Simulation Of Unsteady Heat Transfer Research Articles

Influence of local grid refinement on prediction of impinging jet heat transfer using Scale-Resolving-Simulation methods

A fine near-wall resolution plays a crucial role in simulations of unsteady heat transfer from a wall. A generation of fine structured grids for a complex domain is cumbersome and requires an enormous number of grid points and a local grid refinement is a possible technique for a grid point reduction. The present study addresses numerical simulations of impinging jet heat transfer on locally refined computational grids using the Scale-Resolving-Simulation methods (DES, LES and PANS). The study is performed at a Reynolds number Re = 23000 and two nozzle-plate distances H/D = 2 and H/D = 6. DES and the PANS model with f k = 0.25 predicted results closest to experimental data. However, DES significantly over-predicted heat transfer at the location r / D = 1 − 2 in the small nozzle-plate distance case ( H / D = 2). The decrease of physical resolution in the PANS model resulted in the over-prediction of heat transfer close to the nozzle axis. In conclusion, the influence of the local refinement in the perpendicular direction was rather low compared to the longitudinal direction. Moreover, the local grid refinement improved results predicted by LES and DES in the small nozzle-plate distance case.

Unsteady ballistic heat transport in two-dimensional harmonic graphene lattice

We study the evolution of initial temperature profiles in a two-dimensional isolated harmonic graphene lattice. Two heat transfer problems are solved analytically and numerically. In the first problem, the evolution of a spatially sinusoidal initial temperature profile is considered. This profile is usually generated in real experiments based on the transient thermal grating technique. It is shown that at short times the amplitude of the profile decreases by an order magnitude and then it performs small decaying oscillations. A closed-form solution, describing the decay of the amplitude at short times is derived. It shows that due to symmetry of the lattice, the anisotropy of the ballistic heat transfer is negligible at short times, while at large times it is significant. In the second problem, a uniform spatial distribution of the initial temperature in a circle is specified. The profile is the simplest model of graphene heating by an ultrashort localized laser pulse. The corresponding solution has the symmetry of the lattice and many local maxima. Additionally, we show that each atom has two distinct temperatures corresponding to motions in zigzag and armchair directions. Presented results may serve for proper statement and interpretation of laboratory experiments and molecular dynamics simulations of unsteady heat transfer in graphene.

Simulation of Unsteady Heat Transfer in the Liquid Core of the Earth with Allowance for Internal Heat Sinks

The results of numerical simulation of unsteady convective heat transfer of an electrically conductive liquid in a spherical layer (modeling the Earth’s liquid core) under boundary conditions for a temperature of the first kind and in the presence of internal (negative) heat sinks are presented. The effect of internal heat sinks on the evolution of the structure of a liquid flow, temperature field, magnetic induction, and the distribution of Nusselt numbers in a strong magnetic field is investigated.

Simulation of Heat Transfer in the Liquid Core of the Earth

The results of numerical simulation of unsteady heat transfer of an electrically conductive liquid in a spherical layer modeling the Earth’s liquid core are presented. The evolution of the structure of the flow of an electrically conductive fluid, the temperature field, magnetic induction, and the distribution of local Nusselt numbers is studied. The gravitational acceleration vector is directed along the radius to the center of the spherical layer.

Steady and unsteady calculations on thermal striping phenomena in triple-parallel jet

The phenomenon of thermal striping is encountered in liquid metal cooled fast reactors (LMFR), in which temperature fluctuation due to convective mixing between hot and cold fluids can lead to a possibility of crack initiation and propagation in the structure due to high cycle thermal fatigue. Using sodium experiments of parallel triple jets configuration performed by Japan Atomic Energy Agency (JAEA) as benchmark, numerical simulations were carried out to evaluate the temperature fluctuation characteristics in fluid and the transfer characteristics of temperature fluctuation from fluid to structure, which is important to assess the potential thermal fatigue damage. In this study, both steady (RANS) and unsteady (URANS, LES) methods were applied to predict the temperature fluctuations of thermal striping. The parametric studies on the effects of mesh density and boundary conditions on the accuracy of the overall solutions were also conducted. The velocity, temperature and temperature fluctuation intensity distribution were compared with the experimental data. As expected, steady calculation has limited success in predicting the thermal–hydraulic characteristics of the thermal striping, highlighting the limitations of the RANS approach in unsteady heat transfer simulations. The unsteady results exhibited reasonably good agreement with experimental results for temperature fluctuation intensity, as well as the average temperature and velocity components at the measurement locations.

Numerical Simulation of Unsteady Heat Transfer in a Half-Moon Shape Enclosure with Variable Thermal Boundary Condition for Different Nanofluids

A half-moon shape enclosure which has a very wide range of practical applications in heat transfer is introduced for the first time in this article. The heat transfer is analyzed introducing different commercially available nanofluids such as water–Al2O3, water–Cu, water–TiO2 in this half-moon enclosure. A variable thermal boundary condition is assigned to the model, and the finite-element method is used for the numerical solution of the problem. The effect of solid volume fraction φ, along with a wide range of Rayleigh numbers (Ra = 105–107), are evaluated in various dimensionless times τ. The performance of the shape is described by using streamfunctions, isotherms, charts, and related graphs. It is found that heat transfer in the cavity can be enhanced up to 30% by to the presence of nanoparticles.

