Showing papers on "Convection published in 2019"

3-Dimensional heat transfer modeling for laser powder-bed fusion additive manufacturing with volumetric heat sources based on varied thermal conductivity and absorptivity

Abstract: In this article, a 3-dimensional heat-transfer finite element model for Laser Powder-Bed Fusion (LPBF) was developed for accurately predicting melt pool dimensions and surface features. The sole deployment of trial-and-error experiments for arriving at optimal process parameters is very costly and time-consuming, thus the developed model can be used to reduce the process/material development costs. A literature review of heat source models was presented. Eight commonly used heat source models are evaluated and compared. All of their simulated depths are smaller than the experimental result, which may be due to the melt pool convection and inconstant laser absorptivity in the reality during the experiment. In order to enable the numerical model to predict melt pool dimensions for different combinations of process parameters, a novel model including expressions of varied anisotropically enhanced thermal conductivity and varied laser absorptivity is proposed and verified by both the melt pool dimensions and track surface morphology. It is found that the heat source expressions can be linear while causing the simulation results to be in better agreement with both experimental melt pool dimensions and track surface morphology.

A review of heat transfer enhancement techniques in plate heat exchangers

Abstract: Plate heat exchangers have been widely applied in numerous industrial applications since their first commercial exploitation in the 1920s. Enhancing the thermal-hydraulic performance of plate heat exchangers is of crucial importance for the energy conversion as well as for the improvement of the system economy, through savings in the capital investment. The efficiency of a plate heat exchanger can be improved either by optimizing its geometry or using heat transfer enhancement techniques. This paper provides a comprehensive review of previous works regarding the effects of chevron corrugation geometrical parameters on the performance of plate heat exchangers, and the application of heat transfer enhancement techniques in plate heat exchangers, focusing on passive surface techniques and the use of nanofluids. The objective of the paper is not only to describe relevant studies, but also to provide an understanding of the heat transfer mechanisms governing the results, and to evaluate and compare the different heat transfer enhancement techniques. In addition, prospective directions for future research are provided. The review indicates that for the chevron-type plate heat exchanger, the chevron angle is the most influential geometrical parameter by changing the flow structures in the single-phase heat transfer; meanwhile the chevron angle has a significant influence on the heat transfer regions characterized by convection in the two-phase heat transfer. An analysis based on the performance evaluation criteria suggests that the thermal-hydraulic performances of the studies with different geometrical parameters and enhancement techniques are generally higher at low Reynold numbers. Furthermore, the review and analysis indicate that the capsule-type embossing surface and the microstructured surface with a nano- and microporous layer are the enhancement techniques that present the highest performance in single-phase and two-phase heat transfer, respectively.

Numerical analysis of mixed convection of two-phase non-Newtonian nanofluid flow inside a partially porous square enclosure with a rotating cylinder

Abstract: In this study, mixed convection heat transfer of the non-Newtonian power-law nanofluid including CuO nanoparticles, inside a partially porous square enclosure with a concentric rotating cylinder and a hot side wall is numerically investigated. Two-phase mixture model is utilized for nanofluid flow simulation and the mixture viscosity and thermal conductivity are computed by Corcione’s correlation. The effect of different angular velocity (− 4000 ≤ Ω ≤ 4000) for various Rayleigh (104 ≤ Ra ≤ 106), Darcy (10−4 ≤ Da ≤ 10−1), power-law index (0.8 ≤ n ≥ 1.2) and effective to base fluid thermal conductivity ratio (keff/kf= 16, 4) are studied on heat transfer. Results are presented and compared in terms of the average Nusselt number, and streamline and isotherm contours. Outcomes show that for different kinds of fluid, depending on the value of Ra, Da, keff/kf and the amount and direction of angular velocity, heat transfer can be improved by augmenting heat convection and also can be deteriorated by increasing viscosity. Consequently, optimal values of Ra, Da, keff/kf and Ω exist in order to maximize the average Nu number.

Pre-mission InSights on the Interior of Mars

Abstract: The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans- port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars.

Simultaneous solutions for first order and second order slips on micropolar fluid flow across a convective surface in the presence of Lorentz force and variable heat source/sink

Abstract: This report presents the flow and heat transfer characteristics of MHD micropolar fluid due to the stretching of a surface with second order velocity slip. The influence of nonlinear radiation and irregular heat source/sink are anticipated. Simultaneous solutions are presented for first and second-order velocity slips. The PDEs which govern the flow have been transformed as ODEs by the choice of suitable similarity transformations. The transformed nonlinear ODEs are converted into linear by shooting method then solved numerically by fourth-order Runge-Kutta method. Graphs are drowned to discern the effect of varied nondimensional parameters on the flow fields (velocity, microrotation, and temperature). Along with them the coefficients of Skin friction, couple stress, and local Nussel number are also anticipated and portrayed with the support of the table. The results unveil that the non-uniform heat source/sink and non-linear radiation parameters plays a key role in the heat transfer performance. Also, second-order slip velocity causes strengthen in the distribution of velocity but a reduction in the distribution of temperature is perceived.

