Constant Wall Temperature Research Articles

Numerical simulation of reactive gas-particle flow in a solar jet spouted bed reactor for continuous biomass gasification

Solar steam gasification of beech wood biomass in a novel solar conical jet spouted bed reactor was studied. A 3D numerical model of the reactive two-phase gas-particle flow was developed and the model was experimentally validated. The cavity-type solar reactor was operated in both direct and indirect heating configurations at a constant wall temperature of 1200 °C with a biomass feeding rate that varied from 1.2 g/min to 2 g/min. Experiments showed that directly heating the particles increases the H2 yield and the carbon conversion efficiency. The Computational Fluid Dynamics (CFD) model was developed to thoroughly investigate the biomass conversion process inside the conical jet spouted bed subjected to direct or indirect solar irradiations. The desired vigorous cyclic flow pattern of the biomass particles was accurately predicted by the model with peak particles velocity of 2 m/s in the spout region. The maximum temperature reached by the indirectly-irradiated particles is about 1200 °C while it is above 1500 °C when considering directly-irradiated particles. Direct heating thus increases both particles gasification rates and carbon conversion efficiency. Furthermore, simulations reveal that the reaction zone temperature is 100 °C higher for the direct heating configuration. This promotes the H2 formation at the expense of CH4. The steam oxidant concentration, gas products distribution and particles trajectories inside the cavity were also investigated to identify improvement strategies for enhancing the phase mixing and the gas/solid residence time.

Thermocapillary rivulets in a locally heated falling liquid film

The formation of a three-dimensional rivulet structure in a heated falling liquid film is investigated theoretically. For a description of the dynamics of a non-isothermal film, the theoretical model is developed taking into account the thermocapillary force. Within the framework of the spatial approach, a linear analysis of the stability of the heated film relative to perturbations in the spanwise direction to the flow is performed for the first time. A spatial growth rate of these perturbations down-stream is obtained. Analysis of experimental data for different liquids and different Reynolds numbers has shown that the distance between the rivulets corresponds well to the wavelength of most amplified perturbation. The stationary rivulet structure in the film is simulated by numerical method for two types of heating conditions implemented in our previous experiments: with constant wall temperature and heat flux on the wall. Calculations show, that on a heater with constant temperature of a wall the developed rivulet structure has a quasistationary character and very slightly changes downstream. The development of rivulets on a heater with a constant heat flux on the wall has been calculated for the first time. The good agreement between calculations and our experimental data is found for both types of heating conditions implemented in the experiments.

Numerical and experimental investigation of melting process in spherical pcm capsule used for low-temperature thermal energy storage systems

The present work deals with the experimental and numerical investigation of melting process of phase change material used for low temperature thermal energy storage applications. Thermal energy storage is a good solution to bridge the gap between energy supply and energy demand. The unconstrained melting of paraffin wax having melting range of 570C to 610C was studied in simple spherical glass capsule. The objective of the subject work is to perform the experimental and numerical analysis of melting process in spherical capsule subjected to constant wall temperature. Ansys Fluent 18 is used for computation purpose. In experimental analysis, melting process was observed through visual capturing of interface between solid and liquid. Melting fraction and temperature at the centre of PCM capsule are monitored both experimentally and numerically. Good consistency in results were observed.

Heat transfer and presser drop of copper oxide–thermal oil in upward single-phase flow in inclined microfin tube under constant wall temperature

