Conjugate Heat Transfer Model Research Articles

Numerical Modeling of Subcooled Flow Boiling and Heat Transfer Enhancement: Validation and Applicability to Fusion Reactor Target Design

Abstract Boiling flows are an extremely efficient mechanism for the transfer of ultrahigh heat fluxes and used in numerous industrial applications. In this paper, the accuracy of computational fluid dynamics in predicting the temperature distributions and heat transfer performance is examined within a nuclear fusion reactor divertor. The aim is to establish the role of computational fluid dynamics (CFD) within the design of complicated high heat flux components using a semi-mechanistic approach to flow boiling that is independent of geometry and flow conditions. An Eulerian–Eulerian two-fluid method is developed and a conjugate heat transfer model is validated against the existing experimental data where available. Overall, a satisfactory accuracy is achieved in the prediction of several important quantities. Temperature distribution throughout the divertor is found to be highly accurate and aligns with the physical testing across two expected operating regimes. Additionally, the system heat transfer coefficients and coolant temperatures are close to the assumptions already established within the literature. Heat transfer enhancement is a critical component of the divertor design, and a twisted-tape insert appears to be necessary for the system to withstand ultrahigh heat fluxes encountered within the fusion reactor. The results show that the inclusion of a twisted tape improved the heat transfer coefficient of the system by almost 45% allowing the divertor to withstand the required heat fluxes of 10 MW/m2 and 20 MW/m2.

Accuracy of interface schemes for conjugate heat and mass transfer in the lattice Boltzmann method

The lattice Boltzmann method (LBM) is attractive for conjugate heat and mass transfer modeling due to its capability to satisfy the interfacial conjugate conditions without nested iterations. This paper presents a comparison of the popular interface schemes proposed in the literature with the focus on their numerical accuracy and convergence orders. The various interface schemes examined include the geometry-considered interpolation-based treatment that constructs second-order accurate corrections to the distribution functions across the interface by treating the interface as a shared boundary for the adjacent domains, as well as representative modified schemes that bypass the local geometry and topology consideration by either reformulating the macroscopic governing energy equation with additional source terms, or proposing modified microscopic equilibrium distribution functions in the lattice Boltzmann model. It is recognized that for the interface schemes based on governing equation reformulation, approximation of the discontinuous heat capacitance gradient at the interface is required to account for the interfacial heat flux continuity. Through analysis and numerical tests including both straight and curved interfaces, it is shown that in order to preserve the second-order accuracy in the LBM, the local interface geometry must be considered; and the modified geometry-ignored interface schemes result in degraded convergence orders – at most first order for general cases and only zeroth order is achieved for the schemes requiring discontinuous heat capacitance gradient approximation. In addition, much higher error magnitude is observed for the numerical solutions obtained from using these modified schemes without considering the interface geometry.

Numerical study on the moisture transfer characteristics of membrane-based liquid desiccant dehumidifiers: Resistance distribution and concentration polarization

Membrane-based liquid desiccant dehumidifiers have received increasing attentions due to their main advantage of preventing liquid carryover in air handling process, but the low dehumidification efficiency limits their further applications. The underlying mechanism of their moisture transfer deterioration is not entirely clear yet, and further efforts are needed to effectively identify and reduce the mass transfer resistances in the dehumidification process. In this work, considering impressionable transmembrane permeation flux, the resistance distribution of moisture transfer in the quasi-counter flow flat-plate membrane-based liquid desiccant dehumidifier is studied by a three-dimensional conjugate heat and mass transfer model. The velocity, temperature and concentration fields in dehumidifying process are analyzed in baseline condition, especially on the membrane surfaces. Analogy to a thermal circuit, the normalized moisture transfer resistances through gaseous, solid and liquid phases are defined to describe their relative importance. Causes of these normalized resistances in variable conditions are analyzed in terms of concentration polarization and boundary layer theory. Furthermore, the resistance variation is compared with the latent effectiveness, to reveal their inherent relation. The results suggest that moisture transfer resistances are mostly originated from the membrane and air side, and thus dehumidifying enhancement is more likely to emerge from the air-side concentration polarization alleviation in synergy with the appropriate improvement of membrane porosity. Among all operating parameters, the air inlet temperature and solution flow rate have relatively limited effect on the total resistance, while increasing solution concentration, followed by decreasing solution temperature, can significantly reduce the moisture transfer resistances at a nondecreasing latent effectiveness. In addition, the latent effectiveness shows a similar variation tendency to that of the air-side moisture transfer resistance, but it cannot reflect the total resistance variation. These results can help researchers and designers make decisions on where and how to improve the performance of membrane-based liquid desiccant dehumidifiers.

