Accepted Manuscripts

Wei Li, Yang Luo, Jing-zhi Zhang and W. J. Minkowycz
J. Heat Transfer   doi: 10.1115/1.4040147
This paper presents a fundamental research on hydrodynamics and heat transfer about an elongated bubble flow boiling in a circular microchannel. In present study, continuum Surface Force (CSF) model based on Volume of Fluid (VOF) methodis combined withthermocapillaryforce to explore the effects of thermocapillarity for flow boiling in microchannels. To validate the self-defined codes, a two-phase thermocapillary-driven flow and a Taylor bubble growing in a capillary tube are performed. Results of both test cases show good convergence and are in good agreement with data from other literatures. Bubble motion and local heat transfer coefficient (HTC) on the heated wall with respect to time are discussed in present study. It is found that for large Marangoni number (case 3), variation of surface tension has an influenceon bubble shape and temperature profile. Thermocapillary effect induces convection in liquid film region, which augments the HTCs at specified positions. The numerical investigation also shows that average HTC has increased by 6.7% in case 3 when compared with case 1. Thus, it is very significative to further study the influence of themocapillarity and Marangoni effect on bubble growth in microchannels.
TOPICS: Simulation, Bubbles, Evaporation, Gravity (Force), Microchannels, Boiling, Flow (Dynamics), Hydrodynamics, Heat transfer, Fluids, Bubbly flow, Convection, Heat transfer coefficients, Surface tension, Liquid films, Shapes, Temperature profiles
Jiannan Chen, Rui-Na Xu, Zhen Zhang, Xue Chen, Xiaolong Ouyang, Gaoyuan Wang and Peixue Jiang
J. Heat Transfer   doi: 10.1115/1.4039903
Enhancing spray cooling with surface structures is a common, effective approach for high heat flux thermal management to guarantee the reliability of the many high power, high speed electronics and to improve the efficiency of new energy systems. However, the fundamental heat transfer enhancement mechanisms are not well understood especially for nanostructures. Here, we fabricated six groups of nanowire arrayed surfaces with various structures and sizes that show for the first time how these nanostructures enhance the spray cooling by improving the surface wettability and the liquid transport to quickly rewet the surface and avoid dry out. These insights into the nanostructure spray cooling heat transfer enhancement mechanisms are combined with microstructure heat transfer mechanism in integrated microstructure and nanostructure hybrid surface that further enhances the spray cooling heat transfer.
TOPICS: Cooling, Sprays, Nanowires, Heat transfer, Nanostructures , Reliability, Energy / power systems, Thermal management, Electronics, Heat flux
Marcelo J.S. de Lemos and Paulo Carvalho
J. Heat Transfer   doi: 10.1115/1.4039915
This work presents a study of double-diffusive free convection in a porous square cavity under turbulent flow regime and with aiding drive. The thermal non-equilibrium model was employed to analyze the energy and mass transport across the enclosure. Governing equations were time- and volume averaged according to the double-decomposition concept. Analysis of a modified Lewis number, Lem, showed that for porous media this parameter presents opposite behavior when varying the thermal conductivity ratio or the Schmidt number, while maintaining the same value for Lem. Differently form free flow, the existence of the porous matrix contributes to the overall thermal diffusivity of the medium, whereas mass diffusivity is only effective within the fluid phase for an inert medium. Results indicated that increasing Lem through an increase in Sc reduces flow circulation inside porous cavities, reducing Nuw and increasing Shw. Results further indicate that increasing the buoyancy ratio N promotes circulation within the porous cavity, leading to an increase in turbulence levels within the boundary layers. Partial contributions of each phase of the porous cavity (solid and fluid) to the overall average Nusselt number becomes independent of N for higher values of the thermal conductivity ratio, ks/kf. Further, for high values of ks/kf, the average Nusselt number drops as N increases.
