Heat Transfer Regimes Research Articles

Experimental investigation of the heat transfer characteristics of CO2 at supercritical pressures flowing in heated vertical pipes

Supercritical CO2 shows great potential as a working fluid in power plant cycles due to its moderate critical pressure of 7.38 MPa and critical temperature of 30.98 °C, closely matching typical heat sink temperatures. Understanding the heat transfer characteristics of sCO2 is crucial for designing cycle components. This publication presents a systematic analysis of sCO2 heat transfer in two heated vertical pipes with 4 and 8 mm inner diameters, with both upward and downward flow at pressures of approximately 7.75, 8.00, and 9.50 MPa, and flow inlet temperatures between 5 and 40 °C, based on 196 experiments. The study investigates a range of mass fluxes from 400 to 2000 kg/m2s and heat fluxes from 10 to 195 kW/m2, resulting in a heat to mass flux ratio of 6 to 275 J/kg. The findings reveal enhanced, normal, and deteriorated heat transfer within the experimental dataset. Buoyancy effects are identified as the main cause of deteriorated heat transfer in the investigated parameter range, with flow acceleration showing no significant influence on heat transfer. A new proposed dimensional criterion categorises 36 out of 119 experiments with upward flow in the deteriorated heat transfer regime, accompanied by temperature peaks rising to 47 K. Thermal inflow lengths range from 0 to 480 inner pipe diameters, with some experiments not achieving a thermally fully developed flow over the entire pipe length. Based on a comparison with approximately 8950 experimental data points, two Nusselt correlations are found, capable of reproducing the experimental results with a mean absolute deviation of around 30 %. This publication provides valuable data for validating numerical models and developing correlations to predict the heat transfer of supercritical fluids.

Experimental study of convective heat transfer with a multi-scale roughness

The heat transfer in a turbulent Rayleigh-Bénard convection with a multi-scale roughness at the bottom is studied experimentally. Two different regimes for the heat transfer are found. The first regime has scaling exponent γI=0.4 and corresponds to the reduced values of the Nusselt number. The second regime with enhanced values of the Nusselt number has a scaling exponent γII=0.32, which is noticeably larger than in the case of smooth boundaries. Significant variation in the Prandtl number (from 6.4 to 62) does not change the scaling exponent value of the second regime but increases the values of Nusselt number. The scaling exponent for the relation Re∼Raα is insensitive to the change of the heat transfer regime and is close to 1/2 for all values of Ra. The characteristic ratio of the velocity pulsations to the mean velocity does not depend on the Rayleigh number and is characterized by close values (about 0.8). The local temperature measurements support the mechanism of the transition from the reduced Nusselt number regime to the enhanced one, which is based on the formation of flows between obstacles.

Numerical appraisal of the role of heat transfer regimes on transient response of carbon dioxide based supercritical natural circulation loop during power upsurge

Appearance of steep property gradients with change in temperature is a fascinating feature of any supercritical fluid, which can instigate intricate dynamics in supercritical natural circulation loops by modulating the effective forces. While most of the relevant literature focuses on stability evaluation, anticipation regarding the transient response of the system during power transition is of utmost significance, especially in high-power applications. Present study aims at furnishing insight on the same by developing a one-dimensional numerical model of a rectangular loop with supercritical CO2 as the working medium, and characterizing the temporal trends over a wide range of heating power. Two different profiles of power upsurge have been tested for different regimes of heat transfer, unearthing intriguing characteristics. The combination of initial and final regimes during any transformation is found to be the most crucial factor. Single-step rise in power, in general, is the most vulnerable one, specifically during large-scale change of the order of 1000 W, and better be employed only at low-power regime. Even single-step change ∼ 25 W can inflict instability and flow transition within the transition regime. Power transformation following linear ramp profile with transition periods of 5, 10 and 20 s is identified to be the most suitable one across all the regimes. It can successfully mitigate instability even in the later parts of the transition regime, albeit at the expense of greater time requirement to attain the final stable state and possibly a greater period of transformation. Change through multiple small steps (about 250 W for large change in low power regime and 15 W within transition regime) can also be a feasible option for avoiding the growth of unstable oscillations at higher powers.

