Heat Flux Rate Research Articles

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.

Implementation of the artificial neural network to predict the effectiveness of the solar system using Cu/water-ethylene nanofluid to save energy

A solar heat panel with the nanofluids (NFs) flow is simulated. For this purpose, an 80 W Photovoltaic (PV) with its cooling system (CLS) is simulated. The CLS consists of a copper pipe, and a copper plate with 1 mm thickness is located under the panel. Cu/water-ethylene (70-30) NFs flows with different flow rates in the CLS under heat flux of different hours of the day. The neural network is employed to estimate the thermal behavior of a PV that provides heat flow. For better analysis, an optimization is performed on the variables, including pipe diameter (dPipe), heat flux, and NFs flow rate on the output parameters such as maximum temperature (T-Max) and pressure drop (ΔP). The simulation results show the extraordinary ability of the neural network to estimate the output variables so that MSE < 10−5, R2 = 0.999, and MOD < 0.5%. The results of optimization revealed that the greatest fluid flow should be conducted via the pipe with the smallest (dPipe) and lowest heat flux in order to decrease the T-Max. The maximum undesirable panel temperature occurs at the opposite point. The minimum ΔP occurs when the panel has the pipe with the largest diameter, and the fluid passes through it with the lowest velocity.

Heat transfer characteristics of thermal energy storage system using single and multi-phase cooled heat sinks: A review

• Passive and active cooling techniques for battery and electronic components cooling • Computational and experimental techniques for battery and electronic components cooling • Single phase coolant investigation • Geometry optimization concerning energy management • Flat plate, pin fin, and microchannel heat sink cooling investigation Heat sinks are considered as heat exchangers employed to cool high-temperature devices such as electronic components. They can significantly improve heat dissipation from the base surface. A wide range of heat sink geometries is categorized into three major types: flat-plate, pin-fin, and microchannel heat sinks. Over the past few decades, to keep up with the rate of electronics components heat flux, extensive examinations carried to enhance heat sinks thermal performance include various fabrication materials, single-phase coolants, geometry optimization, and designing complicated heat sink concepts. Heat sink optimization contributes a significant opportunity to improve thermal management and reduce energy consumption. Hence, developing and reviewing different heat sink research methodologies is fundamental. This paper reviews various research methodologies (using single-phase coolant) to aim for heat sink optimization and thermal performance enhancement in three geometry categories (flat-plate, pin-fin, and microchannel). The reviewed articles focused on experimental, numerical, and computational efforts on energy storage thermal managements utilizing single-phase coolant for flat-plate, pin-fin, and microchannel heat sinks design.

Моделирование нестационарной теплопередачи в многослойной биметаллической пластине

When using the method of active thermal non-destructive testing to detect latent defects in multilayer bimetallic products, the adequate choice of technological modes of testing is of great importance. One of the important parameters is the optimum observation time, at which a sufficient amplitude of the temperature signal over the defect is provided. This value depends on the controlled object geometry, its thermophysical characteristics, and also selected testing modes and procedures. Various experimental stu-dies are used to determine the optimum observation time. Preparing bimetallic plates samples for full-scale experiments is a technologically complex and expensive task. Therefore, a large amount of preparatory research is carried out in the form of a numerical experiment, during which, with certain assumptions, changing the thermal state of the control object is simulated. Aim. Determination of optimal parameters of the thermal NDT process for a three-layer steel-aluminum plate with a planar delamination defect between the metal layers. For different variants of mutual location of the temperature registration device and source of heat flux of different power it is necessary to determine the heating time at which the sufficient amplitude of temperature signal is observed. Materials and methods. The methods of mathematical and computer modeling were used. Solving the task by the finite element method with the Agros2D code is given. Results. The calculation data have been obtained, which are used as the basis for constructing the temperature signal distribution over the bimetallic plate surface above the defect under different heating modes, heat flux rate, relative position of the heat stimulation source and temperature registration device, as well as determining the temperature signal dependence over the defect on the heating time. Conclusion. On the basis of modeling the temperature dependences necessary for interpretation of the results of thermal NDT of multilayered bimetallic plates have been determined, as well as the relations between the parameters of the heating process, which contribute to obtaining a temperature signal of sufficient amplitude. The results can be used for further experimental studies of the active thermal NDT processes of multilayer bimetallic materials.

