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In this study, the cooling of a gas turbine (GT) blade leading edge using a single row of five impinging air jets was numerically and experimentally investigated. The inner surface of the blade leading edge was depicted as a semicircle profile. The numerical study was performed using a shear stress transport (SST) k-ω turbulence model. On the other side, the experimental work was conducted using the infrared (IR) thermal camera. Additionally, the heat transfer characteristics were investigated with changing the Reynolds number (⁠Re ⁠) from 5000 to 40,000. The study was carried out for different cases based on the jet diameter (⁠d ⁠), named as fixed diameter (FD) and variable jet diameter in the streamwise flow direction, i.e., ascending diameter (AD) and descending diameter (DD). The local and average Nusselt numbers and heat transfer uniformity were studied at different operating and design parameters. The results revealed that for FD, increasing the jet diameter decreased the average Nusselt number while it enhanced the heat transfer uniformity. Furthermore, for all values of Re ⁠, the thermal performance of the AD was better than the DD, which has the worst heat transfer uniformity. Finally, predicted developed correlations to estimate the average Nusselt number, surface heat transfer uniformity, and performance evaluation factor (PEF) for all cases of jet diameter were introduced. [ABSTRACT FROM AUTHOR]

This article presents new semi-analytical solutions for transient heat conduction in a slab, which exchanges heat at the surface with its surroundings via convection, radiation, or simultaneous convection-radiation mechanisms. The concept of thermal penetration depth together with the integral method of heat balances form the basis of the formulation. Explicit expressions are derived that eliminate the need for numerical integration of the ordinary differential equations (ODEs) obtained from space integration of the heat equation. Verification of the temperature relations with a convection boundary condition is performed using an exact solution. In the case of radiation and combined convection-radiation boundary conditions, the accuracy of the explicit solutions is assessed against a numerical model. In all three cases, the new relations predict the surface temperature with high precision. For the radiation boundary condition, the computed midpoint temperature is also in excellent agreement with the numerical solution. In the case of convection heat exchange at the surface, the temperature at internal locations has close agreement with the exact solution for Biot numbers up to Bi = 1. The model prediction deviates from the exact solution in the interior positions for higher values of Bi with the highest deviation occurring at the center. For example, at Bi = 3, the highest relative error is 7.5%. Likewise, for the case of combined convection-radiation boundary conditions, the predicted midpoint temperature is found to closely follow the numerical solution at early stages of the process only. The predictability of the explicit solution of the convection-only boundary condition is more accurate than other approximate solutions at a Fourier number less than 0.2. Additional relations are presented for determination of the total heat transfer as a function of the Fourier and Biot numbers. [ABSTRACT FROM AUTHOR]

The single bubble dynamics and local thermal effects in a rectangular channel are investigated in this paper under subcooled nucleate flow boiling conditions. A combined effect of Marangoni and microlayer evaporation in flow boiling regimes has rarely been reported in open literature. Therefore, a comprehensive boiling model that combines the three submodels, namely, phase change model, microlayer evaporation model, and Marangoni model, is developed that accounts for small-scale physics such as evolution of superheat layer during bubble growth, microlayer evaporation, and scavenging of the superheated liquid during the bubble departure. The verification of model has been carried out through detailed flow and temperature field validation exercises of various bubble stages with recent experimental data reported in the literature. The effects of varying subcooled conditions and Reynolds number on bubble dynamics and the associated heat transfer rates have been examined. The study reveals a decreasing trend in the bubble diameter with increasing Reynolds number and degree of subcooling. It has also been observed that the bubble shape is affected by the Marangoni phenomena. Bubble shape slightly flattens during inception, gradually becomes spherical while sliding, and later elongates after liftoff. The individual contribution of microlayer heat flow (Qmicrolayer) is estimated to be around 22-40% for flow boiling conditions and it is the second-highest heat transfer contributor after the latent heat transfer. The results obtained from the proposed model show a good match with published data and indicate the significance of microlayer in the single bubble flow boiling heat transfer. [ABSTRACT FROM AUTHOR]

An exhaustive review was undertaken to assemble all available correlations for supercritical CO2 in straight, round tubes of any orientation, with special attention paid to how the wildly varying fluid properties near the critical point are handled. The assemblage of correlations, along with subsequent discussion, is presented from a historical perspective, starting from pioneering work on the topic in the 1950s to the modern day. Despite the growing sophistication of sCO2 heat transfer correlations, modern correlations are still only generally applicable over a relatively small range of operating conditions, and there has not been a substantial increase in predictive capabilities. Recently, researchers have turned to machine learning as a tool for next-generation heat transfer prediction. An overview of the state-of-the-art predicting sCO2 heat transfer using machine learning methods, such as artificial neural networks, is also presented. [ABSTRACT FROM AUTHOR]

Jet impingement finds an important role in several industrial applications which require high thermal-hydraulic performance of heat exchange systems. The self-exciting sweeping jets produced by fluidic oscillators provide superior thermal and hydraulic performance as compared to plain jets. The fluidic oscillator increases the jet impingement area on the target surface by maintaining a steady and controlled oscillatory flow. The continuous advancements in the design improvement of fluidic oscillators have led to further enhancements of heat and fluid flow characteristics. In this context, double feedback fluidic oscillators have shown promising jet flow control features and jet impingement cooling characteristics. Therefore, the current study has been devoted to analyzing and assessing the recent research progress in design improvements of double feedback fluid oscillators and the augmentation of thermal-hydraulic characteristics of impinging sweeping. Moreover, the variations in the heat transfer and hydraulic performance of impinging jets for different shapes of target surfaces have been comprehensively examined. In the end, research gaps for future work have been highlighted. [ABSTRACT FROM AUTHOR]

