1,720,963 research outputs found
Enhancing thermal comfort: a comprehensive review of wearable cooling systems
No full textExposure to hot environments can induce physiological thermal strain in the human body, leading to reduced working endurance, impaired performance, and an elevated risk of heat-related illnesses. Activities such as sports, military training, and physically demanding work like firefighting can worsen these conditions. The increasing demand for energy-efficient solutions and diverse application requirements has driven the development of wearable cooling systems. These systems offer a localized and efficient alternative to conditioning entire environments, especially for individuals working outdoors or in settings where traditional air conditioning is impractical. This review provides a comprehensive overview of wearable cooling systems, covering their operating principles, designs, testing methodologies, applications, benefits, challenges, and classifications. Wearable cooling systems have been categorized into active, passive, and hybrid types, employing various cooling mechanisms, including air cooling, liquid cooling, vapor-compression cycle cooling, thermoelectric cooling, gas cooling, vacuum desiccant cooling, evaporative cooling, phase change materials, and conductive and radiative textile-based cooling. The review assesses these technologies based on cooling capacity, weight, and operating time, offering a rationale for their selection. Additionally, insights into future research opportunities in wearable cooling systems are discussed, emphasizing the need for continued innovation to enhance thermal comfort and safety
Heat transfer enhancement during dropwise condensation over wettability-controlled surfaces
No full textDropwise condensation (DWC) is a complex heat transfer process in which vapor phase changes to liquid phase forming discrete droplets on a surface whose temperature is below the dew temperature of the condensing fluid. DWC mode can strongly enhance the heat transfer compared to filmwise condensation (FWC) mode that usually takes place when a vapor condenses over a metallic surface. The wettability of the surface plays a crucial role on the promotion of DWC instead of FWC. This Chapter is focused on heat transfer measurements and modeling during DWC. The first two Sections are dedicated to a short literature review and to the description of the experimental procedures that can be used for the measurement of the heat transfer coefficient. DWC involves millions of droplets per square meter that form the so called droplet population. Section 3 is dedicated to the description of the droplet size distribution. Section 4 presents selected models that can be used for the prediction of the heat transfer during DWC. Formed droplets can be removed from the condensing surface by gravity or by other external forces. In the literature, most of the DWC experimental data are taken with quiescent vapor and very few works investigate the effect of the vapor drag force on the droplet departing radius and thus on the heat transfer during DWC. Furthermore, the effect of vapor velocity is not accounted for in available DWC models. Therefore, the last Section of this Chapter is focused on heat transfer modeling in presence of vapor velocity. A recent approach proposed by the present authors to account for the reduction of droplets departing diameter due to vapor velocity is here presented. The model is then used to show the effect of the main parameters on the DWC heat transfer coefficient
Drag effect of steam flow on droplet removal during dropwise condensation at different surface inclinations
Full text linkDropwise condensation is a quasi-cyclic process characterized by the nucleation, growth, and removal of discrete liquid droplets on a subcooled surface. The removal of condensate is a critical aspect, usually achieved by exploiting the gravity force, the drag force of vapor or the surface wettability gradient. This paper presents an experimental study of the vapor drag action on condensate removal, with focus on droplet's departing radius (rmax). Specifically, for the experimental campaign, vapor velocity was varied from 3 to 14 m s−1 considering three different surface inclinations: vertical, 45° inclined, and horizontal. The results showed that, as the velocity increases, the difference in departing radii among the three different configurations decreases and, consequently, the difference in heat transfer coefficients decreases too. In fact, at the highest vapor velocity (~14 m s−1), rmax was almost equal for all the inclinations leading to similar heat transfer coefficients (~120 kW m−2 K−1). Interestingly, on a horizontal surface considering vapor velocity equal to 3 m s−1, despite the lack of gravity's contribution to droplet removal, no transition to filmwise condensation was observed
Dropwise condensation mechanisms when varying vapor velocity
No full textThe promotion of dropwise condensation (DWC) has been identified as an effective strategy to significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). Understanding the mechanisms governing dropwise condensation on modified wettability surfaces is crucial for a wide range of energy applications. In the literature, most of the experimental data are collected during DWC with quiescent vapor. On the other hand, in industrial applications, the vapor to be condensed can have a non-negligible velocity which is expected to affect the droplet population on the condensing surface and the heat transfer. However, measurements of heat transfer coefficient and droplet population with flowing steam are rare and the effect of vapor velocity on the drop-size distribution, which is a key parameter in DWC modeling, needs to be investigated.
In the present work, the effect of steam velocity during DWC is experimentally studied on two different specimens: a sol–gel coated aluminum sample and a reduced graphene oxide coated copper sample. HTC, droplet departing radius and drop-size distribution measurements are performed at constant saturation temperature and heat flux, while varying the inlet vapor velocity in the range between 3 and 15.5 m s-1. Due to the increase of the vapor drag force on the droplets, a reduction of the droplet departing radius is observed along with an increase of
the condensation HTC. The vapor flow is found to affect the droplet population and, in particular for the largest droplets radii, the drop-size distribution function is lowered when increasing vapor velocity. The experimental data are used to assess a model for the estimation of the droplet departing radius in presence of vapor velocity previously proposed by the present authors. As a second step, the equation for the calculation of the droplet departing radius is coupled with available models for droplet population and heat transfer through a single droplet to model the whole DWC process. The proposed calculation method is able to predict the effect of vapor velocity on the DWC heat transfer coefficient
Effect of steam velocity during dropwise condensation
No full textDropwise condensation (DWC) is a complex phenomenon involving droplets nucleation, coalescence and motion. Starting from the nanoscale up to the macroscale, DWC involves millions of droplets per square meter. The maximum dimension assumed by a droplet before sliding is characterized by the departing radius: it is the radius at which the droplet starts to sweep through the surface as a consequence of the acting forces (gravity force, adhesion force, drag force induced by the flowing vapor). In the literature, very few works investigate the effect of vapor velocity on the heat transfer coefficient (HTC) and on the droplet departing radius during DWC. Furthermore, the effect of vapor velocity is not accounted for in available DWC models. In the present paper, DWC of steam has been promoted on an aluminum sol-gel coated surface. Heat transfer coefficients and droplets departing diameters have been measured at 107 °C saturation temperature, heat flux of 335 kW m−2 and average vapor velocity between 2.7 m s−1 and 11 m s−1. A method for the estimation of the droplet departing radius in presence of non-negligible vapor velocity is here proposed. The equation accounting for vapor velocity has been included in the model by Miljkovic et al. [1] for heat transfer coefficient prediction during DWC and has been assessed using the present data and two other datasets from independent laboratories
Experimental analysis of drop size distribution and nucleation site density during dropwise condensation from humid air flow
Full text linkCondensation of the water vapor present in the air is a heat and mass transfer process
encountered in many applications as humid air dehumidification and water harvesting.
