Heat and Mass Transfer最新文献

In the context of wire electrical discharge machining (WEDM), determining the fraction of thermal energy transferred to the workpiece (f_c) is crucial for numerical modelling. This information is necessary to anticipate material removal mechanisms and understand thermal behaviour. In this study, two metaphor-less Rao algorithms are modified to solve the inverse heat conduction problem (IHCP) for the estimation of f_c during the WEDM process without knowing any prior information on the transient functional form of f_c. These two algorithms are compared in terms of accuracy and convergence speed. The Rao-1 algorithm stands out with high accuracy and rapid convergence. To evaluate the algorithm applicability in estimating f_c, the following cases are considered: (1) a numerical investigation with artificial Gaussian error in simulated temperature readings and (2) a real-time experiment on WEDM setup with varying discharge currents. The RMS error between the actual and estimated value of fc with SS-304 material during numerical investigation is found to be 562 W/m which is just 0.008 times of heat source. Real-time experiments reveal that the discharge current is directly proportional to the total energy supplied by the wire as well as f_c. The f_c values estimated by the proposed inverse algorithm with various discharge currents fall within the range of 15–18%, aligning with the existing literature. This shows the proposed methodology is accurate and can be extended to incorporate other machining processes.

This paper introduces a novel a CO₂ mechanical subcooling heat pump water heater (MSHPWH) to improve the heating performance in low temperatures. By utilizing a mechanical subcooling (MS) cycle, additional heat is supplied to cooling water, improving system efficiency. The study evaluates the heating COP (COP_h), power consumption and temperature of hot water under various steady-state operating conditions. Results indicate that the COPh of the MSHPWH increases by 44% to 57% compared to conventional HPWH as ambient temperatures range from -25 ℃ to -5 ℃. The MS cycle proves beneficial, with a subcooling range of 4 ℃ to 20 ℃. Adjusting the refrigerant mass flow rate ratio enhances heating output and hot water temperature. Changes in the mass flow rate ratio impact COP_h and the temperature of hot water concurrently. This research highlights the innovative MS cycle’s significant role in enhancing CO₂ heat pump water heater performance in cold climates, showcasing its potential as an eco-friendly and efficient heating solution.

This study delved into the efficacy of enhanced oil production (EOP) within perforated horizontal wellbores across diverse flow profiles. The authors implemented five distinct configurations, encompassing uniform radial air injection (profile 1) and variable radial air injection (profiles 2–5), with a particular emphasis on the concomitant production of liquid and air phases. Additionally, the study examined the frictional behavior along the perforated wellbore. Liquid production was demonstrably amplified throughout the bubble, plug, and slug flow regimes; however, a decline was observed in the stratified, stratified transition, and stratified wave flow regimes. Notably, the liquid product exhibited a direct correlation with both the mixture flow rate and its associated Reynolds number, signifying an increase with holdup and a decrease with void fraction. Conversely, air production displayed a positive association with a higher air flow rate. Overall, profiles 2 and 4 yielded the most favorable production during the bubble, plug, slug, and stratified flow regimes. In contrast, profile 3 emerged as the optimal configuration for the stratified transition and stratified wave flow regimes. The friction factor remained relatively constant with profile 1, experienced a reduction in profile 2, and exhibited an escalation in profile 3. Additionally, it increased in the middle of profile 4 and decreased at the center of the perforated section in profile 5. The friction factor behavior of profile 1 remained stable and smooth due to the invariant air flow rate throughout the perforated section. Conversely, some fluctuation was observed in profile 2 due to the inherent variability of the radial air injection along the perforated section. Importantly, the experimental and numerical results demonstrated satisfactory agreement across all flow patterns, with some minor discrepancies noted in the static pressure drop behavior during the bubble, dispersed bubble, and slug flow regimes.

