Complex Heat Transfer Characteristics Research Articles

The thermal history of the directed energy deposition process monitored by pyrometer and camera

Abstract In the process of directed energy deposition (DED), the local thermal history of a component can change significantly owing to its complex heat transfer characteristics. The main objective of this study is to elucidate the distribution of the thermal history of a static scroll plate by monitoring its manufacturing process through a pyrometer and camera and to provide insights into the factors affecting the variation of its thermal history from the point of view of the part shape and scanning strategy. The melt pool area and temperature were extracted using a camera and pyrometer as the thermal features under study, and the thermal features were mapped in three dimensions. This mapping strategy provides a comprehensive visualization of the distribution of thermal features in parts with equal and variable cross-section structures. In the case of an equal cross-section structure, three factors, printing time, scan vector length, and probability density, are selected to analyze the main factors affecting the distribution of thermal features. In the case of the variable cross-section structure, the effects of the layer printing time, cumulative layer printing time, and probability density on the distribution of thermal features were analyzed. By calculating the correlation between the thermal features of the monitored signals and the factors mentioned above, it was found that the factors affecting the thermal distribution of the parts vary in different structures. In equal cross-section structures, the probability density plays a pivotal role, while in variable cross-section structures, the cumulative layer printing time has the most significant influence on the thermal distribution. The above studies have provided a better understanding of the thermal processes of DED technology, paving the way for the control and optimization of DED manufacturing.

Research on modeling method of non-axisymmetric endwall based on body-fitted coordinates

Due to the large bending of turbine blades, there is strong three-dimensional flow in cascade channel, the three-dimensional flow at the endwall of blades generates extremely complex heat transfer characteristics. The design concept of high heat load turbine blades, particularly comprehensive optimization of flow and heat transfer at the endwall, has gained increasing attention. In this paper, we have developed a three-dimensional endwall parameterized modeling platform based on Body-Fitting Coordinates and introduced a novel approach for streamwise direction and normal direction concerning endwall profile construction. The endwall modeling was optimized using Genetic Algorithm to obtain a modified one with low heat transfer level on the premise that the aerodynamic loss is not increased. The analysis indicates that the reduction of aerodynamic loss with the modified endwall is relatively marginal, whereas the heat transfer intensity on the endwall surface experiences a significant decrease. Additionally, employing an endwall model can effectively mitigate the lateral pressure gradient in the cascade channel, which exerts a pronounced influence on suppressing secondary vortex development in the cascade.

Heat transfer in the argon space of pool-type sodium-cooled fast reactor: Numerical simulation and experimental validation

Liquid sodium serves as the coolant in the pool-type sodium-cooled fast reactor. The primary penetration equipment on the primary side is immersed in a sodium pool. Argon gas is employed as the covering gas above liquid sodium. To determine the temperature distribution of the roof slab of the main vessel for strength analysis, it is necessary to analyze the heat transfer characteristics. Owing to the large volume of the original reactor, it is difficult to establish a full-scale experimental model. Consequently, a scaled-down experiment is required to obtain the temperature distribution of the roof slab and argon space. In addition, the complex heat transfer characteristics within the roof slab and argon space make it difficult to maintain similarities between the experimental and original facilities. Numerical simulations are a useful approach for extrapolating an experiment to the original reactor. In this study, a scaled-down heat transfer experimental facility for an argon space (HEFA) was established based on the similarity principle, and a numerical model was set up according to the HEFA. The validity of the numerical methods was confirmed by comparing the experimental and numerical results for the HEFA. Based on similarity theory, the temperature distribution in the HEFA was extrapolated to that in the original reactor. A numerical model was established based on the original reactor. By comparing the extrapolated temperature distribution and numerical distribution of the original reactor, it was found that they were in good agreement. Therefore, the validity of the extrapolated temperature distribution was confirmed.

