Thermal Capacity Research Articles

Effect of Temperature on the Embryogenesis of Three Geographically Distinct Populations of Meloidogyne incognita Is Driven by Intrinsic Thermal Acclimation Reaction.

Research interest in the mechanisms enabling plant-parasitic nematodes to adjust their physiological performance and cope with changing temperatures has intensified in light of global warming. Here, we show that geographically distinct populations of the root-knot nematode Meloidogyne incognita, which is prevalent in the three main pepper-growing regions in Israel-Carmel Valley (Carmel), Jordan Valley (JV), and Arava Rift (Arava)-possess persistent differences in their thermal acclimation capacity, which affect pre- and postembryonic development. The optimal temperature for embryonic growth completion was 25°C for the Carmel population; 25 and 30°C for the JV population; and 30°C for the Arava population. Cumulative hatching percentages showed variations among populations; relative to hatching at 25°C, the Carmel population experienced hatching reduction at the higher studied temperatures 30 and 33°C, while the JV and Arava populations exhibited an increase in hatching at 30 and 33°C, respectively. Juvenile survival indicates that at the lowest temperature (20°C), the Carmel population gained the highest survival rates throughout the experimental duration, while at the same duration at 33°C, the Arava population gained the highest survival rate. Infective juveniles of the Carmel population demonstrated increased penetration of tomato roots at 25°C compared to the JV and Arava populations. Inversely, at 33°C, increased penetration was observed for the Arava compared to the Carmel and JV populations. Altogether, the Arava population's performance at 33°C might incur distinct fitness costs, resulting in consistent attenuation compared to the Carmel population at 25°C. Precisely defining a population's thermal acclimation response might provide essential information for models that predict the impact of future climate change on these populations.

Comprehensive Performance Analysis of Hybrid Solar Water Heating Systems: Synergizing Photovoltaic and Thermal Technologies

Abstract: Solar water heaters are an efficient and sustainable technology for meeting the growing demand for hot water in residential and commercial applications. This abstract presents a comprehensive overview of the performance evaluation of solar water heater systems through an extensive experimental study. The study aims to assess the effectiveness and efficiency of different solar water heater configurations under varying operating conditions. The experimental investigation involved the measurement and analysis of key performance parameters such as solar collector efficiency, heat transfer efficiency, thermal storage capacity, and overall system efficiency. A range of factors including solar radiation intensity, ambient temperature, water flow rate, and collector tilt angle were systematically varied to examine their impact on system performance. The results of the experimental study demonstrated the significant potential of solar water heaters in providing cost-effective and environmentally friendly hot water solutions. The solar collector efficiency was found to be highly influenced by the solar radiation intensity, with higher levels of solar irradiation leading to increased thermal energy generation. The heat transfer efficiency was optimized by adjusting the water flow rate and collector tilt angle, ensuring efficient transfer of heat from the collector to the water. Furthermore, the thermal storage capacity of the system played a crucial role in maintaining a consistent supply of hot water during periods of low solar radiation or high demand. Overall system efficiency was found to be strongly dependent on the integration of efficient heat exchangers and insulation materials. Based on the experimental findings, recommendations for system design and optimization were formulated, highlighting the importance of proper sizing, selection of suitable materials, and appropriate control strategies. The results also identified areas for future research, such as the integration of advanced heat transfer enhancement techniques and the utilization of hybrid solar water heater systems

Municipal sludge-based low-carbon phase-change composite towards energy storage: Fabrication and investigation

The integration of municipal sludge with phase change materials for composite energy storage material fabrication benefits to sludge significant reduction and recycling, heavy metal fixation, and minimizing environmental pollution. This groundbreaking work proposed municipal sludge-derived phase-change composites utilizing potassium nitrate as phase change material to produce cost-effective, low-carbon thermal energy storage materials. Five samples with varying mass fractions of municipal sludge incineration ash and potassium nitrate were prepared through a cold-compression & hot-sintering process and then thermal properties, microscopic structure, mechanical strength, and chemical compatibility between components was investigated extensively to explore the feasibility of municipal sludge recycling for composite energy storage material fabrication. The results revealed successful encapsulation of heavy metals within the phase change composites, and the composite with a 50:50 ratio of ash to potassium nitrate exhibited superior overall performance, which boasts a thermal energy storage capacity of 330.34 J/g with temperature rising from 100 up to 380 °C, a peak thermal conductivity of 1.04 W/(m·K), remarkable mechanical strength of 153.78 MPa, and excellent thermal stability. In addition, the components of the municipal sludge incineration ash and potassium nitrate displayed exceptional chemical compatibility. This novel phase change composite holds significant potential for engineering application compared to traditional alternatives.

