Convective Heat Research Articles

Thermal management of mixed convection in a partially heated triangular cavity with a cylindrical enclosure

Abstract Numerical Investigations are carried out to study the thermal performance of the magnetohydrodynamics laminar mixed convection in a triangular cavity with a circular enclosure. The present work analysis is carried out on a triangular cavity with circular blockage by varying the Re (200-600), Ri (0.01-1), and Gr (4000-36000), respectively. The working system is a triangular cavity filled with water with a circular block. Non-linear partial differential equations are the governing equations that use the finite element method. The moving upper wall and temperature difference contribute to the convection heat transfer. The upper wall is heated and maintained at high temperatures. The other walls are kept as adiabatic. The obstacle at the center is kept at a low temperature. The physical parameters are non-dimensional numbers like the Reynolds, Richardson, and Hartmann numbers that influence the heat transfer rate. The Richardson and Reynolds numbers impact positively, and the Hartmann numbers tend to decrease heat transfer rates.

Steady and unsteady numerical investigation of mixed convective heat transfer enhancement in a channel with baffles attached to the heated wall

Steady and unsteady numerical investigation of mixed convective heat transfer enhancement in a channel with baffles attached to the heated wall

Homotopic dynamic solution of hydrodynamic nonlinear natural convection containing superhydrophobicity and isothermally heated parallel plate with hybrid nanoparticles

Abstract The purpose of this work is to investigate the effect of thermal radiation on convective heat transfer for an electrically conducting hybrid nanofluid moving perpendicularly through a microchannel with parallel plates heated under isothermal conditions, while subjected to a transverse magnetic field. In this case, one surface exhibited a superhydrophobic slip and temperature jump, whereas the others did not. The objective of the study was to determine the impacts of thermal radiation, heat generation, and magnetism on the volume flow rate, velocity, and bulk temperature when either surface is heated by a constant wall temperature. Our findings reveal that the radiation parameter significantly influences both the Nusselt number and skin friction differently depending on the surface conditions, while also reducing flow rate and bulk temperature. The heat generation parameter similarly affects these variables but varies with the type of surface being heated. Additionally, both the velocity and temperature profiles increase with the porosity parameter and heat generation coefficient, regardless of the heated surface. Statistical analysis confirms the significance of magnetic and heat generation parameters in determining skin friction and the Nusselt number, and streamlines and isotherms illustrating the effects of thermal radiation and magnetism on two different hybrid nanofluids when heated on nonslip and superhydrophobic surfaces were provided. These insights provide valuable information for the design and maintenance of mini- and microdevices in the fields of nanoscience and nanotechnology.

Analysis of Thermal Isotropy of Parallelepiped Shape of Bronze Samples for Mechanical Engineering Applications

Objective: The need to find out whether the specimens are ‘thermally thin’ or not is the basis for applying the law of conservation of energy or the Fourier differential equation of heat conduction in finding the time dependence of the specimen temperature. The aim of this study is to find out the thermal field distribution in a parallelepiped shaped bronze specimen and to verify the Bio number. Methods: A number of external variables make it difficult to maintain a monotonic change in sample temperature during the ‘heating’ stage. These factors led to the use of the cooling method to study tiny samples with low inertia of the thermal process. We used the cooling method to experimentally verify the isotropy of the thermal field in the sample and to study the kinetics of cooling relative to the sample axes over a wide range of temperatures. Results: It was found out that in natural air cooling the main mechanisms are conductive, convective and radiative heat transfer. The contribution of thermal radiation to the cooling process is noticeable at high temperatures. It was found that the typical cooling time increases with sequential radiative, conductive and convective heat transfer. By extending the temperature study area to, for the first time, we were able to observe each component of the heat transfer process independently. The values of the typical cooling time on all axes correspond to the experimental error bounds. It was found that the temperature value of the sample is independent of the coordinates and depends only on time. In this case, the law of conservation of energy can be used to explain the dependence of body temperature on cooling time. Conclusion: It is shown that the sample under study is ‘thermally thin’, i.e. the thermal field is constant in all directions. In this situation, the thermal balance equation should be applied rather than the Fourier differential equation of heat conduction, since the temperature gradient inside the body is practically zero. The results obtained are of great importance for the study of cooling processes of metallic products in thermal power engineering, heat engineering and thermophysics.

