Thermo-hydraulic Characteristics Research Articles

Numerical Study of Hydrothermal Enhancement of Two-Phase Flow in a Vertical Pipe by Using Modified Vortex Generators

The improvement in heat transfer and energy savings in two-phase (gas-liquid) flow in tubes occupies a wide area of research interest. In this work, the effect of modified twisted tape turbulators on heat transfer augmentation in vertical tubes is studied numerically by using the CFD technique. A vortex generator (twisted tape) was used as a passive method to raise the rate of heat transfer. The variants of the water flow rate, air flow rate, heat flux, and twisted ratio of a twisted tape tabulator were considered. This work provides a superficial water Re No. With ranges of 6300–10500, considering three different water and airflow rates. Additionally, the superficial gas Re range was 6000-16000. Three distinct twist ratios (TR) of 7.8, 3.9, and 2.6 were considered for the iron twisted tape to examine the impact of the twist ratio on the tube's thermo-hydraulic characteristics. The test section consisted of an insulated copper vertical tube subjected to three constant heat fluxes. This paper investigated three variants: the friction factor, Nusselt number, and performance evaluation factor. A strong turbulent vortex flow with an increase in secondary flow in the tube's radial direction was significantly intensified in the presence of the twisted tape inserted. Also, an increase in Reynold's number for both water and air and increased heat flux led to a proportional increase in heat transfer while the friction factor decreased. The presence of the modified twisted tape showed an improvement in the heat transfer and the pressure loss reduction when compared with the plain tape for all twisting ratios and for all cases where the highest value of the performance evaluation factor was two at the twisted ratio of 2.6 with an increase percentage of 81%. Conclusively, this investigation concludes that using modified twisted tape can greatly enhance the heat transfer rates and reduce pressure resistance, increasing the performance evaluation factor of heat exchanger systems.

Effect of forced lateral vibration on flow and heat transfer performance within an annular fuel element

The investigation into the thermo-hydraulic characteristics of fuel elements under forced vibration conditions is of paramount importance in the design and application of offshore nuclear power platforms. This study employs a mathematical model of fluid-solid coupling and applies the theory of multi-field synergy to numerically analyze the flow and heat transfer features in an annular fuel element with internal and external double-sided cooling under forced lateral vibration conditions. The synergistic relationship of each flow field parameter is scrutinized for vibration conditions with frequencies ranging from 0 to 10 Hz and amplitudes from 0.005 to 0.2 m. The results demonstrate that increased amplitude and frequency contribute to enhancing the homogeneity of the temperature and velocity field distributions. Notably, the field synergy angles α, β, ψ, and γ attain minimal values at the gaps (X = 8 mm, X = 25 mm). Furthermore, the variation of vibration frequency and amplitude decreases the mean synergistic angle ψ by 5.14 % and 7.9 % and increases the mean field synergistic angle θ by 23.18 % and 31.21 %, respectively. It indicates that the vibration parameters have the least effect on the synergistic angle ψ and the greatest effect on the synergistic angle θ.

Numerical investigation on thermohydraulic characteristics in a circle tube with a novel arrangement of ellipsoidal dimples

A novel arrangement of ellipsoidal dimples for a circle tube was established to improve the tube’s thermal performance. The impacts of axis ratio and depth of an ellipsoidal dimple with an attack angle of 45° on the thermohydraulic characteristics were numerically studied using the realizable k-ε model in the Re range of 10,000 to 40,000. The research shows that increasing the axis ratio of the ellipsoidal dimple from 4:3 to 6:3 leads to the increase in Nu by 15.83 %-18.4 %, with the corresponding increase in f by 19.03 %-30.76 %. The depth of the ellipsoidal dimple also significantly affects the heat transfer performance and pressure drop of the dimpled tube. When the dimple depth increases from 1 mm to 2 mm, Nu and f increase by up to 30.47 % and 64.12 %, respectively. The maximum performance evaluation criteria of 1.68 is achieved when the dimple depth is 2 mm. The proposed ellipsoidal dimple arrangement significantly improves the heat transfer compared with other arrangements in the open literature. The performance evaluation criteria of the studied model with d = 1.5 mm and a:b = 5:3 is up to 19 % larger than the models reported in the references.