The effects of using copper oxide–thermal oil on convective heat transfer and pressure drop in an upward flow in an inclined microfin tube is studied experimentally in this research project. The flow regime and wall temperature are laminar and constant, respectively. The effects of nanofluid, Graetz number, Prandtl number and positive inclination angles on convective heat transfer augment moderately with the boom in nanoparticles mass concentration. The result shows that the forced heat transfer and Darcy friction factor increase with the increment of mass nanoparticle concentration, Reynolds number and inclination angle. The ratio of Darcy friction factor increases up to 64% in microfin tube. The Nusselt number boosts to reach 62 when mass nanoparticle concentration and inclination angle were 1.5% and 30o, respectively. Two correlations are advised to estimate Nusselt number and Darcy friction factor in upward single-phase flow in microfin tube under constant wall temperature and laminar flow in microfin pipe. The maximum aberrations of Nusselt number and Darcy friction factor are 20% and 18%, respectively, which are admissible to predict experimental data. Accompaniment of heat transfer ratio with pumping power ratio is presented in this paper. If the increment of pressure drop is more than heat transfer enhancement, it will not be appropriate to use CuO-thermal oil, inclination angles and microfin tube. The maximum FOM is 59% which is calculated with 1.5% nanoparticle mass concentration and inclination angle of 60o at Prandtl number of 349.

Numerical and experimental investigation of melting characteristics of phase change material-RT58

The present study is focused on experimental and numerical analysis of unconstrained melting of Paraffin wax-RT58 in a horizontally placed cylindrical container. After the validation of numerical model with experimental results, numerical analysis is extended to constrained melting to investigate the process. The experiments are carried out at constant wall temperature maintained on the lateral surface of the cylinder. The influence of initial sub-cooling and lateral surface temperature on the melting rate is investigated. The melting process is better analyzed by the melting phase front and temperature contours as time progresses. The results show that the melting rate decreases by increasing the initial sub-cooling, and increases with increasing lateral surface temperature of the cylinder. In unconstrained melting, heat transfer by conduction governs the melting process initially, but later it is restricted to only the bottom part of the cylinder as the solid PCM at a higher density sinks due to effects of gravity. Heat transfer in the upper half of the cylinder is dominated by natural convection set up in the liquid PCM. In constrained melting, pure conduction phenomenon exists only in the beginning, and later conjugate heat transfer occurs. When subjected to similar boundary conditions, PCM melt-time is lower in unconstrained melting than in constrained melting. A correlation between melt-time and Stefan number is also developed.

Dry spot dynamics and wet area fractions in pool boiling on micro-pillar and micro-cavity hydrophilic heaters: A 3D lattice Boltzmann phase-change study

A three-dimensional lattice Boltzmann phase-change study is carried out on dry spot characteristics in pool boiling on micro-pillar and micro-cavity hydrophilic heaters under constant bottom wall temperature conditions. The degree of superheat and geometry of these micro-structures (i.e., the pitch distance p, the height h, and the width d) are found to have significant effects on the cycle-average dry spot diameter. Pool boiling curves for these micro-structured heaters are obtained, which are compared with a smooth surface under same conditions. The micro-pillar structured hydrophilic heater is found to have the highest CHF because of capillary wicking, and geometric effects (p/d and h/d) on pool boiling curves from micro-pillar structured hydrophilic heaters are investigated. Although p/d and h/d have small effects on the developing nucleate boiling regime in pool boiling, these parameters have significant effects from fully-developed nucleate boiling to CHF and in transition boiling regime. Critical heat fluxes are shown to be related to wet area fractions: the higher is the wet area fraction, the higher is the CHF.

Effect of Constant Heat Flux on Forced Convective Micropolar Fluid Flow over a Surface of Another Quiescent Fluid

Due to the many applications of micropolar fluid such as blood, paint, body fluid, polymers, colloidal fluid and suspension fluid, it has become a prominent subject among the researchers. However, the characteristics of micropolar fluid flow over a surface of another quiescent fluid with heavier density of micropolar fluid under the effect of constant heat flux are still unknown. Therefore, the objective of the present work is to investigate numerically the forced convection of micropolar fluid flow over a surface of another quiescent fluid using constant heat flux boundary condition. In this study, the similarity transformation is used to reduce the boundary layer governing equations for mass, momentum, angular momentum and energy from partial differential equations to a system of nonlinear ordinary differential equations. This problem is solved numerically using shooting technique with Runge-Kutta-Gill method and implemented in Jupyter Notebook using Python 3 language. The behaviour of micropolar fluid in terms of velocity, skin friction, microrotation and temperature are analyzed and discussed. It is found that, the temperature is higher in constant wall temperature (CWT) compared to constant heat flux (CHF) at stretching or shrinking parameter and various micropolar parameter K. Furthermore, as Prandtl number increases, the temperature is decreasing in both CHF and CWT.

