Single-phase Flow Characteristics Research Articles

Numerical and experimental investigation of single phase flow characteristics in stirred tanks using Rushton turbine and flotation impeller

In this study, computational fluid dynamics (CFD) simulations are used to investigate turbulent single phase flow characteristics in lab-scale stirred tanks with different geometric variations. Water at standard conditions is used as operating fluid. Rushton turbine (RT) and flotation impeller (FI) are used to agitate the fluid leading to turbulent flows in the tank. For FI, impeller diameter, d, is varied and three sizes corresponding to d values of 75, 100, and 150mm are considered. Additionally, for 75 and 100mm FI, off-bottom clearance, C, is varied from 100 (D/3) to 60mm (D/5). The impeller based Reynolds number, Re, ranged from 29,000 to 120,000. CFD results are compared with LDA data from the literature for RT and in-house PIV data for FI. CFD predictions for FI are found to match experimental measurements satisfactorily with accurate prediction of flow transition at lower C. The normalized flow properties are observed to be invariant with Re for both impellers in fully turbulent regime. Mean flow characteristics for FI suggests that the flow is characterized by strong radial and tangential velocities close to impeller with peak values along disc level. Turbulence kinetic energy profiles close to impeller are characterized by two peaks suggesting development of trailing vortex which is further verified using swirling strength visualization. For FI with diameter equal to 100mm, flow transition in which mean flow changes from radial flow (double loop) to axial-type (single loop) flow is observed when C is reduced. Both PIV measurements and CFD simulation are able to predict this transition accurately. Using both torque on rotating parts and volume averaged dissipation rate of turbulence kinetic energy, power numbers are calculated for both impellers. The axial-type flow at smaller clearance is marked by significant drop in power number value.

Effect of angle of arch-supplied overfire air on flow, combustion characteristics and NOx emissions of a down-fired utility boiler

A new overfire air (OFA) technology was proposed by the authors for a Foster Wheeler down-fired boiler, and a small-scale cold experimental system for a 660-MWe unit was established. The velocity field distribution was measured in the furnace to study the characteristics of single-phase flow in the furnace at different OFA nozzle angles. The furnace temperature and distributions of the O2 concentration and NO concentration at different OFA nozzle angles were simulated using Fluent 6.3.26, and the average temperature of the furnace outlet, the O2 and NOx concentrations and carbon content in the fly ash were calculated. As the OFA nozzle angle increased, the vertical penetration depth of OFA increased and the horizontal penetration depth of OFA gradually decreased. The carbon content in the fly ash and NOx concentration initially rose and then fell at the furnace outlet, and they were lowest when the OFA angle was set at 20°. Thus, according to the findings of this article, an optimized OFA angle of 20° was chosen.

Pressure drop and heat transfer characteristics for single-phase developing flow of water in rectangular microchannels

Experiments were conducted to investigate the pressure drop and heat transfer characteristics of single-phase flow of de-ionized water in single copper microchannels of hydraulic diameters 0.438 mm, 0.561 mm and 0.635 mm. The channel length was 62 mm. The experimental conditions covered a range of mass flux from 500 to 5000 kg/m2 s in the laminar, transitional and low Reynolds number turbulent regimes. Pressure drop was measured for adiabatic flows with fluid inlet temperatures of 30°C, 60°C and 90°C. In the heat transfer tests, the heat flux ranged from 256 kW/m2 to 519 kW/m2. Friction factors and Nusselt numbers determined from the measurements were higher than for fully-developed conditions, but in reasonable agreement with predictions made using published solutions for hydrodynamically and thermally developing flow. When entrance effects, experimental uncertainties, heat losses, inlet and exit losses, thermal boundary conditions and departure from laminar flow were considered, the results indicate that equations developed for flow and heat transfer in conventional size channels are applicable for water flows in microchannels of these sizes.