Experimental investigation on transitional flow of supercritical hydrocarbon fuel within straight channels of Printed Circuit Heat Exchangers for hypersonic thermal management systems

The onboard supercritical CO2 closed Brayton cycle (SCBC) has established an effective thermal management pathway for hypersonic vehicles operating at high Mach numbers. A Printed Circuit Heat Exchanger (PCHE) is employed within the cycle system to facilitate heat transfer between fuel and carbon dioxide, with its thermo-hydraulic characteristics significantly impacting both power generation and propulsion performance. Given the insufficient research on the flow and heat transfer of hydrocarbon fuels in PCHEs, this study aims to experimentally address the gaps in understanding transition flow for supercritical hydrocarbon fuels, particularly within semi-circular microchannels. We conducted experiments on the flow transitions using a specially designed test piece with a diameter of 1 mm, replicating the PCHE channel characteristics. The experimental Reynolds number range from 300 to 8300 spans laminar, transition, and turbulent flow regimes. Friction factors for n-decane are obtained under adiabatic and heated conditions. The results validate the nomenclature of the quasi-turbulent region and reveal that the critical Reynolds numbers for the transitional, quasi-turbulent and hydraulic smooth turbulent regions under adiabatic conditions are 2200, 2440 and 2980, respectively. A reduction in friction drag attributed to boundary layer viscosity is observed to alter the friction factor under heating conditions, while heating influences both the onset and termination of the transition region, but does not affect its width. Classical correlations are compared with the experimental results, and new empirical correlations for both adiabatic and non-adiabatic conditions are developed, showing superior predictive accuracy compared to existing models. This provides a reliable tool for the engineering design of heat exchangers in the onboard thermal manage systems.

Thermo-hydraulic performance characteristics of novel G-Prime and FRD Triply Periodic Minimal Surface (TPMS) geometries

Efficient thermal management is crucial for numerous applications, from electronics cooling and automobile systems to aerospace systems, necessitating the exploration of advanced heat transfer technologies like TPMS structures. Compared to conventional heat exchangers and heat sinks, lattice structures have improved by around 20–30 % performance. However, the variation in performance of the various lattice structures, such as FRD, G-Prime, etc., is yet unknown. Thus, this study investigates the thermo-hydraulic performance of various Triply Periodic Minimal Surface (TPMS) structures, including novel G-Prime and FRD geometries, through numerical simulations and experimental validation. The TPMS geometries were modeled using LattGen software and voxelized with 20 % relative density. Computational Fluid Dynamics (CFD) simulations were performed using Ansys Fluent with the k-epsilon turbulence model, and experiments were conducted on 3D-printed samples for validation. The numerical results reveal that G-Prime-2 and FRD Prime exhibit the highest heat transfer performance while demonstrating higher pressure drops than other TPMS geometries. Experimental validation agrees with numerical predictions, confirming the superior thermo-hydraulic performance of G-Prime-2 and FRD Prime for heat transfer enhancement applications. The results contribute to understanding TPMS thermo-hydraulic performance and enable informed geometry selection for thermal management applications.

Advancing thermohydraulic performance of micro-grooved flat heat pipes through a combined numerical-experimental approach

A combined numerical-experimental model has been developed to predict the thermohydraulic characteristics of micro-grooved flat heat pipes (FHPs). This model uses a limited number of experimental data for calibration with two parameters, βexp,1 and βexp,2, which account for uncertainties in the accommodation coefficients used to determine evaporation and condensation mass fluxes. This calibration, which is a key feature of the developed model, addresses the uncertainties and challenges in determining these coefficients. Moreover, the model incorporates the effects of interfacial shear stress, axial wall conduction, heat transfer in the liquid block region, and the contact angle of the liquid–vapor interface. Furthermore, the evaporation mass flux from the curved liquid–vapor interface is calculated using a separate multiscale approach developed in two recent works for analyzing evaporation from the extended meniscus in microchannels, considering thin-film evaporation. The model demonstrates high accuracy in predicting the maximum heat transport capacity (Qmax) and wall temperature distribution across various input heat powers, with a maximum error of less than 0.5 % in wall temperature predictions, as validated against experimental data. The validated model is then used to explore a new micro-grooved wick structure featuring converging/diverging microchannels. In this structure, the channel width is constant and equal to Wf1 in the first half of the heat pipe, from the condenser end to the midpoint, and then it decreases or increases to Wf2 by the end of the evaporator section. Results indicate that a flat heat pipe with a converging micro-grooved wick structure (Wf1 = 200 µm and Wf2 = 160 µm) can enhance Qmax from 53 to 70 W, 70.8 to 91.8 W, and 91.2 to 116.5 W at working temperatures of 60 °C, 70 °C, and 80 °C, respectively. This means the new converging micro-grooved wick structure can boost flat heat pipe performance by more than 25 % compared to a flat heat pipe with a straight micro-grooved wick (Wf1 = Wf2 = 200 µm) without increasing the occupied space.

