Convective Heat Transfer Process Research Articles

Effects of the Broken Kernel on Heat and Moisture Transfer in Fixed-Bed Corn Drying Using Particle-Resolved CFD Model

To investigate the pore structure distribution and the coupled heat and moisture transfer during the drying process of the grains, this study focuses on fixed-bed corn drying with varying levels of broken kernel rate. A model of internal flow and conjugate heat and mass transfer was established for the drying process. Random packing models of whole and half corn kernels with different proportions were generated using rigid body dynamics (RBD), and the porosity, airflow distribution, and coupling of temperature and moisture transfer in fixed beds with different levels of broken kernel rate were analyzed. A fixed-bed corn drying device was developed, and the effects of broken particle contents of 0%, 10%, 20%, and 30% on drying characteristics were studied. The research findings reveal that the radial porosity in the fixed bed exhibits an oscillating distribution, with the localized porosity decreasing as the broken kernel rate increases. Increasing the broken kernel rate intensifies the curvature of the airflow paths within the fixed bed, increasing the pressure drop in the bed. The broken kernels fill the gaps between the whole kernels, improving the uniformity of the velocity distribution within the fixed bed. Under various packing models, the average discrepancy between pressure drop obtained from Particle-resolved Computational Fluid Dynamics (PRCFD) simulations with experimental remains below 15%. The increase in broken kernel rate within the fixed bed enlarges the heat transfer area, resulting in an elevation of the transient heat transfer characteristic parameters during drying. Simultaneously, the broken kernel rate increases the surface area of mass transfer, thereby enhancing the moisture transfer rate within the fixed bed. Compared to the fixed bed without broken kernels (0%), which requires 560 min to dry the corn pile to a safe moisture of 14% (d.b.), the drying time is reduced by 60 min, 100 min, and 130 min for the respective broken kernel contents of 10%, 20%, and 30%, respectively. The PRCFD method successfully simulates the processes of convective heat and mass transfer in the fluid phase and thermal and mass diffusion in the solid phase, exhibiting a strong correlation with experimental data.

Correlations for forced convective heat transfer coefficients at the windward building façade with vertical louvers

Vertical louvers are a widely used external shading system to control natural lighting and solar heat gain for buildings. However, when installed on a building façade, they can disrupt the flow field around the façade and affect the external convective heat transfer process of building's exterior surfaces. To this end, this paper conducted CFD simulations to clarify the forced convective heat transfer patterns in a vertical louver-facade system based on the k-ω SST model with a detailed verification. The extended Newton's cooling equation is used to describe the heat exchange process between louvers, building façade, and air, which generates four heat transfer coefficients: hwa, hwb, hba, and hbw. Then the impacts of dimensionless temperature difference (θ), louver angle (φ), louver slat width (B), and installation distance (W) on the heat transfer coefficients are investigated. The results show that θ does not affect the average heat transfer coefficients. hwa reaches a minimum value and hba reaches a maximum value when φ = 75o. In addition, hwa increases by 55.3% and hba decreases by 21.9% as B increases from 0.3 m to 1 m. When W increases from 0.25 m to 1 m, hwa decreases by 5.6% and hba increases by 39.6%. The variation in the heat transfer coefficients (hwb and hbw) between the louvers and building façade is also explained. Finally, new correlations of paired heat transfer coefficients are established as a function of φ, B, W, and wind velocity (U10). The accuracy of these correlations is evaluated by comparing calculated values with simulated results.