Numerical simulation of the heat transfer characteristics of the second and third superheaters in a coal-fired thermal power plant radiant boiler

Superheaters are essential components in a coal-fired power plant because they keep the highest tube wall temperature points in the radiant boiler. The pressurized steam flowing in the superheater tubes is heated via the thermal radiation and convection from the combustion gas. To prevent the bursting of superheater tubes caused by the thermal fatigue for stable and safe plant operation, precisely predicting complex heat transfer characteristics, and estimating the local temperature and heat flux of the superheater are necessary. In this study, a computational fluid dynamics model of the boiler and the second and third superheaters in a coal-fired power plant is developed using radiation model (DO model) and turbulence models (k-ε RNG model). In the second superheater installed near the burner, the influence of radiation from the combustion gas is strong, and the temperature and heat flux of the outermost tube are highest. In the third superheater, the tube wall temperature and heat flux becomes large in the upper and middle parts of the superheater, where the combustion gas flow velocity around the superheater is high, whereas those at the bottom of the superheater are significantly small due to the dead water region of the gas. Therefore, to consider the combustion gas flow is important for prediction of the heat transfer characteristics of the third superheater.

Numerical study on inter-wrapper flow and heat transfer characteristics in liquid metal-cooled fast reactors

Inter-wrapper flow (IWF) is a typical thermal hydraulic phenomenon that exists in narrow gaps among the adjacent assemblies of a liquid metal fast reactor (LMFR) that can dissipate heat from assemblies and transfer heat among adjacent assemblies. Moreover, when blocked or reduced flow accidents occur, the IWF has a significant effect on reactor safety, especially reducing the peak cladding temperature. In this study, a computational fluid dynamics (CFD) model of the Karlsruhe liquid metal laboratory IWF experimental setup (called MYRRHA design) is built. The model contains three fuel assemblies with seven rods using lead–bismuth eutectic as working fluid. The new mesh generation method, correlation of turbulence model, and fluid–solid coupling heat transfer were considered in the CFD model. The established model was initially validated, and the simulation results were found to be in good agreement with experimental data. The influence of IWF on the thermal hydraulic characteristics within the rod bundle was revealed. The results indicated that the IWF rate can affect the heat transfer capability of the bundle under. The amount of heat removed increased by up to 10% with the IWF rate in a single bundle. The occurrence of IWF reduces the wall surface temperature and appropriately balances the temperature of adjacent bundles; however, it cannot effectively reduce the peak temperature of the cladding. The complex heat transfer characteristics and flow mechanism under the influence of IWF and wire mixing were revealed. The work presented in this paper represents an important step toward the IWF research on blocked flow scenarios.

Experimental validation of a continuum model for local heat transfer in shell-and-tube moving-bed heat exchangers

• Local heat transfer coefficients in shell-and-tube moving bed heat exchangers were simulated. • Local heat transfer coefficients were measured using a small heat exchanger mockup. • Experimental results were used to validate a comprehensive thermal model. • Simulation results were in good agreement with the experiments. Moving-bed heat exchangers with a horizontal tube arrangement offer a promising solution to exchange high-temperature heat with a particle stream. However, in order to guide the design of such heat exchangers, accurate and time-efficient simulation tools capable of capturing the complex heat transfer characteristics still need to be improved. In this article we present experimental results of the local heat transfer coefficients around the tube circumference of a heat exchanger mockup. To this end, specially prepared measuring probes were developed and a test campaign was conducted. A comprehensive thermal model was applied to the setup under consideration. This model takes into account the temperature-dependent thermophysical properties of the bulk, variable temperature boundary conditions, and the increased thermal resistance caused by the stagnation and void zones. To validate the model, calculated and experimental results were compared. It turns out that the predicted heat transfer coefficients well match the experimental data, albeit with slight deviations at the stagnation and void zones of the tubes. The prediction of a tube’s area-weighted heat transfer coefficient, which constitutes the most important factor for design purposes, yields results with errors of less than 3%. Therefore, it is concluded that the proposed thermal model is able to well predicts the heat transfer in shell-and-tube moving-bed heat exchangers.

Numerical simulation of the thermal radiation and convective heat transfer characteristics of the second superheater in a coal-fired thermal power plant boiler