Temperature uniformity enhancement of densely packed high concentrator photovoltaic module using four quadrants microchannel heat sink

The dense solar radiation received by a high concentration photovoltaic module (HCPVM) causes a high cell temperature. In this module, multiple solar cells were electrically connected in both series and parallel. The higher temperature of the solar cell in the series string limits the generated power for the whole string. Therefore, it is crucial to employ a uniform cooling mechanism for higher electrical performance along with a longer lifespan. The uniform cooling is required to attain safe operating temperature and prevent the hot spot formation. Hence, in the current work, a four-compartment microchannel heat sink is proposed for the thermal management of HCPVM under high solar concentration of 1000 suns (1 sun = 1000 W/m2). A three-dimensional (3D) conjugate heat transfer model with exergy analysis is developed and validated. This model was used to investigate the effect of inlet and outlet orientation of four quadrants microchannel heat sink as a cooling method for HCPVM. Eight different orientations of parallel-flow and counter-flow conditions were investigated and compared in terms of temperature non-uniformity, module power, and exergy performance. The results showed that the inlet and outlet orientation was a key role affecting the module temperature non-uniformity. For the counter-flow operated heat sinks, the HCPVM can be operated under a temperature non-uniformity of 3.1 °C at total inlet module mass flowrate of 350 g/min and solar concentration ratio of 1000 suns. In addition, the attained HCPVM electrical, thermal, and overall exergy efficiency were 37.2%, 8.2%, and 45.4% respectively at the same conditions.

Effects of oxygen concentration on the thermal and chemical structures of laminar coflow CO/H2 diffusion flames burning in O2/H2O atmosphere

The thermal and chemical structures of laminar coflow syngas diffusion flames burning in O2/H2O atmosphere were experimentally and numerically studied at 1 atm and initial temperature of 400 K. The O2/H2O molar ratio was varied from 20/80 to 100/0 to investigate the effects of oxygen concentration. An intensified CCD was used to capture the OH*-chemiluminescence based on which the measured flame heights were obtained. Numerical modelling was conducted using a detailed validated chemical mechanism including OH* formation, the discrete-ordinates method coupled with the statistical narrow-band correlated-K(SNBCK) model for the radiative properties of combustion products, and the conjugate heat transfer model to account for the heat transfer between the burner wall and the fuel and oxidizer streams. Results show that the measured flame heights agree well with the simulated values. The flame height and the maximum flame temperature decrease and increase respectively with increasing the O2 concentration. With increasing the O2 concentration, the maximum OH mole fraction first increases but finally decreases and peaks at the oxidizer composition of 90%O2-10%H2O. Increasing the O2 concentration has little influence on the flame attachment but significantly enhances the heat release rate of H + O2(+M) = HO2(+M) and this reaction is found to be the primary cause of the strong influence of the oxygen concentration on the burner tip temperature. As the oxygen concentration increases, syngas pyrolysis inside the fuel tube is promoted, mainly through the H consumption reactions due to the enhanced back diffusion of H from the flame front into the burner.