Unsteady flame–wall interaction: Impact on CO emission and wall heat flux

This study investigates the interaction of a two-dimensional (2D) laminar methane-air premixed flame with a constant temperature wall using Direct Numerical Simulation (DNS). The flame is excited using velocity perturbations at the inlet for a range of forcing frequencies. The GRI 3.0 chemical mechanism is used to perform the simulations. The flame behaviour is first analysed by comparing the results to those observed for a steady 2D laminar flame interacting with the wall and a one-dimensional head-on quenching (HOQ) flame under the same conditions as the 2D flame. This is followed by analysing the wall heat flux and CO emission due to the flame quenching at the wall for different forcing frequencies. At low forcing frequencies, the flame is observed to sweep against a large portion of the wall whereas at high frequencies, the flame has an insignificant response to the incoming velocity perturbations. In both these regimes, side-wall quenching (SWQ) is the dominant mechanism of flame–wall interaction. However, at an intermediate frequency, both HOQ and SWQ occur at different stages of flame–wall interaction. The analysis of the total integrated wall heat flux of forced flames shows a large variation of this quantity compared with a steady state flame, with the highest fluctuations occurring for the flame forced at the intermediate frequency. The radical recombination reactions are found to be significant in the vicinity of the wall, contributing to about 50% of the total HRR at the wall at the quenching instant. The analysis of the species transport budget for CO shows that CO transport near the wall close to the flame tip is dominated by convection and diffusion.

Parametric analysis of axial wall conduction in a microtube subjected to two classical thermal boundary conditions

Heat transfer in laminar flow microtube is numerically explored with an objective of discriminating conjugate heat transfer process experienced in a microtube under two different thermal conditions. Two classical thermal conditions – constant heat flux and constant wall temperature – are imposed separately on the outer surface of a microtube. Wide parametric variations are considered in this study, for the two thermal conditions, albeit the problem under consideration being very classical from both geometry and thermal condition point of view. The parametric variations considered in this work include wall thickness, wall conductivity and coolant flow rate. An expression for Nusselt number in terms of radial (or transverse) and axial conduction number is presented and validated against existing theoretical correlation as well as reported experimental data for both circular and non-circular channels. Dominance of axial conduction over radial (or transverse) conduction is explored and it is found that the effect of wall material on conjugate heat transfer plays an important role. Additionally, it is also observed that with the increase in coolant flow rate, the ratio of radial to axial conduction number increases for both thermal boundary conditions.

Numerical investigation of forced convective heat transfer around a solid circular cylinder utilizing nanofluid in unsteady regime

This paper presents a numerical solution for low Reynolds number, unsteady flow around, and heat transfer from a stationary circular cylinder placed in a uniform flow. The fluid is assumed to be incompressible and of constant property. Twodimensional and unsteady mass continuity, momentum, and energy equations have been discretized using finite volume method. A SIMPLE algorithm has been applied for solving the pressure linked equations. The range of Reynolds numbers was investigated which varied from 50 to 300 with volume fraction on Cu nanoparticles varying from 1 to 5 % at the constant wall temperature. The results of the heat transfer characteristics of nanofluid flow over a circular cylinder showed marked improvement comparing with the base fluid. It is found that the vorticity, pressure coefficient, and recirculation length are increased by the addition of nanoparticles into base fluid. Moreover, the local and mean Nusselt numbers are enhanced due to adding nanoparticles into base fluid.