Increasing variable renewables in coal-based energy systems under high electrification in the transport and heating sectors: The case of Kosovo

The increasing penetration of variable renewables poses major challenges in energy systems with limited flexibility. However, there are many solutions to support variable renewable integration when utilizing potential synergies among the different energy sectors. This research examines the technical and environmental effects of electrifying the transport and heating sectors in a coal-based energy system with limited flexibility, aiming to support the integration of variable renewable energy. Scenario analysis is employed to evaluate the use of heat pumps with thermal storage (power-to-heat) for individual heating solutions, electric vehicles, and Vehicle-to-Grid in the transport sector. These measures are evaluated as the primary means for enhancing the energy system flexibility to accommodate a larger share of variable renewables. Additionally, a dynamic analysis was conducted on thermal power plant operational capacities and efficiencies under varying electrification rates and renewable energy shares. The analysis is developed in the EnergyPLAN model using Kosovo energy system as a case study. The results confirm that the electrification of the heating and transport sectors positively impacts the integration of variable renewable energy by reducing critical excess electricity production and CO2 emissions, while power-to-heat and Vehicle-to-Grid facilitate synergies among different energy sectors. However, scenarios with high shares of variable renewable energy present challenges for the nominal operation of thermal power plants, potentially leading to lower operational efficiencies and slightly higher CO2 emissions compared to scenarios where thermal power plant part-load efficiency remains unchanged.

Enhancement of Thermo-Physical Properties of Form-Stable Nano-Enhanced Phase Change Materials: Advancing Maritime Sustainability

This paper explores the application of novel, form-stable eutectic mixtures in thermal energy storage systems for maritime environments. These advanced materials present a significant leap forward, addressing critical challenges faced by conventional phase change materials (PCMs) in marine environments. The stability, eliminating leakage and fluidity issues encountered with liquid PCMs are ensured by incorporating 2-hydroxypropyl ether cellulose (HPEC) as a gelling agent. Additionally, the thermal properties and heat transfer capacities were significantly enhanced, and eventually, overall system efficiency improved by including nano-graphene platelets (NGPs). Notably, NGPs effectively suppress supercooling, minimizing energy losses and guaranteeing consistent performance at elevated temperatures. Further, the eutectic mixture demonstrates exceptional durability through an accelerated thermal reliability test, guaranteeing optimal performance over a projected seventy-year lifespan. This enhanced thermal performance, and enduring stability combination establishes the form-stable eutectic mixture with NGPs as an up-and-coming solution for diverse maritime applications requiring efficient and reliable thermal energy storage. This research provides a compelling case for implementing form-stable eutectic mixtures with NGPs in maritime thermal energy storage systems. Its superior performance and sustainability offer significant advantages for diverse shipboard and offshore applications, contributing to improved energy efficiency, environmental sustainability, and operational resilience within the maritime sector.

Synergistic enhancement of metal–organic framework-derived hierarchical porous materials towards photothermal conversion and storage properties of phase change materials

The design and synthesis of composite phase change materials that integrate photothermal conversion and latent heat storage have guiding significance for the efficient development and utilisation of solar energy. In this study, taking into consideration the advantages of metal–organic frameworks (MOFs) with adjustable structure and properties, HKUST-1-derived CuO/hierarchical porous carbon materials were employed and integrated with palmitic acid (PA) to synthesise composite phase change materials that exhibit exceptional comprehensive properties. The effects of different carbonation temperatures on HKUST-1-derived CuO/hierarchical porous carbon were analysed through a series of characterisations. The hierarchical porous structure in the blended system offers abundant pore structure and high specific surface area to prevent leakage of phase change materials. Composite phase change materials can maintain good shape stability and excellent thermal energy storage capacity. The thermal storage efficiency and photothermal conversion efficiency are 98.28% and 81.83%, respectively. Meanwhile, the thermal conductivity of the composite phase change material is 3.65 times that of pure PA. In conclusion, the MOFs derivative-based composite phase change materials designed in this study exhibited potential for thermal energy storage and can be applied to the field of solar energy conversion and storage systems.