Conjugate heat transfer model for feedback control and state estimation in a volumetric solar receiver

Open volumetric solar receivers (VSRs) are a promising technology for concentrated solar power plants due to their capability to provide heat using ambient air as the working fluid operating at temperatures over 700°C. Nevertheless, VSRs are challenged by the unsteadiness and high intensity of the radiation flux, which may cause unreliable or unsafe outflow temperatures, and may compromise the lifetime of the porous ceramic absorbers due to extreme thermal loads, thermal shock or thermal fatigue. We propose a data assimilation framework to address these matters using blower actuation, measurements from sensors located in the outflow stream of air, and a model for the conjugate heat transfer in an open VSR. We formulate said model and compare it against full three-dimensional CFD simulations to show that it captures the relevant dynamics while reducing the computational cost enough to allow for online calculations. A linear quadratic Gaussian (LQG) controller is used with the model to perform simultaneous state estimation and feedback control in three simulated scenarios. Our framework proves capable of stabilizing outflow air temperatures during the passing of a cloud, estimating the radiation flux hitting the absorber during daily operation, monitoring temperature cycling in the solid matrix, and avoiding extreme temperature gradients during start-up procedures. Artificial noise and disturbances are added to the system for all scenarios and the LQG controller proves to be robust, rejecting disturbances and attenuating noise, as well as compensating for model uncertainty.

How mushy zone evolves and affects the thermal behaviours in latent heat storage and recovery: A numerical study

Mushy zone is where the melting or solidification actually begins and proceeds, accompanying which the latent heat absorbs or releases. Thus, it is critical for the realization of latent heat storage. In this paper, attempts were made to examine the melting and solidification processes from a new perspective, that is, the mushy zone evolution, which tightly couples with the heat transfer, fluid flow, and phase change. A conjugate heat transfer model was developed including the lauric acid in a basic rectangular latent heat storage unit, the surrounded thermal insulations, and the ambient air. The enthalpy-porosity model was used and the mushy zone constant was reasonably evaluated to accurately predict the solid-liquid phase change processes. The model was validated against the experimental data from literature. In addition, a detailed description on the coupling of heat transfer, fluid flow, and phase change within the mushy zone was presented. It has been found that the mushy zone was highly suppressed during melting while widely extended during solidification. Moreover, the effects of the isothermal assumption for non-isothermal phase transition as well as mushy zone constant on the predicted heat storage performance were investigated. The results obtained can give some insights into the enthalpy-porosity method to accurately predict the mushy zone phase change processes. Meanwhile, more clues can be achieved to design more efficient heat storage devices and to develop better phase change materials.

Structure and geometric dimension optimization of interlaced microchannel for heat transfer performance enhancement

A novel interlaced microchannel is designed, and two side walls of the microchannel are served as the main heat transfer surface. The thermal and hydraulic performance of the interlaced and parallel microchannels are numerically investigated using the full-size conjugate heat transfer model, and then are compared with the experimental results. The optimized geometric dimensions of the interlaced microchannel including the depth, width and spacing are obtained. The results show that the maximum surface temperature deviation between the simulation and experiment of the interlaced and parallel microchannel are only 4.8% and 4.7%, respectively. When the cold water flow rate is 600 ml/min, the Nusselt number of the interlaced microchannel is increased by 65.4% compared to the parallel microchannel, and the JF factor reaches 1.438. The uniform surface temperature of the interlaced microchannel is obtained with a maximum temperature difference of 40 °C, which is lower than that of 50 °C in the parallel microchannel. The thermal and hydraulic performance of the interlaced microchannel are improved with the increase of the depth and width of microchannel due to the heat transfer area in the first and second heat transfer directions are increased. However, a large amount of cold water in the central position of the microchannel can’t participate in the heat transfer because of its larger depth and width, so 1.5 mm and 0.5 mm are selected as the optimized depth and width, respectively. Also, it is found that the spacing of microchannel has little effect on the thermal and hydraulic performance. Considering to the compactness and strength, 0.5 mm is the optimized spacing for heat transfer performance enhancement.