Breakup regimes and heat transfer of an isolated bubble and Taylor bubble flow in the T-type microchannel

Current research on Taylor bubble flow heat transfer mainly focuses on straight channels, with less emphasis on the transport characteristics and heat transfer regimes of Taylor bubble flow in branching channels. Understanding the flow and heat transfer regimes of Taylor bubble flow in branching channels is crucial for enhancing microfluidic and microchemical applications. Numerical simulations can provide detailed flow information that experiments might miss, deepening our understanding of Taylor bubble flow for enhanced heat transfer. The research results indicate the presence of three bubble breakup regimes at the T-junction: Non-breakup (NB), Tunnel breakup (TB), and Obstructed breakup (OB). The phase diagrams were established based on Capillary number and bubble length to predict the transitions between these three breakup regimes. Pressure within the channel increases with bubble deformation but decreases after breakup. Bubble breakup converts potential energy to kinetic energy, enhancing the heat transfer coefficient in the branching microchannels. The stagnation of bubbles at the T-junction temporarily reduces the heat transfer coefficient but increases after the breakup. Up to 115 % of the best performance improvement was achieved for Taylor bubble flow when the channel width ratio was 1.2. When the channel width ratio was greater than or equal to 1, the TB regime influenced bubble breakup within the channel, resulting in asymmetric breakups and uneven heat transfer. When employing tree-like branched microchannels and Taylor flow for enhanced heat transfer, a channel width ratio of 0.8 is recommended.

An experimental study on evaporator heat transfer performance of radially rotating heat pipes with water as working fluid

The utilization of radially rotating heat pipes (RRHPs) provides a novel method for gas turbine cooling due to the high thermal conductance and simple structure. Centrifugal force is utilized to return the condensate from the condenser to the evaporator. In this paper, the evaporator heat transfer performance of a RRHP, including heat transfer regime and the effect of centrifugal acceleration on heat transfer, are investigated experimentally. The RRHP evaporator with 45 % filling ratio was heated by electric heating wire with heat input ranging 50 W to 200 W and the condenser was cooled by air. Centrifugal acceleration at the evaporator center ranged from 117 g to 1457 g. Temperature distribution along the RRHP was measured by thermocouples. Experimental results revealed that non-boiling regime and boiling regime could occur in the RRHP evaporator according to heat input while natural convection and nucleate boiling were the dominating heat transfer mechanisms respectively. Geyser boiling phenomenon at 117 g to 344 g and temperature overshoot at 682 g to 1457 g were first observed in RRHPs as the transition stage at which boiling started to occur and the heat transfer performance was enhanced by 23 % in average. The evaporator heat transfer coefficient ranged 5180 to 9866 W/(m2·K) in non-boiling regime and 10,621 to 15438 W/(m2·K) in boiling regime with a boundary around 10000 W/(m2·K). With increase of centrifugal acceleration, transition stage was delayed to higher evaporator temperature and heat input. Nucleate boiling was suppressed by 20.3 % while natural convection was enhanced by 14.9 % as accelerations increased from 117 g to 682g. Interestingly, it was first observed that boiling was not entirely suppressed at 1457 g and the RRHP evaporator showed high heat transfer coefficient of 13065 W/(m2·K) at 200 W under such a high acceleration.

Assessment of Look-up tables for the prediction of heat transfer coefficient distribution in rod bundles cooled by supercritical water

The evaluation of available Look-Up Tables for prediction of heat transfer coefficient distribution in rod bundles cooled by supercritical water with the aim of their further use in computational analyses of various Fuel Assembles of Supercritical Water-Cooled Reactors is made. The comparison between the calculations based on Look-Up Tables with the values from empirical correlations and experimental data for smooth and wire-wrapped rod bundles was presented. The obtained results showed that Look-Up Table of the University of Ottawa, which was created to describe improved and deteriorated heat transfer regimes in round tubes, allows describing available data points with 30 % of the mean square deviation. It is noted that the presence of wire intensifies heat transfer exchange near pseudocritical temperature region but existing versions of Look-Up Tables cannot take into account this effect. Nevertheless, there is potential for further improvement in predicting the heat transfer coefficient using Look-Up Table by introducing additional correction factors.

