Effect Of Heat Transfer Coefficient Research Articles

Numerical Study to Define Initial Thermal Integration Window for Methane Oxidative Coupling with Dehydroaromatization Reactors

Oxidative coupling of methane and methane dehydroaromatization are attractive one-step conversion routes to make valuable platform chemicals more sustainable. Both processes require elevated temperatures above 600°C, good heat management, and the use of heterogeneous catalysts. None of these reactions are yet commercial due to many technical challenges. This work explores the potential of combining these two processes under one umbrella to overcome some of the technical challenges and make these processes more attractive. It focuses on the recuperative autothermal reactor coupling as one of the possible integration options. A tube-in-tube reactor design is proposed in which OCM is in the inner tube and MDA is in the outside. A numerical study is carried out using pseudohomogenous ideal fixed bed reactor models with literature kinetics. A systematic tabulated approach is used to simplify, visualize, and structure the design process and view the design options. Practical constraints such as reactor sizing, pressure drop, reaction performance, and axial temperature profile are investigated. The effect of heat transfer coefficient, diluents, catalyst profiling, and flow direction have been investigated to alter the axial temperature profile, avoid thermal run away, and improve the performance. Multiple thermally coupled OCM-MDA reactor design candidates are identified. This is the first time that the thermal coupling of OCM and MDA has been identified and quantified. These candidates are merely a starting point toward exploring the full coupling opportunities between OCM and MDA toward reaching the ultimate and more attractive option of full mass and heat integration in the same reactor.

Investigation of moving trapezoidal and exponential fins with multiple nonlinearities

In this article, we study moving longitudinal and exponential fins with variable thermal conductivity, thermal emissivity, and heat transfer coefficient. The problem is solved numerically using shooting technique. To compare the performance of trapezoidal fins with exponential fins, we introduce a novel idea of volume adjusted fin heat transfer rate, fin effectiveness, and fin efficiency. The result shows that heat transfer from the exponential fin is higher than from the trapezoidal fin. It is observed that the capability of an exponential fin is higher than the trapezoidal fin. With increasing thermal conductivity and radiation-conduction number as well as convection-conduction number, the fin’s efficiency increases. The results illustrate the effect of heat transfer coefficient, thermal emissivity, radiation-conduction and convection-conduction parameters, dimensionless radiation, and convection sink temperatures on the fin’s performance.

Experimental studies on the effect of cyclic thermal shock and cooling methods on the mechanical properties and fracture behavior of prefabricated fissured sandstone

The construction of sandstone thermal storage systems and the evaluation of their long-term stability are of great significance to the development of geothermal energy. In this study, the effect of the number of thermal shocks and cooling methods on the mechanical properties and fracture behavior of sandstone containing a single prefabricated fissure was investigated. The experimental results show that as the number of thermal shocks increases, the peak stress and elastic modulus of the fissured sandstone gradually decrease, and show a very good exponential functional relationship. The first thermal shock weakens the mechanical properties of fissured sandstone most severely, and the deterioration effect of mechanical properties is significantly slowed down when the number of thermal shocks exceeds 4. In addition, compared with water cooling, the reduction of the mechanical properties of fissured sandstone by air cooling is lower. As the cooling water temperature decreases, the thermal shock effect of the fissured sandstone is more severe, resulting in more significant damage. An empirical formula considering the number of thermal shocks and cooling water temperature is proposed to characterize the damage to the fissured sandstone. The crack initiation of fissured sandstone under uniaxial compression subjected to different thermal shocks is dominated by tensile failure (wing crack). As the number of thermal shocks increases and the cooling water temperature decreases, the initial crack modes gradually change from T1 to T2, or a more complex pattern. Subsequently, the thermal shock process of fissured sandstone is simulated in COMSOL Multiphysics. The effect of heat transfer coefficient and cooling water temperature on the temperature field and stress field in fissured sandstone are discussed, and the reasons for the thermal cracking of the fissured sandstone under thermal shock are revealed.