Large-eddy simulation of thermal striping in unsteady non-isothermal triple jet

Unsteady temperature fluctuations of non-isothermal turbulent jets are encountered in many engineering applications including liquid metal cooled fast reactors (LMFR), and can cause thermal stresses on solid boundaries. An accurate prediction of the temperature fluctuations is important to assess potential thermal fatigue damage to components, and traditionally this has been done by RANS turbulence modelling calculations with limited success. In this study, a large eddy simulation (LES) technique was applied to predict the temperature fluctuations of thermal striping observed in a triple jet. The triple jet model was used as a mock-up of the outlet of fuel subassemblies in a nuclear fast reactor. The results show that LES predicted the highly oscillatory nature of unsteady thermal mixing of the triple jet. The LES results were in good agreement with the available experimental data in terms of mean, RMS, skewness and kurtosis. The large amplitude of the temperature fluctuations associated with the thermal striping was captured correctly, demonstrating that LES can be used to analyse unsteady characteristics of thermal striping. Instantaneous and time mean thermal fields were further analysed to assess the capability and accuracy of LES in the thermal striping study. The Spalart–Allmaras and realisable k − ε turbulence models were also considered along with LES. It is found that these turbulence models produced a very small amplitude of fluctuations, and failed to predict the correct magnitude of unsteady thermal fluctuations, highlighting the limitations of the RANS approach in unsteady heat transfer simulations.

Two-dimensional simulation of unsteady heat transfer from a circular cylinder in crossflow

The heat transfer from the surface of a circular cylinder into a crossflow has been computed using a two-dimensional model, for a range of Reynolds numbers fromRe=200 to 15550. The boundary-layer separation, the local and overall heat-transfer rates, the eddy- and flare-detachment frequencies and the width of the flares were determined from the numerical simulations. In this range of Reynolds numbers, the heat-transfer process is unsteady and is characterized by a viscous length scale that is inversely proportional to the square root of the Reynolds number. To ensure uniform numerical accuracy for all Reynolds numbers, the dimensions of the computational mesh were selected in proportion to this viscous length scale. The small scales were resolved by at least three nodes within the boundary layers. The frequency of the heat flares increases, and the width of each flare decreases, with the Reynolds number, in proportion to the viscous time and length scales. Despite the presence of three-dimensional structures for the range of Reynolds numbers considered, the two-dimensional model captures the unsteady processes and produced results that were consistent with the available experimental data. It correctly simulated the overall, the front-stagnation and the back-to-total heat-transfer rates.

Detection of knock occurrence in a gas SI engine from a heat transfer analysis

This paper focuses on the transient thermal signal around an engine cylinder in order to propose a new and non-intrusive method of knock detection. Numerical simulations of unsteady heat transfer through the cylinder and inside the coolant flow are performed. The amplification of wall heat transfer due to knock is taken into account by a self-developed program. It enables one to fix the instantaneous heat flux on the internal side of the cylinder. The transient component of the thermal signal is then calculated and analysed. The effect of a machining, consisting of a transverse groove, which reduces the cylinder liner thermal resistance, is investigated. This groove disturbs the turbulent coolant flow, and its shape mainly influences the heat transfer from the wall to the fluid. The numerical results show that the use of a square groove would multiply by three the amplitude of the transient thermal signal recorded within the water compared to a smooth wall case. Moreover, the amplitude can be enhanced by using a nanofluid as coolant because of its higher thermal diffusivity.

Unsteady Heat Transfer Enhancement Around an Engine Cylinder in Order to Detect Knock

This paper deals with the transient thermal signal around an engine cylinder in order to propose a new and nonintrusive method of knock detection. Numerical simulations of unsteady heat transfer through the cylinder and inside the coolant flow are carried out to account for heat flux variations due to normal and knocking combustion. The effect of rib roughened surfaces on thermal signal amplification is investigated. The geometric parameters are fixed at Pi/h=10 and w/h=1 with a Reynolds number based on hydraulic diameter of 12,000. The results reveal that square ribs give better performance in term of thermal signal amplification within the fluid. An amplification of the temperature variation up to 20 times higher is found. Finally, flow analysis shows that amplification depends on the position where the thermal signal is collected.

Unsteady laminar flow and convective heat transfer in a sharp 180° bend

Unsteady laminar flow and heat transfer in a sharp 180° bend is studied numerically to investigate a convective heat transfer regime of especial relevance to electronic systems. Due to the high geometrical aspect ratios occurring in the practical application, two-dimensional unsteady simulations are considered. The two-dimensionality assumption adopted is validated by three-dimensional test simulations. Unsteady heat transfer simulations are performed for 50⩽ Re⩽1000. Results show that the flow remains steady until Re≈600. In this steady regime, the re-attachment length increases gradually with the Reynolds number. For Re>600, the flow becomes unsteady with large-scale vortices emanating from the sharp edge dominating the flow field. Flow oscillation causes a substantial reduction in the re-attachment length and a dramatic heat transfer enhancement. As the vortices move downstream, the Nusselt number along the wall oscillates significantly. The correlation between the flow structure and the heat transfer is found to be strong.

Modelling and simulation of unsteady heat transfer for aerospacecraft trajectory optimization

Heat input reduction by optimal trajectory control is considered for the range cruise of a hypersonic flight system propelled by a turbo/ram jet engines combination. A mathematical model is developed for describing the unsteady heat transfer through the thermal protection system. This model is coupled to the equations of motion of the vehicle. An efficient optimization technique is applied for constructing a solution. The results show that a significant heat input reduction can be achieved with only a small increase in fuel consumption.

Numerical simulation of unsteady heat transfer in canned mushrooms in brine during sterilization processes

The proposed numerical model determines the temperature distribution in convectively heated liquid brine and in conductively heated mushroom solid during in-can sterilization. The convective heat transfer in the brine is described by the regular regime equation and the conductive heat transfer in the mushrooms which have an irregular shape by the generalized equation of heat conduction. The predicted results obtained by using this method are compared with experimental results. The proposed approach may also be used for simulating the temperature in heating and cooling of heterogeneous foodstuffs.