Heat Flux Rate Research Articles

Investigation of the cooling enhancement of a single crystal diamond heat sink with embedded microfluidic channels

Single crystal diamond (SCD) owns superior mechanical strength, chemical stability, and the highest thermal conductivity among the well-known materials. In this work, we investigated the cooling enhancement of a cold plate made of SCD with embedded microfluidic channels. In particular, we studied the enhanced heat spreading due to conduction followed by convective dissipation of a locally heated resistor mimicking a linear hot spot within electronic chips. Experiments were carried out with various heat fluxes (9–75 W/cm 2 ) and volumetric flow rates (0.02–0.18 ml/min) under transient state. The results showed that cold plate made of a SCD layer with embedded microfluidic channels exhibited the highest cooling effect obtained for maximum applied power density and flow rate. This indicated that combined effect of conductive spreading and convective heat transfer exhibited a significant cooling enhancement. Simulation results further support the improvement of the cooling capability due to the addition of microfluidic channels and the use of SCD as the substrate of the heat sink. • Single crystal diamond embedded with microchannels (SCD-MC) was fabricated and tested. • A maximum temperature was decreased by 42.1 % when using SCD-MC instead of Silicon. • The cooling enhancement of SCD-MC increased with heat flux and volumetric flow rate.

Evaluation of Heat Flux Distribution Characteristics of a Concentrated Solar Dish Collector with different geometric indices

The concentrated solar dish collector is a promising technology for generating both electricity and thermal energy together and it is termed as concentrated photovoltaic thermal. The important component of the parabolic dish collector is the absorber; where all concentrated lights are falling. The present paper has investigated the heat flux distribution characteristics of the flat plate absorber based solar dish collector by using the ray-tracing simulations. In concentrating dish collector, rim angle and dish diameter are significant factor of the flux distributions. The present study reported the average heat flux distribution, maximum flux intensity and non-uniformity of flux distribution for different geometrical conditions. The maximum heat flux rate attains the rim angle between 35 to 55° for any dish diameter. Where the peak flux intensity raises concerning raises of rim angle and peak flux occurred at rim angle 90°. The increase of heat flux intensity causes the non-uniformity of heat flux distribution over the absorber surface. The non-uniformity factor is mainly influenced by the rim angle, not a dish diameter. When rim angle 15 & 75°, the non-uniformity is 2.5 & 10 respectively for whichever dish diameters. A critical rim angle produces the non-uniformity factor. Results shows that optimization of rim angle is a significant contribution for decreasing the non-uniformity index of concentrator; it most valuable for coupled thermal and electricity generating applications.

Experimental investigation on the thermal performance of a novel thermosyphon heat pipe with truncated cone for heat recovery systems

A Thermosyphon Heat Pipe (THP) is an effective heat transfer device primarily deployed in the field of energy conversion and heat recovery systems. Among the various working fluids, deionized (DI) water exerts higher latent heat of vaporization making it suitable for efficient performance testing of THPs. The novel design of a THP in a non-uniform geometry-based truncated cone structure is suggested to recover waste heat from various sources. A detailed parametric study is carried out on the non-uniform structure THPs to analyze the thermal performance. This study examines the thermal performance of the Convergent Truncated Cone (CTC) THP and the effect of modifying the influencing factors of its diameter, length, heat flux, inclinations, and mass flow rate. Based on the experimental results, it is seen that the CTC helps to reduce the overall thermal resistance by 48.94 %, 35.15 %, and 34.69 % for 90°, 60°, and 30° inclinations, respectively. Similarly, the temperature difference between the evaporator and the adiabatic sections shows an average thermal resistance reduction of 36.38 %, 30.3 %, and 55.82 % under the same operating conditions. Additionally, the results of the current investigation indicate that there are two positive outcomes for the supplied heat input range of 30–300 W. It is observed that the THP is 2.54 % more efficient than a conventional THP at an angle of 90° for a heat input range of 30–150 W. The maximum augmentation of thermal efficiency is noted to be 4.12 % and is attained at 30° inclination for the input range of 180–300 W.

