Search Results (59)

This study presents a comprehensive experimental investigation into the frictional pressure drop of two-phase flows—boiling and condensation—in horizontal minichannels, emphasizing its impact on the energy efficiency of vapor compression systems. A total of 3553 data points were obtained using six low-GWP refrigerants (R32, R134a, R290, R410A, R513A, and R1234yf) across a wide range of operating conditions in multiport aluminum tubes with hydraulic diameters of 0.715 mm and 1.16 mm. The dataset covers mass fluxes from 200 to 1230 $\mathrm{kg}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, saturation temperatures between 5 °C and 55 °C, and vapor qualities from 0.05 to 0.95. Results showed a strong dependence of frictional pressure gradient on vapor quality, mass flux, and channel size. Boiling flows generated higher frictional losses than condensation, and high-density refrigerants such as R32 exhibited the largest pressure penalties, which can directly translate into increased compressor power demand. Conversely, higher saturation temperatures were associated with lower frictional losses, highlighting the role of thermophysical properties in improving energy performance. Additionally, an inverse correlation between saturation temperature and frictional pressure gradient was observed, attributed to variations in thermophysical properties such as viscosity and surface tension. Existing correlations from the literature were assessed against the experimental dataset, with notable deviations observed in several cases, particularly for R134a under high-quality conditions. Consequently, a new empirical correlation was developed for predicting the frictional pressure drop in two-phase flow through minichannels. The proposed model, formulated using a power-law regression approach and incorporating dimensionless parameters, achieved better agreement with the experimental data, reducing prediction error to within ±20%, improving the accuracy for the majority of cases. This work provides a robust and validated dataset for the development and benchmarking of predictive models in compact heat exchanger design. By enabling the more precise estimation of two-phase pressure drops in compact heat exchangers, the findings support the design of refrigeration, air-conditioning, and heat pump systems with minimized flow resistance and reduced auxiliary energy consumption. This contributes to lowering compressor workload, improving coefficient of performance (COP), and it ultimately advances the development of next-generation cooling technologies with enhanced energy efficiency. Full article

This study proposes a new air treatment system that integrates dehumidification, cooling, and water recovery using a Horizontal Air–Ground Heat Exchanger (HAGHE) combined with Peltier cells. The airflow generated by a fan flows through an HAGHE until it meets a septum on which Peltier cells are placed, and then separates into two distinct streams that lap the two surfaces of the Peltier cells: one stream passes through the cold surfaces, undergoing both sensible and latent cooling with dehumidification; the other stream passes through the hot surfaces, increasing its temperature. The two treated air streams may then pass through a mixing chamber, where they are combined in the appropriate proportions to achieve the desired air supply conditions and ensure thermal comfort in the indoor environment. A Computational Fluid Dynamics (CFD) analysis was carried out to simulate the thermal interaction between the HAGHE and the surrounding soil. The simulation focused on a system installed under the subtropical climate conditions of Nairobi, Africa. The simulation results demonstrate that the HAGHE system is capable of reducing the air temperature by several degrees under typical summer conditions, with enhanced performance observed when the soil is moist. Condensation phenomena were triggered when the relative humidity of the inlet air exceeded 60%, contributing additional cooling through latent heat extraction. The proposed HAGHE–Peltier system can be easily powered by renewable energy sources and configured for stand-alone operation, making it particularly suitable for off-grid applications. Full article