Numerical Study of Perturbators Influence on Heat Transfer and Investigation of Collector Performance for a Micro-Combined Heat and Power System Application

The objective of this article is to analyze complex micro-channels with wire-net and S-shaped perturbators and implement a reduced order modeling (ROM) approach to assess the entire heat exchanger performance and validate through experiments. Shifting the critical Reynolds number to lower values using perturbators decreases the pressure losses and enhance the thermal efficiency. There is an optimum mass flow (for both perturbators) where the thermal efficiency reaches maximum. The thermal efficiency of the wire-net perturbator is relatively high compared to S-shaped perturbators. The S-shaped perturbators induces strong wall-normal velocity fluctuations and enhances the heat transfer. Furthermore, the turbulence production term provides a deeper insight into flow attachment and detachment near the wire net intersections. The computational fluid dynamics approach (conjugate heat transfer models and ROM) was introduced to reduce the computational grid size and predict the collector performance. The secondary collector performance is determined by considering the microchannels as porous mediums. Apparently, the primary collector performance is determined by considering both secondary collectors and microchannels as porous mediums. The cylindrical secondary collectors contribute nearly 40–50% of the pressure drop. Experimental validation showed that the ROM predicts the heat exchanger performance with a good (<4.4%) accuracy.

Generalized multirate models for conjugate transfer in heterogeneous materials

We propose a novel macroscopic model for conjugate heat and mass transfer between a \emph{mobile region}, where advective transport is significant, and a set of \emph{immobile regions} where diffusive transport is dominant. Applying a spatial averaging operator to the microscopic equations, we obtain a \emph{multi-continuum} model, where an equation for the average concentration in the mobile region is coupled with a set of equations for the average concentrations in the immobile regions. Subsequently, by mean of a spectral decomposition, we derive a set of equations that can be viewed as a generalisation of the multi-rate mass transfer (MRMT) model, originally introduced by Haggerty & Gorelick. This new formulation does not require any assumption on local equilibrium or geometry. We then show that the MRMT can be obtained as the leading order approximation, when the mobile concentration is in local equilibrium. The new Generalised Multi-Rate Mode (GMRM) has the advantage of providing a direct method for calculating the model coefficients for immobile regions of arbitrary shapes, through the solution of appropriate micro-scale cell problems. An important finding is that a simple re-scaling or re-parametrisation of the transfer rate coefficient (and thus, the memory function) is not sufficient to account for the flow field in the mobile region and the resulting non-uniformity of the concentration at the interfaces between mobile and immobile regions.

Study on Thermoelectric Conversion and Conjugate Heat Transfer for PCBA by Finite Element Analysis

In this study, the optical sphere and the power analyzer were exploited to measure the optical and power parameters for the high-power LED lamps. The results, derived from the experimental data, were used as the power distribution inputs in the finite element analysis (FEA) for the determination of the temperature distribution for the printed circuit board assembly (PCBA) built in the LED lamp. In the finite element analysis, the conjugate heat transfer model was adapted for the calculation of the heat transfer, including thermal conductivity, convection and radiation. Applied on the power chips located on the PCBA, the graphene thermal interface material (TIMs) had been studied for its effects on the temperature distribution. For an accurate simulation about the LED lamp, the model with closed and compact space was built in the analysis. Compared to the experimental data, it showed that the simulation results had a deviation in the range of 3–5% around the main heating source, the light-emitting diodes. It proves the FEA model proposed in this study were well developed for the simulation of the temperature distribution for the high-power LED lamps which have mixed heat transfer mechanisms. The thermal radiation effects by TIMs with graphene were also investigated in this study and proven to be useful for the heat dissipation for the LED lamps.

A conjugate heat transfer model for unconstrained melting of macroencapsulated phase change materials subjected to external convection

A new conjugate heat transfer model for unconstrained melting of macroencapsulated phase change material subjected to external convection is presented. Flow and heat transfer processes are modeled for the phase change material, the capsule wall and the external heat transfer fluid. The solvers for each region operate in a segregated and sequential manner and are coupled by mixed thermal boundary conditions. For the description of the solid body motion in unconstrained melting, the enthalpy-porosity method is modified. The solid body surface is reconstructed from the liquid volume fraction field for every time step by a triangulated isosurface using a marching tetrahedra technique. Based on computation of the forces exerted on the solid, the motion of the solid is determined iteratively in a strong coupling between solid velocity and pressure. The new conjugate heat transfer solver is utilized to simulate the commonly experimentally and numerically investigated scenario of a spherically encapsulated phase change material immersed in a water bath. The coupled flow and heat transfer is investigated for a natural convection dominated mixed convection problem with a Richardson number of 50 · 103 and Stefan numbers of 0.075, 0.1, 0.15, 0.2 and 0.25. Contrary to the commonly made assumption, the resulting capsule wall temperature is transient and highly non-uniform. The differences in melting times range from 50.7 to 67.5% between the consideration of external heat transfer and the assumption of constant uniform wall temperatures.