Natural variability of Southern Ocean convection as a driver of observed climate trends

Abstract: Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simulations are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated with this natural variability show potential for skilful decadal prediction. Sea-ice expansion around Antarctica, and related surface cooling, is shown to be linked to natural long-term variability of Southern Ocean convection. Model simulations reproduce the observed trends, if they start from an active phase of convection.

Convection with Local Thermal Non-Equilibrium and Microfluidic Effects

Abstract: Introduction.- Thermal Convection with LTNE.- Rotating Convection with LTNE.- Double Diffusive Convection with LTNE.- Vertical Porous Convection with LTNE.- Penetrative Convection.- LTNE and Multi-layers.- Other Convection/Microfluidic Scenarios.- Convection with Slip Boundary Conditions.- Convection in a Porous Layer with Solid Partitions.- Convection with Produting Baffles.- Anisotropic Inertia Effect.- Bidispersive Porous Media.- Resonance in Thermal Convection.- Thermal Convection in Nanofluids.- References.

First and second laws of thermodynamics analysis of nanofluid flow inside a heat exchanger duct with wavy walls and a porous insert

Abstract: This paper investigates the combined effects of using nanofluid, a porous insert and corrugated walls on heat transfer, pressure drop and entropy generation inside a heat exchanger duct. A series of numerical simulations are conducted for a number of pertinent parameters. It is shown that the waviness of the wall destructively affects the heat transfer process at low wave amplitudes and that it can improve heat convection only after exceeding a certain amplitude. Further, the pressure drop in the duct is found to be strongly influenced by the wave amplitude in a highly non-uniform way. The results, also, show that the second law and heat transfer performances of the system improve considerably by thickening the porous insert and decreasing its permeability. Yet, this is associated with higher pressure drops. It is argued that the hydraulic, thermal and entropic behaviours of the system are closely related to the interactions between a vortex formation near the wavy walls and nanofluid flow through the porous insert. Viscous irreversibilities are shown to be dominant in the core region of duct where the porous insert is placed. However, in the regions closer to the wavy walls, thermal entropy generation is the main source of irreversibility. A number of design recommendations are made on the basis of the findings of this study.

Phonon heat transfer across a vacuum through quantum fluctuations

Abstract: Heat transfer in solids is typically conducted through either electrons or atomic vibrations known as phonons. In a vacuum, heat has long been thought to be transferred by radiation but not by phonons because of the lack of a medium1. Recent theory, however, has predicted that quantum fluctuations of electromagnetic fields could induce phonon coupling across a vacuum and thereby facilitate heat transfer2–4. Revealing this unique quantum effect experimentally would bring fundamental insights to quantum thermodynamics5 and practical implications to thermal management in nanometre-scale technologies6. Here we experimentally demonstrate heat transfer induced by quantum fluctuations between two objects separated by a vacuum gap. We use nanomechanical systems to realize strong phonon coupling through vacuum fluctuations, and observe the exchange of thermal energy between individual phonon modes. The experimental observation agrees well with our theoretical calculations and is unambiguously distinguished from other effects such as near-field radiation and electrostatic interaction. Our discovery of phonon transport through quantum fluctuations represents a previously unknown mechanism of heat transfer in addition to the conventional conduction, convection and radiation. It paves the way for the exploitation of quantum vacuum in energy transport at the nanoscale. Conventionally, heat transfer occurs by conduction, convection or radiation, but has also been theoretically predicted to occur through quantum fluctuations across a vacuum; this prediction has now been confirmed experimentally.

Quadratic convective transport of dusty Casson and dusty Carreau fluids past a stretched surface with nonlinear thermal radiation, convective condition and non-uniform heat source/sink

Abstract: Here, the nonlinear convective transport of non-Newtonian fluids embedded with dust particles over a stretched surface is investigated. The silent features of non-Newtonian fluid are considered by Casson and Carreau fluid models. The heat transfer mechanism involves the influences of a magnetic dipole, nonlinear radiative heat and non-uniform heat source/sink. The convective condition is also retained at the boundary. The non-linear partial differential equations that model the transport phenomenon was transformed, non-dimensionalized and parameterized. The subsequent boundary value problems were computed numerically for distinct pertinent parameters using Runge–Kutta based shooting techniques. The present results are validated with the existing literature by direct comparison. The heat transfer rate in Casson/Carreau fluid phase is significantly higher than that of dust phase.