Hydroxysulfobetaine foamer for potential mobility control application in high-temperature and ultra-high salt reservoirs

A good foamer should have good solubility in the formation of water, strong foaming ability, and also meet the requirements of thermal stability and low adsorption capacity under high temperature and high salinity conditions. The foam properties of hydroxylsulfobetaine surfactant with different carbon chain lengths were measured at 130 °C, 30 MPa, and in brine with a salinity of 22 × 104 mg·L−1. The results showed that hydroxylsulfobetaine surfactant with long carbon chains exhibited strong foam stability, with foam decay half-lives reaching 13 days for 0.4 wt% EHSB (erucamide propyl hydroxysulfobetaine) and 2.9 days for 0.4 wt% HSB18 (octadecyl dimethyl hydroxypropyl sulfobetaine), respectively. Hydroxysulfobetaine surfactant with short carbon chains had a foam decay half-life of less than 10 h. Considering the poor thermal stability of EHSB at 130 °C, HSB18 was primarily chosen as the foamer. HSB18 and HSB12 (dodecyl dimethyl hydroxypropyl sulfobetaine) were combined in a ratio of 3:1 to form the compound hydroxysulfobetaine MHSB31, which demonstrated good solubility in saline solution with KP (Krafft point) lower than 30 °C. The evaluation, using different concentrations of MHSB31, showed that the foam decay half-life ranged from 88 h for 0.03 wt% MHSB31 compared to 42 h for 0.1 wt% MHSB31. The excellent foaming performance of the compound system is attributed to its high surface dilational modulus and the formation of wormlike micelles. The adsorption capacity test demonstrated that when the concentration was less than 1 wt%, the adsorption capacity of MHSB31 on quartz sand surface was less than 1 mg·g−1, satisfying oilfield requirements. The results of the flow experiment at 130 °C and 10 MPa, in brine with a salinity of 22 × 104 mg·L−1, show that when the concentration of MHSB31 is greater than 0.1 wt%, the foam formed in the core with permeability of 632 mD and 2005 mD has a higher apparent viscosity and resistance factor. Additionally, the relative mobility of the foam formed in the core with a permeability of 2005 mD is lower than that formed in the core with a permeability of 632 mD. This indicates a certain selective reduction effect on mobility. Due to the influence of ultimate capillary pressure, both foam resistance factor and apparent viscosity are lower for the 47 mD core injected with different concentrations of MHSB31 ranging from 0.05 wt% to 0.5 wt%. The evaluation data presented in this paper regarding bulk foam and flow foam, using hydroxysulfobetaine surfactant under high temperature, high pressure, and high salinity conditions, have significant guiding significance for selecting and applying foamers in reservoirs with such harsh conditions.

Decarbonization of Heating and Cooling Systems of Buildings Located Nearby Surface Water Sources: Case Study

The study was carried out to evaluate theoretically and in laboratory conditions the capacity of a hybrid heating and cooling system that sustainably uses thermal energy extracted from surface waters in order to decarbonize buildings located near water sources. The novelty of the research consists in the realization of two experimental systems, one for the rapid evaluation of the performance of the water–water heat pump heating system and one for the evaluation of the operating behavior of a cooling system with fan coil units. Starting with the heating and cooling demand, and the climatic and hydrological local characteristics, a hybrid system model for the heating and cooling of the analyzed building was established and implemented. The forecasted energy consumption and CO2 emissions for the operation of the new equipment were compared with the historical values of the old systems with which the building was equipped (thermal energy supply from the district heating and cooling system with an air conditioning unit). Also, the results were extrapolated for forecasting the energy potential of the surface waters. The study highlights a percentage reduction in annual energy consumption of 67.71% and CO2 emissions of 80.13% through the implementation of the hybrid system.

Model development and validation of a vapor-compression system integrating water-based thermal energy storage using a three-fluid heat exchanger (TriCoil™)

This paper presents a comprehensive simulation suite for a residential heating and cooling system that combines a vapor-compression system (VCS) with a water-based thermal energy storage (TES) using a three-fluid heat exchanger (TriCoil™). The VCS configuration includes the TriCoil™ as the indoor coil, a fin-and-tube heat exchanger as the outdoor coil, a variable-speed compressor, and an isenthalpic expansion device. To minimize computational cost for annual simulations, an artificial neural network (ANN) was employed to develop a data-driven model for the TriCoil™. The suite integrates models of the VCS, TES, and control systems, factoring in utility rates, building load profiles, and weather data. Validation against experimental data demonstrated accurate predictions, with TES temperature profiles having a root mean square error of 0.3 K and VCS heating capacity predictions showing mean absolute errors of 4.3% for the R410A side, 5.0% for the water side, 4.9% for the air side, and 4.3% for the air-sensible side.Simulations for a cooling season in Stillwater, OK, demonstrated significant electricity cost reductions by shifting cooling loads from peak to off-peak hours. The proposed system minimizes expenses using time-of-use rates and optimized load scheduling, showcasing substantial benefits even with a small TES unit.