Optimization of thermoelectric generator power density for cryogen cold exergy recovery

The article presents the optimization of the power density of a thermoelectric generator (TEG) operating in cryogenic temperature conditions. Optimization of TEG power density was performed as a function of TEG leg length and its effect on TEG performance. The figure of merit (ZT) of the TEG was experimentally verified for the temperature range of a cold sink from 160 to 250 K, which corresponds to the temperature of the wall subjected to the boiling film of LNG. The numerical model proposed in the article was verified by comparison with experimental data, and then used to simulate the operation of the TEG at a cold sink temperature of 80 K corresponding to the wall temperature in the process of LH2 regasification. The obtained results showed that the optimal length of TEG legs is less than 10 mm and depends on the boiling and heat transfer regime. The results of the presented research can be used to improve the effectivity of cold exergy recovery from cryogenic systems.

Are man-made hives valid thermal surrogates for natural honey bee nests (Apismellifera)?

Honey bees preferentially occupy thick walled tall narrow tree cavities and attach their combs directly to the nest wall, leaving periodic gaps. However, academic research and beekeeping are conducted in squat, thin walled man made hives, with a continuous gap between the combs and the walls and roof. Utilising a computational fluid dynamics (CFD) model of thermoregulating bees in complete nests in trees and thin walled man made hives, with the average size of tree comb gaps determined from honey bee occupied synthetic tree nests, this research compared the metabolic energy impacts of comb gaps and vertical movement of the thermoregulated brood area. This shows their heat transfer regimes are disparate, including: bee space above combs increases heat loss by up to ∼70%; hives, compared to tree nests, require at least 150% the density of honey bees to arrest convection across the brood area. Tree cavities have a larger vertical freedom, a greater thermal resistance and can make dense clustering redundant. With the thermal environment being critical to honey bees, the magnitude and scope of these differences suggest that some hive based behavioural research needs extra validation to be considered non-anthropogenic, and some bee keeping practices are sub-optimal.

STUDY OF EFFECT TO COLD PILGER ROLLING PARAMETERS ON THE TEMPERATURE REGIME OF PROCESS

The research is aimed at determining the optimal parameters that affect the efficiency of rolling and the quality of the product. The relationship between various parameters such as heat transfer, ultimate strength, friction index and temperature regime is analyzed to improve the technological parameters of cold mill rolling and increase the quality of products. Mathematical modeling of cold mill rolling processes allows not only a deeper understanding of the physical processes occurring during rolling, but also provides an opportunity to analyze the influence of various parameters on the temperature regime and, in general, on the quality of the final product.

Insight of Jeffrey flow over a stretching Riga plate with activation energy and viscous dissipation: Melting heat transfer regime

AbstractPresent article highlights the significance of Arrhenius activation energy along with viscous dissipation in Jeffrey fluid over a Riga plate. Riga plate is basically an actuator made up of array of magnets and electrodes scaled on a plane surface to tackle the weaker electrical conductivity during fluid flow. In order to ensure the novelty, a reliable melting heat surface condition has been incorporated on nonlinear stretching Riga plate of variable thickness to reconnoiter features of heat transfer. Moreover, stagnation point has been retained in this study. Adequate transformations are employed in order to attain system of nonlinear ordinary differential equations. A well known semi analytical technique (Homotopy analysis method) is utilized to obtain series solutions of prevailing dimensionless equations. Influence of several apposite parameters on velocity, thermal and concentration distributions is analyzed graphically. Physical evaluation and graphical sketch is presented for drag force coefficient and rate of heat transfer. Analysis of velocity as well as associated boundary layer thickness gives the growing up impact for the strength of modified Hartmann number. Enhancement of dimensionless reaction rate and endothermic/exothermic reaction parameter results in increment for heat flux over stretching Riga plate. Increase in thermal distribution takes place for higher Eckert number while thermal boundary layer thickness depicts opposite trend in this case. Concentration boundary layer thickness enhances while concentration profile declines for higher Schmidt number. Velocity distribution is found to be incremented for intense melting process. Higher dimensionless activation energy parameter is analyzed to be responsible for growing up concentration field.