Numerical Study on Heat Transfer Characteristic of the Plate-Fin Microchannel Heat Sink for Water-Based Thermal Management of CPU Chip

For effective water-based thermal management of high heat generating CPU chip, a series of numerical simulation has been conducted to study the effects of heat flux, fin height and flow rate on convective thermal performance of the plate-fin microchannel heat sinks. The characteristics of heat transfer and flow resistance have been quantificationally discussed and <i>JF</i> factor is employed to evaluate the comprehensive efficiency of convective heat transfer of microchannel heat sink. Results show that the increase in fin height and flow rate of cooling water is helpful to decrease the maximum temperature of CPU chip. Large flow rate and heat flux and short fin height are benefit to improve Nusselt number <i>Nu</i>, but they lead to large resistance coefficient <i>fRe</i> simultaneously. Analysis of <i>JF</i> factor shows that the microchannel with short fins shows better convective thermal performance when the thermal power of the CPU chip is small. The fins should be heightened when the CPU is operating at higher thermal power. The employment of <i>JF</i> factor in the present work shows its pertinence and convenience in the application of design of microchannel heat sink.

Coupled Fluid-Thermal Investigation on Drag and Heat Reduction of a Hypersonic Spiked Blunt Body with an Aerodisk

In the traditional investigations on the drag and heat reduction of hypersonic spiked models, only the aerodynamic calculation is performed, and the structural temperature cannot be obtained. This paper adopted the loosely coupled method to study its efficiency of drag and heat reduction, in which the feedback effect of wall temperature rise on aeroheating is considered. The aeroheating and structural temperature were obtained by the CFD and ABAQUS software respectively. The coupling analysis of the hypersonic circular tube was carried out to verify the accuracy of the fluid field, the structural temperature, and the coupled method. Compared with experimental results, the calculated results showed that the relative errors of stagnation heat flux and stagnation temperature were 1.34% and 4.95% respectively, and thus the effectiveness of the coupled method was verified. Installing a spike reduced the total drag of the forebody. The spiked model with an aerodisk reduced the aeroheating of the forebody, while the model without an aerodisk intensified the aeroheating. The spiked model with a planar aerodisk had the best performance on drag and heat reduction among all the models. In addition, increasing the length of the spike reduced the drag and temperature of the forebody. With the increase of the length, the change rates of drag, pressure, heat flux, and temperature decreased gradually. Increasing the diameter of the aerodisk also reduced the temperature of the forebody, while the efficiency of forebody drag reduction first increased and then decreased. In conclusion, the heat and drag reduction must be considered comprehensively for the optimal design of the spike.

Investigation on heat transfer performance of a novel active method heat sink to maximize the efficiency of thermal energy storage systems

In this work, a novel electric-body-force-equipped heat sink was investigated to determine the effect of the electric field on the heat sink performance. To introduce the optimal composition of the heat sink and electric field, different fin lengths of the heat sink with various arrangements of electrodes were investigated. Increasing the length of fins improves the transferred heat to a certain extent, but the results of this study show that using the electric field has decreased the limitation of this range. Using an electric field on the fins with several applied voltage levels and heat flux rates demonstrated that increasing the applied voltage had an improving effect on heat transfer until 20 kV, and the maximum efficiency occurred on heat flux of 800(W·m−2). The composition of high voltage and ground electrodes in two sides of fins was the best configuration for heat transfer enhancement in the heat sink. Employing this arrangement could transfer the trapped heat farther from fins by maximum creation of fluid flow near the surface of them. This modern heat sink can be used as an active method in accompany with the passive method advantages to maximize the efficiency of sensible heat energy storage systems.

