Heat Transfer Research Articles

Cooling layout optimization for a turbine blade squealer tip with the application of oval holes

For the purpose to enhance the cooling performance of the squealer tip under stage conditions, an automatic optimization framework was constructed to optimize the cooling holes on a squealer tip, including the utilization of oval-shaped holes. The analysis of the optimization results indicates that the configuration of the cooling holes with positive axial inclination and the reduction in the arrangement interval of holes that are assembled in the front cavity can effectively enhance the film attachment, resulting in augmented film coverage and cooling effectiveness. The coefficient of heat transfer in the region between and downstream the holes is observed to decrease in accordance with the film coverage. Meanwhile, the positive axial inclination guides the jets to accumulate towards the rear of the cavity, enhancing the blocking effect to leakage flow. The film attachment is further improved as the jets outflow along the long axis edge of oval holes, which exhibit low curvature. In general, the implementation of round hole optimization has led to an increase in cooling effectiveness by 54.85 % in comparison to the benchmark. Furthermore, the use of oval holes has resulted in a greater improvement of 67.65 %. The aerodynamic performance has remained uncompromised throughout these modifications.

Investigation of the Arrangement of Aluminum Fins on the Thermal Behavior of Lauric Acid as a Phase Change Material in a Two-pipe Heat Exchanger by CFD Simulation

BackgroundPhase change material (PCM) thermal storage systems store more thermal energy per unit volume than sensible heat storage systems. PCMs offer a potential solution to reduce energy consumption in various thermal engineering applications. This study aimed to examine how fin arrangement affected the thermal efficiency and melting time of PCMs. MethodsA two-dimensional numerical analysis of the melting process of lauric acid in a heat exchanger featuring two pipelines and fins was conducted using CFD simulation. In most previous investigations, the heat transfer fluid was a single-phase liquid. An enthalpy-porosity technique was used to model the solid and liquid phases of PCM. The governing equations were solved using the commercial software ANSYS Fluent 2021, and the pressure and velocity equations were coupled using the SIMPLE algorithm. Significant FindingsThe best model among the 13 tested was Model 5, which featured 6 fins and a consistent angle of 60 degrees. For Model 5, the melting time was 1818.3 seconds. Due to sensible heating, the fin's temperature (Temp) rose gradually from 300 K to 318 K. Temp then gradually increased as the PCM melted in the phase transition zone between 316.5 K and 321.2 K. Once the phase transition was complete, the PCM's Temp steadily rose from 324 K to 340 K. In Model 5, the inner wall Temp and the maximum Temp of the PCM were closest, at 327.34 K and 333.55 K, respectively. The thermal shock between the PCM and the ambient Temp caused a peak heat flux at the beginning of the PCM loading process.

Transient heat transfer characteristics of cooling fluid loop with latent heat storage unit

In the realm of electronic cooling, it is of significance to efficiently manage the transient heat load. In this context, an innovative strategy is introduced by integrating a cooling fluid loop with a latent heat storage (LHS) unit. The effects of strategic positioning of LHS units on the dynamic temperature under varying conditions, such as heat load intensities, filling rates of phase change materials (PCMs), heating durations, and coolant flow rates, are examined and analyzed. The results indicate that the introduction of LHS unit can significantly alleviate the temperature fluctuation of microchannel heat sink (MHS) under instantaneous heat load. When the LHS unit is located upstream and downstream of the cooler, the maximum temperature of the MHS surface is reduced by 10 °C and 42 °C, respectively. The heat load determines the dynamic temperature characteristics of LHS units, and PCM performs best at precise melting states. When the heat storage of LHS unit matches the heat generated by the heating surface, further increasing the PCM filling rate has no positive effect, and extending the heating time is not conducive to cooling MHS. Additionally, increasing flow rate of working fluid has a limit effect on thermal enhancement of fluid loop with LHS.