Experimental investigation on flow and heat transfer characteristics of single-phase flow with simulated neutronic feedback in narrow rectangular channel

Abstract Single phase forced convection flow and heat transfer of deionized water in horizontal and vertical narrow rectangular channel with gap size of 1.8 mm were experimentally investigated. The integration of thermal hydraulic hardware tests with simulated neutronic feedback has been carried out in this experiment. The results indicated that the transition Reynolds number from laminar to turbulent region in the narrow rectangular channel was about 1500–2300. Compared with the conventional channels, the friction factor became larger in narrow rectangular channel. Nusselt number in narrow rectangular channel was higher than that in conventional channels in laminar region while it was lower in turbulent region. Furthermore, the variations of power, heating surface temperature, inlet/outlet temperature, Re number and Nu number in transient process were investigated with the consideration of neutronic feedback. It indicated that both the quantity and rate of introduced reactivity affected the peak of power. The quantity of power raise was not directly proportional to the introduced reactivity. The delay effect of wall and fluid temperature to the power raise could not be ignored. The temperature peak came later than the power peak. The variations of Re and Nu number were considerable due to the appreciable power raise. In addition, there was no significant difference on the variations of friction factor and Nu number versus Re number between transient and steady state.

Prediction of pressure drop using artificial neural network for non-Newtonian liquid flow through piping components

Numerous investigations have shown that ANN can be successful for correlating experimental data sets for macroscopic single phase flow characteristics. The approach proved its worth when rigorous fluid mechanics treatment based on the solution of first principle equations is not tractable. Evaluation and prediction of the frictional pressure drop across different piping components such as orifices, gate and globe valves and elbows in 0.0127 m piping components for non-Newtonian liquid flow are manifested in this paper. The experimental data used for the prediction is taken from our earlier work (Bandyopadhyay and Das, 2007). The proposed approach towards the prediction is done using a multilayer perceptron (MLP), which is trained with backpropagation algorithm because the function approximation is achieved with very good accuracy using MLPs.

Flow characteristics of single-phase flow in narrow annular channels

The characteristics of single-phase flow in narrow annular channels were analyzed and theoretical model was proposed. Based on the present model, the theoretical calculation was performed to predict the flow characteristics for the developed flow in narrow annuli with the gap sizes of 1.0, 1.5 and 2.0 mm, respectively. The results were in good agreement with the experimental data. In addition, the gap size of narrow annuli has great impact upon the flow characteristics. The decrease of gap size reduces friction factor. The higher the Reynolds number, the more remarkable the effect of gap size upon friction coefficient during single-phase flowing through narrow annular channels. The effect of gap size upon friction coefficient is dependent on the Reynolds number, and will decrease with the decrease of the Reynolds number.

Single phase flow characteristics in the headers and connecting tube of parallel tube platen systems

Analytical solutions have been obtained for single phase flow in parallel tube platen systems where the platen configuration (geometric shape and dimensions) of each connecting tube within a platen can vary. Continuous and discrete mathematical models have been derived to study the distributing and combining flow characteristics in platens along header axes and in connecting tubes within each individual platen, respectively. The specific volume of working fluid is assumed to vary linearly along the connecting tube length, in order to close the mathematical equations. Analytical solutions of static pressure and velocity in the headers and connecting tubes have been obtained firstly under the assumption that the friction along the headers can be neglected. Then, these solutions are corrected to account for the effects of friction along the headers. Both sets of solutions are the general solutions for all flow configurations of parallel tube platen systems where the flow parameters in the headers are one-dimensional and those in connecting tubes are three-dimensional. These general solutions can be applied directly to the design and analyses of superheaters and reheaters in power boilers, and other heat exchangers of same configurations.

Thermal–hydraulic characteristics of single-phase flow in capillary pipes

The objective of the present paper is to provide a general overview of the research carried out so far in single-phase heat transfer and flow in capillary (micro) pipes. Laminar flow and laminar-to-turbulent flow transition are analyzed in detail in order to clarify the discrepancies among the results obtained by different researchers. Experiments performed in the ENEA laboratory indicate that in laminar flow regime the friction factor is in good agreement with the Hagen–Poiseuille theory for Reynolds number below 600–800. For higher values of Reynolds number, experimental data depart from the Hagen–Poiseuille law to the side of higher f values. The transition from laminar to turbulent regime occurs for Reynolds number in the range 1800–2500. Diabatic experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional tubes, are not properly adequate for heat transfer coefficient prediction in microtubes.