Heat Transfer Considerations Research Articles

Pool boiling heat transfer enhancement by bi-conductive surfaces

The use of the bi-conductive surface provides a promising prospect for the enhancement of the boiling phase change heat transfer. In this study, a theoretical model of two-phase flow and boiling heat transfer on bi-conductive surfaces is developed. With the consideration of thermal conductivity contrast and conjugate heat transfer, the boiling behavior is numerically investigated based on a modified lattice Boltzmann method. The bubble dynamics during the boiling process on the bi-conductive surface are reproduced and the effect of the structural size of these insets on the boiling performance is explored. The results indicate that boiling performance is enhanced on the bi-conductive surface when compared to homogeneous ones. In-plane temperature variation on the surface contributes to the pinning effect on bubble contact lines, and thus promotes boiling heat transfer. Boiling enhancement gets optimized once the embedding pitch space equal to the capillary length of the boiling fluid. Increasing the width of the insets also improves the boiling heat transfer coefficient on the bi-conductive surface under low superheat conditions, but also leads to the advance of CHF. However, changing the depth of the insets shows almost no effect on the boiling heat transfer performance.

Investigation of Graphene Oxide-Distilled Water Nanofluids with Consideration of Heat Transfer and Flow Structure for Backward-Facing Step Flow

Investigation of Graphene Oxide-Distilled Water Nanofluids with Consideration of Heat Transfer and Flow Structure for Backward-Facing Step Flow

Numerical analysis of the close-contact heat transfer of the electro-thermal drilling probes for glacier-ice exploration

The present study aims to numerically investigate the thermal characteristics of the drilling probe for glacier-ice exploration with consideration of close-contact heat transfer between the electro-thermal probe and the ice. The commercial program of Ansys Fluent (v.17.2) was used, and the enthalpy porosity method was adopted for simulating the phase change heat transfer. The dynamic layering method was also used to move the computational mesh with a given speed for understanding the correlation between the moving speed and the thermal characteristics. It is found that the thin liquid layer increases with the heating power imposed on the probe, and the molten liquid radially moves in the thin liquid layer, which is much thinner than the probe size. Also, the growth rate depends on the heating power, especially in the early stage of drilling. Moreover, the probe velocity significantly affects the evolution of the thin liquid layer.

Inhomogeneous Bonding in Low-TemperatureSoldering: Brief Review

Thermal stresses [1-4] in solder joint interconnections (see, e.g., [5]) used in electronic and photonic packaging are proportional to the thermal contraction mismatch strains T [3-9]. Here  is the effective coefficient of thermal expansion (CTE) mismatch between the soldered materials (the chip or the package to their substrates), and T is the change in temperature from the elevated manufacturing (bonding/fabrication/solder ing) temperature, at which, because of the interaction of shrinkage and stressrelaxation processes, the induced stresses are next-to-zero, to the low, room, testing or operation, temperature, at which the induced stresses are the highest. Clearly, these stresses are lower for lower soldering temperatures, and therefore there is an obvious incentive for using low temperature solders. What is less obvious is that significant stress relief in solder joints can be achieved also by employing, for the same materials and the same thermal mismatch strain T, inhomogeneous bonds [10-19], when, e.g., a solder material with a high soldering temperature and/or with a high Young’s/shear modulus is employed in the mid-portion of the assembly, where the stresses in this material are low (at the mid-cross-section of the assembly these stresses, distributed in an anti-symmetric fashion, are always zero), and a low-melting-point solder and/or a solder with a lower modulus is employed at the assembly’s peripheral portions, where the interfacial shearing and peeling thermal stresses are the highest. Ultimately, when only the elevated stresses, and not heat transfer considerations, are viewed to be critical, even assemblies bonded at the ends only are viable [16,17].

Macroscale Modeling of Solid–Liquid Phase Change in Metal Foam/Paraffin Composite: Effects of Paraffin Density Treatment, Thermal Dispersion, and Interstitial Heat Transfer