Superheaters are critical components in a coal-fired power plant because they sustain the highest tube wall (metal) temperature point in the boiler. The pressurized steam flowing in superheater tubes is mainly heated via the thermal radiation and convection from the combustion gas in the radiant zone. To prevent the bursting of superheater tubes caused by the high-temperature creep, corrosion, and thermal fatigue for proper plant operation and maintenance, accurately predicting complex heat transfer characteristics, estimating the local temperature, and assessing the heat flux of the superheater are essential. In this study, a computational fluid dynamics model of the boiler and the superheater in a coal-fired power plant was developed using radiation and turbulence models. The local metal temperature and heat flux of the superheater were evaluated by calculating the heat exchange between the combustion gas and pressurized steam flows in the tubes. The calculated values of the steam outlet temperature accurately matched the values measured using the equipment at the plant, and thermal radiation was confirmed to be dominant in the boiler. The metal temperature and heat flux of the superheater at the outermost heat transfer tubes, which receive the maximum thermal radiation, were larger than those of the superheater at the inner tubes. Therefore, the outermost tubes were determined to be under the most severe thermal conditions despite having a higher mass flow rate of steam. Additionally, the heat fluxes on the center lines of superheater tube panels were higher due to the large gap between the adjacent heat transfer tubes at the bent part.

Experimental investigation and force analysis of flat-plate type pulsating heat pipes having ternary mixtures

Comprehensive experiments are conducted to analyze complex heat transfer characteristics and flow nature of a flat plate closed loop pulsating heat pipe charged with ternary mixtures. Consecutive channels of the heat pipe have different cross sections. Fluids forming the mixtures are deionized water (W), methanol (M) and pentane (P). Effects of different volume mixing ratios of these fluids (W:M:P = 1:1:1, 1:2:3 and 1:3:2) on thermal performance are evaluated comparatively with pure counterparts. Experimental range also covers other key parameters: filling ratio (30%, 50% and 70%), inclination angle (0° and 90°) and heat input. Flow patterns and bubble-liquid dynamics are examined. Results show that medium filling ratio of 50% presents optimum performance for all mixing ratios. At vertical orientation, ternary mixtures, especially the ones having higher content of pentane or methanol (1:2:3 and 1:3:2), show significantly better thermal characteristics compared to pure fluids. However, heat pipes charged with ternary mixtures are highly dependent on gravity; such that flat plate closed loop pulsating heat pipe having mixtures cannot operate in horizontal position. In usage of ternary mixtures, liquid droplets flowing down through channels are observed; this is called as rewetting phenomenon enhancing thermal performance. Force analysis characterizing thermo-fluid-geometric coupling is conducted.

Heat transfer performance of supercritical pressure CO2 in a non-uniformly heated horizontal tube

The convective heat transfer performance of supercritical pressure CO2 in a horizontal tube under three wall heat flux conditions was numerically investigated. More complex heat transfer characteristics under non-uniform heating conditions than that with uniform heat flux were observed. The more non-uniform the heat flux is in the circumferential direction, the worse heat transfer performance the CO2 has under most circumstances, except for relatively small or extremely high Reynolds number Re conditions. The heat transfer in most regions of the horizontal tube is enhanced, and the bulk heat transfer performance depends on the heat transfer in the top region of the tube mainly. For one-side heating conditions, it is better to place the heat source as closer as possible to the bottom of the tube for larger heat transfer coefficient, especially under relatively high heat flux conditions. When other conditions are the same, improving the Re shrinks the discrepancies of the heat transfer performance among tubes with different sides heated, which also indicates the weakening of the buoyancy effect. The ratio of the secondary flow number to the Reynolds number Se/Re could give good predictions for the buoyancy effect on the heat transfer performance in horizontal tubes with non-uniform heat flux.

Numerical simulation of radiation heat transfer characteristics in a cylindrical fluidized bed

In this paper, a 3D two-phase simulation involving the complex heat transfer characteristics within a vertical cylindrical fluidized bed was conducted numerically. The accuracy of mathematical modeling has been validated by comparing with the experimental results. The discrete ordinate model (DOM) was employed to investigate the radiation heat transfer in a fluidized bed. The user-defined function (UDF) was compiled by considering the effect of the particle flow emissivity on radiation heat transfer, and coupled the two-phase flow field by using the gray and non-gray model respectively. The results show that the non-gray model can better depict the selected absorption of radiation wavelength in two-phase flow heat transfer than gray model. Furthermore, under high heat flux density conditions, the proportion of radiation heat transfer increases gradually with the increase of heat flux, which can reach about 40% of the total heat transfer coefficient. Meanwhile, other factors about radiation heat transfer such as initial solid packing height, inlet gas velocity, and other conditions were studied and consistent with the thermal experimental regularity. The findings demonstrated that the radiation heat transfer and it’s modeling play an important role in two-phase heat transfer of fluidized bed.