AUGMENTATION HEAT TRANSFER IN A CIRCULAR TUBE USING TWISTED - TAPE INSERTS: A REVIEW

Heat transfer enhancement is the process of increasing the heat-transfer coefficient, which enhances the system's performance. Enhancing heat transfer is a major problem for saving energy and is also beneficial economically. Many passive devices are used inside tubes to improve heat transfer such as twisted tape inserts, rough parts, extended surfaces, additives for liquids wire plugs, etc. This research reviewed one of the most effective passive devices which are twisted tape inserts. Since it has many advantages such as simple fabrication, simple operation, and ease of maintenance. The twisted tape inserts generated swirl flow and vortex inside the tube. Therefore, the internal convective heat transfer process is significantly improved. The current research article provides an overview of different twisting tape inserts that can improve heat transfer rates. By reducing boundary layer thickness near tube walls. Which lead to reduce the size and cost of many industrial applications, including heat exchangers, refrigeration systems, air conditioners, reactors, thermal power plants, spacecraft, and automobiles. A summary of previous experimental and numerical studies is presented as well. The primary results indicated that the twisted tape inserts are demonstrated to be efficient in enhancing heat transfer inside the tube for laminar and turbulent flow. But during a turbulent flow, twisted tapes increased pressure loss more than laminar flow because of flow obstruction.

The Effect of Vibrating Inner Cylinder on Natural Convective Heat Transfer in Concentric and Eccentric Horizontal Cylindrical Annuli

In this study, natural heat convection caused by centric and vertically eccentric long horizontal cylinders under the influence of vibration is experimentally investigated. The internal wall of the annulus is heated and kept at a constant heat flux, while the outside wall is cooled and maintained at a constant temperature. The vibration frequency impacts the annular convection heat transfer process and the effects of the Rayleigh number, heat flow, and eccentricity. This work employed a moderate, laminar-ranging Rayleigh number from (5×104 to 6.48×106), while the eccentricity range is varied from (-0.667, 0, and+0.667). The investigation is carried out at five different frequencies (0, 2, 5, 10, 15, and 20 Hz); therefore, it was decided to compare the case under the same circumstances in both the absence and presence of vibration. The present results' verification worked exceptionally well in concordance with previous studies. When heat fluxes are considered, the study demonstrates that the temperature difference along the gap (radial difference) between the two cylinders significantly decreases for negative as opposed to positive eccentricities for each Rayleigh number. For different eccentricities, it was discovered that the temperature difference decreased as the Rayleigh number increased. Along with these reductions, the temperature difference was promoted as the vibration frequency increased, which is significant at (20 Hz) within the range considered for controlling parameters. It was also observed that the decrease in temperature difference is higher for the negative eccentric position than for the centric and positive positions. At vertical positive eccentricity at a low Rayleigh number, the gain in vibrational average Nusselt number caused by applying vibration had a minimum value of 22.50601475%, While at the higher value of the Rayleigh number, the maximum increase in negative vertical eccentricity was 86.66933125%. However, the gain in the average Nusselt number depends on the position of the heated inner cylinder, the Rayleigh number, and the vibrational frequency.

The effect of two main vortexes generator on heat transfer enhancement in mini rectangular ducts of airborne thermoelectric conversion system

To enhance the heat transfer process of supercritical pressure fluid in rectangular channels of the airborne thermoelectric conversion system, the combined effect of the Reynolds stress, properties variation and longitudinal fins on convective heat transfer process in ducts under supercritical pressure is investigated. Then, a method to induce the two main vortexes structure in rectangular ducts is proposed. Results show that, in smooth ducts, the two main vortexes structure is formed due to the effect of density variation. By comparing the flow and heat transfer process in the smooth duct and ridged duct with trapezoidal fins attached on the lower wall, it is found that, due to the effect of new vortexes induced by trapezoidal fins, the two main vortexes structure is inhibited, which makes local wall temperature increases about 50 K. Accordingly, the thermal performance enhancement factor of rectangular duct with trapezoidal fins is lower than 1. To enhance the two main vortexes structure, a method that using triangular fins to induce two main vortexes structure is proposed, and the duct with one triangular longitudinal fin attached on the upper wall is designed, which has better thermal performance than ducts with trapezoidal fins. Due to the effect of triangular fins, the dominant distance of two main vortexes structure increases by 416%. Accordingly, in ducts with triangular fins, local heat transfer coefficient is enhanced about 23.5%, and the maximum of thermal performance enhancement factor is about 1.29.

