Natural Convection Heat Transfer Characteristics Research Articles

Estimation of natural convection heat transfer characteristics of rack server in a cavity: experimental and numerical analyzes

Estimation of natural convection heat transfer characteristics of rack server in a cavity: experimental and numerical analyzes

Natural and Forced Convective Heat Transfer Enhancement for Solid Cylinders with Different Geometrical Shapes

Enhancing heat transfer for both natural and forced convection is a common issue for any heat transfer process. Experimental studies have been carried out for six different geometrical shapes of solid bars for natural convection and forced convection with four different air velocities while keeping the same perimeter and length of the solid bars, which means the lateral surface area of the bars is the same. Results reveal that both the natural and forced convective heat transfer characteristics are greatly influenced by the geometrical shape in terms of Nusselt number (Nu), heat transfer coefficient (h), and heat transfer rate (q). In addition, isosceles and cylindrical shape geometry contribute to the lowest and highest heat transfer, respectively. As well, it is obtained from the results that convective heat transfer characteristics are directly related to the cross-sectional area, even if the perimeters are the same. Moreover, among the different geometrical shapes, the isosceles and hexagonal shapes take the shortest and longest duration to attain the steady-state condition in the conductive heat transfer process. The convective heat transfer characteristics are well-validated, with available results for both natural and forced convection heat transfer.

Numerical computation on cavity natural convection for built-in plate arrays application: Heat transfer analysis and optimization

Natural convection in cavity with built-in plates has been widely used in fluid mechanics and heat transfer engineering, such as heat exchangers, electronics, solar collector, growing crystals, etc. However, the natural convection flow physics and heat transfer characteristics in cavity with plate arrays remain to be investigated further. In this study, numerical analysis and computational fluid dynamics simulation are conducted for natural convection in cavity with plate arrays by finite volume method. Key influence factors impacting heat transfer are analyzed, including the line dip-angle of plate arrays θ, rotary angle of plates to the array φ, with relationships comparison between the Nusselt number and Rayleigh number in different built-in plate arrays. Moreover, for illustrative application example, the natural convection in solar greenhouse with PV arrays roof is investigated, with inside temperature distribution comparison under different design parameters. The preliminary results indicate that plates spacing could remarkably impacts the number of streamlines between ”late’arrays. The dip-angle of arrays θ leads to different shape flow in the cavity, and the rotary angle of plates φ notably affects the fluid flow direction in the vicinity areas of plates, especially when φ = 90°. Solar greenhouse case study indicates that increasing array group numbers can improve air temperature distribution uniformity in the cavity, which is favorable for built environment accurate control and operation. This work can provide modeling method support and reference for natural heat convection applications.

Measurements of natural convective heat transfer of the inclined torus

Experiments were conducted to investigate the natural convective heat transfer characteristics of an inclined torus. The Rayleigh numbers ranged from 4.54 × 106 to 1.23 × 108 and the inclination angle was varied from 0° to 80° Mass transfer experiments were performed based on the analogy between heat and mass transfer using the CuSO4H2SO4 copper electroplating system. The heat transfer of the inclined torus exhibited a trend similar to that of an inclined cylinder, with a gradual decrease in heat transfer with the inclination angle due to the variation of vertical cross-sectional shape. However, the inclined torus demonstrated better heat transfer performance than an inclined cylinder as the curved structure interrupts the formation of the stable boundary layer underneath the torus. The flows were visualized by Particle Image Velocimetry (PIV) and the heat transfers, by the electric plating patterns. New correlations were proposed to predict the heat transfer coefficient of the inclined torus as a function of Rayleigh number, inclination angle, and toroidal ratio.

Natural convection heat transfer characteristics of louver surface at small installation distance

Natural convection heat transfer characteristics of louver surface at small installation distance

Natural and forced convection heat transfer characteristics of single-phase immersion cooling systems for data centers

Improving power usage effectiveness (PUE) in data centers becomes a key challenge for carbon neutrality and carbon peaking strategies. In this context, this paper investigates the single-phase immersion cooling (SPIC) system that promises to improve PUE. Three-dimensional mathematical models of pump-driven and buoyancy-driven immersion cooling tanks are established and numerically solved. The thermal characteristics, flow resistance, and energy consumption levels of immersion cooling tanks in three operating modes are investigated. Moreover, the roles of volume flow rate, inlet temperature, and device power in immersion cooling characteristics are analyzed. The results indicate that the heat dissipation of buoyancy-driven SPIC systems is less effective than pump-driven ones. Compared to buoyancy-driven SPIC systems, the average temperature of immersion coolants and PUE in pump-driven SPIC systems reduces by 55.5 and 11.6%, respectively. Interestingly, although natural convection in mixed convection operation enhances convective disturbance, it induces temperature stratification that weakens temperature homogeneity. Increases in inlet temperature and device power both worsen the immersion cooling performance. However, increasing inlet temperature improves temperature uniformity and reduces flow resistance losses, while increasing device power has the opposite effect. Besides, the increased flow rate enhances the convective heat transport and temperature uniformity but increases the flow resistance loss.

