Heat Transfer Dynamics Research Articles

Performance of the frosting characteristics of fin-and-tube heat exchanger subject to superhydrophobic coating

The condensation and freezing processes of fin-and-tube heat exchangers with a bare surface and coated with a superhydrophobic substance were experimentally studied for various air velocities, relative humidity (RH) values, and fin spacings. The performance of the fin-and-tube heat exchangers in terms of dynamic pressure drop and heat transfer rate were reported in details. The frosting characteristics of these exchangers were visually characterized microscopically. It is found that for a superhydrophobic exchanger with 1-mm fin spacings, water bridges were likely to form, causing droplets to stick between the fins instead of slipping off the surface. At a low RH and a maximum air velocity of 1.0 m s−1, the heat transfer rate of the superhydrophobic exchanger was 12% lower than that of the surface without superhydrophobic coating. For a fin spacing of 3.0 mm, the critical droplet size was smaller than the fin spacing, hence the droplets can easily slip or jump off the fins. At a velocity of 0.6 m s−1, the steady-state pressure drop at low humidity of the superhydrophobic exchanger was 14% lower than that of the bare surface because of the differences in frosting characteristics. The results of this study indicate that the adopted superhydrophobic coating can delay frosting at high humidity and air velocity.

Study on Flight Dynamics and Heat Transfer Solidification of Metal Droplets during Centrifugal Spray Deposition Forming Process

Centrifugal spray deposition forming technology, which is used in the preparation process of near-net-forming billets, not only reduces the macroscopic segregation and refines the microstructures of billets but also has the characteristics of a rapid solidification structure. The trajectory, velocity, heat transfer and solidification of metal droplets granulated by the centrifugal force during flight will affect the shape, precision and microstructure of the billet. Therefore, it is necessary to study the dynamics and thermal history of droplets in flight. In this study, a single droplet is taken as the object. Considering the resistance of ambient gas, Newton’s second law, classical nucleation theory, Newton’s cooling law and the energy conservation equation were used to establish a dynamic model and heat transfer solidification model of liquid metal droplets during flight. The influence of the centrifugal disc speed on the diameter of granulated droplets was analyzed. The variation law of droplet flight trajectory and velocity was explored. The supercooling degree in metal droplet nucleation was quantified, and the influence of droplet diameter, superheat and other factors on heat transfer and solidification was revealed. The results show that the numerical calculation results are basically consistent with the previous research results. The trajectory of the droplet is parabolic during flight. The initial velocity of the droplet, the environmental gas resistance and the convective heat transfer coefficient are positively correlated with the rotating speed of the centrifugal disc; however, the droplet diameter is negatively correlated with the rotating speed of the centrifugal disc. The super cooling degree at the time of droplet nucleation and the flight time required for solidification are negatively correlated with the droplet diameter. Among them, the droplet diameter has a linear relationship with the solidification start time and a quadratic curve relationship with the solidification end time. The effect of superheat on the heat transfer and solidification of droplets is not obvious. The conclusions obtained can provide a theoretical basis for the determination of the preparation process parameters.

Computational Heat Transfer and Fluid Dynamics

Modern advances in numerical methods have led to the rapid development of various fields of human activity, including microelectronics, mechanical engineering and medicine [...]

Comparative dynamics of mixed convection heat transfer under thermal radiation effect with porous medium flow over dual stretched surface

Due to enhanced heat transfer rate, the nanofluid and hybrid nanofluids have significant industrial uses. The principal objective of this exploration is to investigate how thermal radiation influences the velocity and temperature profile. A water-based rotational nanofluid flow with constant angular speed {Omega } is considered for this comparative study. A similarity conversion is applied to change the appearing equations into ODEs. Three different nanoparticles i.e., copper, aluminum, and titanium oxide are used to prepare different nanofluids for comparison. The numerical and graphical outputs are gained by employing the bvp-4c procedure in MATLAB. The results for different constraints are represented through graphs and tables. Higher heat transmission rate and minimized skin friction are noted for triple nanoparticle nanofluid. Skin coefficients in the x-direction and y-direction have reduced by 50% in trihybrid nanofluid by keeping mixed convection levels between the range 3 <upepsilon le 11. The heat transmission coefficient with raising the levels of thermal radiation between 0.5 < {uppi } le 0.9 and Prandlt number 7 < {text{Pr}} le 11 has shown a 60% increase.