A Novel Approach for Performance Analysis of Air–Air Wickless Heat Pipe Heat Exchanger

Abstract Wickless heat pipe heat exchangers (HPHE) consistently showed instabilities and omits many experimental tests. The performance of a heat pipe is governed by many parameters, and the effects of which may influence each other. The objective of this paper is to develop a new approach for an air–air heat pipe heat exchanger that takes into consideration the effect of heat transfer coefficients, saturation temperature, and thermal resistances inside the heat pipes as well as maximum heat transfer limit. The approach is based on analyzing inner heat pipe parameters with HPHE external working conditions. Results are also assessed for higher heat capacities ratio and found that the thermal resistances inside a heat pipe are a limiting factor, leading the HPHE system to perform poorly in particular for Cr ≤ 1. Overall heat transfer coefficients at HPHE sides as well as HPHE effectiveness as a function of Cr are assessed. Trying to follow the Chaudourne (1992, “The Heat Pipe Heat Exchangers: Design, Technology and Applications,” Design and Operation of Heat Exchangers, Springer, Berlin, Heidelberg, pp. 386–396) profile for conventional heat exchangers, HPHE effectiveness is determined and limited to 0.5 number of transferred units. The established model is verified by the existing literature and demonstrates numerical results that agree with the experimental data within a 2.86% error.

Investigating the Effect of Heat Transfer Coefficient and the Heat Flux on the Thermal Performance of the PCM in the Heat Sink

Investigating the Effect of Heat Transfer Coefficient and the Heat Flux on the Thermal Performance of the PCM in the Heat Sink

Effects of Heat Transfer Coefficient and Cell Geometry on Thermal Runaway Behavior of Lithium-Ion Batteries at High-Temperature Thermal Abuse Conditions

Effects of Heat Transfer Coefficient and Cell Geometry on Thermal Runaway Behavior of Lithium-Ion Batteries at High-Temperature Thermal Abuse Conditions

Thermal performance enhancement of phase change material heat sinks for thermal management of electronic devices under constant and intermittent power loads

Metal fin enhanced phase change material (PCM) has been successfully demonstrated as an effective approach for electronic devices cooling, especially in the case of high-power density for only a limited period. Application of PCMs in the heat sink accelerates the heat storage and reduces the heat rejection resulting in a tradeoff effect. In this study, the effects of PCM filling ratio, intermittent power load with different power off time interval and heat transfer coefficient on the performance of PCM-filled heat sinks are studied for electronic devices cooling. A constant power load is firstly studied to reveal the effect of PCM filling ratio. An intermittent power load with different power off time interval is then investigated to enlarge the operation time of electronic devices. Finally, the effect of heat transfer coefficient is discussed to assess the negative and positive effects of PCM. The results show that the effective operation time of PCM-filled heat sinks significantly increases and its peak temperature obviously reduces under intermittent power load compared to the constant power load, and it becomes more apparent with increasing power off time interval. The thermal performance of PCM-filled heat sinks significantly strengthens under natural cooling condition, i.e., h = 10 or 20 w•m−2•K−1. However, during forced convection cooling, e.g., h = 100 w•m−2•K−1, the heat sink without PCM may have a better performance because the negative effect of PCM outweighs its positive effect. The results of this paper are of great significance in the optimal design of PCM-filled heat sink.

Design of a AB5-metal hydride cylindrical tank for hydrogen storage

Hydrogen storage in metal hydrides presents distinct challenges which encourage the study of effective heat management strategies. Hydrogen absorption in metal hydrides is an exothermic reaction, consequently the generated heat must be removed effectively to achieve the desired performance. This work presents a mathematical model describing the adsorption of hydrogen in La Ni4.7Co0.3 metal hydride as a storage material. Heat and mass transfer effects are modeled in detail. The effect of heat transfer coefficient is also estimated. Besides, a heat transfer fluid for cooling is incorporated to the model. The problem is mathematically formulated presenting a numerical simulation of a design of a cylindrical tank for hydrogen storage. The alloy is studied by using pressure-composition-temperature curves which are carried out at different temperatures. Thermodynamic parameters and hydrogen storage capacity are determined. For isotherm's kinetics, the Jonhson-Mehl-Avrami-Kolomogorov model is used, from which the kinetic constant of the hydriding process is determined.

Effect of heat transfer coefficients on evaluation of temperature distribution in membrane type LNG carriers

ABSTRACT The major challenge of designing the hull of an LNG carrier is to reliably predict the hull temperature distributions. The study objective is to introduce a new process for obtaining improved temperature distributions in the multi-layered barriers of a membrane type LNG carrier. An in-house code capable of estimating the heat transfer coefficients, boil off ratio, and the temperature distributions in the LNG carrier compartments was developed based on the fundamental heat transfer theory. The in-house code was verified by comparing hull temperatures obtained from calculations by other researchers. The finite element analyses (FEAs) with newly calculated heat transfer coefficients derived from the in-house code quite much improved the accuracy of the temperature distributions in the membrane-type LNG cargo holds than those with the nominal ones. It was concluded that the in-house code is necessary to obtain more reliable temperature distributions using the FEAs.

