Showing papers on "Critical heat flux published in 2017"

Heat transfer characteristics in nanofluid—A review

Abstract: The heat transfer characteristics of current fluids are tremendously improved by suspending nano-sized solid particles with diameter below 100 nm and are considered as prospective working fluids for the applications such as solar collectors, heat pipes, nuclear reactors, electronic cooling systems, automobile radiators etc. The present paper summarizes the current research in the nanofluid studies on convective heat transfer performance, thermo-physical properties, effect of fluid temperature, inlet velocity, use of surfactant for better stability of nanofluids, particle size, and volume concentration effects. The article also suggests the direction for future developments.

Surface heat loads on the ITER divertor vertical targets

Abstract: The heating of tungsten monoblocks at the ITER divertor vertical targets is calculated using the heat flux predicted by three-dimensional ion orbit modelling. The monoblocks are beveled to a depth of 0.5 mm in the toroidal direction to provide magnetic shadowing of the poloidal leading edges within the range of specified assembly tolerances, but this increases the magnetic field incidence angle resulting in a reduction of toroidal wetted fraction and concentration of the local heat flux to the unshadowed surfaces. This shaping solution successfully protects the leading edges from inter-ELM heat loads, but at the expense of (1) temperatures on the main loaded surface that could exceed the tungsten recrystallization temperature in the nominal partially detached regime, and (2) melting and loss of margin against critical heat flux during transient loss of detachment control. During ELMs, the risk of monoblock edge melting is found to be greater than the risk of full surface melting on the plasma-wetted zone. Full surface and edge melting will be triggered by uncontrolled ELMs in the burning plasma phase of ITER operation if current models of the likely ELM ion impact energies at the divertor targets are correct. During uncontrolled ELMs in pre-nuclear deuterium or helium plasmas at half the nominal plasma current and magnetic field, full surface melting should be avoided, but edge melting is predicted.

Cattaneo-Christov model for radiative heat transfer of magnetohydrodynamic Casson-ferrofluid: A numerical study

Abstract: The knowledge of heat transfer in MHD nanofluid flows over different geometries is very important for heat exchangers design, transpiration, fiber coating, etc. Recent days, heat transfer of non-Newtonian nanofluids plays a major role in manufacturing processes due to its shear thinning and thickening properties. Naturally, magnetite (Fe3O4) nanoparticles move randomly within the base fluid. By applying the transverse magnetic field, the motion of those nanoparticles becomes uniform. This phenomenon is very useful in heat transfer processes. With this initiation, a mathematical model is developed to investigate the heat transfer behaviour of electrically conducting MHD flow of a Casson nanofluid over a cone, wedge and a plate. We consider a Cattaneo-Christov heat flux model with variable source/sink and nonlinear radiation effects. We also considered water as the base fluid suspended with magnetite nanoparticles. R-K-Felhberg-integration scheme is employed to resolve the altered governing nonlinear equations. Impacts of governing parameters on common profiles (temperature and velocity) are conversed (in three cases). By viewing the same parameters, the friction factor coefficient and heat transfer rate are discussed with the assistance of tables. It is found that the boundary layers (thermal and flow) over three geometries (cone, wedge and a plate) are not uniform. It is also found that the thermal relaxation parameter effectively enhances the heat local Nusselt number and the heat transfer performance is high in the flow over a wedge when compared with the flows over a cone and plate.

Thermal behavior of aqueous iron oxide nano-fluid as a coolant on a flat disc heater under the pool boiling condition

Abstract: This paper experimentally focuses on the pool boiling heat transfer characteristics of gamma Fe3O4 aqueous nano-fluids on a flat disc heater. The nano-fluid used in this research was prepared using two-step method and was stabilized using nonylphenol ethoxylate nonionic surfactant, pH setting, and sonication process as well. Influence of different operating parameters such as heat flux (0–1546 kW/m2), mass concentration of nano-fluids (weight concentration 0.1–0.3 %), bubble formation, critical heat flux (1170 kW/m2 for water, 1230 kW/m2 (wt% = 0.1), 1320 kW/m2 (wt% = 0.2), 1450 kW/m2 (wt% = 0.3) and fouling on pool boiling heat transfer coefficient of nano-fluid as a thermal performance index were experimentally investigated and briefly discussed. Results demonstrated that the pool boiling heat transfer coefficient increases with increasing the mass concentration and the applied heat flux. In addition, the rate of bubble formation is significantly intensified at higher heat fluxes and subsequently, larger bubbles detach the surface due to the intensification of bubble coalescence. In terms of fouling formation, it can be stated that fouling of nano-fluids is a strong function of time and rate of deposition is increased over the extended time while the pool boiling heat transfer coefficient was not decreased over the time, as porous deposited layer on the surface are detached from the surface by bubble interactions. In terms of critical heat flux, capillary action of the deposited layer was found to be the main reason responsible for increasing the critical heat flux as liquid is stored inside the porous deposited layer, which enhances the surface toleration against the critical heat flux crisis.

Enhancement of Pool Boiling Heat Transfer Using Aligned Silicon Nanowire Arrays

Abstract: Enhancing the critical heat flux (CHF), which is the capacity of heat dissipation, is important to secure high stability in two-phase cooling systems. Coolant supply to a dry hot spot is a major mechanism to prevent surface burn-out for enhancing the CHF. Here, we demonstrate a more ready supply of coolant using aligned silicon nanowires (A-SiNWs), with a high aspect ratio (>10) compared to that of conventional random silicon nanowires (R-SiNWs), which have a disordered arrangement, for additional CHF improvement. We propose the volumetric wicking rate, which represents the coolant supply properties by considering both the liquid supply velocity and the amount of coolant (i.e., wicking coefficient and wetted volume, respectively). Through experimental approaches, we confirm that the CHF is enhanced as the volumetric wicking rate is increased. In good agreement with the fabrication hypothesis, A-SiNWs demonstrate higher coolant supply abilities than those of R-SiNWs. The longest (7 μm) A-SiNWs have the hig...

Thermal analysis of supercritical pressure CO2 in horizontal tubes under cooling condition

Abstract: Numerical investigation on convective heat transfer of supercritical pressure CO2 in horizontal tubes under cooling condition is carried out by using SST k-ω turbulent model. The effects of heat flux, tube diameter and buoyancy on heat transfer characteristics are discussed. The results show that the temperature stratification and secondary flow are generated, and the radial velocity and turbulent kinetic energy profiles are asymmetric on the cross-section due to buoyant effect. The peak of heat transfer coefficient near the pseudo-critical temperature appears earlier but with a smaller peak value on the bottom surface than on the top surface. The heat transfer discrepancy on the cross-section can be explained by field synergy principle well. The heat flux has no evident influence on the peak value of heat transfer coefficient, but affects its position seriously. The larger the heat flux and the tube diameter are, the more significant the buoyant effect becomes.

Unified correlation for heat transfer during boiling in plain mini/micro and conventional channels

Abstract: A correlation is presented for predicting heat transfer coefficients during saturated boiling prior to critical heat flux in mini/micro channels as well as channels of conventional sizes in horizontal and vertical upward flow. The correlation is verified with a database that includes channels of various shapes (round, rectangle, triangle), fully or partially heated, horizontal and vertical downflow, diameters 0.38 to 27.1 mm, 30 fluids (water, CO2, ammonia, halocarbon refrigerants, organics, cryogens), reduced pressure 0.0046 to 0.787, and mass flux 15 to 2437 kg m−2s−1. The new correlation predicts the 4852 data points from 137 data sets from 81 sources with a mean absolute deviation of 18.6 %. Several other correlations were also compared with the same database; all had significantly higher deviations.