Flow boiling heat transfer mechanisms and flow characteristics of pump-driven two-phase flow systems used in data center cooling

Pump-driven two-phase flow systems are commonly used in data center cooling owing to their outstanding heat-transfer performance. The evaporator is a key component of the cooling system, and its design significantly affects the overall cooling efficiency of the system. The boiling heat transfer process in the evaporator is primarily affected by the boiling heat transfer mechanism. Such a mechanism is mainly influenced by the type of coolant, inner wall structure of the tube (threaded tube or smooth tube), heat flux, and mass flow rate. In this study, we experimentally investigated the heat transfer mechanisms, capacity, and flow characteristics of the boiling process in a pump-driven two-phase flow system commonly used in data center cooling. The heat transfer capacity involved in this experiment was 140–1150 W, and the mass flow rate was 40–270 kg/h. Different coolants (R134a and R410A) and tube structures (threaded and smooth) were tested. The main novelty of this study is as follows. First, under the working conditions in this experiment, the inside-tube flow boiling mechanism was nucleate boiling for both the smooth and threaded tubes, and the boiling heat transfer coefficient remained constant as the mass flow rate decreased and vapor quality increased. Second, although it is commonly believed that a threaded pipe can significantly enhance the heat transfer ability, our study found that under data center cooling applications, the threaded tube had little strengthening effect on the heat transfer performance. Third, commonly used coolants, including R134a and R410A, were tested and the above conclusions were verified.

The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

Abstract. The role of precipitation (P) variability with respect to evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E), from savannas continues to draw significant research interest given its relevance to a number of ecohydrological applications. Our study reports on 6 years of measured ET and estimated T and E from a grazed savanna grassland at Welgegund, South Africa. Annual P varied significantly with respect to amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy-covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T for the 4 wet years with frequent early wet-season rainfall was nearly constant, 326±19 mm yr−1 (T/ET=0.51), but was lower and more variable between the 2 dry years (255 and 154 mm yr−1, respectively). Annual T and T/ET were linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grasses as well as trees' ability to use water from deeper layers. During extreme drought, grasses respond to water availability with a dieback–regrowth pattern, reducing leaf area and transpiration and, thus, increasing the proportion of transpiration contributed by trees. The works suggest that the early-season P distribution explains the interannual variability in T, which should be considered when managing grazing and fodder production in these grasslands.

The distinct morphologic signature of underplating and seamounts in accretionary prisms, insights from thermomechanical modeling applied to Coastal Iranian Makran

The active structures of the Iranian Makran, especially the presence of normal faults, vary laterally in the upper plate of the subduction zone, and their relationship with the deep duplexes and seamounts at depth remains unknown due to poor coverage of data onshore. In this paper, we investigate the relationship between deep structures and the topographic slope using thermo-mechanical simulations. The initial and boundary conditions of the models, including basal heat flux and convergence rate, are calibrated using the depth of the seafloor, bottom seafloor reflectors and the few available well temperature. We specifically test the influence of seamount subduction and thermally controlled changes in rock strength and décollement on the relationship between topography and deep tectonic structures. The inclusion of the brittle-ductile transition and dehydration reactions like the smectite-illite transition produce the three slope segments observed in the Makran accretionary prism. The three segments correspond to a regular accretionary prism, a flat segment that marks the smectite-illite transition, and a rise in topography located above the zone where the tegument reaches its temperature-controlled brittle-ductile transition causing underplating. Nevertheless, the models show that neither the decrease in friction associated with dehydration nor the onset of underplating is sufficient for the observed normal faults to arise in a self-consistent manner from the simulations. Crustal-scale normal faults only emerge in simulations that include subduction of a large seamount. These simulations also produce a large-thrust-slice that is the scar of former subducting seamount and serves as a buttress for the formation of a new imbricated zone. Using offshore seismic reflection data, the published onshore tomographic profiles, and our thermo-mechanical simulations, we propose two onshore-offshore cross-sections of the Iranian Makran accretionary prism.

