Describing Heat Transfer Research Articles

Optimization of cold startup strategy with quasi 2-D model of metal hydride hydrogen storage with fuel cell

In this work, a novel cold startup strategy is presented for a large-scale metal hydride–fuel cell system. Due to the thermal requirement of the system, when the initial temperature of metal hydride and ambient air is low, the system cannot be started because the hydrogen pressure in the metal hydrides storage system is lower than the minimal pressure required by the fuel cell. The cold startup is achieved by maintaining a part of metal hydride storage at a target temperature and using coolant control to transfer heat to the remaining cold metal hydride storage upon operation of the fuel cells. The strategy is studied numerically and a novel Quasi-2-Dimensional approach is developed to describe heat transfer within metal hydrides and validated. By the proper selection of the value of the control variable, both fast startup and immediate uninterrupted fuel cell power supply are achieved. The effect of initial conditions on the startup process is studied and the strategy consumed 30 % less energy than the alternative without control. The proposed strategy is suitable and efficient for the cold startup of large-scale metal hydride-fuel cell systems.

Multiphysics and multiscale simulation for an experimental sodium-cooled fast reactor

This work aims to explore a new upscaling methodology to describe heat transfer simultaneously coupled with the neutronic process, in the core of a sodium-cooled reactor. The upscaling method, as demonstrated in this work, allows the development of models that describe reactor-scale phenomena, starting at the scale of a cell, composed of a nuclear fuel pin and liquid metal. In principle, this methodology allows high fidelity in the description of these multiphysics and multiscale phenomena. The main result is the two-upscaled energy no-equilibrium model to describe the temperature distribution of the fuel and coolant. These upscale models are described in terms of the upscaled heat transfer coefficient, which is presented and developed in this work, for the Experimental Sodium-cooled Fast Reactor. The use of the new proposed models/methodology leads good results within lower computational times compared with CFD codes, which is useful when computations have to be done a large number of times, for instance for sensitivity analyses to parameter choices in the conception phase of the reactor, and other applications such as plant analyzers, and for operator training in full-range simulators.

Imperfectly conducting eigenflows in a vertical fluid layer

The stability of buoyant flow in an infinite vertical fluid layer bounded by imperfectly conducting rigid walls, called imperfectly conducting eigenflows, is discussed. The third kind boundary conditions describing heat transfer to the external environment are applied to perturbations in temperature. The linear stability analysis is carried out numerically by employing the Chebyshev collocation method. Instability arises when the Grashof number G exceeds its critical value, which depends on the Prandtl number Pr and the Biot number Bi. It is found that the onset of instability changes dramatically depending on the magnitude of Prandtl and Biot numbers particularly when the instability is through the traveling-wave mode. The numerical results show that the Biot number plays a pivotal role in determining the transition Prandtl number PrT at which the instability switches over from one mode to another mode. The novel outcomes suggest the presence of a single PrT for Bi<2.1739 while three distinct values of PrT for Bi≥2.1739. The departure from the conventional single value typically observed at isothermal boundaries signifies the complexity of the instability mechanism.

Experimental study on the dynamic wetting of silica nanofluids

Nanofluids are recently discovered nanomaterials with improved thermophysical properties that can enhance the efficiency and reliability of heat transfer systems. Their relevant properties for describing heat transfer, thin film flows, droplet impingements, or microfluidics are surface tension and wettability. A thorough investigation on this topic of research is crucial to understand the root cause of the factors affecting the degree of wetting of nanofluids on solid surfaces. In the present work, silica (SiO2) nanofluids with different mass fractions and types of surfactants based on deionized water were prepared using a simple and cheap two-step method. The effect of surface tension changes on the lateral friction force of the nanofluid was studied using a solid–liquid interface lateral friction force detection system. According to the findings, an increase in the mass fraction of silica nanoparticles led to the larger surface tension of the nanofluid, resulting in a linear decrease in the lateral friction of the latter on the hydrophobic surface of PDMS. The use of ionic surfactants as dispersants caused a dramatic decrease in surface tension and static angle of SiO2 nanofluid (by 20% and 30% after adding SDS and by 40% and 50% after adding CTAB, respectively). This exerted a noticeable impact on the lateral friction force of the fluid. On the contrary, polymeric surfactants (NaPSS) had a small effect on the lateral friction of SiO2 nanofluid. Meanwhile, the lateral friction after stable motion differed from the maximum static friction to a small extent.

