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The fabrication of most current thermal insulation materials from polymeric foams has created an unfavorable impact on the environment. With the aim of reducing the use of foam plastic, this paper focuses on honeycomb paperboard, an alternative thermal insulation material which is recyclable and cost-effective, and evaluates its heat transfer performance under different conditions. The experimental results show the effective thermal conductivity of honeycomb paperboard increases as the honeycomb height or the side length increases. This paper also establishes a novel theoretical model based on a newly developed convective heat transfer correlation to predict the effective thermal conductivity under different conditions. The model's results conform well to the experimental results, and the effect of convective heat transfer is investigated quantitatively using this model. As the vertical adiabatic boundary surfaces are replaced by honeycomb walls, the convection heat transfer strictly depends on the thermal boundary condition of the honeycomb walls and highly depends on honeycomb height and side length. • The novel theoretical models involving convective heat transfer are established under different conditions. • The different heat transfer paths are quantitatively investigated based on the proposed models. • Under the condition of hot-above and cold-below, convective heat transfer in honeycomb paperboard increases as the honeycomb height increases, but decreases as the side lengths increases. • Under the condition of cold-above and hot-below, convection heat transfer in honeycomb paperboard increases as the honeycomb height or the side length increases. [ABSTRACT FROM AUTHOR]

To deal with inverse nonlinear heat transfer problems in a two-dimensional heat conduction problem, this paper presents a hybrid approach combining fuzzy logic and the extended Kalman filter (EKF). The proposed algorithm has been applied to the real-time reconstruction of the temperature field, time-varying convective heat transfer coefficient, and time-varying boundary heat flux using temperature data from a set of sensors. The effects of the covariance of the initial estimation error, the time step of data acquisition by the sensors, and the number of installed sensors on the precision and stability of the estimation results has been investigated by numerical experiments. To compare and validate the results, all these parameters were investigated using the EKF method. The results show that the adaptive fuzzy extended Kalman filter (FEKF) method for estimating temperature, convection coefficient, and heat flux at the boundaries is precise and stable. The comparison shows that the performance of the FEKF method in parameter estimation is better than the EKF method, and the highest increase in precision and stability of the results is related to the convective heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The combination of the high conductivity of nanofluids with the huge accessible surface area of open-cell metal foam would indeed be a superb heat exchange technology. While individual ingredients of this technology have been separately investigated, the combination of the two has received only scant attention. Actually, there are only few studies that are focused on nanofluids flow and heat transfer in metal foam. This paper is intended to serve as an orientation for researchers who are interested in both nanofluids heat transfer and metal foam. The paper thus includes discussion of various aspects of nanofluids and their heat-transfer applications, as well as fundamentals of flow and heat transfer of ordinary fluids in metal foam. A section on recent heat transfer studies in metal foam is included, and another section on pore-scale modeling of transport in metal foam is provided. Nanofluids are presented in terms of their definition, brief history, preparation methods, theoretical models, properties, and engineering applications related to heat transfer. The paper then moves to describe metal foam and its morphology, as well as current understanding and theoretical treatment of flow and heat transfer in this class of highly-porous media. Key heat transfer and flow properties of the foam are outlined. The paper also presents the case for nanofluids forced convection heat transfer in open channels to serve as a background for the more complex case that involves heat transfer in porous media in general. The case of forced convection of nanofluids in metal foam in particular is presented next. Studies including complex effects, e.g. magnetism, are briefly described. Based on the above information, some key findings, conclusions and recommendations are made for interested researches. [ABSTRACT FROM AUTHOR]

This paper presents a model of heat and moisture transfer through multilayer firefighter protective clothing, which for the first time has taken into account all the major heat and moisture transport mechanisms including conductive and convective heat transfer, two-flux radiative heat transfer, moisture bulk flow, moisture absorption/desorption by fibers, moisture diffusion and phase change. The model was solved by Crank–Nicolson scheme of the finite difference method. Numerical analysis showed that our model can accurately predict the second-degree burn time with an error ranging from 0.37% to 3.55% for different types of clothing assemblies. With this model, the distributions of heat and moisture within the protective clothing system are elucidated and the relative importance of key clothing parameters on thermal protection better understood. It revealed that the thickness of the moisture barrier layer has the greatest influence on the skin burn time. This study is significant in providing a theoretical framework for optimizing the design of thermal protective clothing. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants. • Fluid flow and heat transfer in a coiled flow reverser (CFR) were studied. • Effects of tube insert on the heat transfer enhancement of CFR were investigated. • Effects of perforation of tube insert on heat transfer enhancement of CFR were examined. • The effects of tube insert at laminar flow are more than turbulent flow. • Perforation has no significant effects on the thermal performance ratio. [ABSTRACT FROM AUTHOR]