Thermal and mechanical impact of artificial ground-freezing on deep excavation stability in Nakdong River Deltaic deposits

This paper presents a case study of deep excavation using the artificial ground freezing (AGF) method for tunnel restoration work in the Nakdong River deltaic deposits. The study involved detailed construction monitoring and data analysis to assess the thermal and mechanical impacts on surrounding ground and underground structures. Factors influencing heat transfer were identified and evaluated for their effect on ground temperature distribution. The excavation and frost expansion of the ground led to unique lateral deformation of the diaphragm wall. However, the frozen soil effectively resisted earth pressure and suppressed deformation of the wall. The axial stress applied to the braced strut was closely related to the deformation of the diaphragm wall and was influenced by both excavation-induced and frost-expansion pressures. Boreholes near the frozen soil functioned as stress-relief holes, enhancing excavation stability. These comprehensive findings enhance the understanding of AGF techniques and their impact on complex deltaic geological conditions and adjacent structures.

ANALYSIS OF THE EFFECT OF SWIRL FLAME SHAPING ON EMISSIONS FROM THE CO-FIRING OF AMMONIA AND METHANE

This paper presents experimental and numerical insights into the behaviour of lean swirl flames with co-combustion of ammonia and methane. In order to evaluate the accuracy of the emissions modelling for CH4/NH3/air preheated (473K) mixtures with an increasing share of ammonia, experimental investigations were performed, varying the share of ammonia up to 25% in the fuel and the equivalence ratio of air to fuel of 0.71. A burner featuring swirl outflow blades with a 30-degree angle of outflow and a convergent-divergent nozzle was employed in the experimental setup. In order to determine the size of the reaction zone, the concentrations of NO and CO along the probing radius inside the combustion chamber were measured. A complete geometry of the combustion chamber and burner was modelled with RSM and EDC models. The outcomes of the Okafor, Xiao, and San Diego mechanisms were compared to the test data. A further DES simulation was conducted to assess the potential applicability of the models. For the flames tested with 3D RSM, a good agreement was achieved for NO emissions. In the case of a fuel containing between 10% and 25% ammonia, the Okafor mechanism demonstrated the most favourable qualitative and quantitative agreement. The reactor volume, residence time and heat transfer conditions for the CRN reactor network were calculated using numerical CFD methods. A satisfactory correlation was observed between the calculated and experimental NO emissions, with a discrepancy of only 10%. In view of the considerable computational expense associated with the models employed and the high degree of accuracy demonstrated in the emission predictions, the 3D RSM simulation was deemed to be an appropriate means of modelling small-scale applications.

Complex heat transfer analysis in heat exchanger with constructal cylindrical fins

This paper documents a 3-D numerical analysis and theoretical prediction in novel micro-channel heat sinks comprise of three configurations. The first design has solid fins, the second model has half-hollow fins placed on micro-channel, while in the third design, hollow fins are mounted on the heat sink. The objective of the study is to maximise the dimensionless thermal conductance subject to fixed volume. A microelectronic device released high-density heat flux at the bottom of heat sink and liquid of Reynolds number range between 400 and 500, is applied at entrance of the boundary under laminar forced convection scheme to dispel heat deposited in the cooling channels and the internal parts of the hollows. A multi-objective broadscale step-by-step code is used to perform the optimisation and the results shows that the micro-channel with full-hollows has the highest thermal conductance at Rew<475, followed by solid fins before half-drilled fins. Theoretical analysis is conducted on the three configurations using the intersection of asymptotes method, and the results compared favorably with numerical solution. The findings indicate that the theoretical solution tends to over-predict the numerical results within a certain range of Bejan number. Code validation is performed, and the results align with existing literature.