A review of liquid flow and heat transfer in microchannels with emphasis to electronic cooling

Since the realization of microchannel devices more than three and half decades ago with water as the cooling fluid providing heat transfer enhancement, significant progress has been made to improve the cooling performance. Thermal management for electronic devices with their ever-widening user profile remains the major driving force for performance improvement in terms of miniaturisation, long-term reliability, and ease of maintenance. The ever-increasing requirement of meeting higher heat flux density in more compact and powerful electronic systems calls for further innovative solutions. Some recent studies indicate the promise offered by processes with phase change and the use of active devices. But their adoption for electronic cooling still weighs unfavourably against long-term fluid stability and simplicity of device profile with moderate to high heat transfer capability. Applications and reviews of these promising research trends have been briefly visited in this work. The main focus of this review is the flow and heat transfer regime related to electronic cooling in evolving channel forms, whose fabrication are being enabled by the significant advancement in micro-technologies. Use of disruptive wall structures like ribs, cavities, dimples, protrusions, secondary channels and other interrupts along with smooth-walled channels with curved flow passages remain the two chief geometrical innovations envisaged for these applications. These innovations target higher thermal enhancement factor since this implies more heat transfer capability for the same pumping power in comparison with the corresponding straight-axis, smooth-wall channel configuration. The sophistication necessary to deal with the experimental uncertainties associated with the micron-level characteristic length scale of any microchannel device delayed the availability of results that exhibited acceptable matching with numerical investigations. It is indeed encouraging that the experimental results pertaining to simple smooth channels to grooved, ribbed and curved microchannels without unreasonable increase in pumping power have shown good agreement with conventional numerical analyses based on laminar-flow conjugate heat-transfer model with no-slip boundary condition. The flow mechanism with the different disruptive structures like dimple, cavity and rib, fin and interruption, vortex generator, converging-diverging side walls or curved axis are reviewed to augment the heat transfer. While the disruptions cause heat transfer enhancement by interrupting the boundary layer growth and promoting mixing by the shed vortices or secondary channel flow, the flow curvature brings in enhancement by the formation of secondary rolls culminating into chaotic advection at higher Reynolds number. Besides these revelations, the numerical studies helped in identifying the parameter ranges, promoting a particular enhancement mechanism. Also, the use of modern tools like Poincare section and the analysis of flow bifurcation leading to chaotic advection is discussed. Among the different disruptive structures, sidewall cavity with rib on the bottom wall within the cavity plays a significant role in augmenting the thermal performance. Among the different converging-diverging side walls or curved axis, the sinusoidal channel provides the highest mixing by the introduction of secondary vortices or dean vortices to augment the heat transfer with less pressure drop. The optimum geometry in terms of high heat transfer with low pressure plays a major role in the design of heat sink. Directions of some future research are provided at the end.

Convective Effects in a Latent Heat Thermal Energy Storage

Convection within a latent heat thermal energy storage (LHTES) shell-and-tube device filled with phase change material (PCM) has been studied by means of numerical simulations. Both, the heat transfer fluid and the PCM mass, momentum and energy equations are solved and coupled with a conjugate heat transfer model. The study highlights three specific zones within the PCM: the top convective-dominated part, the curvilinear solid–liquid interface, and the bottom conductive-dominated part. The PCM melts from the top to the bottom, therefore the main mechanism of melting appears to be confined in the top part of the solid PCM. However, the flow details reveal a convective cell that includes the whole melted PCM from the top to the bottom of the PCM enclosure. Even though the problem is widely studied by means of experiments and numerical simulations, here the convective flow has been studied quantitatively. During the melting phase the viscous and thermal boundary layers at the walls has been reported at different heights from the bottom of the device. Results show in detail the phenomenology of the melting process within a shell-and-tube LHTES supporting the development of design solutions that could enhance the heat transfer of such device.

