International Journal of Heat and Mass Transfer最新文献

The physical appearance of a flame in a long and narrow confined space, such as a tunnel, is an important factor in determining the development of flame spread in moving bodies and the distribution of the energy field in space. A thermally thin fuel flame spread experiment was carried out in a reduced-size experiment platform. Two types of wind flow conditions were considered, i.e., wind flow field caused by piston effect only and wind flow field caused by longitudinal ventilation system and piston effect. Based on the piston wind calculation model developed by the previous authors, a simplified forced wind flow calculation formula was constructed, and the predicted values were in good agreement with the measured values. An improved image processing method was utilized to quantitatively describe the changes in the geometric features of the flame during the flame spread process. The flame probability plot shows that the characteristic flame tilt angle shows an overall increasing trend with the increase of forced wind flow. The flame height fluctuates within a certain range under the condition of forced flow less than 1.0 m s^-1, and decreases with the increase of moving body velocity and longitudinal ventilation wind speed under the remaining conditions. Finally, based on the previous prediction model of flame tilt during moving fire source and flame spread, the prediction model of flame tilt under lateral flame spread condition for a moving body in a narrow and long confined space is constructed by introducing the velocity factor, and the error is within 20 % in the case of downstream flow.

Liquid film thickness is a dominant feature for understanding boiling heat transfer mechanism in microscale slug flow. Flow boiling in circular microchannels has been extensively studied. Microchannels with non-circular cross-section are more common in industrial applications, but there have been few studies on such complex cross-sections. In the present study, the transient liquid film thickness during flow boiling in non-circular microchannels was experimentally investigated by a laser confocal displacement meter. Non-circular tubes with inner dimension of 0.39 × 0.39, 0.5 × 0.5, 0.6 × 0.6, 0.7 × 0.7 and 0.3 × 0.8 mm² were used for the test section, and water and ethanol were used as working fluids. The variation of liquid film thickness under adiabatic condition in non-circular microchannels was analyzed and an empirical correlation was proposed for predicting initial liquid film thickness. On this basis, a new theoretical model for liquid film thickness variation under flow boiling in non-circular microchannels was developed, considering the effects of evaporation, shear force and transversal flow.

The emerging aerogel fibers, which combine the high porosity of aerogel materials with the softness and weavability of fibers, demonstrate significant potential for the next-generation material for wearable thermal protection. However, there is currently a lack of fundamental understanding regarding their thermal insulation performance. In this work, aramid nanofibers-derived aerogel fibers (ANAFs), which exhibit high strength and excellent thermal insulation properties, are adopted as model aerogel materials for the numerical calculation of thermal conductivity. ANAFs are woven into aerogel fabrics with plain, twill, and satin structures, and their three-dimensional structural models are established based on fabric structure parameters. Building upon this foundation, a relationship model between thermal conductivity of ANAFs and thermal conductivity of aramid aerogel fabrics is proposed by series-parallel hybrid model, which is derived from thermal resistance theory. The model comprehensively considers the effects of thermal radiation and conduction. By an iterative approximation strategy, the thermal conductivity of ANAFs is calculated as $0.0345W/\text{mK}$. During the calculation process, the fabric structure plays a crucial role in determining the volume fraction of different components in the model, thereby affecting the calculation of ANAFs thermal conductivity. Furthermore, prediction models for the thermal conductivity of twill and satin weave fabrics are constructed to verify the accuracy of the calculated thermal conductivity of ANAFs. The results derived from the prediction models proposed in this work demonstrate a deviation of <5 % when compared to experimental measurements. This work provides a convenient and feasible method for calculating the thermal conductivity of aerogel fibers, while also offering a viable approach for optimizing fabric structural parameters to enhance their thermal insulation performance.

Non-Fourier heat conduction plays a dominant role in many extreme transient heat conduction processes, such as laser pulses and heat transfer in biological systems, but the heat wave effect makes it difficult to solve the temperature field accurately and quickly. In order to solve this problem, the first order time derivative enhanced parallel hard constraints physics-informed neural networks (T-phPINN) is proposed. T-phPINN comprises two subnetworks and incorporates a first order time derivative to capture sharp temperature changes. Two numerical cases show that the minimum relative error of T-phPINN is 0.001 % and 0.015 %, which is 1.04 % and 12.30 % of the error of conventional PINN respectively, proving the accuracy of our architecture. A transfer learning framework is established for scenarios of different parameters, the training only requires 1/6 iterations of the basic model, and close accuracy is obtained. The computational cost of T-phPINN is evaluated using the finite element method as the baseline. For the two cases, the single calculation time is 33.43 % and 51.50 % of the baseline, while the multiple calculation time under the acceleration of transfer learning is 11.59 % and 17.75 % of the baseline. This study will be helpful for solving large-scale non-Fourier heat conduction equations precisely and expeditiously.

Establishing the stability of nanofluids is essential in both laboratory and industrial settings. High stability over time is needed to ensure that the suspensions retain their enhanced properties and provide reliable long-term performance. In the current work, a new optical method is proposed for visualizing and quantifying the stability of transparent nanofluids. The time variation of the concentration distribution and the local concentration gradients have been measured in an Al₂O₃-water nanofluid (ϕ=0.16 wt.%) using a Mach-Zehnder interferometer. A nanofluid prepared using standard two-step methods was found to be unstable over a short time interval, despite having a high zeta potential (43.7 mV). The concentration distribution was predicted using a simple gravitational settling model based on Stokes’ flow combined with particle size distribution measurements from dynamic light scattering (DLS). The results indicate that one of the main causes of the sedimentation instability was the presence of a small number of larger particles, which were detected using DLS analysis.