Influence of geometric and operational variables on indirect evaporative cooling using cyclone as heat exchanger

With the increasing environmental impacts of human activities and the demand for new air cooling alternatives, indirect evaporative cooling (IEC) emerges as a sustainable alternative. However, there is a gap in the literature regarding examining cyclone configurations in this context. Despite being good separator devices, cyclones can be used as heat exchangers, enhancing convective heat transfer through turbulence. This opens the possibility of developing improved and economically accessible geometries of IEC systems, especially beneficial for hot and arid regions, and communities with limited resources. This study aimed to investigate the performance of 25 geometric configurations of cyclones used as indirect evaporative heat exchangers. The researchers employed an experimental approach by subjecting cyclones covered with a wet cotton fabric to IEC tests under various geometric and operating conditions, followed by statistical analyses to identify relationships between the variables. The results showed significant thermal variation in the cyclones, with a considerable impact on all variables, particularly the cyclone’s total length, relative humidity, air temperature, and flow rate. The thermal efficiency of the cyclones varied widely, with total length and flow rate being the most significant factors. Additionally, an analysis of the Euler values revealed the importance of the cyclone geometry in selecting the one that reduces energy costs. Among the cyclones tested, the configuration with the optimal geometric design (Cyclone 8) achieved the highest thermal performance and lowest Euler number. The findings of this study could help developing more efficient and sustainable IEC systems, helping to reduce energy consumption and environmental impacts.

Comparação de métodos para determinação do fator solar de vidros

Abstract Solar Heat Gain Coefficient (SHGC) is a thermal property of glass and transparent elements, defined as the ratio between the amount of solar energy that passes through the glass and the amount of solar energy that reaches it. SHGC can be determined using mathematical models, computer simulations, or field and laboratory measurements. This paper compares the methods for obtaining the SHGC, which include laboratory measurements using a portable calorimeter, computer simulation using the WINDOW software and calculated according to ISO 9050 standard. Two solar control glasses and one clear glass, used as a reference, were selected. The SHGCs obtained using laboratory measurements were lower than those obtained using the other methods due to the solar simulator and equipment limitations. Despite this, there was a good approximation of the behaviour of the glasses regarding the increase in air velocity. The heat transfer coefficient on the exterior surface was fixed, so the SHGC obtained for each glass based on ISO 9050 remains the same, regardless of the air velocity. The SHGC results based on ISO 9050 were similar to those measured by means of a portable calorimeter under conditions of higher air velocities.

A new accurate solution method for steady forced convection in an unbounded flow around a circular cylinder in continuum and slip regimes

A new accurate numerical solution method for steady forced convection in an unbounded bidimensional flow around a circular cylinder is presented, addressing both the no-slip and slip regimes for Reynolds numbers and Knudsen numbers within the range 0.01 ≤ Re ≤ 47 and 0 ≤ Kn ≤ 0.1. The challenge of singular behaviour in infinite domains, which degrades numerical precision, is effectively mitigated by our new approach, unlike previous methods that truncate the flow domain. This method provides complete and correct description of the flow near the cylinder and in the far field perfectly reproducing the asymptotic solutions at large distances. We also present a new effective analytical solution for Oseen flow with slip effects, accurately representing flow at low Reynolds numbers. The validity of our numerical solution is confirmed by extensive comparisons with analytical solutions and numerical and experimental data from the literature. Additionally, we derive a new accurate correlation formula for the Nusselt number in slip flow regimes, involving both scenarios with and without temperature jump. This formula is crucial for engineering applications, especially in constant temperature hot-wire anemometry, as it avoids the requirement to correct heat transfer measurements due to slip and temperature jump effects when using very small diameter wires.

