ASSESSMENT OF ANNULAR FLOW BOILING IN THE CONTEXT OF COMPUTATIONAL FLUID DYNAMICS (CFD) SIMULATIONS, EXPERIMENTS, AND EXISTING CORRELATIONS

The main objective of the current work is to achieve a better understanding – through modeling and simulation/programming activities – of annular flow-boiling and its applications based on a synthesis of Computational Fluid Dynamics (CFD) simulations, existing correlations, and experiments. A unique and rigorous 2-D CFD simulations technique was developed for annular flow-boiling to propose a correlation for convective component of Heat Transfer Coefficient (HTC) – defined here as flow-boiling in the absence of nucleation. To provide a context for the correlation structure of convective component of HTC, flow-physics details of annular flow-boiling and correlation structure (based on fundamental considerations) for HTC have also been discussed. Further, other existing correlations for Nusselt number, void-fraction, flow-regime transition, and pressure-drop have been used to develop a general but first order engineering estimates-methodology for design of annular flow-boilers and flow-condensers. The first order estimates-methodology, thus developed for annular flow-boiler operations, was used to: (i) make a priori estimates of flow predictions towards choosing suitable instrumentations for the design of a particular test-section and associated flow-loop needed for a new high heat-flux flow-boiling experiments involving water as a working fluid), and (ii) define a range of experimental operating conditions – for a low heat-flux test-section and flow-loop (involving FC-72 as working fluid) facility from which experimental data needed to be obtained to throw light on the flow-physics being modeled by the CFD code. Furthermore, preliminary results from a different low heat-flux experiments are briefly discussed here and then compared with rigorous CFD simulations to achieve better understanding of the flow-physics

Designing Of Advanced And Original Austempering Processes Based On Thermal Science And Engineering Physics Approaches

In the paper, a small concentration of inverse solubility polymers in water and other liquid media is recommended to eliminate film boiling by means of reducing initial heat flux density. Quenching steel parts and tools in a small concentration of water solutions under pressure allows performing austempering process just using cold liquids. Its essence consists in coinciding martensite start temperature MS with the average temperature of self-regulated thermal process during nucleate boiling mode and further immediate transferring steel parts for tempering at the temperature which exceeds value MS. The new technology increases the service life of austempered workpieces by more than two times, saves alloy elements, is suitable for larger metallic components, improves environmental conditions, since instead of melted salts and alkali, plain water and water salt solutions can be used. The new austempering process can be used in forging shops to obtain super-strengthened materials in order to switch from alloy steel to plain carbon steels. And it can be also widely used for obtaining nano - bainitic structure in plain carbon steels resulting in saving alloy elements and improving mechanical characteristics of materials

Physics of puffing and microexplosion of emulsion fuel droplets

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.This article has been made available through the Brunel Open Access Publishing Fund

Numerical investigation of boiling

In this work, we study different phenomena that occur during nucleate boiling. We numerically investigate boiling using two phase flow direct numerical simulation based on a level set / Ghost Fluid method. This method allows us to follow the interface and to make accurate geometric calculation as for bubble curvature. Nucleate boiling on a plate is not only a thermal issue, but also involves multiphase dynamics issues at different scales and at different stages of bubble growth. As a consequence, we divide the whole problem and investigate separately the different phenomena considering their nature and the scale at which they occur. First we analyse the boiling of a static bubble immersed in an overheated liquid. We perform numerical simulations at different Jakob numbers in the case of strong discontinuity of density through the interface. These simulations permit us to estimate the accuracy of our numerical method dealing with phase change in the context of two phase flow direct numerical simulation. The results show a good agreement between numerical bubble radius evolution and the theoretical evolution found by Scriven(1959). The validation of our code for the Scriven case allows to pursue our study by focusing on the phenomena that take place in the particular case of an interaction with a wall. This interaction is characterised by the angle formed between a solid and a fluid interface, named the contact angle. We implement a method that makes it possible for a droplet, to reach, in the case of a static contact angle, a steady state corresponding to a theoretical equilibrium. Besides this method enables to take into account the contact angle hysteresis model, which considers different angles whether the contact line is advancing or recoiling. We perform simulations of the spreading of a liquid droplet impacting on a plate, and we compare the maximum spreading diameter and the advancing and receding droplet behaviour of our numerical results with the experimental data Son and Lee (2010) have reported

Investigation of Transient Nucleate Boiling Processes and Their Practical Use in Heat Treating Industry

In the paper transient nucleate boiling process is widely discussed. It's unknown previously and investigated by author characteristics create a basis for designing of new technologies which allow receiving super strengthened materials. Obtained results are also used for appropriate software development to be widely applied for control of technological processes and cooling recipes design. A possibility of transition from real heat transfer coefficients (HTCs) to effective HTCs is discussed in the paper too. It is shown that core temperature of steel parts at the end of transient nucleate boiling (self-regulated thermal process (SRTP)) is a linear function of a part dimension when convective heat transfer coefficient during quenching in liquid media is fixed. Also, it is shown that effective Kondrtajev number Kn is a function of part size and convection intensity and is almost linear function for large sizes of steel parts. Surface temperature at the beginning of self-regulated thermal process and at its end is calculated depending on size and intensity of cooling. Based on obtained new results, it is possible to design DATABASE for liquid quenchants using standard Inconel 600 probe combined with the Liscic/Petrofer probe. Obtained results can be useful for engineers and software designers

Forced boiling of nanofluids, effects of contact angle and surface wettability

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Nanofluids are the suspension of ultra fine particles in a conventional base fluid which tremendously changes the heat transfer characteristics of the original fluid. In this paper the boiling characteristics of different nanofluids was studied numerically using a CFD approach. Dispersions of Al2O3, SiO2, and ZrO2 nanoparticles in water at different concentrations (0.1, 0.01 and 0.001% by volume) have been used. Effects of some noticeable parameters such as nanoparticle concentration and temperature profile on the critical heat flux (CHF) have been investigated. The results of CFD simulation based on two-phase models were compared with experimental data. Boiling curves and critical heat flux were measured for the base fluid and the nanofluids. Based on the simulation results, it was concluded that the using of the Zirconium oxide (0.001%) led to modest (up to 31%) increase in the CHF. The minimum enhancement belongs to the aluminum oxide (0.1%) which increases the critical heat flux up to 11%. According to the experimental results, despite of expectation, addition of the nanoparticles causes decreasing the boiling heat transfer coefficient. This reduction is related to the changing of the surface characteristic causing by depositing the nanoparticles. In the Al2O3/water and SiO2/water nanofluids, the surface contact angle increases with increase in the nanoparticle volume fraction, so the CHF decreases