The aim of and inspiration behind this paper was to explain the reasons, also observed by other researchers, of the discrepancy in the results of experimental free convection, which for small Rayleigh and Nusselt numbers in the initial phase of research can sometimes reach several hundred percent. These discrepancies decrease with increasing heating power and plate surface temperature, in proportion to the increase in Ra and Nu , reaching typical values for this type of research. To explain this phenomenon, a comprehensive theoretical and experimental analysis of the influence of the physical properties of a fluid (air and water) as well as primary (t w and t ∞) and secondary (t av and Δ t) temperatures on the Rayleigh number was carried out. The impact was found to be unequal. The plate temperature t w is of greater importance, which is much higher than the much lower and almost constant temperature t ∞ of the undisturbed area, especially since it causes convective movement, generating differences in fluid density and thus driving the phenomenon. Similarly, the direct contribution of the temperature difference Δ t to Ra suggests that it has a greater influence on convective heat transfer than the average temperature of the medium t av. By analysing the effect of each of these temperatures separately, it was possible to show that their mutual, compatible or opposite interaction (t w / t ∞) causes a different scattering of results, or may even lead to unusual Rayleigh numbers (Ra temperature dualism). This study led not only to a better understanding of the phenomenon, but even to a prediction of its unusual behaviour, unheard of in typical experimental studies of free convection. For example, if we consider the theoretical convective heat transfer from a plate l = 0.15 m in air in the context of the interaction of t av and Δ t , it turns out that for the same Δ t = 40 K, the Rayleigh number may assume, depending on t av = (t w + t ∞)/2, different values. So, for t w = 50 °C, t ∞ = 10 °C and t av = 30 °C, Ra = 1.213. 107, whereas for t w = 90 °C, t ∞ = 50 °C and t av = 70 °C it is ≈ 1.7 times smaller (Ra = 0.687. 107). This hypothetical phenomenon, unheard of in typical experimental studies, which could occur, and maybe even does occur in smelting, thermal energy, etc., forces us to think about the values of Nusselt numbers, heat transfer coefficients and heat fluxes for these two cases. This lies beyond the scope of the present paper, but it is a topic for possible future research. • A theoretical analysis of the influence of t w , t ∞ , Δ t and t av on values of Ra has been carried out. • A coefficient of the convective properties of the medium has been introduced. • It has been demonstrated that the influence of t ∞ and t av on convective heat transfer is as important as t w and Δ t. • Theoretically, the possibility of Ra temperature dualism has been demonstrated. [ABSTRACT FROM AUTHOR]

Vehicle braking systems use sliding friction between discs and pads to convert large amounts of kinetic energy into heat. Thus, sufficient cooling of disc brakes is needed. Convective heat transfer is an efficient way to cool disc brakes and prevent very high temperatures. To enhance convective heat transfer, ventilated disc brakes are commonly used. Therefore, the heat transfer characteristics of the internal passages of ventilated discs are of special interest. This paper describes a new experimental method to study the convective heat transfer characteristics of the interior passages of ventilated disc brakes, in which a wheel rolled a ventilated disc with tapered radial vanes on a circular track to simulate the real working conditions of a ventilated disc brake. The naphthalene sublimation method was used to obtain convective heat transfer characteristics by the heat/mass transfer analogy principle. Different data reduction methods were used to present the experimental results. The correlations among the Nusselt number, Reynolds number and other geometric parameters of the tapered radial vanes are reported in this paper. The results indicate that the heat transfer coefficient has a close relationship with Coriolis acceleration. Thus, this paper reports a correlation between Coriolis acceleration and the convective heat transfer coefficient, which can be used to extend the results obtained by the tested discs to realistic cases. • A new model to study the heat transfer ability of the interior passage of ventilated disc brake is provided. • Correlations of the interior passage of a ventilated disc brake with tapered radial vanes is obtained. • Relationship between Coriolis acceleration and heat transfer capacity is addressed. • Relationship between Coriolis acceleration and heat transfer capacity can extend the experimental results to real case. [ABSTRACT FROM AUTHOR]

