International Journal of Heat and Mass Transfer最新文献

We experimentally investigate the near-wall heat transfer at single bubble growth in nucleate saturated pool boiling of water at atmospheric pressure. Our focus is on the evaporation of the micro-metric thin film of liquid (microlayer) that is formed between the heating wall and the bubble. High speed and high resolution optical techniques are employed. Synchronous and simultaneous measurements of the microlayer thickness, wall temperature and bubble macroscopic shape are performed by white light interferometry, infrared thermography and side-wise shadowgraphy, respectively. We measure the wall temperature of an ITO heating film through a transparent to the infrared waves porthole. The heating is provided by an infrared laser. The wall heat flux is numerically reconstructed by using the experimental wall temperature data. We reveal a temporal rise of the thermal resistance of the liquid–vapor interface during the microlayer evaporation, which corresponds to a decrease of the accommodation coefficient. We attribute it to the progressive accumulation of impurities at the interface during evaporation. The contribution of microlayer evaporation to the overall bubble growth is about 18%.

Ultra-thin flat heat pipes (UFHPs) are being explored as a potential thermal management solution to address the heat dissipation challenges of electronic devices. However, the ultra-thin process increases fluid flow resistance and reduces the heat transfer capacity, posing challenges for wick structure optimization. In this study, the effects of wick structure, vapor space, and fluid flow properties on the maximum heat transfer capacity are analyzed by a fluid flow model. Microgroove and microcolumn as wick structures are optimized, and the coupling effects between capillary pressure and fluid flow resistances are analyzed. The maximum heat transfer capacity and the corresponding optimal wick structure dimensions are determined by the vapor space friction and wick structure friction, which are calculated by structural friction coefficients (S_v and S_l) and fluid friction coefficients (F_v and F_l). The models of fluid flow friction chosen in previous literature are validated for accuracy by Fluent. When the height of the wick structure (h) or vapor space (H) increases, the optimal dimensionless wick structure height (h*) increases and decreases under a fixed porosity (ε), respectively. The optimal h* of microgroove decreases and that of microcolumn increases as the dimensionless spacing (l*) increases. When ethanol is used as the working fluid, the optimal h* of microgroove is 1.98 and the optimal h* of microcolumn is 0.48, under the condition of H = 0.3 mm, h = 0.15 mm, and ε = 0.5. This model also emphasizes the importance of working fluid properties on the design of the wick structure, and a higher value of F_l/F_v results in a lower optimal h*. Moreover, methanol and acetone exhibit higher heat transfer capacity compared with ethanol. This study aims to provide comprehensive design principles for optimizing UFHP heat transfer capacity.

The present work utilizes Molecular Dynamics (MD) method to study the physical mechanism of the effect of hybrid deposited nanoparticles (HDNs) on the boiling heat transfer by changing the wettability of the substrate. Three kinds of nanofluid simulation models are established, in which water molecules are used as the base fluid, and nanoparticles are hydrophilic deposited particles, hydrophobic deposited particles and HDNs, respectively. Compared with hydrophilic and hydrophobic deposited nanoparticles, it is found that the equilibrium contact angles of droplet containing HDNs decreases by 2.22° and 6.99° respectively during wetting simulation, indicating that HDNs improve the wettability of the substrate. By simulating the boiling process of three fluids, it is found that HDNs advance the start time of explosive boiling by 0.15 ns at most, that is, accelerate the nucleation time of bubbles, and increase the heat flux by 46.9 % at most, indicating that the heat convection near the substrate is enhanced. In addition, HDNs improve the vibration matching degree of atoms between the solid-liquid interface and enhance the heat transfer between the substrate and the fluid. The results of boiling simulation verify the conclusion that HDNs improve the wettability of the substrate and thus enhance the heat transfer inferred by droplet wetting simulation.

This paper presents a study of a novel type of heat exchanger (HE) whose core is built based on a Triply Periodic Minimal Surface structure. The core of this exchanger is built as a periodic structure based on a gyroid-type lattice and is manufactured by laser powder-bed fusion technology. This solution is distinguished not only by an exceptionally favorable ratio of the heat exchange surface area to the volume occupied but also by a unique geometry that additionally turbulates the flow and intensifies the heat exchange process. This article contains the results of numerical analyses of the entire exchanger under different operating conditions and the results of analyses of small fragments of the core filled with cells of different sizes. Numerical analyzes of the lattice-type exchanger are performed on the basis of the experimentally validated numerical model. The objective of the study is to determine the performance of the gyroid HE under different operational conditions and select the best elementary cell size per exchanger core for the assumed operating conditions. The printed HE was compared with a plate HE that was 30% larger, although the lattice one managed to achieve 10.5% higher values in on average Number of Transfer Units (NTU) and on average 5% higher temperature effectiveness (TE) in the studied range of flow parameters.