The Impact of Circular Holes in Twisted Tape Inserts on Forced Convection Heat Transfer

This study investigated heat transfer and friction characteristics in a forced convection system using wavy twisted tape inserts with circular holes. The inserts, with twist ratios (TR) of 8.5, 7.5, and 6.5, were placed inside a test pipe to create turbulent flow. The tapes measured 700 mm in length and 18 mm in width, while the test pipe had an outer diameter of 35 mm and an inner diameter of 30 mm, with a test section length of 700 mm. Airflow rates were adjusted to achieve different bulk mean temperatures. Experimental data were used to develop new correlations for the Nusselt number and friction factor. The Reynolds number (Re) ranged from 4,000 to 14,000. Comparisons between the wavy twisted tape inserts with varying twist ratios and pitches and a smooth tube showed that the highest heat transfer rate was achieved with a twist ratio of 6.5.

Enhancement of turbulent heat transfer by using CuO nano-particle and twisted tape

AbstractA computational model is developed to investigate the convective heat transfer properties and the fluid flow characteristics of cupric oxide - water nano-fluid in a horizontal circular pipe aiming to provide insights into optimizing heat transfer in such systems. A twisted tape with varied twist ratios is inserted. This quantitative investigation used five Reynolds number from 4,000 to 12,000 under a uniform heat flux scenario of 25,000 W m−2. All experiments were performed as a single-phase fluid with cupric oxide values of 0, 0.4, 1, and 2% by volume. By reducing the twist ratio and increasing volume concentration, the average heat transfer coefficient of cupric oxide-water nano-fluid was improved. For a twist ratio of 4D, the maximum heat transfer improvement was 228% greater than the plain pipe. The presence of twisted tape with modest step ratios causes the friction factor to grow.

Heat transfer in a non-uniformly heated enclosure filled by NEPCM water nanofluid

Purpose This paper aims to focuses on by investigate the heat transmission and free convective flow of a suspension of nano encapsulated phase change materials (NEPCMs) within an enclosure. Particles of NEPCM have a core-shell structure, with phase change material (PCM) serving as the core. Design/methodology/approach The enclosure consists of a square chamber with an insulated wall on top and bottom and vertical walls that are differently heated. The governing equations are investigated using the finite element technique. A grid inspection and validation test are done to confirm the precision of the results. Findings The effects of fusion temperature (varying from 0.1 to 0.9), Stefan number (changing from 0.2 to 0.7), Rayleigh number (varying from 103 to 106) and volume fraction of NEPCM nanoparticles (changing from 0 to 0.05) on the streamlines, isotherms, heat capacity ratio and average Nusselt number are investigated using graphs and tables. From this investigation, it is found that using a NEPCM nano suspension results in a significant enhancement in heat transfer compared to pure fluid. This augmentation becomes more important for the low Stefan number, which is around 16.57% approximately at 0.2. Secondary recirculation is formed near the upper left corner as a result of non-uniform heating of the left vertical border. This eddy expands notably as the Rayleigh number rises. The study findings indicate that the NEPCM nanosuspension has the potential to act as a smart working fluid, significantly enhancing average Nusselt numbers in enclosed chambers. Research limitations/implications The NEPCM particle consists of a core (n-octadecane, a phase-change material) and a shell (PMMA, an encapsulation material). The host fluid water and the NEPCM particles are considered to form a dilute suspension. Practical implications Using NEPCMs in energy storage thermal systems show potential for improving heat transfer efficiency in several engineering applications. NEPCMs merge the beneficial characteristics of PCMs with the enhanced thermal conductivity of nanoparticles, providing a flexible alternative for effective thermal energy storage and control. Originality/value This paper aims to explore the free convective flow and heat transmission of NEPCM water-type nanofluid in a square chamber with an insulated top boundary, a uniformly heated bottom boundary, a cooled right boundary and a non-uniformly heated left boundary.