Earth materials in construction demonstrate significant potential attributed to their accessibility, recyclability, and low energy demands for processing. Modern techniques have enhanced their mechanical strength and durability, enabling their application in load-bearing and infill walls while preserving ecological benefits. However, existing studies on indoor heat–humidity regulation primarily emphasize material parameters and macro-level performance. Moreover, the dynamic interactions between the unique thermal storage–release mechanisms and indoor environments have not been systematically analyzed. With the Kelvin equation, capillary mechanics, adsorption theories, and microstructural analysis were integrated in this study to quantify cyclic capillary condensation and evaporation in microvoids. The results reveal that earth materials contain abundant medium-sized pores (19.85–53.83 nm) sustaining vapor exchange with their surroundings. Capillary condensation occurs 0.86–0.96 times the planar surface vapor pressure, influenced by pore size (negatively correlated) and temperature (negatively correlated). During the daytime, capillary evaporation occurs in the nanopores of the raw earth wall under the influence of the outdoor environment’s cyclical temperature and humidity. This process absorbs heat from the indoor environment and raises the ambient humidity. During the nighttime, capillary condensation occurs in the pores, releasing heat to the indoor area and absorbing moisture from the environment, contributing to the balance of the indoor thermal environment of the earth buildings. The findings lay a scientific foundation for quantitatively evaluating earth buildings’ indoor climate control performance, supporting their integration into green building systems. This research bridges knowledge gaps in micro-to-macro thermal dynamics while advancing the ecological optimization of materials for sustainable architecture. Full article

The paper presents a numerical analysis of a tube-in-tube condenser of a small refrigeration system. One of the challenges in designing such units is to reduce their dimensions while maintaining the highest possible cooling capacity, so the research presented here focuses on the search for and impact of the appropriate flow conditions of these two fluids on condenser performance. The refrigerant is supercritical CO₂, which is cooled by water. Thermal-flow simulations were performed for eight CO₂ inlet velocities in the range of 1–8 m/s, and four cooling water velocities of 0.5–2 m/s. The main parameters of the exchanger operation were analyzed: heat transfer coefficient, Nusselt number, overall heat transfer coefficient, and friction factor, which were compared with selected correlations. The results showed that the condenser achieves the highest power for the highest water velocities (2 m/s) and CO₂ (8 m/s), i.e., over 1000 W, which corresponds to a heat flux on the tube surface of approx. 2.6 × 10⁵ W/m² and a heat transfer coefficient of approx. 4700 W/m²K. One of the most important conclusions is the discovery of a significant effect of water velocity on heat transfer from the CO₂ side—an increase in water velocity from 0.5 m/s to 2 m/s results in an increase in the heat transfer coefficient sCO₂ by over 60%, with the same Re number. The implication of this study is to show the possibility of adjusting and selecting condenser parameters over a wide range of capacities, just by changing the fluid velocity. Full article

When the vapour–gas mixture flow heats the cold walls of a heat exchanger, condensed phase (solid and liquid) precipitation can occur on their surfaces. This study aims to improve a model of thermohydraulic processes in a heat exchanger during condensed phase precipitation on its cold surfaces. The process is considered to occur when a multi-component solid-phase layer and a liquid film are simultaneously formed on the wall. Heat is transferred to the interface surface through radiation and convection and due to the phase transition of diffusing components. The mass flow to the interphase surface is determined for each diffusing component. The developed model allows for the calculation of heat transfer parameters in both steady-state and transient conditions, taking into account the formation of a multi-component condensed phase on cold walls. Full article

Coastal lagoons play an essential role in the energy balance and heat exchange to the atmosphere. Furthermore, at mesoscale Monsoon systems and at local scales, sea breeze influences surface processes; however, there is a lack of information on such processes in arid and semiarid regions. We aimed to characterize the atmospheric conditions during sea and land breeze in different seasons and analyze at different temporal scales the variation of atmospheric stability, turbulent fluxes, lifting condensation level, and atmospheric boundary layer height. The study site is a subtropical semiarid coastal lagoon, Estero El Soldado, located in Northwestern Mexico (27°57.248′ N, 110°58.350′ W). Measurements were performed from January 2019 to September 2020 with an Eddy Covariance system (EC) and micrometeorological instruments over the water surface. Results show that there is a strong seasonality that enhances sea–land breeze dominance; sea breeze was 83% more frequent during the Monsoon, and the land breeze was 55% more frequent in the Post-Monsoon. Specific humidity (23.32 ± 3.84 g kg⁻¹, q), potential temperature (307 ± 2.98 K, θ_p), latent heat (135 W m⁻², LE), and turbulent kinetic energy (0.81 m² s⁻², TKE) were significantly higher during the Monsoon season at sea breeze events. Atmospheric boundary layer (ABL) and lifting condensation level (LCL) were higher in the Pre-Monsoon season (3250 ± 71 m and 1142 ± 565 m, respectively). During the Monsoon, surface conditions lead to lower LCL (~800 m) due to the amount of water vapor (q = 23.3 g kg⁻¹). Full article