Effect of wave aspect ratio of skewed sinusoidal corrugated channel and copper metal foam inserts on thermohydraulic characteristics

The present study examines numerically the wave aspect ratio of skewed sinusoidal corrugated (SSC) channel and partially inserted open-cell metal foam (OCMF) made of copper effects on the forced convective heat transfer and airflow characteristics. The computations are conducted employing ANSYS Fluent 19.2 for various wave aspect ratios (2a/λ = 0.3–0.075) at the laminar Reynolds number range (Re = 700–1700). Additionally, the foam is fixed on the corrugated surfaces' upper and lower wave peaks in staggered arrangements with a pore density range of (10–40) pores per inch (PPI). The results show that the three-dimensional, incompressible and steady airflow through the SSC channel increases the average Nusselt number (Nuavg), friction factor (f), and thermo-hydraulic performance (THP) with increasing the aspect ratio at constant Re due to the increased surface area and due to increased flow restriction associated with the secondary flows. Moreover, using partial OCMF inserts improves fluid mixing, resulting in a higher heat transfer rate and friction along the channel. Results further illustrate that Nuavg, f, and THP increase with increasing PPI, primarily because of the elevated foam surface area and reduced permeability. A maximum THP value of 1.5 is achieved for the SSC channel with 40 PPI.

The preliminary design of emergency core cooling scheme and loss-of-coolant accident analysis for Tsinghua high flux reactor

This paper proposes the preliminary emergency core cooling scheme for Tsinghua High Flux Reactor. According to the thermohydraulic characteristics of high flux reactors, forced circulation needs to be maintained by emergency pumps in the early stage. When the decay power is low enough, natural circulation between core and the reactor pool is initiated to remove the residual core heat. The sources of safety injection are accumulators and reactor pool. Accumulator injection can ensure core safety in the early stage and reactor pool injection can maintain long time stable forced circulation. To avoid emptying the reactor pool, the waterproof zone needs to be built. The waterproof zone consists of reactor pool and several finite volume dry pools. All pipelines and equipment in the reactor coolant system are placed in the dry pools. Once break accident occurs, the dry pool where the break is located can collect the leaked coolant. With the increase of break back pressure, the break flow is restricted. The current scheme is modeled by Relap5 and different size breaks at four locations (namely core inlet, core outlet, primary heat exchanger inlet and main pump inlet) are assumed. According to the response characteristics of the scheme, the accident process can be divided into five stages: non-shutdown stage, high-injection stage, low-injection stage, stable forced circulation stage and natural circulation stage. Critical heat flux predicted by Sudo CHF correlations is adopted as the primary safety criterion. Through analysis, a success path is found to maintain core safety for a wide range of loss-of-coolant accident scenarios.

Experimental Investigation of Thermo-Hydraulic Characteristics and Sustainability Measure of a Novel Multi-Fluid Heat Exchanger for Turbulent Water-Based Nanofluid Flow through the Inserted Brazed Helix Tube

Abstract A Novel Multi-Fluid Heat Exchanger (NMFHE) deployed for simultaneous heating of water and space is experimentally investigated to predict its thermo-hydraulic, exergetic, and sustainability performance for distinct Al2O3, TiO2, and CuO nanofluid flow of 50ppm concentration of each through the inserted Brazed Helix Tube (BHT). The input parameters such as flow rates, helix tube diameters, and nanofluid types are varied throughout the experiments to evaluate their effect on output performance parameters i.e., Nusselt number (Nu), Friction factor (f), entropy generation number (Ns), JF factor (JF), exergy efficiency (εE), and sustainability index (SI). The nanofluid (NF) flowing through the BHT is the heating fluid that simultaneously heated the cold water (CW), and air (CA) flowing through the outer shell and inner conduit of the BHT respectively. A distinct Nusselt number correlation for turbulent nanofluid flow inside BHT was developed, compared, and validated reasonably with the current result. For Al2O3 NF at a Reynolds number of 5698 with a 1/2-inch diameter helix tube, the best results for JF, εE, and SI are found to be 0.009, 0.72, and 3.53, respectively. Furthermore, for Al2O3 and TiO2 NF at a Reynolds number of 14250 and a helix tube diameter of 1/4 and 1/2-inch, f, and Ns are found to be 0.0032 and 0.043, respectively are optimum. It is observed that the use of Al2O3 NF, higher helix tube diameters, and lower flow rates all make the proposed heating application more sustainable.