In this paper, linear and nonlinear properties of thermohaline convection at the onset with the stress-free boundary conditions are investigated using perturbation analysis relevant to oceanic water and ground water. The nonlinear governing equations are expanded using linear stability solutions, related to the temperature, motion, and concentration in the series of nonhomogeneous linear equations. The proposed method yields finite amplitude steady solutions by means of successive approximations. The rate of heat transfer is studied up to sixth-order using an expansion of thermal Rayleigh number (Ra1) as proposed by Kuo (1961, "Solution of the Non-Linear Equations of the Cellular Convection and Heat Transport," J. Fluid Mech., 10(04), pp. 611-630). The heat transfer rate of the system depends on Lewis number, Prandtl number, and thermal (Ra1) as well as solutal (Ra2) Rayleigh number. The result for the heat transport (N) in the system fits a power law of exponent of 0.288, i.e., N≃(Ra1/Ra1o)0.288 for Ra2=0 ⁠, Pr = 10 and Ra1>4Ra1o ⁠. This power law of exponent value of 0.288 is close to the results (0.25) obtained by Kuo (1961, "Solution of the Non-Linear Equations of the Cellular Convection and Heat Transport," J. Fluid Mech., 10(04), pp. 611-630), and also to the experimental results for laminar convection by Jakob (1949, Heat Transfer, Vol. 1, Wiley, New York.). The characteristics of heat transfer and flow field results are depicted by means of isotherms and streamlines, respectively. The path of convective heat transport and the comprehensive analysis of energy distribution by means of heatlines are explained using the concept of heat function. Laminar natural convection using entropy generation analysis due to fluid friction and heat transfer is also being studied. The numerical simulation for total entropy generation has been carried out for Prandtl numbers pertaining to ground water for different Ra1 and Ra2. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cells (SOFCs) generate electricity with high quality waste heat which if harnessed and used as energy source for vapor absorption refrigeration systems (VARS) will address the emission issues related to refrigerated transport infrastructure. The temperature range of the heat source required at the desorber of the VARS is between 120 °C and 200 °C, while SOFCs cathode exhaust heat temperatures are 600 °C and above. Therefore, an internally finned double pipe heat exchanger (DPHX) was used in this study with thermal oil as the coupling fluid to experimentally couple the SOFC cathode exhaust heat with the VARS indirectly. The experimental setup mimics a 5 kWe SOFC stack. Results showed that 1.84 kW of heat was recovered at a cathode exhaust flowrate of 24.64 L s-1 resulting in a heat exchanger effectiveness of 12.22% and overall heat transfer coefficient of 60.15 W m-2 K-1. The recovered 1.84 kW of heat are able to power a single effect VARS on board a small, refrigerated truck to supply 1 kW of cooling load. Results also revealed an exchanger effectiveness and overall heat transfer coefficient increase by 70.2% and 19.4%, respectively, at a reduced exhaust flowrate of 7.347 L s-1. Further improvement of 81.1% and 39.22%, respectively, of exchanger effectiveness and overall heat transfer coefficient was achieved at 4.653 L s-1 exhaust flowrate. [ABSTRACT FROM AUTHOR]

This paper presents an extension of the nonfield analytical method--known as the method of Kulish--to some nonlinear problems in heat transfer. In view of the fact that solving nonlinear problems is very complicated in general, the extension of the method is presented in the form of several important illustrative examples. Two classes of problems are considered: first are the problems, in which the heat equation contains nonlinear terms, while the second type of problems includes some problems with nonlinear boundary conditions. From the practical viewpoint, the case considering asymptotic solutions is of the greatest interest: it is shown that, for complex heat transfer problems, where applications of the nonfield method are practically impossible due to a large volume of necessary computations, it is still possible to analyze the solution behavior and automatically determine similarity criteria for the limiting values of the parameters. Wherever possible the obtained solutions are compared with known solutions obtained by other methods. The practical advantages of the nonfield method over other analytical methods are emphasized in each case. [ABSTRACT FROM AUTHOR]

Fluid flow and heat transfer of a gas stream in various ducts have been studied thoroughly before. However, in real applications, a gas stream usually contains dust particles, whose effects have typically been neglected. In this study, the effects of the dust particles on the flow and heat transfer characteristics in a parallel-plates duct were numerically investigated in detail. A lattice Boltzmann method combined with a modified immersed boundary approach was employed to calculate the velocity and temperature distribution in the duct. The effects of the particles on the development of the hydrodynamic and thermal boundary layers in the duct were predicted. The product of friction factor and Reynolds number (fRe) and local Nusselt number (NuL) along the flow direction were obtained for a particle-laden flow and compared with those for a pure gas flow. The results indicated that for particle-laden flows, the "fully-developed" flow was just an approximation. Both the flow and thermal boundary layers were disrupted by the accompanying particles. The particles would form a stable and dense particulate fouling layer at the walls; this could increase the local (fRe) and reduce the NuL in "fully developed" regions. Moreover, ducts with superhydrophobic properties would be less influenced by the particles due to decreased particle deposition because of the anti-dust property of the surface. [ABSTRACT FROM AUTHOR]

Heat transfer modeling of large industrial ovens such as those used in automotive paintshops is difficult due to the large and multiple scales and long curing times. The flow inside a convective oven is turbulent, and the process includes large temperature gradients. An efficient simulation requires a simplified and localized model of the heat transfer coupling. We present a novel method with three ingredients: localization using local Nusselt numbers of the oven nozzles, projection of Nusselt number profiles onto the target, and efficient conduction modeling on a coarse background mesh. The approach, which was developed in a research project together with the Swedish automotive industry, makes it possible to accurately simulate a curing oven with close to real-time performance. The simulation results are demonstrated to be in close agreement with measurements from automotive production. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the optimization of the geometric parameters and the coolant's inflow states of the perforated micropin-fins (MPFs) heat sink using an elitist nondominated sorting genetic algorithm-II (NSGA-II) coupled with a finite volume-based computational fluid dynamics (CFD) solver. Square-shaped MPFs with two circular perforations were considered for the investigations on the fluid flow and conjugate heat transfer using numerical simulations. Five design variables (two perforation diameters, their respective locations, and the inflow velocity) with the essential constrained equations were optimized to search for the optimal solutions. Two objective functions, viz., Nusselt number (Nu) and friction factor (f), were selected to evaluate the hydrothermal performances of the perforated MPFs heat sink. The optimization was performed for 52 generations with a population size of 30. We obtained the Pareto optimal solutions, which gave the design boundary of the important parameters. Some of the cases of the Pareto solutions were also investigated in detail to understand the underlying thermal physics and structural rigidity under thermal and hydraulic stresses. It is observed that the MPF's stiffness was not compromised upon introducing two perforations. This study identified different thermohydraulic features responsible for optimal performance at different inflow velocity regimes. The present paper demonstrates that this optimization technique has led to a better understanding of the underlying thermal physics of complex electronic cooling equipment while systematically exploring the design space. [ABSTRACT FROM AUTHOR]

The present investigation explores the dynamic effects of wall motion on the turbulent flow and thermal characteristics of a dual jet. The effects of variation in Reynolds number (Re) and offset ratio (OR) on different heat transfer and flow characteristics of turbulent dual jet have also been elucidated. The Reynolds number of flow and offset ratio are considered in the range Re=10000-25000 and OR=5-13⁠, respectively. The wall to jet velocity ratio (⁠Uw⁠) is varied in the range of 0-2 at an interval of 0.5⁠. The detailed study of heat transfer from moving impingement wall has been done by considering either the isoflux or the isothermal boundary condition on the heated wall. The motion of the wall strongly affects the thermal behavior and several flow characteristics of the dual jet. The nature of the velocity profile in the vector diagram changes from parabolic to linear in the far-field region due to the motion of the wall. The dual jet flow exhibits different patterns of self-similar profiles for different values of velocity ratio. The value of maximum longitudinal velocity (⁠Umax⁠) at a given axial position increases with the increase in impingement wall velocity. The exhaustive parametric study of dual jet flow over a moving wall reveals the fact that the process of heat transfer from heated moving wall to fluid is more intense for the higher value of Reynolds number and offset ratio, and for isoflux boundary condition. For the moving wall case, the maximum enhancement in heat transfer with reference to the stationary wall has been found to be 45.31%. [ABSTRACT FROM AUTHOR]