Performance Analysis of Water Heat Pipe for Application of the Passive Cooling System in Nuclear Power Plants

Incorporating heat pipes into passive cooling systems in nuclear reactors offers the benefits of passive operation without external power, a simple design, and high thermal capacity. Accurate thermal performance prediction of the heat pipe is crucial for ensuring safe reactor design and operation. Prior studies on nuclear reactor systems utilizing heat pipes have focused on thermosyphons, which operate by gravity. However, to expand the range of heat pipe applications in reactor systems, experimental investigations of large-scale heat pipes driven by capillary pumping force are required. In this study, a water heat pipe with a 25.4-mm diameter and 4-m length was manufactured to provide thermal experimental results under extreme conditions, such as system rollover or loss-of-cooling accidents. A three-dimensional (3D) printing technique was used to fabricate the high-performance lattice capillary wick structure by combining cubic and diamond lattice structures. The 3D printed wick structure showed 21 to 165 times higher capillarity and enhanced surface properties compared to the screen mesh wick structure. Compared to wickless thermosyphons, the 3D printed wick heat pipe exhibited higher thermal conductivity, stable operation in both vertical and horizontal orientations, and faster startup under extreme conditions.

Cascade Latent Heat Storage for Solar Cooking Applications

The present investigation explores the potential of multiple phase change material (PCM) based storage for real-time solar thermal applications. There are two types of storage configurations tested in this work for the simultaneous charging and discharging operation, i.e., single PCM-based NaNO3 storage and 2-stage cascade latent heat storage having NaNO3 and NaNO2 as the storage mediums in the consecutive stages. The storage is designed to achieve the end-use temperature condition of 513 K (the highest temperature required for cooking) with the latent thermal energy accumulation capacity of 1 MJ. The study envisages that instead of using a single PCM in the storage unit, 2-stage cascading of the PCMs can improve energy accumulation and retrieval in storage. After 8 hours of operation, none of the storage achieves the complete charging state. The energy accumulated in the NaNO3/NaNO2 cascade storage is found to be 2.6 times higher than the energy accumulated in the NaNO3 storage. NaNO3/NaNO2 storage achieves the desired constant outlet temperature of cold heat transfer fluid for chapati making (531 K) after 165 minutes of operation. At this moment, the latent energy accumulated in the NaNO3 and NaNO3/NaNO2 storage is observed as 35% and 14%, respectively

Computational Fluid Dynamic Investigation of Local Flow-Field Conditions in Lab Polymer Electrolyte Membrane Fuel Cells to Identify Degradation Stressors and Performance Enhancers

The use of polymer electrolyte membrane (PEM) fuel cells as an alternative to internal combustion engines can significantly contribute to the decarbonization of the transport sector, especially for heavy-duty applications. However, degradation is still an issue for this type of component, affecting their durability and performance. In this scenario, a detailed analysis of the anodic and cathodic distributors’ flow-field geometry may help to identify some local stressors that trigger the degradation mechanism, such as local hot spots and reactants not having a uniform distribution. A computational fluid dynamic (CFD) methodology is able to provide a volumetric description of a PEM fuel cell so it can be a useful tool to better understand the physical phenomena that govern the component operations. In this work, the open-source simulation library openFuelCell2 is adopted for a detailed analysis of two different PEM fuel cells characterized by standard distributor geometries, namely a parallel channel geometry and a serpentine configuration. The library, based on the OpenFOAM code, has been extended with a novel implementation accounting for the catalytic activity reduction due to the platinum oxide (PtOx) formation occurring under certain particular conditions. The adopted methodology is firstly validated resorting to experimental data acquired for the two different fuel cell configurations. The analysis highlights that the PtOx formation leads to a reduction in the fuel cell performance reaching up to 60–80% when operating at high voltages. Then, the effect of the distributor geometries on the component performance is investigated by resorting to in-plane and through-plane physical quantity distribution, such as reactant concentration, pressure or velocity fields. While the parallel flow channel configuration shows some diffusion losses under the rib, the serpentine channel geometry configuration can achieve some local performance peaks thanks to the convective flow in the gas diffusion layer (GDL) driven by local pressure gradients. Furthermore, the local enhancement in terms of higher current density under the rib is associated with an effective heat removal due to the high thermal capacity of the bipolar plate, avoiding the generation of local hot spots.