Study on configurational operation strategy of ground heat exchangers under the effect of groundwater flow

In large-scale ground source heat pump (GSHP) systems, the accumulation of thermal energy resulting from seasonal load imbalances has emerged as a significant impediment. The implementation of a rational operation strategy can effectively mitigate heat accumulation in soil. This study investigates the impact of the configurational operation strategy for large-scale vertical ground heat exchanger (GHE) on the spatial distribution of ground temperature under the influence of groundwater flow. The internal heat source model is proposed to describe heat transfer process of GHE. Sandbox experiment is conducted to verify the model with maximum deviation of 3 %. The validated model is then utilized to simulate soil temperature profile in the large-scale GHE field over a 10-year operational period, considering a matrix of three configuration strategies and three operation strategies. The results shows that trapezoidal configurations combined with operation Strategy 3 has a minimal temperature increment of 2.9 °C in soil temperature. The contribution of this study lies in provision of a methodology for mitigating thermal accumulation in large-scale GHE field.

Numerical and Dynamic Features for Heat Transfer Through a Longitudinal Rectangular Fin with Temperature-Dependent Thermal Conductivity

In the work reported herein, we adopt a Newton-Richtmeyer iterative scheme to arrive at solutions describing heat transfer in a longitudinal rectangular fin. Both the thermal conductivity and heat transfer coefficients are assumed to be temperature dependent and the governing partial differential equation highly nonlinear. In order to validate the numerical formulation, we consider some analytical results for this class of problems at steady state .Comparisons show convergence to exact solutions as demonstrated by local and global error indicators. In addition,various fin parameters which play a key role for efficient fin operation and design are investigated by carrying out a comprehensive dynamical qualitative study.

Nanothermometry in rarefied gas using optically levitated nanodiamonds.

Heat transfer in gases in the continuum regime follows Fourier's law and is well understood. However, it has been long understood that in the subcontinuum, rarefied gas regime Fourier's law is no longer valid and various models have been proposed to describe heat transfer in these systems. These models have very limited experimental exploration for spherical geometries due to the difficulties involved. Optically levitated nanoparticles are presented as the ideal experimental system to study heat transfer in rarefied gases due to their isolation from their environment. Nanodiamonds with nitrogen-vacancy centers are used to measure temperature. As the pressure decreases so does the heat transfer to the rarefied gas and the nanodiamond temperature increases by over 200 K. These experiments demonstrate the utility of optically levitated nanoparticles to study heat transfer in any gas across a wide range of pressures. In the future, these measurements can be combined with models to empirically determine the energy accommodation coefficient of any gas.

Particle-particle contact heat transfer models in thermal DEM: A model comparison and experimental validation

Models for particle-particle heat transfer are essential in DEM of thermal processes. A recently proposed model that easily calculates particle-particle heat transfer coefficients from effective packed bed thermal conductivity has for the first time been implemented in DEM for the thermal equilibration of particles with initially different temperatures in a rotating flat 2D drum and in a stirred 3D mixer. Experiments have been performed with low thermal conductivity polyoxymethylene and highly thermal conducting aluminum spheres. Comparisons show that it is mandatory to consider heat conduction in both involved phases, particles and gas, when the particle thermal conductivity is low. Derivation of particle-particle heat transfer from bed conductivity works well for both particle types, which is also true for classical, more complicated mixed conduction models. Contrary, the often used model of Batchelor for conduction only through the contact area between particles fails to describe heat transfer at low particle thermal conductivity.

Heat transfer characteristics for multi-silicon ingots irradiation in a typical research reactor

A computational fluid dynamics (CFD) study is conducted for silicon ingot irradiation facility in a typical material testing reactor (MTR). The ingots are placed inside the irradiation device in staggered and in-line bundles. The configurations are varied according to the variations of longitudinal and transverse pitches Sl and St, respectively. For in-line bundles, Sl and St are changed in the range of 1.1, 1.2, 1.3, 1.4, and 1.5 of the ingot diameter. For staggered bundles, Sl is changed in the range of 1.1, 1.2, 1.3, 1.4, and 1.5 of the ingot diameter while St is changed in the range of 0.95, 1.0, 1.05, 1.1, and 1.15 of the ingot diameter. The silicon ingot volumetric heat generation Qv is allowed to change around its nominal value in the range of 0.25, 0.5, 0.75, 1.0, and 1.25 W/g. The effects of rotation of ingots during irradiation at velocities of 15, 30, and 60 rpm are investigated. The computational fluid dynamics code ANSYS FLUENT is employed to carry out the calculations. The temperature profile inside the silicon ingot during irradiation is of significant importance for uniform doping. Therefore, the impact of heat generation and bundle pitches on the temperature profile through the ingot is calculated and reported for all cases of this study. The results show that the maximum ingot temperature does not exceed the limits for each bundle at maximum heat generation. Two correlations for average Nusselt number for both aligned and staggered configurations are generated to describe heat transfer rates during natural convection regime. The rotation of ingots during irradiation showed considerable enhancement of heat transfer rates.