Thermodynamic performance of Al2O3-Cu-CuO/water (W) ternary nanofluids in the full-flow regime of convective heat transfer

To explore the convection mechanisms of ternary nanofluids in the full-flow regime (laminar-transition-turbulent regime), the thermodynamic performance of Al2O3-Cu-CuO/W ternary nanofluids (with a volume fraction of 0.02 vol%) and their corresponding mono nanofluids was investigated in Reynolds number (Re) range of 800–8000 in a cylindrical tube. The results showed that the presence of suspended particles in the Al2O3-Cu-CuO/W nanofluids caused the transition regime to shift to a lower Re value of ∼ 1900. As Re was increased, the heat transfer performance exhibited a fluctuating upward trend, with a maximum at Re = 5250. Further, the ternary nanofluid with a mixing ratio of 30:30:40 showed a Nu 1.73 times that of the base fluid, with the corresponding values for the CuO/W and Al2O3/W nanofluids being only 1.36 and 1.39 times, respectively. Moreover, the thermal conductivity of the ternary nanofluids had a substantial impact on heat transfer performance in the laminar regime, with a correlation coefficient, r, of 0.6429, but less of an influence in the turbulent regime, with an r value of only 0.5. Finally, the optimum mixing ratio for the convective heat transfer process was determined to be 30:50:20 based on the values of Nusselt number enhancement rate and enhanced heat transfer factor, as well as on thermodynamic analysis of the system entropy generation and exergy.

Experimental study on convective heat transfer of an open-loop borehole heat exchanger

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter.

Production of a large volume non-equilibrium region in an atmospheric argon arc plasma with a counter injection of cold gas from an annular anode

In this letter, an annular anode is designed for producing arc plasmas with a large non-equilibrium region by using a counterflow cold gas through the annular anode. The coupled mass-momentum-energy exchange processes in an argon arc plasma are studied numerically and experimentally. The counter-injection of the cold argon gas from the center of the anode leads to a steep gradient of the heavy-particle temperature due to the formation of a thin stagnation layer resulting from the interaction of the high temperature plasma with the cold gas; and in particular, a large volume non-equilibrium ‘dark’ plasma region is obtained above the anode surface. The results show that, with the enhancement of the convective heat transfer process in the plasma core region, the fraction of the non-equilibrium region to the whole arc plasma region reaches 92.2% where the heavy-particle temperature can be reduced significantly, e.g. ∼2300 K, while simultaneously, the electron temperature and number density are remained at high levels greater than 8000 K and 2.4 × 1020 m−3, respectively, under the operating condition studied in this letter. This research not only deepens the understanding to the non-equilibrium synergistic transport mechanisms of arc plasmas, but also provides a method for producing a large volume non-equilibrium plasma region so as to promote various existing applications, or even creating new applications in the future.

Effects of Fin Height, Fin Thickness and Reynolds Number on Heat Transfer Enhancement of Flat-Plate Thermal Collector: A Numerical Analysis

A flat-plate thermal collector combined with a PV panel also called a PV/T collector is a device that converts solar irradiation into thermal energy and electrical energy simultaneously. The unused thermal energy of the PV/T collector is absorbed by the flat-plate thermal collector. This may then contribute directly to an enhancement of the electrical efficiency of the PV/T collector. In the present study, the effect of geometry and Reynolds Number on the thermal performance of flat-plate thermal collectors is numerically investigated. Thereafter, CFD simulation is then implemented to characterize the thermal performance in terms of absorber temperature and convection heat transfer coefficient. To disturb the fluid flow pattern in this work the 45o inclined fins are attached underneath the surface of the collector model and they act as an absorber. Monthly average weather data of inlet fluid temperature and ambient temperature as well as solar irradiation level implemented in this study were obtained from the Meteorological, Climatological and Geophysical Agency of Bandarlampung regency. Several different cases have been considered by varying the fin height from 20 to 80 mm, fin thickness from 1 to 4 mm, and Reynolds number from 1500 to 6000. The results show that increasing the fin geometry (the fin thickness and fin height) and Reynolds Number reduce the flat-plate surface temperature due to the more conductive and convective heat transfer process. However, in terms of the convective heat transfer coefficient parameter, the Reynolds Number implemented has a dominant effect compared to the fin geometry. Moreover, by increasing Reynolds Number by four times, there is a decrease in the mean surface temperature by 26% and an increase in the mean convective heat transfer coefficient of 146% compared to the initial conditions