Preparation and properties of ultra-thin-walled energy storage paraffin microcapsules for naval applications

Due to the special nature of weather conditions at sea, the tricky status quo of heat loss in readiness or operational clothing, there is an urgent need for special insulated clothing materials to withstand harsh environments, maintain body temperature, and increase comfort for naval personnel who have been at sea for a long period of time in preparation for battle or combat. Paraffin is a special kind of phase change energy storage material, And the preparation of paraffin microcapsules is an effective way to solve the above problems, in which the ultra-thin-walled energy storage paraffin capsules have the advantages of large volume, accommodating more paraffin phase change materials, good energy storage effect and low cost. In this study, three kinds of paraffin microcapsules with core-shell structure were prepared by encapsulating a membrane with stable performance on the surface of paraffin particles using microencapsulation and layer-by-layer self-assembly techniques. We characterized their thermal storage capacity, composition and morphology by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier infrared spectroscopy, and optical microscopy, and compared their performances. The results show that the paraffin microcapsules prepared by layer-by-layer (LBL) self-assembly technique in this study have good thermal storage performance and stability, and their thickness increases with the increase of the number of layers, and the materials used in the experiment wrapped the liquid paraffin layer by layer, which is expected to be practically applied in the field of naval energy storage materials.

3D internal crack propagation in brittle solids under non-uniform temperature field: Experimental and numerical simulation

In the field of engineering, elevated temperatures often induce thermal stress. Especially in brittle materials, the propagation of internal cracks under such thermal stress is frequently the primary cause of failure. We utilize the 3D internal laser-engraved crack technology to create internal cracks at various locations inside the half-Brazilian disk specimens. By precisely controlling the power of the heating device, we achieved temperature non-uniformity of the external heat source, resulting in uneven heating of the material’s surface. Simultaneously, by adjusting the geometry of the material and using semi-Brazilian disk specimens, we generated uneven temperature gradients and thermal capacities along different heat conduction paths. The internal cracks at different locations also affect the distribution of thermal stress in the material, further influencing the formation of the non-uniform temperature fields (NUTF). Through the above methods, we successfully conducted physical experiments on the 3D propagation of internal cracks in brittle materials under NUTF. Additionally, we utilized J-integral and the finite element method for thermal analysis to conduct thermo-mechanical coupled 3D numerical simulations. Furthermore, we employed a polarising microscope to analyze the microscopic morphology of 3D internal cracks. Finally, we revealed the mechanism of 3D internal crack propagation under NUTF. The results show that prefabricated internal cracks in group A produce “n-shaped” crack at a height of 15 mm and “u-shaped” cracks at heights of 25 mm and 35 mm. Meanwhile, in group B, the eccentric prefabricated internal cracks ultimately produce “s-shaped” cracks. Through analysis, it was determined that the differential formation occurred due to relative sliding between the upper and lower surfaces of the crack under longitudinal shear stress induced by NUTF. Group A cracks have smooth surfaces, which are mode I-II cracks. group B generated “lance-like” fractures with obvious “anti-binary tree”, which are mixed mode I-II-III cracks. These findings provide an experimental and theoretical basis for the study of 3D internal crack propagation modes under NUTF and deepen the understanding of fracture mechanics in brittle materials.

Seeking non-Fourier heat transfer with ultrabroad band thermoreflectance spectroscopy

Studying superdiffusive thermal transport is crucial for advanced thermal management in electronics and nanotechnology, ensuring devices run efficiently and reliably. Such study also contributes to the design of high-performance thermoelectric materials and devices, thereby improving energy efficiency. This work leads to a better understanding of fundamental physics and non-equilibrium phenomena, fostering innovations in numerous scientific and engineering fields. We are showing, from a one shot experiment, that clear deviations from classical Fourier behavior are observed in a semiconductor alloy such as InGaAs. These deviations are a signature of the competition that takes place between ballistic and diffusive heat transfers. Thermal propagation is modelled by a truncated Lévy model. This approach is used to analyze this ballistic-diffusive transition and to determine the thermal properties of InGaAs. The experimental part of this work is based on a combination of time-domain and frequency-domain thermoreflectance methods with an extended bandwidth ranging from a few kHz to 100 GHz. This unique wide-bandwidth configuration allows a clear distinction between Fourier diffusive and non-Fourier superdiffusive heat propagation in semiconductor materials. For diffusive processes, we also demonstrate our ability to simultaneously measure the thermal conductivity, heat capacity and interface thermal resistance of several materials over 3 decades of thermal conductivity.