Experimental and numerical study of mixed convection heat transfer in a vented cavity partially filled with a porous medium: Effects of reynolds and rayleigh numbers on Nusselt number and flow regimes

This article presents a comprehensive investigation of mixed convection flow and heat transfer within a vented, partially side-heated cubical cavity, incorporating a porous medium with low-conductivity square-shaped inclusions. The study encompasses an extensive range of Rayleigh numbers (106 < Ra < 6 × 106) and Reynolds numbers (200 < Re < 4000) while maintaining a fixed Prandtl number of Pr = 0.71. This investigation spans over three decades in Richardson numbers (Ri = Ra/(Re2 Pr)), aiming to discern the interplay between the Nusselt number, Nu, and the Richardson number. Our understanding of Nusselt number dependencies is enhanced by combining PIV measurements of heat transfer and velocity fields with temperature field data. The study uses both experimental and numerical PIV data, incorporating porous media representation to reveal heat transfer scaling with both Reynolds and Rayleigh numbers. Computational Fluid Dynamics (CFD) methods are used to explore the complex physical behavior under different flow conditions. Three distinct flow and heat transfer regimes have been identified, predicated upon the Richardson number. For Ri < 25, the flow structure and Nusselt number exhibit similarities with pure forced convection, where the Nusselt number scales as Nu ∼ Re0.6, independent of the Rayleigh number. Conversely, for Ri > 70, natural convection dominates the vicinity of the heating wall, rendering the Nusselt number less sensitive to Reynolds number variations and predominantly dictated by the Rayleigh number. Notably, the intermediate regime, ranging from 25 < Ri < 70, witnesses a competition between upward-directed natural convection flow at the heating wall and downward-directed forced flow, culminating in a minimum effective Nusselt number.

Dynamic characterization of supercritical natural circulation loop under periodic excitation

Characterization of supercritical natural circulation loop under the influence of periodic power profile has not been explored in open literature, despite the promise of producing intriguing dynamics. Present study employs a one-dimensional numerical model to explore the transients on imposing a sinusoidal excitation on the heater power. The resultant dynamics are found to be strongly correlated with the heat transfer regime. The time period of excitation plays a more dominating role compared to the amplitude in deciding the characteristics. System attains dynamic steady-state and persists with a periodic profile at both low and high power levels. But introduction of natural frequency emanates into flow reversal at power levels within the transition regime. Chaotic oscillations can be identified for an intermediate band of imposed frequency, transforming to bidirectional pulsing at higher time periods. Frequency locking within the chaotic zone and frequency pushing for double-periodic nature is also identified, and the appearance of chaos is substantiated using the Poincaré map.

Lattice Boltzmann method study of pool boiling on an electrowetting substrate

Electrowetting has attracted significant attention in condensation, but its application in enhancing pool boiling heat transfer is very limited. In this paper, the lattice Boltzmann method is adopted to investigate the pool boiling heat transfer on electrowetting surfaces with different electrode configurations. The wettability of the baseline hydrophobic surface above the electrode will change to hydrophilicity once a voltage is applied. The differences in heat transfer and boiling phenomenon between the hydrophilic and hydrophobic surfaces without electrodes are illustrated firstly by the density contours and boiling curves. Then, the possibility of electrowetting in improving the heat transfer coefficient (HTC) and critical heat flux (CHF) is verified by comparing with baseline hydrophobic and patterned-coating surfaces, and different boiling heat transfer regimes are illustrated in detail. Furthermore, based on the heat transfer characteristics of the electrowetting surfaces with different electrode configurations, a surface with temperature-dependent electrodes is obtained to comprehensively enhance heat transfer from the onset of nucleate boiling (ONB) to CHF.

