Heat Transfer Resistance Research Articles

Numerical investigation on heat transfer and vapour layer geometry of a droplet impacting on a spherical particle in Leidenfrost regime

Understanding the heat transfer mechanism and dynamic behaviors of droplet-particle collision in Leidenfrost regime is essential for the pyrohydrolysis process of pickling liquor, which is sprayed into a fluidized bed and then decomposes to oxides and acid vapour. In this study, the droplet deformation, heat transfer characteristics and vapour layer geometry of Leidenfrost droplet impacting spherical particle are investigated by constructing a microscale phase change model in VOF framework. The applicability of the computational model in capturing droplet dynamics and thermodynamics is verified by comparing it with experimental outcomes. The results indicate that the vapour layer geometry is relatively uniform in spreading but shows instability in recoiling. The vapour layer thickness increases rapidly in both spreading and early recoiling, while decreases in the late recoiling phase. Furthermore, the heat transfer and droplet evaporation depend on the vapour layer thickness strongly. Specifically, the thinner the vapour layer, the lower the heat transfer resistance. The increasing surface temperature, We and Re can effectively enhance heat transfer and raise droplet temperature. In addition, the maximum spreading factor and liquid–solid contact time play an important role in heat transfer and they both show a clear power-law dependency on We and Re for viscous droplets. The effect of droplet size on contact time is consistent with first-order vibration theory but the contact time is independent of surface temperature and impact velocity.

Study of Influences of the Level of Zr Oxidation and Volumetric Heat Generation on Heat Transfer and Crust Formation Within CANDU Corium

A full-station blackout at a nuclear power plant can lead to fuel failures and radiological release to the environment if there is a breach of the reactor vessel. For Canada Deuterium Uranium (CANDU) reactors, the expected failure mechanism is through thermal stress concentration at the calandria vessel wall, and is thus influenced by local heat flux values. The current study uses computational fluid dynamics to simulate heat transfer, fluid flow, and crust formation within a CANDU geometry. Sensitivity to critical parameters, including the volumetric decay heat generation rate and the percentage of Zr oxidation is explored. The results show that as the volumetric heat generation rate decreases, the crust is thicker, and the wall heat flux is lower. This suggests that the activation of mitigating measures that delay the accident progression result in more favorable outcomes. The percentage of Zr oxidation primarily influences the thermal conductivity, which impacts the crust formation and wall heat flux rates. Specifically, corium with a lower percentage of Zr oxidation has higher thermal conductivity, and thus lower heat transfer resistance. This results in lower corium temperatures, which reduces the radiation heat transfer from the top surface and also increases crust thickness. Higher rates of heat removal from the vessel wall thus occur. In contrast, a higher percentage of Zr oxidation results in lower thermal conductivity, which leads to lower wall heat flux and a thinner crust at the vessel wall. Overall, these findings highlight the importance of considering the effects of sensitivity parameters on the heat flux distribution in the event of severe accidents.

Heat transfer and thermal resistance analysis under various heat transfer fluid flow rates based on a medium-temperature pilot-scale latent heat storage system

Assessments often rely on experimentally observed factors when examining heat transfer variations due to different heat transfer fluid (HTF) flow rates. To better comprehend limiting factors and guide operational parameter design for shell-and-tube latent heat system (LHS) systems, a quantitative analysis of thermal resistance hindering heat transfer is crucial. Hence, this article conducted experimental evaluations at various flow rates to critically analyze its thermal performance based on a large-capacity (1000 MJ) medium-temperature LHS system loaded with 4130 kg commercial nitrate mixture (peak melting temperature: 219.90 °C). The heat transfer coefficient and thermal resistance during charging and discharging were quantitatively investigated to identify the main factors influencing heat transfer and dominant mechanisms. The results suggested that augmenting the charging flow rate from 5 to 15 L/min resulted in 2.65 times increase in average power (from 24.01 to 63.64 kW). Besides, during charging, the overall thermal resistance primarily came from the phase change material (PCM) side initially. As PCM melted, the thermal resistance on the PCM side became equivalent to that on the HTF side. Increasing the charging flow rate enhanced heat transfer through two mechanisms: reducing the thermal resistance on the HTF side and increasing the driving force of heat transfer by elevating the average HTF temperature. During discharging, the overall thermal resistance was predominantly derived from the PCM side after solidification. Increasing the discharging flow rate caused a limited influence on decreasing thermal resistance. Enhancing the discharging flow rate only bolstered the driving force for heat transfer by elevating the average HTF temperature. This study provides valuable insights into the factors influencing heat transfer in LHS systems and contributes to optimizing LHS technology.