Research on indoor dynamic temperature based on circulating water heating

During the heating season, cities in northeast China primarily emphasize the implementation of large-scale CHP central heating systems. However, the heat-electricity constraints associated with these heating units hinder the integration of clean energy sources such as wind power, photovoltaic, and other renewable energies. Despite efforts to reduce carbon emissions and address concerns related to urban haze formation, energy conservation, and emission reduction, coal remains the predominant energy source for heating.In light of these challenges, this paper aims to investigate the heat transfer characteristics of buildings during the heating season. It also examines the feasibility of implementing heating regulations and provides a theoretical foundation for establishing a regional heat model that incorporates low-carbon energy islands. The proposed model combines wind power, photovoltaic power generation, and other renewable energy sources supplemented with coal power.By employing the concentrated heat capacity method, this study develops a mathematical model to simulate the dynamic thermal process involved in heating buildings. The model encompasses various components, including the dynamic heat transfer model of the building’s envelope structure, which considers its heat storage characteristics, as well as the radiator model and the indoor air heat balance equation. Furthermore, the model comprehensively accounts for the influence of different indoor and outdoor disturbances on the heat transfer process, including the effects of solar radiation.To validate the model, a simulation program is implemented using Matlab. This program is capable of hourly calculations to determine the indoor temperature. By comparing the simulated temperature values with the measured ones, the rationality and accuracy of the model are verified.

Dynamic heat transfer through a hollow block wall in a naturally ventilated dwelling

Este estudio presenta una investigación experimental sobre la transferencia dinámica de calor a través de un muro exterior de bloques huecos de concreto en una vivienda sin aire acondicionado en un clima cálido-seco. El estudio busca determinar si la parte sólida del bloque hueco, que se considera un puente térmico en este tipo de sistema constructivo, es o no el camino principal para la transferencia de calor en condiciones climáticas cálidas y secas. Los resultados muestran que la proporción entre el calor transferido por la trayectoria de la cavidad y el correspondiente a la trayctoria sólida varía a lo largo del día. Durante la noche, por la trayectoria dentro de la cavidad se transfiere casi el mismo calor que por la trayectoria de la parte sólida, y durante el di?a, por la trayectoria dentro de la cavidad se transfiere menos calor que por la trayectoria de la parte sólida, mientras que por lapsos cortos esta última proporción se invierte. Por lo tanto, se concluye que el camino sólido de un bloque hueco no puede considerarse automáticamente un puente térmico, ni el camino en la cavidad como la parte de baja transferencia de calor, debido a que esta proporción varía a lo largo del día. Este trabajo proporciona un análisis dinámico de la transferencia de calor a través de un material ampliamente utilizado en viviendas de bajos ingresos en México para una mejor comprensión del fenómeno.

Application of a Distributed Element Roughness Model to Additively Manufactured Internal Cooling Channels

Abstract Design for cooling effectiveness in turbine blades relies on accurate models for dynamic losses and heat transfer of internal cooling passages. Metal additive manufacturing (AM) has expanded the design space for these configurations, but can give rise to large-scale roughness features. The range of roughness length scales in these systems makes morphology resolved computational fluid dynamics (CFD) impractical. However, volumetric roughness models can be leveraged, as they have computational costs orders of magnitude lower. In this work, a discrete element roughness model (DERM), based on the double-averaged Navier–Stokes equations, is presented and applied to additively manufactured rough channels, representative of gas turbine blade cooling passages. Unique to this formulation of DERM is a generalized sheltering-based treatment of drag, a two-layer model for spatially averaged Reynolds stresses, and explicit treatment of dispersion. Six different AM rough surface channel configurations are studied, with roughness trough to peak sizes ranging from 15% to 60% nominal channel passage half-width, and the roughness Reynolds number ranges from Rek = 60 to 300. DERM predictions for spatially and temporally averaged mean flow quantities are compared to previously reported direct numerical simulation results. Good agreement in the mean velocity profiles, stress balances, and drag partitions are observed. While DERM models are typically calibrated to specific deterministic roughness morphologies at comparatively small roughness Reynolds numbers, the present more generalized DERM formulation has wider applicability. Here, it is demonstrated that the model can accommodate random roughness of large scale, typical of AM.