Pulmonary tuberculosis is a chronic respiratory disease and lung infection that can be fatal if left untreated, as severe cases lead to compromised oxygen exchange at the alveolar level. This study uses a dual-scale porous medium model and computational methods to understand the nature of tuberculosis infection spread within the lungs and its effects on the alveolar oxygen exchange. The entire lung is modelled as a global, equivalent, heterogeneous porous medium comprising three zones with varying permeabilities that correspond to 23 generations of airflow branches. Airflow during each breathing cycle is simulated by solving transient mass and momentum transfer equations across the three zones of the global model. A separate local model is invoked in zone 3, to analyse oxygen exchange between the alveolar airflow and incoming capillary blood via mass transfer equations. The transient mass exchange equations are solved in the local model to yield the percentage of oxygen transferred to the blood. Tuberculosis spread – and hence, the congestion of the lung – is introduced by modifying the permeability and porosity of the global porous medium model. The impact of infection on the overall bloodstream oxygen content is evaluated by concurrent use of the global and local models. For the case with sudden reduction in immunity, severe infection condition is observed at (varvec{86%}) of the total infection spreading time and at (varvec{75%}) for the case with gradual reduction in immunity. For (varvec{40%}) increase in immunity beyond the (varvec{50% Gamma }) stage, it is observed from the simulations that the severe infection situation is completely avoided, preventing any further tuberculosis spread.

The conditions and influencing factors of hydrate formation is significant for hydrate technology. Combining with the existing literatures and the experimental data of this work, the phase equilibrium of CO₂ hydrate in (NaCl/CaCl₂/MgCl₂) ionic solutions, pure water-sediment system and (NaCl/CaCl₂/MgCl₂) ionic solution-sediment systems under the static magnetic field (0.39 T) was studied. Moreover, the effect mechanism of magnetic field on hydrate phase equilibrium in different systems was analyzed in terms of intermolecular interaction. Under the same pressure, the magnetic field increased the phase equilibrium temperature of CO₂ hydrate by 2.0–2.8 K in the three ionic solutions, which improved the hydrate formation conditions. This is mainly due to that the magnetic effect increases water activity and weakens the ionic hydration shells, thus promotes hydrate formation. In addition, compared with the ionic solution systems without magnetic field, the magnetic field increased the hydrate phase equilibrium temperature by 0.1–2.5 K in the ionic solution-sediment systems. However, the degree of temperature increase is less than that in the magnetic field-ionic solution systems, which is because the magnetic field enhances the binding between ions and the sediment particle in sediment-bearing systems. Compared with the magnetic field-ionic solution systems, the water activity in the magnetic field-ionic solution-sediment systems is lower, which makes hydrate formation more difficult. Moreover, with the movement of cations and anions in magnetic field, the crystals may be formed due to ion collisions, enhance the capillary action in ionic solution-sediment systems, and then hinder the hydrate formation. Therefore, the sediments can weaken the magnetic field promotion to hydrate formation.

This article deals with the application of the temperature oscillation method (TOIRT method) to the laminar flow of water in a pipe. This dynamic and contactless method was derived for the assumption of homogeneous temperature on the fluid side, but this assumption is violated in the case of laminar flow. Numerical simulations were used to discover the fundamental influence of the amount of incident heat flux, which is modulated by the sine function, on the resulting local values of the heat transfer coefficient. The frequency of the transmitted signal, on the other hand, has no effect. The experimental measurement confirmed the numerical results even with a deviation of 25%, which is still a good result due to the sensitivity of the experimental method in this area.

Microwave assistance has the potential to reduce the energy input required for mechanical rock breaking. This study systematically investigated the changes in electrical properties (specifically resistivity, capacitance, and impedance) of gabbro after microwave heating during the graded loading process, as well as its internal fracture mechanism. The findings indicate that the variations in resistivity, impedance, and capacitance of gabbro can be divided into three stages during the graded loading process: the compaction stage, elastic-steady cracking stage, and nonlinear crack propagation stage. When the strain level exceeds 70%, the resistivity and impedance start to increase, and the capacitance begins to decrease. The study also identifies a significant positive correlation between microwave power and the rate of temperature increase on the rock surface. A critical power threshold of approximately 2 kW is observed, below which achieving rapid temperature rise becomes challenging, but beyond which the temperature escalates swiftly with the energy input. Once the temperature exceeds 350 °C, rupturing mineral inclusions generate numerous microcracks, causing resistivity and impedance to exponentially increase. Furthermore, microwave heating induces a temperature differential exceeding 200 °C between the internal and external regions of the rock. Under the same radiation energy, high-power short-duration radiation is more likely to generate thermally induced cracks within the rock. The rapid expansion and heating of absorbent minerals, as well as the rupture of inclusions, further intensify the propagation of microcracks, greatly reducing the mechanical properties of the rock. This study will provide theoretical guidance for microwave-assisted mechanical rock excavation.