Determination of heat transfer mechanisms during vacuum drying of solid-moist and liquid-viscous materials

Most drying methods combine convective, conductive and radiation heat transfer processes. The share of each type of heat transfer may vary depending on the type and mode of drying, type of product, etc. In this study, the problem of determining the heat transfer mechanism of vacuum drying of solid-moist and liquid-viscous materials is solved. The objects of the study are Jerusalem artichoke tubers, camel and mare milk. The numerical values of the heat transfer components are found experimentally and their shares in the total heat flux are determined. During vacuum drying of Jerusalem artichoke at a medium pressure of 4 kPa and a temperature of 55 °C (with a layer height of 0.01 and 0.02 m), the convective component predominates (58.55 and 67.65 %). The share of thermal conduction (18.96 and 29.39 %) and radiation (13.39 and 12.05 %) is much lower. The mechanism of thermal conduction begins to prevail with an increase in the height of the material layer (0.03 and 0.04 m). The convective component is also dominant for vacuum drying of milk: at medium pressures of 6÷10 kPa and a temperature of 40 °C, its value for mare milk reaches 78.21 %, for camel milk – 73.33 %. The second most important is the share of radiation (19.45 and 22.58 %). Conductive heat transfer has the minimum indicators (5.66 and 6.17 %). The large values of the share of thermal conduction during drying of Jerusalem artichoke compared to milk are explained by the fact that heat transfer occurs inside the tubers due to conduction, and inside milk – due to convection. Insignificant shares of radiation are explained by low and medium vacuum values in the chamber. In the studied range, heat and mass transfer occurs due to molecular diffusion and convection. The results obtained can be used to formulate criterion heat transfer equations, in engineering calculations, and optimization of the vacuum dryer operation.

Optimization and experimental verification of microchannel heat sink based on heat transfer and flow balance regulation

The heat transfer performance and flow performance of microchannel convective heat transfer processes are usually mutually constrained. However, we expect microchannel heat sinks to have both excellent heat transfer and good fluidity. In order to achieve a balance between heat transfer and fluidity, we established an optimization objective function of a complex network structure based on the linear programming method. This composite objective function included both the entransy dissipation rate model to express the heat transfer performance of the fractal microchannel heat sink and the pump power model to express its flow performance. Under the same initial values and constraints, we optimized the objective function. Finally, the comprehensive performance of the optimized structure (OPTS) and the randomly designed initial structure was compared through experiments. The experimental results show that the comprehensive performance of the OPTS was always better than the initial design structure (INDS) under any conditions, and the performance of the optimized structure with fewer fractal levels was better. Within the scope of the study, the average comprehensive performance of the OPTS was improved by a minimum of 14% and a maximum of 30% relative to the INDS. This validates the rationality of the optimization method proposed in this study. We have theoretically and experimentally solved the active optimization problem of how many levels of fractal structure should be designed in a constant space and what the channel size of each level is when the heat sink has optimal performance.

Calculation and Analysis of Heat Transfer Characteristics of Internal Threaded Tube at High Reynolds Number

In the actual operation of a heat storage steam boiler heat exchanger, an internal threaded tube is used as a heat exchange element. Usually, the heat exchanger volume is large and there are many consumables because of the complex composition of flue gas in the pipe so the internal threaded tube in the actual long operation process will suffer from ash accumulation and serious wear phenomenon. In this paper, the heat transfer process of internally threaded tubes is taken as the main research object, and the convection-enhanced heat transfer of internally threaded tubes at a high Reynolds number is deeply studied. For gas under the condition of high Reynolds number, the strengthening of turbulent flow inside of inner grooved tubes convective heat transfer process of deduction and analysis is derived. A fluid calculation model which is more in line with the heat transfer mechanism of threaded pipe is proposed, which provides a reference for the subsequent theoretical studies of heat transfer enhancement of threaded pipe.