CO2, H2O and energy fluxes from chickpea crop grown under residual soil moisture condition in rainfed peninsular India using eddy covariance techniques

Present study is the first to report the ecosystem CO2, H2O and energy exchanges (at diurnal, daily and seasonal-scale) from rainfed winter chickpea crop (Cicer arietinum L.), grown under residual soil moisture over semi-arid peninsular India, using eddy covariance system. The available energy to the ecosystem was found to be predominantly partitioned into sensible heat flux (H), except during the peak vegetative stage. Asymmetric response of the daytime (forenoon and afternoon) CO2 and energy fluxes were observed due to limited soil moisture condition. The peak vegetative stage of the crop i.e., 40–60 Days after Sowing (DAS) showed highest rate of Net Ecosystem CO2 Exchange (NEE; −9 to −11 μmol m−2 s−1) and Latent Heat Flux (LE; ∼ 300 W m−2). The response of daytime Gross Primary Production (GPP) to light intensity was found to be poor during 1–16 DAS, improved steadily in 17−34 DAS, remained high in 35–69 DAS, and deteriorated thereafter till harvesting (70–106 DAS). The chickpea crop was net CO2 sink on daily basis during 17–69 DAS. Peak daily NEE of −2.52 gC m−2 day−1, GPP of 4.2 gC m−2 day-1 and Evapotranspiration (ET) of 2.38 mm day−1 were observed during the growing season. Over the entire season, rainfed chickpea was found to be a weak carbon sink with cumulative NEE of -32.39 ± 4.21 gC m−2; GPP of 163.03 ± 15.04 gC m−2; Ecosystem Respiration (Reco) of 127.45 ± 12.85 gC m−2; ET of 177 ± 18.13 mm. The maximum ecosystem water use efficiency (GPP/ET) and crop coefficient were found to be 1.7 gC m−2 mm−1 and 0.6 respectively at peak vegetative stage, which are sub-optimal compared to well managed chickpea crop. The results help us to understand the ecosystem processes and exchanges from rainfed winter legume-based cropping system, which intern would significantly contribute in global carbon and moisture budget.

The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness

AbstractThe thickness of the brittle lithosphere—the outer portion of a planetary body that fails via fracturing—plays a key role in the geological processes of that body. The properties of both a planet and its host star can influence that thickness, and the potential range of those properties exceeds what we see in the Solar System. To understand how planetary and stellar parameters influence brittle lithospheric thickness generally, we modeled a comprehensive suite of combinations of planetary mass, surface and mantle temperature, heat flux, and strain rate. Surface temperature is the dominant factor governing the thickness of the brittle layer: smaller and older planets generally have thick brittle lithospheres, akin to those of Mercury and Mars, whereas larger, younger planets have thinner brittle lithospheres that may be comparable to the Venus lowlands. But certain combinations of these parameters yield worlds with exceedingly thin brittle layers. We predict that such bodies have little elevated topography and limited volatile cycling and weathering, which can be tested by future telescopic observations of known extrasolar planets.

Thermal transmittance prediction based on the application of artificial neural networks on heat flux method results

Deep energy renovation of building stock came more into focus in the European Union due to energy efficiency related directives. Many buildings that must undergo deep energy renovation are old and may lack design/renovation documentation, or possible degradation of materials might have occurred in building elements over time. Thermal transmittance (i.e. U-value) is one of the most important parameters for determining the transmission heat losses through building envelope elements. It depends on the thickness and thermal properties of all the materials that form a building element. In-situ U-value can be determined by ISO 9869-1 standard (Heat Flux Method - HFM). Still, measurement duration is one of the reasons why HFM is not widely used in field testing before the renovation design process commences. This paper analyzes the possibility of reducing the measurement time by conducting parallel measurements with one heat-flux sensor. This parallelization could be achieved by applying a specific class of the Artificial Neural Network (ANN) on HFM results to predict unknown heat flux based on collected interior and exterior air temperatures. After the satisfying prediction is achieved, HFM sensor can be relocated to another measuring location. Paper shows a comparison of four ANN cases applied to HFM results for a measurement held on one multi-layer wall – multilayer perceptron with three neurons in one hidden layer, long short-term memory with 100 units, gated recurrent unit with 100 units and combination of 50 long short-term memory units and 50 gated recurrent units. The analysis gave promising results in term of predicting the heat flux rate based on the two input temperatures. Additional analysis on another wall showed possible limitations of the method that serves as a direction for further research on this topic.