Convective heat transfer characteristics of microalgae slurries in a circular tube flow

Continuous hydrothermal pretreatment in tubular reactors is a key process for the production of biofuel from microalgae biomass. Complex heat transfer characteristics of microalgae slurries, which consist of micron-sized cells suspended in water, are a challenge for the industrialization of hydrothermal pretreatment systems. In this study, the convective heat transfer characteristics of microalgae slurries flowing through a uniformly heated circular tube were experimentally investigated for the first time. The results revealed that the microalgae cell concentration and cell motion in the solid–liquid two-phase flow were the two main factors affecting the convective heat transfer characteristics of microalgae slurries in laminar tube flow. Considering the effects of the cell concentration, cell motion, viscosity variation with temperature and shear rate, and Peclet number, a novel correlation was proposed to predict the Nusselt number of microalgae slurries in laminar tube flow. In addition, a heat transfer deterioration phenomenon was observed under forced convection subcooled boiling conditions. This study provides guidance for the design and optimization of tubular heat exchangers in continuous hydrothermal pretreatment systems.

Numerical investigation of local thermal non-equilibrium effects in coal porous media with cryogenic nitrogen injection

The technology of fire prevention and extinguishing using liquid nitrogen or cryogenic nitrogen has been widely applied in underground coal fire. In this paper, a multi-scale model has been proposed to investigate the complex heat transfer characteristics in coal porous media (CPM) with cryogenic nitrogen injection. In the model, the fluid energy equation at representative elementary volume (REV) scale is coupled with the heat conduction equation of a solid particle at pore scale considering the temperature variation in the coal particle, and the equivalent diameter is employed to simplify the heat conduction process in the solid particle. The experiment has been carried out and the result is consistent with the simulation obtained by the multi-scale model. The influences of equivalent diameter, mass flow rate, inlet fluid temperature and initial temperature on the local thermal non-equilibrium (LTNE) effects have been investigated numerically by using the multi-scale model. The results show that the time to reach local thermal equilibrium (LTE) between fluid and solid center is longer due to the small thermal conductivity of coal matrix, and it is affected dramatically by equivalent diameter and mass flow rate. The results also indicate that the inlet fluid temperature and initial temperature of CPM have important influence on the temperature difference between solid center and fluid.

Development of a thermal model for a hybrid photovoltaic module and phase change materials storage integrated in buildings

The performance of building integrated photovoltaic modules (PV) situated outdoors suffers from attained high temperatures due to irradiation as a negative temperature coefficient of their efficiency. Phase change materials (PCMs) are investigated as an option to manage the thermal regulation of photovoltaic modules and, hence, enhance their electrical efficiency. In this study a transient one-dimensional energy balance model has been developed to investigate the thermal performance of a photovoltaic module integrated with PCM storage system. Possible all heat transfer mechanisms are described to have a basic and step by step fundamental knowledge to analyze and understand the complex heat transfer characteristics of the PV-PCM system. Finite difference scheme is applied to discretize the energy balance equation while fully implicit scheme is applied to discretize the heat balance in the PCM module. Three different PCM of different melting temperatures were investigated. The numerical result is validated with experimental studies from the literature. The result indicates that PCM are shown to be an effective means of limiting the temperature rise in the PV devices thus increasing the thermal performance up to 5%.

베인과 블레이드 사이의 상대위치 변화에 따른 터빈 블레이드 표면에서의 열/물질전달 특성

The present study investigated the effect of relative position of the blade on blade surface heat transfer. The experiments were conducted in a low speed wind tunnel with a stationary annular turbine cascade. The test section has a single turbine stage composed of sixteen guide vanes and blades. The chord length of the blade is 150 mm and the mean tip clearance of the blade is of the blade chord. The Reynolds number based on blade inlet velocity and chord length is and mean turbulence intensity is about . To investigate the effect of relative position of blade, the blade at six different positions in a pitch was examined. For the detailed mass transfer measurements, a naphthalene sublimation technique was used. In general, complex heat transfer characteristics are observed on the blade surface due to various flow characteristics, such as a laminar flow separation, relaminarization, flow acceleration, transition to turbulence and tip leakage vortices. The results show that the blade relative position affects those heat transfer characteristics because the distributions of incoming flow velocity and turbulence intensity are changed. Especially, the heat transfer pattern on the near-tip region is significantly affected by the relative position of the blade because the effect of tip leakage vortex is strongly dependent on the blade position. On the pressure side, the effect of blade position is not so significant as on the suction side surface although the position and the size of the separation bubble are changed.

