Convective Heat Transfer Process Research Articles

THERMAL-HYDRAULIC PERFORMANCE OF Al2O3 NANOFLUIDS UNDER LAMINAR FLOW IN A MINITUBE

The improvement of performance and applications that include the convection heat transfer process is extremely related to the thermophysical properties of the fluids. However, it was found that adding nanoparticles to the conventional fluids (known as nanofluids) is an advanced way to enhance their thermophysical properties leading to an enhanced heat transfer process. This study presents a numerical approach to investigate the laminar flow-based convective heat transfer of Al2O3 nanofluids in a mini-tube under constant heat flux boundary conditions (15 kW/m2) and at an inlet temperature of 25°C. For this study, the nanofluid samples are prepared for several concentrations of Al2O3 nanoparticles and ethylene glycol/water mixture as a base fluid. Also, the thermal properties of nanofluids and base fluid are characterized. Computational fluid dynamics tools are used to develop a numerical model and validate it in comparison to experimental data. The results are discussed based on the heat transfer coefficient, considering the entrance area, pressure drop, and friction factor to determine the performance of nanofluids in comparison to the base fluid. A considerable enhancement in the heat transfer, especially in the entrance area up to 17.8%, with a corresponding increase in the pressures drop, friction factor, and the required pumping power of the nanofluids was observed.

Effect of a rotating cylinder on the 3D MHD mixed convection in a phase change material filled cubic enclosure

This study displays an exhaustive numerical analysis of entropy production and mixed convection in a 3D cavity filled with a phase change material (PCM) and incorporating a rotating cylinder. The temperature gradient was created between the hot right vertical wall and the cold left while the remaining walls are considered adiabatic. The impacts of various parameters such as the angular rotational velocity (−10 ≤ Ω ≤ 10), vertical position of the cylinder ((0.025 ≤ H ≤ 0.075)) and Hartmann number (0 ≤ Ha ≤ 10) on the convective heat transfer and entropy production were numerically studied in detail. The analysis found evidence to consider the rotating cylinder speed and position as good controlling parameters for the convective heat transfer and melting process in the enclosure. Imposing an angular velocity on the cylinder was found to enhance the heat transfer rate by 21.2% compared to motionless cylinder. In addition, it was noticed that the vertical position of the cylinder can be considered as optimizing parameter of the heat transfer. The application of an external magnetic field lead to the reduction of the heat transfers due to the generated Lorentz force that oppose the Buoyancy effect.

Simulation analysis of a single well geothermal system with open-loop structure

The relatively small heat exchange power limits the promotion and application of deep borehole heat exchanger (DBHE) with the closed loop due to the main heat transfer pattern of heat conduction through rocks with poor thermal conductivity coefficient. Furthermore, geothermal water is prohibited to extract in some places even with 100% reinjection. To solve the above problems, here a novel single-well enhanced geothermal system (SEGS) is studied with an open-loop structure having the convection heat exchange process. The heat transfer performance, mechanism and main influential factors of SEGS are studied. Result shows that the heat transfer performance of SEGS is greatly improved compared to DBHE, and the average heat transfer power of SEGS reaches 1603.6 kW and 1204.6 kW respectively with different operation modes. In addition, the study also finds that the convective heat transfer process inside the geothermal reservoir is simultaneously affected by the driving effect of the fluid in the screen and the buoyancy effect. Further, the above two effects act in the same direction when the annulus is used as the inlet, while they act in the opposite direction when the inner tube is used as the inlet.

Experimental investigation of flow and exergy transfer characteristics in the air-cooled randomly packed particle bed based on second law analysis

Randomly packed particle beds (RPPB) are widely used in industry, and their internal flow and heat transfer characteristics have a significant impact on the energy efficiency of the overall system. Based on the second law of thermodynamics theory, the correlation equation to predict the exergy transfer coefficient (ETC), exergy transfer Nusselt number (Nuex) within the RPPB is developed, which can reflect temperature and pressure exergy transfer. It effectively reveals the relationship between the role of heat transfer and pressure loss. The experiments are then carried out using stainless-steel particles packed bed with an electric induction heating system to provide a uniform internal heat source. The ambient air is used as the test fluid to cool the heated particles. The pressure drops and forced convection heat transfer characteristics of air-cooled high-temperature particles are investigated. The experiments are performed for flows with modified Reynolds number Redh in the range of 811–6810. The effects of cooling air inlet velocity (ua,in) and electromagnetic induction heating power (Qe) on the experimental results are investigated under different particle sizes (dp = 6 and 8 mm). The results show that when dp and Qe are constant, the overall Nuex decreases exponentially with increasing Redh, and even becomes negative. It means that the heat gain of the air through the RPPB is lower than the pressure loss. The critical Redh are 2650 and 5300 when dp = 6 and 8 mm, respectively. At fixed dp and ua,in, Nuex,T decrease linearly with the increase of Redh. In the case of low Qe, the exergy transfer between the particles and air is mainly based on pressure exergy transfer. As the Qe increases, the temperature exergy transfer gradually increases and dominates, and Nuex shifts from negative to positive. This study delves deeper into the forced convective heat transfer process in RPPB, comprehensively considering the effects of heat transfer and pressure loss.