3D coupled conduction-convection problem using in-house heat transfer experiments in conjunction with hybrid inverse approach

Purpose Many a times, the information about the boundary heat flux is obtained only through inverse approach by locating the thermocouple or temperature sensor in accessible boundary. Most of the work reported in literature for the estimation of unknown parameters is based on heat conduction model. Inverse approach using conjugate heat transfer is found inadequate in literature. Therefore, the purpose of the paper is to develop a 3D conjugate heat transfer model without model reduction for the estimation of heat flux and heat transfer coefficient from the measured temperatures. Design/methodology/approach A 3 D conjugate fin heat transfer model is solved using commercial software for the known boundary conditions. Navier–Stokes equation is solved to obtain the necessary temperature distribution of the fin. Later, the complete model is replaced with neural network to expedite the computations of the forward problem. For the inverse approach, genetic algorithm (GA) and particle swarm optimization (PSO) are applied to estimate the unknown parameters. Eventually, a hybrid algorithm is proposed by combining PSO with Broyden–Fletcher–Goldfarb–Shanno (BFGS) method that outperforms GA and PSO. Findings The authors demonstrate that the evolutionary algorithms can be used to obtain accurate results from simulated measurements. Efficacy of the hybrid algorithm is established using real time measurements. The hybrid algorithm (PSO-BFGS) is more efficient in the estimation of unknown parameters for experimentally measured temperature data compared to GA and PSO algorithms. Originality/value Surrogate model using ANN based on computational fluid dynamics simulations and in-house steady state fin experiments to estimate the heat flux and heat transfer coefficient separately using GA, PSO and PSO-BFGS.

The impact of employing insulation with variant thermal conductivity on the thermal performance of buildings in the extremely hot climate

This paper illustrates the impact of transient thermal conductivity of the insulation layer embedded in a typical residential building on cooling effect and energy performance. The test was performed applying expanded polystyrene (EPS), as an example of insulation material, in the extremely hot climate of UAE at different levels of densities, and three moisture content levels (10%, 20%, and 30%) compared to dry insulation material for Low density (LD) material. Thermal wall resistance was evaluated by applying a conjugate heat-transfer model based on enthalpy-based formulation. The thermal performance of the building incorporating expanded polystyrene with variable thermal conductivity was compared to non-variable thermal conductivity by quantifying the additional cooling demand and capacity due to the λ relationship with time using the e-quest as a building energy analysis tool. The results show that, when the λ value is modeled as a function of operating temperature, its effect on the temperature profile during daytime is significant. The LD expanded polystyrene insulation leads to the lowest heat resistance across the wall section. Applying different levels of moisture showed that 30% moisture content has the lowest heat resistance compared with 10% and 20%, resulting in the highest change of 6.5% and 8.9% in cooling demand and capacity, respectively.

Effects of Nanoparticle Enhanced Lubricant Films in Thermal Design of Plain Journal Bearings at High Reynolds Numbers

Performance investigation of oil journal bearings is of particular importance given the growing use of them as a support for rotary components in a wide range of industrial machines. Frictional forces and shear stresses, which are proportionate to the velocity of lubricating layers at different points in the bearing space, provide the basis for changing temperature conditions. Various factors such as rotational velocity increase, slip width reduction, and small heat transfer coefficient of lubricant cause intensification of lubricant temperature changes. In the present study, with using computational fluid dynamic (CFD) thermohydrodynamic (THD) numerical simulations, the effect of nanoparticles on the performance features of plain journal bearings is evaluated. Particularly, 3D simulation of a journal bearing is implemented using CFD which considerably improves the accuracy of results, coupled with conjugate heat transfer model for metal parts of bearings. Reynolds equation model is used to calculate the oil-film pressure developed in hydrodynamic journal bearings by applying the nano-based lubricants. The configuration of thrust bearing consists of six pads in this study. In order to reduce the modeling complexity and computational cost and because of the symmetrical geometry of the pads, simulation of a single pad is considered instead of the entire domain. In this study, TiO2 nanoparticle with different volume fraction percentages are used. The parameters that are changed to evaluate the performance of the bearing include volume fraction percentage of the nanoparticle, type of lubricant, and rotational speed. Based on the results, for all different lubricant types, the dissipation power, average shear stress, and temperature rise are increased with augmenting the rotational speed. By increasing the rotational speed from 500 to 1500 rpm, the average shear stress increases by more than 100%, 120%, and 130% for DTE 26, DTE 25, and DTE 24 lubricant types, respectively. Moreover, by increasing the rotational speed from 500 to 1500 rpm, the dissipation power, and temperature rise are increased around 600% and 800%, respectively. Furthermore, increasing nanoparticles volume fraction from 0% to 10%, increases all parameters by approximately 10% for all lubricant types and in all rotational speeds.