The current work enlightens the flow pattern and thermal scenario of a nanofluid spraying on an inclined permeable rotating disk. The whirling disk is assumed to revolve with the angular speed Ω. The water-based alumina (Al2O3) nanofluid is considered as a functioning liquid. The nanofluid spraying is treated to be magnetically influenced and thermally radiative. The perception of the nanoparticles' diameter and solid-liquid interfacial layer is incorporated precisely at the nanolevel to observe the thermal variations of the nanofluidic motion. How the magnetic effect, permeability, and nanolayer affect nanofluidic transportation is revealed in detail. The leading flow equations are altered nondimensional using apposite similarity translation, and the spectral quasi-linearization method (SQLM) is instigated to tackle those multi-ordered nonlinear equations. Various three-dimensional figures, graphs, and tables are described to detect and analyze the hydrothermal variations. The linear regression slope technique is addressed to extract the reduction or enhancement rate of heat transference. Also, the probable error is estimated statistically to assure that hydrothermal characteristic is correlated with physical parameters. The consequences indicate enhanced heat transport for nanolayers, but reduced heat transmission for nanoparticles' diameter. Thermal profile intensifies for thickness parameter and impermeable surface, whereas decreases for nanoparticles' diameter. [ABSTRACT FROM AUTHOR]

Fluid velocity or flow rate measurement, by flow sampling or bypassing flow from main pipelines, offers several advantages, like reducing the footprint of the flowmeter and avoiding the stoppage of a process during flowmeter repair or replacement. The flow measurement technique could be thermal mass flowmetry, a popular flow measurement method. A major limitation in the bypassing technique is the low flow rate through the bypass pipe resulting in a weak flowmeter thermal signal or reduced sensitivity to flow. This paper describes the usage of a validated computational fluid dynamics (CFD) model to investigate several bypass pipe designs to maximize fluid flow rate through the bypass pipes installed on the main pipeline and consequently maximize the thermal signal generated by an elementary thermal mass flowmeter installed on the bypass pipe. Investigations reveal that bypass design modification can improve thermal signal strength by as much as 38% with respect to that obtained in the baseline bypass design. [ABSTRACT FROM AUTHOR]

This paper presents a comparison between the results of three thermoconvective flows of a Newtonian fluid over uniformly heated, undulated horizontal surfaces in a porous medium against the background of the results of a flat plate. The undulations are assumed to have sinusoidal, sawtooth, and triangular waveforms. A system of nonlinear coupled partial differential equations arising in the study is solved using the Keller-Box method. Streamlines and isotherms have been plotted and analyzed to examine the effect of parameters on the fluid dynamics and heat transfer. At large surface amplitudes, secondary flow is observed in the cases of sinusoidal and triangular waveforms, but not in the cases of a sawtooth surface and a flat plate. The magnitude of the slip velocity at the horizontal surface is greatest for the sine waveform, while it is least in the case of triangular. The flat plate does not support slip in the velocity to the extent seen in the case of undulated surfaces. The variation of the mean Nusselt number and mean skin friction with surface amplitude and the Rayleigh number indicate that heat transfer and viscous friction at the boundary increase with individual and collective increases in the values of the amplitude and the Rayleigh number. Further, the mean Nusselt number and mean skin friction are found to be maximum for the sinusoidal surface and minimum for the triangular one. The heat transfer and skin friction by the flat surface are much less than that of all three undulated surfaces. [ABSTRACT FROM AUTHOR]

A computational work is performed on laminar free convection from an isothermally heated spherical shaped open cavity with negligible wall thickness suspended in the air. Fluid flow and heat transfer are analyzed in detail by solving governing differential equations (continuity, momentum, and energy) numerically over wide ranges of the relevant dimensionless parameters, namely, Rayleigh number, 104 = Ra = 108; and height to diameter ratio, 0.15 = h/D = 0.95. The detailed behavior of thermal and flow fields is delineated by suitable visualization techniques for different Ra and h/D. The influence of Ra and h/D on the local and average Nusselt number is also predicted and it is observed that the average Nusselt number on both outer and inner surfaces decreases with the increase of h/D for a constant value of Ra. A suitable correlation for the net average Nusselt number is obtained for the spherical-shaped open vessel surface as a function of Ra, and h/D based on the computed data points, which is expected to be relevant for various academic and industrial operations. This study can be helpful in various industrial operations, such as heat treatment of foodstuffs, shield surfaces, thermal insulations, melting of polymer pellets, and fluidized reactors. [ABSTRACT FROM AUTHOR]

Triple-diffusive convection for nanofluids in which differences in density are derived because of triple diffusion process, i.e., with three diffusing components: nanoparticles, solute, and heat, using more realistic two temperature model with separate thermal energy equations for the two phases of the considered nanofluid has been investigated by making use of the method of superposition and one term Galerkin technique. A complex system with local thermal non-equilibrium (LTNE) effects along with the Brownian motions and thermophoresis to account for nanoparticles has been considered. The problem is solved for top-heavy arrangement of nanoparticles leading to stationary mode of convection and numerical computations are carried out by using Mathematica software. An additional solute concentration equation supplements the conservation equations due to the existence of solute, which introduces two additional nondimensional parameters, whereas three additional parameters came into existence due to the consideration of LTNE effects. The critical Rayleigh number remains constant for smaller values of interphase heat transfer parameter, whereas it diminishes for the intermediate range and approaches to constant value for higher ranges of the parameters. What all this means is that only the intermediate range of Nield parameter shows stabilizing/destabilizing effects. Interestingly, the additional parameters Lewis number and solute Rayleigh number enhance the effect of destabilization of considered nanofluid layer. [ABSTRACT FROM AUTHOR]

The frost mass in different rows of the outdoor heat exchanger of an air source heat pump is uneven when the ambient temperature is below 0 °C, which leads to a shorter defrosting interval and reduces the heating capacity of the heat pump. In order to improve the problem of uneven frost along the airflow direction in the evaporator, an experimental study is conducted on the frosting and heat transfer performance of three rows of variable pitch (VP) finned coils in different fin pitch combinations under low ambient temperature conditions. The results show that the frosting time of the sample with VP of 5*3*2 is extended by 46.67%, the frost mass per unit of bare tube surface area is reduced by 14.54%, and the average heat transfer power per unit mass and volume is increased by 37.64% and 33.56%, respectively. [ABSTRACT FROM AUTHOR]