Experimentation, Taguchi and machine learning-based optimisation of parameters concerning heat losses from the trapezoidal box-type solar cooker

ABSTRACT The optimal design parameter combinations for a trapezoidal solar cooker that yield the lowest heat loss coefficient have been determined using a thorough Taguchi and Machine learning-based optimization approach that has been established for this study. Data for the previously described optimization have been generated through the use of an experimental approach in a laboratory environment. Numerous influencing factors are taken into account, including the temperature of the bottom plate, the depth of the cavity, the emissivity of the bottom cover, and the wind-induced heat transfer coefficient on the top glass cover. For the current investigation, the ideal factor combination is found to be a parametric combination plate temperature of 350K with 0.4 plate emissivity, 15W/m2 convective heat transfer coefficient, and 0.25 m height. To determine the percentage contribution of each control element to the heat loss coefficient, analysis of variance (ANOVA) has been used for the collected data and signal-to-noise ratios. Plate temperature at 74.72% and emissivity at 23.84% are the primary contributing variables found. Likewise, the least contributing factors like height (0.57%) and convective heat transfer coefficient (0.32%) are discovered. A regression equation is used to represent the connection between the response and the control factor. This formula is suggested as a forecast formula for calculating the heat loss coefficient.

Influence of the thermal insulation position in standard and cool roofs on their contribution to urban heating during heatwaves

The urban heat island, coupled with increasingly intense, frequent and long heat waves, leads to a significant risk of excess mortality and increased energy requirements in urban areas during the summer season. Various cooling techniques are therefore being studied to moderate the intensity of this phenomenon during the hot season. The aim of this study is to quantify the contribution of roofs to urban climate during heatwaves, depending on the position of the insulation in the structure and its radiative properties. This study is carried out in the laboratory, using an experimental set-up to submit roof samples to heatwave conditions. The experimental set up includes a temperature and humidity-controlled climate chamber with a solar simulator. Two roofing configurations are studied: roofs with external thermal insulation (ETI) and roofs with internal thermal insulation. Each configuration is studied for two types of roof: standard (low albedo), and reflective (high albedo). It appears that ETI roofs heat the outside air during the day, but not at night. While ITI roofs heat the ambient air less during the day, they release some of the stored daytime heat during the night. Whatever the structure, increasing solar reflectivity significantly reduces the contribution of roofs to urban heating, although the impact is stronger for internally-insulated roofs, since it reduces the amount of heat released. Finally, for the externally insulated structure, the increase in solar reflectivity reduces daytime convective exchange by 41% and heat entering the building by 75%. For the internally insulated structure, daytime convective heat exchange is reduced by 43%, and the heat stored in the concrete layer is reduced by over 50%.

Investigating Enhanced Convection Heat Transfer in 3D Micro-Ribbed Tubes Using Inverse Problem Techniques

The improved heat dissipation observed in 3D micro-ribbed tubes is primarily influenced by the intricate interplay of multiple structural parameters. Nevertheless, research into the coupling mechanisms of these multi-structural parameters remains constrained by the absence of effective methodology in numerical solutions. In the present work, a new 3D micro-rib structure based on discrete adjoint method is established. Firstly, the research examines the interplay of different parameters (such as arrangement, relative roughness height, angle of attack, and circumferential rows) on the thermo-hydraulic performance. It is noted that the heat transfer efficiency is notably impacted by the relative roughness height. And the arrangement methodology dictates the optimal positioning for heat transfer efficiency. An increase in the number of circumferential rows enhances fluid mixing, while the angle of attack plays a crucial role in the formation of longitudinal vortices. Secondly, the coupling optimization technique is employed to obtain the optimal structure featuring non-uniform relative roughness height by the developed numerical solution. Overall, in comparison to the smooth tube, the optimized ribbed tube exhibits a remarkable 64.9% enhancement in performance evaluation criteria. Finally, a notable enhancement of 10.65–22.78% is observed when comparing with the prevailing micro-rib structures.

High-Quality Conductive Network Films Constructed from Carbon Nanotube/Carbon Nanofiber Composites via Electrospinning for Electrothermal Applications.