Preliminary CFD simulations of a lab-scale novel design of a particle receiver for CSP applications

In the present work, a preliminary CFD analysis of a novel design of particle receiver for CSP applications was performed. The particle receiver is a new lab-scale prototype consisting in a thin fluidized bed in which the walls are made of glass to permit thermal radiation to pass through. The working fluid is air and the thermal performance has been studied for the case in which i) only gas flows through the receiver (no particles) and ii) particles are placed in the receiver. The addition of the particles enhances the heating process by increasing the heat transfer coefficient from the heated wall to both the bed of particles and the gas resulting in an increase of more than 55% in the air temperature at the outlet of the receiver. Then, the bed dynamics have been studied in detailed as a function of the superficial gas velocity and the static bed height of the system. The analysis comprises the time evolution of the temperature of the gas and solid phases, the position of the centre of mass (that can be increased by more than 25% during the heating process) and the bubble characteristics. It has been found that when the gas velocity is increased by 66% the average temperature of the solids is only decreased 10%; while, for the static bed height, when the number of particles is increased 3 times the average temperature of the particles is increased by 18%.

Electro-Osmotic Mechanism of Ellis Fluid With Joule Heating, Viscous Dissipation, and Magnetic Field Effects in a Pumping Microtube.

The dynamics of electro-osmotically generated flow of biological viscoelastic fluid in a cylindrical geometry are investigated in this paper. This flux is the result of walls contracting and relaxing sinusoidally in a magnetic environment. The blood's viscoelasticity and shear-thinning viscosity are the primary causes of its non-Newtonian characteristics. Hence, the rheology of the fluid (blood) is accurately captured with the Ellis fluid model. Both Joule heating and viscous dissipation are accounted for during thermal analysis. The electric potential induced in the electric double layer (EDL) is obtained by applying the Debye-Huckel linearization to the nonlinear Poisson-Boltzmann equation. Mathematical modelling is incorporated in cylindrical coordinates in wave frame of reference. Assuming a long wavelength and creeping flow characterized by a low Reynolds number, the Ellis fluid model's governing equations are simplified. The resulting differential equations are evaluated numerically via the built-in tool NDSolve of the Mathematica. Graphical representations are utilized to visually and comprehensively assess the thermal characteristics, flow features, heat transfer coefficient, and skin friction coefficient. Various factors are taken into consideration, including the impact of Ellis fluid parameters, electric double layer, magnetic field, Brinkman number, and Ohmic dissipation. Ellis fluid's axial velocity boosts with a rise of the electro-osmotic parameter and power-law index while decreasing with an increase in the Hartmann number and material fluid parameter. The fluid temperature is directly proportional to EDL parameter and parameters of Ohmic and viscous dissipation. The presence of both electric and magnetic fields may aid in the management and control of Ellis fluid (blood) mobility at different temperatures, which is helpful in controlling bleeding during surgeries. The current model may be used in clinical scenarios involving the gastrointestinal system and capillaries, electrohydrodynamic therapy, delivery of drugs in pharmacological, and biomedical devices. This research creates a theoretical model that can predict the effects of different parameters on the characteristics of fluid flows that are like blood.

Deep Fat Frying Characteristics of Malpoa: Kinetics, Heat, and Mass Transfer Modeling

This article investigated deep-frying characteristics of malpoa for varied frying time (2–10 min) and temperature (170–190 °C). The evaluation encompassed a comprehensive analysis of textural and color kinetics and heat and mass transfer modeling during deep fat frying of malpoa balls. Such investigations confirmed an enhancement in fat content from 10.2 to 41.65%. On the other hand, textural properties such as hardness, cohesiveness, and springiness varied from 3.14 to 22.59 N/mm, 0.22 to 0.76, and 15.5 to 49.56, respectively. Similarly, color parameters such as b*/a* and ΔE varied from 3.31 to 1.55 and 55.36 to 75.48. For the textural and color kinetics, the activation energies ranged between 58.65 and 85.82 kJ/mol and 31.34 and 64.34 kJ/mol. Similarly, for a variation in frying time from 2 to 10 min, responses (hardness, cohesiveness, springiness, and overall color) varied across the following ranges: 3.15–13.57 N, 0.22–0.66, 15.5–35.5, and 55.63–63.50 and 5.60–20.60 N, 0.30–0.77, 22.35–49.56, and 62.26–75.65 for temperatures of 170 and 190 degrees, respectively. On the other hand, heat and mass transfer analysis indicated a Biot number and heat transfer coefficient within the range of 0.31–0.65 and 25.58–34.64 for 170–190 °C. Thus, this investigation provides a deeper insight of the deep fat frying characteristics of malpoa. This provides a guideline for the food processing sector for such products.