Vapor compression refrigeration and heat pump (VCRHP) systems consume large amounts of energy annually. Therefore, improving the VCRHP system efficiency holds paramount significance in pursuing the dual-carbon goal. The heat transfer performance of heat exchangers, as an essential component of the VCRHP system, has a significant impact on the VCRHP system efficiency. Compared with traditional large-size heat exchangers, microchannel heat exchangers (MCHXs) have garnered substantial attention due to their advantages, such as compact structure, high heat transfer coefficient, and small refrigerant charge. In actual VCRHP systems, oil in compressor is inevitably carried away by refrigerant. Oil can either form a mixture with refrigerant liquid, or exist as a separate oil film in various components of VCRHP system. Depending on internal geometry and operation conditions, MCHX exhibits distinct different oil retention (OR) characteristics. Oil retained in inlet header of MCHX changes the flow pattern within header and affects the pressure of downstream tube, consequently impacting refrigerant distribution characteristics. During flow boiling and cooling/condensation processes, oil changes flow behavior and heat transfer characteristics. However, previous review work has not comprehensively summarized and discussed the impact of oil in MCHX. Therefore, in this study, to provide valuable insights for assessing OR characteristics of MCHX, action mechanism of oil on refrigerant distribution behavior, and effect of oil on flow behavior and heat transfer characteristics during flow boiling and cooling/condensation, a comprehensive review of literature from the past 20 years has been conducted. Considering the limitations of current research, several potential future research directions are proposed: research on the effect of oil on flow and heat transfer in microgravity environment, design of super-oleophobic surface with nanoscale structure, optimal design of MCHX header and development of refrigerant-oil distributor, measurement of thermophysical properties and heat transfer performance of non-azeotropic refrigerant-oil mixture, as well as numerical simulation of flow and heat transfer of refrigerant-oil mixture. This review is intended to serve as a reference for the practical design and optimization of MCHX under oil-bearing conditions.

Due to the widespread presence of heat and mass transfer phenomena in industrial applications, numerous studies have been devoted to the accurate solution of energy equations, providing a foundation for the analysis of heat and mass transfer processes in practical applications. In this study, a Green's Function Markov Superposition Monte Carlo (GMSMC) for solving general energy equations has been developed based on probability and statistical principles owing to its advantageous features of insensitivity towards dimension and geometric complexity as well as the capability to handle multiple integrals in complex domains. The energy equation is first decomposed, and corresponding probability models are established for each component, considering their interrelationships. Subsequently, a solution framework for solving the general energy equation is constructed by integrating these probability models based on a Markov chain structure. The mathematical principles and formulae of the proposed method are derived in detail. The performance of the proposed method is validated by several heat transfer systems with different combinations of boundary conditions and features, which mainly include the distribution of the internal heat source and whether the convection or transient term is included. Results of the validation show that the temperatures obtained by the proposed method are in good agreement with the FEM based on a fine grid, no matter whether the calculation is for a single point or a distribution.

A non-invasive, real-time monitoring, spectral analysis heat flux sensor with single measuring point based on chirp fiber Bragg grating (CFBG) has been proposed in this paper. When CFBG senses heat flux, the linear increase of axial thermal expansion leads to spectral broadening. The measurement of heat flux can be realized by the measurement of the full width at half maximum (FWHM). The theoretical model and working principle of CFBG heat flux sensor are established to determine the heat flux sensing characteristics of CFBG. The numerical analysis and theoretical calculation show that CFBG heat flux sensor can realize stable transformation of heat flux and FWHM. A heat flux calibration platform was built and its feasibility was verified by simulation. The sensitivity of CFBG sensor to monitor heat flux in a wide temperature range is 1.078 pm/(W/m²). The heat flux of the pipe wall at the outlet of the axial variable piston pump in the hydraulic system was quantitatively measured by the CFBG heat flux sensor. The sensor has a spatial resolution of 1 cm, a length of 10 mm, a diameter of 0.125 mm, and a sensitivity of 1.078 pm/(W/m²). It has the advantages of high spatial resolution, single measurement point, real-time measurement, small installation space, and non-invasiveness. The theoretical model can provide theoretical guidance for the design of fiber grating heat flux sensor, and it is convenient to design a series of sensors for specific measurement requirements.

Due to the high heat transfer coefficient, flow boiling has applied in various industrial fields. Previous literature mainly focuses on the enhancement methods of flow boiling for water or refrigerants, with few reports on the enhancement of flow boiling for hydrocarbon fuels. Therefore, in this study, we design different types of micro-finned surface with strong capillary force to increases the nucleate sites and keep the wall in a superwetting state during the flow boiling to postpone the appearance of “annular bubble” flow. The influence of fin width, fin height, fin spacing and coolant type on boiling heat transfer through experiments and numerical simulation. The results show that the wider the fin width, the lower the fin height, the smaller the fin spacing, the stronger the heat transfer enhancement effect, and the more stable the flow boiling. The best heat transfer performance of the finned surface is achieved with the average wall temperature 27 °C lower and heat transfer coefficient 1.9 times higher than the smooth surface. Moreover, the heat transfer coefficient of hydrocarbon fuel is 43.3% larger than that of deionized water.