Thermal contact resistance measurement between two intersecting PAN type carbon fibers using the T-type 3ω method

The prediction of the effective thermal conductivity of composites filled with carbon fibers requires the knowledge of the microstructure, the relative amount and thermal properties of filler and matrix materials, the orientation of non-isometric filler phases and the thermal contact resistances between fibers and matrix and also between fibers. However, information at small scale are often missing especially thermal contact resistances. The present work describes the measurement of the thermal contact resistance (TCR) between two crossed carbon fibers which is performed by an adaptation of the T-type 3ω method for thermal conductivity measurement of wires. The first carbon fiber is connected to two copper blocks supplied by a modulated electrical current providing a volumetric heating. The second fiber fixed on a U-shape holder is delicately implemented over the first one. After performing a secondary vacuum, the 3ω voltage V3ω of the first fiber is recorded as function of the frequency of the modulated current. Knowing the applied force, the contact area between the two carbon fibers is calculated from bending and deformations then a 3D numerical model describing heat transfer in two crossed carbon fibers is used to estimate the TCR at their intersection. In addition, a detailed sensitivity analysis of the unknown parameter TCR is performed allowing to find optimal operating conditions especially the frequency range. Measurements are performed with PAN type carbon fibers (FT300B) picked up from the same bundle. Different TCR estimations were performed by fitting numerical and computed V3ω voltage as function of frequency. Finally, values of TCR between crossed carbon fibers were found equal (10.4 ± 10.1) 105 kW−1 which provides an order of magnitude for such phenomenon.

TEMPERATURE PREDICTION OIL PIPELINE IN FROST AREAS

Pipeline operation in frost areas is essential thing in the present days. Changes of low-temperature profile of frozen soil along the length of a pipeline lead to changing conditions of the pipeline. That’s why it is vital to properly forecast and model thermo-hydraulics regimes and evaluate pipeline conditions to sustain correct positioning of the pipeline and to prevent incidents. That’s why this paper describes a simple forecasting method. It is based on heat balance equation, where coefficient k describing heat transfer from a pipeline to the environment considers that soil gets to technogenic state. Actual efficiency and temperature data from an active non-isothermal pipeline over 2 years period after its commissioning are used to derive actual value of the coefficient k. Pipeline type – underground, insulated, initial petrol temperature is up to 45 °С. Using Shukhov’s formula it is possible to calculate average value of the coefficient k. Based on this coefficient we can then derive forecasted values and get the following data: temperature profile, temperature value at specific points and cross sections of a pipeline, thawing area, critical locations of a pipeline. This method allows to timely evaluate conditions of a pipeline in permanent frost soil. Results can be used to forecast and predict thermo-hydraulics regimes not only for a specific pipeline but also for other Northern ones. It also allows to evaluate conditions of critical parts of a pipeline when taking in account actual size of thawing areas.

Thermal equilibrium modeling of cutting zone cooled by supercritical CO2: A case study on Ti-6Al-4V validated with experimental data

In recent years, cryogenic cooling techniques such as supercritical carbon dioxide (scCO2) cooling have been widely used for the machining of difficult-to-machine materials. Different from conventional homogeneous flood cooling cutting, the jet field temperature in cryogenic cooling is non-uniform. Additionally, the common modeling of cryogenic cooling describes heat transfer mainly from a transient perspective, making it hard to characterize the time-dependent dynamic impact of gas jet parameters on thermal equilibrium. This limitation set barriers to online monitoring of the thermal equilibrium in the cutting zone, resulting in its poor guidance on industrial applications. Therefore, this study aims to propose a modeling method that can guide industrial practice to evaluate the dynamic impact of gas jet parameters on thermal equilibrium. To consider the balance between model accuracy and industrial usability, the thermal equilibrium model with jet mass flow and cutting parameters as variables was established based on thermodynamic theory, analyzing the input and output of material and energy of the system from a continuous time domain perspective, i.e., enthalpy change of scCO2 and energy change of chips. Considering that the boundary conditions were simplified and assumed in modeling process, the reliability and applicability conditions of the model were further investigated, and the model was verified by experimental data of turning Ti-6Al-4V with scCO2 cooling. The results show that, jet distance affects the applicability of the model and that the model is acceptable within a certain jet distance (e.g. 15 mm). Meanwhile, the model can calculate the theoretical mass flow rate of scCO2 in variable cutting speed machining, where the model errors are less than 1.4 %. In summary, under the jet distance less than 15 mm, the modeling approach proposed in this study can provide a reliable estimate of the dynamic thermal equilibrium in the cutting zone and offer useful instructions for industrial applications.