This review article highlights the critical impact of surface roughness in modifying the structure of two-phase flow within mini- and microchannels, particularly in processes such as boiling and condensation. Channel surface roughness enhances flow resistance, affects the distribution of vapor bubbles, and enhances heat transfer by providing additional nucleation sites. Several experiments have shown that while increased surface roughness enhances the efficiency of heat transfer, increased flow resistance may hurt system performance. This is so because too high a surface roughness negatively impacts flow resistance, a factor of importance in the optimization for a balance between heat transfer and flow resistance, especially in high-performance compact heat exchangers. Furthermore, the review identifies that higher-degree measurement and characterization techniques of the surface roughness are increasingly required, as traditional 2D parameters may not fully represent the actual physics of complex surface interactions in two-phase flow systems. Consequently, the article calls for further research that can examine the exact relationship between roughness, flow structure, and thermal performance with the aim of improving design strategies for future heat exchanger technologies. Full article

Due to its good corrosion resistance and high rigidity, fluoroplastic steel heat exchangers have become one of the main feasible solutions to the problem of flue gas waste heat recovery. Therefore, a heat transfer performance test of a fluoroplastic steel heat exchanger was carried out, and a new heat transfer correlation (Nu = 0.11Re^0.72Pr^0.36, 1900 < Re < 4100, Pr ≈ 0.8) of a fluid transverse tube bank suitable for smooth fluoroplastic surfaces was proposed. Comparing the results of the proposed correlation and the Zhukauskas correlation, the calculation discrepancy was decreased when using the proposed correlation, and the average discrepancy dropped from 16.8% to 3.3%. As the thickness of the fluoroplastic film was reduced, the gap between the two correlations widened. Thus, the Zhukauskas correlation was not applicable to fluoroplastic steel heat exchangers with a thin fluoroplastic film. However, the deviation between the two correlations decreased as the condensing heat transfer area increased. For the fluoroplastic steel heat exchanger with a large condensation heat transfer area, the Zhukauskas correlation could still be utilized. It was interesting to observe that the deviation between the two correlations was reduced when only flue gas condensation was present, but it showed an increasing trend with the increase in condensation intensity. Full article

The study describes experimental data on thermal tests during the condensation of HFE7100 refrigerant in a compact heat exchanger. The heat exchanger was manufactured using the additive 3D printing in metal. The material is AISI 316L steel. MPCM slurry was used as the heat exchanger coolant, and water was used as the reference medium. The refrigerant was condensed on a bundle of circular tubes made of steel with an internal/external diameter of d_i/d_e = 2/3 mm, while a mixture of water and phase change materials as the coolant flowed through the channels. Few studies consider the heat exchange in condensation using phase change materials; furthermore, there is also a lack of description of heat exchange in small-sized exchangers printed from metal. Most papers deal with computer research, including flow simulations of heat exchange. The study describes the process of heat exchange enhancement using the phase transition of coolant. Experimental data for the mPCM slurry coolant flow was compared to the data of pure water flow as a reference liquid. The tests were carried out under the following thermal and flow conditions: G = 10–450 [kg m−² s⁻¹], q = 2000–25,000 [W m⁻²], and t_s = 30–40 [°C]. The conducted research provided many quantities describing the heat exchange in compact heat exchangers, including heat exchanger heat power, heat exchange coefficient, and heat exchange coefficients for working media. Based on these factors, the thermal performance of the heat exchanger was described. External characteristics include the value of the thermal power and the heat exchange coefficient as a function of the mass flow density of the working medium and the average logarithmic temperature difference. The performance of the heat exchanger was presented as the dependencies of the heat exchange coefficients on the mass flux density and the heat flux density on the heat exchange surface. The thickness of the refrigerant’s condensate film was also determined. Furthermore, a model was proposed to determine the heat exchange coefficient value for the condensing HFE7100 refrigerant on the outer surface of a bundle of smooth tubes inside a compact heat exchanger. According to experimental data, the calculation results were in good agreement with each other, with a range of 25%. These data can be used to design mini condensers that are widely used in practice. Full article

