Numerical investigation of the effectiveness of effusion cooling for plane multi-layer systems with different base-materials

Within Collaborative Research Center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants” at RWTH Aachen University, an effusion-cooled multi-layer plate configuration is investigated numerically by the application of a three-dimensional in-house fluid flow and heat transfer solver, CHTflow. CHTflow is a conjugate code, which yields information on the temperature distribution in the solid body. This enables a detailed discussion of the effects of a change in materials. The geometrical set-up and the fluid flow conditions derive from modern gas turbine combustion chambers and bladings. Within the SFB, two different multi-layer systems, one consisting of substrate made of CMSX-4 (a single-crystal super-alloy), an MCrAlY-bondoat and a ZrO₂ thermal barrier coating (TBC), and the other consisting of a NiAl-alloy and a graded bondcoat/TBC, have been investigated. The grading will increase the life-span of the TBC as it can better compensate the different thermal expansion coefficients of different materials. The main focus in this study is on the different substrate materials, because the thermal conductivity of the NiAl is considerably higher than that of CMSX-4, which leads to different temperature profiles in the components. The numerical grid for the simulations contains the coolant supply (plenum), the solid body for the conjugate calculations, and the main flow area on the plate. The effusion-cooling is realized by finest drilled shaped holes with a diameter of 0.2mm. The investigation is concentrated on a cooling hole geometry with a laterally widened fan-shaped outlet, contoured throughout, and one without lateral widening that is only shaped in the TBC-region of the system. Two blowing ratios, M=0.28 and M=0.48, are investigated, both for a hot gas Mach number of 0.25. The results for the lower blowing ratio and the fully contoured hole are discussed as well as those of the higher blowing ratio and the non-laterally widened hole. These represent two characteristic cases.     

The effect of start-up cycle in ceramic coating used as thermal barrier for a gas turbine bucket

The unsteady aerodynamic and aero-thermal performance of a first stage gas turbine bucket with thermal barrier coating (TBC) and internal cooling configuration were investigated by application of a three dimensional Navier–Stokes commercial turbomachinery oriented CFD-code. Convection and conduction were modeled for a super alloy blade with TBC.The CFD simulations were configured with a mesh domain including the nozzle and bucket interstage in order to accurately predict the fluid parameters at inlet and outlet of bucket. Comparisons to the gas turbine manufacturer data have permitted validation of the flow conditions at the inlet of the rotor.The effects of blade TBC surface temperature changes during a start-up cycle were simulated by means of an unsteady simulation, with unsteady inlet/outlet boundary conditions specified according to test data. The calculations include not only the fluid but also the solving of conduction within the blade, allowing for a correct modeling of the large difference of thermal inertia between the fluid and solid.The role of thermal barrier coatings (TBC) is, as their name suggests, to provide thermal insulation of the blade. A coating of about 100–400 μm can reduce the temperature by up to 200 °C. A TBC can be used either to reduce the need for blade cooling (by about 36%) increasing the turbine efficiency, while maintaining identical creep life of the substrate; or to increase considerably the creep life of the blade while maintaining level of blade cooling (and therefore allowing the blade to operate at a lower temperature for an identical turbine inlet temperature).     

Influence of thermal sensitivity of the materials on temperature and thermal stresses of the brake disc with thermal barrier coating

The time-dependent frictional heating of a disc with applied thermal barrier coating (TBC) on its working surface was investigated. To determine the temperature fields in the coating and the disc a one-dimensional friction heat problem during braking was formulated, with taking into account the dependence of thermal properties of materials from temperature. A model was adopted for materials with a simple non-linearity, i.e. materials whose thermal conductivity and specific heat are temperature dependent, and their ratio – thermal diffusivity is constant. The linearization of the corresponding boundary-value heat conduction problem was made by the Kirchhoff transformation and the linearizing multipliers method. A numerical-analytical solution to the obtained problem was found by Laplace transform method. Knowing the temperature distributions, quasi-static thermal stresses in the strip (TBC) with taking into account change in temperature mechanical properties, were determined. The distribution of temperature and thermal stresses in the strip made from ZrO₂ deposited on the UNS G51400 steel disc, was investigated.     

