Dimensionless Heat Research Articles

Magnetic williamson hybrid nanofluid flow around an inclined stretching cylinder with joule heating in a porous medium

The study of an inclined stretching cylinder in a porous medium holds significant implications for understanding complex fluid dynamics and heat transfer phenomena, mainly when influenced by a hybrid nanofluid containing Cu and Al2O3, Joule heating, and a magnetic field in the presence of Williamson fluid. The nonlinear ordinary differential equations are derived by applying pertinent similarity transformations to partial differential equations. Subsequently, these non-dimensional ordinary differential equations are transformed into a system of first-order ODEs and numerically solved using Maplesoft's symbolic computation software MAPLE 2023. The influence of governing parameters on dimensionless velocity, temperature, concentration, skin friction, heat, and mass transfer rates is investigated. This investigation is crucial for various practical applications, including geothermal reservoirs, enhanced oil recovery, and environmental processes involving porous media. It is demonstrated that Biot and Eckert numbers and nanofluid parameters contribute to increased surface temperature, while the Lewis number leads to a decrease in dimensionless concentration. Moreover, the curvature parameter is associated with an increase in dimensionless concentration, and skin friction demonstrates a direct relationship with the magnetic parameter while inversely correlating with the cylinder's curvature.These results contribute to the scientific understanding of these complex interactions and provide valuable insights for engineering applications, such as designing efficient heat exchangers, optimizing cooling systems, and advancing technologies involving fluid flow and magnetic fields.

Experimental investigation on hydrogen jet flame behavior and temperature distribution with different shapes of outlet

Hydrogen's wide flammability range causes its high-pressure jet flames to exhibit large-scale, high-temperature characteristics, posing risks of casualties and property damage. This study investigates the impact of different nozzle shapes and release pressures on flame behavior and temperature distribution through high-pressure hydrogen jet flame experiments. Circular outlets (0.5–4 mm diameter) and rectangular outlets (aspect ratios 1 to 8) are used, covering release pressures from 20 to 220 bar. High-pressure hydrogen is released through a nozzle into ambient air, igniting to form a flame. Flame length, width, and axial temperature are measured. Findings indicate that visible flame size increases with release pressure. The models for dimensionless visible flame length and width, considering the dimensionless heat release rate, are proposed for different nozzle shapes. Additionally, the dimensionless temperature rise along the flame axis is observed to be a function of the dimensionless distance and dimensionless heat release rate.

Experimental study on flow characteristics and burning behavior for fire of flammable gases leakage underwater

The occurrence of subsea pipeline leakage incidents has gradually increased with the rapid development of the offshore oil and gas industry. Research on the fire dynamics of flammable gases leakage underwater is very limited, particularly in understanding the coupling mechanism of flow and combustion on the water surface. This paper presents a systematical investigation on flow characteristics and burning behavior for fire of flammable gases leakage underwater. A series of laboratory-scale fire behavior experiments were conducted through a self-designed experimental platform, consisting of the leakage control module, water tank and measurement apparatus. It was revealed that the leakage flow rate and water depth would influence the flow characteristics and burning behavior significantly. Two different burning conditions were found for the flame burning above the water surface (FBWS), i.e., the stable and self-extinguishing (unstable) burning states. The dimensionless analysis is conducted for the burning conditions, where the critical criterion is developed to distinguish the stable and unstable burning states. The base diameter of stable FBWS is almost 1.6 times of the diameter of bubble plume on the water surface. The flame pulsation frequency and dimensionless flame height (the ratio of height to base diameter of flame) of the stable FBWS are different from thoes of liquid pool fires and gas diffusion fires. Consequently, a correlation between the Strouhal number and the Froude number is correlated. Finally, the correlation between the dimensionless flame height and the dimensionless heat release rate is proposed for the stable FBWS.