Phase Change Materials (PCMs), among the existing thermal storage technologies, are characterized by higher storage densities than conventional storage systems, and absorb and release thermal energy at nearly constant temperatures. In recent years, the potential advantages that can be obtained by the integration of these materials into refrigeration machines have attracted the attention of specialized literature. Indeed, PCMs can allow a more efficient operation through an appropriate increase in thermal inertia, for applications relative to air conditioning in both internal residential environments and inside vehicles for the transport of people, and also in the case of machines used in the field of food refrigeration. Furthermore, in recent years, innovative solutions with integrated PCM have also been analyzed, aiming at enhancing the usability and transportability of refrigeration systems, as well as increasing the energy efficiency and reducing environmental impact. In this context, the present work focuses on the experimental characterization and numerical simulation of a cooling system with integrated PCM. In particular, the cooling system, designed for a personal cooling application, is experimentally analyzed by varying the configuration of the PCM-based condenser, while the numerical simulations have been realized to validate a simulation tool that could be used for the design and optimization of the PCM condenser configuration. The results allow us to identify the main characteristics of the analyzed personal cooling system, namely, the cooling capacity and operating autonomy, and to point out the utility and the limits of the developed simulation tool. Among the various configurations analyzed, the best one in terms of refrigeration power and autonomy is the one characterized by the highest heat transfer surface of the heat exchanger, with the refrigerant compressor at 50% power. Full article

ZE41 is a magnesium alloy used in heat exchangers, condensers, reactors, and pressure vessels where good surface qualities are required. This current research focuses on the investigation of the striation angle (SA), surface roughness (SR), and striation zone (SZ) in ZE41, using abrasive waterjet cutting. Significant variables in the investigation were jet pressure, traverse speed, mass flow rate, and stand-off distance. In accordance with Taguchi’s L18 orthogonal array, the responses for each cut test were studied. In addition, the principal component-based grey incidence (PGI) technique successfully combined the strengths of the optimization tool to identify the ideal parameter condition. The confirmation results revealed that the PGI technique improved SR by 4.02%, SZ by 6.67%, and 1.48% in the SA. Full article

The low-low-temperature electrostatic precipitator (LLT-ESP) is considered one of the mainstream technological approaches for achieving ultra-low ash emissions and has already been applied in many coal-fired power plants. Particulate matter and SO₃ can both be removed by LLT-ESP. However, the removal performance of SO₃ is relatively lower than that of particulate matter, which is caused by the condensation characteristics of SO₃. In this paper, the condensation characteristics of SO₃ were investigated on a simulated experimental system, and several measurement and characteristic methods were used to investigate mechanisms. After reducing the flue gas temperature with a heat exchanger, the size distribution of particulate matter, the mass concentration of SO₃ on different sizes of particulate matter, as well as the microscopic morphology and elemental composition of particulate matter, were all experimentally studied. The results indicate that gaseous SO₃ transformed into a liquid phase by heterogeneous or homogeneous condensation and then adhered to the surface of particulate matter through nucleation–condensation, collision–coalescence, and adsorption reactions. Furthermore, the removal efficiency of SO₃ in LLT-ESP was also investigated under various conditions, such as ash concentration and flue gas temperature drop, suggesting that a higher ash concentration and a more significant temperature drop were beneficial for improving SO₃ removal efficiency. Nevertheless, it is worth noting that the impact was limited by a further increase in ash concentration and a drop in flue gas temperature. Full article