Analysis on the convective heat transfer process and performance evaluation of Triply Periodic Minimal Surface (TPMS) based on Diamond, Gyroid and Iwp

Heat dissipation capacity is one of the bottlenecks in developing high heat release devices such as electronic chips and laser generators. TPMS exhibits good thermophysical properties and is expected to provide better heat dissipation solutions for high heat-releasing devices. However, studies on the convective heat transfer properties of TPMS structures are still insufficient. In this manuscript, research focuses on evaluating the convective heat transfer performance of several representative TPMS structures and elucidating their mechanism of enhanced heat transfer. The TPMS studied in this paper include Gyroid, Diamond, and Iwp, and their convective heat transfer performance is compared with that of Fins-structure. In addition, an experiment was carried out to verify the accuracy of the numerical simulation. The results showed that the numerical simulation results were in good agreement with the experiment, with an error of less than 6%. In the numerical simulation, the lattice size of the three TPMS is 20 × 20 × 20 mm3, the fluid is air, the heating surface is heated at a constant wall temperature of 373.15 K, and the Reynolds number is in the range of 166–940. Compared with the Fins-structure model, the Nusselt number of TPMS-Diamond is increased by 9–196%, that of TPMS-Gyroid is increased by 5.8–149%, while the TPMS-Iwp is increased by 6.8–43.5%. Among the three TPMS structures, the convective heat transfer performance of TPMS-Dimond is the best, which can be attributed to its geometric structures without the “through-holes,” which leads to a more substantial disturbance of the wall to the fluid, thereby enhancing the heat transfer.

Hybrid nanofluid convection and phase change process in an expanded channel under the combined effects of double rotating cylinders and magnetic field

Convective heat transfer and phase changing process are analyzed for flow over facing step in a channel equipped with double rotating circular cylinders and phase change- packed bed (PCM-PB) under magnetic field. The favorable and opposing effects of using magnetic field with rotations on phase change and thermal performance for separated flow in PCM-PB installed system are numerically explored by using the finite element method. Base fluid with hybrid particle loading of 2% solid volume fraction is used. The analysis is performed for various values of rotational Reynolds number (Rew: −1000 and 1000), cylinder sizes (R:0.01 and 0.3), size ratio (SR:0.5 and 2) and Hartmann number (Ha: 0 and 20). When rotations are active, phase change becomes inefficient due to the establishment of the vortices near the PCM-PB zone. Complete phase transition time (tC) depends upon the rotational direction of the cylinders and fluid type. As compared to case at Rew = 0, tC rises up to 175% at Rew = 1000 by using pure fluid while using nanofluid reduces the tC value. The size and size ratio of the cylinders are highly influential on the vortex size behind the step which affects the phase transition dynamics. There are 185% and 172% variations in the tC values are observed when size and size rations are varied. The case with the PCM + nanofluid gives the highest Nusselt number (Nu) while lowest Nu is obtained when pure fluid without PCM is used. The average Nu rises with larger cylinders for counter clockwise rotations at the highest speed. Magnetic field suppresses the vortices and contributes positively to the phase change process and thermal performance while reduction of tC is up to 74% is obtained at Rew=-1000 by using magnetic field at the highest strength. Proper orthogonal decomposition (POD) is used for fluid and solid temperature reconstruction in the whole computational domain. The model captures the dynamic behavior of phase change and heat transfer under combined effects of double rotations and magnetic field with PCM-PB embedded channel with area expansion.