전기자동차 배터리팩 내부 자연대류 열전달 특성에 관한 수치해석적 연구

This study primarily aimed to explore the natural convection heat transfer characteristics inside the battery pack of electric vehicle equipped with effusion holes and rectangular fins. Three-dimensional numerical simulations were performed to examine the effects of effusion hole diameter, rectangular fin thickness and height, and effusion hole and rectangular fin arrangements. The local and average heat transfer coefficients on the inner surface of the battery pack upper cover were obtained from numerical simulations. The results showed that the flow velocities between the battery modules and under the battery pack upper cover increased with the aid of the effusion holes and rectangular fins. This increased flow velocity has a significant influence on the improvement of the natural convection heat transfer of the battery pack upper cover.

Natural Convection and Radiative Heat Transfer From Constant Surface Area Vertical Helical Coils: Effect of Pitch and Height

Abstract In this work, numerical simulations are carried out to delineate the natural convection and surface radiation heat transfer characteristics of vertically oriented isothermal helical coils having a constant surface area. Numerical computations using the finite-volume method are carried out in the laminar regime for the following non-dimensional parameter ranges: Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity of the coil (0 ≤ ɛ ≤ 1), pitch to the rod-diameter of the coil (3 ≤ p/d ≤ 7.5), and coil-height to the rod-diameter (40 ≤ H/d ≤ 60). Temperature-dependent fluid properties have been implemented to obtain accurate results. The impact of Ra and ɛ on both convective and radiative heat losses is discussed in detail. At a high Ra of 108, when H/d varies from 40 to 60, the mass flowrate inducted through the coil reduces from 40.6% at p/d = 3 to 11.4% at p/d = 7.5. As a result, the relative strength of convection heat loss declines with a rise in H/d. For a higher emissivity of the coil surface of 0.9 and a lower Ra of 104, heat transfer by convection contributes only 12.66% of the total heat transfer. In contrast, the contribution of radiative heat transfer is only 7.46% for a lower emissivity of the coil surface of 0.1 and a higher Ra of 108.

Numerical investigation of buoyancy-induced thermo-fluid characteristics of different types of infrared suppression (IRS) devices

The present work reports numerical investigation of natural convection heat transfer characteristics for three different types of infrared suppression (IRS) devices (i.e., conical, cylindrical, and hybrid) by varying Rayleigh number and diameter ratio in the range of 1010-1012, and 1.025–1.5, respectively. The effect of relevant parameters on average Nusselt number and induced air-flow rate through the IRS device has been elucidated graphically. Numerically computed results indicate that the average Nusselt number from IRS device is higher for cylindrical IRS devices for all Rayleigh number when DR>1.05. The average Nu initially increases with diameter ratio, attains the maximum (1.1 ≤ DR ≤ 1.15), and decreases with a further increase in diameter ratio. The Nu is significantly influenced by the diameter ratio of cylindrical and hybrid IRS devices than the conical IRS devices. It has been found that the dimensionless induced mass-flow rate at a particular diameter ratio decrease with the Rayleigh number and is found to be more for conical IRS device. In addition, correlation for the average Nusselt number as function of relevant input parameters has been proposed. The cooling time estimation based on lumped parameter analysis shows a faster cooling rate for cylindrical IRS devices followed by hybrid and conical IRS devices.

Prediction of 3D natural convection heat transfer characteristics in a shallow enclosure with experimental data

This study presents an inverse computational fluid dynamics (CFD) simulation combined with the least-squares method and overdetermined experimental temperature data to predict the unknown heat transfer rate Qh and three-dimensional (3D) natural convection heat transfer characteristics in a closed shallow enclosure. This novel study belongs to the inverse convection-conduction conjugate heat transfer problem. The results show that the zero-equation turbulence model is more suitable for this study compared to other flow models. Thus, the zero-equation model is chosen as the suitable flow model because its estimates are the closest to the experimental data and existing correlation. An important finding is that, for H/L = 1/15 and Ra = 1820, Bénard convection cells and eight vortices appear in 3D and two-dimensional (2D) temperature contours, respectively. This Ra value is close to the existing critical Rayleigh number of 1708. The arrangement of these cells and vortices can be merged with each other and reorganized as H/L increases. The obtained 2D velocity patterns and air temperature contours agree with existing patterns and isotherms. The obtained h‾hi values also agree well with the existing correlation. Thus, the present results have good accuracy. A review of the literature indicates that a few studies have been conducted on using the inverse method along with overdetermined experimental temperature data to study the present problem.

