This study investigates thermo-hydraulic and second-law performance of ethylene glycol - water (EG/W) mixtures in a heat exchanger tube with conical wire inserts. Experiments were performed under turbulent conditions for Re = 3300- 21,000 using three EG/W ratios and five insert spacings. Nusselt number, friction factor, THPF, entropy generation number, Bejan number, and second-law efficiency were evaluated. Increasing EG content enhanced Nusselt number but increased frictional irreversibility. Maximum entropy generation (0.9221) occurred for CW-0 with 40:60 EG/W at Re = 21,000, whereas maximum second-law efficiency (0.428) was obtained for CW-0 with water at Re = 3300.
Hybrid nanofluid-assisted micro-channel heat exchangers offer an innovative solution for automotive and electronic cooling systems, addressing the challenge of limited space and the need for efficient dissipation of high thermal loads. This research study investigates the enhancement of heat exchanger thermal behavior through the use of a (60:40) water/ethylene glycol base fluid combined with a hybrid nanofluid (Al2O3-Cu) at varying concentrations (2%, 4%, and 6% by volume) at a flow rate of 0.1 L/min. The findings indicate that the request for a hybrid nanofluid with a 6% volume of Al2O3/Cu surpasses the thermal behavior of the base fluid system (water). With the hybrid Al2O3/Cu nanofluid at a 6% volume concentration, they achieve a maximum thermal conductivity of approximately 0.60 W/mK at 75 degrees C and a fluid temperature of 89.5 degrees C, while improving the heat transfer capacity, Nusselt number, & thermal index by around 70.4%, 6.78%, and 27.8%, better than the base fluid. However, the pressure drop was marginally increased to 16% compared to the base fluid. Ultimately, this research underscores the significant improvement in the microchannel heat exchangers thermal behavior, signaling the usefulness of hybrid nanofluids in advancing thermal management within electronic cooling systems.
In this work, the heat transfer characteristics of a multi-turn pulsating heat pipe in three commonly used vertical arrangements in space are investigated using infrared thermography. The influence of the local channel structure on the overall pulsating heat pipe operation is revealed by means of temperature oscillations. The temperature distribution of the condenser section was investigated to verify the trend of working fluid flow at different inclination angles and the effect of different inclination angles on the start-up characteristics. Influenced by gravity and heating mode, there is a symmetry in the working fluid flow mode for a certain heat input load at an inclination of 0 degrees (bottom heating), and the symmetrical flow tendency is not broken until the heat input load is increased to 160 W. The flow tendency remains uninterrupted until the heat input load is increased to 160 W. When the inclination angle reaches 90 degrees, the working fluid facilitates unimpeded directional circulation. Conversely, at an inclination of 180 degrees, counteracting gravity, fluid flow becomes substantially hindered. At a 0 degrees inclination, where the system is horizontal, fluid reflux is observed within the communication pipe, accompanied by a slight increase in heat transfer resistance. However, the PHP system exhibits a substantial temperature overshoot. With the maximum thermal resistance reaching 0.34 degrees C/W, heat transfer efficiency is compromised during the startup and unstable oscillatory phases, especially when the inclination angle is set at 180 degrees. Conversely, at an inclination of 90 degrees, the working fluid achieves a circulating state at minimal heat power input and temperature thresholds, thereby reducing temperature overshoot to less than 1 degrees C. Hence, for this study, a 90 degrees inclination angle is shown to provide enhanced thermal stability and superior heat transfer characteristics.
Ultra-thin finless flat tube heat exchangers are investigated through combined numerical and experimental methods. A parametric numerical analysis is conducted to examine the effects of tube configuration, inlet air velocity, and tube spacing on air-side heat transfer and pressure drop. Four heat exchanger configurations are tested under various inlet air velocities and water-side flow rates. Compared with the conventional fin and tube heat exchanger, the finless flat tube heat exchangers yield lower friction factors and up to 6% lower pressure drop. When Re exceeds 200, they also exhibit higher j/f1/3 values, indicating better overall thermo-hydraulic performance.