Parametric analysis on thermal-hydraulic characteristics in variable-direction twisted-oval tube bundle in cross-flow

In this paper, numerical investigations on heat transfer and flow resistance characteristics of a variable-direction twisted-oval tube (VDTOT) were performed. The effects of different geometric parameters (transverse and longitudinal tube pitches, ratio of major to minor axis, twist angle and pitch) on the heat transfer coefficient, pressure drop and heat transfer capacity per pump work were investigated. Contribution ratios of different factors were also analyzed. The results show that the transverse tube pitch and twist angle have a great impact on the overall performance of the tube bundle. VDTOT realizes the synergistic enhancement of flow and heat transfer performance. Compared to the circular tubes, the heat transfer capacity of VDTOT bundles is increased by 13.6%–20.6 % under the same pump power consumption. The average deviations of Nu and Eu correlations fitted by the multiple regression method are 0.99 % and 1.64 %, which is helpful for the rapid design and optimization of VDTOTs.

Study on the convective heat transfer characteristics of supercritical CO2 in mini-channels under unilateral heating conditions for application in a compact solar receiver

The mini-channel in compact solar receiver deliver the high performance in both heat transfer (HT) and pressure resistance. However, the severe variation of thermophysical properties of supercritical CO2 (S-CO2) coupling with the complex nonuniform distribution of solar flux leads to the abnormal flow and heat transfer (FHT) performance of S-CO2 in mini-channels. The present work numerically investigated the detailed FHT of S-CO2 in the horizontal mini-channel with one-side heating. The impacts of buoyancy and flow acceleration on FHT of S-CO2 are demonstrated in detail. The results show that the buoyancy effects can lead to the enhancement on circumferentially average heat transfer performance of S-CO2 while the local deterioration at the top. In contrast, the thermal acceleration effects can worsen the S-CO2 HT. The buoyancy and thermal acceleration effects can be ignored when Ri ≤ 2.7 × 10−3 and Ac ≤ 7.8 × 10−8, respectively. The empirical formula for S-CO2 HT in the mini-channel under single-side heating conditions is developed, with a maximum error of 4.5%.

Design errors during major repairs of apartment buildings

The main design errors during the overhaul of the heating system of apartment buildings on the example of a 5-storey building made of expanded clay concrete panels are considered and analyzed. A comparative analysis of the values of the heat transfer resistance of external walls obtained by calculation and based on the results of experimental data is performed. Based on the obtained values of the heat transfer resistance of the exterior walls, the calculation of the heat losses of the building was performed. As a result of the identified shortcomings, the amount of thermal energy that the residents of the surveyed house located on the territory of the city of Samara did not receive was calculated.

Study on heat transfer and flow resistance characteristics of cracking furnace tube by axial width of opening combination rotor blade

Based on the passive enhanced heat transfer mechanism, this paper proposes the new opening combination rotor blade, which aims to improve heat and mass transfer performance by changing internal structure of cracking furnace tube, thus achieving efficient production of ethylene. Through mathematical modeling and simulation of heat transfer properties of opening combination rotor blade, heat transfer performance and fluid flow characteristics of ordinary rotor blade tubes and opening combination rotor blade with axial widths of W (W = 0.25 R, 0.5 R, 0.75 R) are analyzed using field synergy principle and comprehensive heat transfer evaluation coefficient (PEC).The results demonstrate that the Nusselt number of opening combination rotor blade is 19%-23% lower than that of ordinary rotor blade. The resistance coefficient of opening combination rotor blade with axial width of 0.25 R is significantly lower by 33%-43%. Opening combination rotor blade with axial width of 0.25 R has a significantly lower resistance coefficient of 72%-78%. Its PEC value is between 1.23–1.28 with significantly improved overall heat transfer performance. In summary, opening combination rotor blade with axial width of 0.25 R has the most significant heat transfer effect and has a high reference significance for engineering design.