Effect of arrhenius activation energy and medium porosity on mixed convective diluted ethylene glycol nanofluid flow towards a curved stretching surface

The present investigation is a computational attempt to explore the impact of Arrhenius activation of the energy on the mixed convective binary chemical nanofluid dissipative flow over a curved stretching surface through a porous medium. Curvilinear coordinates are opted for mathematical interpretation of the physical flow problem and governing equations are transformed into a set of coupled non-linear differential equations by invoking similarity analysis. The nonlinear coupled system is tackled numerically with the aid of computational software MAPLE. The MAPLE routine program bvp midrich is incorporated for numerical solutions subject to the fixed values of emerging parameters. The impact of prominent parameters on velocity, temperature, concentration distributions along with physical quantities of interest such as surface drag force, heat and mass flux rates are explored through graphs and tables. The key findings of the present investigation include that the Darcy-Forchheimer parameter contribute prominently in fluid flow along with heat and mass transfer.

Turbulent cylinder-stirred flow heat and momentum transfer research in batch operated single-phase square reactor

Heat transfer and momentum transfer due to water flow in a cylinder-stirred reactor of square cross section were investigated. Important industrial processes like mixing depend on combined effects of heat transfer and fluid flow which can be understood by analyzing the turbulence structure. A reactor of aspect ratio η = 0.21 (diameter of cylinder to tank side wall) was considered. It uses a small-diameter cylinder to stir the water in batch condition with rotating Reynolds number in a range 1.68 × 104 <Re < 10.1 × 104. The cylinder released a constant heat flux rate of 158 W/m2 through its surface. The steady state mean flow was numerically simulated using two models for turbulence, isothermally with a k-ε and thermally with the RSM model. The results revealed a fluid motion like Taylor-Couette vortex which was validated with PIV streamlines for Re = 6.1 × 104 and 10.1 × 104. Turbulent angular momentum and shear rate revealed differences for the corner direction as the Re number increased compared with the wall direction. Temperature fluctuations and thermal gradient in the gap allowed to analyze the turbulent heat diffusivity coefficient and the ratio to predicted diffusivity for momentum. The results revealed that heat and momentum diffusivity increase for higher Re number and show that heat diffusivity is faster than momentum in the gap. The main frequencies of fluid motion showed large-scale structures and secondary cells of fluid motion. The averaged heat transfer as a function of Re number indicate that this reactor is an enhanced mean for mixing process compared to concentric cylinders reactors.

Kinetic theory of polydisperse gas–solid flow: Navier–Stokes transport coefficients

The particulate phase stress and solid–solid drag force in the multifluid modeling of polydisperse gas–solid flows are usually closed using kinetic theory. This research aims to establish the hydrodynamic equations and constitutive relations of the multifluid model for polydisperse systems via species kinetic theory, in which the non-equipartition of energy and interphase slip velocity between different species are considered. Whereas previous studies have used approximations, such as Taylor series expansions, to simplify the calculation of collision integrals, the present study, for the first time, solves the collision integrals analytically without any approximations to obtain accurate constitutive relations. Explicit expressions for the constitutive laws are obtained, including the particle stress tensor, solid–solid drag force, heat flux, and energy dissipation rate up to the Navier–Stokes order. The present study offers more complete and mathematically rigorous constitutive laws for the multifluid modeling of polydisperse gas–solid flows.

Feasibility analysis of low thermal conductivity asphalt concrete bridge deck deicing by using geothermal energy

Deicing of bridge deck is studied based on ground source heat. In this paper, a COMSOL finite element numerical model is established, it considers the generalized heat flux, convective radiation heat flux and snow melting heat rate as the cold source. The designed pipe spacing of the model is 20cm, 25cm and 30cm respectively. The influence of low thermal conductivity of asphalt bridge deck on deicing effect is considered in the model, and the temperature and stress of bridge deck are simulated, and the relevant data results are obtained. The actual heat flux required for deicing is calculated, and it is concluded that the deicing effect increases gradually with the increase of pipe spacing, but the increase of 20cm spacing is smaller than that of 25cm spacing. 25cm spacing has good effect and is the most economical.