Devices utilizing high voltage direct current (HVDC) necessitate effective heat dissipation using heat sinks to ensure stable and dependable operation. This paper introduces a novel spiral grooved cooling path heat sink concept, which uses a combination of propylene glycol and water as a coolant. A numerical analysis thoroughly explores how modifications to the cooling path structure can affect fluid dynamics and heat transfer characteristics. A series of simulations are conducted to scrutinize the performance of these innovative geometric modifications according to Friction factor, Nusselt number and Thermal performance factor. The heat sink's performance is also examined in the context of the working fluid's flow rate. Results show that the novel spiral grooved cooling path heat sink design exhibits the best thermal performance compared to a plain cooling path heat sink. Among the spiral grooved cooling path designs, SGCP 2 shows the highest thermal performance factor, indicating higher convective heat transfer rates. The thermal performance factor of SGCP 2 is 2.6–5.0% higher than SGCP 1 and 1.3–1.4% higher than SGCP 3. The proposed numerical approach provides a comprehensive understanding of cooling path configurations for heat sinks used for the purpose of cooling HVDC devices. [Display omitted] • A novel heat sink is presented for the cooling purpose of HVDC devices. • Experimental results are well matched with simulation results. • SGCP heat sinks exhibit higher heat transfer rate as compared to PCP heat sink. • Among SGCP heat sinks, SGCP 2 shows enhanced thermal performance factor. [ABSTRACT FROM AUTHOR]

Deep reinforcement learning (DRL) has gradually emerged as a novel and effective method for intelligent control of conjugate heat transfer. Through proper training, DRL agent usually can find a better control strategy than the one optimized manually. For numerous numerical simulation-based investigations, the promising results have only been obtained on 2-dimensional models due to the heavy burden of data acquisition. This paper proposes a novel strategy of transferring the control policy learned in 2-dimensional environment into a new 3-dimensional environment. PDD-DQN (Prioritized Dueling Double Deep Q-Networks) algorithm is used to recognize the underlying relationship between the forced fluid flow and heat transfer performance to produce a control strategy. The DRL controller is first trained on a simple 2-dimensional cavity with one heat source, and the DRL controller is able to reduce the maximum temperature to 315 K which is 2 K lower than the manually optimized control strategy; then the control strategy is transferred to 3-dimensional models where the maximum temperature is further reduced to 308 K; and compared to training the DRL agent directly on 3-dimensional model, the policy-transfer strategy requires only 10% computational expenditure. The proposed strategy is then applied to a more complex testbed with multiple heat sources for further investigating its ability; in both 2D and 3D environment, the DRL controller can cool the maximum temperature to be 307 K which is 8 K lower than the manually optimized control policy; and the computational cost drop to be 3%. The generalization ability of the trained DRL agent in inter-dimensional geometries is confirmed. For the cases with huge grid size, the policy-transfer strategy can economize exponential computational cost, which is of great significance for applying DRL methods to practical thermal control problems. [ABSTRACT FROM AUTHOR]

In this paper we carry out a numerical investigation of forced convection heat transfer from a heated elliptical cylinder in a uniform free stream with angle of inclination θ °. Numerical simulations were carried out for 10 ≤ R e ≤ 120 , 0 ° ≤ θ ≤ 18 0 ° , and P r = 0. 71. Results are reported for both steady and unsteady state regime in terms of streamlines, vorticity contours, isotherms, drag and lift coefficients, Strouhal number, and Nusselt number. In the process, we also propose a novel method of computing the Nusselt number by merely gathering flow information along the normal to the ellipse boundary. The critical R e at which flow becomes unsteady, R e c is reported for all the values of θ considered and found to be the same for θ and 18 0 ° − θ for 0 ° ≤ θ ≤ 9 0 ° . In the steady regime, the R e at which flow separation occurs progressively decreases as θ increases. The surface averaged Nusselt number (N u av ) increases with R e , whereas the drag force experienced by the cylinder decreases with R e. The transient regime is characterized by periodic vortex shedding, which is quantified by the Strouhal number (S t). Vortex shedding frequency increases with R e and decreases with θ for a given R e. N u av also exhibits a time-varying oscillatory behavior with a time period which is half the time period of vortex shedding. The amplitude of oscillation of N u av increases with θ. • For steady state, flow for θ is a mirror image of 18 0 ° − θ. • The surface averaged Nusselt number increases with Re for a given θ. • At steady state, drag on cylinder decreases with Re and increases with θ. • Vortex shedding frequency increases with Re, and decreases with θ for a given Re. • N u av varies periodically with time, its time period = 1 / 2 × the time period of vortex shedding. [ABSTRACT FROM AUTHOR]