As an efficient heat and mass transfer technology, falling film evaporation has attracted more and more scholars' attention. In this paper, a falling film evaporation platform was established to discuss the mechanism of two-phase flow in narrow channels. The width of the channel varied from 0.5 to 2.0 mm. We focused on the relationship between heat transfer characteristics with flow patterns, channel width, and pressure loss. Three kinds of flow patterns were observed: bubble flow, restricted bubble flow, and dry area. In channels with the same width, the initial heat flux of subcooled evaporation increased linearly with the growth of flow rate. But at the same flow rate, the initial heat flux of subcooled evaporation decreased when the channel width became larger. The heat transfer coefficient on the wall was influenced by the bubble generation frequency and bubble separation diameter. Higher bubble generation frequency led to smaller bubble separation diameter. Through mathematical analysis, it was found that the heat transfer coefficient changed with the channel size. And a boundary was observed between the strengthening and weakening status. The comparison of theoretical and experimental values showed good agreement. [ABSTRACT FROM AUTHOR]

Having published some of the most widely cited publications over the last 30 years, Professor Satish G. Kandlikar has been among the most prominent researcher in the field of heat transfer. Throughout these years, he has coauthored over 200 journal paper in various areas of heat transfer. This paper provides a comprehensive look at Professor Kandlikar's research work over the years. The research work has been broadly categorized into (1) flow boiling correlations, (2) fluid flow and heat transfer in microchannels, (3) roughness effect at microscale, (4) pool boiling heat transfer and critical heat flux modeling, (5) surface enhancements for pool boiling, (6) numerical modeling of bubble growth in boiling, (7) water transport in proton exchange membrane (PEM) fuel cells, and (8) infrared imaging to detect breast cancer. The research conducted in each of these areas has produced some landmark findings, some of the most widely used theoretical models and an abundance of high-quality experimental data. The focus of this paper is to collate major finding and highlights some of the common themes that guided the research in Professor Kandlikar's group. This will help the readers gain a broad understanding of each of the areas of study in Professor Kandlikar's group and place the findings of the papers in a larger context. [ABSTRACT FROM AUTHOR]

This work investigates flow and heat transfer under an array of annular jets impinging on a heated moving surface. Numerical solutions of the full Navier-Stokes equation were attempted with a highly refined mesh. This study reports results for Reynolds numbers up to 500. In the surface movement direction, a periodic element from a jet-bank configuration was chosen, and the nondimensional surface velocity was considered from zero (i.e., a stationary plate) to two times the jet velocity. The impact of annular jet impingement over a moving surface on flow and heat transfer characteristics, including the development of the flow field, velocity profiles, skin friction coefficient and topology of skin friction lines, and local as well as surface averaged Nusselt number distribution are presented. It is observed that both the flow field and thermal performance are strongly affected by the surface motion. Heat transfer from the surface initially increases with the increasing surface motion, and after attainment of the highest value, heat transfer reduces with a further increase in surface velocity. However, higher surface velocity leads to higher uniformity in heat transfer, which may be beneficial for situations demanding uniformity in heat transfer. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of random suspensions of 0.25 aspect ratio (AR) cylinders are investigated for Reynolds numbers (Re) between 10 and 300 and solid fraction (φ) ranging from 0.1 to 0.3 using particle resolved simulations. The effect of particle inclination with respect to flow and particle clustering on heat transfer is investigated. The Nusselt number decreases with an increase in inclination angle and the dependence becomes stronger as φ and Re increase. On the other hand, while prolate ellipsoid suspensions of AR 2.5 follow the same trend, the Nusselt number increases with inclination angle as AR increases to 5 and 10 and as φ increases. Local particle clustering nominally decreases the Nusselt number because of the dominance of thermal wakes. At low φ⁠, this effect is felt only at low Re⁠, but as φ increases, the effect spreads to higher Re⁠. Similar but weaker trends are also found in suspensions of prolate ellipsoids of AR 2.5, 5, and 10. High AR, low Re prolate ellipsoids exhibit the greatest dependence of Nusselt number on local solid fraction. Implementation of two independent definitions of reference length, i.e., volume equivalent sphere diameter deq for ellipsoids and diameter dp of the cylindrical particle in the correlation of Tavassoli et al. (2015, "Direct Numerical Simulation of Fluid-Particle Heat Transfer in Fixed Random Arrays of Non-Spherical Particles," Chem. Eng. Sci., 129, pp. 42-48) provides good estimates of the respective suspension mean Nusselt numbers. [ABSTRACT FROM AUTHOR]

Advanced energy systems propose use of supercritical fluids (SCFs) as working fluids owing to various advantages associated with SCF. Supercritical water (SCW), supercritical carbon-dioxide (SC-CO2), and supercritical helium (SC-He) find proposed applications in advanced nuclear reactor designs too. Heat transfer behavior of SCFs is still not understood completely. Though there exist many heat transfer correlations and criterion in the literature, they miss having a general consensus among them. In spite of having many reported studies in literature for SCFs under forced convection conditions, studies related to SCF under natural circulation (NC) mode are very limited. In the present study, extensive experiments have been performed with SC-CO2 under NC conditions in tube geometry. The experimental data generated for vertical heated test section at various pressures between 8.0 and 13.0 MPa have been utilized to bring out salient findings related to fluid flow and heat transfer. The data have also been utilized to make comparison with generalized NC flow correlation available in the literature. Besides, few popular heat transfer correlations have been compared with experimentally obtained heat transfer coefficients. [ABSTRACT FROM AUTHOR]

The technological opportunities enabled by understanding and controlling microscale systems have not yet been capitalized to disruptively improve energy processes, especially heat transfer and power generation. The main limitation corresponds to the laminar flows typically encountered in microdevices, which result in small mixing and transfer rates. This is a central unsolved problem in the thermal-fluid sciences. Therefore, this work focuses on analyzing the potential of supercritical fluids to achieve turbulence in microconfined systems by studying their thermophysical properties. In particular, a real-gas thermodynamic model, combined with high-pressure transport coefficients, is utilized to characterize the Reynolds number achieved as a function of supercritical pressures and temperatures. The results indicate that fully turbulent flows can be attained for a wide range of working fluids related to heat transfer applications, power cycles and energy conversion systems, and presenting increment ratios of O(100) with respect to atmospheric (subcritical) thermodynamic conditions. The underlying physical mechanism to achieve relatively high Reynolds numbers is based on operating within supercritical thermodynamic states (close to the critical point and pseudo-boiling region) in which density is relatively large while dynamic viscosity is similar to that of a gas. In addition, based on the Reynolds numbers achieved and the thermophysical properties of the fluids studied, an assessment of heat transfer at turbulent microfluidic conditions is presented to demonstrate the potential of supercritical fluids to enhance the performances of standard microfluidic systems by factors up to approximately 50×. [ABSTRACT FROM AUTHOR]