Laser technologies for processing metals used as heat exchange surfaces are unrivaled to solve a number of problems in the energy industry. This is explained by the fact that after laser radiation treatment, metal surfaces gain unique surface functional properties (extreme wettability properties, high resistance to corrosion in contact with traditional coolants, high abrasive and cavitation resistance). The study of the processes of evaporation, boiling, and condensation on such surfaces is hampered by one of the unsolved problems, which is the lack of the ability to predict the configuration of microtextures, for example, in the form of micropillars and microchannels with predetermined sizes. In this work, a graphic–analytical technique based on the use of ablation spot sizes for the formation of a given configuration and microtexture dimensions on traditional structural materials of heat exchange surfaces is developed. Based on experimental data, regime maps were constructed for the formation of microtextures on the surfaces of aluminum alloy AlMg6 and steel AISI 310. The prospects for using metal surfaces with a given microtexture formed by laser radiation to intensify the phase transition of coolants and control convective flows in a droplet lying on a heated surface were assessed. The obtained results can be used in the development of spray (drip) irrigation systems to provide thermal protection for heat-stressed equipment. Full article

The subseasonal variability of the low-cloud fraction (LCF) over the southeastern North Pacific (SENP) and northwestern North Pacific (NWNP) was studied using satellite observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is found that subseasonal variability of the LCF was closely related to variations in the estimated inversion strength (EIS), sea surface wind speed (SSW), sensible heat flux (SHF), sea surface temperature (SST), surface temperature advection (Tadv), relative humidity (RH), surface level pressure (SLP) and surface air temperature (SAT). An increase in the LCF over the SENP is associated with the development of an anomalous anticyclonic circulation, which is located on the west coast of America. The cold advection, together with the subsidence warming associated with the anticyclonic circulation, strengthens the temperature inversion, favoring the development of the LCF. In the NWNP, the maximum LCF anomaly was also correlated with the stable boundary layer. The southerly wind blows airflow over the Kuroshio Extension from the subtropics, which brings warm and moist air. When air flows to the colder sea surface, it is cooled and condensed by the intensified heat exchange. A lead-lag composite analysis indicates that the mechanisms are different between the SENP and the NWNP, possibly due to the different types of low-level clouds over these two regions. In the SENP, the trade cumulus dominates under a strong capping inversion over the subtropics, whereas fog and stratus often occur under a shallow capping inversion in the NWNP. The effects of atmospheric circulation are also discussed. Full article

Cogeneration or combined heat and power (CHP) has found wide application in various industries because it very effectively meets the growing demand for electricity, steam, hot water, and also has a number of operational, environmental, economic advantages over traditional electrical and thermal systems. Experimental and theoretical investigations of the afterburning of fuel oil in the combustion engine exhaust gas at the boiler inlet were carried out in order to enhance the efficiency of cogeneration power plants; this was achieved by increasing the boiler steam capacity, resulting in reduced production of waste heat and exhaust emissions. The afterburning of fuel oil in the exhaust gas of diesel engines is possible due to a high the excess air ratio (three to four). Based on the experimental data of the low-temperature corrosion of the gas boiler condensing heat exchange surfaces, the admissible values of corrosion rate and the lowest exhaust gas temperature which provide deep exhaust gas heat utilization and high efficiency of the exhaust gas boiler were obtained. The use of WFE and afterburning fuel oil provides an increase in efficiency and power of the CPPs based on diesel engines of up to 5% due to a decrease in the exhaust gas temperature at the outlet of the EGB from 150 °C to 90 °C and waste heat, accordingly. The application of efficient environmentally friendly exhaust gas boilers with low-temperature condensing surfaces can be considered a new and prosperous trend in diesel engine exhaust gas heat utilization through the afterburning of fuel oil and in CPPs as a whole. Full article