Experimental study on heat transfer and flow resistance characteristics of integral rolled spiral finned tube bundles heat exchangers

The integral rolled spiral finned tube (IRSFT) represents a novel variant of low carbon steel heat exchanger components. The unique feature of IRSFT is the integral structure of its fins and base tube, with the fin section exhibiting an isosceles trapezoidal shape, thereby enhancing its heat transfer performance. However, the dearth of comprehensive studies on the heat transfer and flow resistance characteristics of IRSFT has impeded its widespread application in engineering projects. To address this gap, this study undertakes an experimental investigation into the heat transfer and flow resistance properties of IRSFT bundles. The findings reveal that the Nusselt number (Nu) escalates with an increase in both transverse and longitudinal pitch. Furthermore, Nu initially increases and subsequently decreases with a rise in fin pitch and fin height. Concurrently, the Euler number (Eu) diminishes with an increase in fin pitch, fin height, transverse pitch, and longitudinal pitch. Within the parameters of this experimental study, IRSFT demonstrated superior heat transfer performance and reduced flow resistance compared to serrated spiral finned tube and spiral finned tubes. This study also proposes a novel experimental correlation for predicting Nu and Eu for IRSFT bundles, thereby providing a theoretical foundation for the industrial application of IRSFT.

Solar adsorption refrigeration system: Comparison between equilibrium, universal and transient model

Three models of different level of complexity are detailed in order to predict the cold production of a solar cooling device. The transient model consists on a heat and mass transfer model of the absorber, coupled with a solar collector model which conceives the ambient temperature as variable along the day and comprehends the evolution of the solar radiation throughout the day according the locations of the prototype to be analyzed in addition to the ideal models of the condenser and evaporator. The Equilibrium and Universal Model are both lumped models in which the spacial distributions of the variables and the heat and mass transfer resistance in the system are disregarded. The advantages and disadvantages of each model are discussed.The results of the models are compared with experimental data obtained from four methanol/activated-carbon adsorption refrigerator prototypes. The cold generated is underestimated 12%–20% by the transient model, while it is overestimated 22%–46% by the equilibrium model and 53%–66% by the universal model. Even when the transient model allows some improvements, is a useful tool to analyze the impact that different variables of a solar adsorption refrigeration system have in the generation of cold. The simpler models can be used for preliminary design and feasibility studies of solar adsorption refrigeration devices (SARS) whose results can be extrapolated on the premise of its overestimation of cold generation approximately given by this work.

Synthesis of vertically aligned and interconnected BN fiber skeleton for significantly improving thermal conductivity of PVDF

Seeking efficient thermal management materials for electronic components is urgent and important. BN has high thermal conductivity and good insulating properties, making it an ideal polymer filler for improving thermal conductivity. Currently, because of the weak interaction between BN and its high melting point, it is extremely difficult to construct a BN three-dimensional skeleton by direct sintering to achieve efficient heat transfer channels. In this work, vertically aligned BN three-dimensional skeletons with fiber walls were synthesized in one step by combining the in-situ conversion of precursor fibers and directional freeze-drying technology. The precursor fibers were expelled by the directionally grown ice crystals, forming vertically aligned fiber walls. Importantly, molten B2O3 produced during air calcination made the fibers bond to each other, thus obtaining the vertically aligned and interconnected BN fiber skeleton. Benefiting from the short-path fast heat transfer and low interfacial thermal resistance of the fiber walls, the obtained vertically aligned BN skeleton exhibited obvious structural advantages, resulting in composites with a unidirectional thermal conductivity of 1.372 W/(m·K) with the filling content of 25.7 wt% and high enhancement efficiency per unit volume of 32.6 %. This approach is expected to open a new avenue for developing highly thermally conductive composites with great potential.