Experimental characterization of a self-adaptive shape memory alloy cooling approach to regulate temperature under varying heat loads

Electronic cooling devices typically aim to provide low thermal resistance, however, for many applications, temperature non-uniformity across the cooled object is also a key parameter, as it impacts reliability. Tailored microchannels or fin layouts are examples of cooling devices that can be designed to reach uniform wall temperatures but require known and constant heat load distributions. In this work, a self-adaptive liquid cooling device is proposed to provide constant and uniform temperature distributions under spatially and time dependent heat load scenarios. The working principle is based on the use of the Two-Way Shape Memory Effect (TWSME) of trained shape memory alloy (SMA) wings that elevate into the flow to reduce the convective thermal resistance when a critical temperature is reached. The conditions for proper operation are first defined analytically. Then, the impact of SMA wings on thermal resistance is experimentally investigated. Finally, the ability of the self-adaptive concept to maintain a constant wall temperature when the heat flux increases, is demonstrated. In the studied case, for fixed inlet coolant temperature and flow rate, the liquid cooling device was able to maintain the temperature of the cooled object within an interval of 7 °C for a heat flux varying from 32.8 to 67 W/cm2. Without self-adaptation, a temperature variation over 85 °C was expected for the same cooling device, corresponding to an order of magnitude improvement.

Coupled conjugate heat transfer and heat production in open-cell ceramic foams investigated using CFD

Combining low pressure drop and remarkable heat transport properties, open-cell foams offer a combination that makes them a highly attractive option as monolithic catalyst support. The coupled thermal behavior of foams and fluids during heat production, e.g., exothermic chemical reactions, is still not thoroughly described despite their potential to optimize temperature control in fixed-bed reactors. Hence, the aim of this study is to get deeper insight into coupled conjugate heat transfer and heat production in open-cell foams used in a tubular reactor with constant wall temperature. Therefore, we conducted - μCT-based CFD simulations of open-cell foams with artificial heat sources that mimic the heat of reaction during an exothermal chemical reaction and allow to study the effect of heat production on heat transfer while requiring lower computational cost than simulating actual chemical reactions. We implemented a range of heat source intensities, that covers typical exothermic reactions, to study their effect on heat flows as well as temperature fields and used CO2 methanation as a case study. We further quantified the influence of superficial velocity, heat source intensity, and material on the temperature fields inside the foam and found conduction being the dominant heat transport mechanism. We further evaluated the feasible range of even more simplified pseudo-homogeneous models and found high thermal conductivities and low superficial velocities to be appropriate. In conclusion, the presented approach offers the possibility to study thermal effects regarding catalytic supports and give valuable insight in heat transport mechanisms under relevant process conditions in heterogeneous catalysis (heat sources: 5–150 W for a reaction volume of 1.2 × 10−5 m3, superficial velocities 0–0.51 m s−1, thermal conductivities 5–50 W m−1 K−1).

Thermal Performance Analysis of Slug Flow in Square Microchannels

Thermal performance enhancement of microchannel heat sinks for electronics cooling is becoming more and more necessary. To this end, microchannels with noncircular cross-sections conveying immiscible droplets have been employed in this study and geometric manipulations are applied to enhance fluid mixing and consequently achieve a higher rate of heat removal. Three-dimensional numerical simulations are performed using volume of fluid method for channels of 100 µm hydraulic diameter. Constant wall temperature is chosen as the boundary condition for heating section of the channels. Effects of parameters such as adding curvature to the side walls and varying the entering velocity of the base liquid on heat transfer rate are studied. A performance coefficient is used to evaluate the relative impact of increase in both the Nusselt number and pressure drop as a result of adding curvature to the channel walls. Results of the study showed that slug flow in curved channels is capable of improving the thermal performance in comparison with single liquid flow in straight channels and in best case, can improve the performance up to 50%.