The conventional pyramid solar still (PSS) suffers from low thermal efficiency and limited freshwater yield due to inadequate solar energy utilization and heat losses. To address these challenges, this study introduces the Floating Absorber Pyramid Solar Still (FAPSS), a novel design that employs buoyant cork elements as floating absorbers to enhance solar energy capture. Each absorber consists of a black-coated sheet metal panel and a hydrophilic wick that ensures continuous water transport via capillary action. Three configurations with four, six, and eight absorbers (FAPSS-4A, FAPSS-6A, FAPSS-8A) are evaluated, and the optimal FAPSS-6A design is further enhanced by external mirrors to amplify incident radiation, an electric fan for vapor extraction, and a silver-nanoparticle-enhanced phase change material (Ag-nano-PCM) for thermal energy storage. The dual-phase thermal management stabilizes the system under varying irradiance and extends operation into nighttime hours. Experimental results show that FAPSS-6A with reflectors and fan achieves a daily freshwater yield of 10,600 mL/m(2) & centerdot;day - an increase of 194% in yield over the conventional PSS - and a thermal efficiency of 62%. Incorporating Ag-nano-PCM further raises the yield to 11,300 mL/m(2) & centerdot;day, representing a 182% improvement. Economic analysis reveals a substantial reduction in water production cost: 0.011 $/L for FAPSS with reflectors and fan, and 0.012 $/L for the configuration with PCM, compared to 0.024 $/L for the conventional PSS. The proposed FAPSS system thus offers a highly efficient, cost-effective, and sustainable solution for decentralized solar desalination. The work demonstrates a strong commitment to advancing the United Nations SDGs (Sustainable Development Goals), particularly Clean Water and Sanitation (SDG 6).
Fouling is termed as the accumulation of undesirable substances on the surface of heat exchangers. It usually forms from the precipitation of dissolved mineral salts, which reduces the heat transfer rate and introduces corrosion on the heat transfer surface. Thus, it is mandatory to explore sustainable sources to inhibit the fouling and corrosion on heat exchanger surfaces. The present study investigated heat transfer improvement and retardation of CaCO3 fouling and corrosion by applying a thin graphite layer coating on the heat transfer surface. The results revealed that the graphite-coated surface enhanced the heat transfer and retarded fouling deposition and corrosion.
Efficient heat removal is essential to ensure the reliability of electronic components. This study experimentally investigated the thermal performance of copper metal foam (CMF) heat sinks with different configurations and pores per inch (PPI) under forced convection. Three configurations were tested: block metal foam (BMF), finned metal foam (FMF), and pin-fin metal foam (PFMF), with pore densities of 5, 10, and 20 PPI and a porosity of 90%. Tests were performed using air velocities of 2.5, 3.0, and 3.5 m/s, heat inputs values of 80, 100, and 120 W, and an inlet air temperature of 27 degrees C. Results were all compared with a conventional aluminum heat sink (CHS), as a baseline for performance evaluation, under identical operating conditions. Thermal performance was evaluated using base temperature, Nusselt number, and thermal resistance. In comparison to CHS, results showed that the FMF heat sink with 5 PPI achieved better performance at 80 W and 3.5 m/s, with a 29.5% increase in Nusselt number and 17% reduction in thermal resistance. At PPI 10, the PFMF configuration provided superior performance, improving the Nusselt number by 39.3% and reducing thermal resistance by 26% when compared to CHS. However, PPI 20 did not enhance heat transfer due to restricted airflow within the fine pore structure. Overall, pore density and geometric influenced thermal performance.
The influence of a localized transverse magnetic field (MF) and nanoparticle mass fractions on the thermo-hydraulic performance of water-based Fe3O4 magnetofluids over laminar flow in both plain tube (PT) and eight-start spirally corrugated tube (SCT) heat exchangers has been experimentally investigated. Under the most intensified operating condition (omega = 3.0% Fe3O4 and B = 200 mT), the hybrid enhancement strategy yielded a maximum Nusselt number augmentation of 73.69% for the PT and 151.30% for the SCT. The corresponding peak performance evaluation criteria (PEC) values reached 1.69 and 1.74, respectively. indicating significant thermo-hydraulic superiority.