Predictive control of reactor network model using machine learning for hydrogen-rich gas and biochar poly-generation by biomass waste gasification in supercritical water

Supercritical water gasification (SCWG) technology can convert biomass into hydrogen rich gas and biochar. Fluidized bed reactor is promising for the industrialization of this technology, and the reactor dynamic performance study is of great significance for its scaling up. However, current simulation studies mainly focus on steady-state analysis using Computational Fluid Dynamics (CFD) software, it is difficult to conduct dynamic study due to its high computational costs. To this end, a reactor network model (RNM) that accounts for the flow, heat transfer, and kinetic dynamics for fluidized bed reactor is firstly developed based on the partitioning theory, which can reduce the computation time of dynamic simulation from several days to seconds. Additionally, extensive open-loop simulations of the RNM are carried out to generate the dataset for the development of a recurrent neural network (RNN) model. Additionally, current studies mainly focus on open-loop simulation, closed loop optimization is absent. To this end, a framework for building a machine learning (ML) model and a ML-based nonlinear predictive control scheme is developed. Model predictive control (MPC) schemes based on the RNN model is used to optimize the SCWG process to achieve multiple objectives, such as maximizing hydrogen yield and carbon yield. The open loop simulation results demonstrate that H2 yield decreases from 0.00022 to 0.0002 kg s−1 with temperature reducing from 700 to 600 °C. To increase H2 yield to the setpoint, MPC increases temperature and mass flowrate to 708 °C and 417.57 kg h−1. Additionally, MPC increases carbon yield and minimizing CO2 yield by decreasing temperature to 475 °C and increasing mass flowrate to 407.38 kg h−1.

Numerical investigation of the effect of carotid bifurcation stenosis degree on pulsatility characteristics.

Arterial bifurcations are regions that are susceptible to hemodynamic effects and thrombus formation. In the current study, the hemodynamic effects of a simplified 3D model of an arterial bifurcation were simulated using the commercial computational fluid dynamics software FLUENT. The non-Newtonian properties of blood were modeled using the Carreau model, and the pulsation dynamics and heat transfer characteristics of blood at different degrees of stenosis in the arterial bifurcation were analyzed. The results indicate that arterial stenosis caused by a thrombus when the pulsation velocity reaches its peak has an essential impact on blood transport. The stenosis of the bifurcation increases the peak pulsatile flow pressure drop, and each 0.5mm stenosis of the arterial bifurcation increases the mean wall shear stress of the bifurcated segment by approximately 0.25Pa. From the heat transfer perspective, arterial stenosis has little effect on the heat transfer coefficient. The heat transfer coefficient measured inside the bifurcation is much larger than that measured outside the bifurcation. The stenosis of the arterial bifurcation causes an increase in the mean velocity of the arterial cross-section, and the volume-averaged absolute vorticity is introduced to quantify the secondary flow effect during the pulsation cycle, where the arterial stenosis causes an increase in the mean absolute vorticity at pulsation velocity and accelerates the decay of the vorticity at uniform velocity. In this paper, the hemodynamics of carotid bifurcation pulsation is analyzed in conjunction with flow field properties to reveal the flow field dynamics factors and heat transfer characteristics of local stenosis of the carotid bifurcation and to conduct an exploratory study for the diagnosis and treatment of carotid bifurcation thrombosis.

Numerical simulations and modeling of MHD boundary layer flow and heat transfer dynamics in Darcy-forchheimer media with distributed fractional-order derivatives

In this study, we extend the application of an existing Cattaneo heat flux model integrated with magnetohydrodynamic (MHD) Maxwell boundary layer flow using distributed fractional-order derivatives. This offers a novel approach for the use of disturbed order along with the midpoint quadrature rule, not previously considered in the literature for such phenomena. Moreover, the application of a variable heat flux with distributive order fractional constitutive equation, also an unexplored area in the literature, adds to the novelty of our study. We investigate the dynamics of the model over a flat surface in the presence of variable heat flux and Darcy-Forchheimer media. The governing equations, along with their respective initial and boundary conditions, are derived and solved using the finite difference method employing the L1 scheme. A benchmark 'exact solution' is obtained for a simplified version of the problem and a numerical solution is provided for a more complex, realistic version. The accuracy of our numerical scheme is validated by comparing results with the benchmark solution, and a mesh independence study further validates our code. Our study uncovers unique insights into the influence of various physical parameters on the velocity and temperature boundary layer thickness, as well as the flow and heat transfer mechanisms, elucidating the rheological characteristics of the system.