Experimental investigation of the natural convection heat transfer characteristics of cylinder walls with a DBD actuator as the heat source

Conventional electric heating methods applied when evaluating the natural convection heat transfer parameters at the outer walls of cylinders produce uneven heat flows and large measurement errors. The present work addresses these issues by developing a new heating method using a dielectric barrier discharge (DBD) actuator as the heat source. An experimental system is constructed, where the thermal power output of the DBD actuator is first measured by calorimetry, and the natural convection heat transfer characteristics of a representative cylinder are analyzed with an explicit accounting of the temperature distribution at the outer wall of the cylinder. The results demonstrate that the DBD actuator can provide more uniform heat flow than conventional electric heaters. The temperature difference between the top and the bottom of the cylinder wall initially increases with increasing thermal power, decreases to a minimum at a thermal power of about 400 W, and subsequently increases with increasing thermal power. In addition, and the feasibility of the DBD actuator as a heat source is demonstrated according to the high degree of conformation between the calculated values and empirically established correlations between Nusselt number, which generally deviate by less than 10%.

Effect of Al2O3–SiO2/Water Hybrid Nanofluid Filled in a Square Enclosure on the Natural Convective Heat Transfer Characteristics: A Numerical Study

In this study, heat transfer enhancement by natural convection Al2O3/water mono and Al2O3–SiO2/water hybrid nanofluid in an enclosure cavity have been performed utilizing the finite element method. For numerical computations, the homogeneous nanofluid approach was considered. The cavity was heated from the left vertical wall and cooled from the right vertical wall while the top and bottom walls were taken as adiabatic. The effects of some related factors such as the Rayleigh number (103 ≤ Ra ≤ 106) and nanoparticles’ volume fraction (0 ≤ φ ≤ 0.05) on the heat transfer by natural convection were examined. To discuss fluid characteristics of mono and hybrid type nanofluid under natural convection effect, the obtained results were presented as streamlines and isotherms. Also, variations of local and average Nusselt numbers were examined in detail. It was obtained that an increase in the nanoparticle volume fraction leads to the enhancement of convective heat transfer for all Rayleigh numbers. It was also indicated that the highest increment in heat transfer by convection occurs in the nanoparticle volume fraction of 2% for Al2O3/water and 4% for Al2O3–SiO2/water. The present study results are also consistent with the literature results.

Numerical study of natural convective heat transfer of nanofluids within a porous corrugated triangular cavity in the presence of a magnetic field

In the present study, a nonorthogonal multi-relaxation time (MRT) lattice Boltzmann method (LBM) was realized to numerically study the natural convective heat transfer characteristics of Al2O3-water nanofluid inside a porous corrugated triangular cavity in the presence of a magnetic field. The comparison of the nonorthogonal MRT-LB model with the orthogonal MRT-LB in simulating the natural convective heat transfer considering the magnetic field effect and the filling of porous media was presented. The effects of Ra (Rayleigh number), Da (Darcy number), Ha (Hartmann number), φ (nanoparticle volume fraction), and the d (structural parameter of the corrugated wall) on the natural convective heat transfer characteristics inside the corrugated triangular cavity were investigated in detail. The results showed that the nonorthogonal MRT-LB model can accurately simulate such problems and it has higher computational accuracy than the orthogonal MRT-LB in simulating the natural convective heat transfer in porous cavities under the magnetic field effect. The magnetic field has no effect on the conduction but suppresses the convection process. The increase of Ha number significantly weakens the heat transfer performance when the convection dominates. When Ra = 104, the heat transfer performance weakens with increasing d. When Ra = 105, the heat transfer performance fluctuates with increasing d, among the structural parameters d in the current study, the optimal is reached at d = 24 while the worst is at d = 72. The average Nusselt number increases with increasing the φ for different Ha numbers at Ra = 104 and Ra = 105.