Magnetohydrodynamic Effects in Mixed Convection Copper-Water Nano Fluid Flow at Lower Stagnation Point on a Sliced Sphere

The study of simulation and applications of mathematics in fluid dynamics continues to grow along with the development of computer science and technology. One of them is Magnetohydrodynamics (MHD) which is closely related to its implementation in engineering and industry. And given the importance of magnetic fluid flow has attracted researchers to study and explore its benefits and uses in the industrial field, especially in convective flow and heat transfer processes. This paper therefore considers mathematical modeling on mixed convection MHD viscous fluid flow on the lower stagnation point of a magnetic sliced sphere. The study began with transforming the governing equations which are in dimensional partial differential equations to non-dimensional ordinary differential equations by using the similarity variable. The resulting similarity equations are then solved by the Keller-Box scheme. The characteristics and effects of the Prandtl number, the sliced angle, the magnetic parameter, and the mixed convection parameter are analyzed and discussed. The results depicted that the uniform magnetic field produced by Lorentz force and slicing on the sphere act as determining factors for the trend of nano fluid movement and controlling the cooling rate of the sphere surface. In addition, the viscosity depends on the copper particle volume fraction.

Aerodynamics and convective heat transfer processes in cyclone chambers with external recirculation of gases

The paper presents the results of an experimental and theoretical calculation study of aerodynamics and convective heat transfer on the side surface of cyclone recirculation furnace devices. The possibility of controlling the main aerodynamic characteristics without changing the geometric parameters of the cyclone devices by organizing external gas recirculation is shown.

Numerical Simulation on Enhanced Heat Transfer of Metal Foam Based on LBM

Open cell foam metal has the characteristics of high porosity and large specific surface area. And it has developed rapidly in the related research of heat exchanger. Aiming at the convective heat transfer process of open cell metal structure with high porosity, a two-dimensional stochastic distribution model is established. Numerical simulation is carried out using the single-relaxation-time dual-distribution-function lattice-Boltzmann-method (BGK-DDF-LBM). For the non-ideal solid particles with unequal diameter and incomplete circular structure, the flow field is analyzed by taking the porosity of 0.964 as an example, and the dimensionless permeability is calculated. When the porosity is constant, the Nusselt number of the porous section increases with the increases of Reynolds number in the range of 10 to 100, which shows heat transfer performance. In addition, the Nusselt number of the porous section increases with the increase of porosity in the range of porosity from 0.900 to 0.980.

Theoretical analysis of unsteady convective heat transfer from a flat plate with time-varying and spatially-varying temperature distribution

Convective heat transfer due to laminar, incompressible flow past a flat plate has been extensively researched due to applications in a variety of engineering systems. In several cases, such as thermal management of Li-ion cells and phase change based energy storage, the flat plate temperature may vary as a function of both time and space, and the resulting plate heat flux is of interest. This paper presents an integral method based analytical model for predicting the heat flux distribution from a flat plate with a time-varying and spatially-varying temperature. Third-order Karman-Pohlhausen polynomial forms of velocity and temperature distributions are used in the integral energy equation to derive an ordinary differential equation that predicts the plate heat flux distribution in response to an arbitrary time- and space-dependent plate temperature distribution. The generalized results derived here are shown to reduce to previously reported results for special cases of only time-dependent or only spatially varying or constant plate temperature. Results are also shown to be in good agreement with finite-element simulations. The analytical model developed here is used to predict the plate heat flux in response to realistic plate temperature distributions that may be encountered in practical applications. These results improve the fundamental understanding of an important convective heat transfer process, and may help in the design and optimization of a broad variety of engineering systems involving convective heat transfer.