Abstract This work focuses on macroscale modeling of solid–liquid phase change in metal foam/paraffin composite (MFPC), addressing the treatment of paraffin density (under distinct paraffin filling conditions in metal foam), thermal dispersion effect, and influence of thermal diffusion-dominated interstitial heat transfer. To this end, a macroscale thermal non-equilibrium model for melting in MFPC with fluid convection is developed by employing the enthalpy-porosity technique and volume-averaging approach. Meanwhile, visualized experiments on the melting of the MFPC sample are carried out to validate the modeling results. Comparing the numerical modeling and experimental visualization results, it is found that for MFPC with an initially saturated filling condition in metal foam using solid paraffin, the varied paraffin density is preferred to be employed for developing accurate phase change model. However, for MFPC that can be just filled with liquid paraffin after melting (i.e., non-saturated filling condition using solid paraffin), the Boussinesq approximation is preferred to achieve satisfying phase change simulation. Thermal dispersion effect in MFPC is proved to be negligible, which should not be overvalued to avoid inducing physical distortions of heat transfer and fluid flow. Consideration of diffusion-dominated interstitial heat transfer in the thermal non-equilibrium model is vital to accurately capture phase interface evolutions as well as to reasonably simulate the mushy zone of paraffin, and the model only incorporating the convection-induced interstitial heat transfer will predict quite inaccurate phase change process. This study can provide useful guidance in macroscale modeling of phase change in MFPC associated with the thermal energy storage applications.

Turbulent convective heat transfer in helical tube with twisted tape insert

Enhancement of heat transfer has been a main focus in thermal-fluid engineering field among the other aspects in industrial for decades. Despite numerous studies have been done on turbulent flows, helical tubes, and twisted tapes related to enhancement, no study has been done on the combination of all three components in augmentation of heat transfer. The objective of this study is to numerically investigate heat transfer of turbulent flow in helical tube with twisted tape insert subjected to constant wall temperature. A three-dimensional computational fluid dynamic model was developed and validated against experimental measured data and correlations. The model was then used to evaluate the turbulent convective heat transfer in straight and helical tube with and without twisted tape insert. To obtain comprehensive view on the heat transfer performance, the effect of twist ratio and inlet Reynolds number are evaluated as well. The insertion of twisted tape is found to increase the intensity of secondary flow in the radial direction. Helical tube with twisted tape insert was found to have higher heat transfer performance. The results show that for every case, the higher the inlet Reynolds number, the heat transfer increases proportionally while friction coefficient drops with decreasing amount of decrement. It is found that helical coiled tube with twisted tape at twist ratio of 7.86 has the highest performance among the configuration studied after taking consideration of heat transfer, friction coefficient and entropy generation. Finally, correlations of Nu and f are developed to predict the heat transfer and pressure drop within considered range of parameters in this study. This study serves as a foundation on the development of a high-performance heat exchanger for industrial application.

Heat Transfer Considerations on the Spontaneous Triggering of Vapor Explosions—A Review

Vapor explosions have been investigated both theoretically and experimentally for several decades, focusing either on the vapor film, or on mechanical aspects. Where the main interest for industry lies in the safety risks of such an event, fundamental research is focusing on all partial processes that occur during a vapor explosion. In this paper, vapor explosions are discussed from a heat transfer point of view. Generally accepted knowledge of heat transfer between hot surfaces and liquids is compared to early investigations regarding the origin of vapor explosions. Both steady state and transient models are discussed. The review of available literature suggests that vapor explosions trigger spontaneously by the collapse of the boiling film. Better understanding of the fundamental aspects of vapor explosions might give rise to future ideas on how to avoid them.

A volatile spray zone model and experimentation in a gas-solid fluidized bed with liquid injection

In a gas-solid fluidized bed, volatile liquid injection results in complex flow behaviors in the spray zone. To characterize the liquid existence forms (droplets and liquid films) and fluidization properties in the volatile spray zone, a mesoscale-theory based volatile spray zone model is developed with the consideration of hydrodynamics, heat transfer, and mass transfer. A new energy-based stability condition is established according to two competition and coordination relationships, consisting of the energy dissipation based on the liquid-solid interface change and suspension and transporting of particles. The modeling results agree well with the experimental bed temperature as well as void fraction data, and show that the variation of liquid existence forms is dependent on the ratio of different energy consumption rates under the stability condition. Moreover, the gas-solid drag coefficient in the volatile spray zone is modified. Simulation results also reasonably describe the bed pressure drop fluctuation and particle velocity distribution.