Experiemental Study on Heat Transfer of Supercritical Water in Triangular Channel of Reactor Core with Spacer Grid

The experimental study on supercritical water flow heat transfer is carried out for the vertical upward triangular sub-channels in the supercritical water cooled reactor core with spacer grid. The fuel bundle diameter and pitch ratio of this sub-channel used are 8 mm and 1.4 respectively. The parameters adopted in the research include heat flux (q) ranging from 600 kW/m2, pressure (P) ranging from 23-28 MPa, and mass flow rate (G) ranging from 700-1,300 kg/(m2·s). This study analyzes the effect of such thermal parameters as heat flux, pressure and mass flow rate on the heat transfer characteristics of supercritical water. According to the experimental results, with the mass flow rate at the spacer grid increasing, the fluid disturbance increases and the heat transfer coefficient rises significantly; under the supercritical pressure, the inner surface temperature increases with the increasing pressure, and consequently, the peak heat transfer coefficient decreases; as the peak heat transfer coefficient reduces at an excessively high heat flux, the heat transfer performance can be improved when the heat flux is reduced properly; increasing the mass flow rate causes the drop of inner surface temperature and the increase of peak heat transfer coefficient, and thus can significantly improve the heat transfer performance; and the pressure change has little effect on the heat transfer characteristics in the vicinity of spacer grid, and however, the system safety can be improved by increasing the pressure properly.

Experimental study on flow boiling heat transfer characteristics of multi-channel modified filled foam metal small channel

This experiment reports the experimental study on the heat transfer performance of water in the flow-boiling heat transfer process of different modified foam metal-filled channels. In the experiment, the foam metal has subjected to surface hydrophobicity treatment, surface roughness modification treatment, and gradient pore density stacking treatment. The mass flow rate is 50–150 ml/min, and the heat flux is 15 kW/m2 – 25 kW/m2. The pore density of foam metal is 20 PPI, 50 PPI, and 110 PPI. A high-speed camera, micro camera, and electron microscope are used to record and observe the internal flow of the channel, the results after surface treatment, and the related reaction mechanism. The experiment uses different modifications as the basis for grouping and uses heat flux and mass flow rate as variables to perform multiple sets of orthogonal experiments. At the same time, the prediction model of matching heat transfer coefficient and the correlation of pressure drop gradient are proposed. Construct a comprehensive evaluation index of heat exchange of porous foam metal, which is evaluating the heat exchange performance of foam metal that matches the change of resistance.

The Influence of Convection Heat Transfers for Vertical Mini-Tubes Using Solvent Carbon Dioxide and Porous Media at Supercritical Pressure

Porous media and solvent CO2 at supercritical pressure were investigated experimentally to study the effect of convection heat transfer in vertical mini-tubes. Mini-tubes diameter (5 and 8 mm) with medium porosity of 0.5 are proposed in experimental investigation. Experimental conditions consisted of bulk fluid, wall temperatures ranged from 33 to 55 oC, and 8 to 10 MPa of pressure. Reynolds number, Mass flow rate, and heat flux were 1750 to 21000, 0.5 to 4.5 Kg/h, and 3.25×104 to 1.1×105 W/m2 respectively. Some chemical additives like Ethanol, Chloroform, Acetone, Dimethyl sulfoxide, and Methanol were considered. A special focus was dedicated to studying the influence of heat flux, inlet temperature, and mass flow rate at measured values of wall and fluid bulk temperatures, and coefficients of local heat transfer for mini-tubes and porous media. A higher effect was noticed on the convection heat transfer by buoyancy and properties of the thermophysical variable of solvent CO2 in mini-tube at vertical position. However, when these results were compared with the controls (empty tube) shoewn dramatically different results. Heat transfer coefficient was bigger about 4 times when using the porous media tube compared with the empty type in the case of using a 5% of acetone solvent.