This paper studied the buoyance-aided mixed convective heat transfer of air-cooling flow in a 5 m long and 1.2mX0.3 m cross-section vertical channel. One of the channel walls was uniformly heated and the rest three walls were unheated and insulated. Heat was transferred from the heated wall by convection and radiation, and heat was also transferred from unheated walls to the air flow by convection originated from their net radiation gain from the heated wall. The air flow rate, the inlet and outlet air flow temperatures, and local surface heat fluxes and temperatures on the heated wall were measured. The Reynolds number of air flow was from 2900 to 221,000 and Richardson number was changed from 0.006 to 90.6. The results showed that for walls with larger emissivity the total convection heat transfer was not deteriorated in mixed convection zone. Therefor a modified heat transfer correlation was developed for the aiding mixed convection in the asymmetrically heated channel considering both mixed convection and radiation. It predicted the experiment results with a deviation less than 18%. [ABSTRACT FROM AUTHOR]

In order to improve the heat transfer performance of microchannels, considering from balance flow resistance and increasing heat transfer area, a non-closed droplet pin fin array microchannel has been designed. The purpose of this paper is to study the inner influence of non-closed droplet structure on convection heat transfer by means of simulation. Through the simulation tool ANSYS Fluent, the working conditions of heat flux density from 5 W/cm2 to 85 W/cm2 are simulated respectively, with the inlet flow as 1.5 kg/h and the inlet fluid temperature as 40 °C unchanged. Along with the increase of heat flux density (5 W/cm2 ⩽ q ⩽ 85 W/cm2), the pressure drop, the bottom temperature as well as the heat transfer coefficient continue to rise, but the rising rates are various. Such variation could be explained by the different heat exchange processes: single phase heat transfer, bubble nucleation, growth and migration and then dry phenomenon, which can be observed from the vapor phase distribution results. The distribution of vapor phase under multiple heat flux explains the variation of pressure and temperature in different stages. It could be concluded that the non-closed droplet pin fin has a strong fractal effect on the fluid. After multiple shunting and confluence, the flow velocity of the fluid on both sides is significantly greater than that between the fins, whose average velocity is 0.135 m/s faster than that between the pins. Besides, the heat transfer effect of the corresponding walls on both sides is also better than that between the pin fins. Because the pin fin has the characteristics of droplet structure, the boundary layer on both sides of the pin fin will be separated and the heat transfer performance will be enhanced. The temperature of the pin fin head is lower, and the temperature of the pin fin tail is higher, with an average difference as about 9.32 °C. The comparison research between non-closed droplet and traditional droplet pin fin has been conducted, as well. It could be concluded that when 5 W/cm2< q < 60 W/cm2, the heat transfer coefficient of the opening droplet microchannel is higher, with pressure drop lower. The main reason is that the improvement of the non-closed structure reduces the kinetic energy loss caused by the collision, and promotes the emergence and growth of the vaporization core. • With the increase of heat flux, the pressure drop increases slowly, rapidly and slowly. • With the increase of heat flux, the initial point of phase transition gradually moves forward from the rear section. • The non-closed structure has a strong fractal effect on the fluid. • The boundary layer on both sides of pin fin will be damaged and the heat transfer performance will be enhanced. [ABSTRACT FROM AUTHOR]