Review on research progress in boiling acoustics

Ever-growing miniaturization of electronic devices and space-conserving endeavours of heat transfer systems pose a challenging task to the current cooling strategies. Boiling acoustics is one of the most potent and efficacious methodologies to reliably predict the boiling regime inside the cooling systems to assay the safety and address the emergency conditions. Boiling acoustics is gripping attention out of the few foreseeable technologies for the future cooling requirements. Though the potentiality of boiling acoustics was unravelled in the late 1990s, the research data present in this arena is however lacking. This paper presents a comprehensive review of the literature reported from the 1970s to the present date. Furthermore, this paper also details the evolution of boiling acoustics from the initial application of boiling incipience detection to regime identification. Much focus is given on salient features of boiling acoustic characterization dealing with the regime detection. Effects of various parameters such as thermos-physical properties of the heater surface and the boiling liquid that directly or indirectly influence the acoustic spectra are also presented. The prediction of the boiling regime constitutes the first necessary step in producing autonomous cooling systems. Hence, the detection and characterization of boiling noise under various conditions such as pressure, heat flux, and flow rate is essential.

Development of resistor-capacitor and finite difference models to evaluate green roof thermal performance

Modelling green roof physics has mainly involved developing complex numerical models to simulate physical processes that occur between the many surfaces and materials that define a green roof system. However, a recent review of these models declares that (1) increasing model complexity may not necessarily translate into better predictability of key thermal performance metrics (e.g., interior temperature), and (2) researchers should consider developing parsimonious models and alternate modelling techniques that can predict variables or processes that are more indicative of green roof thermal performance. In this paper, two inverse models – a resistor-capacitor (RC) thermal network model and an implicit finite difference (FD) model – are developed. The models are calibrated with multi-year sensor data from a green roof in Ottawa, Canada by employing the genetic algorithm. The calibrated models are then evaluated based on their ability to predict hourly rates of heat flux. Our results demonstrate that characterization of green roof thermal properties is affected by differences in spatial resolution between the models. Predictability of hourly heat flux by the RC and FD models resulted in a root-mean-squared error that ranged between 0.51 and 1.04 W/m2 and 0.42–0.81 W/m2, respectively, across five separate months: May through September 2016. Percent reductions in total monthly heat exchange relative to a conventional roof were better predicted by the FD model each month. Validation of each model using five continuous months of data from 2017 demonstrates the inverse models generated realistic thermophysical green roof properties.

Thermal Willis Coupling in Spatiotemporal Diffusive Metamaterials.

Willis coupling generically stems from bianisotropy or chirality in wave systems. Nevertheless, those schemes are naturally unavailable in diffusion systems described by a single constitutive relation governed by the Fourier law. Here, we report spatiotemporal diffusive metamaterials by modulating thermal conductivity and mass density in heat transfer. The Fourier law should be modified after homogenizing spatiotemporal parameters, featuring thermal Willis coupling between heat flux and temperature change rate. Thermal Willis coupling drives asymmetric heat diffusion, and the diffusion direction is reversible at a critical point determined by the degree of spatiotemporal modulation. Moreover, thermal Willis coupling stands robustly even when only thermal conductivity is modulated. These results may have potential applications for directional diffusion and offer insights into asymmetric manipulation of nonequilibrium mass and energy transfer.

Optimization of MLP neural network for modeling flow boiling performance of Al2O3/water nanofluids in a horizontal tube

In this paper, a multilayer perceptron (MLP) artificial neural network (ANN) with a back-propagation (BP) training algorithm is applied for modeling thermophysical properties and subcooled flow boiling performance of Al2O3/water nanofluid in a horizontal tube. The influence of nanofluid concentration, heat flux, and flow rate on different thermophysical parameters, including thermal conductivity, thermal conductivity enhancement, viscosity, viscosity enhancement, and heat transfer coefficient, are investigated. Specifically, flow boiling of Al2O3/water nanofluid in a horizontal tube is modeled with the MLP neural network optimized by three novel swarm-based optimization algorithms: namely, Equilibrium Optimizer (EO), Marine Predators Algorithm (MPA), and Slime Mould Algorithm (SMA). To evaluate the effectiveness of different models, the MSE (Mean-Square Error) of the ANN model with varying optimization algorithms is calculated and compared. Additionally, the optimal network and regression values for each parameter are determined. The results show that the applied neural network and optimization algorithms could model the thermal conductivity, thermal conductivity enhancement, and viscosity better than the viscosity enhancement and heat transfer coefficient. The MSE of the best network for the thermal conductivity is 2.693 × 10−7, while the MSE of the best network for the viscosity enhancement is 0.0598. Also, the EO algorithm achieves the best optimization for the first three outputs, thermal conductivity, thermal conductivity enhancement, and viscosity. In comparison, the MPA algorithm extracts the optimal network for the other two outputs, viscosity enhancement, and heat transfer coefficient.