Symmetry and self-similar analysis of natural convection of van der Waals gases over a vertical plate

In this work, the symmetry method was first applied to the study of the problem of free convection of van der Waals gas with physical properties that differ from an ideal gas under steady-state conditions. Using the previously obtained simplified form of the equation of state for a van der Waals gas, the nature of the influence of its parameters on the process of convective heat transfer is studied in comparison with the case of using the equation of state for an ideal gas. While solving the problem, it was revealed that nonlinear parameters in the equation of state for van der Waals gas can intensify or worsen the heat transfer process. This feature is associated with the influence of the parameters of the van der Waals gas equation of state on the flow in the boundary layer. The data obtained based on the transport equations in a self-similar form are compared with the results of the previously obtained analytical solution. It is shown that the trends of the influence of the parameters of the simplified equation of state for a van der Waals gas on the Nusselt number agree well with the analytical studies.

Experimental and numerical analysis of naturally ventilated PV-DSF in a humid subtropical climate

Photovoltaic double-skin façades (PV-DSFs) have great potential to simultaneously achieve power generation, thermal insulation and natural lighting when applied in buildings. Among them, crystalline silicon PV-DSF is one of the common types due to its high efficiency and mature technology. However, the alternate arrangement of opaque crystalline silicon and transparent glass in crystalline silicon PV-DSF complicates its heat transfer characteristics. Furthermore, the ventilation air cavity in crystalline silicon PV-DSF could induce a vertical convective heat transfer process. It is difficult to accurately predict the complex multidimensional heat transfer process in crystalline silicon PV-DSF by simplified 1-D or 2-D thermal models. Therefore, a comprehensive numerical model of PV-DSF including the optics, electricity and 3-D heat transfer sub-models is developed in this paper to estimate the annual electrical and thermal properties of PV-DSF in a humid subtropical climate. The effects of structure, orientation, air cavity depth and solar cell coverage on the overall performance of PV-DSF are analyzed. In comparison with single-glazed semi-transparent photovoltaic, PV-DSF provides a 30.4% reduction in heat gain and a 50.3% reduction in heat loss. The preferred installation orientation is due south for PV-DSF in a humid subtropical climate.

Comparative analysis of heat loss performance of flat plate solar collectors at different altitudes

Flat plate solar collectors (FPSC) are a vital component in the medium and low temperature utilization system of solar energy and are widely used at different altitudes throughout China. Changes in altitude can cause changes in outdoor meteorological conditions, which can affect radiation and convection heat transfer processes on the surface of FPSC. Research into the heat loss characteristics of FPSC, when used at different altitudes, is relatively inadequate. To analyze the comprehensive impact of the change in altitude on the heat loss performance of FPSC, a model for the calculation of heat losses of FPSC was developed in this study. The influence of air pressure on air convection and long-wave radiation heat transfer was considered, and the accuracy of the model was verified by experimental test data. The variation of convection heat loss (CHL) and radiation heat loss (RHL) caused by environmental conditions such as atmospheric pressure, air density, solar irradiance and water vapor partial pressure with changes in altitude is investigated using the numerical calculation method. The results show that with increasing altitude, CHL and RHL show opposite trends, with the former gradually decreasing and the latter gradually increasing. The maximum variation of CHL and RHLs as a percentage of total heat loss can exceed 50%. Two opposite processes of heat loss play a combined role in the variation of the total heat loss, resulting in a decreasing and then an increasing trend in the altitude of the total heat loss coefficient, with a maximum variation of 10%.