Investigation of Tool Cooling and Heat Transfer Using Computational Fluid Dynamics Simulations

Thermal errors remain one of the biggest challenges for the precision of cutting machine tools. Aside from optimizations in the machine tool design and behaviour, optimal cutting process parameters and targeted usage of cutting fluid or alternative methods of tool cooling are required for improved process efficiency with minimal energy demand and maximal tool life. A simulation-based study is presented which compares both different methods of tool cooling, specifically air cooling, flooded cooling and minimum quantity lubrication and also different simulation methods and models. Using a case study, which models an existing thermal test stand comprised of motor spindle, tool holder, tool and coolant nozzle, different cooling scenarios were tested and compared. The simulations were performed in ANSYS CFX. Comparisons were made between simulations with and without buoyancy, with and without tool rotation, transient and steady-state, with laminar flow and with different turbulence models and between the different cooling scenarios. Some insight on different time step sizes and the resulting increase in simulation time and precision was also gained. These results will make future studies on the thermal behaviour of both tool and cutting process easier by showing suitable simulation techniques and viable model simplifications.

Ultrafast Laser-Induced Thickness-Limited Sintering of Metal Nanoparticle Film for Ultrathin Flexible Microelectronic Devices.

Laser sintering of metal nanoparticles (NPs) has been widely used in flexible microelectronic device fabrication, wherein the sintered layer thickness is a key factor affecting the mechanical stability and conductivity. In this work, ultrathin flexible electronic circuits on flexible substrates with robust bonds and excellent conductivity have been fabricated through ultrafast laser-induced thickness-limited sintering of the metal NP film. When the laser fluence is below the damage threshold of the metal NP film, sintered layer thickness can be controlled by the laser parameters. The maximum thickness of the dense sintered NP layer is limited to 355, 421, 491, 527, and 647 nm at laser pulse durations of 0.3, 5, 10, 15, and 20 ps, respectively. This thickness-limited sintered layer is mainly determined by the plasmonic photothermal absorption of metal NPs and heat transfer within the NP layer. Due to the nonthermal process under intense ultrafast laser irradiation, the metal-polymer interaction can be further enhanced with minimal damage on substrates. The resistivity of the as-received Ag NP film decreases to 6.32 μΩ·cm after laser sintering at a pulse duration of 20 ps. Meanwhile, the relative resistance of the NP film increases to 2.7, 1.7, and 1.03 after 105 bending cycles, 500 tape peeling cycles, and water flow impinging for 60 min, respectively. This thickness-controlled ultrathin Ag NP film fabricated by ultrafast laser sintering exhibits excellent mechanical robustness and electrical conductivity, which shows great promise in ultrathin flexible microelectronic devices.

Radiation in the coffee cup

Abstract It is commonly assumed that radiative heat transfer only plays a relevant role in situations involving temperatures of several hundred degrees Celsius and above. Other common misconceptions relate to the cooling of metallic vs non-metallic objects, and the effect of their colour. A series of alternative demonstrations that counteract these beliefs and show the role played by radiation in the cooling of common tableware containing hot liquids are described here. The experiments require basic equipment, easily available even at home. The cooling curves obtained can be modelled with Newton´s law of cooling and the corresponding heat transfer coefficients estimated. The results elucidate the influence of surface emissivity on radiative heat transfer, and that this mechanism has a non-minor share on cooling processes even at temperatures below the normal boiling point of water. A final demonstration concerns heating of the same systems and corroborates the previous conclusions.