This study experimentally compares open-loop and closed-loop thermoelectric air-channel configurations composed of five TEC1-12706 modules operating under forced convection. The prototype was tested at air velocities over a 30 min operating period. The thermal response, power consumption, and coefficient of performance were interpreted using air-side energy balances. The open-loop arrangement was the only configuration that produced useful net cooling, with a maximum air temperature reduction of about 5 degrees C at 1.5 m/s and a peak COP of about 0.82 at 2.5 m/s. By contrast, the closed-loop produced progressive reheating, which reported air-side heat gain reaching 1083.7 W at 3.5 m/s.
This study aimed at investigating the influence of increasing and decreasing mass and heat fluxes based on modified surfaces. Test channel was 4 mm in height with a heating surface (20 $ \times $& times; 50 mm2) settled at the bottom center. HFC245fa at a saturated pressure of 250 kPa (39.9 degrees C) was used as the working fluid. Thermal spray coating (TSC) and perpendicular micro-V-shaped grooves were applied. The pitch and angle of the V-grooves were 0.5 mm, and 60 degrees, respectively. Influence of increasing and decreasing mass and heat fluxes on boiling behaviors by modified surfaces was analyzed by evaluating the heat transfer coefficient (HTC), hysteresis temperature deviation. Temperature overshoot at the onset of nucleate boiling was observed for all the surfaces by thermal history. Although V-grooved surface also enhanced the nucleation sites by increasing heat transfer area, temperature deviation was only observed on the TSC surface. At an inlet subcooling of 30 K, mass flux and heat flux were 300 kg/(m2 center dot s) and approximatively 320 kW/m2, respectively, the HTC difference was 4.58 kW/(m2 center dot K) (24%). Balance effect by mass fluxes was emphasized because bubbles departure was accelerated while the heat removed by generated bubbles was reduced due to weakened nucleation sites and bubble diameter.
This study addresses the trade-off between heat transfer enhancement and pressure penalty by investigating three novel bio-inspired turbulators (Wavy-Cell, Fish-Scale, Dragon-Egg). Comprehensive experiments were conducted under fully turbulent conditions (Re = 10,500- 15,000), integrated with Second-Law thermodynamic analysis and multi-objective optimization. The primary innovation lies in combining biomimetic geometries with entropy generation minimization. Results indicate that the 3-piece Wavy-Cell configuration achieved the highest thermal performance (TPF = 1.11) and a 15.4% reduction in total entropy generation compared to a plain pipe. Response Surface Methodology (RSM) yielded high-accuracy predictive models (R2 > 98%). Furthermore, a multi-criteria decision-making framework using Pareto-TOPSIS was employed to simultaneously optimize conflicting objectives: maximizing Nusselt number while minimizing friction factor and entropy generation. This analysis identified the 2-piece Fish-Scale configuration at Re = 15,000 as the optimal solution, offering the best thermodynamic balance despite having a slightly lower TPF than the Wavy-Cell. These findings validate the efficacy of bio-inspired designs for next-generation energy-efficient thermal systems.
Artificial ground freezing faces challenges under groundwater seepage. This study experimentally investigates thermal evolution in water-rich sandy gravel strata during freezing and thawing processes. Results show that 34.6 m/d seepage extends frozen wall closure to 175 min, with the vault and upstream sidewall becoming critical weak zones failing design requirements. Forced thawing reduces duration by 83.1% compared to natural thawing. Under coupled seepage and forced thawing, a dual-directional thawing mechanism is observed. The upstream zone thaws faster, suggesting that terminating heating in upstream pipes early can optimize energy efficiency. These findings provide quantitative benchmarks for underground engineering in high-seepage strata.
This study investigates nucleate boiling under subcooled forced flow conditions on aluminum using distilled water, water-50% ethylene glycol aqueous solution, and Long-Life Coolant. Bubble dynamics, surface morphology, coolant viscosity effects, and corrosion-related changes in heat transfer were analyzed. Distilled water initially showed efficient boiling, but surface oxidation reduced performance over time. Water-50% ethylene glycol aqueous solution exhibited bubble sliding, limiting vapor film formation. Long-Life Coolant's high viscosity and hydrophobicity led to bubble stagnation and poor heat transfer. Findings emphasize the roles of coolant properties and surface degradation, suggesting that surface treatments are key to sustaining efficient boiling in long-term thermal management applications.
