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Flow boiling stands out as an efficient mode of heat transfer with significant potential for reducing carbon footprint. Based on a coupled Volume-of-Fluid and Level Set (VOSET) method, this numerical study reveals the microlayer and its evaporative heat flux distributions, as well as the effects of microlayer evaporation and depletion on bubble dynamic behavior, flow pattern, wall superheat distribution, and boiling crisis for the flow boiling in a mini-channel. These aspects are firstly comprehensively reported by the numerical method. Results indicate that the rapid evaporation of a thin microlayer positioned beneath elongated bubbles significantly contributes to heat dissipation during flow boiling in a mini-channel. The heat flux through the microlayer surpasses 300 kW/m2, accounting for approximately 42% of heat dissipation at 200 kW/m2. Furthermore, this contribution increases to over 70% as the heat flux exceeds 500 kW/m2. As a result, in comparison to scenarios without microlayers, the presence of microlayers results in an average heat transfer coefficient (HTC) that is 3.97 times larger within the range of 200–500 kW/m2. Additionally, microlayer evaporation delays the happening of flow boiling crisis. Without microlayers, the wall superheat exceeds 180 K at 500 kW/m2, whereas under microlayer influence, the maximum local wall superheat is only 37.09 K at 1000 kW/m2. With further increases in heat flux, the emergence of large dry patches resulting from microlayer depletion induces the beginning of heat transfer deterioration. Finally, the ongoing coalescence of elongated bubbles necessitates a prolonged period for rewetting dry patches at the exit of the mini-channel, triggering the flow boiling crisis at 1300 kW/m2. The study results provide crucial guidance for the development and application of the flow boiling heat transfer technology for the thermal management of data centers. [ABSTRACT FROM AUTHOR]

• A comprehensive review on numerical simulations of boiling heat transfer from atomistic to mesoscopic and macroscopic scales is presented. • Critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling are highlighted. • Potential solutions and future research in the field of boiling heat transfer using the MDS, LB and CFD methods are proposed. Boiling is an efficient heat transfer mode with significant potential for thermal management in high-power electronic equipment. However, a comprehensive understanding of the boiling process, which encompasses bubble nucleation, growth, coalescence, slipping, and detachment across various scales, remains challenging. Molecular dynamics simulation, lattice Boltzmann, and computational fluid dynamics methods are popular and powerful tools for investigating boiling heat transfer phenomena at microscopic, mesoscopic, and macroscopic scales. These methods enable researchers to uncover the underlying boiling mechanisms and propose heat transfer enhancement techniques. Therefore, this paper provides a comprehensive review of boiling heat transfer, spanning from atomistic to mesoscopic and macroscopic scales, utilizing these three numerical methods. It addresses critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling, and proposes potential solutions and future researches, supplementing our previous review [Some advances in numerical simulations of multiscale heat transfer problems and particularly for boiling heat transfer, Annu. Rev. Heat Transf., 6 (2022) 217–269]. Besides, by shedding light on the characteristics of these numerical methods in studying boiling heat transfer, this paper aims to foster their development and advance enhanced heat transfer technologies. [ABSTRACT FROM AUTHOR]

Highlights • Relative humidity has little effect on the temperature distribution of the fin. • No condensation occurs near the inlet area. • The maximum condensation mass flux is detected in the second row of pipe. • Relative humidity has little influence on the sensible heat transfer rate. • But manifests profound effects on the latent and total heat transfer rate. Abstract The heat and mass transfer of a five-row fin-and-tube heat exchanger under dehumidifying conditions is numerically studied in this paper, and the distributions of vapor condensation mass rate for both fin and tube surfaces are obtained. The influences of vapor mass percent of humid air and inlet velocity on heat and mass transfer are investigated. Numerical results reveal that relative humidity has little effect on the temperature distribution of the fin under humid conditions. It is also noticed that no condensation occurs near the inlet area, and the maximum condensation mass flux is detected in the second row of pipe; this phenomenon is more obvious at higher inlet velocities. Relative humidity has little influence on the sensible heat transfer rate; however, it manifests profound effects on the latent heat transfer rate and total heat transfer rate. The dimensionless heat and mass transfer factors are also obtained from the numerical study and compared with the correlations in literature. [ABSTRACT FROM AUTHOR]