Three-dimensional numerical simulation of an inclined jet with cross-flow

Three-dimensional numerical simulation of fluid flow and heat transfer characteristics for an inclined jet with cross-flow impinging on a heating plate is presented. The turbulent governing equations are solved by a control-volume-based finite-difference method with power-law scheme and the well-known k– ε turbulence model and its associate wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE method. In addition, body fitted curvilinear coordinates system is employed. The parameters studied include the same inlet temperature of cross-flow and inclined jet ( T in = 30 and 40 °C), the inclined impinging jet to cross-flow velocity ratio (VR = 3–7), the heat flux ( q w ″ = 340 – 1000 W/m 2), the Reynolds number of the cross-flow and the angle between the inclined jet and the bottom surface are fixed at Re = 5000 and θ = 45°. The theoretical model developed is validated by comparing the predictions with available experimental data. The generation of a pair of counter-rotating longitudinal vortices is clearly observed from the computations. Heat transfer enhancement can be attained over wider span-wise region as the VR value is increased. The analysis provides a fundamental insight into the complex heat transfer characteristics for an inclined jet with cross-flow.

환형 캐스케이드 내 고정된 터빈 블레이드 및 슈라우드에서의 열/물질전달 특성 (I) - 블레이드 끝단 인접 표면 -

For the extensive investigation of local heat/mass transfer on the near-tip surface of turbine blade, experiments were conducted in a low speed stationary annular cascade. The turbine test section has a single stage composed of sixteen guide vanes and blades. The chord length and the height of the tested blade are 150 nun and about 125 mm, respectively. The blade has flat tip geometry and the mean tip clearance is about 2.5% of the blade chord. Detailed mass transfer coefficient on the blade near-tip surface was obtained using a naphthalene sublimation technique. The inlet flow Reynolds number based on chord length and incoming flow velocity is changed ftom l.OxlOs to 2.3xlOs. Extremely complex heat transfer characteristics are observed on the blade surface due to complicated flow patterns, such as flow acceleration, laminarization, transition, separation bubble and tip leakage flow. Especially, the suction side surface of the blade has higher heat/mass transfer coefficients and more complex distribution than the pressure side surface, which is related to the leakage flow. For all the tested Reynolds numbers, the heat/mass transfer characteristics on the turbine blade are the similar. The overaIl averaged She values are proportional to Reeo.s on the stagnarion region 뻐 d the laminar flow region such as the pressure side surface. However, since the flow is fully turbulent in the near-tip region, the heat/mass transfer coefficients are proportional to Rec 0.8.

Local Heat/Mass Transfer Characteristics on a Rotating Blade With Flat Tip in Low-Speed Annular Cascade—Part I: Near-Tip Surface

The present study focuses on local heat/mass transfer characteristics on the near-tip region of a rotating blade. To investigate the local heat/mass transfer on the near-tip surface of the rotating turbine blade, detailed measurements of time-averaged mass transfer coefficients on the blade surfaces were conducted using a naphthalene sublimation technique. A low speed wind tunnel with a single stage annular turbine cascade was used. The turbine stage is composed of sixteen guide plates and blades with spacing of 34 mm, and the chord length of the blade is 150 mm. The mean tip clearance is about 2.5% of the blade chord. The tested Reynolds number based on inlet flow velocity and blade chord is 1.5×105 and the rotational speed of blade is 255.8 rpm for the design condition. The result at the design condition was compared with the results for the stationary blade to clarify the rotational effect, and the effects of incoming flow incidence angle were examined for incidence angles ranging from −15 to +7deg. The off-design test condition is obtained by changing the rotational speed maintaining a fixed incoming flow velocity. Complex heat transfer characteristics are observed on the blade surface due to the complicated flow patterns, such as flow acceleration, laminarization, transition, separation bubble and tip leakage flow. The blade rotation causes an increase of the incoming flow turbulence intensity and a reduction of the tip gap flow. At off-design conditions, the heat transfer on the turbine rotor changes significantly due to the flow acceleration/deceleration and the incoming flow angle variation.