Numerical simulation of hydrothermal performance of minichannel heat sink using inclined slotted plate-fins and triangular pins

In the current paper, the effect of an inclined slotted plate-fin mini-channel heat sink with triangular pins on laminar convection heat transfer and fluid flow was numerically investigated using a conjugate heat transfer model. To optimize the hydro-thermal performance of the heat sink, a parametric study on the geometric design of the slots and pins were carried out. The study was performed by varying the height of the inclined slot, the angle of the slot, and the position of the pin with respect to slot leading edge. The Reynolds number was ranged from 100 to 1600. The CFD results showed that the full height slot with an inclination angle of 55° enhanced the Nusselt number and the hydrothermal performance factor (JF) up to 1.5 and 1.43, respectively, in comparison with the simple mini-heat sink. Moreover, the Nusselt number and JF factor of the heat sink with using both slots and pins outperformed the straight channel by about 1.84 and 1.54, respectively.

Effect of cooling hole arrangements of micro-effusion cooling on the development of thermal boundary layer on a flat plate

As the next-generation gas turbine efficiency increases, it is required to develop an improved turbine cooling technology to gradually increase the inlet temperature of turbine from 1400 °C to 1600 °C. For this purpose, various studies have been conducted to enhance the external film cooling and the internal cooling performance. This paper proposes the numerical conjugate heat transfer model that is strictly validated with experiment data in order to investigate the heat transfer characteristics of micro-effusion cooling on a flat plate. First, the development of thermal and velocity boundary layer on the flat plate was simulated and analyzed in three kinds of cooling hole sizes. The smaller cooling hole diameter of 0.5 mm maintains a more stable boundary layer along to flow direction than the conventional larger hole size. The second is to compare and evaluate the cooling effectiveness of micro-effusion cooling in two types of cooling hole arrangements, such as aligned and staggered. The staggered array of cooling hole shows the more improved cooling effectiveness by about 350 % from 0.18 to 0.63 than aligned one by 270 % from 0.18 to 0.50 in micro-effusion cooling.

Investigations on a Variable-Channel-Width Double-Layered Minichannel Heat Sink

Incessant development of electronic components necessitates the exploration of efficient cooling devices to meet the demand of high heat dissipation with less pressure drop penalty. In view of that, a novel liquid-cooled variable-channel-width double-layered minichannel heat sink (VW $_{C}$ DL-MCHS) is proposed in this paper. A detailed numerical investigation on VW $_{C}$ DL-MCHS is carried out using 3-D solid–fluid conjugated heat transfer model for the counter and parallel flow configurations. The influence of the dimensionless stepped fin length ( $\lambda$ ) on the thermal performance of a VW $_{C}$ DL-MCHS is investigated by considering the same values of $\lambda $ (0.25, 0.50, 0.75, 0.8, 0.85, 0.9, 0.95) in both layers. After obtaining the optimal value of $\lambda $ , the investigations are carried forward to study the influence of different values of $\lambda $ in each layer of VW $_{C}$ DL-MCHS by considering 0%, 25%, and 50% overlapping of the dense fin region of the upper layer over the dense fin region of the lower layer. The results show that a counter-flow VW $_{C}$ DL-MCHS with zero overlap (0%) turns out to be the best geometry with the overall improvement in thermal performance of 8.68 times than the conventional single-layered-MCHS and 3.7318 times higher than the conventional DL-MCHS.