In this work, rarefaction effect, heat transfer, and drag coefficient for gas flow around a square cylinder in transition flow regime are numerically studied using unified gas kinetic scheme (UGKS). To reduce computational cost, a mirror symmetry boundary treatment for UGKS is proposed and applied in this study. It is found that: (1) velocity slip is not obvious on upwind and downwind surfaces of square cylinder due to the tangential gas flow being weak in these surfaces; (2) with the increase of local Knudsen number, velocity slip on upper surface always increases, but temperature jump can decrease, which indicates that Knudsen number is not the decisive parameter to characterize temperature jump; (3) the heat transfer between gas and square cylinder enhances with the increase of inflow Knudsen number and cylinder temperature due to the increase of temperature jump and deteriorates with the increase of inflow Mach number on account of gas stagnation; and (4) the drag coefficient increases with the increase of inflow Knudsen number, the decrease of inflow Mach number, and the increase of cylinder temperature. To further predict the variation of average Nusselt number and drag coefficient, correlations for average Nusselt number and drag coefficient with inflow Mach number ranging from 0.05 to 0.3, inflow Knudsen number ranging from 0.1 to 10, and cylinder temperature ranging from 320 K to 380 K are proposed. This research can improve the understanding for mechanisms of gas flow and heat transfer in micro-electromechanical system (MEMS) devices. [ABSTRACT FROM AUTHOR]

Impinging jet nozzles are omnipresent in industrial applications. Innovative impinging jet nozzles, such as radial jet reattachment (RJR) and slot jet reattachment (SJR) nozzles, have been proven to be highly efficient tools to enhance heat and mass transfer, compared to traditional in-line jet and slot-jet nozzles. However, the heat and mass transfer in the region immediately underneath these reattachment nozzles are relatively inefficient. The ionic wind is a promising technique for heat transfer enhancement. The ionic flow is induced when free ions are accelerated by an electric field, and exchange momentum with neutral air molecules. In this numerical study, the performance of the SJR nozzle is improved by the application of an electric field, specifically, ionic wind, which is generated in the region directly between the nozzle and the exposed impingement surface. The two-dimensional numerical model is based on the flow field generated by an ionic wind-assisted SJR nozzle. The simulation results show a significant secondary flow induced under the nozzle, due to ionic wind. A significant enhancement of local and average heat transfer coefficients is achieved. The heat transfer increases with the applied potential and nozzle exit velocity. However, the SJR flow pattern is altered when the air exit velocity is below a certain threshold. The simulation results provide an in-depth understanding of the heat transfer characteristics under various operating conditions and pave the way for developing this novel impinging nozzle design. [ABSTRACT FROM AUTHOR]

This study investigates the unsteady, two-dimensional flow and heat transfer past a rotationally oscillating circular cylinder in linear shear flow. A higher order compact (HOC) finite difference scheme is used to solve the governing Navier-Stokes equations coupled with the energy equation on a nonuniform grid in polar coordinates. The hydrodynamic and thermal features of the flow are mainly influenced by the shear rate (K), Reynolds number (Re), Prandtl number (Pr), and the cylinder oscillation parameters, i.e., oscillation amplitude (αm), the frequency ratio (fr). The simulations are performed for Re=100, Pr=0.5-1.0, 0.0≤K≤0.15⁠, and 0.5≤αm≤2.0⁠. The numerical scheme is validated with the existing literature studies. Partial and full vortex suppression is observed for certain values of shear parameter K. The connection between heat transfer and vortex shedding phenomenon is examined where a pronounced increase in the heat transfer is observed for certain values of oscillation parameter, relative to the nonshear flow case. [ABSTRACT FROM AUTHOR]

In the present paper, numerical study of flow and heat transfer properties of RP-3 kerosene in an impingement model are conducted with SST k-ω turbulence model and a 10-species surrogate of kerosene. The independence of grids is first studied and the numerical results are compared with experimental data for validation. Characteristics of flow and heat transfer of kerosene flow in the impingement model are studied with different inlet temperatures, dimensionless open areas, and separation distances. The heat transfer coefficients and pressure losses increase with decreasing dimensionless open areas. The separation distance H/d = 2 gets the best heat transfer performance and the separation distance H/d = 3 gets the best heat transfer uniformity on the target plate. Higher inlet temperature gets a higher heat transfer performance and heat transfer uniformity on the target plate. [ABSTRACT FROM AUTHOR]

The two-temperature model of local thermal nonequilibrium (LTNE) is utilized to investigate a weakly nonlinear stability of thermosolutal convection in an Oldroyd-B fluid-saturated anisotropic porous layer. The anisotropies in permeability, thermal conductivities of the porous medium, and solutal diffusivity are accounted for by second-order tensors with their principal directions coinciding with the horizontal and vertical coordinate axes. A modified Darcy-Oldroyd model is employed to describe the flow in a porous medium bounded by impermeable plane walls with uniform and unequal temperatures as well as solute concentrations. The cubic-Landau equations are derived in the neighborhood of stationary and oscillatory onset using a modified perturbation approach and the stability of bifurcating equilibrium solutions is discussed. The advantage is taken to present some additional results on the linear instability aspects as well. It is manifested that the solutal anisotropy parameter also plays a decisive role on the instability characteristics of the system. It is found that the stationary bifurcating solution transforms from supercritical to subcritical while the oscillatory bifurcating solution transforms from supercritical to subcritical and revert to supercritical. The Nusselt and Sherwood numbers are used to examine the influence of LTNE and viscoelastic parameters on heat and mass transfer, respectively. The results of Maxwell fluid are outlined as a particular case from this study. [ABSTRACT FROM AUTHOR]

Since 1933, the vortex tube (VT) has been used as a device for cooling, heating, and creating separation process (particles, hydrocarbons, energy, etc.). The VT efficiency is very relevant to the characteristics of the inside fluid flow patterns. The most important part in the VT that directs the fluid flow is the "navigator," which has an angle and a rounded edge. In this study, the effects of change in the radius of the rounded edge (R* = 0, 0.5, 1, 1.5, 2, and 2.5 mm) on flow patterns and thermal performance of the VT are studied experimentally. The results show that with a rounded radius variation from 0 to 2 mm, the VT cooling efficiency increases sharply (34.84%) and with a further increase, there will be a decreasing trend for the cooling efficiency (10.48%). Also, this radius affects the location of the effective cold mass fraction (CMFs related to the highest thermal performance). In addition, variations in the gas velocity (axial/swirl at two positions Z/L = 0.3, 0.7) were studied with complete details and compared with other researchers' valid reports. As the results, the maximum axial velocity near the chamber and the maximum swirl velocity drop along the tube are effective on better cooling/heating performance. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of heat transfer and pressure drop during single-phase flows inside diverging, converging, and uniform microgaps using distilled water as the working fluid. The microgaps were created with a plain 10 mm × 10 mm heated copper surface and a polysulfone cover that was either uniform or tapered with an angle of 3.4 deg, and average gap heights of 400 and 800 µm. Experiments were conducted with single-phase water flow with an inlet temperature of 30 °C for flow rates varying from 57 to 498 mL/min and heat flux from 27 to 153 W/cm² depending on the flowrate and microgap configuration. The uniform configuration resulted in the lowest pressure drop due to the less constricted flow. A slight decrease of pressure drop with heat flux was observed due to temperature dependent properties. The best heat transfer performance was obtained with the converging configuration, which was especially significant at low flow rates and shorter average gap. This behavior could be explained by an increase in the heat transfer coefficient due to flow acceleration in the converging gaps, which compensates for the decrease in temperature difference between the fluid and the surface along the flow length. Overall, the converging microgaps have better performance than uniform or diverging ones for single-phase flows, and this effect is more pronounced at lower flow rates, where the fluid experiences higher temperature changes. [ABSTRACT FROM AUTHOR]