Two-phase flow and heat transfer on a cylinder via low-velocity jet impact

Combining the benefits of jet impact and phase-transition heat transfer, jet impact boiling has been widely adopted in industrial scenarios. Notably, owing to limitations of high-velocity jets, such as high impact stress and pumping pressure in liquid circuits, low-velocity jet impacting deserves more attention and explorations. In this paper, under an atmospheric environment with wall heating temperatures ranging from 36 °C to 128 °C, the two-phase flow and heat transfer characteristics (local surface temperature, boiling curves and liquid loss rate) of low-velocity jet impacting on a cylindrical surface are investigated by experiment, followed by the analysis on the effects of jet outlet velocity and impact height. The findings reveal that, with the increase of heat transfer rate, the heat transfer regimes in sequence are non-phase transition (single-phase convection and air-liquid two-phase convection) and nucleate boiling. The influences of jet impact height and outlet velocity on local surface temperatures are pronounced at non-phase transition stage. In impact region, along with average superheat, the growth rates of heat transfer rate and liquid loss rate increase significantly from non-phase transition to nucleate boiling stage, and slow down at anaphase of nucleate boiling stage. Interestingly, the increase in outlet velocity delays the onset of slowdown in growth of liquid loss rate to some extent. Overall, the heat transfer rate and liquid loss rate are significantly enhanced at higher jet outlet velocity of 0.60 m/s, but not affected significantly by impact height.

A novel thermal management system for cylindrical lithium-ion batteries using internal-external fin-enhanced phase change material

This study introduced a phase change material (PCM)-based battery thermal management system (BTMS) using an innovative combination of internal and external fins to create PCM silos around the battery surface, addressing the low thermal conductivity of PCM. The lumped-capacitance thermal model and enthalpy-porosity approach were employed for the battery heat generation and PCM melting process, respectively. The BTMS was evaluated under transient and constant heat generation of the battery at various discharge rates. The effects of fin quantity on thermal management performance, energy density, heat storage capacity, and PCM melting time were investigated. The developed BTMS showed superior performance compared to the battery system with natural air convection cooling and the PCM cooling system without fins. At the end of the PCM complete melting process, the BTMS with 4 internal-external fins decreased the battery surface temperature by 9.90 and 17.45 K compared to the PCM cooling system without fins at the discharge rates of 3C and 5C, respectively. The BTMS with 4 internal-external fins kept the battery surface temperature under the optimum temperature of 318.15 K even at a high ambient temperature of 308.15 K while providing up to 10.02% higher energy density, 11.11% higher heat storage capacity, 32.79% higher fin efficiency, and only 1.08% lower overall fin effectiveness compared to greater fin quantities. The proposed BTMS provided an almost uniform cooling effect on the battery surface compared to the system without fins, which has been largely overlooked in previous studies on BTMSs. Various heat transfer regimes were identified during the PCM melting process. Correlations for predicting the PCM liquid fraction and Nusselt number were proposed as a function of a group of dimensionless numbers. These findings provided valuable insights for designing efficient PCM-based BTMSs.

Quantifying heat losses from experimental setup and their effect on annular channel heat flux using CFD