Thermal and hydraulic characteristics of a single reverse nanofluid jet in a double-wall cooling configuration

AbstractThis work investigates the thermo-hydraulic characteristics of a novel reverse jet impingement cooling 3-D module for high-power systems, entailing a single jet impinging upon a concave hemispherical surface within a double-wall cooling configuration. Water-based nanofluids, with Al2O3 and multi-wall carbon nanotubes (MWCNT), were subject to a comprehensive examination at a range of Reynolds numbers from 10,000 to 40,000. The computational framework utilized the volume of fluid (VOF) model for precise phase interface tracking, along with the $$k-\omega$$ k - ω SST turbulence model. The results demonstrated that nanofluids have remarkable heat transfer enhancement, compared to water. The maximum Nusselt number augmentation reached 61.76% and 77.47% for 2.0% Al2O3 and 0.04% MWCNT nanofluids, respectively. The thermal performance improved when escalating nanoparticle concentration and Reynolds numbers, reaching a cooling module’s thermal resistance of only 0.0266 K W−1. However, mild increments in pressure drop of up to 7.80 and 3.04% were noticed at the lowest Reynolds number for the two nanofluids, respectively. MWCNT nanofluids exhibited superior thermal enhancement over their Al2O3 counterparts despite their lower concentrations. The greatest combined thermo-hydraulic performance was attained with a 0.04% MWCNT nanofluid at a Reynolds number of 20,000, where the energy performance was boosted by 77%, compared to water. The study identified significant conjugate heat transfer effects, demonstrating how temperature gradients within the solid walls directly influenced heat transfer coefficients and thermal resistances within the fluid phase. These findings provide a deeper understanding of thermal management strategies for high-power systems. Graphic abstract

Determining of the thermo-hydraulic characteristics and exergy analysis of a triple helical tube with inner twisted tube

An innovative technique to enhance heat transfer as a passive method was investigated. The combination of the twisted tube and helical coil to increase the swirl intensity is presented. The current investigation showed experimentally and numerically the exergy and thermal performance analysis of a new design of a triple tube design called a triple helical tube with inner twisted tube, THTITT. The new design is a modified design of a double helical tube with inner twisted tube, DHTITT which is created by twisting the inner tube. Besides the benefits of adding the third fluid to DHTITT that achieve extra contact surface area per unit length between the intermediate tube and the new passage in the outer tube. In which expected to enhance the temperature gradient between the three fluids and consequently, the thermal characteristics augment occurred. The twisted tube increased the intensity of the swirl flow and more intense disturbance of the fluid in the tube occurred. A 3d CFD model was established to get more insight at a level of detail not always available in the experiment. The effects of twisted pitch ratio, ξ hydraulic diameter, ω, helical coil torsion, α, helical coil inclination angle, φ, as well as Dean number, NDn, h were explored. Four groups of test specimens include various ξ of 5.32, 7.97, and ∞, various ω of 3.8, 6.8, and 9.9 mm, various α of 0.068, 0.095, and 0.121, and various φ of 0°, 45°, and 90° were established, manufactured and examined in this investigation. The investigation covered Reynolds number, NRe,h, for a range of 2450:29300 corresponding to Dean number, NDn, h, for a range of 570:4800. The results showed a higher Nusselt number, NNu, h, of THTITT compared to DHTITT by 61.8 %%, while the increase in fh is approximately negligible. Also, by decreasing ξ from ∞ to 5.32 increases NNu, h by 33.4 %, with an increase in friction factor, fh by 57.6 %. Furthermore, with a decreasing ω from 9.9 mm to 3.8 mm, a significant increase in NNu, h occurs by 20.6 %, at the expense of increasing fh by 36.4 %. In addition, with decreasing α from 0.121 to 0.068, a significant increase in NNu, h occurs by 27.6 %, at the expense of increasing fh by 17.1 %. Finally, the THTITT decreases the heat loss (exergy destruction) between the hot and cold fluids. New correlations to predict NNu,h, and fh are presented.