Unsteady numerical simulation and optimization of 499.8 MHz superconducting cavity cooling process at the High Energy Photon Source (HEPS)

Superconducting 499.8 MHz β single cell ellipsoidal cavities of quarter-wave geometry have been chosen for the High Energy Photon Source (HEPS). The cooling process of the superconducting cavity from the room temperature to 4.2 K is an unsteady forced convective heat transfer process. Its heat transfer mechanism analysis and efficient cooling strategy in the cooldown process is insufficient in present studies. In this paper, a three-dimensional coupled heat-flow model of 499.8 MHz superconducting cavity was built. The unsteady temperature distributions of the superconducting cavity in different inlet temperature condition were studied, serving as the training samples for the neural networks. The agent models based on the two sets of Back-Propagation (BP) neural networks (BPNet1 and BPNet2) were designed to obtain a set of quick calculation model. Genetic algorithm (GA) was used to perform the results optimization. The cooling time of the optimized cooling method could be reduced from 480 min to 290 min, with a reduction of about 40% compared the traditional cooling method. The standard deviation σsk of the modified cooling method was 0.56. The maximum temperature difference in each cooling stage approximately approached the allowed temperature difference 20 K, indicating the potential of the cooling method was maximized. The modified cooling method could shorten the total cooling time and greatly improve the efficiency during the cooldown process.

Heat and mass transfer of multicomponent gas mixtures in cryogenic tank of automotive equipment

The use of liquefied gas as a motor fuel for automotive equipment has both certain advantages and significant problems. The paper deals with the solution of one of the main problems, reducing the speed of the phase transition of liquefied methane in a cryogenic tank in the mode of drainage-free storage. In solving the above problem, the process of convective heat and mass transfer caused by the chemical and physical state of natural gas and the external heat flow was investigated. The two-phase state of the gas is unstable in the event of an increase in heat input from the environment, which causes an imbalance of pressure and temperature in the volumes of the liquid and gaseous parts of the gas and creates the risk of an emergency. To prevent the formation of critical gas pressure in a cryogenic tank, a method is proposed for calculating the phase transition of liquefied methane to determine the volume fraction of vaporized gas using equilibrium constants, which will allow developing an algorithm for the technological process of gas recirculation in a specially designed tank design. This will also allow you to choose the best option for a thermal insulation layer that can reduce the rate of penetration of heat from the environment and increase the period of drainage-free storage of liquefied natural gas by 1.5-2 times.

Analysis about enhancement in thermal characteristics of viscous fluid flow with induction of ferrite particles by using Cattaneo Christov theory

The elevated convective heat transfer process plays vital role in performance of electronic and engineering equipment’s. Over the years various attempts have been executed in this regards, including the insertion of nano elements in poorly conducting liquids. Initially, improvement in thermophysical characteristics of ordinary fluids was observed but with advancement in nanoparticles structuring new classifications in nano elements are found. Among these discoveries experimentations have explored highly fascinating and intrinsically featured class of nanomaterials renowned as ferromagnetic nano constituents. So, the motivation regarding this investigation is execution about change in thermal features of base liquid with insertion of different ferrite particles. Here, water is considered as based liquid and Nickel Zinc Ferrite (NiZnFe2O4) and magnetite ferrite (Fe2O4) as solid particles are inserted. Impact of magnetic dipole is also envisioned to produce optimized effectiveness of ferrite particles. Energy transmission in flow domain is depicted by incorporation of Cattaneo-Christov heat flux model. Mathematical formulation containing thermo mechanical features of ferrite particles are attained in complexly structured partial differential system and afterwards similarity transformations are implemented for transmutation into ODES. Constructed problem is simulated by implementing numerical approaches. Influence of involved variables on associated distributions are displayed through graphs and tables. It is demonstrated that momentum as well as heat transfer of base fluid augments with inclusion of Nickel Zinc ferrite as compared magnetite ferrite. It is inferred that velocity shows declining behavior against Curie temperature whereas reverse behavior is seen for temperature profile. It is divulged that viscous dissipation imparts diminishing impact on momentum whereas contrary behavior is depicted in case of temperature profile. In addition, increment in wall drag magnitude and thermal flux is manipulated by incorporation of (NiZnFe2O4) rather than (Fe2O4).