Peculiarities of thermal calculation of cross-flow plate air recuperators

Application of modern energy-saving technologies is the most important task in development of construction industry. In the conditions of exhausting decrease of energy consumption of public, administrative buildings due to reduction of transmission heat losses the implementation of mechanical ventilation with recuperation of exhaust air heat becomes a rational way to increase energy efficiency. One of the key problems in the study of plate-type air recuperators is to find the temperature efficiency of the device. The value of this characteristic is influenced by such parameters as the area of the branched heat exchange surface, initial temperatures of heat carriers, volume flow rates, flow rate ratio, type of fins, etc. The purpose of the study is to compare coefficients of thermal efficiency of outside air as a function of Reynolds number obtained by engineering calculation, mathematical modelling and manufacturer’s data for the recuperator ISIS Recover HR-A-05-V-G4-E-1-60. The article substantiates the calculation method using the existing algebraic dependence describing heat transfer in the cross-flow scheme of coolant motion on the basis of new results. An empirical dependence is obtained, which adequately describes the results of numerical simulation using the ANSYS Fluent solver and engineering calculation.

Determination of Thermocline Heat Transfer Coefficient by Using CFD Simulation

This article deals with a thermal energy storage system in the form of a water tank with a thermocline. The well-known thermocline phenomenon is modeled using computational fluid dynamics (CFD). However, the reservoir model proposed in this article is zero-dimensional. This is due to the fact that the aim of this article is to build a mathematical model that will be more useful in mathematical models of complex energy systems in which a hot water tank is one of many elements of the system. In such a zero-dimensional mathematical model, the hot water tank will be modeled using equations describing heat transfer, and the thermocline itself will be treated as a heat transfer surface with known dimensions and heat transfer coefficient. A novelty of this paper is that it addresses heat loss across the thermocline as defined in this manner. A CFD model of a thermal storage tank is created, validated with available experimental data, and used to obtain the heat transfer coefficient U. The resulting value is then analyzed quantitatively and qualitatively and the changes in the thickness of the thermocline are accounted for in the equation. The results from this groundbreaking work can be used to analyze heat storage in the form of thermocline water tanks at the level of system modeling, e.g., for the purpose of configuring the structure of other devices and control systems.

Radiative and dissipative flow of hybrid nanofluid between two coaxial cylinders: A comparative numerical study

Radiation in dissipative flow of hybrid nanofluid TiO2+ Pb/water bounded between two coaxial cylinders is addressed. The hybrid nanomaterial is synthesized by adding nanoparticles lead and titanium dioxide TiO2and Pb in water as conventional fluid. Geometrically the outer cylinder exhibits rotation and the inner cylinder fixed. Constant strength magnetic field is taken radially. Thermal radiation and viscous dissipation describe heat transfer analysis. Mathematical equations (continuity, momentum, and energy) are obtained via mathematical modeling. Bvp4c method is used for solutions development. Influences of flow and heat related parameters on skin friction, velocity, temperature and Nusselt number have been shown. Velocity of fluids (hybrid nanofluid and nanofluid) is controlled by higher magnetic parameter while temperature and skin friction intensifies with it. Higher nanoparticle volume fractions (for both nanoparticles) reduce velocity while skin friction and Nusselt number are intensified. Moreover, higher radiation parameter causes temperature drop. In comparative analysis, the effects of hybrid nanofluid TiO2+ Pb/water are dominant when compared with nanofluid TiO2+ water.