Numerical study on the heat transfer characteristics of hydrocarbon fuel in the cooling channel filled with porous media

Regenerative cooling channel is only heated by a single wall in practical applications, resulting in serious thermal stratification. Under the action of porous media, the wall temperature decreases, which avoids the heat transfer deterioration caused by velocity distortion near the high temperature wall. Compared with the cooling channel without porous media, the porous media improves the homogeneity of the coolant heat sink, and the physical and chemical heat sink non-uniformity coefficient at the outlet is close to 0 with the change of aspect ratio. The increase of the height of the cooling channel increases the heat transfer distance in the height direction, which increases the heat transfer resistance of the heat flux and makes the uniformity of the heat sink worse. The positive feedback between the coolant flow rate and the non-uniform heat flux on the parallel filled channel wall can be effectively suppressed. The porous media greatly reduces the radial transfer shortens the axial distance between the upper and lower walls maximum temperature difference, and reduces the thermal stress of channel during the cooling process.

Effect of Hole Geometry Shape in Vortex Generators on Fluid Output Temperature: Computational Fluids Dynamics Validation

<p>Several methods to enhance heat transfer can be classified into three categories: active, passive, or hybrid. Among these methods, Vortex Generators (VGs) are one passive heat transfer enhancement device widely used in heat exchangers. This study aims to explore the geometric shapes of VGs equipped with longitudinal holes and examine their influence on the outlet temperature of the fluid. For the analysis in this research, a three-dimensional Computational Fluid Dynamics (CFD) simulation using ANSYS Fluent software was employed. The increased heat transfer and flow resistance in the VG geometry were evaluated based on previous research for validation. The study results demonstrate that the simulation produces fluid outlet temperature values and velocity contours that closely resemble the results obtained from the reference study. The validation error of this research was found to be only 0.02%, indicating high-quality and accurate simulation results. Furthermore, the study compared various geometries of the VG holes in the system. Among these geometries, hexagonal-shaped VG holes exhibited high-velocity contours on the VG side while achieving the lowest fluid outlet temperature at approximately 303.53 K. The findings of this study serve as a basis for further developments in enhancing the efficiency and performance of heat exchangers using VGs.</p>

Tech Note. Analysis of practices of layer-by-layer use of expanded polystyrene in the operation of residential buildings

The purpose of this paper is to consider three aspects of the use of polystyrene as an effective heat-insulating material in three-layer load-bearing and enclosing structures of reinforced concrete residential buildings. The first aspect is related to the dependence of the enclosure heat transfer resistance degree on the method of combining expanded polystyrene with stone structural materials. For example, prefabricated expanded polystyrene concrete blocks for the construction of walls of low and medium-rise buildings and cast-in-situ expanded polystyrene concrete poured into the voids of the well brickwork in high-rise buildings. It is shown that the layer-by-layer application of pure expanded polystyrene and the concrete layers protecting it multiplies the degree of thermal insulation of the enclosing structure while retaining its strength and equal consumption of raw materials. The second aspect is associated with the fire resistance of reinforced concrete load-bearing and enclosing structures of residential buildings. A sufficient degree of fire resistance when protecting expanded polystyrene with cast-in-situ reinforced concrete layers is shown using examples. The third aspect shows the feasibility of using outer three-layer reinforced concrete walls with polystyrene insulation for panel heating of the buildings.

Derivation of a new temperature calculation formulation for heavily fire insulated steel cross-sections

Equations are given in the Eurocode standard EN 1993-1-2 on the estimation of temperature in insulated steel cross sections when assuming uniform steel section temperature, sometimes called a lumped heat capacity assumption. When deriving these equations the temperature of the exposed insulation surface is assumed equal the surrounding fire gas temperature. That is a simplification that may provide inaccurate results for heavily insulated steel sections, when the heat capacity of the insulation is considerably higher than that of the steel section. Therefore, a new lumped mass formulation suited for heavily insulated steel sections is proposed in this paper. It accounts for the heat transfer resistance between the fire and the insulation surface as well as for the heat capacity of the insulation. The accuracy of the predictions made with the proposed new formulation are compared with results of accurate 1-D finite element numerical analyses considering insulation materials with several combinations of thermal properties and thicknesses. Analyses with the proposed formulation is shown to yield accurate and generally on the safe side estimations of steel temperature in comparison to the finite element (FE) calculations. The new formulation is applicable to lightly as well as heavily insulated steel sections.