Simulation of forced convective heat transfer in Kelvin cells with optimized skeletons

The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) jv/fof the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the jv/fof the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices.

Effect of Heat Transfer Coefficient on Cooling of Gas Turbine Blades

Effect of Heat Transfer Coefficient on Cooling of Gas Turbine Blades

The Effect of Heat Transfer Coefficient on Ice layer Thickness on Water Surface (Communication)

The Effect of Heat Transfer Coefficient on Ice layer Thickness on Water Surface (Communication)

Effects analysis on catalytic combustion characteristic of hydrogen/air in micro turbine engine by fuzzy grey relation method

In order to enhance catalytic combustion efficiency, a premixed hydrogen /air combustion model of the micro turbine engine is established under different excess air ratio, inlet velocity and heat transfer coefficient. And effects of inlet velocity, excess air coefficient and heat transfer coefficient on the catalytic combustion efficiency of the hydrogen have been analyzed by the FLUENT with CHEMKIN reaction mechanisms and the fuzzy grey relation theory. It is showed that inlet velocity has a more intuitive influence on the catalytic combustion efficiency of the hydrogen. A higher efficiency can be obtained with a lower inlet velocity. The optimum excess air coefficient is in the range of 0.94 to 1.0, the catalytic combustion efficiency of the hydrogen will be declined if the excess air coefficient exceeded 1.0. The effect of heat transfer coefficient on the catalytic combustion efficiency of the hydrogen mainly embodies in the case of the excess air coefficient exceeded 1.0, however, the effect will be declined if the heat transfer coefficient exceeded 4.0. The fuzzy grey relation degrees of the inlet velocity, heat transfer coefficient and excess air coefficient on the catalytic combustion efficiency of the hydrogen are 0.640945, 0.633214 and 0.547892 respectively.

Thermo-mechanical analysis of investment casting ceramic shell: A case study

In investment casting (IC) process, the mould is subjected to stress due to both internal pressure of the molten metal and the thermal stress generated due to the temperature variation across the mould boundary at the time of pouring and during solidification. The solidification pattern and solidification time have predominant effect on the material properties of the casting produced from this process. In the present work, an IC ceramic shell is used as mould for the numerical simulation of the thermal stresses generated and solidification process of Al-7%Si alloy inside it using COMSOL Multiphysics software. The effect of heat transfer coefficients between the casting and ambient air on the solidification pattern and solidification time have also been simulated. The results obtained from the simulation were found to be in good compliance with the data obtained from the experimentation and literature survey. Therefore, this modeling provides promising possibility for the predetermination of casting defects in mould as well as in casting.

Effect of heat transfer coefficient, draw ratio, and die exit temperature on the production of flat polypropylene membranes

In this work, a stable numerical scheme has been developed for the 1.5-dimensional film casting model of Silagy et al. [Polym. Eng. Sci. 36, “Study of the stability of the film casting process,” 2614–2625 (1996)] utilizing the viscoelastic modified Leonov model as the constitutive equation and energy equation coupled with the crystallization kinetics of semicrystalline polymers taking into account actual temperature as well as cooling rate. The model has been successfully validated on the experimental data for linear isotactic polypropylene taken from the open literature. Drawing distance, draw ratio, heat transfer coefficient, and die exit melt temperature were systematically varied in the utilized model in order to understand the role of process conditions in the neck-in phenomenon (unwanted film width shrinkage during stretching in the post die area) and crystalline phase development during flat film production. It is believed that the utilized numerical model together with the suggested stable numerical scheme as well as obtained research results can help to understand a processing window for the production of flat porous membranes from linear polypropylene considerably.