Generalized model of thermal boundary conductance between SWNT and surrounding supercritical Lennard-Jones fluid – derivation from molecular dynamics simulations

Using classical molecular dynamics simulations, we have studied thermal boundary conductance (TBC) between a single-walled carbon nanotube (SWNT) and surrounding Lennard-Jones (LJ) fluids. With an aim to identify a general model that expresses the TBC for various surrounding materials, TBC was calculated for three different surrounding LJ fluids, hydrogen, nitrogen, and argon in supercritical phase. The results show that the TBC between an SWNT and surrounding LJ fluid is approximately proportional to local density (ρ_L) formed on the outer surface of SWNT and energy parameter (ε) of LJ potential, and inverse proportional to mass (m) of surrounding LJ fluid. In addition, the influence of the molecular mass of fluid on TBC is far more than other inter-molecular potential parameters in realistic range of molecular parameters. Through these parametric studies, we obtained a phenomenological model of the TBC between an SWNT and surrounding LJ fluid.     

Use of La2O3 with 8YSZ as thermal barrier coating and its effect on thermal cycle behavior,microstructure, mechanical properties and performance of diesel engine operated by hydrogen-algae biodiesel blend

In this present work, the effect of lanthanum oxides (La₂O₃) on the thermal cycle behavior of TBC coatings and mechanical properties such as adhesion strength and microhardness of 8% Yttria Stabilized Zirconia (8YSZ) TBCs were investigated. CoNiCrAlY and aluminium alloy (Al–13%Si) were used as bond coat and substrate materials. 8YSZ and different wt % of La₂O₃ (10, 20, and 30%) top coatings were applied using the atmospheric plasma spray (APS) method. The thermal cycling test for TBC coated samples were conducted at 800 °C in the electric furnace. The XRD pattern shows that the La₂O₃ doped 8YSZ material transformed to cubic pyrochloric structured La₂Zr₂O₇ during thermal cycling. Further, the Taguchi-based grey relation analysis (GRA) method was applied to optimize the TBC coating parameters to achieve better mechanical properties such as adhesion strength and microhardness. And the optimized La₂O₃/8YSZ TBC coating was coated on CRDI engine combustion chamber components. The engine was tested with microalgae biodiesel and hydrogen, and the results were promising for the TBC-coated engine. The engine performance increased while using La₂O₃/8YSZ coated components, and the emissions from engine exhaust gas such as CO, HC, and smoke reduced considerably. It was found that there was no separation crack and spallation of the coating layer in the microstructure. Ultimately, the microstructural analysis of the optimized TBC coated piston sample after 50 h of running in the diesel engine confirmed that the developed coating had a superior thermal insulation effect and longer life.     

3-D CONJUGATE ANALYSIS OF COOLED COATED PLATES AND HOMOGENIZATION OF THEIR THERMAL PROPERTIES

To predict the aerothermal behavior of a transpiration cooled plate, a multiscale approach based on the homogenization method of periodic material structures is presented here. This method allows calculation of effective equivalent thermophysical properties either for each layer or for the multilayer of superalloy, bondcoat, and thermal barrier coating (TBC). From the 3-D conjugate flow and heat transfer analysis, the stationary state is extracted and transferred to the microscale unit cell discretized by finite elements. The analysis proves for different cooling configurations a significant decrease in the amount of cooling fluid to obtain a desired superalloy temperature. Beyond, the hole outlet shaping leads to a reduction of the thermal gradients on the multilayer. The effect of the different cooling designs on the effective conductivities are discussed then. Finally, the influence of the selection of the unit cell position on these effective thermal properties is investigated.     

Experimental study of Fe2O3/water and Fe2O3/ethylene glycol nanofluid heat transfer enhancement in a shell and tube heat exchanger

Heat transfer characteristics of Fe₂O₃/water and Fe₂O₃/EG nanofluids were measured in a shell and tube heat exchanger under laminar to turbulent flow condition. In the shell and tube heat exchanger, water and ethylene glycol-based Fe₂O₃ nanofluids with 0.02%, 0.04%, 0.06% and 0.08% volume fractions were used as working fluids for different flow rates of nanofluids. The effects of Reynold's number, volume concentration of suspended nanoparticles and different base fluids on the heat transfer characteristics were investigated. Based on the results, adding nanoparticles to the base fluid causes a significant enhancement of the heat transfer characteristics and thermal conductivity. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and temperature of fluids. In this paper, the effect of Fe₂O₃ nanoparticles on the thermal conductivity of base fluids like ethylene glycol and water was studied. The thermal conductivity measurement was made for different concentrations and temperatures. As the concentration of the nanoparticles increased, there was a significant enhancement in thermal conductivity and overall heat transfer due to more interaction between particles. It was also observed that there was an improvement in the thermal conductivity of the base fluid as the temperature increased. The measurements also showed that the pressure drop of nanofluid was higher than that of the base fluid in a turbulent flow regime. However, there was no significant increase in pressure drop at laminar flow.     