Characterisation of a plate heat exchanger working as condenser. Comparative analysis

This paper presents a method to evaluate the heating capacity of a plate heat exchanger working as condenser. As in previous works with phe evaporators and single phase heat exchangers, the proposed methodology relays on a discretisation of the heat exchanger in cells, in such a way that the outlet temperature is assumed and the heat transferred is calculated by integrating along the heat exchanger by means of a 1D approach which allows us to determine the outlet variables iteratively based on a heat transfer area calculation.The input variables of the model are pressure, enthalpy, and flow rate of both fluids at the inlet of the heat exchanger .The study presents the method, pays attention the heat transfer coefficient correlations utilised, and is used to comparatively determine the heat transferred by the refrigerant to the secondary fluid. Experimental results are used for validation and to analyse the goodness of the different correlations tested. Some conclusions are finally drawn.

Experimental and numerical estimation of thermal conductivity of bio-based building composite materials with an enhanced thermal capacity

The paper tackles the important problem of estimating and predicting the thermal conductivity of bio-based building materials that can help decarbonize the building sector. Knowledge and the possibility to optimize their insulating properties is the first criterion for determining the ability of their use in the building sector. The novel yet not well-studied hemp shives and magnesium binder composites with improved thermal mass by microencapsulated phase change material (PCM) were considered. The samples of composites without PCM having different densities as well as with different amounts of microencapsulated PCM and the same density were manufactured and tested experimentally in different states, i.e., dry and after conditioning at relative humidity (RH) of 50, 75, and 90 %, and in the case of samples with PCM also at different average measurement temperatures selected with respect to the phase change range of the PCM. A novel method of predicting bio-based composites' effective thermal conductivity tensor was also developed. The method is based on the numerical solution of the heat conduction equation at the micro-scale considering real composite microstructure. The method was tuned using micro-computed tomography (μCT) data with the microstructure of composites without PCM and then applied to predict the thermal conductivities of composites with PCM, utilizing computational domains with an artificially distributed PCM in the binder. The experimental testing of composites without PCM revealed an apparent effect of increasing thermal conductivity with rising sample density and RH applied during conditioning. With an increase in density from 394 to 576 kg/m3, their thermal conductivities varied from 0.105 to 0.159 W/m/K for the dry state and from 0.157 to 0.241 W/m/K for RH 90 %. A similar effect of sample state and RH was observed for composites with microencapsulated PCM, which had the highest thermal conductivity, equal to 0.265 W/m/K, for the lowest PCM amount and RH 90 %. Moreover, a decrease in composites' effective thermal conductivity with an increase in PCM fraction was observed, except for the lowest PCM fraction, for which it increased. It also rose with increasing the average measurement temperature. The developed micro-scale-based numerical method allowed for predicting the thermal conductivities of composites with accuracies below 4.1 and 7.2 % for composites without and with PCM, respectively, except for the composite with the lowest amount of PCM, which behaved differently. Thus, its suitability for designing and optimizing bio-based composites was shown.

Investigation on the dynamic thermal response of jet evaporators to short-term thermal excitation

This study investigates the dynamic thermal response of a jet evaporator under short-term thermal excitation in a mechanically pumped two-phase loop (MPTL) with a special focus on the temperature response characteristics of the jet evaporator under different flow and heat flux conditions. Two dimensionless numbers, Ja and Re, were proposed to characterize the thermal response. In addition, the time constant and transient time were introduced to evaluate the temperature response rate, whereas thermal stability was characterized by the temperature overshot amplitude. The results indicate that the gradual response mode, single-jump overshoot response mode, and twice-jump overshoot response mode can be sequentially obtained with an increase in the applied heat load, that is, an increase in Ja. The Re number influences the threshold value of the Jacob number for the response-mode transition. Elevating the heat load accelerates the thermal response rate of the jet evaporator and reduces its time constants, whereas increasing the flow rate enhances the thermal capacity and anti-interference ability. Furthermore, as the flow rate increased, the thermal capacity and anti-interference ability of the MPTL were enhanced, leading to an increase in the time constant and transient time. Notably, the twice-jump overshoot response exhibited a smaller temperature overshoot than the single-jump overshoot response, suggesting improved thermal stability.