Non-Local Formulation of Heat Transfer with Phase Change in Domains with Spherical and Axial Symmetries

Describing heat transfer in domains with strong non-linearities and discontinuities, e.g. propagating fronts between different phases, or growing cracks, is a challenge for classical approaches, where conservation laws are formulated as partial differential equations subsequently solved by discretisation methods such as the finite element method (FEM). An alternative approach for such problems is based on the non-local formulation; a prominent example is peridynamics (PD). Its numerical implementation however demands substantial computational resources for problems of practical interest. In many engineering situations, the problems of interest may be considered with either axial or spherical symmetry. Specialising the non-local description to such situations would decrease the number of PD particles by several orders of magnitude with proportional decrease of the computational time, allowing for analyses of larger domains or with higher resolution as required. This work addresses the need for specialisation by developing bond-based peridynamic formulations for physical problems with axial and spherical symmetries. The development is focused on the problem of heat transfer with phase change. The accuracy of the new non-local description is verified by comparing the computational results for several test problems with analytical solutions where available, or with numerical solutions by the finite element method.

Heat Transfer Calculations during Flow in Mini-Channels with Estimation of Temperature Uncertainty Measurements

The main aim of this work was to provide heat transfer calculations of flow boiling in mini-channels with an application for the Trefftz functions. The test section comprised five parallel mini-channels with a depth of 1 mm, with a common heated wall. For the estimation of the temperature uncertainty, during the experiment the temperature measurement was performed with the use of K-type thermoelements and an infrared camera in two mini-channels simultaneously. According to the Guide to the Expression of Uncertainty in Measurement, the Monte Carlo method is a practical alternative to the GUM uncertainty framework. Since the uncertainty components are not approximately the same magnitude, the Monte Carlo method was indicated to estimate the uncertainty of the surface temperature measurement. The results obtained from this simulation method were compared with the results of the computation related to the uncertainty propagation method. The results of both methods of temperature measurement were found to be consistent. The results of the statistical analysis were used to describe heat transfer calculations. The heat transfer investigations concerning the subcooled boiling region were performed during the other experiment. The local heat transfer coefficients on the contact surface between the working fluid and the heated wall were calculated from the Robin boundary condition. The mathematical model described by the heat equation in the mini-channel wall and by the Fourier-Kirchhoff equation in a flowing fluid leads to an inverse heat transfer problem. This problem was solved using the FEM with the Trefftz-type basis functions. The estimation of temperature uncertainty measurements due to the Monte Carlo method was included in the final results of the heat transfer coefficient.

Comparative analysis of film boiling correlations for steam explosion problem

The article considers the existing models and correlations for describing heat transfer during film boiling. Film boiling is an important thermophysical process that determines the course of interaction of the melt of reactor core materials with the coolant, which can potentially occur during a severe accident at a nuclear power plant with a pressurized water-cooled reactor (VVER/PWR). The values of the Nusselt number predicted by these models are compared. A fairly significant dispersion of calculated parameters has been established, making it difficult to unambiguously choose one or another correlation. However, in the range of parameters typical for the interaction of a high-temperature melt of reactor core materials with a coolant, several correlations give quite close values, which allows them to be recommended for use in calculation codes for modeling thermal-hydraulic processes during a severe accident at a nuclear power plant with VVER/PWR type.

Effect of Channel Size on Air-Cooling Performance in Interior Permanent Magnet Motor Driving Aviation Fans at High Altitude

Abstract To enhance worldwide environmental conditions, the air transport industry must drastically reduce carbon dioxide emissions. Electrification of aircraft propulsion systems is one way to meet this demand. In particular, the focus is on obtaining single-aisle aircraft with partial turbo-electric propulsion and approximately 150 passenger seats by the 2030s. To develop a single-aisle aircraft with partial turbo-electric propulsion, an air-cooled interior permanent magnet (IPM) motor with an output of 2 MW is desired. One of the most difficult problems in air cooling is that air-cooling performance decreases with increasing altitude because the air density decreases. To investigate the effect of altitude on air-cooling performance in the IPM motor, the authors formulated mathematical system equations to describe heat transfer inside the target air-cooled IPM motor, and mathematical analytical solutions were obtained. The most severe condition is the top-of-climb condition. For this condition, a designer should choose cooling air mass flow rates that keep the temperature of the permanent magnets below the maximum temperature limit of 100 °C and the temperature of the coils below the maximum temperature limit of 250 °C. Here, the sizes of the air-cooling channels strongly affect air cooling with the IPM motor. In this paper, the authors briefly review the mathematical formulations and their solutions, investigate the effect of channel size on air-cooling performance in an IPM motor, and explore the optimum configuration and settings for the air cooling channels.

A numerical study for describing heat transfer, pressure loss, and overall performance of twisted-tube with variable cross-sectional area

A numerical study for describing heat transfer, pressure loss, and overall performance of twisted-tube with variable cross-sectional area