A thorough investigation of geometric and thermohydraulic features in pillow-plate heat exchangers

Determining the thermohydraulic characteristics in pillow-plate channels (PPCs) is challenging due to their intricate structure. This work introduces novel geometric parameters specific to a flow cross-sectional area in PPCs and employs a two-stage optimized algorithm for artificial neural networks (ANNs) to enhance its thermohydraulic feature predictions significantly. These parameters enable practical consideration of various PPC structures and help develop a new mean hydraulic diameter (MHD) correlation. Besides the general parameters in PPCs, the novel parameters are incorporated into two two-stage optimized ANNs to predict thermohydraulic characteristics. The traditional method for approximating MHD neglects the actual channel structure and only considers the maximum channel height, leading to deviations of −16.93% to +52.00% from finite element simulated PPCs. Our new MHD correlation and the first ANN, which consider the channel structure, yield more accurate results with deviations of −6.97% to +13.07% and −7.85% to +13.21%, respectively. The second ANN predicts Nusselt numbers and Darcy friction factors with errors of less than 13.04% and 15.83%, respectively, across a wide range of geometric and thermohydraulic conditions. These promising results and presenting the maximum and minimum possible MHD in PPCs by the novel correlation pave the way for more accurate future studies and industrial development of PPCs.

Thermo-hydraulic transport characteristics of non-Newtonian nanofluid flow through wavy channel: Influence of nanoparticle volume fraction and Prandtl number

The present study numerically investigates the thermal-hydraulic transport characteristics of non-Newtonian nanofluid flowing through the trapezoidal wavy channel. Constitutive behavior of non-Newtonian fluid was used to elucidate the power law model. The average Nusselt number, pressure drop, and performance factor are evaluated for varying volume concentration of nanoparticles (0 ≤ ϕ ≤ 0.04) and Prandtl number (Pr = 0.76, 1, 6.39, 20, 50, and 100) at different values of Reynold number (Re = 50 and 500), power index values (n = 0.5, 1, 1.5) and wave phase shift (θ = 0°, 90°and 180°). An increase in average Nusselt number and pressure drop was observed for increasing volume concentration of nanoparticles, Prandtl number, and Reynold number. At the same time, the performance factor decreases with an increase in ϕ, Pr, and Re. The heat transfer rate was observed to be more for shear thinning fluids than shear thickening fluids.

Effect of ultrasonic on thermo-hydraulic characteristics of shell and tube heat exchangers with twisted tape

Ultrasonic technology was combined with twisted tape insert technology to improve the performance of shell and tube heat exchangers. The effects of sound intensity, sound frequency, and ultrasonic transducer position on thermal transfer, resistance reduction, and overall performance of shell and tube heat exchangers with twisted tape (STHXTT) at different Reynolds numbers were investigated compared to the non-ultrasonic conditions. The experimental results show that applying ultrasonic can significantly enhance thermal transfer, reduce flow resistance, and improve the overall performance. The maximum increase of Nu is 36.53 %, the maximum decrease of f is 12.50 %, and the maximum value of PEC is 1.41. With the increase in Reynolds number, the performance is enhanced but the resistance reduction and overall performance are weak. When the ultrasonic intensity increases, the thermal transfer and resistance reduction performance is enhanced, but a critical ultrasonic intensity exists. When the ultrasonic intensity is large enough, the ability to reduce resistance is enhanced with the decrease of ultrasonic frequency. When the ultrasonic transducer is installed vertically, the thermal transfer performance is the best, however, the resistance reduction effect is the worst. When installed horizontally, the STHXTT has the best resistance reduction performance but the worst thermal transfer effect.