Simulation and experimental study on temperature field and flow field of laser cladding high-temperature Ni-based alloys

Abstract The multiphase liquid's development during the laser cladding process involves complex processes such as energy input, mass input, heat transfer, multi-phase liquid flow, and rapid solidification. Based on the coupled theory of temperature field and velocity field, a multi-field coupling model for Ni-based high-temperature alloy on the surface of 40Cr has been established. Throughout the modeling procedure, the surface tension coefficient's impact o.During the modeling process, the effect of the surface tension coefficient on the flow velocity of the molten metal in the melt pool is considered, along with the tracking of the gas/liquid free boundary through the use of the dynamic mesh technique. By comparing experimental and simulation results, it is found that the prediction error of the model ranges from -11.79% to 12.08%, demonstrating that the model has certain explanatory and predictive capabilities for laser cladding of Ni-based high-temperature alloys&amp;#xD;

Numerical analysis of slip phenomena on heat and mass transfer in a vertical channel with immiscible fluids with variable viscosity and thermal conductivity

Numerical analysis of slip phenomena on heat and mass transfer in a vertical channel with immiscible fluids with variable viscosity and thermal conductivity

A Current‐Adaptive Evaporator with Optimized Energy Utilization for Self‐Cleaning High‐Salinity Desalination

AbstractSolar steam generation (SSG) shows great potential in brine desalination to address the global water crisis. However, the salt accumulation has become a severe bottleneck that destroys devices and weakens efficiencies. Traditional evaporators are difficult to simultaneously realize clean salt‐rejection and efficient operation during high‐salinity desalination. In this work, a weaved cylinder evaporator with both efficient static and dynamic energy utilization is fabricated for self‐cleaning high‐salinity desalination. The energy utilization performance is optimized through a balanced water supply, omnidirectional solar absorption, infiltration content regulation, and dynamic current adaptation. Hence, an outstanding SSG rate of 1.91 kg·m−2·h−1 is achieved under 1 sun radiation. The dynamic self‐cleaning characteristics ensure competitive and durable high‐salinity desalination (≈11 kg·m−2·day−1) during weekly practical operation without salt deposition. Overall, the design overcomes the trade‐off between desired mass transfer and undesired heat loss.

Nanofluid dissipative Reiner-Philippoff model with thermal radiation: numerical investigation using the modified Adomian decomposition method associated with Mohand transforms

Abstract The flow of nanofluids over a stretched sheet situated within a porous medium is the main subject of this work. The Reiner-Philippoff model, which includes a magnetic field, chemical reaction, thermal radiation, viscous dissipation, and variable thermal conductivity, is examined. The study investigates how these complex processes affect the system’s heat transfer characteristics and flow dynamics. A system of partial differential equations describes the physical model. We arrive at a system of ordinary differential equations that, due to its highly nonlinear nature, requires numerical treatment by employing the proper similarity transformations. The governing equations are solved numerically, namely by combining the Mohand transform and the Adomian decomposition method. For computer-based solutions, complicated equations are simplified using the sophisticated Modified Decomposition Method (MDM). To guarantee convergence, it combines the Mohand transform with Adomian decomposition methods, yielding a series solution that almost matches the precise solution to the issue.

Thermal and structural improvements of a motorcycle piston using a fully coupled thermo-mechanical FEM simulation

This paper aims to conduct thermo-mechanical simulations using the finite element method (FEM) to enhance the design of the piston in a single-cylinder, water-cooled motorcycle engine. The finite element model of the piston was developed to represent its thermo-mechanical behavior under combustion processes and transient heat transfer phenomena. Realistic engine boundary conditions were applied using the non-linear dynamic tool in Solidworks design software. Two different design configurations were investigated concerning the oil jets and the upper face of the piston pins. The results revealed lower piston temperatures by 12.2% as three oil jets targeted the upper region of the piston bosses. Additionally, an optimum radius fillet in the upper face of the piston holes showed potential for minimizing thermo-mechanical stresses. This finding suggests that optimizing the piston design can reduce developed thermo-mechanical stresses while addressing elevated temperatures.