Entropy generation analysis of nanofluid forced convection in MHD plane diffuser

Fluid flow and forced convection heat transfer inside a plane diffuser with Cu–water nanofluid in the presence of magnetic field is numerically studied with the finite volume method. The steady nanofluid flow is assumed incompressible, laminar, and Newtonian. The cooling nanofluid with low temperature enters the diffuser with constant wall and higher temperature. The influence of Reynolds number (100 < Re < 500), Hartmann number (0 < Ha < 10), and volume fraction of nanoparticles (0 < < 0.05) on the local and average Nusselt numbers, skin friction coefficient, dimensionless velocity and temperature profiles, total entropy generation, static pressure variation, and pressure recovery coefficient are examined. The results show that increasing the volume fraction of nanoparticles, Reynolds, and Hartmann numbers would increase the total entropy generation and average Nusslet number. In contrast, the pressure recovery coefficient of diffuser decreases by increasing the Reynolds and Hartmann number. Furthermore, numerical results show that, for each Reynolds number there is a critical Hartmann number. So that, the effect of the nanoparticle addition on the pressure recovery coefficient in all Reynolds number is depended to the flow Hartmann number and its value relative to the critical Hartmann number.

Entropy generation analysis of convective turbulent flow in alternating elliptical axis tubes with different angles between pitches; a numerical investigation

This paper presents a numerical analysis for studying entropy generation and the convective turbulent flow of an alternating elliptical axis (AEA) tube with different angles between the pitches at constant wall temperature boundary conditions. The turbulent flow of water fluid is simulated at Reynolds numbers of 10,000 to 60,000. The numerical results indicated that the multi-longitudinal vortices have a major influence on convective heat transfer, pressure drop and entropy generation. Two symmetrical vortices having the same direction of rotation close to the tube wall cause local wall shear stress and the local total entropy generation of the tube wall decrease. However, in some parts of the domain of the AEA tubes, these symmetrical vortices are absent. Therefore, there are different results for the local wall shear stress and the local total entropy generation in all domains of the AEA tubes. The Nusselt number and friction factor increases with an increase in Reynolds number and the angle between pitches of the AEA tube. The entropy generation analysis showed that the main source of irreversibility is by thermal entropy generation. The total entropy generation increases when there is an increase in the Reynolds number, as well as the angle between pitches of the AEA tube. Also, at Reynolds numbers of 20,000, the ratio of the total entropy generation of AEA tube to that of the circular tube is at its minimum.

Numerical investigation of turbulent Cu-water nanofluid in heat exchanger tube equipped with perforated conical rings

Numerical analysis of the nanofluid flow characteristics of perforated conical rings in a heat exchanger tube has been investigated under constant wall temperature condition. The pitch ratio of the perforated conical rings is 4 and the number of holes is varied from 0 (typical conical ring) to 10. The flow regime is fully turbulent with the Reynolds number is varied from 5000 to 14,000 and Cu-water nanofluid 0<ϕ<1.5% is selected as the working fluid. The main novelty of this paper is to perform a 3D simulation of this problem because some previous studies using similar geometry were restricted to experimental analysis. The Reynolds averaged Navier Stokes (RANS) equations are solved with the finite volume method using (RNG) k-∊ model. A significant heat transfer enhancement is obtained by using compound perforated conical rings and nanofluids. The heat transfer enhancement provided by PCR with N = 4 is 278.2% higher than plain tube. The maximum thermal performance of 1.10 is obtained by using Cu-water nanofluid with ϕ=1.5% and perforated conical rings with N = 10 at Re = 5000.