CFD-based coupled multiphase modeling of biochar production using a large-scale pyrolysis plant

This paper presents a combined CFD simulation of the thermal conversion of biomass to biochar and the co-combustion of air and biomass volatiles with non-premixed swirl air and volatiles in an industrial pyrolysis plant. The main objective of this study is to investigate thermal conversion process in indirect biochar plants taking into accounts the main challenges associated with biochar production. The model is based on the implementation of calculations on the thermal conversion of biomass in a computational fluid dynamics (CFD) environment. Several sub-models have been introduced to simulate the thermal conversion of biomass to biochar, taking into consideration heat and mass transfer, drying, pyrolysis and volume shrinkage. A non-uniform heat flux obtained by simulating the non-premixed co-combustion of the swirl propane burner and the wood volatile gas has been implemented as an inlet boundary condition in the pyrolysis section. The feasibility of the self-ignition of wood volatiles has been investigated. Uniformity of heat transfer rate between thermal oxidizer and pyrolysis section and lower moisture content in the feedstocks play a remarkable role in producing higher quality of biochar and minimizing residence time in indirect slow pyrolysis plants.

Effect of Geometric and Operational Parameters on the Performance of a Beta-Type Stirling Engine: A Numerical Study

This paper describes the development of a mathematical model for a beta-type Stirling engine with rhombic drive mechanism. This model is based on thermodynamics, heat transfer, mass transfer, and machine dynamics considerations, and it estimates the power output and efficiency of a beta-type Stirling engine depending on different geometric and operational variables. The model is used to conduct a parametric investigation on the effect of various geometric and operational parameters on the overall performance of the engine and to shed light on the underlying mechanisms responsible for the observed trends. The results show that the geometric parameters such as the height of the displacer and crank length affect the engine performance by altering the compression ratio of the engine, whereas the impact of width of the regenerative channel is through pumping losses for fluid flow between the hot and cold chambers of the engine. The operational variable, engine speed, is shown to influence the amount of heat transfer to and from the working fluid since it directly affects the residence time of fluid in the hot and cold chambers and accordingly it has a bearing on the engine performance.

Prediction of lack-of-fusion porosity in laser powder-bed fusion considering boundary conditions and sensitivity to laser power absorption

This paper proposes an analytical modeling method for the prediction of lack-of-fusion porosity of parts fabricated by laser powder-bed fusion (LPBF), with the consideration of boundary heat transfer and sensitivity to laser power absorption. The temperature distribution of the part was first predicted by an analytical thermal model, which consists of a linear heat source solution and a heat sink solution. The temperature increase due to laser power input was calculated by the point moving heat source solution. The temperature drop due to thermal conduction, convection, and radiation at part boundaries was calculated by the heat sink solution. The coefficient of laser power absorption was inversely obtained by comparing predicted molten pool widths with experimental measurements. The lack-of-fusion area was then calculated by plotting molten pool shapes of multi-tracks and multi-layers on a transverse cross-sectional area of the part. The powder bed porosity was calculated by an advancing front method with the consideration of powder size distribution and packing pattern. Finally, the lack-of-fusion porosity was obtained by multiplying the lack-of-fusion area with powder bed porosity. The predicted results were close to the measurements of Ti6Al4V in LPBF. The maximum deviation for porosity prediction is 5.91%. The presented model shows high computational efficiency without relying on iteration-based numerical method. The computational time for five consecutive layers is less than 100 s. The acceptable accuracy, short computational time, and the ability to consider complex boundary effects make the proposed method a good basis for future research and a promising tool for optimization of process parameters in LPBF of complex parts.

Fluid–thermal–structural coupling investigations of opposing jet in hypersonic flows

As one of active cooling method, opposing jet shows its advantages on the aerodynamic thermal reduction for hypersonic flight. To accurately predict its thermal protection features, a fluid-thermal-structural coupling strategy is adopted for a cone body with opposing jet, with a nose radius of 6 mm and jet radius of 0.6 mm. The related turbulent model and coupling method is validated by comparison of the numerical and experimental results. The interaction among flow, heat transfer, and the structural responds are gained in 60 s flight, and the influence of total pressure ratio of opposing jet (PR) is also discussed. The analysis indicates the necessary of fluid-thermal-structural coupling investigation for the details understanding of thermal protection procedure. The results show that the adoption of opposing jet can decrease the structure temperature and reduce drag obviously, and increase the stress at the tip region meanwhile, but still presents its advantage for its large enough safe factor. Besides, the adoption value of PR should be an integrated consideration of flow, heat transfer, structural response and coolant consume.