Natural convection heat transfer characteristics of sinusoidal cavities filled with nanofluids

To improve the heat transfer of free convection of cavity in the field of photothermal conversion and cooling of electronic components, new sinusoidal cavities were developed. The heat transfer properties of Fe3O4-H2O nanofluids in the novel cavities were studied experimentally. The influence of three amplitudes (A=1 mm, A=2 mm, A=3 mm) and three wave numbers (n = 1, n = 2, n = 3), four mass fractions (wt%=0.0%, wt%=0.1%, wt%=0.3%, wt%=0.5%) and four heating powers (Q=10 W, Q=15 W, Q=20 W, Q=25 W) on heat transfer behavior was researched. The results showed that when the amplitude is A= 3 mm and the wave number is n = 2, the Nusselt number is the largest, and the heat exchange cavity exhibits the best heat transfer performance, the Nusselt number increases with nanofluids mass fraction and heating power. It provides important guidance for the design of cavities in the field of photothermal conversion and cooling of electronic components.

Experimental study of natural convection heat transfer characteristics affected by electrical field with periodically changed direction

The effects of electrical field on natural convection heat transfer have been extensively investigated. While, the heat transfer characteristic enhanced by the electrical field with periodically changed direction is rarely involved. Therefore, the heat transfer performance of transformer oil, the mechanism analysis and prediction correlation for heat transfer characteristic in the electrical field with periodically changed direction are studied in the present study. Individually, the heat transfer coefficient enhancement is compared with different switch periods. Meanwhile, the prediction correlation for heat transfer enhancement is analyzed and proposed. Results show that the heat transfer is significantly enhanced by the direction changed electrical field with the decrease of switch period. Furthermore, mechanism analysis indicates that the positive and negative charges in the fluid, which form a dipole, move in a certain range by the electrical field with periodically changed direction. It results in the violent fluctuations in the fluid, which is beneficial for the internal local heat transfer. Finally, a prediction correlation is proposed for heat transfer in the electrical field, which agrees well with the experimental data.

Review on Thermal Performance of Nanofluids With and Without Magnetic Fields in Heat Exchange Devices

Addition of nanoparticles into a fluid can improve the heat transfer performance of the base fluid in heat exchangers. In this work, the preparation method and process of nanofluids are introduced, and thermal properties of nanofluids, such as thermal conductivity and viscosity, are discussed deeply. This paper summarizes various theoretical models of thermal conductivity and viscosity of nanofluids. A comprehensive literature survey on applications and limitations of nanofluids has been compiled. This paper also aims to review the natural and forced convective heat transfer characteristics of nanofluids with and without magnetic fields. The discussion for the natural convective heat transfer of nanofluids focuses on the heat transfer performance of non-conventional enclosures and electric heaters. The effects on heat transfer due to variations of heated walls are also investigated. Specific applications of nanofluids in a tube with trapezoidal ribs, double-tube heat exchangers, and plate heat exchangers have been reviewed and presented in a discussion about forced convective heat transfer. The previous results show that the inlet temperature of nanofluids obviously affects the heat transfer characteristics of double-tube heat exchangers, whereas a multi-walled carbon nanotube–water nanofluid shows significant advantages in plate heat exchangers. Finally, this paper studies natural convective heat transfer of magnetic fluids in a square cavity and forced convection heat transfer in a straight tube and a corrugated structure under the action of magnetic fields. It is found that the heat transfer performance of an Fe3O4–water nanofluid is enhanced when a magnetic field is applied to the corrugated plate heat exchangers, and the pressure drop can be reduced by around 10%. It is recommended that natural convection of magnetic fluids needs to be investigated experimentally in a real cavity and a corrugated channel under the influence of a magnetic field. In addition, studies of alternating magnetic field are recommended to reveal any improvements of thermal performance of magnetic fluids in heat exchange devices. This review puts forward an effective solution for improvement of the thermal performance of heat transfer equipment and serves as a basic reference for applications of nanofluids in heat transfer fields.

Natural convection phenomena in a liquid metal pool due to relocated and heap of heat-generating core debris: Numerical study

In the present numerical study, authors have investigated the natural convection heat transfer and fluid flow characteristics in a pool due to the heat generating core debris, i.e. decay heat in a typical fast breeder reactor. Two significant aspects of decay heat removal from debris under core disruptive accidents have been analyzed. These key aspects are (i) relocation of heat-generating core debris at the different locations inside the main vessel and (ii) effect of debris heap on heat transfer from source to ambient. These cases are analyzed extensively on full-scale three-dimensional reactor model with all major components. Multiple tray-type core catcher has incorporated with passive pipes over one collection tray. The present numerical study provides a robust and practical feasible core debris retention arrangement after a severe accident in fast breeder reactors. Also, the study is focused on a feasible array of core collection trays that maintained heat-generating debris and heat shield trays under safe thermal designed limit (∼1200 K for debris and ∼ 923 K for trays). This eliminates the further advancement of accidents and leakage of radioactive material into environments. During our analysis, it has been found that single tray with passive pipes maintained the heat-generating debris and tray under safe thermal limit for a fraction of 50 % destroyed core relocated to lower plenum while remaining heat-generating debris is present in its original position. Multiple tray arrangements accommodate the whole core debris within safe thermal design limits. With the possibility of debris heap after its relocation to the lower plenum, it has been found that heap angle less than 3° does not affect the integrity of core catcher.