Numerical study on the dynamic process of reciprocating liquid hydrogen pumps for hydrogen refueling stations

Reciprocating liquid hydrogen pump operates under alternating pressure, temperature, and flowrate with dramatic changes in the thermodynamic states of liquid hydrogen. The thermodynamic state in the pump is influenced by the dynamic interactions between fluid flow, heat transfer, piston motion, and valve dynamics. This paper presents a numerical study for reciprocating liquid hydrogen pumps based on a coupled simulation of the dynamic processes between the alternating flow, unsteady heat transfer, and valve dynamics with a given piston motion as the input. A pump for hydrogen refueling stations with a nominal flowrate and delivery pressure of 50 kg/h and 87.6 MPa is selected as the research object. The appropriate design parameters of the pump and its valves are determined to avoid cavitation in the cylinder and oscillation of the valves. The effects of the frequency and delivery pressure on the valve motion and pump performance are further analyzed. Finally, the in-cylinder heat transfer leads to an extra evaporation loss of 0.012 kg/day, and the isentropic and volumetric efficiencies of the liquid hydrogen pump are 97.30% and 90.76% respectively. The work presented here would be beneficial for the design and optimization of reciprocating liquid hydrogen pumps.

Electrodialysis membrane desalination for water and wastewater processing: irregular attack angles of membrane spacers.

Electrodialysis desalination is constructed with a number of anion exchange membranes (AEM), cation exchange membranes (CEM), anode, cathode, adjacent silicon gasket integrated membrane spacers, and inlet/outlet holes per cell. At the boundary among an ionic solution and an ion exchange membrane, concentration polarization develops. Spacers placed in between channel's walls function as stream baffles to increase turbulence, improve heat and mass transfer, diminish the laminar boundary layer, and lessen fouling problems. The current study offers a systematic review of membrane spacers, spacer-bulk attack angles, and irregular attack angles. Spacer-bulk attack angle is accountable for variations in the pattern and direction of stream which impact heat-mass transfer and concentration polarization. Irregular attack angles (e.g., 0°, 15°, 30°, 37°, 45°, 55°, 60°, 62°, 70°, 74°, 80°, 90°, 110°, 120°) in the present study were found to provide unique stream patterns due to the spacer's filaments being less or more transverse in respect to the primary solution direction, which may significantly alter heat transfer, mass transport, pressure drop, and overall flow dynamics. Spacer applies shear stress resulting by continuous stream tangent to the membrane exterior, which lessens polarization. In the end, 45° is concluded as the preferred attack angle that offers balanced rates of heat transfer, mass transport, and pressure drop throughout the feed channel while greatly lowering the rate of concentration polarization.

Convection heat transfer of stepped basin single slope solar still: A numerical investigation

Solar-powered desalination is a quick and easy way to make drinking water. Numerous solar distillation systems have been investigated for various parameter modifications based on local resource availability. In this work, a solar still with a modified stepped basin is investigated to raise the rate of internal evaporation and, therefore the output yield of the solar still. Stepped basin solar stills are taken into consideration for the study since they are very effective because the water's surface exposure to radiation is greater. Thus, increasing the rate of internal water evaporation enhances the rate of convective heat transfer between the evaporating and condensing surfaces, leading to improved and consistent output. The two-dimensional stepped basin single slope solar still was investigated and contrasted with the traditional single slope still in terms of heat transfer and fluid dynamics. To optimize the configuration for better performance in various types of climatic and operational circumstances that imitate the scenario of daily life, design elements such as the number of steps and the different heights of the basin are also taken into account. Each step was added with far more success than the one before it, up until the length was reduced to 1.16% of the entire shorter length. This numerical study enables us to draw the conclusion that the rise in natural heat transfer rate with the addition of steps is mostly caused by the increased surface area and the inherently restrictive nature within the domain. Additionally, with an increase in Rayleigh number, Ra, the gradient variations of the traditional single slope solar still overheat transfer features have been greatly regulated and successfully raised for the modified stepped basin Solar still.