In this study, carbon nanotube (CNT)/carbon nanofiber (CNF) composite electrothermal films were prepared by electrospinning, and the effects of the CNT content and carbonization temperature on the electrothermal properties of the CNT/CNF composite films were investigated. The experimental results demonstrated that the conductivity of the CNT/CNF composite electrothermal film (0.006-6.89 S/cm) was directly affected by the CNT content and carbonization temperature. The electrothermal properties of the CNT/CNF positively correlated with the CNT content, carbonization temperature, and applied voltage. The surface temperature of CNT/CNF can be controlled within 30-260 °C, and continuously heated and cooled 100 times without any loss. The convective heat transfer with air is controllable between 0.008 and 31.75. The radiation heat transfer is controllable between 0.29 and 1.92. The prepared CNT/CNF exhibited a heat transfer efficiency of up to 94.5%, and melted a 1 cm thick ice layer within 3 min by thermal convection and radiation alone.

Air-cooling process modelling for cord steel based on fusion approach of mechanism and artificial neural network

In this study, a modelling approach is introduced that fuses the mechanistic model with an artificial neural network (ANN) during the cord steel manufacturing. This involves replacing certain mechanistic calculations with ANN representations, optimizing mechanistic parameters through ANN techniques, and implanting mechanistic equations into the ANN framework to achieve more accurate and efficient calculations of the cooling process. The mechanistic parameters come from the models including Natural Convection Heat Transfer, Forced Convection Heat Transfer, Radiation Heat Transfer and Phase Transition during the air-cooling process. The model demonstrated an average error of 8.6 °C and 5.4 °C, compared to measurement values from two temperature sensors on the air-cooling line, and an average error of 3.3 °C relative to manually obtained temperature.

Experimental and numerical investigation of heat transfer modes on pot surfaces in a power range of a cooking porous burner

The investigation of convective and radiative heat transfers to a cooking pot in a porous burner, coupled with an analysis of the pot's bottom and side contributions to total heat transfer, remains an underexplored area of research. In this study, we adopt a comprehensive approach, combining experimental measurements and 3D numerical models, to explore the practical application of a Silicon carbide porous burner fueled with natural gas in cooking pots with power ranging from 12.15 kW to 31.04 kW. Three primary aspects are examined: differentiating between radiative and convective heat transfer contributions, investigating porous mixing characteristics, and analyzing the burner's thermal and combustion efficiencies. To facilitate this investigation, a dedicated test bench is meticulously designed and constructed, equipped with state-of-the-art measuring instruments to effectively monitor the thermal behavior of the heating process. The results reveal that the higher Damköhler number at higher power, driven by the dominance of chemical time over mixing time, accentuates the importance of chemical reactions in heat transfer to the pot. Moreover, increasing the power leads to a reduction in the burner's overall thermal efficiency, from 29.2% to 18.8%, due to a higher energy waste percentage, escalating from 45.12% at P = 12.15 kW to 65.31% at P = 31.04 kW. This increase in power also strengthens the downward movement of the flame and its detachment from the pot surface. The pot's bottom contribution to total heat transfer ranged from 76.7% to 95.5%, with convection alone accounting for over 98% in all cases. Consequently, optimizing the pot bottom geometry for the purpose of enhancing convection presents promising avenues for significantly boosting thermal efficiency. These findings highlight the potential of the investigated porous burner for efficient and clean combustion in cooking appliances.

Determination of heat transfer characteristics of tube bundles over heating plate by machine learning

In this study machine learning is used with the aid of a deep neural network algorithm to predict convective heat transfer characteristics for inline and staggered tube bundles, and correlation equations for Nusselt number and friction factor are derived. Machine learning algorithm's data were obtained from experimental work for various transverse pitch of the tube bundles, longitudinal pitch of the tube bundles and Reynolds number. 276 experimental data points were taken for both inline and staggered tube bundles. However, considering that the data obtained from the experimental study may be insufficient for training, a two-step data augmentation method and retraining with cross-validation was used to prevent data deficiency in the deep neural network structure. Thus, the unseen data in the experimental work were also predicted. The coefficient of determination for the DNN model predictions was obtained greater than 0.96. One correlation equation for Nusselt Number and three correlation equations for friction factor were proposed from the augmented data with machine learning. The R2 values of the correlation equations varied between 89 % and 99 %. As a result, machine learning methods successfully applied to predict the Nusselt number and friction factor of tube bundles consistent with the experimental data.