Effect of nano-sized aluminum additive on wall heat transfer characteristics of the liquid-fueled scramjet engine

Hydrocarbon fuel loaded with highly energetic nanoparticles possesses superior energy density and has great potential to significantly improve scramjet engine performance. The addition of nanoparticles also leads to the increase of combustion temperature and solid deposition on the inner surface of the engine, which affects the wall heat transfer characteristics. In this paper, the stable combustion of JP-10 slurry fuel containing aluminum nanoparticles was achieved in a scramjet engine for the first time, and conspicuous deposition was observed in the engine inner surface. Convective heat transfer models for both pure fuel case and slurry fuel case were established to quantitatively investigate the wall heat transfer characteristics. Calculation results illustrated that the convective heat transfer coefficients of the sensor surface in slurry fuel case are 28.74% higher than that of in pure fuel case, which was the direct reason for the high heat flux results in slurry fuel cases. Further research indicated that the higher wall friction coefficient is the origin of the higher convective heat transfer coefficient. This study found that the rough surface of the deposition in slurry fuel case changes the wall heat transfer characteristics of the engine, resulting in higher heat flux by enhancing the convective heat transfer process.

Optimal flow layout and current allocation for improving the thermoelectric refrigeration system based on heat current method

The integration analysis, modeling, and improvement of the multiple thermoelectric cooling components are significant for promoting thermoelectric cooling technology. Based on the heat current method, this paper studied a single thermoelectric cooling component and constructed its equivalent circuit model by integrating the coupled thermoelectric conduction and conversion processes with the convective heat transfer processes of both the hot side and cold side. The equivalent circuit model is used to build system-level energy transfer and conversion models of the thermoelectric refrigeration systems with parallel-flow, counter-flow, and cross-flow layouts. On this basis, the corresponding calculation process is provided by combining with the Gauss-Seidel Algorithm based on MATLAB software. The simulation results show that the cooling capacity and coefficient of performance under the counter-flow layout are the maximum, and the average end temperature difference is the minimum when the working current is uniform. Besides, the reasonable non-uniform working current allocation can effectively improve the cooling capacity of the thermoelectric refrigeration system by 6.6%, 32.3%, and 1.7%, respectively. Finally, experimental results verify that the proposed heat current model of the thermoelectric cooling unit is appropriate and feasible for modeling and improving the performance by counter-flow layout and non-uniform working current allocation.

Experimental study of convective heat transfer characteristics of fractures with different morphologies based on fractal theory

In the Enhanced Geothermal System (EGS), the flow state of the fracture of the thermal reservoir (granite) is important for heat transfer. In this paper, the effects of reservoir temperature, flow rate and fracture shape on the thermal convection that occurs in the fracture of rough granite are studied based on the fractal dimension theory. The rough fracture is manufactured regularly by carving technology. The shear flow experiments were carried out at confining pressures of 10, 15 and 20 MPa to analyze the flow state. The convective heat transfer experiments were conducted at 60 and 70 °C to simulate different heat reservoir conditions. The results indicated that the outlet temperature and overall heat transfer coefficient (OHTC) increased with the increase in the rock temperature. A higher flow rate can enhance the thermal performance. The complexity of the fracture surface is also an important factor that affects the convective heat transfer process. The larger fractal dimension of the fracture surface can more easily excite turbulence, affecting the thickness of the boundary layer. The OHTC will increase as the fractal dimension increases.

Natural convection of nanofluids in a wavy porous cavity

In this piece of work, a numerical investigation of natural convection is carried out on the buoyancy-driven flow of nanofluids and heat transfer through porous media packed inside a wavy cavity. The cavity is placed horizontal, and its right vertical wall is of wavy nature, the bottom and top walls of the cavity are adiabatic, and there is a temperature difference between the left and right vertical wall. The dimensionless governing equations for the flow of nanofluids through the Darcian porous media are solved iteratively by using finite difference method. The study is conducted for wide range of governing parameters, such as Rayleigh-Darcy number [Formula: see text], nanoparticle volume fraction [Formula: see text] for three types of nanofluids [Formula: see text]-[Formula: see text], Cu-[Formula: see text], TiO2-[Formula: see text], the waviness of the vertical wall controlled by dimensionless length of amplitude of the wave [Formula: see text] and number of undulations per unit length ( N = 1, 3, 5). The simulated results reveals that the presence of nanoparticles enhances the convective heat transfer process at low Ra, and the wall affects the local convection rate and it also controls the overall heat transfer rate. For a cavity with N = 3, [Formula: see text] is increased by about 68% at Ra = 10, and at [Formula: see text] the increment is about 11% as the a is increased from 0.05 to 0.25 having 20% of nanoparticles volume friction.