Experimental and numerical study on charging processes of an ice‐on‐coil thermal energy storage system

In this study, an external melt ice‐on‐coil thermal storage was studied and tested over various inlet conditions of secondary fluid—glycol solution—flow rate and temperature in charging process. Experiments were conducted to investigate the effect of inlet conditions of secondary fluid and validate the numerical model predictions on ice‐on‐coil thermal energy storage system. The total thermal storage energy and the heat transfer rate in the system were investigated in the range of 10 l min ^?1?V??60 l min ^?1. A new numerical model based on temperature transforming method for phase change material (PCM) described by Faghri was developed to solve the problem of the system consisting of governing equations for the heat transfer fluid, pipe wall and PCM. Numerical simulations were performed to investigate the effect of working conditions of secondary fluid and these were compared with the experimental results. The numerical results verified with experimental investigation show that the stored energy rises with increasing flow rate a decreasing tendency. It is also observed that the inlet temperature of the fluid has more influence on energy storage quantity than flow rate. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of the impact of physical parameters on a water-based Al2O3 nanofluid using the KKL model

A nanofluid is treated as a smart fluid that is useful for heat transfer enhancement, which is paramount in several industrial applications, transportation, biomedical as well as electronics. This is due to the enhanced thermophysical properties, such as Brownian motion and thermal conductivity of the suspended nanoparticles in the base fluid. The present investigation explores an electrically conducting flow of a water-based nanofluid past a thin film placed horizontally. In particular, the Al₂O₃ nanoparticle is merged into the water for the preparation of the nanofluid. For the enhancement in heat transfer properties, the Brownian thermal conductivity based on the Koo-Kleinstreuer-Li model is introduced. The Adomian decomposition method, a semi-analytical technique, is employed to handle the system of ordinary differential equations. The originality of the current investigation is the statistical analysis of the various characterizing parameters governing the flow phenomena. The influences of these physical parameters on the flow phenomena are displayed in graphs and tables. The major findings of the outcomes are listed as follows: an increase in particle diameter decreases the Brownian conductivity, whereas fluid temperature enhances it significantly. Also, the increase in volume concentration leads to a decrease in the fluid temperature, resulting in faster cooling processes for the production of materials in industries.     

Uncertainties in physical property effects on viscous flow and heat transfer over a nonlinearly stretching sheet with nanofluids

The purpose of this paper is to investigate a numerical analysis for the flow and heat transfer in a viscous fluid over a nonlinear stretching sheet utilizing nanofluid. The governing partial differential equations are converted into highly nonlinear ordinary differential equations by a similarity transformation. Different water-based nanofluids containing Cu, Ag, CuO, Al₂O₃, and TiO₂ are considered in our problem. Furthermore, four different models of nanofluid based on different formulas for thermal conductivity and dynamic viscosity on the flow and heat transfer characteristics are discussed. The variations of dimensionless surface temperature, dimensionless surface temperature gradient as well as the flow and heat transfer characteristics with the governing parameters are graphed and tabulated. Comparison with published results for pure fluid flow is presented and it is found to be in excellent agreement.     

Numerical investigation on the single phase forced convection heat transfer characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger

In this study, forced convection flows of nanofluids consisting of water with TiO₂ and Al₂O₃ nanoparticles in a horizontal tube with constant wall temperature are investigated numerically. The horizontal test section is modeled and solved using a CFD program. Palm et al.'s correlations are used to determine the nanofluid properties. A single-phase model having two-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the nanofluid flow. The numerical investigation is performed for a constant particle size of Al₂O₃ as a case study after the validation of its model by means of the experimental data of Duangthongsuk and Wongwises with TiO₂ nanoparticles. The velocity and temperature vectors are presented in the entrance and fully developed region. The variations of the fluid temperature, local heat transfer coefficient and pressure drop along tube length are shown in the paper. Effects of nanoparticles concentration and Reynolds number on the wall shear stress, Nusselt number, heat transfer coefficient and pressure drop are presented. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid in accordance with the results of the experimental study used for the validation process of the numerical model.     