Depending on the wettability characteristics of the surface, condensation can take place in
filmwise mode or in dropwise mode with the formation of discrete liquid droplets over the
condensing surface. While dropwise condensation (DWC) of pure steam was found to promote
a considerable enhancement of the heat transfer compared to filmwise condensation, when
dealing with humid air DWC more investigation is needed. Modeling of DWC from humid air
requires the calculation of the heat flow rate through a single droplet and the determination of
the drop-size distribution. The heat exchanged through a single droplet depends on the heat and
mass transfer resistances, while the drop-size distribution is also affected by nucleation site
density and droplets mobility. Therefore, to better understand the DWC phenomenon with humid
air and for the validation of the models, it is necessary to measure the heat flux (total and latent),
droplet population and nucleation site density. In the present work, condensation tests from
humid air are performed over two square (40 mm x 40 mm) aluminum samples that display
different wettability. The experimental apparatus consists of a closed air loop with two main
components: the environmental chamber and the test chamber. The air is conditioned in the
environmental chamber and then it flows inside the test section where the vapor present in the
humid air is condensed over the vertical metallic sample. Two variable speed fans are used to
circulate the air. The test section is designed for heat and mass transfer measurements and for
simultaneous visualization of the condensation process. As a peculiar characteristic of the present
experimental technique, all the test section assembly is suspended on a high precision balance
allowing a precise measurement of the mass of condensate. The effect of surface wettability on
the heat and mass transfer during DWC is investigated. Time-lapse videos of the condensation
process are acquired at different magnifications. By using a homemade MATLAB® program for
droplet detection, recorded images are analysed allowing the determination of both the drop size
density distribution (small and large droplet population) and the nucleation sites density
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The advantage of running a direct expansion CO2 heat pump with solar-and-air simultaneous heat sources: experimental and numerical investigation
Full text linkDual-source solar-air heat pumps represent a promising solution for overcoming the limitations associated with single-source utilization, thereby enhancing heat pump performance. However, running the heat pump by alternatively employing the more advantageous source requires the integration of a controller capable of continuously monitoring and predicting the heat pump 's performance in response to dynamic environmental and operational variables. Even so, a selective alternate operation does not allow to get the maximum possible performance from the use of the two heat sources. A different approach to address this challenge is the simultaneous utilization of the two sources, by properly combining two evaporators in the CO 2 circuit. This paper presents an experimental investigation of a dual-source heat pump using CO 2 as refrigerant, which can operate in three different evaporation modes: air-mode (using a finned-coil evaporator), solar -mode (using a photovoltaicthermal PV-T evaporator), and simultaneous-mode (using both the evaporators simultaneously). The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators. Experimental data indicate that the heat pump operating in simultaneous-mode allows to increase the evaporation pressure and the coefficient of performance compared to operation in air-mode or solar -mode. The measurements have been employed for validating a model of the system, capable of predicting steady-state and dynamic performance under various environmental and operational conditions. Simulation results show that the simultaneous-mode operation can be outperformed by the solar -mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature. Finally, the impact of the number of PV-T collectors and solar irradiance on the heat pump performance has been simulated and discussed. On this regard, the simultaneous use of the two heat sources adds more flexibility to the system and its design, because even the availability of a small solar area can contribute enhancing the performance over the mere air source heat pump
Investigation of dropwise condensation of water through an efficient individual-based model
Full text linkIn recent years, researchers have directed their studies towards solutions aimed at enhancing heat exchangers effectiveness. In this context, dropwise condensation (DWC) has been identified among the most promising solutions to increase the condensation heat transfer coefficient (HTC). In fact, DWC provides heat transfer coefficients up to ten times higher than those achievable during filmwise condensation (FWC), resulting in both economic and energy benefits. The DWC phenomenon is usually modelled by combining the heat exchanged through a single droplet and the drop-size distribution. The latter can be divided into a distribution of large droplets N(r), determinable analytically by semi-empirical models, and a distribution of small droplets n(r), typically determined through statistical approaches called population-based models. Another possibility for the determination of the droplet-size density is to simulate the DWC process by an individual-based model (IBM). In this case, each drop is tracked throughout its entire life cycle (nucleation, growth, coalescence, sliding), and the drop-size distribution is obtained as a result. In this paper, a new IBM for the simulation of DWC of steam is proposed. The developed model allows for the simulation of more than 10 million droplets while keeping an acceptable simulation time thanks to the implementation of parallel computing. The predictions obtained from the model, in terms of drop-size distribution and condensation heat flux, are compared against both PBM results and experimental data
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