EXPERMENTAL INVESTIGATION OF TEMPERATUREFOR STIR FRICTION WELDING DISTRIBUTION

Friction stir welding (FSW) is a solid state joining method for metal and is widely used for Al-Alloys. In the present work ,temperature distributions were investigated for Aluminum alloy (7008), the effect of the main welding parameters such as rotational speed (tool speed) and liner speed (welding speed) on the temperature distribution were investigated .The results show that the rotational speed has a strong effect on the temperature distribution and this effect increases with temperature increase, but the linear speed has an (inverse proportion) with the temperature increase. This effect was less than the rotational speed. Peak temperature was estimated theoretically by using (Bakingham ) theorem to derivate the relationship between the dimensionless heat input and dimensionless peak temperature. In this study a mechanical test (tensile test) for welded specimen was investigated to study the effect of temperature distribution on the mechanical properties of the alloy, results show that Aluminum alloy (7008) was weldable on the (FSW) process and obtain an maximum weld efficiency (81%) with parameters (880 rpm) rotational speed and (1 mm/sec) welding speed. By comparing test results with peak temperature to the Al-Alloy the obtained result was that the welding process is successful when welding temperature be (75-80%) of melting point of the used material.

A semi-analytical temperature solution for multi-segment deep coaxial borehole heat exchangers

A semi-analytical and a finite-difference scheme are presented for the simulation of temperature and the heat transfer in a multi-segment coaxial borehole heat exchanger. The single-segment solution on closed-form is extended to a semi-analytical multi-segment solution, where each segment may have unique properties. These properties are such as different casings, widths of the annulus, radius of the inner tubing, material properties, rock properties and geothermal gradients. The multi-segment model is a simple and powerful alternative to numerical methods for simulating a complex coaxial borehole heat exchanger with a constant flow rate. It is demonstrated with a deep coaxial borehole heat exchanger made of three different segments. The analytical and semi-analytical models are validated by comparison with numerical solutions obtained with an upstream finite difference scheme. The match between the solutions is excellent. The solution on a closed-form is used to study the temperature difference between the outlet and the inlet regarding two dimensionless numbers. It is found that the maximum temperature difference occurs when the dimensionless heat transfer coefficient for the casing-rock is much larger than one. A second necessary condition is that the dimensionless heat transfer coefficient for the insulator between the inner tube and the annulus must be much less than one. The power leakage from the inner tubing to the annulus is also at a maximum under these conditions.

An Experimental Study of Pool Fire Characteristics under the Effects of Cross Winds and Baffles

The pool fires that occur behind obstructions in a ventilated environment are very different from other wind-blown pool fires. The pool fire formed by fuel leakage in an engine nacelle is a typical example of a pool fire influenced by cross winds and baffles. Mastering the combustion characteristics of this type of fire is of great significance for fire prevention and control. In this study, the burning rate, flame length, and flame tilt angle of heptane pool fires behind a baffle under different cross wind velocities (ranging from 0 to 5 m/s) were experimentally investigated. Square pool fires with dimension of 8 cm and 12 cm with baffle height from 4 to 12 cm and different distances between fire and baffle (0, 20, 30 cm) were tested in a wind tunnel. The experimental results show that the burning rate increases with the increase in cross wind velocity for each baffle height. As wind velocity exceeds 2 m/s, the burning rate first decreases and then increases with the increase in baffle height. The flame length initially increases and then decreases with increasing wind velocity. The upper flame tilt angle is mainly affected by the cross wind, while the bottom flame tilt angle is influenced by the combined effects of cross wind velocity, baffle height, and distance between baffle and flame. The empirical correlations under different distances between baffle and flame, with wind velocity and baffle height accounted for, are then proposed for the dimensionless heat release rate and the flame length of heptane pool fires.

Numerical study on heat and flow transfer of biomagnetic fluid with copper nanoparticles over a linear extended sheet under the influence of magnetic dipole and thermal radiation

