless frequency and inclination angle on heat transfer characteristics in a square enclosure, such as flow distribution, isotherm, averaged Nusselt number on each wall, and time-averaged Nusselt number, are discussed, with specified value for Rayleigh number = 10⁸, Darcy number = 10^?4, Prandtl number = 7, porosity = 0.4, and specific heat ratio = 1. It is found that when the internal heat source varies by cosine, the Nusselt numbers of the four walls oscillate with the same frequency as the internal heat source; however, phase difference occurs. Moreover, frequency has little impact on time-averaged Nusselt number of the four walls, which is different from the phenomenon discovered in natural convection with suitable periodic varying wall temperature boundary condition. Moreover, inclination angle plays an important role in the heat transfer characteristics of the walls studied.     

Experimental Study of Air Pressure Effects on Natural Convection From a Horizontal Cylinder

The topic of this research is the importance of convection heat transfer coefficient and the effect that ambient pressure has on it. Toward this end, an experimental study was performed to investigate the effects of air pressure on natural convection heat transfer from a horizontal cylinder. Pressure was varied from 1 to 220 kPa and the convection coefficient was obtained in the temperature range from 50 to 100°C. Various diagrams and tables were obtained to show the dependence of natural convection on pressure. A correlation was derived to describe the heat transfer coefficient and Nusselt number as a function of pressure and Knudsen number or Rayleigh and Knudsen number. The rate of heat transfer by radiation was also compared with convection at different temperatures and pressures.     

Concerning the magnitude of the maximum heat flux and the mechanisms of superintensive bubble boiling

We have developed a stabilizer of the temperature of a thermoresistor wire electric heater based on a PID controller. Using this stabilizer, we investigated heat exchange of subcooled water in pool boiling. We found that on stabilization of the heater temperature up to that of the subcooled water, transition from convection to the regime of bubble boiling and vice versa occurs spontaneously and is accompanied by a jumpwise change in heat transfer. It is shown that in the regime of stable bubble boiling, the law of heat transfer is independent of the liquid temperature and the heater diameter and that the maximum heat loading may attain 50 MW/m², which is much above the values cited earlier in the literature. Based on the results obtained, a mechanism of implementation of bubble boiling for the regimes of a constant heat flux and a constant temperature is suggested. The assumption is made that the regime of heterogeneous vapour generation is possible only in the case of the heater constant temperature. In the regime of a stabilized heat flux on the heater, the spatially inhomogeneous regime of heat transfer is established. This regime represents a spatially distributed combination of three regimes: convective heat transfer, homogeneous boiling, manifesting itself in periodic boiling-up of overheated layers of the liquid near the surface and an unstable regime of heterogeneous vapour generation.     

Experimental study on the influence of the aspect ratio of square cavity on natural convection heat transfer with Al2O3/Water nanofluids

Cavity design is an important aspect in thermal systems, and proper cavity design saves plenty of energy as losses are minimised through better design. In this work, the influence that the Aspect Ratio (AR) of a rectangular cavity filled with nanofluids has on the natural convection process is studied experimentally. Three different cavities with the AR of 1, 2 and 4 are fabricated, and the heat transfer performance is studied using two different fluids namely de-ionised water and Al₂O₃/Water nanofluids. It is found that the AR of the cavity has a significant effect on the heat transfer coefficient and Nusselt number. More importantly, the optimum nanofluid concentration for maximum heat transfer varies with the AR of the cavity. It also found that the Rayleigh number has a strong effect on the Nusselt number as well as nanofluid buoyancy.     

Natural convection in fluid–superposed porous layers heated locally from below

A numerical study of two-dimensional natural convection in fluid–superposed porous layers heated locally from below is reported based on the one-domain formulation of the conservation equations. The effects of five dimensionless parameters on overall Nusselt number are investigated: Rayleigh number based on overall layer height, heater-to-cavity length ratio, porous layer-to-cavity height ratio, domain aspect ratio, and Darcy number. Streamline and isotherm patterns indicate that convective motion is restricted to the overlying fluid layer with some penetration into the porous layer. Nusselt numbers increase with a decrease in the heater length and height ratio, and increase with the Darcy number. The size of the heat source does not affect the dependence of the heat transfer coefficient on height ratio and Darcy number. For domains with large aspect ratios, complex flow restructuring is observed with an increase in Rayleigh number. The present results represent an extension of the well studied problem of buoyant convection in fluid–superposed porous layers with a fully heated lower surface.