Amputees often experience high temperatures between the amputated limb and the prosthetic socket, necessitating the use of cooling devices to mitigate this issue. However, challenges arise with the location and size of conventional heat sinks. This research proposes a novel heat sink utilising a phase change material (PCM) to dissipate heat. The leg was chosen as the site for the heat sink, designed in a cylindrical shape. Coolant flow pipes were arranged in a branched configuration inspired by constructal theory, constrained by the dimensions of the artificial leg. The degrees of freedom for the constructal design are branches akin to arterial and venous branching, aiming to minimise pressure drop. Four heat sinks with varying degrees of branching were compared based on temperature reduction, heat dissipation, pressure drop, phase change material melting capacity, and operational efficiency. The cylindrical heat sink measures 50 mm in diameter and 300 mm in length. Ice was employed as the PCM, with water served as the working fluid. The working fluid's temperature and flow rate were maintained at 40 °C and 0.2 L/min, respectively. The experimental work was prepared to validate the theoretical model. The study revealed that the proposed heat sink design, with increased branching, led to a significant temperature reduction, achieving up to 39.62%. Moreover, heat dissipation increased by 236% compared to a single-tube heat sink. The use of branched pipes resulted in a manageable increase in pressure drop, peaking at 39.9 Pa, well within pump specifications, while markedly enhancing heat dissipation. The melting time of the PCM and the melting area increased as the number of branches of the heat sink increased. Ultimately, applying constructal theory in heat sink design for PCM demonstrated its superior performance within spatial constraints, providing a promising solution for prosthetic cooling.

The thermocapillary motion of droplet in a vibrating fluid in a cylinder heated from the top and sides and cooled from the bottom is studied, using a three-dimensional computational fluid dynamics (CFD) model based on volume of fluid (VOF) created with Ansys-Fluent software. The outcomes support the accuracy of the Marangoni phenomenon and are in line with data published in literature. The behavior of the drop is not only impacted by the temperature difference between the top and bottom, but also by heated side surfaces and mostly by vibration. Different flow patterns are observed which directly impact the droplet’s arrival time. The results proof that the neglected frequency and amplitudes of vibration in the presence of gravity have a significant and evident impact on the behavior of fluids in a zero-gravity environment. The change of vessel height also has a significant influence especially on the host fluid properties.

Electrical equipment extensively uses Microchannels (MCs) for cooling. Due to their complexity, it is challenging to evaluate the features of the fluid flow and heat transfer processes in MC pin-fin heat sinks. Numerical approaches have been frequently employed in MC design to enhance efficiency. Machine learning methods have recently enabled the assessment of flow and heat transfer research in these devices. In this study, numerical calculations have been made to obtain outlet fluid temperature, the average Nusselt number, and pressure drop, using the computational fluid dynamics (CFD) software, ANSYS Fluent. Previous experimental work validates the numerical model by examining the average Nusselt number and the apparent friction factor. Three distinct ratios of fin spacing to fin diameter (l/d = 2, 4, and 6) and five different values of Reynolds number (Re = 50, 75, 100, 125, and 150) are considered. A constant ratio of fin height to channel height (h/H = 0.25) is maintained, and the inlet fluid temperature is set to 291.15, 294.15, 297.15, and 300.15 K. Numerical calculations have been conducted for cases of uniform and non-uniform heating, where bottom wall temperatures of 323.15 K and 317.15 K were considered, respectively, for a fixed fin surface temperature of 323.15 K. Using the results of the numerical simulations, a multi-layer perceptron (MLP)-structured artificial neural network (ANN) is trained. The Levenberg-Marquardt (LM) training method is employed in the hidden layer, using 17 neurons for the training procedure. The results of the numerical simulations show that the average Nusselt number increases linearly with the Reynolds number, except for the non-uniform heating case of Re = 50. The average Nusselt number and pressure drop are inversely proportional to fin spacing for all cases. There is also a linear increase in pressure drop with the Reynolds number, since the flow regime considered in this study is laminar. The ANN model predicts the outlet fluid temperature, the average Nusselt number, and the pressure drop, with variation rates of -0.0027%, -0.075%, and − 0.0004%, respectively.