A steady, two-dimensional flow of biomagnetic fluid namely blood flow with copper nanoparticles across a stretchable sheet that is affected by a strong magnetic field and thermal radiation is investigated in this study. Copper nanoparticles (Cu-NPs) were used for this study because of their important applicability in biomedical research. Thus, the properties of copper nanoparticles render it an antibacterial, antimicrobial, and anti-fungal material. Similarity substitutions were applied to reduce the nonlinear partial differential equations to ordinary differential equations. Utilizing the MATLAB R2018b software bvp4c function technique, the physical solution was established. This model's pertinent dimensions, such as the ferromagnetic parameter, the magnetic field parameter, the radiation parameter, the suction parameter, the ratio parameter, the slip parameter, and the Prandtl Number, were computationally and graphically inspected about the dimensionless velocity, temperature, skin friction, and heat transfer rate. One of the pivotal observations was that a rise in the ferromagnetic parameter and Prandtl number drops the temperature and velocity, correspondingly. A cross-case analysis with the outcome of other published research is also executed for divergent parameter values. Based on the investigations, copper nanoparticles may be advantageous for biomedical purposes and lessen the hemodynamics of stenosis. Owing to the research, copper nanoparticle-concentrated blood exhibits a reduced flow impedance and a larger temperature changeability compared to sheer blood.

Thermal analysis of thermoelectric active cooling including external thermal resistances

Thermoelectric active cooling uses nontraditional thermoelectric materials with high thermal conductivity, high thermoelectric power factor, and relatively low figure of merit (ZT) to transfer large heat flows from a hot object to a cold heat sink. However, prior studies have not considered the influence of external thermal resistances associated with the heat sinks or contacts, making it difficult to design active cooling thermal systems or compare the use of low-ZT and high-ZT materials. Here, we perform a non-dimensionalized analysis of thermoelectric active cooling under forced heat flow boundary conditions, including arbitrary external thermal resistances. We identify the optimal electrical currents to minimize the heat source temperature and find the crossover heat flows at which low-ZT active cooling leads to lower source temperatures than high-ZT and even ZT→+∞ thermoelectric refrigeration. These optimal parameters are insensitive to the thermal resistance between the heat source and thermoelectric materials, but depend strongly on the heat sink thermal resistance. Finally, we map the boundaries where active cooling yields lower source temperatures than thermoelectric refrigeration. For currently considered active cooling materials, active cooling with ZT < 0.1 is advantageous compared to ZT→+∞ refrigeration for dimensionless heat sink thermal conductances larger than 15 and dimensionless source powers between 1 and 100. Thus, our results motivate further investigation of system-level thermoelectric active cooling for applications in electronics thermal management.

Unified ANN model for heat transfer factor (j) and friction factor (f) prediction in offset strip and wavy fin PFHEs

Accurate prediction of the dimensionless heat transfer (j factor) and friction factor (f factor) is crucial for optimizing plate-fin heat exchanger (PFHE) performance. However, current models, including existing ANN models, have difficulty predicting these factors for complex fin geometries such as offset strip fins and wavy fins. To address this challenge, this research proposes a novel, unified artificial neural network (ANN) model, specifically a multilayer perceptron (MLP), capable of accurately predicting both factors across various fin types, including offset strip fins and wavy fins. The ANN was trained and validated using a comprehensive dataset that encompasses a wide range of fin configurations and Reynolds numbers, from 300 to 10,000, covering both laminar and turbulent flow conditions. Advanced learning techniques, like Bayesian regularization (BR), were employed to enhance the model's performance. The proposed model surpasses existing models, achieving a predictive accuracy with a deviation of just 1–2% from actual values, compared to the typical ±20 % seen in previous models. This significant improvement implies the new ANN model could enhance PFHE performance, energy efficiency, and cost-effectiveness across diverse applications. Introducing a unified ANN model, the research underscores machine learning's potential in thermal engineering and paves the way for future advancements.

Dimensionless analysis of flow and heat transfer characteristics in a high-speed rotor–stator disk cavity based on similarity criteria

Turbine disk cavity system is of great importance for improving the rotating turbine blades cooling efficiency and preventing gas invasion. This study focuses on turbine disk cavity characteristics at high-speed rotational conditions due to complicated flow and heat transfer mechanisms. Then, a novelty method is proposed to reveal multiple similarity criterion numbers of rotor–stator disk cavity between actual engine and experimental case conditions, based on the dimensionless analysis. Especially, a new evaluation standard including the dimensionless adiabatic wall temperature and dimensionless heat flux is put forward and proved to be equally important with the Nusselt number. Results show that the dimensionless heat flux in the turbine disk cavity mainly depends on Reynolds number and Mach number, while the dimensionless adiabatic wall temperature is dominated only by the same Mach number. When only Reynolds numbers or Mach numbers are considered, the relative error can reach 14.5% on the similarity of dimensionless heat flux. Noteworthily, the effect of thermal boundary of turbine disk cavity should not be ignored in the rotor–stator disk cavity, which will bring 28.8% deviation to the similarity of the Nusselt number. Therefore, this study can provide a theoretical method and engineering application to obtain flow and heat transfer characteristics of actual engine turbine disk cavity under experiment case with normal temperature conditions.