Refrigerant direct cooling is currently being considered as an efficient thermal management technology in power battery systems. In this paper, four types of liquid cooling plates for power battery modules are designed and the computational model is constructed. With the model being validated, it is applied to analyze the effects of the cooling plate structure and cooling channel on the cooling and heat dissipation performances. The results reveal that for the conventional cooling method, i.e., when the cooling plate is placed on the bottom of the battery pack, a significant temperature gradient in the vertical direction is observed. On the contrary, adopting a serpentine cooling plate structure in the main wall or parallel cooling channels in the narrow side wall could significantly reduce the temperature difference of the battery pack, which can keep the temperature difference within 5 °C even under extreme conditions. Further, a detailed analysis of the heat dissipation performance based on the optimal cooling plate structure is performed to determine the safe operating range of the battery pack. It is shown that as the humidity is less than 0.73 and the evaporation temperature is below 15.43 °C, the battery pack can operate safely. However, beyond this condition range the heat dissipation characteristics of the battery pack cannot satisfy the operating requirements. This work sheds light upon the potential of refrigerant direct cooling strategy in power battery thermal management systems by properly arranging the cooling plate.

To improve the thermal management performance of high heat flux components in confined spaces, two three-dimensional oscillating heat pipes (3D-OHPs) with different adiabatic section lengths were designed in this work. 3D-OHPs with surfaces of different hydrophilicity was fabricated using alkaline-assisted oxidation technology, and the impact of surface hydrophilicity on the heat transfer performance of 3D-OHPs was investigated experimentally. The results indicated that the greater the hydrophilicity of the 3D-OHP, the better its start-up and heat transfer performance. A 3D-OHP with a shorter adiabatic section demonstrates slightly inferior start-up performance under identical hydrophilicity conditions but exhibits better overall heat transfer performance. It is also found that the 3D-OHP can initiate at 20 W under four distinct hydrophilicity conditions. Compared to the untreated 3D-OHP, the super-hydrophilic 3D-OHP reduces start-up temperature by 17.76 % and start-up time by 35.31 %. Under high-power conditions, the super-hydrophilic 3D-OHP exhibits a 37.6 % increase in thermal conductivity and a 61.5 % improvement in temperature uniformity compared to the untreated 3D-OHP. Furthermore, the thermal resistance and evaporation section temperature of the super-hydrophilic 3D-OHP are reduced by 56.69 % and 14.33 %, respectively. This study can broaden the approach to enhance the heat transfer performance of 3D-OHP and provide more application scenarios for the thermal management of power devices.

In the present study we use two-dimensional direct numerical simulations (DNS) to understand the coupled heat transfer to fluid flow in a liquid target for the production of nuclear medicines. Fluid motion is driven by buoyancy created by heat generated by a proton beam. The internal heat source has a gaussian distribution in the vertical direction and a rapidly growing intensity in the horizontal direction until it reaches a range at the Bragg peak where the heating drops to zero. The structure of the heating imposes two convective cells, separated at the location of the range. We solve the governing fluid flow and energy equations in a square cavity subject to highly nonuniform internal heating generated by the energy deposition of a proton beam. While most studies of convection driven by an internal heat source in a fluid layer have been focused on a uniform heating of the fluid, our study shows that the nonuniformity in the heat source has important implications for the temperature and flow fields, the boundary heat fluxes, and the growth of convective instabilities in the flow. Interestingly, the scalings of the maximum and averaged temperatures with the Rayleigh number compare similarly to previously found power laws for uniformly heated fluid layers. At higher power levels, the layer of fluid near the top cold boundary becomes convectively unstable via Rayleigh–Taylor instabilities. By comparing the rate of growth of these instabilities to their rate of advection to the boundaries of the cavity, a model is developed that predicts the instability of the convective cells for different values of the range of the beam and the Rayleigh number. Crucially, we demonstrate that the disturbances in the production of isotopes due to convective instabilities and the design of the cooling system is dependent on the location of the Bragg peak and must be considered in design of future generation of this class of target.