Effects of bidirectional rib arrangements on turbulent flow structure and heat transfer characteristics of a two-pass channel for turbine blade internal cooling

The thermal efficiency of gas turbine engines depends heavily on the cooling performance of the internal channels of turbine blades. In the present study, the effects of bidirectionally arranged ribs on the flow and heat transfer mechanism in a two-pass cooling channel are investigated numerically. The channel aspect ratio is kept constant at 1:2 (W: H), and the rib pitch-to-height ratio (P/e) is 10. Comparative analysis is performed for three different cross-sectioned (Square, triangular and Curved) ribs with Reynolds numbers ranging from 10,000-50,000. The results indicate that the bidirectional ribbed channel provided 69% better thermal performance compared to horizontal-only ribs due to its potential to induce secondary flow in both vertical and horizontal directions. The square and triangular ribs show similar characteristics, but the thermal performance of curved ribs drops by 11%. The pressure loss phenomenon is also lowest in the case of the curved ribs. The obtained Nusselt number and friction factor for all cases are normalized with the Dittus-Boelter correlation, Blasius correlation and the smooth channel. The detailed analysis of area-averaged normalized Nusselt number indicates that the best thermal performance occurs at the bend region due to the combined contributions of the ribs and the 180° turn. Furthermore, in terms of the overall thermohydraulic characteristics, the curved-bidirectional ribbed channel is noticed to have the best results with values of 1.38 and 1.25 for TEFo and TEFs respectively.

Optimising low-temperature district heating networks: A simulation-based approach with experimental verification

Fifth generation district heating and cooling systems are becoming increasingly popular due to their ability for working with low temperature of heat transfer fluids. Among the other benefits, this characteristic allows for a better exploitation of renewable energy sources. On the other hand, these networks require a fine design and precise management to exploit their full potential. Both these requirements can be met by using advanced simulation and optimisation tools. This research proposes a simulation tool purposely conceived for the design and the optimisation of fifth-generation district heating and cooling systems. This tool is capable of assessing the effects of each building-plant system on the whole district heating and cooling water loop, and to evaluate the effectiveness of diverse network morphology. These capabilities are due the level of detail of the mathematical modelling which takes into account the thermohydraulic characteristics of the network, each building thermo-physics properties, and the heat pump/chiller detailed operation. The described tool has been adopted to simulate an existing experimental network prototype (consisting of a central heat pump, behaving as thermal energy balancing station, and eight users), and the achieved results were compared to those experimentally obtained for validation aims. The capabilities of the validated tool have been demonstrated by investigating an innovative control logic (representing a further novelty of this research) for a “proof-of-concept” fifth-generation district heating and cooling network. In particular, by adopting a predictive control logic, the water loop temperature is dynamically optimised to minimise the entire network energy demand. The adopted control strategy has yielded significant primary energy savings, amounting to 10.3 MWh/year, with a rate of 6.5 % compared to the reference case characterised by a fixed network temperature. These results underscore the potential of the proposed method and demonstrate the effectiveness of the developed tool.

Numerical energy and entropy analyses of a tube with wavy tape insert including CoFe2O4/water nanofluid under laminar regime

The primary objective of this investigation is to assess the impact of vortex generator geometry and nanofluid on thermohydraulic and irreversibility characteristics within a laminar flow regime. The study introduces an original CoFe2O4/H2O (1 % vol.) nanofluid and employs a wave tape insert to induce forced convection in a tube, accompanied by first and second-law thermodynamic analysis. The novelty of this research lies in the numerical exploration of heat transfer and flow profiles for a nanofluid in a tube, varying the wave rate (y = 4-5-6). The investigation considers the laminar model and single-phase approach in all analyses under constant heat flux (q” = 2000 W/m2). The study observed that the nanofluid flowing in the tube with a wave ratio of 4, 5, and 6 resulted in an average enhancement in the Nusselt number of 93.25 %, 86.26 %, and 80.06 %, respectively. The optimal performance evaluation criterion (PEC) for water flowing in the tube with a wave ratio of 6 at Re = 500 exhibited an increase of 11.0 %, whereas the CoFe2O4/H2O flow showed a 9.14 % increment in the average PEC along the Reynolds number. Moreover, the total entropy generation values for water flowing in tubes with wave ratios of 5, 6, and 4 exhibited increases of 101.88 %, 94.66 %, and 51.34 %, respectively, in comparison to the smooth tube.