TOPICS: Porous materials, Turbulence, Equilibrium (Physics), Buoyancy, Convection, Cavities, Flow (Dynamics), Fluids, Thermal conductivity, Boundary layers, Thermal diffusivity, Natural convection
Salam Hadi and Mustafa Rahomey
J. Heat Transfer   doi: 10.1115/1.4039642
Numerical simulations are carried out for fluid flow and natural convection heat transfer induced by a temperature difference between a hot inner cylinder with different geometries (i.e. circular; triangular; elliptic; rectangular; and rhombic) and a cold outer square enclosure filled with nanofluid superposed porous-nanofluid layers. The Darcy-Brinkman model is applied for the saturated porous layer with nanofluid. Moreover, the transport equations (mass, momentum, and energy) are solved numerically using the Galerkin weighted residual method by dividing the domain into two sets of equations for every layer with incorporating a non-uniform mesh size. The considered domains in this investigation are closely examined over a wide range of Rayleigh number (103 = Ra= 106), Darcy number (10-5 = Da = 10-1), the thickness of porous layer (0% = Xp = 100%), thermal conductivity ratio (1 = Rk = 20) and nanoparticle volume fraction (0 = ? = 0.1), respectively. The nanofluid is considered to be composed of Cu-nanoparticle and water as a base fluid. The results showed that the obtained total surfaces-averaged Nusselt numbers of the enclosure, in all cases, at the same operating conditions, the rate of heat transfer from the enclosure which the triangular cylinder is located inside is better. Also, as the thickness of the porous layer is increased from 20% to 80%, the free convection performance will decrease significantly (to about 50%) due to the hydrodynamic properties of the porous material.
TOPICS: Natural convection, Circular cylinders, Cylinders, Nanofluids, Heat transfer, Nanoparticles, Thermal conductivity, Momentum, Fluid dynamics, Temperature, Fluids, Porous materials, Computer simulation, Rayleigh number, Water
Je-Chin Han
J. Heat Transfer   doi: 10.1115/1.4039644
Gas turbines have been extensively used for aircraft engine propulsion, land-based power generation, and industrial applications. Power output and thermal efficiency of gas turbines increase with increasing turbine rotor inlet temperatures (RIT). Currently, advanced gas turbines operate at turbine RIT around 1700°C far higher than the yielding point of the blade material temperature about 1200°C. Therefore, turbine rotor blades need to be cooled by 3-5% of high-pressure compressor air around 700°C. To design an efficient turbine blade cooling system, it is critical to have a thorough understanding of gas turbine heat transfer characteristics within complex 3-D unsteady high-turbulence flow conditions. Moreover, recent research trend focuses on aircraft gas turbines operate at even higher RIT up to 2000°C with a limited amount of cooling air, and land-based power generation gas turbines (including 300-400 MW combined cycles with 60% efficiency) burn alternative syngas fuels with higher heat load to turbine components. It is important to understand gas turbine heat transfer problems with efficient cooling strategies under new harsh working environments. Advanced cooling technology and durable thermal barrier coatings (TBCs) play most critical roles for developments of new-generation high-efficiency gas turbines with near-zero emissions for safe and long-life operation. This paper reviews basic gas turbine heat transfer issues with advanced cooling technologies and documents important relevant papers for future research references.
TOPICS: Cooling, Gas turbines, Turbines, Heat transfer, Temperature, Rotors, Blades, Energy generation, Thermal barrier coatings, Aircraft engines, Emissions, Combined cycles, Thermal efficiency, Turbine components, Syngas, Aircraft, Flow (Dynamics), Heat, Cooling systems, Fuels, Turbulence, Compressors, Stress, Propulsion, Turbine blades, High pressure (Physics), Design
Francisco Valentin, Narbeh Artoun, Masahiro Kawaji and Donald McEligot
J. Heat Transfer   doi: 10.1115/1.4039585
High pressure/high temperature forced and mixed convection experiments have been performed with helium and nitrogen at temperatures and pressures up to 893K and 64 bar, respectively. The test section had a 16.8-mm ID flow channel in a 108-mm OD graphite column. Flow regimes included turbulent, transitional and laminar flows with the inlet Reynolds numbers ranging from 1,500 to 15,000. Due to strong heating, the local Reynolds number decreased by up to 50% over the 2.7-m test section. In addition, heat transfer degradation and flow laminarization caused by intense heating led to Nusselt numbers 20~50% lower than the values given by the modified Dittus-Boelter and modified Gnielinski correlations. Flow laminarization criteria were considered based on a dimensionless acceleration parameter (Kv) and buoyancy parameter (Bo*). Upward turbulent flows displayed higher wall temperatures than downward flows, due to the impact of flow laminarization which is not expected to affect buoyancy-opposed flows. Laminar Reynolds number flows presented an opposite behavior due to the enhancement of heat transfer for buoyancy-aided flows. At low Reynolds numbers, downward flows displayed higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases. In the entrance region of downward flows, convection heat transfer was reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancy-induced mixing caused higher convection heat transfer and lower wall temperatures.
TOPICS: Flow (Dynamics), High pressure (Physics), Graphite, High temperature, Buoyancy, Reynolds number, Wall temperature, Heating, Heat transfer, Turbulence, Convection, Mixed convection, Entrance region, Laminar flow, Temperature, Helium, Nitrogen

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