Parametric investigation of radiation heat transfer and evaporation characteristics of a liquid droplet radiator

The liquid droplet radiator (LDR) as a frameless heat removal system is considered as a promising solution for the waste heat dissipation of megawatt aerospace applications. The radiation heat transfer and evaporation characteristics of the LDR working fluid are tightly coupled with the operational performance of the liquid droplet generator. In this paper, the effects of operational parameters of droplet generator on the radiative heat transfer and evaporation loss rate are clarified, including the pressure disturbance frequency, pressure difference between the inside and outside of the droplet generator. It is observed that higher coolant mass flow rate does not assure higher heat transfer power for the LDR. For cases with higher pressure difference, where the coolant flow rate is higher consequently, the evaporation loss rate increases continually with pressure difference while the heat transfer power does not increase any more. The disturbance frequency has inconspicuous impact on the evaporation characteristics. Besides, cases with higher pressure differences have wider suitable frequency ranges according to the droplet formation restriction. Generally, a relatively higher pressure difference coupled with a higher disturbance frequency (such as 0.3 MPa and 12 kHz) is suggested to achieve a higher heat transfer power and low evaporation loss rate. This paper may provide favorable reference for determining the operational parameters of the droplet generator in consideration of radiation heat transfer and evaporation characteristics.

Thermal response study of carbon epoxy laminates exposed to fire

AbstractIn this article, a three‐dimensional thermal response model is developed to investigate the thermal behavior of carbon epoxy composite impacted directly by propane flame. The model is established in consideration of heat transfer and energy conservation in which the heat transfer is in the form of anisotropic heat conduction, absorption by matrix decomposition, and diffusion of gas. Arrhenius equation is utilized to present the decomposition process of the materials. The diffusion equation for the decomposition gas is included for mass conservation. The thermal response model is implemented with the UMATHT and USDFLD subroutines via ABAQUS code, from which the temperature, density, decomposition degree, and decomposition rate can be extracted to analysis the process of material decomposition by finite element simulation. The model shows its capability to analysis the evolution of a carbon epoxy composite in fire by the comparison between the numerical and experimental results. Furthermore, the numerical results show that thermal conductivities in different directions of fiber have a significant influence on the heat transfer. In addition, the relationship between the decomposition degree and temperature is correlated with depths, as well as the peak value of decomposition rate and the time to reach that.

Multiphysics Simulation of Nucleation and Grain Growth in Selective Laser Melting of Alloys

Abstract Selective laser melting (SLM) builds parts by selectively melting metallic powders layer by layer with a high-energy laser beam. It has a variety of applications in aerospace, medical device, and other low-volume manufacturing. Nevertheless, the lack of fundamental understanding of the process-structure-property relationship for better quality control inhibits wider applications of SLM. Recently, a mesoscale simulation approach, called phase field and thermal lattice Boltzmann method (PF-TLBM), was developed to simulate microstructure evolution of alloys in SLM melt pool with simultaneous consideration of solute transport, heat transfer, phase transition, and latent heat effect. In this paper, a nucleation model is introduced in the PF-TLBM framework to simulate heterogeneous nucleation at the boundary of the melt pool in SLM. A new method is also developed to estimate the thermal flux out of the SLM melt pool model given a constant cooling rate. The effects of latent heat and cooling rate on dendritic morphology and solute distribution are studied. The simulation results of AlSi10Mg alloy suggest that the inclusion of latent heat is necessary because it reveals the details of the formation of secondary arms, reduces the overestimation of microsegregation, and provides more accurate kinetics of dendritic growth.

Effects of flow structure on heat transfer of squealer tip in a turbine rotor blade

Consideration heat transfer while organizing the flow structure in the turbine blade tip region is important to improve the turbine aerodynamic performance and turbine blade reliability. Numerical methods were conducted to investigate the flow physics and the heat transfer characteristic in high pressure turbine squealer tip. The impacts of some geometric parameters and aerodynamic conditions on heat load distribution were also discussed. Based on the above results, a heat transfer model for squealer tip was developed. The results show that the high heat transfer occurred in certain tip region is mainly caused by the leakage jet impingement on these surfaces. The velocity direction and magnitude of the leakage are determined by the characteristic of flow structures, especially the scraping vortex in the cavity. Different squealer height, relative casing velocity and blade loading can change the heat load distribution of tip region by varying the characteristic of the scraping vortex. Through simplifying the process of the leakage jet flowing into the cavity as a backward step flow, a heat transfer model is established which provides a useful tool to predict heat load distribution of the squealer tip.