Homogeneous-heterogeneous chemical reactions and effectiveness of thermo-sloutal time’s relaxation concept in Carreau fluid flow

This study includes Carreau fluid flow at a stagnation point under MHD influence. The Christov-Cattaneo double diffusion formula is to manipulate the heat and mass flux rate with the generation of heat. The flow is manifested by the stretched sheet. The heterogeneous-homogeneous reactions are carried out. The admissible transformations are invoked to alter the flow narrating coupled partial differential framework into a coupled ordinary differential framework. These equations are sorted out in a numerical framework with the aid of the Runge–Kutta Fehlberg technique supported by the shooting scheme. The parameter of thermal and concentration relaxation diminishes fluid temperature and concentration, respectively. The numerical results for the drag force are examined.

Near-wall dynamics of premixed methane/air flames

This work focuses on sidewall quenching (SWQ) for premixed methane/air flames that are forced by a periodic oscillatory inflow with excitation frequency f = 100 Hz. The effects of steady-state and transient flame stretch on the near-wall flame dynamics are evaluated using two-dimensional direct numerical simulation (2D-DNS) and the GRI 3.0 reaction mechanism. The velocity fluctuations lead to significant changes in flame speed and flame stretch, as well as the associated Markstein numbers. The phenomenon of SWQ is analyzed using flame quenching distance, wall heat flux and heat release rate. For steady-state conditions, there is a strong correlation between the maximum wall heat flux WHFmax and the flame quenching Peclet Number (Peq), as well as between the flame speed and the flame stretch; for transient conditions, the flame quenching distance (dq) increases continuously from phase angles of 1/4f−1 to 3/4f−1 in one cycle as time progresses, and the fluctuation of the quenching distance (Δdq) decreases gradually with increasing equivalence ratio (∅). The flame stretch changes from negative to positive in the process from 1/4f−1 to 3/4f−1, while the heat release rate and fuel reaction rate near the wall gradually decrease. Furthermore, the FWI region is dominated by negative flame stretch while positive flame stretch is present at the base of the flame. Moreover, the methane/air flame has a nearly twofold increase in the consumption speed during the oscillation from phase angle 3/4f−1 to the next cycle at 1/4f−1 at ∅ = 0.5 and ∅ = 1.0. These results show that flow field perturbations are not negligible in elucidating the effects of flame-wall interactions.

Minimizing entransy dissipation for heat transfer processes withq∝Δ(Tn)and heat leakage

Heat transfer process with heat leakage and generalized radiative heat transfer law (HTL) [q∝Δ(Tn)] is investigated. Optimality condition for the minimum entransy dissipation (MED) is derived by applying average optimal control theory. On the basis of the general optimization result, the results for four special cases of Newtonian [q∝Δ(T)], linear phenomenological [q∝Δ(T−1)], radiative [q∝Δ(T4)], and mixed [q∝Δ(T4) with q∝Δ(T)] HTLs are further obtained. Optimal paths are compared with the common strategies of constant heat flux rate and constant hot fluid temperature operation. Numerical examples are presented, and influences of heat transferred amount and heat leakage on optimization performances are analyzed.

Secondary Flow Effect on Supercritical Kerosene Oxidation Deposition and Heat Transfer in a Coiled Tube.

Deposition in fuel cooling systems remains a challenge to the development of active cooling technologies for air-breathing engines. We experimentally and numerically investigated the influence of the secondary flow and heat-transfer characteristics of supercritical kerosene in a coiled tube on oxidation deposition. A coiled heated tube reactor (3000 mm long, 23 cycles) under constant heat flux and flow rate was applied to simulate the conditions of the fuel side in the heat exchanger of an aero-engine cooling system. The coupling characteristics of coking distribution with the development of secondary flow were studied along the whole pipe. The dynamic pressure, temperature, and velocity were analyzed in two specific circular cross sections located in the bend of the tube. The secondary flows induced in the coiled tube greatly enhance the heat transfer and slightly decrease the deposition rate, resulting in linear wall temperature profiles and a uniform coking distribution along the tube compared to the long straight tube. There is no obvious heat-transfer enhancement or deterioration in the whole coiled tube. The modified heat-transfer correlation of the supercritical RP-3 in the coiled tube was fitted at different flow rates and heat fluxes with an error of ±10%.