MHD hybrid nanofluid convection and phase change process in an L-shaped vented cavity equipped with an inner rotating cylinder and PCM-packed bed system

In this study, convective heat transfer and phase change process are analyzed for an L-shaped vented cavity equipped with an inner rotating cylinder and phase change material-packed bed (PCM-PB) system under magnetic field during hybrid nanofluid convection. The numerical work is performed for different values of Reynolds number (Re between 200–1000), rotational Reynolds number (Rew between −1000–1000), size of the cylinder (R between 0.05H-0.15H) and Hartmann number (Ha between 0–40) while hybrid Ag/MgO nanoparticle loading amount in water is 2%. It is observed that the vortex size and their distributions in the cavity and within the PCM-PB system can be controlled by varying rotating cylinder size and rotational speed along with the magnetic field. With higher cylinder size, phase change becomes fast while complete phase transition time (tP) is reduced by about 22% and average Nusselt number (Nu) rises by about 86% at Rew = −1000. Rotational direction of the cylinder is effective for phase transition dynamics while at Rew = −1000, tP rises up to 27% when compared to non-rotating cylinder case. Magnetic field strength is a good parameter for vortex suppression. At the highest strength, phase change becomes fast and average Nu rises up to 26.5% at Rew = −1000. ANFIS based modeling approach is used for impacts of rotating cylinder on the phase change dynamics in the L-shaped vented cavity.

Evaluation and development of a predictive model for conjugate phase change heat transfer of energy storage system partially filled with porous media

The present study proposes a predictive model to explore the effect of partially filled porous media on the conjugate heat transfer characteristic of phase change material (PCM) with interfacial coupling conditions between pure fluid region and porous region. The enthalpy-porosity method, local thermal non-equilibrium model and Darcy-Forchheimer law are comprehensively considered to describe the convective heat transfer process in porous media. The modified model is then validated by benchmark data provided by particle image velocimetry (PIV) experiments. The phase change behavior, heat transfer efficiency and energy storage performance are numerically investigated for different partial porous filling strategies in terms of filling content, position, height of porous foam and inclination angles of cavity. The results indicate that due to the resistance in porous region, the shear stress exerted by the main vortex (natural convection) in pure fluid region and the momentum transferred, a secondary vortex phenomenon appears in the porous region near the fluid/porous interface. Moreover, such discontinuity of permeability and fluid-to-porous thermal conductivity results in the cusp of phase change interface at the horizontal fluid/porous boundary. Among four partial porous filling cases, the lower porous filling one has more desirable heat transfer performance, and the 3/4H lower porous filling configuration is the best solution for optimization of the latent heat thermal energy storage (LHTES) systems. For tilted cavity, the increase of inclination angle positively affects the heat transfer efficiency as well as the energy storage rate of the LHTES system, where the performance of 3/4H lower porous filling configuration is further highlighted.

Study on the three-dimensional heat flow field of supercritical nitrogen in a micro-channel plate heat exchanger

Micro-channel plate heat exchanger (MCPHE) has been recognized as a new high-efficiency heat transfer device and widely utilized in the energy field due to its compactness and high heat exchange rate. In this paper, the three-dimensional heat flow field of supercritical nitrogen (SCN2) in a simplified MCPHE is numerically simulated using computational fluid dynamics (CFD) simulation technology. The numerical approach predicts the thermo-hydraulic performance of SCN2 with a relatively good accuracy, by comparing with the published experimental data. Furthermore, the effects of pressure (3.6 ∼ 7 MPa) and mass flux (800 ∼ 1200 kg/(m2∙s)) on the convective heat transfer characteristics of SCN2 are thoroughly examined, especially the heat flow field of SCN2 in different circumferential directions of the micro-channel is revealed. The obtained results show that under the condition of low pressure and high mass flux, the maximum circumferential inner wall temperature appears at 90° of the micro-channel at the same axial position. With an increased mass flux, the maximum inner wall temperature and minimum heat transfer coefficient gradually shifts from 180° to 90°. When the buoyancy coefficient Gr*/Re2 > 1, the buoyancy force is conducive to enhance the fluid heat transfer capacity gathered at the bottom of the micro-channel. Meanwhile, the buoyancy force could make the distribution of thermo-physical properties of SCN2 uneven at the same time. Finally, in view of the obtained data, a new dimensionless correlation is proposed to predict the convective heat transfer process of SCN2 inside MCPHE, and the prediction error is less than 20%. The research outcomes could provide a reference for the optimal design and safe operation of MCPHE.