An advanced experimental setup in the form of a double-pipe heat exchanger has been developed to investigate various flow and heat transfer regimes in an annular channel. The experimental setup is designed for heat transfer measurements and flow visualization in a transparent annular channel with temperature-controlled heating of the inner tube. It allows measurements in both, single-phase and two-phase (boiling) flow regimes. The transparent annular channel is connected at the front and rear end by specially designed inlet and outlet headers, which ensure a uniform velocity field of the coolant (primary fluid) in the annular channel. The heating water (secondary fluid) heats the inner tube made of copper, which contains unique turbulators positioned equidistantly. These are designed to enhance the heat transfer on the inner side of the copper tube and to accommodate the thermocouples measuring the surface temperature of the tube.The main purpose of this study is to assess as accurately as possible the influence of heat losses in the experimental setup on the heat fluxes in the transparent annular channel. Therefore, it is necessary to accurately estimate the heat losses at the inlet and outlet header, upstream and downstream of the annular section. To this end, a detailed three-dimensional conjugate heat transfer CFD model has been developed and validated with a series of single-phase measurements, including the measured temperatures in both headers and the temperature profile along the heated inner copper tube. The simulated velocity and temperature fields have been used to quantify the heat flux distribution in the experimental setup. This study clearly demonstrates that properly validated numerical simulations can provide missing experimental information and significantly improve the interpretation of complex experimental results.

Flow and heat transfer regimes in Rayleigh–Bénard convection with a melting boundary

This paper analyses the flow and heat transfer regimes arising in Rayleigh–Bénard convection with a melting boundary by means of numerical simulations based on the lattice Boltzmann method. By systematically varying the Rayleigh and Prandtl numbers, we unveil an intriguing spectrum of patterns and transitions. Our results provide critical insight into the mutual interplay between thermal convection, melting, and fluid dynamics, shedding light on the complex behavior of such systems.

Diagnostics of Transient Heat-Transfer Regimes during Pool Boiling Based on Wavelet Transform of Temperature Fluctuations

Diagnostics of Transient Heat-Transfer Regimes during Pool Boiling Based on Wavelet Transform of Temperature Fluctuations

Bilayer thermal metadevices that mold transient heat flows

Thermal manipulation has been widely researched due to its potential in novel functions, such as cloaking, illusion and sensing. To avoid the spatially inhomogeneous and anisotropic heat conductivity tensors (introduced by transformation thermodynamics), several thermal cloaks based on scattering cancellation (SC) approach are designed and experimentally demonstrated. However, the current SC cloak is only effective in the steady-state heat transfer condition as the heat capacity terms are neglected by the requirement of constant external fields. In order to expand the SC cloak to the transient regime, a bilayer scheme is introduced to achieve the accurate solutions of the camouflage equations considering time factors. Indeed, there are two equations to solve with two unknown quantities: first the thermal conductivity and second the product of specific heat capacity by the density. We construct thermal camouflage devices and verify their ability to mold dynamic heat flows both by simulations and experiments. Our proposed method paves an efficient avenue to extend SC approach to dynamic heat transfer regime.

Numerical analysis of mixed convection phenomena in heat transfer to supercritical pressure carbon dioxide inside a horizontal miniature tube

To clarify and evaluate the influence of buoyancy force on the heat transfer to SCO2 flowing in a horizontal miniature tube, in this study, mixed convection phenomena with strong variations in thermophysical properties were numerically studied using an in-house code. The numerical model was validated by comparing the predicted wall temperature and available experimental data obtained for SCO2 at different heat fluxes, and the results showed good agreement. The effects of the heat-to-mass flux ratio on the heat transfer characteristics and heat transfer mechanism were investigated under operating conditions having mass flux, heat flux, pressure, and inlet temperature of 600–1200 kg‧m−2‧s−1, 25–300 kW‧m−2, 8 MPa and 303 K, respectively. A transition in the heat transfer regime from enhanced to normal was found as the heat-to-mass flux ratio increased to 83.33 J·kg−1. A pseudo-trans-critical process occurred in the tube at a heat-to-mass flux ratio above 125 J‧kg−1, and the heat transfer to SCO2 could be classified into three stages based on the reduced bulk fluid temperature. Particularly, the wall temperature was inversely proportional to the mass flux within the reduced bulk fluid temperature ranging of 0.992–1.0. Analysis of the heat transfer mechanism showed that the location of the large heat capacity in the boundary layer and the buoyancy force were responsible for the variation in the heat transfer characteristics. The applicability of the existing buoyancy criterion was examined, and BuP = 6 was found to be reasonable for predicting the onset of the influence of the buoyancy force on heat transfer.