Wetsuit Thermal Resistivity Measurements.

In recent years, attention to the realization and characterization of wetsuits for scuba diving and other sea sports or activities has increased. The research has aimed to establish reliable and standardized measurement methods to objectively assess wetsuit quality, particularly focusing on their mechanical and thermal properties. In this work, we describe and compare two different measurement methods for the characterization of neoprene wetsuit thermal resistivity. The first method follows the existing regulations in the field, while the second one, which we are originally proposing in this paper, offers an alternative yet accurate way based on a simplified experimental set-up and easier measurements. In both cases, the wetsuit sample under testing was shaped in the form of a cylindrical sleeve of proper dimensions and wrapped around a phantom containing water at a higher temperature and surrounded by water at a lower temperature. The wetsuit's cylindrical surface allows heat flow from the warmer water on the inside to the colder water on the outside through the wetsuit area. In the first case, a thermal steady state was achieved, with constant heat flow from the phantom to the exterior. This was obtained with a power balance between two homogenous quantities. Electrically supplied thermal heating within the phantom was used to balance the thermal energy naturally flowing through the wetsuit's surface. In this first case, a stable and fixed temperature difference was obtained between the inner and the outer surfaces of the wetsuit sample. In the second case, a thermal transient was analyzed during the cooling process of the phantom, and the thermal time constant was measured, providing the sample thermal resistance once the phantom thermal capacity was known. In both cases and methods, the heat flow and thermal resistance of other elements than the wetsuit must be evaluated and compensated for if they are not negligible. Finally, the thermal resistivity per unit area of the wetsuit material was obtained with the product of the wetsuit sample's thermal resistance and the wetsuit area. The measurements, conducted until now by immersing the phantom in a free surface tank, show that both methods-under stationary and under transient temperature conditions-were valid to assess the wetsuit's thermal resistivity. The stationary method somehow provided better accuracy while involving less well-known parameters but at the expense of a more complicated experimental set-up and additional energy consumption. The transitory method, on the other hand, is quite easy to implement and, after careful characterization of the phantom's parameters, it provided similar results to the stationary one. An uncertainty budget was evaluated for both methods, and they did provide highly compatible measurement results, with resistivity values of 0.104(9) m2·K/W (stationary method) and 0.095(9) K·m2/W (transient method) for the same wetsuit sample under testing, which is also consistent with the values in the literature. We finally propose that the novel method is a valid alternative for characterization of the thermal insulation properties of a scuba diving wetsuit.

A transient formulation of entropy and heat transfer coefficients of Newton's cooling law with the unifying entropy potential difference in compressible flows

The original Newton's law of cooling is considered an ideal convection model of a point source ignoring conduction, but its unknown convection conductance and inconsistent thermal potential difference make it difficult to further elucidate the transient heat transfer mechanism of convection because of the no-slip condition at the interface between a body and a fluid. For this purpose, both the convective entropy transfer theory and the hypothesis of the isothermal velocity sublayer are developed to recast Newton's law of cooling into a standard Ohm's law form via the unified entropy driving force and the unambiguous entropy transfer coefficient. An explicit expression for the transient heat transfer coefficient is presented theoretically and experimentally in compressible flows by virtue of the product of the free-stream velocity, the volumetric thermal capacity of the fluid, and the near-wall velocity constant. The unified thermal potential difference is established between the body temperature and the total temperature (reduced to the ambient temperature for incompressible fluids). The physical mechanism of convection mode in the original Newton's cooling law is reasonably explained by the entropy transfer theory and entropy transfer efficiency, and the formulae for the entropy flux vector driven by the standard entropy potential difference and the rate of entropy generation are obtained. The analytical heat transfer coefficients as well as the standard thermal driving forces are validated by comparison with the forced convection experiment with a maximum error of 4.6 % in incompressible flows and the previous Newton's benchmark cooling experiments with the maximum error of 6.7 % in compressible flows. The effects of radiation on the total heat transfer coefficient are also analyzed quantitatively.