The use of transport membrane condenser (TMC) technology to recover heat and mass from the flue gas has been increasing recently. The heat and mass transfer from the TMC tube bundle have been studied experimentally and numerically, and several numerical models have been proposed. Although many heat transfer and pressure drop correlations are available for single-phase flows over tube bundles of solid walls, to the best of our knowledge, there is a lack of heat and mass transfer and pressure drop correlations for the porous membrane tubes with condensing flue gas that cover a wide range of parameters. In this study, the heat transfer, mass transfer, and pressure drop imposed by the crossflow ceramic nanoporous tubes in TMC have been studied numerically within wide ranges of tube diameters (4.57-7.62 mm), number of rows (2-24 rows), and Reynolds number (170-8900), under flue gas condensation. The turbulent flow of the flue gas mixture was modeled by the shear stress transport SST k-ω turbulence model. A hybrid/mixed condensation model written in user defined functions was employed to calculate the water vapor condensation rate. Numerical results with condensing flue gas are compared to available correlations for single-phase Nusselt number and pressure drops in the literature. It was found that except for selected conditions, the single-phase correlations noticeably differed from the TMC numerical results. Empirical TMC correlations for heat transfer and pressure drops with respect to condensation rate, number of rows, and the nanoporous membrane geometrical properties were derived thereby. The derived correlations for TMC show a good agreement with numerical data for all investigated parameters and can predict the 96% of the convective Nusselt number, overall Nusselt number, and friction factor inside the TMC within ±10%, ±10%, and ±15%, respectively. The effects of key parameters on the heat transfer, mass transfer, and pressure drops are illustrated and discussed in detail. [ABSTRACT FROM AUTHOR]

The local thermal nonequilibrium (LTNE) model has been used widely for analyzing heat transfer during internal flow through porous media, including when a channel is only partially filled with a porous medium. In such problems, the Biot number describes the rate of convective heat transfer between solid and fluid phases. While uniform Biot number models are commonly available, recent advances in functionally graded materials necessitate the analysis of spatially varying Biot number in such geometries. This paper presents LTNE-based heat transfer analysis for fully developed flow in a channel partially filled with porous medium and with spatially varying Biot number to describe solid-fluid interactions in the porous medium. Fully uncoupled ordinary differential equations for solid and fluid temperature distributions are derived under three different boundary condition models. Solid and fluid temperature fields are presented for a variety of Biot number distributions, including quadratically and periodically varying functions. An explanation of the nature of temperature distribution predictions for such problems is provided. For special cases, the results presented here are shown to reduce to past work on constant Biot number. This work improves the theoretical understanding of porous media heat transfer and facilitates the use of such theoretical models for functionally graded materials. [ABSTRACT FROM AUTHOR]

A comparative study on the enhancement mechanism by various twisted tapes (TTs) in association with the local flow pattern is investigated in detail via computational fluid dynamics (CFD). The effect of gap between TT and tube is further examined based on prior experimental validation. Result shows that the Nusselt number and friction factor will be increased by 12.6% and 18.1%, respectively, and the comprehensive thermal performance (CTP) will be improved from 1.05-1.07 to 1.12-1.15 when the gap is reduced from 1.0 mm to zero. The obtuse V-cut design in the experimental study shows superior performance, and the related enhancement mechanisms by windward, isosceles, and leeward V-cut configurations are numerically investigated. By comparing the local streamline at V-cut regions and the velocity distribution on the cross section, the windward V-cut might notably introduce fluid into the cut region and toward the tube wall subsequently, thereby resulting in effective heat transfer augmentation near the tube wall. Compared to the isosceles V-cut TT, the CTP of windward V-cut TT can be augmented from 1.15-1.18 to 1.16-1.21. However, the leeward V-cut TT will impair the CTP due to its inconsistent turbulence direction to the main helical flow. On the other hand, it is also indicated that fabricating three windward V-cuts at each helical pitch shows the highest improvement on CTP. Further increment of V-cut number may reversely reduce the CTP because of the rapid increase of flow resistance. Moreover, fabricating windward V-cuts on bilateral edges of the TT shows limited improvement. [ABSTRACT FROM AUTHOR]

Dropwise condensation (DWC) has the potential to enhance heat transfer compared to filmwise condensation (FWC). The heat transfer rates achieved by DWC depend on the drop size distribution, which is influenced by nucleation processes of newly formed drops. In DWC modeling, the nucleation site density Ns is used as an input parameter to obtain the drop size distribution of small drops. However, due to the small scale of the condensate nuclei, direct observation is difficult, and experimental data on the nucleation site density are scarce. In the literature, values in the range of 109 m-2 to 1015 m-2 can be found for Ns. In this paper, we report DWC experiments on SiO2 and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) thin hydrophobic coatings that show significantly different nucleation site densities. Nucleation site densities are estimated from high-speed imaging of small drops during initial condensation and from model calibration using established DWC theory. We have found the values for Ns to be in the range from 1.1×1010 m-2 to 5.1×1011 m-2 for the SiO2 coating and 1011 m-2 to 1013 m-2 for the PFDTES coating. Our results show that there can be large differences in the nucleation site density under similar conditions depending on the surface properties. This underlines the importance of investigating nucleation site density specifically for each surface and under consideration of the specific process conditions used for DWC. [ABSTRACT FROM AUTHOR]

The influence of wall-corrugation-induced swirl flow on enhanced forced convection in wavy-plate-fin cores has been investigated. Three-dimensional computational simulations were carried out for steady-state periodically developed air flow (Pr ∼ 0.71; 50 ≤ Re ≤ 4000) with channel walls subject to constant-uniform temperature conditions. The recirculation that develops in the wall troughs and grows to have an axially helical character is scaled by the Swirl number Sw. As Sw increases with higher flowrate and/or corrugation severity, tornado-shaped vortices appear in the wave trough region midway of the interfin channel height, then extend longitudinally to encompass majority of the flow channel. The local wall-shear and heat transfer coefficient variations indicate that boundary-layer thinning upstream of the wave peak aids in intensifying momentum and heat transfer. However, the flow recirculation in wall trough impedes heat transfer at low Sw due to flow stagnation but promotes it at high Sw because of the vortices-induced augmented fluid mixing. The effects of this secondary flow are quantified by Φf(or j), which is seen to increase log-linearly as fin corrugation aspect ratio γ and/or fin spacing ratio ζ increases; the influence of cross section aspect ratio α is marginal. Moreover, the pressure drag penalty due to swirl critically affects overall pressure loss, and its proportion remains nearly constant when α varies, but grows as Sw, γ, and/or ζ increases and can be as much as 80% of the total pressure drop. [ABSTRACT FROM AUTHOR]