Unsteady Magnetohydrodynamic Flow with Heat and Mass Transfer between Two Parallel Walls under Variable Magnetic Field

This paper presents a semi-analytical solution using homotopy perturbation method of magnetohydrodynamic unsteady flow of fluids between two inclined parallel permeable walls with the consideration of heat and mass transfer. The flow is subjected to variable magnetic field normal to the walls. The solution for the governing nonlinear partial differential equations coupled with equations of motion, concentration, and energy equation including the viscous and Joule dissipations is adopted. The effect of Prandtl and Eckart numbers, Reynolds magnetic number, Schmidt and Grashof numbers, and modified Grashof number, and magnetic field strength on velocity, temperature, concentration, and induced magnetic field are discussed. Results are represented graphically for the considered problem.

Dynamic propagation of ice-water phase front in a supercooled water droplet

A three-dimensional (3-D) theoretical model is proposed to take the crystallization kinetics into the consideration of heat transfer, which allows for the description of the entire freezing process of a supercooled water droplet with the inclusion of the description of recalescence. The temporal evolution of the temperature and solid mass fraction at various stages during freezing are numerically studied, which shows a good agreement with the experimental results of the water droplet frozen in cold nitrogen gas stream. In particular, the ice-water phase front dynamics initiated from a single ice nucleation site is numerically investigated in recalescence stage, which shows that the ice-water phase front spreads over the entire water droplet with a diameter of 1.4 or 2.0 mm within about 16.0 or 23.0 ms governed by crystallization kinetics, leading to the drastic increase of the temperature and solid mass fraction of the droplet under the effect of thermodynamics. The present theoretical model can be useful to describe the dynamic behavior of ice crystallization in many other relevant cases.

Revisiting Gradient Layer Heat Extraction in Solar Ponds Through a Realistic Approach

Abstract In this paper, the concept of heat extraction from the gradient zone (GZ) in solar ponds has been analyzed in a more realistic manner to overcome the drawbacks of previously conducted studies. For this purpose, a net heat transfer coefficient has been invoked to investigate the heat transfer occurring from the GZ to the exchanger installed in this zone, in addition to the storage zone (SZ). Analytical solutions for temperature profiles in the GZ and the corresponding exchanger have been obtained which are further used to investigate various aspects of the thermal performance of the pond. The consideration of realistic heat transfer across the GZ exchanger reveals that the ideal thickness of GZ yielding maximum power output is always under-predicted by the idealized assumption of the literature. Unlike intuitive perception, the conventional assumption of an infinite heat transfer coefficient does not affect the pond stability because, for all practical purposes, the critical salt diffusion rate predicted by it is always larger than the actual critical value required for ensuring stable pond operation. However, as expected, the rate of exergy destruction caused by the pond’s operation is found to be underestimated by the idealized assumption. This study provides a useful analytical tool to make more realistic predictions on various performance parameters of solar ponds utilizing the heat stored in their GZ.

A simplified method to evaluate geothermal storage in an aquifer with consideration of heat transfer between aquifer and caprock/baserock

Storing and extracting heat during different seasons of the year is possible through the utilization of a ground aquifer with an open loop Ground Source Heat Pump (GSHP) system. Being able to predict the hydrothermal performance of geothermal storage is required for an efficient operation of the system for cooling and heating of buildings. Complex 2D and 3D hydrothermal numerical models can simulate the thermal performance of geothermal storage accurately but often lack the desired computational speed for conducting large number of simulations for performance optimization. Instead, a 1D radial model can be used to conduct fast evaluation. However, it is important that the model computes the amount of heat loss from an aquifer into the overburden and underlying layers accurately to evaluate the amount of geothermal storage in the aquifer at different times. In this study, a source term is introduced into a 1D model to simulate the heat transfer between the aquifer and caprock/baserock in the vertical direction. The following two heat loss models are introduced in the heat advection-conduction equation: (i) Newton’s heating/cooling law, which leads to a closed form solution, and (ii) a conduction-based semi-analytical model, which requires a 1D finite element solution. When compared to a full 2D axisymmetric simulation result, it was found that the Newton’s heating/cooling law model with a constant heat transfer coefficient works well in cases of fast heat flow rate in thick aquifers of around 100 meters. But large errors in estimating heat dissipation are observed in cases with low heat flow rate in thin aquifers, especially for simulations exceeding two to five years. On the other hand, the model with the conduction-based semi-analytical solution gives a better match for these conditions.