International Communications in Heat and Mass Transfer最新文献

Many natural and practical applications, from macro- to microscale, entail the evaporation of an array of droplets on a substrate surface. A simplified point source model (PSM) was developed to simulate the evaporation of a pair of sessile droplets on a substrate surface, surrounded by still air. The model is for a purely diffusive isothermal quasi-steady-state evaporation process. The model determines the initial evaporation rate as a function of the separation distance between two droplets. A threshold separation distance between two droplets, beyond which the evaporation rate of the droplets is very similar to the evaporation of a single isolated droplet, is determined. The effect of the evaporation mode (Constant Contact Angle (CCA) mode and Constant Contact Radius (CCR) mode) was considered. Two expressions to determine the evaporation time under CCA and CCR evaporation mode are presented. The model is a simple and computationally inexpensive tool. An experimental setup was designed and built to investigate the validity of the proposed model. The effect of droplet volume, ambient relative humidity, surface wettability, and liquid volatility are tested experimentally. The predictions from the model were in excellent agreement with the experimental results regarding the droplet volume, ambient relative humidity, and surface wettability.

The high temperature of PV panels during power generation not only reduces their efficiency but also leads to increased heat transfer to the room. The impact of using PV panels on the indoor thermal environment should not be overlooked. A novel roof structure is proposed in this paper, which is composed of PV panels, phase change materials (PCMs) and ventilation layer from top to bottom. The thermal performance of PV-PCM- ventilation roof room is compared with that of PV roof room and PV-PCM roof room under active and passive conditions respectively. Results show that under passive conditions: Compared to PV roof, the PV-PCM-ventilation roof internal surface temperature fluctuation decreases by 44.12%, the correlation coefficient between the back surface temperature of PV panel and outdoor air temperature decreases from 0.79 to 0.69, and the correlation coefficient with solar radiation decreases from 0.66 to 0.60. Compared to PV-PCM roof, the back surface temperature of the PV panel decreases by 3.31 °C during the day. Under active conditions, when the set temperature is 26 °C and 28 °C, the energy consumption of the PV-PCM-ventilation roof room decreases by 15.61% compared to the PV roof room. In summary, the new roof demonstrates good insulation and energy-saving performance.

This study numerically investigates the effectiveness of concurrently applying nano-enhanced phase change material (NEPCM) and an ultrasonic field for the thermal management of a pin fin heat sink. The role of the NEPCM is to absorb heat from the heat sink wall, while the ultrasonic field generated by ultrasonic transducers facilitates the accelerated melting of the NEPCM. The study investigated how varying the number of ultrasonic transducers positioned near each side wall of the square cross-section heat sink, along with adjusting the concentration of nanoparticles in the NEPCM, impacts the heat sink's performance. The total power consumption of the transducers is assumed to be constant and an increase in their number is associated with a decrease in the power consumption of each transducer. It was observed that raising the number of transducers and lowering the nanoparticle concentration both contributed to a decrease in the CPU's highest temperature. Additionally, it was found that by using the combination of ultrasonic field and NEPCM, the average temperature of CPU can be reduced by 12.33–15.91 °C to the case without the ultrasonic field. Moreover, raising the number of transducers and lowering the nanoparticle concentration both contributed to a decrease in the CPU's average temperature.

Heat exchangers(HX) based on triple periodic minimal surfaces (TPMS) show superior heat transfer performance to conventional structures due to extremely high surface area-to-volume ratios and complex geometrical topologies. While current studies primarily examine flow and heat transfer within original TPMS structures, research on hybrid TPMS structures is limited. Therefore, this study aims to construct flexible hybrid TPMS structures based on the original TPMS structures by sigmoid transition parameters and to analyze their internal flow and heat transfer mechanisms in depth, as well as to comprehensively evaluate the heat transfer performance of several TPMS original structures and hybrid structures based on the complex proportional assessment (COPRAS) method. The results show that the unique topology of TPMS induces a continuous change in the internal fluid flow direction, which significantly enhances the convective heat transfer. Gyroid-Diamond TPMS HX had the highest heat exchange efficiency of 37.78%. the convective heat transfer coefficients of Primitive-Gyroid and Primitive-Diamond HX were increased by 47.31% and 67.38%, respectively, compared with the Primitive HX. In addition, Gyroid-Diamond HX exhibits the highest convective heat transfer coefficient of 2380.15 W/m²⸳K at a = 0.5, relative density of 40%, but also results in a relatively large pressure drop.

The stability of a stationary fully developed vertical flow across a porous pipe is investigated. The heating due to viscous dissipation is assumed to be non–negligible and also to be the only effect triggering the onset of thermal convection. An innovative scaling is employed to study the case of small Gebhart number. The viscous heating term present inside the energy balance equation yields a basic stationary flow characterised by dual branches of solutions: for a given vertical pressure gradient, two possible velocity profiles are obtained. The linear and nonlinear stability of the stationary dual solutions is performed. The linear stability is investigated in a usual fashion by employing the normal modes method and then solving the eigenvalue problem obtained by using the shooting method. The nonlinear stability is investigated by simulating numerically the evolution in time of the perturbed system. The linear stability analysis allows one to conclude that the critical wavenumber for the onset of instability is zero and the critical dimensionless velocity at the pipe axis is equal to $3.43631$. The results of the nonlinear analysis display subcritical instabilities. Indeed, the onset of instability is obtained for values of the governing parameters which are lower than the critical values obtained by the linear analysis. This feature occurs when the amplitude of the initial disturbance applied to the nonlinear problem is sufficiently high, namely when the dimensionless amplitude is larger than ${10}^{-2}$.