This paper examines the influence of the distance between vertical plates on the intensity of free convective heat transfer along with the optimization of this distance. Experimental tests were carried out for one model channel of such an heat exchanger with widths s = 0. 045 , 0.085 and 0.18 m. This channel, open at the top and sides, was formed by two isothermal symmetrically heated parallel vertical plates of dimensions H = 0. 5 m and B = 0. 25 m. The influence of the heating surface temperatures t w = 30 , 40, 50, 55, 60 and 70 °C on the convective temperature fields and velocity generated inside the channels was investigated. Directly measured temperature fields, as well as velocity fields measured indirectly using the NRP, enabled the thermodynamic parameters of the heat exchanger to be determined. Based on the temperature gradient distribution ∂ T w ∂ x on the wall, its average value was determined for each of the plates and for the entire channel, after which the heat flux Q w transferred from the plates was calculated. The heat flux transferred with the air Q and the efficiency η = Q / Q w of heat transfer in the channel were determined using the balance method based on the average temperatures and air velocities at the inlet t ̄ b o t t and u ̄ i n and at the outlet t ̄ t o p and u ̄ o u t of the channel obtained from the temperature and velocity fields. A grid placed vertically in the channel, halfway across the panel width and perpendicular to the heating surfaces was used to detect the temperature field in air. The image and matrix of these temperatures were determined using a thermal imaging camera. The numerical reconstruction procedure (NRP) was used to determine the velocity field. For technical reasons and lack of time it was not possible to test more channels. Since three values of the width s were insufficient to determine s o p t , it was decided to use numerical research. For this purpose, numerical results were compared with those obtained experimentally for the channels already tested. As the compliance verification was successful, further studies of the channels were continued numerically. As a result of optimizing the heat exchange intensification, optimal widths of a single channel s o p t = 0. 035 m and n channels s 0 = 0. 0123 m, included in the heat exchanger, were obtained. • Experimental tests for one model channel of an heat exchanger were carried out. • Numerical method of reconstructing convective velocity fields in channel was used. • Verification of the NRP with experimental CTM and SNC results was conducted. • The optimal width of the isothermal, vertical channel was obtained. • The results confirmed usefulness of NRP for heat fluxes determination. [ABSTRACT FROM AUTHOR]

The thermal protection of the rocket engine combustion chamber is one of the fundamental challenges in supersonic flight. This paper investigates serpentine and furrowed wavy cooling channels with a rectangular cross-section to enhance heat transfer performance and hydrogen turbulent fluid flow characteristics. Ansys fluent was used to investigate the effects of configuration, wavy amplitude, and channel wavelength on the heat wall temperature, Nusselt number, coolant velocity, pressure drop, turbulent kinetic energy, and thermal stratification phenomena. Four different wave amplitudes and three wavelengths are compared to determine the optimal structure required to achieve the best heat transfer enhancement with acceptable pressure drop. Compared to the straight channel, the maximum temperature of the furrowed channel (Case 6) was decreased by 15.38% at an amplitude of 0.2 mm. Furthermore, the thermal performance factor is evaluated for eight furrowed wavy structures. The result is shown that the structure with an amplitude of 0.2 mm and a wavelength of 1 mm (Case 6) can demonstrate better thermal performance, which is increased by 31.55% compared to the straight channel. The wavy surfaces cause separation in diverging zones and secondary flows in converging zones, which disturbs the boundary layer, enhances particle mixing and improves convective heat transfer. • The effect of wavy surfaces on heat transfer and fluid flow properties is studied. • The pressure drop for all configurations is compared. • Thermal Performance Factor is used to obtain the optimum configuration channel. • The thermal performance factor is considerably enhanced by a maximum of 31.55%. • Wavy channels decreased non-uniform temperature distribution behaviour. [ABSTRACT FROM AUTHOR]

For the cooling of transformer in the grid, the heat transfer efficiency of mixed convection is relatively low due to the low thermal conductivity and large viscosity of transformer oil. Therefore, this paper aims to study the PCD (periodically changed direction) electrical field to enhance the mixed convective heat transfer of transformer oil. Then, a semi empirical correlation is developed to calculate the Nusselt number affected by electrical field. Specially, the influences of heat flux, electrical field intensity and switching period on the heat transfer characteristics are discussed experimentally. As a result, the result reveals that the mixed convective heat transfer is enhanced increasingly with the decrease of switching time and improvement of voltage. Thus, the PCD electric field can effectively enhance the mixed convection heat transfer. In addition, the mechanism analysis shows that the PCD electrical field leads to the continuous change of fluid mode, and the polarized molecules continuously vibrate and move within a certain range, which leads to the violent disturbance of internal fluid. Meanwhile, the secondary flow induced by the electric field results in the disruption of thermal boundary layer, which strengths the heat transfer. Finally, the predicted results of the semi-empirical correlation are in good agreement with the experimental results, and the effect of electrical field intensity and switching periods are comprehensively considered in the correlation. The present study is of great importance for the transformer cooling. [ABSTRACT FROM AUTHOR]