Research on the radiation scaling criteria of film-cooled gas turbines

The escalating gas turbine (GT) inlet temperature and the amplifying radiation effect of multi-component radiation gases aggravate the thermal radiation phenomena. In addition, the omission of radiation effect in the classical film cooling scaling criteria makes the research data obtained under relative low temperature conditions invalid. This paper provided a mechanistic-level explanation of the influence of the thermal radiation term on the energy equation, and subsequently introduced two novel dimensionless parameters (radiation number and optical thickness) that can effectively reflect the radiation impact on scaling criteria. Furthermore, three parameter matching schemes were proposed to enable the data match of high and low temperature conditions for adiabatic cooling effectiveness, overall cooling effectiveness, and dimensionless radiation heat flux. Compared to the high temperature test method, the scaling criteria can provide valuable insights into the film cooling behavior under realistic GT conditions. The method proposed in this paper can not only help prevent material loss and energy waste but also align with the principles of energy conservation, emission reduction, and sustainable development.

Experimental investigation on hydrodynamic characteristics of 1,1,1,2-tetrafluoroethane (R134a) in a horizontal tube at supercritical pressures

In this study, we experimentally assessed the hydrodynamic characteristics of supercritical-pressure R134a in a horizontal circular tube at pressure, inlet temperature, and heat flux ranges of p = 4.2 – 5.4 MPa, Tin = 20 – 50 °C, and q = 60 – 300 kW/m2, respectively. The hydrodynamic curves of R134a were plotted under various working conditions to assess the underlying mechanisms. The hydrodynamic characteristic curve of supercritical-pressure R134a does not possesses a negative-slope region but has a transition region compared with that under subcritical pressure. The transition region becomes more evident with increasing heat flux, while it weakens with increasing pressure and inlet temperature. Moreover, the onset of the transition region shifts in the direction of high mass flow rate with increasing heat flux. The transition region, generated by the relative changes in viscosity and density, is indicated by a dimensionless heat quantity Qratio, whose boundary is 1.8–2.2.

Viscoelastic effect on the triple diffusive oscillatory flow in a fluid-saturated porous layer

The combined heat and mass transfer, the so-called thermosolutal convective problem, has become an attractive field of research in many diversified areas. In this paper, for the first time, oscillatory flow analysis has been carried out for triple diffusive viscoelastic fluid flow in a porous medium. A comprehensive model is developed for the modified Darcy–Brinkman–Oldroyd-B fluid, porous medium, Boussinesq approximation, heat and mass transfer across a finite temperature and concentration difference in the chemical potential of two salts. Triple diffusive viscoelastic fluid flows through porous media have grown significantly as this situation occurs in more than a few applications such as improved oil recovery filtration, liquid complex molding, solidification of liquid crystals, cooling of metallic plate in a bath, exotic lubricants and colloidal solutions, polymer processing, chemical and bioengineering industries, among others. The governing coupled nonlinear partial differential equations with boundary constraints represent the modeled flow problem. In addition, these equations are converted into non-dimensional form by employing suitable non-dimensionalizing quantities. The impacts of the pertinent parameters and related dimensionless numbers on the dimensionless velocity, temperature, concentrations, shear stress, heat and mass transfer are examined for both suction and injection cases. It has been found that when the injection level on the heated plate is increased, the shear rate increases for each channel plate. Furthermore, we recognized that the viscoelastic parameters exhibit an opposite kind of behavior on the velocity, temperature, and concentrations fields.