A three-dimensional model of a partially melted powder bed with particles stochastically distributed in size and space coordinates has been developed. Numerical simulation of temperature distributions in stainless steel AISI 316 L and Al-12Si powders in vacuum, air, and argon has been performed to analyze unsteady heat transfer in a porous medium. The numerical model demonstrates a large effect of heat transfer through the gas phase in case of powders with low thermal conductivities like stainless steels. At the porosity level of 65% and above, the mechanism of heat transfer drastically changes and a linear dependence of thermal conductivity on porosity frequently used in literature becomes incorrect. The effects of the consolidation coefficient and size distribution on effective heat transfer in powders are discussed. The obtained dependencies of the effective thermal conductivity on porosity and the consolidation coefficient could be used in additive manufacturing applications. [ABSTRACT FROM AUTHOR]

This study is the examination of magnetohydrodinamics (MHD) flow and heat transfer between microparallel plates with constant wall heat flux using two different boundary condition coefficients, widely known in the literature, in a second-order slip flow and temperature jump boundary condition model. The stream of incompressible viscous liquid between rigid microtwin plates with electrical conductivity was assumed to be steady, laminar, hydrodynamically and thermally fully improved. The effects of dilution, viscous dissipation, and axial transmission, which are significant at the microscale, are also contained in the analysis. Momentum and energy equations are solved analytically under second-order slip velocity and temperature jump boundary conditions. Closed type answers for the temperature field and the fully developed Nusselt number are derived as a function of the magnetic parameter (MHD), Knudsen number and modified Brinkman number. Particular attention has been paid to the effects of the magnetic field on the second-order slip flow conditions. The use of the second-order boundary condition coefficients proposed in the study varies according to the proposed boundary conditions and tends to increase and decrease the heat transfer. Otherwise, MHD increases the heat transfer for both boundary conditions. [ABSTRACT FROM AUTHOR]

High-temperature supercritical CO2 Brayton cycles are promising possibilities for future stationary power generation and hybrid electric propulsion applications. Heat exchangers are critical components in supercritical CO2 thermal cycles and require accurate correlations and comprehensive performance modeling under extreme temperatures and pressures. In this paper (Part I), new Colburn and friction factor correlations are developed to quantify shell-side heat transfer and friction characteristics of flow within heat exchangers in the shell-and-tube configuration. Using experimental and computational fluid dynamics (CFD) data sets from existing literature, multivariate regression analysis is conducted to achieve correlations that capture the effect of multiple critical geometric parameters. These correlations offer superior accuracy and versatility as compared to previous studies and predict the thermohydraulic performance of about 90% of the existing experimental and CFD data within ±15%. Supplementary thermohydraulic performance data are acquired from CFD simulations with supercritical CO2 as working fluid to validate the developed correlations and demonstrate its capability to be applied to supercrtical CO2 heat exchangers. [ABSTRACT FROM AUTHOR]

Recent advancements in surface nano-engineering have spurred intense interests in their implementation for enhancing condensation heat transfer. When appropriately designed, nano-engineered surfaces not only lead to highly efficient transport mechanisms not achievable with conventional dropwise condensation (DWC), they also demonstrate the possibility of augmenting condensation of low surface tension fluids widely used in industry. These advantages are further enhanced by the development of highly scalable nanofabrication methods, which enable the potential transition from laboratory-scale prototypes to real-world industrial applications. In this review, we discuss the progress, opportunities, and challenges of enhancing condensation heat and mass transfer with nano-engineered surfaces. This article provides an overview of the recent developments in micro/nanoscale coating and structure fabrication techniques and performs a thorough comparison of their condensation performance, elucidating the complex interfacial transport mechanism involved. Surface structuring methods that are durable, scalable, and low-cost are essential attributes for large-scale industrial implementation. Here, the methods used to improve surface durability and demonstrations of nanostructure-enhanced meter-scale condensers are presented. Limitations are discussed and the potential techniques to overcome these challenges are summarized. Given the recent development of metal additive manufacturing (AM) technology and its growing relevance in manufacturing processes, we end this review by providing our perspectives on the opportunities in enabling surface nanostructuring of metal additive manufactured materials and the potential of nanometric-millimetric codesign optimization for the development of next-generation additively manufactured condensers. [ABSTRACT FROM AUTHOR]

Boiling is a multiscale phenomenon. Nucleation and rapid bubble growth at the heated wall provide a highly localized mechanism for heat transfer to the surrounding liquid. The liquid-vapor interface of the growing bubble supplies latent heat needed to evaporate the liquid and sustain the bubble activity. Although the boiling process is efficient in removing large amounts of heat, further improvements are needed to increase the critical heat flux (CHF) as well as heat transfer coefficient (HTC) in many applications. Recent developments in enhancing boiling heat transfer have mainly focused on small-scale heaters, typically on the order of a centimeter, that are particularly relevant in electronics cooling application. Many of these developments are based on fundamental understanding of the microscale processes of bubble nucleation, bubble growth and removal from the heater surface, and supply of liquid to the active nucleation sites. Some of these microscale enhancement techniques have set new records in heat dissipation (both CHF and HTC). This paper explores the potential of these microscale enhancement techniques in large-scale boiling equipment, such as boilers, reboilers, and evaporators in power generation, refrigeration, air conditioning, cryogenic, desalination, chemical, petrochemical, pharmaceutical, and other industries. Implementation of the microscale enhancement technologies in macroscale boiling equipment will provide the next level of energy efficiency and energy savings in the face of climate change and environmental concerns. [ABSTRACT FROM AUTHOR]

In this paper, we report on experimental results to measure the total temperature of nitrogen gas at the inlet and outlet of microtubes with constant wall temperature and to quantitatively determine the heat transfer rates. Experiments were conducted with nitrogen gas flowing in a stainless steel microtube with a diameter of 524 µm and a copper microtube with a diameter of 537 µm. The temperature differences between the inlet and the wall were maintained at 3, 5, and 10 K by circulating water around the inlet and the wall. The stagnation pressures were also controlled so that the flow, with atmospheric back pressure, could reach Reynolds numbers as high as 26,000. To measure the total temperature, a polystyrene tube with a thermally insulated exterior wall containing six plastic baffles was attached to the outlet. Heat transfer rates were obtained from the gas enthalpy difference by using the pressures and the total temperatures measured at the inlet and outlet. Heat transfer rates were also compared with those obtained from the ideal gas enthalpy using the measured total temperatures and from the Nusselt number of incompressible flows. It was found that the measured total temperature at the microtube outlet was higher than the wall temperature. Also, the heat transfer rates calculated from the total temperature difference were higher than the values obtained from the incompressible flow theory. [ABSTRACT FROM AUTHOR]