Magnetic Dipole and Thermophoretic Particle Deposition Impact on Bioconvective Oldroyd-B Fluid Flow over a Stretching Surface with Cattaneo-Christov Heat Flux.

This study emphasizes the performance of two-dimensional electrically non-conducting Oldroyd-B fluid flowing across a stretching sheet with thermophoretic particle deposition. The heat and mass transfer mechanisms are elaborated in the presence of a magnetic dipole, which acts as an external magnetic field. The fluid possesses magnetic characteristics due to the presence of ferrite particles. The gyrotactic microorganisms are considered to keep the suspended ferromagnetic particles stable. Cattaneo–Christov heat flux is cogitated instead of the conventional Fourier law. Further, to strengthen the heat transfer and mass transfer processes, thermal stratification and chemical reaction are employed. Appropriate similarity transformations are applied to convert highly nonlinear coupled partial differential equations into non-linear ordinary differential equations (ODEs). To numerically solve these ODEs, an excellent MATLAB bvp4c approach is used. The physical behavior of important parameters and their graphical representations are thoroughly examined. The tables are presented to address the thermophoretic particle velocity deposition, rate of heat flux, and motile microorganisms’ density number. The results show that the rate of heat transfer decreases as the value of the thermal relaxation time parameter surges. Furthermore, when the thermophoretic coefficient increases, the velocity of thermophoretic deposition decreases.

ВИКОРИСТАННЯ ВОДНЯ В ЕНЕРГЕТИЦІ

As a result of the conducted literature research the experimentally fixed positions about features of the hydrogen-induced deformation of a palladium plate are described. It is determined that when hydrogen saturation in the metal, a temporary gradient material "metal-hydrogen" is formed and hydrogen stress concentrations always occur. This in turn provides effective planning and determination of the time of penetration of hydrogen into the metal. The obtained experimental facts allow to control the change of shape and regulate the modes of operation of the fuel cell during the operation of the products. It is described that the maximum bending of the plate occurs at a constant temperature, and is determined by two fundamental properties of the palladium system - hydrogen, namely, the diffusion coefficient and equilibrium solubility of hydrogen in palladium. However, when the temperature changes, the diffusion coefficient of hydrogen in palladium and the equilibrium concentration of hydrogen in palladium changes, which determines the temperature dependence of the final bending of the plate and its change) when interacting with hydrogen. The scientific novelty is the use of the known material palladium, which in contact with hydrogen becomes a temporary gradient alloy with variable physical properties. It is established that the heat exchange in the plate and the energy balance around the plate depends on the rate of interaction of hydrogen and material, as well as the rate of heat flux that occurs during chemical reactions in fuel cells and heat loss in the fuel cell. The practical significance of the work lies in the possibility of using certain physical bases in practice in the manufacture of specific devices that work as a fuel cell with hydrogen. You can also model this process in MaCad and improve the operating conditions of fuel cells and hydrogen sensors, because at low temperatures there is a relatively small residual bending of the main components, and accordingly there are changes in the shape of these components.

Turbulent Heat Fluxes in a Mediterranean Eddy Quantified Using Seismic and Hydrographic Observations

Mediterranean eddies (meddies) play an essential role in transferring heat, salinity and momentum into the Atlantic Ocean. The rate of heat (and salt) flux from the meddy and its ultimate lifetime are key proxies to understanding how meddies impact the redistribution of heat and salt in the ocean system. A Mediterranean eddy was observed in the Gulf of Caidz in 2007 using seismic and hydrographic data. The spatial distribution of turbulent dissipation rates around the meddy is estimated from the seismically derived internal wave spectra subrange using fine-scale parameterization. Turbulent dissipated rates are lowest (10−11 W/kg) within the core of the meddy but rise by nearly two orders of magnitude at the upper and lower boundaries, where signs of double diffusive convection are observed. Along the left flank of the meddy, thermohaline intrusions and interleaving of water masses are found in inverted temperature and salinity profiles, transporting heat laterally from the warm core to the Atlantic water with a flux of around 470 Wm−2. The meddy presented in this study is shown to decay in 2 years, primarily due to the heat loss associated with thermohaline intrusions. For the first time, heat fluxes around the meddy and its lifetime are quantified using seismic oceanography data, and the methods proposed here can be applied to more seismic datasets in the global oceans.