Experimental and numerical investigation on natural convection heat transfer characteristics of vertical 3-D externally finned tubes

Tubular heat exchangers for natural convection can be found in many industrial applications. In order to develop the high-efficiency natural convection radiator, the heat dissipation characteristics of vertically oriented 3-dimensional (3-D) finned tubes under the condition of non-forced convection were studied experimentally. The airflow and temperature distribution outside the tubes were analyzed by computational fluid dynamics. Results showed that the 3-D finned tube with fin height of 7 mm achieved the highest Nusselt number, which was up to 207% higher than that of a smooth tube. Nusselt numbers increased with the increasing of the fin height and the decreasing of the axial fin pitch, but were minimally affected by fin width and circular fin pitch. For comparison, the most important fin geometric parameter that affects the heat dissipation is the fin height. Finally, an empirical correlation for predicting Nusselt number was obtained based on the experimental data, which can be used in practical design.

Experimental and numerical study of innovative plate heat exchanger design in simplified hot box of SOFC

The inverse computational fluid dynamics simulation combined with experimental temperature measurements is applied to investigate the three-dimensional natural convection of an innovative plate heat exchanger in a hot box. This natural convection conjugate heat transfer separated by all insulating walls of the hot box is solved by couping. The upper low-temperature duct is located between the lower high-temperature duct and the top wall of the hot box. If the root mean square error between the obtained numerical results of the air temperature and the experimental data at the selected measurement locations is the smallest among all flow models, then the appropriate flow model and wall function can be predicted. Furthermore, more accurate natural convection heat transfer characteristics and the heat transfer rate applied to the two ducts can be determined. An important finding is that the realizable k-ε model combined with the standard wall function and the zero-equation model are suitable for problems with smaller and larger duct spacings, respectively. This means that the location of the two ducts may influence the selection of an appropriate flow model. In addition, increasing the number of grid points does not necessarily lead to more accurate results. An optimal total grid points can be obtained. The obtained heat transfer coefficients on the upper surfaces of the two ducts are also in good agreement with the existing correlation. This comparison further verifies that the present results and the selected flow model are accurate. Due to the blocking, buoyancy and heat dissipation effects of the two ducts, the formation of the main vortex, the velocity patterns and air temperature contours obtained by the two selected appropriate flow models are slightly different. The two gaps between the edges of the ducts and the side walls of the hot box can destroy the structure of natural circulation and improve the heat accumulation under the two ducts. The air flows slightly in the direction of the larger gap so that the velocity pattern and the temperature contour become asymmetric. As far as we know, these findings have not yet appeared in the public literature. Therefore, this study is novel and innovative.

Numerical investigation of nanoparticles slip mechanisms impact on the natural convection heat transfer characteristics of nanofluids in an enclosure

This study intends to give qualitative results toward the understanding of different slip mechanisms impact on the natural heat transfer performance of nanofluids. The slip mechanisms considered in this study are Brownian diffusion, thermophoretic diffusion, and sedimentation. This study compares three different Eulerian nanofluid models; Single-phase, two-phase, and a third model that consists of incorporating the three slip mechanisms in a two-phase drift-flux. These slip mechanisms are found to have different impacts depending on the nanoparticle concentration, where this effect ranges from negligible to dominant. It has been reported experimentally in the literature that, with high nanoparticle volume fraction the heat transfer deteriorates. Admittingly, classical nanofluid models are known to underpredict this impairment. To address this discrepancy, this study focuses on the effect of thermophoretic diffusion and sedimentation outcome as these two mechanisms turn out to be influencing players in the resulting heat transfer rate using the two-phase model. In particular, the necessity to account for the sedimentation contribution toward qualitative modeling of the heat transfer is highlighted. To this end, correlations relating the thermophoretic and sedimentation coefficients to the nanofluid concentration and Rayleigh number are proposed in this study. Numerical experiments are presented to show the effectiveness of the proposed two-phase model in approaching the experimental data, for the full range of Rayleigh number in the laminar flow regime and for nanoparticles concentration of (0% to 3%), with great satisfaction.