Rational design process for gas turbine exhaust to supercritical CO2 waste heat recovery heat exchanger using topology optimization

Advances in additive manufacturing (AM) technologies and topology optimization methodologies are enabling sophisticated novel designs for heat exchanger performance. These tools have been demonstrated for development of high-performance heat sinks considering local or component-level performance factors (e.g., heat transfer per volume). To leverage such capabilities in larger-scale energy systems, structured design methodologies are needed that consider system-level factors, such as production cost, cycle-level efficiency, and operational constraints. This study seeks to develop and assess a rational approach for designing thermo-economically optimal heat exchangers for such applications. The methodology is illustrated through development of the Primary Heat Exchanger (PHX) for a supercritical carbon dioxide (sCO2) power cycle recovering exhaust heat from a 6 MW-scale natural gas turbine. The proposed approach begins with a detailed thermodynamic cycle model, which is then extended to account for techno-economics. Next, an optimal PHX heat transfer capacity target is identified, and a high-level geometry is selected based on operating characteristics. This geometry is then divided into repeating 2D prismatic unit cells, for which topology optimization is applied to identify high-performance heat transfer geometries. A key aspect of this process is that the unit cell geometries are optimized using the total PHX mass as the objective function, which represents a surrogate for production cost. This leads to distinct designs compared with approaches that optimize local heat transfer and flow resistance factors. A second topology-optimized design is developed using a representative local thermal-fluid performance objective function and is found to require 1.6× the mass of the design generated with the system-level techno-economic objective for the same unit cell size. Conventional-type PHX designs with simple longitudinally finned tubes are developed for comparison, and are found to require total masses 1.5× or greater than the design obtained with the proposed process. Integrating this approach with detailed additive manufacturing costing models and experimentally validated fabrication constraints can yield a streamlined workflow for HX design for future energy systems.

Initial Multidisciplinary Study of Oxidized Chromium-Coated Zirconium Alloy for Fuel Cladding of SCW-SMR Concept: Weight-Gain and Thermal Conductivity Measurements and Coating Cost Evaluation

One of the challenges of small modular reactors (SMRs) in comparison with large reactors is the greater difficulty in achieving high burnups in smaller cores. With greater neutron leakage through the periphery, a key factor is the neutron economy of the fuel cladding. However, all large supercritical water-cooled reactor (SCWR) concepts have employed neutron-absorbing stainless steels and nickel-based alloys in order to meet all the requirements in terms of corrosion and thermalhydraulics. In order to achieve higher burnups and extend the time between refueling in a SCW-SMR, the use of chromium-coated zirconium alloy as a potential fuel cladding candidate has been explored. Chromium coatings up to a few micrometers thick have shown improved oxidation resistance of zirconium-based claddings under operating conditions relevant to SCWR concepts. In this study, Zr-2.5Nb alloy (UNS R60904) from pressure tube samples was coated using a physical vapor-deposition (PVD) method. Oxidation tests were performed on coated samples at 500 °C and approximately 25 MPa in a refreshed autoclave. The effects of the oxide on heat transfer and hydraulic resistance are also discussed in this study. Last, but not least, this study evaluates the coating cost of the fuel cladding with chromium in a vacuum plasma spray process.