In this paper, a new method of estimation of a convective heat transfer coefficient on a hot spot is presented. First of all, a complete numerical model is developed in order to simulate the convection on a wall heated at a hot spot. Then, an analytical solution of the configuration is developed assuming simplified assumptions. Finally, an inverse method is performed to estimate the local convective heat transfer on a hot spot using noisy simulated data. The results of the estimation procedure are accurate and validate this new approach of convection measurement. • A 3D completed model of forced convection of a heated plate at a hot spot is developed. • An explicit analytical calculation using the Fourier and Hankel integral transforms. • Identification of the local convective coefficient is performed using simulated data. [ABSTRACT FROM AUTHOR]

In this paper, simulations of supercritical heat transfer of R134a in horizontal tubes are performed to study the effects of circumferential heat conduction on the heat transfer deterioration, and the abnormal phenomena of higher temperature distribution of non-gravity supercritical flow is explained. The results show the heat transfer deterioration is caused by the impairment of specific heat and heat conduction in the boundary layer while the following recovery of heat transfer is due to the enhancement of thermal conduction and turbulent convection. A dimensionless parameter of Biot number is defined to characterize the thermal resistance ratio of the circumferential conduction to the convective heat transfer. The wall temperature redistribution due to the circumferential conduction affects the supercritical convection in horizontal tubes, and non-gravity supercritical flow may have a higher wall temperature when the Bi number is small. The deterioration of the top surface can be significantly alleviated in a tube with a larger wall thickness or thermal conductivity. • Biot number characterizes the circumferential conduction in supercritical flow. • Non-gravity flow has higher wall temperature at small circumferential Bi number. • High thermal conductivity or wall thickness alleviates heat transfer deterioration. [ABSTRACT FROM AUTHOR]

This paper reports an experimental investigation on the convective heat transfer and flow performance of silica-zinc oxide (SiO 2 –ZnO)/water hybrid nanofluid (HNF) flowing inside a tube with constant heat flux at different Reynolds number (Re) from 7743 to 23,228. A well-stabilized SiO 2 –ZnO/water HNF with 0.025–0.10weight fractions(φ) was produced via two-step method. Thermal conductivity and viscosity of HNF were measured. The convective heat transfer and flow characteristics of the HNF were determined. Thermal conductivity and viscosity of HNF showed notable enhancements as compared to water. The convective heat transfer coefficient (CHTC) of HNF enhanced with increasing φ and Re. A maximum of 29.44% enhancement in CHTC was obtained with φ of 0.10 at Re = 20,131 when compared to water. Further, the friction factor and pressure drop of HNF increased with increasing φ. A maximum of 17.45% increase in friction factor and a maximum increase of 24% in pressure drop were recorded with φ of 0.10 at Re = 23,228 as compared to water. New CHTC and friction factor correlations of high accuracies (R2>90%) have been proposed. A Figure of Merit (FOM) was drawn to describe the thermal performance of HNF. Based on the analysis, the optimum thermal performance was achieved with φ of 0.10 at Re = 20,131. [ABSTRACT FROM AUTHOR]

This study focuses on the innovative integration of different cooling techniques to maximize the thermal performance in a 3D microchannel heat sink (MCHS). The paper discusses nanofluid flow and thermal fields in the MCHS with protrusions subjected to double synthetic jets (SJs). A validated multiphase mixture model is used to study the effect of Al 2 O 3 particles in water. The effect of particle concentration, orifice spacing, orifice height, oscillation frequency, membrane amplitude, and phase actuation on convective and conductive heat transfer is analyzed. Heat transfer is significantly influenced by the particle concentration in both active and inactive SJs. Overall, 180° out-of-phase jet configurations show better performance in terms of convective heat transfer, while in-phase jet arrangements are more efficient in conductive heat transfer enhancement. In terms of thermal performance, 180° out-of-phase cases show a higher figure of merit (FOM) than in-phase SJs. Increasing the frequency and amplitude enhances the heat transfer. In the case of nanofluids, the maximum degree of convective enhancement of 1.23 demonstrates the effectiveness of SJs and protrusions. However, the maximum degree of overall thermal enhancement of 54% is obtained at in-phase actuation under the same conditions. [ABSTRACT FROM AUTHOR]