Numerical Investigation of Boiling Flow in Oval Tube with Different Ellipticity

Improving heat transfer has been the continuous quest in thermal engineering field. With higher transfer rate required, many researchers have shifted their attention towards boiling heat transfer that offer superior transfer rate. Despite its potential, study on this heat transfer process have not been as extensive as its single-phase counterpart. Particularly, to the best of our knowledge, no study on the boiling heat transfer in oval tube with different ellipticity. Thus, numerical investigation using RPI wall boiling model and RNG k-ε turbulence model is conducted for different ellipticity of the oval tube. It is found that the heat transfer performance drops as the ellipticity increases with constant perimeter. Decrease of heat flux and increase of mass flow rate have positive impacts toward the heat transfer performance. The vapor generation resulting by the heat flux and mass flow rate is having high influence in the heat transfer coefficient and pressure drop.

FLOW BOILING BEHAVIORS OF VARIOUS REFRIGERANTS INSIDE HORIZONTAL TUBES: A COMPARATIVE RESEARCH STUDY

In this study, convective boiling behaviors of refrigerants (R22, R134a, R290, R404a, R410a, R600, R507, R717, and R744) are compared with existing flow boiling correlations. Flow boiling heat transfer rates under the primary influences of vapor quality and mass flux are plotted and compared with the experimental results that are extracted from the literature. A statistical approach is introduced for error analysis and a valid correlation is proposed for each refrigerant. Correlation of Wojtan et al. with 30% of experimental data in 20% error region is selected as the best correlation while Shah Correlation with 10% of experimental data in 20% error regions performs the worst. The result of the analysis indicates that flow boiling heat transfer values are strongly dependent upon mass velocity and heat flux. It is observed that existing correlations are in good agreement with experimental works at low heat flux rates whereas deviations are getting bigger for high heat flux rates.

Significance of Rosseland's Radiative Process on Reactive Maxwell Nanofluid Flows over an Isothermally Heated Stretching Sheet in the Presence of Darcy-Forchheimer and Lorentz Forces: Towards a New Perspective on Buongiorno's Model.

This study aimed to investigate the consequences of the Darcy–Forchheimer medium and thermal radiation in the magnetohydrodynamic (MHD) Maxwell nanofluid flow subject to a stretching surface. The involvement of the Maxwell model provided more relaxation time to the momentum boundary layer formulation. The thermal radiation appearing from the famous Rosseland approximation was involved in the energy equation. The significant features arising from Buongiorno’s model, i.e., thermophoresis and Brownian diffusion, were retained. Governing equations, the two-dimensional partial differential equations based on symmetric components of non-Newtonian fluids in the Navier–Stokes model, were converted into one-dimensional ordinary differential equations using transformations. For fixed values of physical parameters, the solutions of the governing ODEs were obtained using the homotopy analysis method. The appearance of non-dimensional coefficients in velocity, temperature, and concentration were physical parameters. The critical parameters included thermal radiation, chemical reaction, the porosity factor, the Forchheimer number, the Deborah number, the Prandtl number, thermophoresis, and Brownian diffusion. Results were plotted in graphical form. The variation in boundary layers and corresponding profiles was discussed, followed by the concluding remarks. A comparison of the Nusselt number (heat flux rate) was also framed in graphical form for convective and non-convective/simple boundary conditions at the surface. The outcomes indicated that the thermal radiation increased the temperature profile, whereas the chemical reaction showed a reduction in the concentration profile. The drag force (skin friction) showed sufficient enhancement for the augmented values of the porosity factor. The rates of heat and mass flux also fluctuated for various values of the physical parameters. The results can help model oil reservoirs, geothermal engineering, groundwater management systems, and many others.