Impact of the abrasive solution heating process with different techniques on the etching parameters for the SSNTD (CN-85)

Abstract The presented work aims to compare and analyze the impact of the abrasive solution heating process using three different techniques on the etching parameters for the solid-state nuclear track detector, SSNTD, (CN-85). Water bath technique (WB) is using water as a conduction medium of heat transfer to heat the etching solution. The thermal oven technique (TO) is using air as a convection heat transfer medium to heat the etching solution. Finally, the microwave technique (MW) as electromagnetic radiation no need for a medium to heat the etching solution. That comparison will be achieved at different irradiation times to an Americium-241 source (241Am) of 5.486 MeV energy and 10 μCi activity as an alpha particle's emitter. Two irradiation times 10 sec, and 20 sec are applied. Small pieces of CN-85 detectors (1×1) cm2 were used. CN-85 detector parameters of the etching process, with the three applying techniques (WB, TO and MW), are estimated and compared. The obtained results from the three techniques are compared. The etching process time is developed to nearly 5 min for MW with heating by radiation mechanism compared to 25-35 minutes for etching techniques (WB and TO) with heating by conduction and convection mechanisms respectively. The efficiency of MW technique with range of from 70 % to 75 % are five times greater than that for WB and TO techniques.

Thermal spreading in a multilayer geometry with convective cooling along the side wall of each layer

Two-dimensional thermal conduction between a source/sink and a body of larger size results in thermal spreading and constriction, which are of much technological importance in problems such as semiconductor thermal management. While the traditional analysis of thermal spreading and constriction assumed adiabatic side walls, there is growing interest in convective heat removal from the side walls, such as in multilayer three-dimensional integrated circuits (3D ICs) and stacked Li-ion cells in a battery pack. This work presents theoretical analysis of thermal spreading in a multilayer body with distinct convective heat transfer coefficients along the side wall for each layer, in addition to anisotropic thermal conductivity in each layer and thermal contact resistance at interfaces between layers. A series solution for the temperature distributions in each layer is derived, with a unique set of eigenvalues for each layer. A set of linear algebraic equations that govern the coefficients of the series solutions is derived. Results are shown to agree well with past work for special cases, as well as with numerical simulations for general problems. The impact of source size, thermal anisotropy and Biot numbers of various layers is analyzed in detail. In particular, the interplay between Biot numbers along the sidewall and at the sink plane in determining total thermal resistance is investigated in detail. This work contributes towards a fundamental understanding of theoretical thermal conduction in problems of practical relevance, such as advanced microelectronics and Li-ion cells. Results may find application in the design and optimization of these and other engineering systems where thermal spreading or constriction play an important role.

Unsteady numerical simulations of aircraft icing based on the dynamic grid method

In this study, an unsteady numerical simulation method based on dynamic grids was established to solve unsteady aircraft icing processes. The dual-time step method was used to achieve the time advancement of the transient airflow and droplets fields. The droplets trajectory was calculated using the Euler method, which incorporates a supercooled large droplets model. The transient evolution of the water film and ice layer on the substrate surface was considered in the icing thermodynamic model. The accuracy of the unsteady method for ice-shape prediction was verified via comparison and analysis of numerical examples. Additionally, the rime and glaze ice formation processes were explored, and the coupling effects of ice horn growth and the air convective heat transfer coefficient, water collection coefficient, and water film flow on the substrate surface were studied. The ice shapes calculated by unsteady and quasi-steady methods were compared. The results indicated that the latter predicts a larger icing range than the former. Moreover, there may be more obvious distinctions between these two methods in predicting glaze ice.