Investigation on the thermal and hydraulic characteristics of the micro heat sinks with grooves and pin fins by Taguchi-based sensitivity analysis

The heat dissipation issue of the micro electronic devices seriously restricts their application in the aerospace, national defense, communication, etc. The novel heat sinks with trapezoidal grooves and rectangular pin fins are proposed to meet the cooling requirement of the high-power devices. The cooling effect, thermohydraulic characteristics, total efficiency and thermodynamic performance of the heat sinks are studied by three-dimensional numerical simulation. Firstly, the influence of the relative position between the grooves and the pin fins on the thermohydraulic performance is explored in detail. Secondly, Taguchi optimization and analysis of variance are adopted to quantitatively analyze the impact degree of the pin fin width, length and height on the Nusselt number, friction factor and total performance index. The optimal structures are obtained for different objectives. Lastly, the temperature performance, total performance and the irreversibility of the optimal structures are compared thoroughly. New findings include that: the best arrangement of the grooves and pin fins is obtained, which enhances the Nusselt number by 1.5 times compared to the smooth heat sink. The fin width is the most critical factor for the heat transfer and flow, while the fin length shows the greatest importance in the overall performance. The optimized heat sink with the fin width of 50 μm, length of 60 μm and height of 150 μm reduces the peak temperature by 12.3 ℃ compared to the smooth heat sink for Reynolds number of 303. This heat sink yields the highest total performance index with 1.43 for Reynolds number of 368 and significantly reduces the irreversible loss. Moreover, the optimized heat sink with the fin width of 10 μm, length of 60 μm and height of 100 μm reduces the friction factor by 61.39 % compared to the original structure for Reynolds number of 635, which is beneficial to reduce the energy consumption. The quantitative analysis of the parameter sensitivity for novel heat sinks reveals the mechanism of the heat transfer enhancement by modifying the geometric parameters and provides an effective solution for thermal management of the high-power devices.

Numerical Investigation of Thermohydraulic Characteristics in Supercritical CO2 Natural Circulation Loop with Spatially Varying Temperature

Investigation of thermohydraulic characteristics is highly significant for any natural circulation loop (NCL) applications. The physical properties of supercritical fluids are highly sensitive to temperature when they cross pseudo-critical points. In the current study, steady state thermohydraulic characteristics of NCL with a spatially varying temperature distribution on the source along with different orientations of the source and sink are numerically investigated. A three-dimensional rectangular NCL with a uniform diameter and constant length of source and sink is modeled. The temperature of the sink is held constant, while the temperature of the source varies spatially with local maximum and minimum temperatures. The system performance is investigated for four different operating pressures ranging from 8 to 12 MPa. The mass flow rate is observed to increase with increasing operating pressure for all orientations and heat load distribution at the source. The results also indicate that heat transfer degrades when the fluid crosses the pseudo-critical point. It has been noticed that spatially varying temperature distribution has a maximum of 48% heat transfer variation compared to the constant temperature distribution. Furthermore, a source or sink located in the horizontal leg performs better than the vertical leg.

Thermo-hydraulic characteristics and structural optimization of supercritical CO2-cooled microchannel heat sink

Supercritical carbon dioxide (sCO2) serves as an efficient working fluid in microchannel heat sinks (MCHS), effectively mitigating heat dissipation challenges in high heat flux microelectronic devices. Nonetheless, the issue of flow acceleration affecting micro-scale heat transfer, especially in channels with diameters <0.2 mm, warrants additional investigation. Flow acceleration exacerbates temperature non-uniformity, consequently diminishing heat transfer efficiency. Numerical simulations were conducted to examine the thermo-hydraulic characteristics of sCO2 in microchannels featuring complex structures including ribs and cavities. The channel had a diameter of 0.133 mm, with heat flux to mass flow rate (q/G) ranging from 0.1 to 0.5 kJ/kg. Three-dimensional entropy production rate (Sg) maps and corresponding heat maps were utilized to establish boundary conditions and evaluate heat transfer optimization. Additionally, structural optimizations were performed to rectify temperature non-uniformity in sCO2 within MCHS. The results indicate that the entropy production rate (Sg) of sCO2 initially decreased and then increased with the increase in q/G and dimensionless enthalpy (h*). The optimized MCHS demonstrated enhanced temperature uniformity, with a maximum temperature difference of 1.8 K, notably lower than that of the non-optimized MCHS (5.3 K). This study offers a theoretical basis for the design and implementation of sCO2 in micro-scale MCHS.