Emmons' solution does not include a development length in front of the flame, which causes the thermal and velocity boundary layers to have different starting locations. In this paper, the effect of the unheated starting segment on solid burning was first experimentally investigated over a horizontal 10 cm × 10 cm × 1.5 cm Poly (methyl methacrylate) sample under 0.75 m/s concurrent airflow with varying unheated lengths L 0 (5, 10, 20 cm). The flow field, flame structure, and gas-phase temperature profile were experimentally recorded, and the subsequently produced flame standoff distance and dimensionless temperature gradient were combined to reveal the effect of the unheated segment on the boundary layer combustion. The parameters directly related to fire hazard, like heat transfer coefficient (burning rate), and mass loss rate were also measured and theoretically deduced, to obtain the effect of the unheated segment on wind-driven fire development. Results demonstrated that owing to the presence of the unheated segment, the pressure gradient is misaligned with the density gradient to be prone to produce vortices and form streaky flames. The streak structure distributed in the flame is associated with a stronger heat transfer, resulting in a larger convective heat transfer coefficient than the theoretical ones. Due to the viscous drag effect, with increasing unheated length, the flame standoff distance increases while the convective heat transfer coefficient decreases. Additionally, coupled effects of the unheated length and forced airflow, solid burning is a transient process. As time progresses, with the valley and vortex effect, the flame standoff distance increases and decreases in the leading and trailing section, respectively, and the differences in flame standoff distance and heat transfer coefficient for varying unheated lengths are enlarged. Eventually, the mass-loss rate has an increasing tendency accompanied by a decreasing burning contribution of the leading section. [ABSTRACT FROM AUTHOR]

The widely used fin-and-tube heat exchangers have a low air-side heat transfer capacity. Enhancing the air-side thermo-hydraulic performance is a key point of the relevant research. Vortex generators have been proven to be one of the most promising techniques to improve the air-side heat transfer capacity of the fin-and-tube heat exchangers. Herein, this paper proposed an original sinusoidal wavy winglet type vortex generator to achieve high heat transfer performance with moderate increased pressure drop penalty. A three-dimensional numerical simulation was conducted to explore the air-side thermo-hydraulic performance with Reynolds number ranging from 1027 to 2054. The longitudinal vortex is generated in the channel so as to enhance the intensify of convective heat transfer. Especially, the wave-shaped vortex generators can produce many local small-scale secondary flows. And it is confirmed that the frontal area of the vortex generator can be effectively extended by such wavy shape and a more intensified disturbance can be caused in the air-side. Furthermore, the effects of the attack angle (α), dimensionless height (H p) and especially arrange position of the vortex generator were investigated. Additionally, the ranges of the Nusselt number ratio, friction factor ratio and surafece goodness factor in this study are 1.09–1.52, 1.09–2.31, and 1.06–1.24, respectively. Moreover, it is also very crucial to evalute the grade of energy and to explore the irreversible loss druing the heat tranfer process in view of the second law of thermodynamics. And the thermal dissipation can be reduced by up to 5.85% compared with the plain fins from the perspective of exergy analysis. [ABSTRACT FROM AUTHOR]

the surface-modified film cooling, which can be realized by paving thermal barrier coatings (TBCs), is widely studied and is proven to be effective in improving cooling performance. In this paper, micro-riblets, a type of bio-inspired microscale surface patterns, were introduced for surface modifications of TBC-film cooling system and its improvement effects on cooling performance were mainly analyzed. In order to more accurately evaluate the flow field and temperature field of the model, the heat transfer effects of heat conduction, heat convection and heat radiation were considered, and the correctness of the calculation method was verified by compared with the experimental results. For the cooling performance under the condition of adiabatic wall, the model of micro-riblets surface could effectively improve the adiabatic film effectiveness compared with the flat-plate surface model, and the advantage was more obvious as the blowing ratio increased. In addition, the characteristics of conjugate heat transfer in the real serving environment were further considered. By analyzing the surface temperature of the TBC and the superalloy, the results showed that the micro-riblet structure could reduce the temperature of each layer of the model. The surface temperature of TBC-film cooling system with bio-inspired micro-riblets structure was cooled down with maximum amplitude of about 50K–60K. As the thickness of the micro-riblet increased, its cooling performance first increased and then decreased. Therefore, there is an optimal height of the micro-riblet structure that makes the cooling performance optimum. [ABSTRACT FROM AUTHOR]