Heat transfer and fluid flow analysis of porous medium solar thermochemical reactor with quartz glass cover

In this paper, heat transfer and fluid flow of porous media solar thermochemical receiver with quartz glass cover were investigated. The Surface-to-surface radiation model and Rosseland approximation for radiation heat transfer were adopted for the transport of diffused solar irradiance and radiative transfer in the fluid phase and porous medium. An experimental test was conducted on a laboratory-scale solar thermochemical reactor. The effects of structural parameters in term of diffused irradiance intensity, the mass flow rate, heat transfer coefficient, quartz glass and inner cavity wall surface emissivity, the porosity and extinction coefficient that could affect heat transfer and fluid flow performance of the proposed solar cavity receiver were sufficiently investigated. It was found that the substantial drops in temperature were mainly attributed to the thermal losses by radiative, convective and conductive heat transfer. The numerical results are compared with the experimental data for the model validation. The thermal loss at the solar flux inlet of the receiver was obviously inevitable due to the stronger effect of heat transfer coefficient that altered the over increasing temperature and heat flux at the surface of diffuse irradiance. However, the use of optimum pore size and higher porosity material could significantly enhance the thermal performance of porous media solar thermochemical reactor.

Heterogeneous bubble nucleation model on heated surface based on free energy analysis

Heterogeneous bubble nucleation on heated surfaces, as the first stage of boiling, is a fundamental and prevalent process in industrial systems. A nucleation model based on chemical potential energy is revisited and a Gibbs free energy based model is developed. The nucleation models cover two types of bubble nucleation, i.e. nucleation with or without prefer nucleation site on heated surfaces which are denoted as type I bubble nucleation and type II bubble nucleation, respectively. The effect of surface wettability is also considered. Analytical solutions such as active nucleation size boundary, critical nucleation radius and heat flux at onset of bubble nucleation are obtained without the assumption that the temperature at the tip or center of nucleation bubbles is higher than saturated temperature which was commonly applied in previous studies. The high wall superheat usually found for type II bubble nucleation is illustrated. Bubble nucleation under flow condition is studied by analyzing the effects of heat transfer coefficient, fluid subcooling and pressure on bubble nucleation. For surfaces that cannot provide relative large initial bubbles, bubble nucleation transits from type I to type II with the increase of heat transfer coefficient, fluid subcooling or pressure due to the decrease in the critical bubble radius. Comparisons with previous nucleation models and experiments indicate that the Gibbs free energy analysis is a very promising method in the prediction of bubble nucleation.

Coupled electro-thermal field in a high current electrolysis cell or liquid metal batteries.

Coupled electro-thermal field exists widely in chemical batteries and electrolysis industry. In this study, a three-dimensional numerical model, which is based on the finite-element software ANSYS, has been built to simulate the electro-thermal field in a magnesium electrolysis cell. The adjustment of the relative position of the anode and cathode can change the energy consumption of the magnesium electrolysis process significantly. Besides, the current intensity has a nonlinear effect on heat balance, and the effects of heat transfer coefficients, electrolysis and air temperature on the heat balance have been released to maintain the thermal stability in a magnesium electrolysis cell. The relationship between structure as well as process parameters and electro-thermal field has been obtained and the simulation results can provide experience for the scale-up design in liquid metal batteries.

Design of flexible thermoelectric generator as human body sensor

Flexible thermoelectric generator (TEG) became an attractive technology that has been widely used especially for curved surfaces applications. This study aims an optimal design of a flexible TEG for human body application. The flexible TEG is part of a sensor and supplies required electrical power for data transmission by the sensor. The TEG module includes ink based thermoelements made of nano-carbon bismuth telluride materials. One flexible fin conducts the body heat to the TEG module and there are two fins that exchange the heat from the cold side of the TEG to the ambient. The proposed design is supposed to produce 10 μV to feed the used sensor in the thermoelectric system. In this design, the effect of heat transfer coefficient due to natural convection on the power generation is studied.

Design of a AB2-metal hydride cylindrical tank for renewable energy storage

Metal hydrides have the ability to reversibly absorb and desorb relatively large amounts of hydrogen at a certain temperature and pressure. The absorption of hydrogen in metal hydrides is an exothermic reaction, therefore, the generated heat has to be removed effectively in order to achieve the desired charging rate.A 3D numerical model has been developed for predicting the hydrogen absorption process performance of cylindrical metal hydride tank (MHT) filled with Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5hydrogen storage material. Heat exchange arrangement considered between the MHT and the heat transfer fluid for cooling is helical coil internal heat exchanger in the powdered metal hydride bed. The performance of the MHT is analyzed based on the numerical simulation. The effect of heat transfer coefficient on characteristics of the helical coil reactor is also analyzed. The numerically predicted results are validated with the experimental data available in the literature. The predicted MHT hydrogen storage capacity is about 1.8wt% at the operating conditions of 20bar supply pressure and heat transfer fluid temperature of 293K.