Hybrid Approach to Modeling the Process of Convective Radiative Heat and Mass Transfer in Closed Differentially Heated Cavities

Hybrid Approach to Modeling the Process of Convective Radiative Heat and Mass Transfer in Closed Differentially Heated Cavities

Heat Transfer Enhancement of Indirect Heat Transfer Reactors for Ca(OH)2/CaO Thermochemical Energy Storage System

The efficiency of a thermochemical energy storage system can be improved by optimizing the structure of the thermochemical energy storage reactor. We proposed two modified structures for indirect heat transfer thermochemical energy storage reactors for a Ca(OH)2/CaO system to improve their heat transfer performance. Our results showed that improving convective heat transfer offered varying effects on heat transfer performance in different reaction processes. For a half-plate pin fin sinks (HPPFHS) reactor and a plate pin fin sinks (PPFHS) reactor, enhancing the convective heat transfer process could improve the heat transfer performance in the dehydration process for a porosity of 0.5, and the time needed to complete reaction was reduced by around 33% compared with plate fin sinks (PFHS) reactor. As for the hydration process, because heat conduction along the bed dominated heat transfer performance, this method had little effect. Furthermore, we found that enhancing heat conduction along the bed and convective heat transfer had different effects on reaction process at different reaction areas. The HPPFHS reactor had a lower pressure drop along the HTF channel and exorbitant velocity of heat transfer fluid (HTF) was unnecessary. Under the condition of the bed porosity of 0.8, due to the lower thermal conductivity of material, both modified reactor structures had little effect on dehydration. However, because the temperature difference between bed and HFT was bigger, the PPFHS reactor could reduce the time of completing the hydration reaction by 20%. Above all, when planning to modify the reactor structure to improve the heat transfer performance to enhance the reaction process, the heat conditions along the bed, convective heat transfer between HTF and the bed and material parameters should be considered totally.

Modeling and Analysis of Human Heat transfer in Cryogenic Protective clothing based on the principle of Heat transfer

In this paper, the simulation of low temperature protective clothing with phase change material is taken as the research object, and the model based on heat conduction equation is established through Fourie heat transfer law, various heat transfer modes and three kinds of boundary conditions. Based on the principle of heat transfer, two kinds of heat transfer modes, heat conduction and heat convection, are mainly considered in this paper. First of all, through the analysis of the heat transfer mechanism of low temperature protective composites in low temperature environment, the boundary conditions of thermal insulation layer and fabric layer are the third kind of boundary conditions, which can be regarded as a convective heat transfer process. Then the partial differential extraordinary model can be established according to the boundary conditions. The interior of the thermal insulation layer, the functional layer and the fabric layer are heat conduction, which can be solved by establishing a model according to Fourier law. Then this paper simulates the above two models and uses the implicit post-difference scheme (Crank method) to solve them in parts.

Molecular dynamics study of convective heat transfer in ordered rough nanochannels

Nanostructures can enhance the convective heat transfer process in nanochannels. The ordered rough nanochannels with different periodic sinusoidal structures are established using the molecular dynamics method to explore the internal mechanism of the effects of nanostructures on convective heat transfer. The potential energy distribution, the mass density distribution, the temperature distribution, and the velocity distribution are obtained to analyze the effects on heat transfer and fluid flow caused by different types of ordered rough surfaces. Moreover, the velocity slip length, the thermal length, and the Nusselt number are also calculated. The results show that with the increase in surface roughness, the thermal slip length decreases, and the Nusselt number increases, indicating that the heat transfer process is improved. However, the decrease of velocity slip length indicates that the fluid flow process becomes worse. The expansion of the low potential energy region and the high mass density region enhances the heat transfer process at the solid–liquid interface. The accumulation of more atoms at the solid–liquid interface hinders the development of the flow field and makes the flow process worse. This research provides an understanding of the convective heat transfer in ordered rough nanochannels, which can guide the design of electronic devices' cooling and fluid transport.

Numerical Optimization of a CPU Heat Sink Geometry

During its operation CPU dissipates undesirable heat. Therefore, heat sink is very essential in modern computing system to absorb extra heat dissipated by the CPU. Forced convection air cooling is common approach. In the present work, a steady-state convective heat transfer process is analyzed for CPU cooling. A rectangular fin heat sink equipped in a computer chassis is numerically investigated and simulated using the software ANSYS CFX. A two-equation based turbulence model is chosen to capture the turbulence of the flow inside the domain. The overall dimension of the heat sink is optimized for three different parameters of the heat sink such as fin quantity, fin height and baseplate thickness. An optimum fin quantity, fin height and baseplate thickness are found, and, respectively. Two different orientations of fins are also compared. Better thermal performance is achieved when the fin channel is perpendicular to the surface parallel to the outlet. The average temperature of the heat sink is found. It is also predicted that the heat sink studied here is capable to keep the CPU temperature underthat is reasonably acceptable.