The paper presents an analytical and a numerical approach to studying the effectiveness of the cooling of a granular target by gaseous helium. The increase in the target temperature is caused by an impinging beam which deposits a very high power of 138 kW inside the limited volume of the target. A porous domain approach is used to model the flow of helium through the packed-bed target. An efficient one-dimensional analytical model is proposed to describe the transverse flow of gas helium, which accounts for its compressibility and the heat exchange between the target spheres and the cooling gas. The predictions of this model are shown to be in good agreement with more complex numerical studies done with Fluent. [ABSTRACT FROM AUTHOR]

Minichannels are a key technology that results in energy, space, material, and cost savings in shell and tube heat exchangers (STHEs) because they improve performance by increasing compactness and reducing the dimensions, volume, and mass, as well as the amount of working fluids used. This paper employed the Computational Fluid Dynamics (CFD) method to investigate the effect of minichannels on the thermal and hydrodynamic performance of a STHE. The reliability and accuracy of numerical results were corroborated with experimental results. Numerical solutions were performed under constant heat flux conditions at 2650 < Re < 9500, the range in which internal flows are considered transition and turbulent flows. The results showed that the higher the Reynolds (Re) number, the higher the convective heat transfer coefficient (CHTC) and pressure drop. The performance benchmark (ψ) is defined as the variations in CHTC per unit of pressure drop. The results showed that the higher the Re number, the lower the ψ. Based on numerical solutions regarding ψ, the flow in the minichannel tube reached optimal conditions both thermally and hydrodynamically at Re ≅ 5900. The deviation between the numerical and experimental CHTCs ranged from −15 % to +15 %, while that between the numerical and experimental friction factors ranged from −13 % to +13 %. These results can be used for engineering designs and applications. • The heat transfer and pressure drop of a minichannel were numerically investigated. • Numerical and experimental results were compared. • The developing flow in the 2700 < Re < 7500 and 15 < x/D < 31 region increased the average convective heat transfer coefficient in the minichannel. • The optimum operating conditions were obtained at Re~6000 for ψ = 1. [ABSTRACT FROM AUTHOR]

Theoretical analysis of heat transfer in a multilayer body is relevant for several engineering applications where heat transfer occurs through a stack of thermally dis-similar materials. In most of such literature, a constant convective heat transfer coefficient is assumed as a boundary condition at one end of the body. In contrast, there is a lack of work to model a problem with spatial variation in the convective heat transfer coefficient. This paper presents an analytical solution for thermal conduction in a multilayer rectangular body with spatially varying convective heat transfer coefficient at one end. In addition, this solution also accounts for internal heat generation and inter-layer thermal contact resistance and spatially varying heat flux on the other end of the body. The solution is derived in a series form, and it is shown that the coefficients of the series can be determined by solving a well-defined set of linear algebraic equations. The analytical solution is verified by comparison with numerical simulations, as well as with standard solution for a simplified special case. The impact of spatially varying cooling provided by jet impingement is analyzed using the analytical solution. A resource optimization problem pertaining to the optimal use of jet cooling is solved. Results presented here may benefit thermal management analysis of a broad variety of engineering applications involving heat transfer in multi-layer bodies. [ABSTRACT FROM AUTHOR]

With the development of heat engines, in some fields the heat wall suffers from dynamic heat flux, thus some cooling control techniques are needed to meet this dynamic cooling requirement. The traditional stationary vortex generators have not met the cooling demand in these cooling fields with dynamic heat flux. Existing studies have shown that the capacity of convective heat transfer depends on the distribution characteristics of the velocity and temperature field of the cooling fluid. Therefore, considering to the factors of single-side heat transfer and fluid flow boundary layer, new and efficient mixing heat technology is increasingly demanded for many industrial fields. In this paper, the dynamic turbulence generator such as spherical cylinder is adopted to study the influence of flow vortexes on heat transfer and achieves a better distribution of velocity field and facilitates the enhancement of convective heat transfer. Research results indicate that compared to traditional stationary vortex generator technologies, vibration spherical cylinder can effectively improve heat transfer performance and leads to a lower flow resistance. As a conclusion, the flow and heat transfer performance increase with the vibration frequency increasing. When Re = 7255, the thermal hydraulic performance is increased by a maximum 24.55% than traditional stationary vortexes generator. When Re = 14510, the performance is better than stationary cylinder structure. And the vibration frequency can be adjusted timely according to cooling demand. [ABSTRACT FROM AUTHOR]