Coupled characteristics and performance of heat pipe cooled reactor with closed Brayton cycle

Heat pipe cooled reactors with closed Brayton cycle (CBC) have the potential of achieving both high efficiency and high safety. However, the coupled characteristics and performance of CBC coupled with heat pipe reactors have not been well studied. Few works consider both the neutron dynamics in heat pipe cooled reactors and the dynamic heat and mass transfer in CBC. In this paper, a novel nuclear power system configuration, heat pipe cooled reactor with CBC, is proposed and comprehensively analyzed. A dynamic model is built based on the modular modeling idea, and verified using a Brayton cycle test bench. The proposed system has load following capability within certain motor speed, and its maximum thermal-electric energy conversion efficiency exceeds 20%. The dynamic charge amount control strategy is further proposed to improve efficiency. The maximum efficiency improvement is about 5% at 2kWe. The self-power-regulation and inherent passive safety of the reactor are verified through dynamic simulation. The power and temperature overshoots of the dynamic responses are selected as quantitative indicators of system dynamic performance. MAPs of the temperature and power overshoots for unit changes in output power are obtained, and can be used to evaluate the system safety during dynamic operations.

Experimental study of performance enhancement strategies for hydrogen production-purification systems in packed-bed reactors under power fluctuations

Methanol reforming for hydrogen production and purification is a key technology in current fuel cell hydrogen source systems. The packed-bed reactor has a simple structure and is widely used, yet it has disadvantages such as slow dynamic response and limited heat and mass transfer. In this paper, an experimental approach is used to simulate the reaction characteristics of the packed bed during power fluctuation and start-up phase. It is found that the combined catalysts are the best in terms of both economical and purification effects, especially the precious metal combined copper-based catalysts. Meanwhile, the methanol conversion rate can seriously affect CO-PROX during the fuel cell start-up phase, and the use of combined catalysts can mitigate the impact of power fluctuation on the system and reduce the CO concentration. The combination of catalysts in such a way that the noble metal should be ensured in the front section, the optimal reaction temperature of copper-based combination noble metal-based catalysts is explored, with reforming temperature of 200 °C and CO-PROX temperature of 180–200 °C, and optimal O2/CO ratio between 0.5 and 1.5, when methanol conversion reaches more than 70%, CO concentration as low as 0.42% and hydrogen concentration higher than 40%.

Differential heat transfer characteristics of coal macerals and their control mechanism: At the mesoscale

The dynamic heat transfer process of coal with different macroscopic types was characterized by a thermal imaging acquisition system, and the differential control mechanism of maceral composition and distribution on coal heat transfer was revealed. The results show that the temperature distribution in bright and dull coal samples are unimodal compared with the bimodal temperature distribution in semibright and semidull coal. With the extension of the heating time, the difference of the temperature distribution in the samples are first decreased, then increased and finally decreased. According to the evolutionary characteristics of the temperature difference, the heat transfer process can be divided into four stages: rapid temperature rise, slow down, fluctuating temperature rise and dynamic equilibrium. Inertinite is heated up faster, forming a high-temperature region, and vitrinite is heated slowly resulting in a low-temperature region. The temperature rise rate and equilibrium temperature in the low-temperature region are inversely proportional to the distance from the high-temperature region. It is difficult for vitrinite to heat up as vitrinite has the largest specific heat capacity and the smallest thermal conductivity and thermal diffusion coefficient. Therefore, the heat transfer capacity of vitrinite is generally weak, featured by a lower rate of temperature transfer disturbance.