• A method for studying microlayer effects on flow boiling is proposed. • The availability of combining radius-based and taylor models in describing microlayer formation is verified. • The implementation procedure of the microlayer model is fully demonstrated. • The effects of different microlayer models on flow boiling are illustrated. • The contribution of microlayer evaporation to heat transfer is 75%−83%. The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel. [ABSTRACT FROM AUTHOR]

A thermal conductivity test apparatus based on transient plane source method is built and developed to measure effective thermal conductivity of open porous materials at different gas pressures. The effective thermal conductivity of open nano-porous silica materials with porosity of 88.5% is measured under gas pressures ranging from 0.001 Pa to 1 MPa. The contribution of gaseous heat conduction to the effective thermal conductivity of materials is decomposed by subtracting the effective thermal conductivity at ultimate vacuum from that at different gas pressure. It is found that the contribution of gaseous heat conduction is much different with the gas thermal conductivity in nano-porous materials and that in free space. The result is also demonstrated by theoretical analysis and numerical simulation. [ABSTRACT FROM AUTHOR]

Abstract: To improve the thermal efficiency of gas fired equipment, latent heat recovery from the flue gas is a very important concept. Based on the analysis of heat transfer enhancement mechanism in an internally finned tube, in this paper, a new type left–right symmetric internally finned tube is proposed. The heat transfer characteristics of the new type left–right symmetric internally finned tube, the centrosymmetric finned tube and the smooth tube is numerically and experimentally investigated. The numerical simulation results indicate that the left–right symmetric internally finned tube has a promising potential for enhancing the convection heat transfer, in particular, its heat transfer coefficient is 4–8 times higher than that of the reference smooth tube and 1.3–1.8 times higher than that of the centrosymmetric finned tube. The experimental results show that the excess air coefficient, the cooling water flow rate, the water inlet temperature, and the Re number have prominent effects on the convection-condensation heat transfer. The left–right symmetric internally finned tube can effectively decrease the radial temperature gradient, reduce the thickness of the non-condensable gas layer and significantly strengthen the water vapor condensation. The convection-condensation heat transfer coefficient of 10 fins left–right symmetric internally finned tube is about 4.1 times higher than that of the forced convection without condensation. A dimensionless number Tv is derived, which takes into account the effects of the water vapor saturation temperature, the average flue gas temperature, the average internal wall temperature and the inlet water temperature. They are represented in an empirical formula for the condensation heat transfer process. [Copyright &y& Elsevier]

Abstract: The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger with delta winglets are investigated numerically. The three-dimensional simulations are performed with renormalization-group (RNG) k − ɛ model to lay the foundation for the design of the high-performance heat exchanger. The wavy fin-and-tube heat exchangers which have three-row round tubes in staggered or in-line arrangements are studied. The numerical results show that each delta winglet generates a downstream main vortex and a corner vortex. For the in-line array, the longitudinal vortices enhance the heat transfer not only on the fin surface in the tube wake region but also on the tube surface downstream of the delta winglet; for the staggered array, longitudinal vortices are disrupted at the first wavy trough downstream from the delta winglet and only develop a short distance along the main-flow direction, and the vortices mainly enhance the heat transfer of the fin surface in the tube wake region. The longitudinal vortices generated by delta winglet cause considerable augmentation of heat transfer performance for wavy fin-and-tube heat exchanger with modest pressure drop penalty. When , compared with the wavy fin, the j and f factors of the wavy fin with delta winglets in staggered and in-line arrays are increased by 13.1%, 7.0% and 15.4%, 10.5%, respectively. [Copyright &y& Elsevier]

The enhancement of airside heat transfer in fin-and-tube heat exchanger is crucial for reducing the pumping power, saving materials and so on. In this paper, the authors propose to arrange the winglets radiantly around the tubes on the fin to enhance the airside heat transfer in fin-and-tube heat exchanger. The airside flow and heat transfer characteristic are numerically investigated. The position and the geometric dimensions of the vortex generators are also optimized. The simulation results show that the Colburn j -factor and the friction factor f both increase with VG number. The fin structure has the best comprehensive performance when its VGs are uniformly arranged both in the upwind and backwind regions of the tubes, and at the same time make the enhanced zones the widest in the tube radial direction. With the increase of the VG length, the comprehensive performance factor JF gets lower. The comprehensive performance of the proposed fin with five rows of tubes are demonstrated to be better than a reference fin used in industry with the same fin thickness, tube diameter, fin pitch and tube pitch. [ABSTRACT FROM AUTHOR]