Improved heating floor thermal performance by adding PCM microcapsules enhanced by single and hybrid nanoparticles

A heating floor is a low-temperature emitter consisting of pipelines in which a fluid circulates between 35°C and 45°C. To ensure energy efficiency, occupant comfort, and building material durability, proper heat management is crucial in buildings. By using phase change materials (PCMs) in building envelopes, the indoor temperature can be regulated through the storage and release of thermal energy, which reduces energy consumption and enhances occupant comfort. In this study, we evaluated numerically a heating floor that incorporates a PCM enhanced by nanoparticles (NePCM). The aim of the numerical analysis is to assess the impact of the addition of single and hybrid nanoparticles in different proportions to the PCM layer on the thermal performance of the PCM-based floor. Therefore, two main objectives are defined. The primary is to take advantage of the storage capacity of a PCM layer by integrating it into the ground; second, to evaluate the hot water temperature levels effect on the floor's performance. Additionally, we address the low thermal conductivity of PCM by enhancing PCM microcapsules with single and hybrid nanoparticles and comparing them to pure PCM. The numerical results obtained show that positioning the PCM microcapsules above the heating tubes (upper position) provides an optimum improvement in thermal performance. Moreover, the addition of hybrid nanoparticles within the base PCM, 1% of Cu mixed with 4% of Al₂O₃, allows an increase of 4°C, which relates to a reduction of 18% in the internal temperature amplitude and a phase shift of 6 h 30 min compared with the conventional heated floor in which there is no PCM.     

Two-dimensional interactions of non-isothermal counter-flowing streams in an adiabatic channel with aiding and opposing buoyancy

A numerical investigation of two-dimensional interaction of two non-isothermal opposing jets/streams of different fluids (same phase, miscible) in an adiabatic channel is carried out in the mixed convection regime. The thermal buoyancy as well as intrinsic buoyancy (inherent differences in the densities of the two fluid streams) has been considered through the Boussinesq approximation. The two buoyancy forces give rise to thermal and intrinsic Richardson numbers, Ri_T and Ri_C, respectively as the important dimensionless numbers governing the problem. Two configurations, namely; (i) the upper stream of heavier fluid at lower temperature and lower stream of lighter fluid at higher temperature (aiding buoyancy forces), and (ii) the upper stream of lighter fluid at lower temperature and lower stream of heavier fluid at higher temperature (opposing buoyancy forces), are considered. For both the scenarios, the simulations have been carried out for various combinations of Ri_T and Ri_C. The numerical experiments reveal the existence of three flow modes depending upon the flow configuration and the magnitude of the buoyancy forces. These are (i) steady asymmetric flow (ii) steady symmetric flow and, (iii) the unsteady, periodic, non-symmetric flow with formation of standing waves and vortex-shedding in the channel. At sufficiently large buoyancy levels, the unsteady periodic flow mode with standing waves is observed for the aiding configuration only. The mixing process is quantitatively monitored through a scalar mixing index that represents the mean square deviation of fluid temperature/mixture concentration from the bulk values at a given channel section. It is shown that in the steady symmetric flow mode, the buoyancy has a very slight effect on the mixing characteristics. However, the mixing is significantly enhanced for the unsteady, periodic standing wave mode.     

Experimental investigation of a new thermal energy storage system using thermo‐sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

The effect of copper coated metal hydride at different ratios on the reaction kinetics

In this study, the process parameters that affect the improvement of hydrogen storage material properties were investigated. In order to accelerate the hydrogen charge/discharge processes and to obtain the required hydrogen at the desired flow rates in a short time, the thermal conductivity of the storage materials has been improved, and density analyses have been made. The ideal grinding time has been determined for the LaNi₅ material. Within the scope of the experimental studies, the thermal conductivity coefficients of LaNi₅ coated with copper and LaNi₅ ground for 5 h and coated with copper were increased by 500–750%, and the copper plating ratios were optimized. The materials obtained were characterized by XRD, SEM, and their density was measured with the Helium Pyknometer device and their thermal conductivity coefficients with the Hot Disk Thermal Constants Analyzer. In addition, the hydrogen storage of materials with increased thermal conductivity was investigated experimentally in the metal hydride reactor at the determined pressure. In the study, it was seen that the storage material coated with copper increases the heat transfer, reduces the hydrogen charging time in the metal hydride reactor, and increases the stable discharge time.     