Heating efficiency enhanced by combination of phase change materials and activated carbon for dry floor heating system

Phase change materials (PCMs) have diverse applications in energy storage, buildings, cooling, and heat exchangers. In this study, n-docosane and activated carbon (AC) were combined using the vacuum impregnation method to form an aggregate, which was then injected into a Poly vinyl chloride container and applied to a dry floor heating system. Thermal storage, gravimetric, conductivity, and specific heat analyses were performed to evaluate the thermal performance of n-docosane and AC-based heat-storage aggregate (D-HSA). The dynamic heat transfer performance and power consumption of the dry floor heating system after D-HSA application were then evaluated. Pure n-docosane and D-HSA showed latent heat capacities of 242.7 J/g and 235.1 J/g and 92.10 J/g and 92.66 J/g during heating and cooling. In terms of thermal durability, D-HSA was more stable than pure n-docosane. Thermal conductivity analysis revealed that D-HSA had a high thermal conductivity (approximately 597 %) in the liquid state. In the field-based dynamic heat transfer analysis experiment, the PCM dry floor heating system filled with D-HSA had the advantages of both dry and wet floor heating systems, namely, a fast surface temperature increase rate and the time-lag effect, respectively. Therefore, the PCM dry floor heating system can compensate for the disadvantages of the dry and wet floor heating systems and can be considered an efficient heating system in terms of energy savings.

Multi-objective optimal dispatch of integrated heat and electricity energy systems considering heat load energy quality

As the energy crisis and consumption revolution advance, traditional energy service providers gradually transform into customer-centric integrated energy service providers. However, the customer’s experience with energy depends on the quality. This paper proposes an energy quality model for heat load of the integrated heat and electricity energy systems, from which the derived energy quality factor for heat load participates as one of the objectives of the multi-objective optimization model. The considered heat transfer dynamics accurately describe the heat transmission through the heat network. Finally, the effectiveness of the proposed multi-objective optimal dispatching model is verified by numerical simulations, in which a decision-making model established based on exergy economics enables the selection of a compromise solution from the Pareto front as the basis for dispatching.

The dynamic thermophysical properties evolution and multi-scale heat transport mechanisms of 2.5D C/SiC composite under high-temperature air oxidation environment

The variations in thermophysical properties and dynamic heat transfer mechanism of 2.5D C/SiC ceramic matrix composites (2.5D C/SiC-CMC) under a high-temperature air oxidation environment are investigated by experiments and numerical simulations. A new multi-scale thermal analysis model of 2.5D C/SiC-CMC under high-temperature oxidation is proposed to predict its equivalent thermal conductivity and analysis its multi-scale heat transport mechanisms. The multi-scale thermal analysis model is developed by blending the multi-scale asymptotic thermal analysis method and oxidation kinetics theory, which includes the micro-scale model and mesoscale model. The micro-scale model simulates the oxidation process of the carbon fibers and PyC interface through oxidation kinetics, which predicts the varying equivalent thermal conductivity of yarns with oxidation. The mesoscale model is constructed based on the experimental statistics which reflect the multi-scale structure within the 2.5D C/SiC-CMC with oxidation. The mesoscale model introduces oxidation characteristics by considering dynamic variations in yarn thermal properties. The dynamic thermophysical properties of the 2.5D C/SiC-CMC with oxidation are tested in the experimental study, which verifies that the multi-scale thermal analysis model has highlighted prediction accuracy. The results show that high-temperature oxidation causes significant variations in the dynamic thermal behavior of 2.5D C/SiC-CMC which is critical for the practical thermal protection design of 2.5D C/SiC-CMC hot components in engineering applications.

Experimental validation of the dynamic thermal network approach in modeling buried pipes

The transient behavior of buried pipe systems plays a significant role in many heating and cooling systems, particularly in thermal energy networks and ground heat exchangers. In this study, the dynamic thermal network (DTN) approach’s validity as a response factor method in modeling dynamic conduction heat transfer in a buried pipe system is experimentally validated. A lab-scale representation of a buried pipe system has been excited by step changes in boundary temperatures and heat fluxes measured up to times approaching steady-state conditions. This data is used to derive weighting factors and also evaluate the validity of numerical representations of the buried pipe and to verify that the DTN method can reproduce the heat flux responses. It is demonstrated that the weighting factors required in this method can be derived from both numerical and experimental step-response time series data. The DTN method is found to be both accurate in reproducing the heat fluxes in the validation experiments but also significantly more computationally efficient than a conventional numerical model when simulating long timescale responses in buried pipe systems.