Semi- Analytical Study on Non-Isothermal Steady R-D Equation in a Spherical Catalyst and Biocatalyst

The Lane-Emden Boundary Value Problem as it appears in chemical applications, science, and biochemical applications are employed. Two specific models are solved by applying the Ananthaswamy-Sivasankari method (ASM). The model in first problem is a reaction–diffusion equation of a spherical catalyst and the model in second problem is the reaction–diffusion process of a spherical biocatalyst. Obtain a reliable semi-analytical expression of the effectiveness factors and the concentrations. A graph is constructed for the obtained semi-analytical solutions.The effects of several parameters like dimensionless activation energy, Thiele modulus and dimensionless heat of reaction are shown in graphical representation. Our semi-analytical solution is compared with numerical simulation by using MATLAB and finds good fit in all parameters. The new analytical method ASM is helpful to solve many non-linear problems mainly Reaction-Diffusion equation.

Heat transfer in a fluidized bed tubular solar receiver. On-sun experimental investigation

Using particles as heat transfer fluid in a solar receiver is an attractive way to increase the global efficiency of solar power plants. In the case of the particle-in-tube solar receiver concept, the fluidized particles circulate vertically in tubes thanks to a controlled pressure gradient and an additional air injection. Experiments are conducted with olivine particles in a one-tube mock-up at the 1 MW CNRS solar furnace (France). The tube of high aspect ratio (more than 3 m height over 48 mm internal diameter) is irradiated along a 1-m height with several solar flux configurations that correspond to realistic operation conditions of a solar thermal power plant. The novelties of this paper lie in extending the operating parameters compared to earlier studies and in linking the fluidization regimes to wall heat transfer. Slugging, turbulent fluidization and fast fluidization regimes are detected within the receiver tube. It is shown that the system can tolerate high solar flux densities, up to 800 kW/m2. The system proves to be highly controllable and flexible with respect to particle mass flux variation and transient operations. Particle temperature increase ranges from 100 to 650 °C depending on the particle mass flux. Maximum thermal efficiency and particle outlet temperature of respectively ∼ 60 % and 680 °C are obtained, proving that this technology can be combined with highly efficient thermodynamic cycles. Global wall-to-particle heat transfer coefficient varies strongly with particle mass flux. It reaches a quasi-plateau above 15 kg/(m2s) with a mean value of 1000 ± 200 W/(m2K) and peaks data up to 1500 W/(m2K). A dimensionless heat transfer index is derived to account for the effects of the particle mass flux on the experimental results. It highlights that both increasing the temperature and working in the turbulent fluidization regime result in the increase of the heat transfer between the tube walls and the particles.

Natural convective loops heat transfer scaling analysis

Heat transfer arising in natural convective loops is analyzed numerically and theoretically. Combining 3D direct numerical simulations, unidirectional heat-transfer semi-analytical computations, scaling arguments and asymptotic analysis new universal scaling laws are proposed to account for dimensionless heat transfer behavior of natural convective loops. It is found that the Reynolds number Re resulting from the buoyancy driven rotating convection (as well as the Péclet number) scales as Grashof number Gr as Re∼Gr3/4 for imposed temperature boundary conditions and Re∼Gr3/5 for mixed imposed temperature/flux heating conditions. These scaling laws are successfully confronted with experimental measurements in both heating configurations. It is shown that these scaling result from boundary layers dominated transfers without noticeable influence of possibly complex 3D flow patterns found in the fluid flow. These simple scaling laws constitute a progress over previously existing correlations which have disregarded the effect of boundary layers, heating configurations as well as fluid to solid variable diffusivity/conductivity.

Partial Slip and Cross-Diffusion Effects on Magnetohydrodynamic Mixed Bioconvection Flow in a Channel with Chemical Reaction

The main objective of the paper is to explore the effects of Soret and Dufour on MHD mixed bioconvection of nanofluid in a flat channel with chemical reaction, radiation, Joule heating and partial (velocity) slip. The related ordinary differential equations are comprised of velocity, energy, nanoparticle concentration, solutal concentration and microorganisms density are solved analytically subject to physically appropriate boundary conditions using homotopy analysis method (HAM). The dimensionless skin friction, heat and mass transport are discussed through plots by varying different physical parameters. The transport of heat enhances with Brinkman number but it increases gradually with thermal radiation. The fluid velocity reduces by the velocity slip, while it increases by raising the Hartmann number. The temperature of the fluid lowered due to the surplus thermal radiation. The dufour number and velocity slip create opposite effect in solutal concentration.