The freezing process of droplet impact on cold cylindrical surfaces are experimentally investigated using high-speed photography. Effects of Weber number (9.93–357.44) and surface temperature (−25 ∼ −5 °C) on the freezing characteristics are analyzed. The results show that, there are five different freezing morphologies after impacting cold cylindrical surfaces: semi-sphere, cone, single-bridge double-ear, double-bridge double-ear, and central concave ring. As Weber number (We) increases or surface temperature decreases, the freezing delay time, freezing time, and total freezing time are shortened, while the freezing velocity and the exterior freezing front moving speed increase. When the freezing morphology is semi-sphere or cone, the exterior freezing front moving speed in axial direction is smaller than that in circumferential direction. When the freezing morphology is single-bridge double-ear, double-bridge double-ear, or central concave ring, the movement of exterior freezing front can be divided into two stages: rapid development stage and slow convergence stage. The exterior freezing front in axial direction moves faster than that in circumferential direction during the rapid development stage. In this paper, the differences of icing in axial and circumferential directions on cylindrical surfaces are analyzed. The freezing characteristics and mechanisms under the influence of curvature are revealed.

This study experimentally investigated the performance of a novel micro-thermomagnetic pump system with a series connection. In this system, a heater and magnetic field are used to drive temperature-sensitive magnetic fluid with a high flow rate. Experiments were performed to examine the performance of a single thermomagnetic pump without the cooling chip, a single thermomagnetic pump with the cooling chip, and two thermomagnetic pumps connected in series. For the single pump, the flow rate increased with the heat flux from the heater and with the use of a cooling chip. When two pumps were connected in series, the flow rate achieved was considerably higher than that achieved with the single pump, and the highest flow rate achieved in the series configuration was 118.17 μL/min. However, as the heat flux increased, the energy efficiency of the series configuration decreased. The increase of flow rate resulted in the rapid heat exchange and made the overall microchannel temperature drop comparing with the single thermomagnetic pump. Finally, temperature measurements were conducted to calculate the heat dissipation of the aforementioned three configurations.

This study investigates heat transfer in rotating leading-edge impingement channels with asymmetrically curved target surfaces. Experiments cover jet Reynolds numbers from 5000 to 15,000 and a maximum jet rotation number of 0.39, considering rotating effects, channel orientations, and jet hole shapes on heat transfer. Numerical simulations elucidate flow mechanisms. The results revealed that the asymmetry of the target surface leads to jet preferentially turning towards the trailing side after reaching the stagnation point, resulting in differences in heat transfer between the trailing side and leading side. Furthermore, flow rate redistribution due to rotation significantly enhances heat transfer at lowest and highest radial positions. Coupling flow rate changes and rotating force exhibit similar heat transfer variation trends with rotation at mid-radius position—initial enhancement, attenuation, then further enhancement. Moreover, changes in channel orientations induce Coriolis force components, altering flow rate distribution and jet deflection direction, influencing heat transfer differences among models under rotation. Furthermore, under rotational conditions, the elliptical jet hole model with higher flow rates exhibits significantly weaker rotational enhancement effects in the stagnation region compared to the circular jet hole model.

Efficient thermal management, especially for cooling electronic components in high power context, is crucial to keep devices operating in a safe temperature range. In the two-phase cooling field, capillary flow through porous systems combined with evaporation is one of the promising solutions. As established in the literature, evaporation occurring within the porous structure reduces the thermal performance. Here, we address this issue in a fundamental way, and propose a theoretical framework to design porous networks with porosity gradients maintaining the water/vapor interface at the top surface where the liquid, pumped by capillarity, evaporates. We model evaporation at the top surface of multiscale tree-like porous networks with minimum volume, by coupling an evaporation model at pore scale and a network model relying on capillary pressure, friction, and gravity balance. The evaporation model is validated through experimental data. We present some case studies and discuss how the geometrical features of the hierarchical networks, such as pore size and number of pores, impact the heat transfer coefficient. At the scale of the porous material, we show how the permeability is related to the heat flux to maintain evaporation. This study lays the foundation for designing efficient graded porous structures for evaporative cooling.

The moist condensation and thermal comfort are two important issues for the radiant cooling systems. To investigate the safe temperature without condensation, and the comfort of the composite system, experiments and simulations are conducted in this work. As the temperature of the floor surface is decreased to approximately 13 °C, slight condensation appears underneath the table. For which, the relative humidity is in the range of 88% ∼ 92%. The dew point temperature is measured especially considering the high humidity environments. Simulations, considering moist air infiltration on rainy days, are performed to evaluate both condensation and thermal comfort of various systems with different positions of fan coil and radiant surface. Of nine cases, case 5 has the maximum humidity, representing the most easily condensed. Out of the 54 localized PMVs, containing two people's ankle, waist and head level in 9 cases, 61.1% are in the range of −0.5 to 0.5. Of these, cases 3 and 5 have the largest proportions. For cases 1, 4, and 7, PMVs of the waist level of person 1 are less than 1, influenced by convective airflow. Increasing the upward angle of airflow can improve thermal comfort, but it is not suitable for the ceiling cooling because the humidity is increased to 89.6%, to condense easily, when the upward inclination is 30°. Multi-perspective findings provide a powerful basis for the optimized configuration of composite cooling systems. Especially, the locations of the minimum temperatures on the radiant surfaces should be given special attention to avoid water condensation for high-humidity environments.