For the first time, an integral form of one-dimensional heat transfer equation in a semi-infinite domain with a boundary, moving arbitrarily in time, and a heat source, depending arbitrarily on time and space location, is obtained. The obtained integral equation relates time histories of the temperature and its gradient at the boundary of the domain with the temperature at any given point inside or at the boundary of the domain. In the latter case, it delivers closed form integral equation for the rate of boundary movement in nonlinear problems where the time history of boundary movement is one of problem unknowns. The obtained equation accounts explicitly for the presence of an arbitrary heat source in the domain, while other existing methods do not allow a closed integral formulation to be obtained in such a case. The equation may be used for an analytical investigation of several types of boundary value problems (BVPs), as well as for numerical solution of such problems. Particular cases of this equation with a trivial heat source are known to demonstrate chaotic behavior. It is expected that the same is true for some nontrivial heat source functions, and this conjecture will be explored in subsequent publications. [ABSTRACT FROM AUTHOR]

A novel nonintrusive technique based on an air-coupled ultrasonic transducer was used to study the hydrodynamic behavior of falling film over metal foam layered horizontal tube. Copper foam having a porosity of 90.5%, brazed over a copper tube of 25.4 mm diameter was used in this study. Falling film thickness distribution in the circumferential direction and the dynamic characteristics of falling film were studied in the falling film Reynolds number range of 356-715, and at a tube spacing of 5 mm and 15 mm. The falling film characteristics over metal foam layered horizontal tubes were compared with that over a plain horizontal tube surface. Heat transfer studies of falling film over metal foam layered tube were studied in an evaporator of a multi-effect desalination system by experiment. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and were found to be in good agreement. Metal foam properties required for the computational model were obtained using a microcomputed tomography-based study. [ABSTRACT FROM AUTHOR]

A convection system can be designed as an energy-efficient one by making a considerable reduction in exergy losses. In this context, entropy generation analysis is performed on the infrared suppression system numerically. In addition, results due to heat transfer are also shown. The numerical solution of the Navier-stokes equation, energy equation, and turbulence equation is executed using ansysfluent 15.0. To perform the numerical analysis, different parameters such as the number of funnels, Rayleigh number (Ra), inner surface temperature, and geometric ratio are varied in the practical range. Results are shown in terms of heat transfer, entropy generation, irreversibility (due to heat transfer and fluid friction), and Bejan number with some relevant parameters. Streamlines and temperature contours are also provided for better visualization of temperature and flow field around the device. Results show that heat transfer and mass flow rate increase with the increase in the number of funnels. Entropy generation and the irreversibility rise with an increase in the number of funnels and geometric ratio. Also, the Bejan number decreases with an increase in Ra and the number of funnels. A cooling time is also obtained using the lumped capacitance method. [ABSTRACT FROM AUTHOR]

Impingement cooling can effectively disperse local heat load, but its downstream heat transfer is always reduced due to crossflow effect. In this study, the flow and heat transfer characteristics of impingement cooling with semi-circular (SC), semi-rectangular (SR), semi-diamond (SD), and semi-four-pointed star (SFS) crossflow diverters are compared over the ReD ranging from 3,500 to 14,000 by solving three dimensional Reynolds-averaged Navier-Stokes equations with SST k-ω turbulence model. It is found that the arrangement of crossflow diverters changes the distribution of local jet Reynolds number (ReD,j/ReD) and reduces the mass velocity ratio of downstream crossflow to jet (Gcf/Gj), so the impingement heat transfer is enhanced significantly. However, friction loss also increases. Overall evaluation reveals that all crossflow diverters can improve the comprehensive heat transfer performance parameter (Φ), and the maximum increases of Φ are 11.0%, 14.3%, 12.2%, and 14.7% for SC, SR, SD, and SFS cases, respectively. It is noted that the Nusselt number of heated SFS-shaped diverter surface is also the highest. Besides, the influences of streamwise location (L) and thickness (t) of SFS-shaped diverter are also investigated. Results show that when the L increases from 2D to 3D, the heat transfer and friction loss change slightly; when the L increases from 3D to 4D, the heat transfer decreases sharply, and friction loss increases seriously. As for the t, it has almost no effect on the flow field and heat transfer. [ABSTRACT FROM AUTHOR]

Many engineering applications consist of rotating components that experience high-heat load. Applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low-pressure region created due to the radially outward pumping of fluid close to the rotating surface. The present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas-turbine disk. The experiments were conducted at rotational Reynolds numbers (⁠ReR⁠) of 5487-12,803 based on the disk diameter (⁠D⁠) and jet Reynolds numbers (Re) of 5000-18,000 based on the jet diameter. The effects of nozzle to target spacing (⁠z/d=2-6⁠), eccentricity of impingement (⁠ε = 0-0.67⁠), angle of impingement--both toward and away from the center (⁠θi and θo=0 deg-20 deg⁠), and the pin fin height (⁠Hf=3.05 mm-19.05 mm⁠) were studied. Steady-state temperature measurements were taken using thermocouples embedded in the disk close to the target surface, and area average Nusselt number (⁠Nu⁠) was calculated. The results have been compared with those for a smooth aluminum disk of equal dimensions and without any pin-fins. The average Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower values of Re⁠, and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re=5000⁠. In the impingement dominant regime, the effect of disk rotation was minimal for a smooth disk, but the heat transfer increased with rotational speed in case of pin-fins. There was no impact of eccentricity on Nu for ε = 0 and 0.33⁠. For ε = 0.67⁠, the maximum reduction in enhancement over a smooth surface (21.95%) was observed when compared with coaxial impingement for stationary impingement for Re=18,000 and z/d=4⁠. The effect of inclination angle was insignificant, and no clear trend could be established. Higher heat transfer rates were observed for z/d=6 with the increasing Re⁠, and this effect diminished with the increase in the rotational speed. With the increase in pin-fin height, especially at higher values of Re⁠, there was in increase in the value of Nu. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the heat transfer enhancement. [ABSTRACT FROM AUTHOR]

A numerical study of heat transfer between circular jet arrays and the flat moving surface is carried out. Two jet patterns, inline and staggered, are chosen. A total of nine circular jets are used in both jet patterns. The analysis is carried out for steady and transient conditions with the turbulent flow of jet fluid. The steady-state analysis analyzes the effect of surface motion on the flow field and heat transfer through the jet arrays. The surface-to-jet velocity ratio (r) varies from 0 to 2. In transient analysis, the effect of jet pattern on the cooling of hot moving plate is analyzed. The two-equation shear stress transport (SST) k-ω turbulence model is used to solve Reynolds-averaged Navier-Stokes (RANS) equations of conservation of mass, momentum, and energy for incompressible turbulent flow. The steady-state analysis shows that surface motion has a significant effect on the flow field and heat transfer. The results of the transient analysis show that a staggering jet pattern cools the plate more uniformly than an inline jet pattern. [ABSTRACT FROM AUTHOR]