Rapid microfluidic thermal cycler for polymerase chain reaction nucleic acid amplification

Polymerase chain reaction (PCR) is widely used in biochemical analysis to amplify DNA and RNA in vitro. The PCR process is highly temperature sensitive, and thermal management has an important role in PCR operation in reaching the required temperature set points at each step of the process. The goal of this research is to achieve a thermal technique to rapidly increase the heating/cooling thermal cycling speed while maintaining a uniform temperature distribution throughout the substrate containing the aqueous nucleic acid sample. In this work, an innovative microfluidic PCR thermal cycler, which utilizes a properly arranged configuration filled with a porous medium, is investigated. Various effective parameters that are relevant in optimizing this flexible heat exchanger are investigated such as heat exchanger geometry, flow rate, conductive plate, the porous matrix material, and utilization of thermal grease. An optimized case is established based on the effects of the cited parameters on the temperature distribution and the required power for circulating the fluid in the heat exchanger. The results indicate that the heating/cooling temperature ramp of the proposed PCR heat exchanger is considerably higher (150.82 °C/s) than those in the literature. In addition, the proposed PCR offers a very uniform temperature in the substrate while utilizing a low power.     

A numerical model for heat sinks with phase change materials and thermal conductivity enhancers

The effectiveness of thermal conductivity enhancers (TCEs) in improving the overall thermal conductance of phase change materials (PCMs) used in cooling of electronics is investigated numerically. With respect to the distribution of TCE and PCM materials, the heat sink designs are classified into two types. The first type of heat sink has the PCM distributed uniformly in a porous TCE matrix, and the second kind has PCM with fins made of TCE material. A transient finite volume method is used to model the heat transfer; phase change and fluid flow in both cases. A generalized enthalpy based formulation and numerical model are used for simulating phase change processes in the two cases. The performance of heat sinks with various volume fractions of TCE for different configurations is studied with respect to the variation of heat source (or chip) temperature with time; melt fraction and dimensionless temperature difference within the PCM. Results illustrate significant effect of the thermal conductivity enhancer on the performance of heat sinks.     

Anisotropy Effects on Heat and Fluid Flow by Unsteady Natural Convection and Radiation in Saturated Porous Media

ABSTRACT

This article deals with a numerical study of fluid flow and heat transfer by unsteady natural convection and thermal radiation in a vertical channel opened at both ends and filled with anisotropic, in both thermal conductivity and permeability, fluid-saturated porous medium. The bounding walls of the channel are gray and kept at a constant hot temperature.

In the present study we suppose the validity of the Darcy law for motion and of the local thermal equilibrium assumption. The radiative transfer equation (RTE) is solved by the finite-volume method (FVM). The numerical results allow us to represent the time–space variations of the different state variables. The sensitivity of the fluid flow and the heat transfer to different controlling parameters, namely, the single scattering albedo ω, the temperature ratio R, the anisotropic thermal conductivity ratio R_c, and the anisotropic permeability ratio R_k, are addressed. Numerical results indicate that the controlling parameters of the problem, namely, ω, R, R_c, and R_k, have significant effects on the flow and thermal field behavior and also on the transient process of heating or cooling of the medium. Effects of such parameters on time variations of the volumetric flow rate q_v and the convected heat flux Q at the channel's outlet are also studied.     

Numerical prediction of performance of a low-temperature-differential gamma-type Stirling engine

Abstract

In this study, a numerical simulation model is used to analyze thermodynamic performance of a low temperature-differential gamma-type Stirling engine by adjusting some values of the operating and geometrical parameters around a designated baseline case. The influences of these operating and geometrical parameters on engine performance such as working fluid materials, the stroke of piston and displacer, charged pressure, the heating temperature, and so on, are concerned. A numerical simulation model is established based on turbulent flow assumption and the realizable k – ε model is employed to solve the flow and thermal fields in the engine. In regard to flow in regenerator, Darcy–Forchheimer model was used to depict dynamic behavior of working fluid. Besides, thermal equilibrium model was used for solving the energy equation. Finally, working fluid in the engine undergoes a wide range of pressure and temperature so the effects of temperature and pressure on the viscosity and thermal conductivity of the working fluid are required to include. Thermal conductivity of porous medium matrix is affected by wide range of temperature as well.