The two-phase immersion cooling of high heat flux generating electronic components is becoming essential as it enables high heat dissipation at lower wall superheat temperature. In the present study, pool boiling heat transfer performance of SES36 refrigerant on copper heater surface is investigated under saturated boiling conditions. The effect of saturation pressure on critical heat flux and heat transfer coefficient is studied by varying saturation pressure from 75 kPa to 200 kPa. The critical heat flux and heat transfer coefficient enhanced by 3.5% and 92.3%, respectively, with an increase in saturation pressure from 75 kPa to 200 kPa.
Additive manufacturing (AM) enables the production of complex components with different materials, making it a promising approach for compact heat exchangers. This study investigates the feasibility of AM for fabricating polymer-based heat exchangers using the LCD photopolymerization technique with ABS resin. Three heat exchangers were manufactured: one with straight channels and two with complex 3D geometries (V-shape and Honeycomb). Experimental tests were performed to evaluate thermal performance under different mass flow rates and temperatures, measuring both heat transfer rates and pressure drops. An analytical model, based on the epsilon-NTU method, was developed to predict the thermal behavior of the straight-channel exchanger. The model demonstrated good agreement with experimental data, with an average difference of about 10%. In contrast, the heat exchangers with complex channels exhibited lower thermal efficiency than the straight-channel exchanger. The reduced performance of the chaotic geometries was attributed to the large wall thickness and low thermal conductivity of the polymer material. Although the chaotic channels increased surface area and induced turbulence, the low thermal conductivity limited the overall heat transfer. This study demonstrates that optimizing channel geometry alone is insufficient to ensure high performance; wall thickness and material selection are critical factors for achieving efficient heat exchangers. The results suggest that AM is a viable technology for fabricating compact polymer-based heat exchangers, with potential for further improvement through enhanced print quality, reduced wall thickness, and the use of materials with higher thermal conductivity.
This study evaluates the thermal performance of an additively manufactured mini-channel cooler produced via stereolithography. The heat sink features internal aluminum mini-channels encased in ABS. Six models were tested: three with varying outer casing geometries (A1-A3) and three with different heat sink geometries (B1-B3) - under constant heat flux from an electronic chip, using water as coolant. Numerical simulations and experiments assessed chip temperature, Nusselt number, pressure drop, Performance Evaluation Factor, and thermal resistance across mass flow rates of 0.001-0.007 kg/s. The model A3 exhibited a significant improvement in the average Nusselt number, reaching an increase of 53.3% at a mass flow rate of 0.001 kg/s. In contrast, Model B2 showed an improvement of 8.78% compared to Model B1 at the same flow rate, despite a substantial increase in pressure drop, which was approximately 300% higher than that of Model A3. The highest thermal resistance was observed in Model A2, with an increase of 32%, whereas Model B2 demonstrated the lowest thermal resistance, with a decrease of 23.6%. The findings demonstrate the promise of polymer-based, 3D-printed coolers for compact electronics, combining low weight, durability, and effective heat management.
With the rapid advancement of electronic and high-power devices in terms of performance and miniaturization, the need for efficient heat dissipation technology has become urgent. This research experimentally explores the effects of fin array shapes (circular, diamond, and drop-shaped) on boiling heat transfer and pressure drop characteristics of the dielectric fluorinated fluid Novec-649 under various mass flow rates, inlet temperatures, and vapor qualities. The results from high-speed camera show that the circular fin achieves the longest two-phase flow region, which is about 8.3% and 3.5% greater than that of diamond fin and drop fin. The extended two-phase region increases the phase-change heat transfer area and heat transfer coefficient, thereby improving the overall thermal performance of the circular fin. However, due to the shorter leading edge, drop-fin shows the lowest pressure loss across the heat sink among the three types of structures. Improved correlations for Nusselt number and pressure drop based on experimental data are proposed in this study with mean absolute percentage errors less than 10%.