REDUCED HEAT LOSS THROUGH WINDOW STRUCTURES

The results of theoretical and experimental studies of heat transfer through the window structures of the building are presented, the thermal characteristics of the windows are established and the level of heat loss realized through them is estimated. To determine the heat transfer resistance of two-chamber double-glazed units, numerical modeling of radiative-convective heat transfer is performed, taking into account the thermal conductivity of the gas layer between the glasses. Natural free convection of the gaseous medium in the double-glazed chambers occurs as a result of the temperature difference between the side glass surfaces of the chamber. Modeling is performed by numerically solving the system of fluid dynamics and energy equations for the air layer and glass. On the inner surfaces of the chambers, conditions of the fourth kind are set, which take into account the radiation and conductive components of the total heat flow coming from the glass surface. The results of simulation of heat transfer through double-glazed windows with ordinary glass indicate that about 60 % of heat is transferred by radiation. Therefore, an effective measure to reduce heat loss through windows is to reduce the radiation component of the total heat flow by applying a low-emissivity coating to the inner surfaces of the glass unit. This makes it possible to reduce the total heat flow (and, accordingly, heat loss to the environment) by 20–34 %, depending on the number of glass surfaces with this coating. Conducted independent experimental studies confirm the modeling results. In order to comply with the current requirements of state building regulations and to achieve the energy efficiency class of type C buildings (specific heat consumption for heating 1 sq. m of usable area is less than 75 kWh), for the climatic conditions of Kyiv, it is recommended to use double-chamber windows with two low-emission coatings with the formula of the glass unit: 4M1-10-i4M1-10-i4M1. Bibl. 22, Fig. 8.

Numerical Investigation Into the Gas–Liquid Two-Phase Flow Regime and Heat Transfer Characteristics in a Gravity Heat Pipe

Abstract In order to reveal the gas–liquid distribution, heat and mass transfer characteristics inside a gravity heat pipe, the bubble behavior and the flow regime transition during the phase-change process were examined by employing a copper-water heat pipe, with the length of 500 mm and Φ22 × 1.5 mm. The results indicate that in the process of phase change, the typical flow regimes of bubble flow, slug flow, and churn flow can be observed in the evaporator, and the presence of bubbles has an obvious disturbance on the flow field. In addition, the discontinuous liquid film plays an important role in the heat transfer mechanism in the condenser, which allows the vapor to contact the wall directly and reduces the heat transfer resistance. The temperature difference between the evaporator and the condenser can be reduced by adjusting the saturation temperature, so as to effectively improve the heat transfer performance of the heat pipe and contribute to the practical engineering design.

UHPC-XPS insulation composite board reinforced by glass fiber mesh: Effect of structural design on the heat transfer, mechanical properties and impact resistance

Extruded foamed polystyrene (XPS) and ultra-high performance concrete (UHPC) are the ideal insulation and structural materials for preparing sandwich composite boards, respectively. However, how to design the multilayer structure of composite board to maximize thermal insulation, mechanical properties and impact resistance has not been clarified yet. In this study, styrene-acrylic emulsion was coated on the surface of XPS to improve its wettability, and glass fiber mesh-reinforced bonding mortar with vac-ethylene copolymer emulsion was used to improve the adhesive properties between UHPC and XPS layers. Then the contribution of each layer in heat-transfer, load-bearing, and impact resistance were investigated and simulated by finite element analysis. By separating XPS and UHPC into serval layers, the midspan displacement at peak load and fracture energy of composite boards increased by 689.4% and 39.1% respectively, then roles of each layer in composite board were discussed. The results provide an effective method to prepare high performance composite board by multilayer structural design, and then promote energy-saving and emission-reduction of prefabricated wall system and even building industrialization.

Grafted Alkene Chains: Triggers for Defeating Contact Thermal Resistance in Composite Elastomers.

The pursuit of enhancing the heat transfer performance of composite elastomers as the thermal interface materials (TIMs) is a compelling and timely endeavor, given the formidable challenges posed by interfacial thermal transport in the domains of energy science, electronic technology, etc. Despite the efficacy of phase change materials (PCMs) in enhancing composite elastomers' interfacial compatibility, thereby reducing contact thermal resistance for heat transfer improvement, their leakage post-transition has impeded the widespread adoption of this approach. Herein, a strategy is proposed for developing a solid-solid phase change composite elastomer by grafting alkene chains onto the crosslink network to eliminate the possibility of leakage. A series characterization suggest that the resulting material possesses a self-adjusting interfacial compatibility feature to help reduce contact thermal resistance for heat transfer facilitating. The investigations on adhesion strength and surface energy reveal that the presence of amorphous grafted alkane chains at the interface facilitates easier absorption onto the contacting solid surface, enhancing intermolecular interactions at the interface to promote across-boundary heat transfer. By integrating these findings with the thermal performance evaluation of composite elastomers using a real test vehicle, valuable insights are gained for the design of composite elastomers, establishing their suitability as TIMs in relevant fields.