Numerical analysis reliability control for a double‐pipe heat exchanger with virtual entropy generation method

AbstractThere are two primary laws including the first and second laws of thermodynamics that should be used to assess a process. Generally, only the first law of thermodynamics is investigated in numerical solutions, so it is possible to exist some numerical results that do not satisfy the second law of thermodynamics because of numerical errors. To achieve reliable numerical outcomes, it is better to apply two indexes of HEAT BALANCE ERROR and VIRTUAL ENTROPY GENERATION, which come from the second law of thermodynamics. In other words, an approach to develop computational fluid dynamics investigations is to take second law of thermodynamics into consideration. In this study, two different models including counterflow double‐pipe heat exchanger and single‐pipe with constant wall temperature are simulated in various cases with different efficiencies and temperature ratios. It is found that 46 cases of total 523 double‐pipe models and 24 cases of total 402 simulations of single‐pipe models had unacceptable results regarding to two mentioned criteria. The results revealed that it is less likely to gain unreliable results in smaller efficiency and lower inlet temperature for double‐pipe heat exchanger and single‐pipe respectively.

Wall Temperature and Suction Control of Linear Modes in Compressible Corner Flow

Linear stability behavior of self-similar streamwise corner flows is investigated under the influence of constant wall temperatures and suction rates perpendicular to each wall. For three Mach numbers, 0.1, 0.95, and 1.5, representing subsonic, transonic, and supersonic flow regimes, modes that propagate in the streamwise direction, i.e., with a wave number at the lateral boundaries, are studied first. Then, the influence of and on the stability of oblique modes with is investigated for the transonic flow. It is shown that the effect on the leading viscous mode with is the same for cooling and suction in the case of small variations in temperature and suction relative to the reference flow without control. However, the increase in stability of the inviscid corner modes depends on both the control method and the Mach number. The stability of oblique viscous modes increases as well and it is shown that at the critical point is independent of the wall temperature and the suction rate. In addition, a maximum spanwise wave number is observed for which the most critical viscous mode is still amplified.

Modeling of Mixed Convection Between Vertical Parallel Plates in Electric Equipment Immersed in High-Pr Liquids

Natural-convection cooling with oil or other fluids of high Prandtl number plays an important role in many technical applications such as transformers or other electric equipment. For design and optimization, one-dimensional (1D) flow models are of great value. A standard configuration in such models is flow between vertical parallel plates. Accurate modeling of heat transfer, buoyancy, and pressure drop for this configuration is therefore of high importance but gets challenging as the influence of buoyancy rises. For increasing ratio of Grashof to Reynolds number, the accuracy of one-dimensional models based on the locally forced-flow assumption drops. In the present work, buoyancy corrections for use in one-dimensional models are developed and verified. Based on two-dimensional (2D) simulations of buoyant flow using finite-element solver COMSOL Multiphysics, corrections are derived for the local Nusselt number, the local friction coefficient, and a parameter relating velocity-weighted and volumetric mean temperature. The corrections are expressed in terms of the ratio of local Grashof to Reynolds number and a normalized distance from the channel inlet, both readily available in a one-dimensional model. The corrections universally apply to constant wall temperature, constant wall heat flux, and mixed boundary conditions. The developed correlations are tested against two-dimensional simulations for a case of mixed boundary conditions and are found to yield high accuracy in temperature, wall heat flux, and wall shear stress. An application example of a natural-convection loop with two finned heat exchangers shows the influence on mass-flow rate and top-to-bottom temperature difference.

Transient conjugated heat transfer in thick walled pipes with axially periodic surface temperature in downstream region

Thermal entrance region transient conjugate heat transfer is investigated involving fluid axial heat conduction for laminar pipe flows. Constant outer wall temperature boundary condition is assumed in the upstream region of a thick walled, two regional pipe. In the downstream region, the outer wall temperature is considered changing spatially in a periodical manner. The problem is solved numerically by a finite difference method. A parametric analysis is conducted in order to determine the effects of Peclet number, wall thickness ratio, wall-to-fluid thermal conductivity ratio, wall-to-fluid thermal diffusivity ratio and axial frequency on heat transfer characteristics. It is seen that, the results are highly dependent on the parameter values and the most effective ones are the Peclet number and the wall thickness ratio. It is observed that heat is transferred towards upstream due to the axial conduction in the wall and in the fluid and with increasing values for high axial frequency.