Numerical study on plug-holing of double fires in tunnel with centralized smoke exhaust

In tunnel fires, smoke is the main fatal factor causing casualties. The technical parameters of tunnel smoke extraction are particularly critical. However, the parameters related to smoke control have rarely been experimentally and quantitatively studied in a tunnel caused by two adjacent fire sources. Numerical simulations were performed to study the plug-holing phenomenon in the centralized smoke exhaust tunnel with multi-fire sources. A 1:10 small-scale tunnel (length 25 m; width 1.2 m; height 0.8 m) was built using FDS. The effects of different fire heat release rates Q̇, fire source separation distances, and smoke exhaust velocities (v) on the plug-holing were considered. Firstly, the smoke layer height beneath the exhaust vent gradually increases as the exhaust velocity becomes higher and then remains almost unchanged until the plug-holing phenomenon occurs. Additionally, the smoke layer height becomes lower as the fire source separation distance increases for the fires with constant heat release rate. For all Q̇ values, the critical plug-holing velocity increases with the increasing separation distance S and Q̇. In addition, by modifying the critical Froude number (Frc), a predictive model of the critical plug-holing velocity considering the dimensionless fire source heat release rate and separation distance is obtained. Furthermore, the modified critical Froude number model, accounting for two adjacent fires with different separation distances, is established. The correlation between the new model and the simulation results shows quite good agreement, indicating that the proposed model can provide a satisfactory prediction for the critical smoke exhaust velocity in plug-holing conditions of double fire sources. This work can provide valuable guidance for tunnel design concerning fire safety.

Numerical Investigation on the Head-on Quenching (HoQ) of Laminar Premixed Lean to Stoichiometric Ammonia–Hydrogen-Air Flames

The Head-on Quenching (HoQ) of laminar premixed ammonia–hydrogen-air flames under lean to stoichiometric condition is numerical investigated. Detailed chemistry including 34 reactive species and detailed multi-component transport model including thermal diffusion (Soret effect) are applied. The quenching distance is considered as a representative quantity for the HoQ process, and the influence of different system parameters on it has been investigated. These parameters involve fuel/air equivalence ratios, hydrogen content in gas mixture and pressure. It was found that an increase of quenching distance can be caused by a lower hydrogen addition and a leaner mixture condition. Furthermore, it was found that, regardless of the gas mixture, the quenching distance decreases monotonically with increasing pressure, obeying a power function with the exponent -\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document} 0.7. Moreover, numerical results show a relation between the quenching Peclet number and the dimensionless wall heat flux normalized by the flame power. Additionally, sensitivities of quenching distances with respect to the transport model, considering the heat loss in the wall and the chemical kinetics are studied.

Conjugate heat transfer from a discrete-source heated plate by offset jet impingement

Convection/conduction simulations have been performed to determine flow and heat transfer phenomena in steady-state, for a plate carrying few embedded discrete heaters, exposed to by a turbulent offset cooling jet. This configuration is relevant, for example, in thermal design of vehicle body segments in aerodynamics conditions: when realized in certain alloys, their configuration will be determined based their discrete structural state prompted by supplied electrical heating. In order to approach an appropriate thermal design of the plate and obtain their precise state, the space of geometric and operating variables needs to be explored by a thorough parametric study based on the governing momentum and energy equations.In this paper, the interaction between the impinging boundary layer and the conductive effects produced by such discrete heaters is explored, giving rise to a conjugate heat transfer situation. With a flexible notation for discretized heating and temperature-dependent conduction, dimensionless heat transfer is discussed spanning the entire range of design parameters. Results report considerable variation in heat transfer: as an example, air jet over polypropylene plates brings about peak Nusselt numbers between 2000 and 4000 for an inclined configuration at Re≈1×106 based on nozzle width, with related strong temperature gradients that must be addressed during design, depending on discrete heaters' configuration and properties; whereas for alumina plates the peak Nusselt numbers fall down between 600 and 750, featuring much smoother temperature variations and consequent more stable structural states.