Smoldering combustion process has been suggested as a new method potentially useful in the treatment of biosolids. Natural forest duff (FD) often consists of organic fuel layers with varying particle sizes, yet the influence of the size-fractioned particles on the smoldering combustion dynamics in terms of the heat and mass transfer is not well understood. In this study, the oxidative pyrolysis and smoldering behavior of FD samples with four particle sizes (0 < d₁ ≤ 0.425 mm, 0.425 < d₂ ≤ 1 mm, 1 < d₃ ≤ 2 mm, 2 < d₄ ≤ 4 mm) were experimentally and theoretically investigated. Micro-scale thermo-gravimetric (TG) analysis, and a four-step kinetic model incorporating water evaporation, FD pyrolysis, FD oxidation and char oxidation showed that the activation energy of the FD pyrolysis and the ash content are negatively correlated, while the activation energy of the char oxidation increases from 87.26 kJ mol⁻¹ to 119.22 kJ mol⁻¹ with the particle size increasing from d₁ to d₄. Furthermore, the order of the combustion performance of the FD samples is shown as d₁<d₂<d₄<d₃. A series of laboratory-scale smoldering experiments revealed that the peak smoldering temperature increases with the fuel depth while the horizontal spread rate decreases with the fuel depth. Both the peak mass loss rate and the smoldering duration presented a reverse order (d₁>d₂>d₄>d₃) of the combustion performance found in the TG tests. A simplified heat transfer analysis qualitatively revealed the beneficial effects of the size-fractioned particles on the smoldering spread rate, while a mass transfer analysis revealed the favorable and adverse influences of the particle size on the kinetic-controlled and diffusion-controlled char oxidation rate, respectively. These findings verify that particle sizes alter the FD physicochemical properties (e.g., specific surface area, bulk density, porosity, permeability, chemical components, and lower calorific value), which in return impact the chemical kinetics, heat and mass transfer process in smoldering combustion. This work provides new insights into the effects of the size-fractioned particles on smoldering combustion, ultimately improving the fundamental understanding for optimizing particle sizes for energy conversion and usage.

Thermal characterization of honeycomb core sandwich panels with GFRP skins by theoretical formulations, and experimental methods followed by corroborating numerical models help identify the effective thermal conductivity in both the out-of-plane and in-plane directions. Swann–Pitman’s semi-empirical model formed the basis for the theoretical understanding of heat transfer in honeycomb core sandwich panels. Experimental approach involved a setup based on guarded hot plate (GHP) apparatus, and transient laser flash technique while the numerical approach involved FE modeling of the woven composite fabric face sheet by multi-scale modeling using the actual physical dimensions of the yarn e.g., crimp angle, width and height of the yarns etc. obtained using microscopic analysis. The fiber volume fraction of the experimental samples was estimated both by image analysis and burn-off method using Thermogravimetry (TGA). Various numerical models based on Mechanics of Structure Genome (MSG) and randomized fiber distribution in the yarns at the micro-level are compared with the experimental results to arrive at the model that most closely mimics real situation. Then, the numerical model is used to predict the thermal conductivity of the composite as the fiber volume fraction and crimp angle varies.

This article reports on the heat transfer performance of a compact heat exchanger based on metal foam and thermal interface material (TIM), which was built in-house. The heat exchanger was 20 x 18 cm, almost the size of a car radiator. The assembly method mimics the plate-type heat exchanger. This study investigates the effects of different commercially available TIMs, different pores per inch (PPI) foams, and compressive loads. The experiments were conducted with copper foams of 20 and 40 PPI with 1 mm thick plate fins and three commercially available TIMs (pad types): TIM 1 (4 W/mK), TIM 2 (5 W/mK), and TIM 3 (12.8 W/mK). Various configurations of the compact finned copper foam heat exchanger were tested at different Reynolds numbers in the range from 0 to 45,000. Nusselt number and pressure drop ratio of the heat exchanger with 3% (5 mm) and 6% (10 mm) compression were measured with a self-built test rig. In general, the Nusselt numbers ratio were dependent on the Reynolds number; they increased as the Reynolds number increased. The results show that the 20PPI_TIM 3 finned copper foam heat exchanger has the highest increase in Nusselt number ratio at a compression of 3%, which increases by 29% at a Reynolds number of 25,000 (compared to 20PPI_NoTIM). The heat exchanger configuration with the highest thermal conductivity (TIM 3, 12.8 W/mK) achieved the best heat transfer performance among the heat exchanger configurations tested. On the other hand, the pressure drop ratio in the finned copper foam heat exchanger of 40PPI_TIM 1 at 6% compression was 33% higher than that of 40PPI_NoTIM at a Reynolds number of 20,000 (the highest pressure drop ratio), which is due to the limitation of airflow through the heat exchanger caused by the smaller pores of the foams. This study shows that the combination of metal foam and fins with TIM in a heat exchanger can increase the surface area between the mating parts and thus increase heat transfer. It also provides an insight into an alternative way of bonding metal foams with other metals (or a base plate) to develop high performance heat exchangers.