Variable density and heat generation impact on chemically reactive carreau nanofluid heat-mass transfer over stretching sheet with convective heat condition

The present study focuses on the physical significance of heat generation and chemical reaction on Carreau nanofluid with convective heat conditions. Heat transfer is characterized using convective boundary conditions. The governing partial differential equations (PDEs) are transformed into ordinary differential equations (ODEs) by using well define stream functions and similarity transformations. Using a shooting methodology, the Keller-box method with Newton Raphson scheme is used to elaborate the numerical solutions of physical phenomena. Utilizing a similar technique to find the impact of physical parameter such as the production of heat δ, the rate of reaction Λ, Biot numbers γ, Brownian motion variable Nb, the thermophoresis parameters Nt, the Weissenberg quantity We, Prandtl number Pr, and Lewis number Le on velocity profile, temperature profile and mass transmission profile are determined graphically. The skin-friction coefficient −f″(0), local Nusselt −θ′(0), and Sherwood numbers −ϕ′(0) are analyzed numerically. Increment in fluid velocity and slip temperature are depicted with high Biot number. Maximum magnitude of fluid temperature and fluid concentration function are depicted at high value of temperature dependent density. The magnitude of heat and mass transportation enhanced with maximum choice of Brownian motion.

Natural convection magneto hydrodynamic filled with TiO2 nanofluid with a right-angled triangle obstacle within square cavity

The goal of this study is to examine the result of natural convection, magnetohydrodynamics, and nanofluids within a square cavity with an obstacle of a right-angle triangle within it. Among the various nanoparticles, TiO2 nanoparticles blended with water have shown promise in improving heat transfer. The Finite Element Method is used for the simulation. By utilizing the commercial software 'COMSOL Multiphysics 6.1' graphical illustrations, the influence of prominent parameters on streamline, isotherm contours and the local Nusselt number is effectively portrayed. The key factors that play a dominant role include the Prandtl number (Pr = 6.2) and Rayleigh number (104 <= Ra <= 106), Hartmann number (Ha = 0, 30, 50), and the heat source/sink (Q = -6, 0, 6) employed. The result confirms that the decline in fluid velocity and convective heat transfer is due to the enhancement of the Lorentz force. This outcome of the study affirms that the appropriate amalgamation of nanoparticles significantly improves the transmission of heat properties of fundamental fluids. The discoveries of this investigation could aid in comprehending the hydrothermal aspects of thermal systems, including electronic cooling, solar power and the utilization of nanofluid to regulate liquid movement and thermal exchange attributes within the annular space.

Effects of charge motion on knock and thermal efficiency under high load condition of a heavy-duty natural gas engine

Nowadays, the carbon-neutral trend and the crude oil crisis have brought natural gas into increasing focus. However, heavy-duty engines fueled by natural gas have been plagued by thermal efficiency and knocking limitation problems. Well-structured charge motions can enhance thermal efficiency and concurrently mitigate knock, however, there is a lack of comprehensive work involving charge motions. In this paper, numerical simulation is employed to investigate the impact of charge motion on knock and thermal efficiency under high load conditions for a heavy-duty spark-ignition natural gas engine. The findings indicate that in the configuration with intense swirl, the diminished turbulent kinetic energy (TKE) at the combustion chamber’s base lessens the convective heat exchange between the fresh charge and the heated exhaust gases, resulting in a noticeable high-temperature zone and triggering hot spots. In configurations with inclined swirl and strong tumble motion, the extensive tumble motion near the spark plug region close to top dead center leads to an asymmetric flame front shape, determining the location of hotspots. The research also revealed that excessive tumbling motion and TKE can increase the likelihood of engine knock, while appropriate cylinder charge motion can improve the knock resistance of the engine to some extent, the maximum reduction in knocking intensity can reach 92.5%. The optimized combustion systems can significantly improve the initial combustion rate, shorten the ignition delay period, and advance the combustion center. With the combined effects of ignition timing and charge motion, the total indicated specific power per unit of fuel mass is increased in the optimized systems. Compared to the original combustion system, the indicated thermal efficiency within the knocking boundary can be improved by 1.11%.